Quantitative element analysis by isotope dilution in diode laser graphite furnace atomic absorption spectrometry

Spectrochimica Acta Part B 63 (2008) 539–560

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Spectrochimica Acta Part B j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s a b

Review

Quantitative element analysis by isotope dilution in diode laser graphite furnace atomic absorption spectrometry ☆ H.D. Wizemann ⁎ Institute of Physics and Meteorology, University of Hohenheim, Garbenstrasse 30, 70599 Stuttgart, Federal Republic of Germany

A R T I C L E

I N F O

Article history: Received 7 February 2008 Accepted 4 March 2008 Available online 15 March 2008 Keywords: Atomic absorption spectrometry Graphite furnace Diode laser Isotope dilution Doppler-free spectrometry

A B S T R A C T The paper reviews the application of the isotope dilution technique in optical atomic absorption spectrometry by use of a low-pressure graphite tube furnace as atomizer and diode lasers as radiation sources. The principles and the methodology to obtain accurate quantitative results despite of the occurrence of interferences are presented. The successful application of different Doppler-limited and Doppler-free spectrometric techniques is also presented. The perspectives but also the limitations of this new method are discussed. © 2008 Elsevier B.V. All rights reserved.

Contents 1. 2.

3.

4.

5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Matrix effects in GFAAS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Definition and classification . . . . . . . . . . . . . . . . . . . . . . . 2.2. Cause of interferences and their forms of appearance . . . . . . . . . . . 2.3. Conventional methods for elimination of interferences . . . . . . . . . . The isotope dilution method . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Isotope dilution in mass spectrometry . . . . . . . . . . . . . . . . . . 3.2. Isotope dilution in optical spectrometry . . . . . . . . . . . . . . . . . Diode lasers in graphite furnace atomic absorption spectrometry . . . . . . . . . 4.1. General remarks about the use of diode lasers in AAS . . . . . . . . . . . 4.2. Measurements applying isotope selective diode laser GFAAS . . . . . . . . 4.2.1. Preconditions for isotopic resolution . . . . . . . . . . . . . . . 4.2.2. Simultaneous measurement of two isotopes . . . . . . . . . . . 4.2.3. Application of 2f-WM in isotope selective spectrometry . . . . . . 4.2.4. Limits of detection obtained for Li, Rb and Pb . . . . . . . . . . 4.2.5. Application of saturation spectrometry . . . . . . . . . . . . . . Applications of isotope dilution in isotope selective GFAAS . . . . . . . . . . . . 5.1. Lead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.1. Isotope dilution measurements . . . . . . . . . . . . . . . . . . 5.2. Lithium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1. Calibration by isotope dilution in presence of strong matrix effects . 5.3. Determination of the Rb and Li content in Standard Reference Material SRM

☆ Dedicated to my wife Barbara who passed away this March after a great fight against cancer. ⁎ Tel.: +49 711 459 22144; fax: +49 711 459 22461. E-mail address: [email protected]. 0584-8547/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2008.03.003

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5.4. 5.5.

Some conclusions from the discussed measurements. . . . . . . . . . . . . . . . . . . . . Determination of large 7Li/6Li isotope ratios by IDA . . . . . . . . . . . . . . . . . . . . . 5.5.1. Isotope ratio determination applying Doppler-limited absorption spectrometry . . . . 5.5.2. Isotope ratio determination applying resonant Doppler-free two photon spectrometry . 5.5.3. Limits of detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Further elements suitable for isotope dilution GFAAS . . . . . . . . . . . . . . . . . . . . . . . . 6.1. Light elements (Z = 1–18) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2. Medium heavy elements (Z = 19–56). . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3. The rare earth elements (Z = 57–71) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4. Heavy elements (Z = 72–92) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5. Detection sensitivity and limits of detection of REE isotopes . . . . . . . . . . . . . . . . . 7. Evaluation of line overlapping in saturation spectrometry . . . . . . . . . . . . . . . . . . . . . . 7.1. Line profiles in saturation spectrometry . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2. Saturation spectrometry applying double modulation . . . . . . . . . . . . . . . . . . . . 8. Summary and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Atomic absorption spectrometry (AAS) is still the most frequently used method for the determination of element concentrations in a sample [1]. The method has a high detection power, is specific and robust and thus often preferred to other efficient techniques as, e.g., inductively coupled plasma mass spectrometry (ICP-MS) [2]. The application fields of AAS are inter alia the clinical use, biology, environmental analysis, geochemistry and petrochemistry. In AAS, the sample is supplied usually in the form of an aqueous solution to a thermal atomizer, an analytical flame, or a plasma where it is atomized. The concentration of the element of interest is determined by absorption of resonant radiation with calibration to an aqueous solution which contains only a known amount of the element. However, the accurate determination of the content of an element in an unknown sample is often disturbed by a multitude of physical and chemical reactions and effects in the atomizer [1]. For example, the element under investigation can react with the hot wall of a graphite tube to stable carbide. Such a reaction is influenced by the kind and the concentration of concomitant elements which can cause a stronger loss of free atoms in comparison to the reference solution. The concomitant elements and their compounds in the sample, the matrix, can lead the analyte to form volatile or stable molecules in the liquid as well as in the vapor phase and influence the absorption signal. Furthermore, the spatial distribution of the elements in the atomizer may be modified and the conditions of the flame or plasma may be influenced. In order to minimize signal depression and to obtain reproducible results for every new analytical problem, chemical admixtures, socalled matrix modifiers, have to be found. Such investigations are time consuming and make up the largest part of research in analytical chemistry. Even if chemical modifiers are used in order to minimize signal depression and even if the measurements are reproducible, calibration still remains a problem because a reliable analysis requires an identical chemical environment in the sample and standard solution. In consequence, the concomitants and their concentrations must already be known before the analysis is carried out. However, this is not the case in practice when so-called real samples have to be investigated. A way out is the chemical separation of the concomitants before the measurement takes place. This is a time consuming task and contains the risk of contamination with the element under investigation at every stage of work [3]. Thus, there is an urgent need to have a methodology which provides accurate results by the simple addition of an internal standard. How this analytical calibration problem can be solved in AAS has been demonstrated by the author for the first time in 1997 [4]. It requires the combination of two proven methods: isotope dilution analysis and advanced laser spectrometric techniques. Isotope dilution is usually applied in mass spectrometry especially in combination

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with ICP-MS [5]. A known amount of one isotope of the element under study is added to the sample while a second isotope serves as an internal standard. Isotope dilution is the method which is the least susceptible to chemical and physical interferences and therefore provides accurate results. In the last decade isotope dilution has proven to be the best choice when the accuracy of an analysis is of primary interest. Isotope dilution is therefore nowadays on the threshold to become a routine method in mass spectrometry [5]. Diode laser spectrometry is an excellent possibility for the specific excitation of single isotopes. Applications are the determination of atomic quantities such as isotope shifts and hyperfine structure constants (e.g. [6]), isotope separation (e.g. [7]) and the use in highly selective techniques using multi-step excitation as, e.g., resonance ionization mass spectrometry (RIMS) (e.g. [8]). Isotope dilution combined with diode laser spectrometry is presented here by use of a low-pressure graphite tube furnace for atomization. In such a device, atomization takes place at reduced pressure where the contribution of pressure broadening to the spectral line width is generally small compared with Doppler-broadening. Since, in general, the isotope shifts of spectral lines from heavy and light elements are larger than the Doppler-broadened line widths of the corresponding lines, isotope selective measurements by excitation with laser radiation are possible for these elements. The first isotope selective measurements using a low-pressure graphite furnace as atomizer in diode laser atomic absorption spectrometry (DLAAS) were reported by K. Niemax et al. in 1993 [9]. A low-pressure graphite furnace atomizer combined with diode lasers as radiation sources for isotope selective excitation of free analyte atoms can be viewed as an ‘optical mass spectrometer’. In comparison with isotope dilution in mass spectrometry there are some important advantages of the optical method. The instrumentation is cheaper and the specificity of atomic transitions excludes the occurrence of isobaric interferences, whereas, e.g., in ICP-MS isobaric interferences are a problem and require either a cumbersome correction of the measured data or an expensive improvement of mass separation. Especially the combination of isotope dilution with electrothermal graphite furnace atomization seems to be best suitable for clinical purposes since only a few microliters of sample solution are needed and accurate results are obtained. The power of isotope dilution graphite furnace atomic absorption spectrometry (ID-GFAAS) is demonstrated by measurements on aqueous solutions which contain lead or lithium, respectively, as analyte element and sodium salt in the matrix (Section 5). Sodium salt gives rise to strong matrix effects. Although a chemical modifier is not used in the investigations presented here and in spite of signals varying from measurement to measurement, an accurate analysis is obtained by the isotope dilution method while the conventionally employed element addition technique fails. Thus, it can be expected that the method guarantees fast and accurate analyses even for

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complex matrices, without a time consuming search for the most suitable modifier and without searching the optimum temperature program for furnace atomization. The accuracy and precision of results obtained by isotope dilution in combination with analytical AAS is demonstrated by the determination of the lithium and rubidium content in SRM 1640 standard water from the US National Institute of Standards and Technology (NIST). We give now a survey of the present review of our work. New, previously non-published material (Sections 3, 6, 7 and parts of Sections 4 and 5) is presented in depth while other material is described in a concise form. Section 2 gives a brief characterization of matrix effects in graphite furnace atomic absorption spectrometry (GFAAS) and the conventional strategies to minimize the occurrence of interferences. Section 3 presents the principles of the application of isotope dilution in mass spectrometry as well as its new application in combination with high-resolution diode laser spectrometry. An appropriate methodology and a formula for evaluation of isotope dilution measurements by optical spectrometry are developed therein and presented for the first time. The beginning of Section 4 deals with the advantages of diode lasers for the use as radiation sources in GFAAS at atmospheric pressure followed by the presentation of isotope selective diode laser GFAAS which is carried out in the low-pressure regime. The applicability of the detection sensitive wavelength modulation (WM) technique is extended to isotope selective analytics. Some examples for the limits of detection of individual isotopes (Li, Rb, Pb) obtained by direct absorption measurements and by application of WM with detection of the second harmonic order (2f-WM) are given. The detection limits of the Li and Rb isotopes were also determined by application of Doppler-free saturation spectrometry. The feasibility and the successful application of isotope dilution in presence of severe matrix effects is demonstrated in Section 5 for the elements lead (Section 5.1), lithium (Sections 5.2 and Section 5.3) and rubidium (Section 5.3). Isotope dilution was also applied to measure the large 7Li/6Li isotope ratio of a 7Li standard solution by use of Doppler-limited spectrometry as well as of two-photon spectrometry with a resonant intermediate level (Section 5.5). The results of the investigation to find out ‘which elements are suitable for isotope selective applications by use of a low-pressure graphite furnace atomizer’ can be found in Section 6. The Doppler-free lines obtained by saturation spectrometry are generally broadened in dependence on pressure by so-called velocity changing collisions. These collisions complicate the evaluation of saturation spectra, especially for overlapping lines. The access of such signals for quantitative analysis is the topic of Section 7. The reduction of the contribution of velocity changing collisions to the signal by application of a double modulation technique is demonstrated. This technique uses modulation of the radiated intensity as well as of the radiated wavelength. Section 8 gives a summary of the work and an outlook to applications of isotope dilution in optical atomic absorption spectrometry which will become possible in the near future. 2. Matrix effects in GFAAS A conventional GFAAS instrumentation consists out of a radiation source in form of a hollow cathode lamp (HCL) or an electrodeless discharge lamp (EDL), the electrothermal graphite tube atomizer, a grating monochromator and usually of a photomultiplier tube as detector for the transmitted radiation followed by an amplifier and a unit for data recording. The liquid sample (10–50 µl) is supplied through a small introduction hole at the upper side of the tube onto the inner wall of the tube or onto an integrated platform. Before atomization takes place, the analyte has to be dried followed by a further process called pyrolysis or ashing in which the volatile substances are separated from the analyte. The pyrolysis step has to be carried out at the highest

