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Element selective detection of molecular species applying chromatographic techniques and diode laser atomic absorption spectrometry
Spectrochimica Acta Part A 60 (2004) 3393–3401
Element selective detection of molecular species applying chromatographic techniques and diode laser atomic absorption spectrometry夽 K. Kunze∗ , A. Zybin, J. Koch, J. Franzke, M. Miclea, K. Niemax Institute of Spectrochemistry and Applied Spectroscopy (ISAS), Bunsen-Kirchhoff-Strasse 11, 44139 Dortmund, Germany Received 30 September 2003; accepted 3 November 2003
Abstract Tunable diode laser atomic absorption spectroscopy (DLAAS) combined with separation techniques and atomization