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temperature possible without loss of analyte. In dependence on the element the analyte is atomized at a temperature of 2000 to 2800 K and atmospheric inert gas pressure followed by a final cleaning step in which condensed material should be removed from the colder ends of the tube. It is easy to understand that each of the three stages of operation, drying, ashing and atomization has its own potential problems. Therefore each of these stages must be optimized. Moreover, the optimization depends on the element under investigation as well as on the other constituents accompanying the analyte element. Even when an optimized temperature–time program is applied, severe interferences from the matrix can occur and cause severe problems with accuracy and precision. This is in particular the case when the matrices of sample and standard solution are not identical. In the worst cases calibration is not possible or the signal is completely suppressed. The matrix effects and the common strategies to avoid interferences are briefly discussed below. 2.1. Definition and classification If a specific component can be identified as causing an effect then this is referred to as interference. If the cause of an effect cannot be identified or must be attributed to a complex combination of all constituents of a sample, then the interference is called a matrix effect [10]. According to the International Union of Pure and Applied Chemistry (IUPAC) one distinguishes between spectral interferences and non-spectral interferences. Spectral interferences are based on an incomplete isolation of the radiation absorbed by the analyte element from other radiation or radiation absorption detected by the measuring system. Non-spectral interferences, in contrast, affect directly the number of free analyte atoms in the absorption volume. These interferences are most conveniently classified according to the place, stage or process of their occurrence. Relevant in GFAAS are volatilization interferences, vapor-phase interferences and spatial-distribution interferences. The term volatilization or vaporization interference is used when in presence of a concomitant the temperature or the time of vaporization is changed. Vapor-phase or gas-phase interferences are caused by a change in the fraction of analyte dissociated, excited or ionized in the gaseous phase. Spatial-distribution interferences stem from an inhomogeneous spatial distribution of the analyte in the atomizer. 2.2. Cause of interferences and their forms of appearance Because in AAS the light emitted by the radiation source is usually modulated and the detected signal is processed at the same frequency, all radiation emitted by the atomizer as well as environmental light is eliminated. Genuine spectral interferences caused by absorption due to electron excitation in foreign atoms or gaseous molecules are very rare and mostly of little significance in the graphite furnace technique since, even at atmospheric pressure, absorption lines are relatively narrow. Almost all vaporization interferences are caused by formation of compounds of the analyte element in the condensed phase. Most of the volatile compounds are extracted at the charring stage and produce a loss of analyte. Carbides formed by a reaction of the analyte element with the carbon of the graphite tube are only referred to as interference in case of different carbide formation in sample and standard. Vapor-phase interferences are due to a shift of the dissociation equilibrium in the absorption volume caused by the concomitants. These interferences are mainly observed when the analyte is atomized from the tube wall. If the atoms are vaporized into the colder buffer gas, recombination, analyte trapping on condensed matrix particles as well as analyte molecule formation takes place. The formed compounds as, e.g., stable chlorides are often hard to dissociate and analyte is lost. Losses by ionization and excitation can, by contrast, be neglected in graphite tube atomizers [1]. In practice, it is difficult to differentiate

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whether a matrix effect is caused by volatilization interference or by vapor-phase interference. The most important interfering compounds are the halides, in particular the chlorides, and then the sulfates [1]. Inhomogeneities in the spatial distribution caused by convection due to temperature gradients occur predominantly for longitudinally heated graphite tubes. Vaporization–condensation–re-atomization processes during the atomization stage are influenced by the matrix and lead also to spatial distribution interferences. It is suggested that the lateral non-homogeneity of the analyte distribution is the main cause of the curvature of analytical working curves in GFAAS [11]. The spatial distribution of radiant intensities from hollow cathode as well as from electrodeless discharge lamps has been shown to be highly non-uniform [12,13]. This fact will probably enhance interference effects caused by a matrix-induced redistribution of analyte atoms within the probing beam [14]. A. Gilmutdinov et al. [15] showed that the absorbance recorded by a common detection system is dependent not only on the number of free analyte atoms but also on the gradient of their distribution and the cross-sectional distribution of the radiant intensity. They propose to overcome the resulting non-linearity of the calibration curve by use of a spatially resolving detection system. 2.3. Conventional methods for elimination of interferences The most effective method to eliminate interferences is the socalled STPF (Stabilized Temperature Platform Furnace)-concept [16]. The STPF-concept bases on the platform-technique introduced by B. L'vov [17] with the goal to bring the graphite furnace atomizer as near as possible to an ideal isothermal atomizer. The items of STPF are (1) to atomize as complete as possible into a thermally stabilized atmosphere by vaporization from a graphite platform inserted to the graphite tube with application of a heating rate as high as possible, (2) an extensive separation of the concomitants from the analyte by application of chemical modifiers, (3) the use of well controllable graphite surfaces, i.e., use of pyrolytic graphite coated tubes and (4) an effective background correction. The STPF-concept recommends peak area evaluation instead of peak height evaluation. The application of the STPF-concept results in a higher effective temperature which provides by its own a reduction of background absorption which is given to a great extent by non-atomized and nondissociated parts of the sample. State of the art for further background correction in GFAAS is the use of the Zeeman effect. For the elimination of non-spectral interferences a complete separation of the analyte from its concomitants is necessary. In order to get the accompanying substances completely volatilized at higher ashing temperatures without any loss of analyte, the analyte has to be transformed into a more stable form. These conditions are mostly obtained by the use of chemical admixtures. The technique is called chemical modification and the admixtures are named chemical modifiers or simply matrix modifiers. An idealized modifier stabilizes all species of an analyte. It is easy to understand that a great number of varying modifiers are described in the literature. A survey is given by Tsalev et al. [18]. However, it is desirable to have a universal modifier. The most powerful and the widest applicable modifier is the Pd–Mg-modifier [19]. The choice of the optimum chemical modifier and the optimization of each stage of analysis, in particular to find the best configuration of the temperature–time program for the graphite tube furnace, are time consuming and may require several weeks of work [20]. Although the use of matrix modifiers is successful, a calibration against an aqueous standard solution should be avoided for complex sample matrices. A reliable analysis requires the identity of the chemical environment in analyte and standard solution. This requirement for an accurate and precise analyte determination can only be fulfilled if the concomitants and their concentrations are known before the analysis is started. In other words, there will remain a large number of so-called real samples for which an accurate and precise determination by GFAAS is currently not possible.

Often the element addition method [1] is applied to circumvent the problem. However, this method can only be successful if the interferences are eliminated by a long lasting procedure as described above, but, in many cases the analyst should provide a result ‘before he has received the sample’. Moreover, even when the STPF-concept is used not all interferences may be completely eliminated (e.g., the persistent chloride interference). In consequence, a calibration method is required which (1) provides accurate and precise results by application of the internal standard principle, (2) is fast and needs practically no preparation, (3) minimizes the risk of contamination and (4) can universally be used. The application of the isotope dilution technique in GFAAS promises to be an appropriate technique. The next section represents the isotope dilution method as used in mass spectrometry and discusses its applicability for optical spectrometry, especially for its use in graphite furnace atomic absorption spectrometry. 3. The isotope dilution method 3.1. Isotope dilution in mass spectrometry Isotope dilution analysis (IDA) with stable isotopes or long-lived radioisotopes is a powerful and widely used technique for the quantitative determination of elemental concentrations in a sample [2,21]. IDA is an internal standard method and thus compensates for transport losses and other matrix effects after the isotope dilution step has occurred. The technique provides very accurate and precise results and it is, when performed properly, superior to other calibration methods. In the first applications IDA was combined with thermal ionization mass spectrometry (TIMS), but in the last decade inductively coupled plasma mass spectrometry has become more and more the major application field of IDA [5]. An isotope dilution analysis is based on the addition of a known amount of a stable isotope (the ‘spike’) of the element under study to the sample and the measurement of the altered isotope ratio of the isotope-diluted sample (in the following text called ‘the mixture’). The principle of IDA is illustrated in Fig. 1. When sample and spike are fully mixed all isotopes are in the same way affected by chemical and physical matrix effects. Thus, matrix effects have no influence on the measured isotope ratio and, in turn, no effect on the analytical result. This is the major advantage of IDA. Isotope dilution is therefore the best choice if the accuracy of an analysis is of top priority [5]. In isotope dilution mass spectrometry (IDMS) the elemental concentration CY,X in the sample X is determined by [2,21] CY;X ¼

CY;S WS AS  BS RM K : WX BX RM  AX

ð1Þ

Here, CY,X and CY,S are the concentrations of element Y in sample X and spike S, respectively. WX and WS are the weights of sample X and spike S, AX and BX are the atom fractions of the isotopes A and B of Y in the sample X while AS and BS are the atom fractions in the spike S. RM is the experimental value of the A/B-isotope ratio in the mixture and K is the ratio of the natural atomic weight and the atomic weight of the enriched material. Since mass spectrometers generally show a mass dependent transmission, the measured ratio, RM, has to be multiplied with a mass correction factor in order to obtain the true isotope ratio. The mass correction factor is usually determined by use of an isotopic standard of known composition. If the mass correction factor is known, an isotope dilution analysis can be performed with only one single measurement. The isotope ratios of the spike solution are provided by the supplier of the enriched material while the isotopes in the sample normally have natural abundance. For some elements (e.g. Li, B, Pb, U) the isotopic abundance varies in nature and an isotopic standard has to be used for calibration. Isotope dilution is not only suitable for trace element analysis, it is also suitable for element-speciation analysis when ICP-MS is coupled

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Fig. 1. Principle of isotope dilution; the atom fractions of isotope A and B are (panel (a)) AX and BX in the sample X and (panel (b)) AS and BS in the spike solution; (c) the measured ratio of the isotopic signals in the mixture Mi depends on the quantities of sample X and spike S applied to the mixture. NA,X and NB,X (NA,S, NB,S) are the numbers of atoms of isotope A and isotope B, respectively, from sample X (spike S) in the mixture Mi.

with a separation method as, e.g., high-performance liquid chromatography (HPLC), gas chromatography (GC) or capillary electrophoresis (CE) [22–24]. The state of the art of isotope dilution ICP-MS was recently reported in a critical review by K. Heumann [5]. In general, IDA is not restricted to mass spectrometry; it can be combined with any technique resolving isotopic signal contributions. A potential application field is analytical atomic absorption spectrometry (AAS) where the accuracy of the analytical results is affected by matrix effects. 3.2. Isotope dilution in optical spectrometry Atomic absorption spectrometry is very sensitive to matrix effects. As already discussed in Section 2, in graphite furnace atomic absorption spectrometry the most important interferences occur by analyte losses due to reaction with the carbon of the graphite tubes to stable carbides

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and due to formation of volatile molecules in the liquid or the vapor phase. Moreover, the occurrence and strength of these interferences are dependent on the analyte's chemical environment. The use of matrix modifiers and the application of the platform-technique [17] improve the repeatability and reproducibility of the measurements but an accurate calibration of the obtained signals requires often a matrix matched standard which is generally not available. The application of IDA in GFAAS simplifies the calibration process and improves the accuracy of the results. Instead of a cumbersome search for the right modifier and the best experimental conditions, the universal Pd–Mg modifier and a standard temperature–time program can be used. IDA in optical spectrometry is feasible by the simultaneous excitation of two isotopic transitions using two radiation sources of small bandwidth [25]. Single mode laser diodes are best suited for this application. The typical non-uniformity of the atomic distribution in the atomizer is best compensated by exact overlapping of the exciting beams in the interaction zone (an appropriate experimental arrangement is described in Section 4.2.2 and illustrated in Fig. 5). In order to resolve the required isotopic features in the absorption spectra, the measurements have to be carried out at reduced pressure where collision line broadening can be neglected. This requirement excludes flame-AAS and restricts the applicability of IDA in AAS to electrothermal atomizers and plasmas. The main advantages of isotope dilution optical spectrometry (IDOS) over IDMS were already mentioned in the introduction. First, the cost of the instrumentation is much lower. For example, mass spectrometry needs an expensive high vacuum technique while the vacua required for isotope selective optical spectrometry can be generated by inexpensive rotary pumps. Second, because of the specificity of atomic transitions, isobaric interferences cannot occur in highresolution optical spectrometry. By contrast, IDMS requires that the two measured isotopes are free of isobaric interferences. The main drawbacks of IDOS are first the limited number of elements whose spectra include transitions with fully resolved isotopic components at the required conditions and, second, the fact that the overlap of the line profiles of adjacent isotopic components can usually not be neglected. This complicates the evaluation of the measured ratios. We will show that a correction for the overlap is possible, if the shape of the line profiles is known. Before a methodology for overlap correction is developed, some general remarks on isotope ratio measurements in optical absorption spectrometry will be made. We assume at first that the transition under investigation shows two components which are related to two isotopes and which are free of interferences. Even in this idealized case, the obtained absorbance ratio will not match the isotope ratio since Doppler-broadening depends on mass. This applies in particular to the light elements where the isotopes have large mass differences. If a Doppler-free technique or the more sensitive wavelength modulation technique is applied, the observed intensity ratio of the isotopic signals depends also on the ratio of the radiation densities of the used diode lasers, in the case of WM additionally on the wavelength modulation amplitudes. These effects are comparable to mass discrimination in mass spectrometry. In consequence, in IDOS the measured intensity ratios have to be calibrated by an isotopic standard and at least two measurements have to be carried out (in contrast to one in IDMS). The correction for the line overlap bases on the fact, that the profile of an absorption line is a Voigt profile which can be calculated numerically (see, e.g., Humliček [26]) if Doppler width and pressure broadening are known. With the additional knowledge of the isotope shifts and the hyperfine structure splitting of the used transition a correction for the overlap can be performed. References for isotope shift and hyperfine structure data are given, e.g., by Heilig [27–30] and Fuller [31]. The Doppler width, ΔνD, is obtained by DmD ¼

1 k

rffiffiffiffiffiffiffiffiffiffi 2kT : M

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Here, λ is the transition wavelength, k is Boltzmann's constant, T is the temperature in K and M is the mass of the atoms or molecules. The full width at half maximum (FWHM) of a Doppler-broadened line profile is expressed by pffiffiffiffiffiffiffi CD ¼ 2 ln2DmD :

where the index i denotes one of the contributing isotopes. The relative sensitivity of two isotopes A and B at the frequencyν, EA,B(ν), will then be EA ðmÞ IA ðmÞ IB ðmB Þ ¼ : EB ðmÞ IB ðmÞ IA ðmA Þ

m X

Iij ðmÞ and Ii ðmi Þ ¼

ð2Þ

The above considerations are in general also valid for isotopic line profiles showing hyperfine structure. In that case, one has to sum for each of these isotopes over all hyperfine structure components j

m X

Iij ðmi Þ;

j¼1

respectively, where m is the number of hyperfine structure components. We now assume that for IDA the intensity ratio of the signals at the frequencies ν1 and ν2 are considered for evaluation. These frequencies correspond normally to maxima in the sum spectrum of all isotope lines, but, as we will see, this is no prerequisite. With this assumption, the intensity IX(ν1) of the analyte signal at the frequency ν1 (see Fig. 2) can be expressed on the basis of the known line profiles and use of the above Eq. (2) by IX ðm1 Þ~NA;X qX ðm1 Þ and qX ðm1 Þ ¼

n X i¼1

Ni;X 1 : EA;i ðm1 Þ NA;X

ð3aÞ

The corresponding relation for the spike solution S is IS ðm1 Þ~NA;S qS ðm1 Þ with qS ðm1 Þ ¼

n X i¼1

Ni;S 1 : EA;i ðm1 Þ NA;S

ð3bÞ

Here NA,X and NA,S are the numbers of atoms of isotope A (of the element Y) in the analyte X and the spike S, respectively, and i = 1... n stands for the number of isotopes of Y. At the frequency v2 the analyte signal will have the intensity IX(v2) while the signal intensity of the spike will be IS(v2). Dividing for the analyte as well as for the spike solution the intensity at v1 by the intensity at v2, we obtain the calculated intensity ratios RX;C ¼

Ii ðmÞ ; Ii ðmi Þ

EA;B ðmÞ ¼

Ii ðmÞ ¼

j¼1

In the following text ΓD is called the full Doppler width. Values for pressure broadening are available, e.g., from the Atomic Spectral Line Broadening Bibliographic Database [32] which is made available by the NIST. The evaluation of isotope selective measurements performed by application of wavelength modulation spectrometry is described below in Section 4.2.3. If Doppler-free techniques as saturation spectrometry or resonant two-photon spectrometry are applied, the form of the line profiles generally deviates from a pressure broadened Lorentz profile due to the occurrence of velocity changing collisions [33]. Here, the best choice to get information about the profiles of the isotope lines is to fit an experimental spectrum obtained under the same conditions as in the isotope dilution analysis. A fitting procedure is described by Inguscio et al. [34] and below in Section 7.1. As discussed above, a formula which is suitable for the evaluation of isotope dilution measurements in optical spectrometry must take into account the overlap of the isotopic line profiles. Such a formula is not known from the literature and will be derived in the following sections. Fig. 2 illustrates the example of a spectrum which is composed by the overlapping line profiles of three isotopes A, B and C showing no hyperfine structure (for profiles with hyperfine structure see below). The profiles of the individual isotopic lines have their maximum at the frequencies νA, νB and νC. The frequency dependent contributions of the three isotopes to the sum signal will be IA(ν), IB(ν) and IC(ν). For each isotope line we can define the entity Ei ðmÞ ¼

occurring in the spectral transition used for detection. The intensities at the frequencies ν and νi are then given by

IX ðm1 Þ IX ðm2 Þ

ð4aÞ

for the analyte, and RS;C ¼

IS ðm1 Þ IS ðm2 Þ

ð4bÞ

for the spike solution. Both ratios are independent on the analyte concentrations. If for the measurements the sensitive wavelength modulation technique or a high-resolution Doppler-free method is applied, the measured ratios, RX,M and RS,M, will depend on the output power of the used diode lasers and not match the calculated ratios, RX,C and RS,C, respectively. Measured and calculated ratios are correlated by a factor κ: RX;C ¼ jRX;M

and RS;C ¼ jRS;M :

ð5Þ

After performing isotope dilution, the corresponding measured ratio for the mixture, RMi,M, will be RMi;M ¼ j1

IX ðm1 Þ þ IS ðm1 Þ : IX ðm2 Þ þ IS ðm2 Þ

ð6Þ

Using Eqs. (4a), (4b) and Eq. (5), the intensity IX(v1) of the analyte signal at the frequency v1 can be written to IX ðm1 Þ ¼

IS ðm1 ÞRX;M RMi;M  RX;M

1

! RMi;M  RX;C : RX;M RS;C

ð7Þ

Insertion of Eqs. (3a), (3b) will give the number NA,X of atoms of isotope A in the analyte X: Fig. 2. Overlap of the line profiles of three even mass number isotopes A, B and C. IA(ν1), IB(ν1), IC(ν1) are the contributions of each isotope to the sum signal at frequency ν1. The line centers of the single isotope lines are indicated as νA, νB and νC.

NA;X ¼

  RMi;M RX;C NA;S RX;M qS ðm1 Þ 1 : RMi;M  RX;M qX ðm1 Þ RX;M RS;C

ð8Þ

H.D. Wizemann / Spectrochimica Acta Part B 63 (2008) 539–560

The concentration CY,X of the element Y in X is finally obtained to CY;X ¼

NA;X MY;X u ; AX WX

ð9Þ

where MY,X is the relative atomic mass of Y in X and u is the atomic mass unit. Besides the correction for the spectral overlap of isotope lines, the application of the above derived equation has some more practical advantages: - even for polyisotopic elements not more than two spectral features of the transition have to be resolved, - the features used for the measurements may show contributions of more than one isotope, - the intensities at these features may be measured by different spectroscopic methods, e.g. by 2f-wavelength modulation and by saturation spectrometry, - a possible intensity discrimination is automatically corrected and has no influence on the result. However, to compensate for intensity discrimination the intensity ratios of the mixture (RMi,M) as well as of the sample (RX,M) have to be measured. Furthermore, it must be noted, that Eq. (8) as well as below Eq. (10) is only correct for linear calibration curves. The mass dependence of Doppler-broadening can be neglected for heavy elements and the absorbances at the transitions of the investigated isotopes become independent on mass as well as on laser intensity when saturation is avoided. In this case RX,M equals RX,C and Eqs. (8) and (9) can be combined to CY;X ¼

CY;S  WS AS  BS RMi;M qS ðm1 Þ ; K WX BX RMi;M  AX qX ðm1 Þ

CY;S ¼

deviation from the true value with decreasing relative sensitivity EA,B. For the illustration RX = RX,M = RX,C is assumed, i.e., evaluation by Eq. (1) is compared with evaluation by Eq. (10). If RX,M is different from RX,C, the deviation from the true value additionally depends on the amount of spike and on EB,A. It is obvious that the accuracy of an IDA performed in optical spectrometry as presented above is dependent on the quality of the calculated isotopic line profiles used for evaluation. The accuracy of the relative sensitivities EA,i(v1) inserted in Eq. (2) is influenced by various parameters. For Doppler-broadened line profiles it depends on the accuracy of the parameters for collision broadening as well as on the accuracy of the determination of the detection temperature of the evaporated atoms. In Doppler-free spectrometry techniques, the accuracy of EA,i(v1) is determined by the quality of the fit curve to an experimental spectrum which was obtained under the same conditions as in the isotope dilution analysis. A look at Eq. (3b) shows that the error of qS(v1) is minimized when the isotope ratios Ni,S/NA,S in the spike solution are as low as possible (i.e., the spike has to be enriched as much as possible). On the other hand, the ratios Ni,X/NA,X in the sample X are given and cannot be influenced. However, the error of qX(v1) is minimized when the relative sensitivities EA,i(v1) are maximized. This is the case, when (if possible) the center frequency of an isolated line of one single isotope is set to be v1. The choice of the ratio RMi,M/RX,M determines the precision of the measurements. Under the assumption that the spike was prepared without weighing and dilution errors, the relative statistical experimental error of the number NA,X of atoms of isotope A in the analyte X, (dNA,X/NA,X)stat, will be given by

ð10Þ

with NA;S MY;S u : AS W S

MY,S is the relative atomic mass of Y in spike S. The concentration as given by Eq. (10) corresponds to the formula of isotope dilution mass spectrometry (Eq. (1)), weighted by qS(v1) / qX(v2). As long as the relative sensitivities are large the line overlap can be neglected and for the determined elemental content no significant difference is obtained; either the measurements are evaluated following Eq. (1) or following Eqs. (8) and (9) (or Eq. (10), if RX,M = RX,C). Fig. 3 shows for three sample isotope ratios (RX = AX/BX = 5; 1; 0.2) the increase of the

545

dNA;X ab   j c NA;X stat ða  1Þ b  RX;C =RX;M

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  2  2 dRMi;M dRX;M þ ; RX;M RMi;M

ð11Þ

where R RS;C N1, b ¼ RMi;M N1 and RS,C NN RX,C are assumed. a ¼ RMi;M X;M Eq. (11) tells us that the errors become large if the mixture is underspiked (RMi,M ≈ RX,M, i.e. a ≈ 1) or overspiked (b = RS,C/RMi,M ≈ RX,C/ RX,M). These are the same requirements as for IDMS. The experimental errors, dRX,M and dRMi,M, are smallest when the measured signals are large compared with the detection limit while, on the other hand, optical thickness is avoided. This will set the upper limit of the ratio RMi,M/RX,M to about 10. The feasibility of isotope dilution for calibration in GFAAS requires the resolution of single isotope components, i.e. the application of isotope selective GFAAS. Isotope selective GFAAS makes use of laser diodes as radiation sources. These tools and the realization of isotope selective measurements in GFAAS are the topic of the next section. 4. Diode lasers in graphite furnace atomic absorption spectrometry 4.1. General remarks about the use of diode lasers in AAS

Fig. 3. Deviation (in %) of the determined elemental content from the true value when the equation derived for IDMS (Eq. 1) is taken for evaluation. Solid line: RX = AX/ BX = 0.2; dashed: RX = 1; dot and dash: RX = 5. Isotope A is enriched in the spike to RS =AS/BS = 99.

There are two types of radiation sources which in the future may replace the conventionally used hollow cathode lamps and electrodeless discharge lamps in atomic absorption spectrometry. One type is the continuum source (CS) and the other type is the tunable single mode semiconductor diode laser. A big advantage of the CS [35] is the feasibility of multi-element detection. However, a complicated polychromator in the form of an échelle spectrometer is required. The spectral resolution of an échelle polychromator with a charged coupled device (CCD)-array is at the current state not sufficient to isolate isotopic components in low-pressure GFAAS and thus the continuous source is not suitable for isotope selective GFAAS. In contrast, a free-running single mode diode laser emits a prominent single, narrow line of 20–25 MHz width. This is two orders of

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magnitude less than the atomic line widths as found in hot flames or graphite furnaces at atmospheric pressure. The wavelength of the emitted diode laser radiation can be tuned continuously over a small spectral range by temperature and current to match exactly the desired spectral transition [36]. The range of continuous tuning is limited by so-called mode-hops and wavelength gaps, both are characteristic for free-running diode lasers. The wavelength gaps can be reduced or even be closed by optical feedback from a grating [36] or a simple glass plate in front of the laser diode [37]. Diode lasers are more powerful than other radiation sources and the selective excitation enabled by their spectral purity simplifies the spectral isolation of the absorption signal and thus a monochromator becomes redundant. Diode lasers have proven to be exceptionally stable radiation sources, both in terms of wavelength and power. For atomic absorption applications mostly inexpensive devices from mass production can be used. Nearly all metals can be measured by DLAAS either by use of the fundamental wavelength of the diode lasers or when frequency conversion as second harmonic generation (SHG) or sum frequency generation (SFG) is applied. A list of the detectable metals and the respective transition wavelengths is given by Zybin et al. [38]. The full detection sensitivity in analytical diode laser spectrometry is obtained with the application of wavelength or frequency modulation techniques [39,40] which can easily be implemented. Since the wavelength of a laser diode is in linear dependence on the injection current (within the limited ranges of stable single mode operation), one has simply to connect a waveform generator to the laser diode supply. The sinusoidal modulated output signal of the monitoring detector is usually processed by a lock-in amplifier referenced to the modulation frequency or a higher order harmonic thereof. Often the second harmonic is used for detection in the kHz range and the technique is called 2f-wavelength modulation (2f-WM). If the center frequency (i.e., the center of modulation) is slowly scanned across the absorption line a signal resembling a broadened second derivation curve of this line is obtained. The signal is maximized when the peakto-peak modulation depth amounts to about two times the full line width [39,41]. The application of 2f-WM significantly reduces lowfrequency flicker noise in the baseline and provides an ideal correction for non-specific absorption of zero and first order. In comparison to intensity modulation, where the exciting radiation is periodically interrupted by a chopper blade, the limits of detection are improved up to three orders of magnitude [42,43]. A general treatment of the fundamentals of wavelength modulation is given by P. Kluczynski et al. [44].

these elements if the pressure at atomization in the graphite furnace is reduced to a few hPa. At this condition, the full pressure (homogeneously) broadened line width, ΓL, is generally be small compared with the (inhomogeneously broadened) full Doppler width ΓD. If the isotope shift of a spectral line is smaller than the full Doppler width, high-resolution Doppler-free laser spectroscopic techniques, such as saturation spectrometry or Doppler-free two-photon spectrometry, have to be applied [46]. Fig. 4 shows the profile of the rubidium D2 line at (a) atmospheric pressure (1013 hPa Ar) and (b) at reduced pressure (10 hPa Ar). Two ground state hyperfine transitions are fully resolved at the reduced pressure condition. 4.2.2. Simultaneous measurement of two isotopes Fig. 5 illustrates the setup for the simultaneous measurement of two isotopes by graphite furnace atomization. By use of a mirror (Mi) and a beam splitter (BS) two laser beams are overlapped and directed through the graphite tube atomizer. Each laser beam excites one isotope. To avoid saturation, the beams are attenuated by a neutral density filter (not shown). The radiation emitted by the diode lasers DL1 and DL2 are modulated with different frequencies f1 and f2, respectively, and the transmitted intensities are recorded by one photo diode. The output of the photo diode is connected to two lock-in amplifiers LI1 and LI2 for phase sensitive processing with the reference frequencies f1 and f2. For example, this configuration was recently used [47] to demonstrate the preferential binding of the smaller 7Li+ isotope by an organometallic macrocycle containing ruthenium. 4.2.3. Application of 2f-WM in isotope selective spectrometry The advantages obtained in GFAAS by application of the 2fwavelength modulation technique are already described in Section 4.1. The wavelength of the radiated light is modulated around a center frequency νc with the frequency fmod (a few kHz) and the amplitude Δνmod. The typical 2f-WM line profile is obtained by tuning the center frequency across the absorption line. The signal height depends on the so-called modulation index m, defined by the ratio m = 2Δνmod / Γ (Γ: full line width). For a Voigt profile a flat maximum around m ≅ 2.1 is

4.2. Measurements applying isotope selective diode laser GFAAS 4.2.1. Preconditions for isotopic resolution Isotope selective optical spectrometry makes use of the relative small wavelength shifts of the center of gravity of the individual isotope contributions within an atomic line as well as of the hyperfine splitting of the contribution of a single isotope, i.e., of isotope shift and hyperfine splitting. The isotope shift originates from the differences in mass, volume and charge distribution of the nuclei of the isotopes of a chemical element. Hyperfine splitting originates from the interaction of the electron shell with the magnetic dipole moment and the electric quadrupole moment of the nucleus. The observed isotope shift is the sum of the contributions by the Bohr mass shift (BMS), the specific mass shift (SMS) and the volume or field shift (FS) [45]. The mass effects both show a 1/M2A (MA: mass of the nucleus) dependency and are dominating for light elements, the field shift is dominating for the heavy elements while the isotope shifts are generally small for the medium heavy elements. The isotope shifts of spectral lines of light and heavy elements are comparable or even larger than the full Doppler widths of the corresponding lines. Thus, isotope selective GFAAS becomes feasible for

Fig. 4. Doppler-limited spectrum of the D2 line of rubidium for atomization in a graphite tube furnace; (a) argon buffer gas pressure: 1013 hPa; (b) argon buffer gas pressure: 10 hPa; the observed lines are referred to the two isotopes and the ground state hyperfine structure. Two of the lines (85Rb, F = 3 and 87Rb, F = 2) overlap.

H.D. Wizemann / Spectrochimica Acta Part B 63 (2008) 539–560

547

Fig. 5. Principle of simultaneous measurement. The two laser diodes are each tuned to one isotope line and modulated with different frequencies. The signals are separately recorded by phase sensitive lock-in detection. DL diode laser; Mi Mirror; BS beam splitter; PD photo diode; f frequency.

obtained [39,41]. The applicability of the 2f-WM-technique for isotope selective diode laser spectrometry will be discussed in the following text. In order to express the relative sensitivities as defined in Section 3.2 when wavelength modulation is applied, the entire 2f-WM signal at frequency νc is viewed as the sum of the contributions of the individual isotope components [25]. The relative intensities can be determined in accordance to Section 3.2. However, the ratio of the Voigt profiles have to be replaced by the ratio of the 2f-WM profiles. It must be noted that the relative sensitivities become negative when the sign of the 2f-WM signal changes at zero-crossing. For the decision whether intensity modulation (IM, e.g., using a chopper or an electro-optical modulator) or 2f-WM will provide a better selectivity, we assume that the lines related to the two isotopes A and B show identical Voigt profiles and that the same number of atoms is detected. Fig. 6 shows the result of the calculated relative sensitivity of isotope A with respect to isotope B in dependence on the frequency separation. For wavelength modulation (solid line) a singularity occurs at zero-crossing of the signal of the interfering isotope. This singularity moves with increasing natural absorbance Ae to higher frequency separation between the isotope peaks. It is thus not suitable for analytical applications since the density of absorbing atoms (which have to be determined by the measurement) should be

known before starting the experiment. Disregarding this singularity, IM provides a larger relative sensitivity up to a limiting frequency separation Δνcr. Beyond Δνcr, a larger relative sensitivity is obtained by 2f-WM. If the transition of interest is the outermost component of a multipeak spectrum (as, e.g., the 6Li D1 component in the Li D spectrum), a further enhancement of the relative sensitivity by 2f-WM is achieved when 2f-WM is performed with the center frequency positioned at the outermost extremum of the respective 2f-WM line profile [25,43]. Moreover, all contributions to the measured sum signal are now of the same sign. Thus, a compensation of the contributions due to different signs is excluded. The dot and dash line in Fig. 6 shows the gain in relative sensitivity when νc is tuned from the line center to this outer subsidiary extremum. This procedure is often feasible in quantitative isotope selective optical spectrometry. However, one has to take care of the dependency of the position of the subsidiary extremum on the value of the modulation index m. The amount of the 2f-WM signal is related to the absorptance and is thus for absorptions exceeding a few percent no longer in linear dependence to the analyte concentration [43]. Since the 2f-WM signal gives no information about the absorptance, no accurate determination of the analyte content seems to be possible at higher concentrations. Moreover, the absorption signals become broader due to their

Fig. 6. Dependence of the relative sensitivity on the separation between neighbored isotope components calculated for identical Voigt profiles (ΓL: 0.07 GHz, ΔνD: 1.2 GHz, Ae: 0.001). Solid line: 2f-WM (m: 2.1, νc: line center); dashed: intensity modulation; dot and dash: 2f-WM (m: 2.1, νc: “outer“ subsidiary extremum, see text).

Fig. 7. Decrease of the modulation index m (solid line, left ordinate) and shift of the position of the subsidiary extremum (dashed line, right ordinate) in dependence on the natural absorbance Ae; m = 2.1 for Ae bb 1; ΓL: 0.07 GHz; ΔνD: 1.2 GHz.

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Table 1 Characteristic masses mchar and limits of detection obtained with isotope selective diode laser GFAAS and classical GFAAS using hollow cathode lamps Isotope selective GFAAS Isotope

6

Li Li Rb 87 Rb 208 Pb 206 Pb 207 Pb 7

85

λ/nm

670.81 670.78 780.03 780.03 405.78 405.78 405.78

mchar/pg

39 16 23 24 1400 1400 3000

Classical GFAAS Limit of detection/pg IM

2f-WM

5.5 2.2

0.29 0.11

600 600 1300

Element

λ/nm

mchar/pga

Li

670.8

1.4

Rb

780.03

2.3

Pb

283.31

10

IM — intensity modulation, 2f-WM — 2f-wavelength modulation. a [1].

dependence on increasing natural absorbance. This causes a dramatic reduction of the modulation index m which was optimized for small concentrations (Fig. 7, solid line). Further increasing concentrations can create a double peak structure in the transient 2f-WM signals since the analyte density passes through a maximum during the atomization stage [43]. If the measurements are performed using an outer subsidiary extremum of the 2f-WM spectrum, this outer extremum shifts with increasing absorbances to larger distances from the line center (Fig. 7, dashed line). These limitations can be circumvented when the photo diode signal is first logarithmically amplified by use of a logarithmic detector before phase sensitive processing with a lock-in amplifier is carried out [48]. Just the height of the logarithmically amplified signal depends on the analyte concentration and not the width. By doing this, even the singularity in Fig. 6 is in principle applicable for analytical measurements. 4.2.4. Limits of detection obtained for Li, Rb and Pb The detection sensitivity is of primary interest in isotope selective GFAAS not only for the determination of traces of isotopes but also for the applicability of the isotope dilution technique for calibration. In isotope selective GFAAS the detection limits are expected at higher values because the atoms will diffuse faster out of the interaction region due to the reduced pressure. Second, single isotope and hyperfine structure lines are measured and third the predominantly linear polarized light excites mainly transitions between states of identical magnetic quantum number. For example, the detection limit of lithium at the resonance line of 671 nm increased by about a factor of

six when the pressure conditions were reduced from 1000 hPa to 10 hPa [49]. Table 1 shows the characteristic mass mchar (1% absorption) of the naturally occurring isotopes of Li, Rb and Pb (except of 204Pb) as well as, if measured, the detection limit (3σ-criterion) obtained with intensity modulation and 2f-wavelength modulation. In comparison, the characteristic masses as tabled for longitudinally heated furnaces [1] are also given. The results indicate that the characteristic masses lie at about one order of magnitude higher values if the same atomic transition is excited. If 2f-WM detection is applied, and as we will see in the next section also for the application of Doppler-free saturation spectrometry, the limits of detection of the isotopes are comparable with the ones obtained for the elements with conventional GFAAS because of a better signal-to-noise ratio. Note, for the lithium isotopes different fine structure transitions were used. The values obtained for lead are relatively poor. Since no lead ground state transition can be matched with diode laser radiation, the Pb atoms had to be excited from a metastable level at 10 650.5 cm− 1. Moreover, only about 20 nW of blue light could be used for excitation. For the 207Pb isotope one single hyperfine component was measured. More of the experimental details of the lead measurements are given in Section 5.1.1. 4.2.5. Application of saturation spectrometry Saturation of a transition is observed when the grade of absorption is decreasing with increasing radiation power. This is caused by an increasing depopulation of the lower transition state and a correlated population of the upper state. In the worst case the sample will be fully transparent. A measure of the saturation is the saturation parameter S. It is defined by the ratio of the pumping rate P divided by the relaxation rate R . The saturation effect is advantageously useable for spectrometry. In saturation spectrometry [46] the radiation emitted by a tunable laser is split into two beams of different power and counterpropagating orientation. The stronger pumping beam saturates the transition in a small frequency interval inside the Gaussian shaped velocity distribution of the analyte atoms. The weaker probe beam measures the change in the population difference of the involved energy states. Fig. 8 displays the measuring arrangement we designed for saturation spectrometry using a graphite furnace atomizer. The saturation parameter S(ν0) can be adjusted to a desired value by inserting neutral glass filters to the pump beam. A bandpass filter in front of the detector suppresses radiation emitted by the hot walls of the graphite furnace. A polarizer which is orientated parallel with the polarization of the laser light and positioned between furnace and detector will minimize the background caused by non- or de-polarized scattered light.

Fig. 8. Arrangement for saturation spectrometry. 1 Pump beam; 2 probe beam; DL diode laser; NG neutral glass filter; BS beam splitter; BP bandpass filter; PD photo diode (according to [51]).

H.D. Wizemann / Spectrochimica Acta Part B 63 (2008) 539–560

It turned out that saturation spectrometry provides better detection limits than Doppler-limited spectrometry with intensity modulation. If saturation spectrometry can be carried out at the respective resonance lines (this is, e.g., the case for Li and Rb), the detection limits obtained for the isotopes are comparable with the ones obtained for elemental lithium with classical GFAAS. The detection limits for Li using saturation spectrometry were evaluated to 1.1 pg (6Li) and 0.4 pg (7Li), and for Rb we obtained 1.4 pg (85Rb) and 1.3 pg (87Rb) [49]. However, for many elements the wavelengths of the resonance transitions cannot be matched by diode laser radiation or the isotope components are not sufficiently separated for these lines. In these cases, weaker transitions have to be used for the isotope selective diode laser measurements. In a recent work [51], we showed that, especially by application of Doppler-free techniques but also in the Doppler-limited regime, highresolution spectra may be affected by relaxation sidebands which occur in the emission spectrum of some laser diodes. Thus, a careful selection of the laser diodes is recommended. 5. Applications of isotope dilution in isotope selective GFAAS The topic of this section is the application of the isotope dilution methodology in isotope selective GFAAS. Isotope dilution was carried out for the determination of analyte contents as well as of isotope ratios. The analyte elements investigated were lead, lithium and rubidium. Lead is the most analyzed element in AAS. Lithium is considered to be the element which is most critical for isotope dilution GFAAS. The lighter isotope 6Li diffuses faster out of the interaction zone [49] and the large relative mass difference between 6Li and 7Li may cause a different spatial distribution of the isotopes. Both effects can influence the measurements. Rubidium is still an almost unnoticed element. However, it was found that Rb has a biological essentiality as an ultra-trace element [52]. For isotope selective determinations of lithium and rubidium contents by optical spectrometry the respective resonance lines can be used. Thus, IDA combined with isotope selective GFAAS and the applicability of the highly sensitive wavelength modulation technique should provide accurate and precise results for these alkali metals even at very low traces. This is demonstrated with the help of standard reference material. Each of these three elements is dealt with in its own section beginning with a brief outline of characteristics and routine analytics of the considered element. 5.1. Lead Lead has the four naturally occurring isotopes 204Pb (1.4% abundance), 206Pb (24.1%), 207Pb (22.1%) and 208Pb (52.4%) [53]. 204Pb is a very long-lived radioactive nuclide (half-life period: 1.4 × 1017 years). The lead isotopes 206Pb, 207Pb and 208Pb are the end products of three of the four alpha decay series. For this reason, the natural deposits of lead comprise various lead isotope ratios which thus contain information about the origin of the lead found in a sample. Because of the toxicity of lead an accurate determination of the contents in environment and food chain is of particular importance. The fastest and most sensitive methods for lead detection are electrothermal GFAAS and ICP-MS. GFAAS combines high sensitivity, low detection limits and a low sample volume requirement. The latter makes GFAAS best suitable for the monitoring of lead pollution in humans with samples provided from whole blood. The application of isotope dilution with addition of a known amount of one lead isotope before storage will compensate analytical errors by losses during storage and due to the volatility of lead compounds. Moreover, isotope selective GFAAS provides the lead isotope ratios and will thus give information on the origin of the detected lead. 5.1.1. Isotope dilution measurements The first demonstration of the applicability of IDA in isotope selective GFAAS was the measurement of lead isotopes in a low-

549

pressure graphite tube atomizer with integrated discharge, known as Furnace Non-thermal Atomization Spectroscopy (FANES) [54]. FANES is known to be very sensitive to chemical and physical effects if real samples are measured. In this experiment [4], the metastable Pb level at 10,650.5 cm− 1 was populated by the discharge of the FANES-system during atomization of aqueous samples. The wavelength of 405.78 nm for probing this level was generated by frequency doubling the fundamental near-infrared radiation of a laser diode emitting at 810 nm using an LiIO3 crystal (laser diodes with fundamental emission at 405 nm were still not available at the time of the measurements). Typically about 20 nW of blue radiation was generated from about 20 mW fundamental radiation focused into the crystal. In the spectrum, 206Pb, 208Pb, and one hyperfine component of 207Pb were fully resolved. Addition of NaCl to a 10 µg/ml sample prepared from a stock solution caused severe signal depressions up to 80% but it was possible to calibrate the measurements when a part of the sample was spiked with 206Pb. The signals of 206Pb, 208Pb and the hyperfine component of 207Pb measured for the spiked and unspiked sample were also used to determine the isotope ratios in the stock solution. The spiked 206Pb amount was used for signal calibration. For 204Pb, which was not measured, 1.4% abundance was assumed. The results are summarized in Table 2. Because data for collision broadening were not available for the 405.78 nm transition, Gaussian line profiles (line width: 1.4 GHz) were assumed for evaluation (first line in Table 2). For comparison, the results obtained by ICP-MS using an isotope standard are also displayed (line 3). After the parameter for collision line broadening in argon was measured in a later work [55], the isotope ratios were evaluated once more with consideration to the Voigt profiles and to all line overlaps. The obtained isotope composition is given in the second line of Table 2. The relatively large discrepancies and statistical errors are referred to the poor signalto-noise ratio which is due to the low radiation power. In a separate work [55], the impact broadening and shift rates of all 6p2 3P0,1,2 → 7s 3P°0,1 transitions of lead (see Fig. 9) induced by collisions with argon and helium were measured, and, in a further work [56], the population of the metastable 6p2 3P1,2 lead levels in a low-pressure argon discharge was investigated for optimization. It turned out that, when lead is atomized in a FANES furnace at approximately 10–40 hPa argon buffer gas pressure, the relative optical depths at the 368.35 nm, 363.96 nm and 405.78 nm lines are expected to be 1:0.5:0.14, respectively. Taking into account this result as well as the separations of the isotope and hyperfine lines of the discussed transitions, we can conclude that the transition at 363.96 nm should be preferred if a maximum isolation of isotope components is required as, e.g., for isotope ratio investigations. For the application of isotope dilution, however, at least two components of the transition spectrum have to be resolved. Thus, for isotope dilution the isotope resolution obtained by use of the more sensitive 368.35 nm transition is sufficient. 5.2. Lithium Lithium is composed of two naturally occurring isotopes, 6Li (7.59% abundance) and 7Li (92.41%) [53]. Significant artificial depletion in 6Li Table 2 Isotope composition of the lead stock solution obtained using Gaussian line profiles (line 1) and Voigt profiles (line 2) for evaluation Abundance in % 208

Gaussian profile Voigt profile ICP-MS

Pb

51.0 ± 2.6 49.9 ± 1.1 51.6 ± 0.5

207

206

20.3 ± 1.0 21.0 ± 0.7 21.3 ± 0.2

27.3 ± 1.4 27.7 ± 1.2 25.7 ± 0.3

Pb

Pb

For comparison, results obtained by ICP-MS are given in line 3.

204

Pb

1.4 1.4 1.40 ± 0.01

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Fig. 9. Partial term diagram of lead. The air wavelengths are given. The numbers in brackets are the corresponding transition probabilities in 108 s− 1. The dashed lines indicate dipole forbidden transitions. According to [55].

due to isotope enrichment has been observed for commercially available chemical reagents [57]. Lithium finds application in many fields of science and technology including biomedicine, geology and nuclear industry. Lithium is somewhat more toxic than other alkali metals. However, ingestion of small amounts of lithium is generally not considered harmful and there are some clinical applications of this metal. However, the ingestion of larger amounts of lithium salts may cause lithium intoxication [58]. Therefore, a periodical monitoring of the lithium level is prescribed for clinical applications, e.g., for the treatment of bipolar disorders. In Germany, the Li concentration in the plasma of these patients should amount to about 0.6 to 0.8 mmol/l for prevention and 0.8–1.2 mmol/l in acute maniac phases [59]. Such concentrations are easy to determine by application of flame-AAS [60]. But flame-AAS is not sensitive enough for the determination of the Li content in the plasma of healthy persons (b1 µmol/l). In contrast, GFAAS provides the required sensitivity and the required sample quantities are small. GFAAS seems therefore to be the best choice for the determination of lithium in biological fluids. A big problem by use of GFAAS for Li determinations is the occurrence of a lot of matrix interferences. For biological material the alkali and earth alkali salts are known to be the most responsible factors. Chlorine causes the strongest interferences, but even the simple presence of the metals without halogen suppresses the signal [61]. Additionally to these interferences a further interference caused by the reaction of lithium with the carbon of the graphite tube to stable lithium carbide (Li2C2) occurs.

Fig. 10. 2f-WM signals, in panel (a) without and in panel (b) with a salt containing matrix (1% table salt). The 6Li signal heights are multiplied by a factor of 4; (a) 230 pg 7Li, 17 pg 6Li; (b) 2000 pg 7Li, 110 pg 6Li.

5.2.1. Calibration by isotope dilution in presence of strong matrix effects With the measurements on lead the applicability of isotope dilution for analysis in optical spectrometry using a graphite furnace for atomization was demonstrated in principle. However, these measurements were carried out sequentially using one laser diode whereas accurate results can only be obtained by a simultaneous recording of the different signals of two isotopes; one of the spike isotope and a second one of a reference isotope of the same element. Lithium was chosen as analyte for such simultaneous measurements. As mentioned above, the correct determination of lithium contents is a great challenge for isotope dilution optical spectrometry. The transition best suitable for isotope selective detection of Li is the resonance line at 670.8 nm. We prepared four samples each containing 130 ng/ml Li and 1% table salt in the aqueous matrix. Note, the total Li content of this sample was truly unknown since table salt also contains a distinct amount of Li. Three of the samples were spiked with increasing amounts of 7Li. In our experiment, the signals of both isotopes were measured by 2f-WM. 7Li was measured at the line center of the 7Li D2 transition. Since the relative sensitivity for 6Li becomes larger when the 2f-WM center frequency is shifted to the subsidiary extremum in the red wing of the 6Li profile, this spectral position was used for measuring the 6Li signal. The parameters used provided a relative sensitivity of approximately −56 for 7Li and 4900 for 6Li, respectively. Assuming natural isotope abundance, the contribution of 6Li to the 7Li signal was approximately 2.7 × 10− 3 and 1.4 × 10− 3 in the reversed case. Fig. 10a shows simultaneously recorded transient 2f-WM signals of a sample containing no salt while Fig. 10b represents signals obtained for a sample containing 1% table salt. The sample aliquots (10 µl) were atomized from a Ta foil which was inserted to the graphite furnace in order to reduce carbide formation. The double peak structure of the signals in Fig. 10b indicates that in the presence of salt the analyte Li is found at atomization in at least two chemical forms. Metallic Li is first atomized and creates the first peak while stable LiCl dissociates at higher temperatures and causes the broad second maximum. In all spectra, the second feature was less pronounced for 6Li than for 7Li, an indication that 6Li and 7Li behave differently. Although a different behavior was expected due to the relatively large mass difference of both isotopes, the exact cause for the observed different behavior is not known and needs further investigation. This observation let one assume that a calibration is not possible. However, when the first peak for both isotope signals is normalized to the same height, identical curves (Fig. 11) are produced from the onset of the signal across the maximum up to the tail, where the 7Li signal becomes more expressed due to the slower diffusion of the heavier 7Li isotope. On the basis of

Fig. 11. 2f-WM signals of 6Li (0.11 ng) and 7Li (1.35 ng) in presence of 1% table salt contained in the matrix. The first occurring peak of both isotopes is normalized.

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this result, a calibration will be possible if the signal heights of the first appearing peaks are used for evaluation. Besides a determination of the Li content, the measurements allow also a comparison of calibration by isotope dilution with calibration by application of the conventionally used standard addition technique. In the ladder case, no calibration by the 6Li signal strengths is carried out. Fig. 12 demonstrates clearly the superiority of the isotope dilution method. For comparison, the result of a measurement carried out by ICP-MS was 156 ng/ml. 5.3. Determination of the Rb and Li content in Standard Reference Material SRM 1640 Naturally occurring rubidium has two isotopes, 85Rb (72.17% abundance) and the weakly radioactive 87Rb (27.83%) [53]. The number of papers dealing with the determination of Rb by AAS is small and thus there is also little information on interferences and the use of modifiers [1]. However, Rb is biologically essential and it was stated that this element deserves in the future more scientific attention and correct determination [52]. Because of the good characteristics of Rb used in GFAAS and since its resonance transition can easily be matched by diode laser radiation, Rb is used as analyte for basic research in wavelength modulation diode laser AAS, e.g., in the groups of K. Niemax (Institute for Analytical Sciences (ISAS) in Dortmund and formerly at University of Hohenheim, Germany) and of O. Axner (University of Umeå, Sweden). The reliability of the isotope dilution technique applied to DLAAS was proved by verifying the Li and Rb content in the NIST (National Institute of Standards and Technology, USA) Standard Reference Material 1640 [62]. The 85Rb/87Rb isotope ratio of SRM 1640 was determined by comparing the 85Rb/87Rb signal ratios obtained for SRM 1640 with the respective ratios measured for a sample of known natural abundance. The ratios agreed within the limits of experimental uncertainty. A comparison of the signals obtained for the SRM 1640 sample with the signals obtained for an aqueous 2.5 ng/ml Rb sample enabled a rough estimation of the SRM 1640 Rb content. The content was estimated to be 1.60 ± 0.07 ng/ml and 1.85 ± 0.06 ng/ml when the peak height and the peak area, respectively, were evaluated. Both values were smaller than the referenced value. The signal de-

Fig. 12. Isotope dilution curves by addition of 7Li to a sample containing Li with presence of 1% NaCl in the matrix. The results are displayed in panel (a) without and in panel (b) with calibration by the 6Li signal strengths. The evaluated total Li contents are: (a) 97 ± 42 ng/ml and (b) 148 ± 2 ng/ml. From [25].

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Table 3 Values used in Eq. (8) for the determination of the Li content in SRM 1640 and result of the analysis NA,S (1015 cm− 3)

RX,C

RS,C

RX,M

RMi,M

qs(ν1)/qx(ν1)

CX,Li (ng/ml)

3.90

0.0786

170.7

0.0154

0.0338

1.045

51.7

pression was attributed to the large amount of dissolved salts in the SRM 1640 water sample. In contrast to the above described experiment with Li as analyte, the behavior of the two Rb isotopes was identical. The pure SRM 1640 sample and five samples spiked with different amounts of 87Rb were measured five times one after the other. The mean value obtained by evaluation of all the diluted samples by use of Eq. (8) and Eq. (10) was found to be 1.99 ± 0.04 ng/ml Rb (this deviates a little from the 1.97 ± 0.03 ng/ml given in [63] where the experimental data were evaluated by an approximation method [25]). This result is in close agreement with the referenced content of 2.00 ± 0.02 ng/ml Rb. For the verification of the Li content in SRM 1640 by isotope dilution GFAAS, 10 ml of the standard water were spiked with 20 µl of a 2 µg/ml 6Li spike solution. A 50 ng/ml Li sample diluted from the lithium standard solution with known isotope ratio, the pure SRM 1640 and a blank solution were also measured. The figures of merit and the result of the determination (51.7 ± 2.8 ng/ml), which agree well with the referenced value of 50.7 ± 1.4 ng/ml, are given in Table 3. Fig. 13 shows simultaneously recorded transient signals of the Li isotopes before and after spiking of 20 µl SRM 1640 with 40 pg 6Li. 5.4. Some conclusions from the discussed measurements • Although the signals fluctuated from atomization to atomization and although the two Li isotopes showed a different behavior in the experiment of Section 5.2.1, isotope dilution provided the means to determination of the Li content, despite the absence of a matrix modifier and without cumbersome work to find the best temperature–time program. On the other side, the occurrence of such a strong matrix effect, as observed in this experiment, indicates a severe loss of analyte atoms due to chloride formation which decreased the sensitivity of the measurement (but not the accuracy). Thus, even when the isotope dilution technique is applied, it makes sense to use a standard chemical modifier, e.g. the Pd–Mg modifier. Analyte losses due to formation of stable carbides can be reduced by the use of graphite tubes modified with high-melting carbides, socalled carbide-modified graphite tubes. A general review [64] as

Fig. 13. Transient Li isotope signals of 20 µg SRM 1640 before (solid lines) and after spiking (dashed lines) with 0.04 ng 6Li (the 6Li and 7Li isotope intensities are not in scale to each other).

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well as a review focused on practical aspects of this technique is given by A.B. Volynsky [65]. • In all above-mentioned measurements the pyrolysis stage was carried out at reduced pressure. This had some practical advantage, since the valves as well as opening and closing of the used FANESsystem must be operated by hand. Thus, the entire measuring process is shortened when furnace evacuation is started at begin of the pyrolysis stage and the final working pressure for atomization is established at the end of this stage of work. However, to avoid analyte loss, low pressure also requires low pyrolysis temperatures. In the case of lithium, analyte losses were observed at pyrolysis temperatures as low as 550 K. Further investigations at this stage of work should be carried out. • The synchronous increase of the transient signals of different isotopes up to the peak maximum and across will give peak height evaluation in isotope dilution analysis the same quality as peak area evaluation has. Moreover, in special cases, as seen by the Li measurements in Section 5.2.1, peak height evaluation will be superior. 5.5. Determination of large 7Li/6Li isotope ratios by IDA The experiments in this section deal with the determination of the Li/6Li isotope ratio of the 7Li isotope standard by application of isotope dilution diode laser GFAAS. It is the same 7Li isotope standard as used for the isotope dilution experiments of Section 5.2.1. The principle of the experiments is the addition of increasing amounts of a sample with an unknown isotope ratio for determination to a reference sample of known isotope ratio. The latter sample is also used for signal calibration. In a first experiment [25] presented in the following Section 5.5.1, Doppler-limited spectroscopy using the resonance transition at 671 nm was applied while in a second experiment [66] Doppler-free two-photon spectrometry with a resonant intermediate state was performed (Section 5.5.2). 7

5.5.1. Isotope ratio determination applying Doppler-limited absorption spectrometry For the isotope ratio determination of the 7Li standard, samples with increasing Li content were prepared by adding defined quantities of the 7Li stock solution to a diluted Li standard solution with known isotope ratio. Since the 7Li standard solution contained small amounts of 6Li ((0.06 ± 0.02)%), the 6Li content of the samples increased also slightly. From the given error, a 7Li/6Li isotope ratio between 1250 and 2500 had to be expected. For evaluation the 7Li atom concentrations added to the standard solution were displayed against the corresponding simultaneously measured and evaluated 6Li atom concentrations. The slope of the regression line represented by the 7Li/6Li isotope ratio of the 7Li stock solution was found to be 2380 ± 180. This value is of the same order as the relative sensitivity of 4300 for 6Li at the chosen experimental conditions. Therefore, the dependence on the uncertainty of the relative sensitivity is high. This uncertainty was mainly determined by the uncertainty of the value for collision broadening of 50% given in the literature [67]. Taking this into account, a minimum value for the 7 Li/6Li isotope ratio of 1900 and maximum value of 3300 was found. For comparison, the isotope ratio determined by ICP-MS amounts to 2450 ± 106. The uncertainty for collision broadening would be of minor influence if the value for the relative sensitivity is significantly increased. Thus, the determination of the isotope ratio of the 7Li isotope standard was repeated by applying resonant Doppler-free two-photon spectrometry. 5.5.2. Isotope ratio determination applying resonant Doppler-free two photon spectrometry In resonant Doppler-free two photon spectrometry [68] a distinct velocity group of analyte atoms is pumped by a laser beam from a low

lying state E1 into a resonant intermediate state E2, while the radiation of a second laser excites the same velocity group into a final third level E3. The two laser beams can be adjusted in parallel as well as in opposite direction. By application of Doppler-free two-photon spectrometry one has to take special care to collision processes. A complex example is the 2s 2S1/2 → 2p 2P°1/2,3/2 → 3s 2S1/2-transition in the lithium spectrum. The involved energy levels of both isotopes and the applied excitation scheme (solid lines) are illustrated in Fig. 14. An impairing collision induced energy transfer between the intermediate 2PJ-levels [69] of the two isotopes is indicated by double arrows. Energy transfer is also possible between the two isotopes by 6Li – 7Li collisions [70] because the 2PJ-levels of both isotopes are of similar wave number. Thus, the collision energy transfer between the 2PJ levels excluded a simultaneous detection of both natural Li isotopes by resonant two-photon spectrometry. Therefore, both isotopes were measured separately. 6Li was measured using the 2S1/2 → 2P°1/2 → 3S1/2-transition (670.808 nm and 812.625 nm) and 7Li by use of the 2S1/2 → 2P°3/2 → 3S1/2-transition (670.777 nm and 812.645 nm). Taking into account the uncertainty of the value for collision broadening, for counter propagation a relative sensitivity of 46,000 was evaluated, with 31,400 as lower limit and 82,000 as upper limit. These values are all much larger than the expected isotope ratio and will cause an uncertainty of less than two percent which is smaller than the statistical error. The evaluated 7Li atom concentrations added to the standard solution were as before displayed against the corresponding 6Li atom concentrations. The 7Li/6Li isotope ratio was found to be 2470 ± 110. This value agrees well with the formerly measured ratio of 2380 ± 180 when Doppler-limited spectrometry was applied (see Section 5.5.3) as well as with the value measured by ICP-MS which is 2450 ± 106. 5.5.3. Limits of detection As for the application of Doppler-free saturation spectrometry the exclusive modulation of the pump beam by a chopper enabled the best signal-to-noise ratio also for resonant two-photon spectrometry [66]. With parallel beams and an Ar buffer gas pressure of 10 hPa the limits of detection (3σ -criterion) were 0.5 pg 6Li and 0.25 pg 7Li. Here, the saturation parameter S was 4 for the 6Li measurement and 8 for 7 Li, respectively. The results are comparable with those measured by application of 2f-WM in Doppler-limited spectrometry (Section 4.2.4) and by Doppler-free saturation spectrometry (Section 4.2.5). A direct absorption measurement of the 2P → 3S transition showed the detection sensitivity being reduced by about two orders of magnitude in comparison with resonance line absorption. The excellent detection limits

Fig. 14. Schematic diagram for the levels involved in the resonant 2S → 2P → 3S-twophoton transitions in 6,7Li. Solid lines: transitions used for excitation; dashed lines: transitions due to collision transfer. From [66].

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obtained with resonant two-photon spectroscopy are therefore assigned to a very low noise level which corresponds to about 10− 6 absorption in the second transition. 6. Further elements suitable for isotope dilution GFAAS This section presents the result of our investigation of the periodic table for elements which are suitable to be determined by isotope dilution GFAAS. Required is the existence of transitions in the elemental spectra which are sufficiently resolved to perform isotope

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selective measurements by graphite furnace atomization. The basic requirements are that the element of interest can be generally atomized by electrothermal heating [1] and the existence of at least two natural isotopes of this element. Further criteria for the selection of suitable transitions are - a large isotope shift or hyperfine splitting; - the availability of data concerning these effects; - the transition wavelength should be matched by the fundamental radiation of free-running single mode laser diodes, by the application of second harmonic generation, sum or difference frequency generation or by use of an ECDL-system (Extended Cavity Diode Laser); - the lower level has to be the ground state or a deep lying metastable state which is thermally populated or can be populated in a discharge (e.g. a FANES-system); - a large oscillator strength. The following discussion is split into four groups, the light elements (Z = 1–18), the medium heavy elements (Z = 19–56), the rare earth elements (Z = 57–71) and the heavy elements (Z = 72–92). Spectra expected to be suitable for isotope selective GFAAS and not presented in previous work [4,25,43,49,50,63,66] were calculated and are displayed here (Fig. 15 and Fig. 16). Since the working pressure in the isotope selective application amounts only to a few hPa, the temperatures for graphite furnace atomization given by B. Welz and M. Sperling [1] were reduced by 200–300 K for the present calculations. This was regarded to be justified. If no data concerning collision broadening were available, a homogeneous line width of 70 MHz was assumed. This value is in the range usually obtained for the homogeneous line width at the present conditions. 6.1. Light elements (Z = 1–18) The elements from hydrogen (Z = 1) up to argon (Z = 18) will here be classified as light elements. Considering the above-mentioned criteria, the only element of this group suitable for isotope dilution measurements in GFAAS is lithium (resonance line, data see [71,72]). The transition at 208.9 nm in the spectrum of boron can certainly be matched by SHG of laser diode radiation emitted at 418 nm, but the full Doppler width of the isotope lines as well as the isotope shift both amount to approximately 17 GHz and, thus, a Doppler-free technique should be applied. However, the power of actually available laser diodes emitting in the blue spectral range is too poor to generate sufficient SHG intensity for saturation. 6.2. Medium heavy elements (Z = 19–56)

Fig. 15. Calculated Doppler-limited spectra of antimony, strontium and barium for natural isotope abundances; (a) 372.28 nm line of Sb, T = 2000 K and ΓL = 70 MHz. Solid: sum spectrum; dashed: 121Sb; dot and dash: 123Sb. Resolved F → F' transitions are indicated; (b) 679.10 nm line of Sr, T = 2 200 K and ΓL = 70 MHz. Solid: sum spectrum; dashed: even isotopes; dot and dash: 87Sr. The resolved hyperfine components of 87Sr are indicated; (c) 791.13 nm line of Ba, T = 2 300 K and ΓL = 125 MHz. Solid: sum spectrum; dashed: even isotopes; dot and dash: 135Ba; dotted: 137Ba. Features occurring from the odd isotopes are separated from the even isotopes.

The second group encompasses the elements from potassium (Z = 19) up to barium (Z = 56). These elements show (as already mentioned) very low isotope shifts. Nevertheless, rubidium is best suitable for isotope selective measurements using a graphite furnace. The isotopes 85Rb and 87Rb have a different nuclear spin and the ground state magnetic dipole constant A is large and very different for the two isotopes [72]. That is why the hyperfine lines of the isotopes are well separated in the ground state resonance transitions D1 (794.76 nm) and D2 (780.03 nm). The conditions are similar for the 372.28 nm transition of antimony [73] (Fig. 15a) which starts from the metastable 5p3 2P°1/2 state (16 395.8 cm− 1). However, it must be proved whether this state can be sufficiently populated in a FANES-system. Transitions with small isotope shifts between the even numbered isotopes but large hyperfine structure splitting for the odd numbered isotopes can be used for isotope dilution applications. Here, the isotopes of even mass number are separated from isotopes of odd mass number. An example is the strontium 679.10 nm transition (Fig. 15b) starting from the metastable 5s 5p 3P°0 state (14,317.5 cm− 1) [74]. In the barium spectrum the intercombination line at 791.13 nm (Fig. 15c) can be used [75,76].

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Fig. 16. Calculated Doppler-limited spectra of osmium, platinum, thallium and uranium for natural isotope abundances; (a) 426.08 nm line of Os, T = 2 200 K and ΓL = 70 MHz. Solid: sum spectrum; dashed: even isotopes; dot and dash: 189Os. Two features of 189Os are resolved; (b) 340.81 nm line of Pt, T = 2 200 K and ΓL = 70 MHz. solid: sum spectrum; dashed: even isotopes; dot and dash: 195Pt. The hyperfine components of 195Pt are resolved; (c) 535.05 nm line of Tl, T = 1 900 K and ΓL = 74 MHz. Solid: sum spectrum; dashed: 203Tl; dot and dash: 205Tl; (d) 404.28 nm line of U, T = 2 200 K and ΓL = 70 MHz. The isotope lines are indicated.

6.3. The rare earth elements (Z = 57–71) The rare earth elements (Z = 57–71) are of particular interest in nuclear physics because of their complex core structure and large core deformations. Therefore, they are very well investigated and, in contrast to many other elements, numerous measurements concerning isotope shifts (they give information on the nuclear deformation) and hyperfine structure are reported. The rare earth elements are also of particular interest for trace analysis. They are used in many technical products and manufacturing processes as well as in agriculture [77,78] and medicine [79]. In a recent work [50] the suitability of these elements for isotope selective GFAAS and isotope dilution applications was intensively discussed and demonstrated by experimental spectra. The transitions found to be suitable and which can be matched with the radiation of currently available laser diodes are given in Table 4. 6.4. Heavy elements (Z = 72–92) Seven elements of the heavy elements can be taken into consideration for isotope selective GFAAS. The ground state transition of Os at 426.08 nm [90] shows two hyperfine lines of 189Os well separated from the other lines which allows the determination of Os by application of isotope dilution GFAAS (Fig. 16a). For each of the two Ir isotopes, 191Ir and 193Ir, one hyperfine component in the 380.01 nm transition (0 cm− 1 → 26 307.5 cm− 1) [91] should be resolved if saturation spectrometry is applied. The Doppler-limited spectrum of the 340.81 nm line of platinum (823.7 cm− 1 → 30156.9 cm− 1) shows two strong hyperfine lines of

195

Pt well separated (Fig. 16b) [92,93]. The transition is matched by SHG radiation of a laser diode emitting in the red spectral range at 681.6 nm. If saturation spectrometry is applied the even numbered main isotopes 194Pt, 196Pt, 198Pt should appear resolved. However, the application of this technique requires high power fundamental radiation and effective SHG, e.g. by use of a ring resonator. Because of its toxicity, the heavy metal thallium is very interesting for analytics. The hyperfine components of the odd isotopes 203Tl and 205 Tl are not sufficiently resolved in the Doppler-limited spectrum of the resonance transition at 377.57 nm [94]. Thus, a Doppler-free technique is required for the determination of Tl at its resonance transition. A better separation of the Tl isotopes is obtained by use of the 535.05 nm transition (7792.7 cm− 1 → 26 477.5 cm− 1, Fig. 16c) [95] which, unfortunately, cannot be excited with fundamental radiation of commercially available diode lasers. However, by use of a commercial ECDL-system which incorporate a tapered amplifier and second harmonic generation, 535 nm can be matched at an output power up to 200 mW. Table 4 REE transitions suitable for isotope selective GFAAS using diode lasers Element

λ/nm

Transition/cm− 1

References for hyperfine structure

References for isotope shift

Sm Eu Gd Er Er Lu

672.59 686.46 419.08 840.99 658.35 396.85

0 → 14 863.9 0 → 14 563.6 999.1 → 24 854.3 0 → 11 887.5 0 → 15 185.4 0 → 25 191.6

[6] [80,81] [82,83] [85,86] [85], misseda [88,89]

[6] [80] [84] [86] [87] [89]

a

15,185.4 cm− 1.

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Transitions of the lead spectrum suitable for isotope selective GFAAS applications are discussed in Section 5.1. Three of these transitions can be excited by the fundamental radiation of laser diodes emitting in the blue spectral range or by application of SFG. By use of Doppler-free techniques, a full isotope analysis will be possible at 363.96 nm and 405.78 nm. Uranium shows numerous transitions for which the isotopes are resolved by use of a low-pressure graphite furnace atomizer. The isotope shifts of the involved levels are tabled by Engleman et al. [96]. Hyperfine structure data of 235U are not so common. The ground state transitions at 394.38 nm [97,98] and 639.54 nm [97,99] as well as the 404.28 nm line (Fig. 16d) [98,99] with the lower level at 620 cm− 1 can be investigated by use of commercially available laser diodes. Besides isotope shift data, also data for the hyperfine structure constants are available for these lines. The oscillator strengths of the transitions in the blue spectral range are larger by approximately two orders of magnitude. The ground state transition having the longest wavelength in the mercury spectrum is the intercombination line at 253.65 nm. The next higher levels are beyond 37 645 cm− 1. R. Uhl et al. [100] excited the strong 365.48 nm transition by use of SFG and application of the cold vapor technique [1] for detection. The metastable level (was populated using a radio frequency discharge. Some of the 199,201Hg hyperfine components were well resolved. 6.5. Detection sensitivity and limits of detection of REE isotopes As can be seen in Table 5, the detection sensitivities for the isotopes of Sm, Eu and Er are generally three orders of magnitude worse in relation to the ones obtained for the elements at atmospheric pressure with classical GFAAS. The difference is predominantly attributed to the much weaker transitions in the isotope selective measurements. One must also take into account the causes discussed in Section 4.2.4, i.e., the relative abundance of the isotopes, a faster analyte diffusion and the linear polarization of the exciting laser light. The discrepancy is smaller in the case of Gd, since the used transition is of comparable strength with the conventionally used line. With application of saturation spectrometry, the limit of detection of 154Sm at 672.59 nm was found to be 40 ng and 14 ng, respectively, when the peak heights and the peak areas were evaluated. The Ar pressure was 5 hPa and the radiated power density yielded 120 mW/cm2. The detection limit of 170Er measured at 840.99 nm by use of saturation spectrometry was 7 ng/ml. Here, the radiated power density was 400 mW/cm2. We conclude that the application of Doppler-free saturation spectrometry provides not only a better spectral resolution of isotopic components but also a better limit of detection in comparison with Table 5 Characteristic masses mchar and limits of detection of the rare earth elements obtained with isotope selective GFAAS compared with classical hollow cathode lamp GFAAS Isotope selective GFAAS Isotope

λ/nm

152

672.59 678.10 686.86 686.86 840.99 658.53 419.08

Conventional GFAAS

mchar/ng

IM Sm 152 Sm 151 Eu 153 Eu 170 Er 170 Er 160 Gd

220 950 50 50 260 400 400

Element

λ/nm

mchar/nga

Sm

429.7

0.24

2

Eu

459.4

0.02

2.3b

Er

400.8

0.07

Gd

368.4

11

Limit of detection/ng

20

2f-WM

IM — Intensity modulation; 2f-WM — 2f-wavelength modulation. a [1], b Measured at the subsidiary extremum of the blue wing.

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Doppler-limited direct absorption measurements. This agrees with the observations made for Li and Rb in Section 4.2.5. 7. Evaluation of line overlapping in saturation spectrometry 7.1. Line profiles in saturation spectrometry Doppler-free lines obtained by application of saturation spectrometry are not only broadened by pressure and radiation power, also so-called velocity changing collisions (vcc) with buffer gas atoms contribute to the observed line profiles [33]. These collisions distribute analyte atoms between different velocity groups visible in form of a broad Doppler background. For higher analyte concentrations where the partial pressure due to analyte atoms is no longer negligible in comparison with the buffer gas partial pressure of a few hPa, absorption of fluorescence radiation as well as mutual collisions of analyte atoms also take place. The latter effects result in a concentration dependence of line width and line shape (e.g., collisions among identical particles can cause asymmetrical line profiles [101]). Due to all these broadening mechanisms the relative sensitivity in saturation spectra can no longer be estimated in the same unproblematic way as in Doppler-limited spectra. However, beyond the line core a Lorentzian profile with decrease proportional 1/Δν2 can be assumed. When the homogenous line width is substantially smaller than the Doppler width a superposition of a Lorentzian profile L(ν) with a weak Doppler background G(ν) is often used to approximate the profile in the line core [34,102]: Isat ðmÞ ¼ j½LðmÞ þ CGðmÞ:

ð12Þ

Here, Isat(ν) is the intensity of the measured saturation signal, κ is a constant for normalization and C is a weight factor for the Gaussian profile of the Doppler background. Values for κ and C as well as the homogenous width, which is mainly determined by pressure and saturation broadening, can be obtained by a fit to the experimental curve. J. Tenenbaum et al. [103] investigated velocity changing collisions where they made a difference between ‘weak’ collisions and ‘strong’ collisions. For weak collisions the velocity of a particle is only slightly modified and a large number of collisions is necessary to invert the velocity component from +vz to −vz (z is the axis of laser light propagation). For strong collisions one single process is sufficient. Predominant weak collisions produce a line shape with an exponential decrease on both sides of the line center which is positioned on a broad Doppler background. Predominant strong collisions form a Lorentzian distribution on a Doppler-broadened pedestal. The signal contribution of the Gaussian profile depends on the kind of collisions and on buffer gas pressure. Tenenbaum et al. derived an expression similar to Eq. (12) with the Lorentzian function L(ν) replaced by a function F(ν) which is an exponential function for weak collisions and a Lorentzian function for hard collisions. In the following, we present the attempt to fit signals calculated with this theory to experimental saturation spectra. For this attempt, we used the double peaks formed by the isotopes 152Sm and 154Sm at different working pressures (0.5 hPa; 1 hPa; 5 hPa; 10 hPa). In general, fits applying an exponential function F(ν) produced the better results. This was unambiguous for the lower pressures (0.5 and 1 hPa) whereas for the higher pressures (5 and 10 hPa) a contribution of hard collisions and thus a Lorentzian behavior could not be excluded. Exponential fits to the experimental spectra are presented in Fig. 17a (0.5 hPa Ar) and Fig. 17b (1 hPa Ar) while in Fig. 17c the experimental spectrum of Fig. 17b is fitted with a Lorentzian function. The discrepancy in the small region at the center of the spectra displayed in Fig. 17a and b can be attributed to a neglect of the atoms which avoided collisions [103]. On account of the result, it can be concluded that an estimation of values for the relative sensitivity is possible also

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Fig. 17. Signals measured by saturation spectrometry for the isotopes 152Sm and 154Sm. Solid lines: experiment, dashed lines: fit curves; (a) F(Δν) (see text) is an exponential function, Ar buffer gas pressure 0.5 hPa; (b) F(Δν) is an exponential function, Ar buffer gas pressure 5 hPa; (c) F(Δν) is a Lorentzian function, Ar buffer gas pressure 5 hPa.

for saturation spectra and thus a full quantitative analysis can be carried out just as in the Doppler-limited regime. 7.2. Saturation spectrometry applying double modulation In contrast to the Doppler-free signal contributions, the Doppler background in saturation spectra varies only slightly with frequency. Thus, wavelength modulation with small modulation amplitude added to the pump beam intensity modulation should significantly reduce the contribution of velocity changing collisions and stress the Dopplerfree signal. This combination is expressed in the following text as double modulation technique. With the frequency of the intensity modulation set to fCh and the wavelength modulation frequency set to

fWM, the transmitted probe beam signal is processed with the sum frequency 2fWM + fCh or the difference frequency 2fWM − fCh. Detection on an even higher order of the wavelength modulation may reduce the Doppler background much stronger; however, the spectra will become more confusing. The spectra of the Sm 672.59 nm transition are displayed in Fig. 18a–c for the application of double modulation at different wavelength modulation amplitudes Δνmod. The line width in Fig. 18b (Δνmod: 370 MHz) was estimated to about 250 MHz in good agreement with the value for the homogenous width which was used for the fit of the experimental spectrum in Fig. 17c. The same modulation depth as in Fig. 18c was used for Fig. 18d while the intensity modulation of the pump beam was turned off. In this figure,

Fig. 18. (a)–(c) Signals obtained for 30 µg of Sm by saturation spectrometry with double modulation, λ = 672.59 nm, fCh = 8.6 kHz, fWM = 13.7 kHz, (a) 2Δνmod = 2 GHz, (b) 2Δνmod = 750 MHz, (c) 2Δνmod = 370 MHz; (d) only 2f-wavelength modulation is applied, 2Δνmod = 370 MHz. The Ar buffer gas pressure was 10 hPa.

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the Doppler-free lines can be regarded as Lamb-dips positioned in a Doppler-limited 2f-WM spectrum. For the measurements of Fig. 18, a graphite tube was used which was contaminated with samarium due to numerous atomization cycles with large amounts of Sm. For this contamination, the absorption signal was present for a longer time. Thus, a lower scan frequency of only 0.3 Hz and also a longer time constant (1 ms) for data processing by the lock-in amplifier could be used which resulted in an improved signal-to-noise ratio. It must be noted, that the relative intensities of the single lines in Fig. 18 were distorted under these experimental conditions. The double modulation method certainly provides a better spectral resolution, however, on the cost of a lower signal-to-noise ratio and, in turn, a worse detection limit. Therefore, it must be assumed that the Doppler background due to velocity changing collisions produces an unavoidable background signal in high-resolution 2f-WM spectra, even for low modulation amplitudes. For example, the detection limits obtained for rubidium by application of the double modulation technique (85Rb: 7.9 pg, 87Rb 8.6 pg) were approximately six times the ones obtained by exclusive intensity modulation (85Rb: 1.4 pg, 87Rb 1.3 pg). 8. Summary and outlook In this review, isotope dilution optical spectrometry (IDOS) is presented, a new technique for calibration in quantitative trace element analysis. Isotope dilution is introduced to optical atomic absorption spectrometry by use of low-bandwidth single mode laser diodes as radiation sources and a low-pressure graphite furnace of the FANES-type for atomization. It is demonstrated that the introduction of isotope dilution overcomes the calibration problem in graphite furnace atomic absorption spectrometry. Isotope dilution GFAAS is a potential alternative to mass spectrometry. Mass discrimination effects, which are e.g. very unfavorable for the lithium determination by ICP-MS or TIMS are of low significance in isotope selective GFAAS. Moreover, isobaric interferences by isotopes of different elements (e.g. 87Rb and 87Sr), typical for mass spectrometry, cannot occur. A further advantage over mass spectrometry are the smaller costs. Isotope dilution is also applicable with other techniques of isotope resolving optical spectrometry as for instance optogalvanic spectrometry. Especially GFAAS suffers from matrix effects. Despite careful consideration of the STPF-concept in GFAAS, there often remain disturbing interferences by concomitants which usually produce signal depressions (frequently combined with a double peak structure) resulting in an incorrect analysis. The reduction of such interferences without analyte loss or contamination is a difficult and very time consuming task and does not guarantee accurate results. The reduction of working pressure in GFAAS down to a few hPa combined with the replacement of the classically used hollow cathode and electrodeless discharge lamps by tunable single mode laser diodes enables isotope selective excitation, the prerequisite for the applicability of the isotope dilution method in GFAAS. In mass spectrometry, isotope dilution has proved to be the best choice for accurate and precise results [5]. With the transfer of the isotope dilution method from mass spectrometry to optical spectrometry an internal standard method becomes available for GFAAS. While in mass spectrometry the isotope lines are well separated, this is generally not the case in optical spectrometry. The Voigt profiles of the resolved isotope lines are more or less overlapping at temperatures of 1500–2300 K which are obtained at detection in isotope selective GFAAS. Thus, a simple transfer of the formula used in mass spectrometry for evaluation of the measurements into optical spectrometry will in principle provide incorrect results. We demonstrate that with the knowledge of detection temperature, pressure broadening and of the respective Voigt profiles, the contribution of

557

each isotope to the sum signal at a given frequency can be estimated. With the definition of a relative sensitivity for single isotopes, a formula for the evaluation of isotope dilution analyses in optical spectrometry is derived. For a given spectral transition, the formula includes the spectral overlap of all isotope contributions. Moreover, this definition allows to record two or more isotope lines simultaneously by application of very different spectroscopic techniques. For example, the Li and Rb content in a NIST standard water was reproduced by isotope dilution GFAAS performing direct absorption measurement for one isotope signal and 2f-WM for the other one. Calibration by isotope dilution succeeded even for the element lithium, which is, because of the high isotopic mass difference, supposed to be the most critical element for determinations by isotope dilution GFAAS. As our experiments show, isotope dilution is clearly superior to the classical element addition method. In order to reduce analyte losses and to keep a good detection sensitivity, we recommend the use of a standard chemical modifier. In contrast to a full isotope ratio analysis, isotope dilution applications need only two features to be resolved in the sum signal of single isotope or hyperfine structure signal contributions. Thus, also transitions in the blue and even in the UV spectral range are accessible, although Doppler-broadening will be 1.5 to 2 times higher in comparison with the red and near-infrared range. Disregarding a few exceptions, the elemental spectral transitions are 1–3 orders of magnitude stronger in the blue and UV. Thus, improved limits of detection are expected there. If in isotope selective diode laser GFAAS the same transitions as in classical hollow cathode lamp GFAAS can be used, the detection limit obtained for a single isotope with the lowpressure high-resolution method is comparable with the detection limit for the element and use of the conventional technique. The availability of appropriate radiation sources is a basic requirement for isotope dilution optical spectrometry. Laser diodes which are of interest for trace element analysis are available for the near-infrared, the red and the blue spectral range. By the use of frequency conversion techniques as second harmonic generation or sum or difference frequency generation, spectral regimes become accessible, which are still not covered by commercially available single mode laser diodes. It should be noted at this place, that only a limited number of elements can be determined by isotope dilution GFAAS taking into account the present state of diode laser availability. The applicability of the presented method is demonstrated for the elements Li, Rb and Pb as well as for the rare earth elements Sm, Eu, Er and Gd. Isotope dilution analyses are certainly also possible for Ba, Lu, Os, Ir, Pt and U. This will be also the case for Sb, Sr and Hg, provided, the respective metastable lower state can be populated in the FANES-system. Fortunately, the increasing availability of diode laser sources with fundamental emission in the blue spectral range is combined with the publication of new isotope shift and hyperfine structure data in this spectral range. This lets us assume that on the one hand stronger transitions and on the other hand new elements will be accessible for isotope dilution optical spectrometry in the near future. The 2f-wavelength modulation technique is frequently applied in diode laser AAS since it provides better limits of detection in comparison with direct absorption measurements. Unfortunately, the WM-technique gives worse isotope selectivities for close neighbored lines. At first sight, it seems to be a technique less appropriate for isotope selective applications. However, as closer consideration in this work reveals, when the line separation exceeds a distinct frequency difference, considerably higher relative sensitivities can be obtained than with application of the direct absorption method. This is in particular the case when that subsidiary extremum in the 2f-WM spectrum of the respective isotopic component can be used for the measurement, which is positioned opposite to the interfering line. As expected, a significantly better selectivity is obtained by application of Doppler-free spectrometric techniques. First of all,

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saturation spectrometry comes into consideration. However, at a working pressure of a few hPa in the graphite furnace, velocity changing collisions determine the width of Doppler-free line profiles. An attempt to fit experimentally obtained saturation signals with theoretical profiles succeeded. This enables the calculation of relative sensitivities. Thus, the requirement for performing quantitative trace element determinations by isotope dilution analysis is also fulfilled when Doppler-free saturation spectrometry is applied. If besides the intensity of the strong pump beam the wavelength is also modulated with small amplitude, the impairing signal contribution by velocity changing collisions is reduced. This double modulation method competes with other techniques to suppress the influence of velocity changing collisions to the measured signal. The described method is the best suitable for high-resolution isotope selective applications in GFAAS and is generally applicable in highresolution spectrometry. However, in the present experiments, the gain in selectivity was at the cost of a decrease in signal-to-noise ratio. In comparison with Doppler-limited spectrometry, the application of a Doppler-free technique requires a more extensive instrumentation. Simultaneous Doppler-free measurements of two isotope transitions need four instead of two laser beams directed through the absorption volume. Saturation spectrometry requires two diode lasers and resonant two-photon spectrometry even four lasers. Doppler-free two-photon spectrometry with resonant intermediate state provides even higher selectivities than saturation spectrometry. Resonant Doppler-free two-photon spectrometry was used for the determination of the large 7Li/6Li isotope ratio (N2000) in a 7Li isotope standard. Increasing amounts of this standard were added to a reference sample of known isotope ratio. In current of these measurements it became evident, that collisional and radiative processes not only have influence on the shape of the Doppler-free lines, but they may also influence the analytical result. This must be kept in mind when such measurements are performed. The working stages of isotope selective GFAAS, starting with insertion of the sample and ending with data evaluation can be automated. Therefore, isotope dilution GFAAS is generally suitable for routine analytics. It will in particular become an interesting and reasonable alternative to mass spectrometry, if the sample content of one single element has to be checked-up in routine cases as, e.g., in monitoring of patients in the clinical use. GFAAS requires only a few microliters of sample volume and isotope dilution guarantees accurate results. Isotope dilution optical spectrometry is in principle also a method for accurate species specific determinations. Here, a low-pressure microwave induced plasma [104], a radio frequency discharge or a dielectric barrier discharge [105] has to be combined with a separation technique as, e.g., gas chromatography, high-performance liquid chromatography or electrophoresis. The separated species have to be atomized in the plasma or in the discharge and detected by isotope selective diode laser spectrometry. The poor commercial availability of isotope-labeled spike compounds limits currently the application of species specific isotope dilution methods [5]. With the expected increasing availability of such compounds, the number of demands for accurate and precise species specific determinations will also increase. This will expand the application field of the isotope dilution technique; in combination with optical spectrometry as well as in combination with mass spectrometry. We can state that isotope selective GFAAS has been developed from a method, which displays absorption signals from individual isotopes, to an instrument for accurate quantitative element analysis and isotope ratio determination. Thus, element content determinations by isotope dilution become possible. Isotope dilution optical spectrometry, the combination of isotope dilution with sensitive and selective techniques of diode laser spectrometry, solves the calibration problem in optical atomic absorption spectrometry. In particular, the technique provides a fast and accurate quantitative element analysis by diode

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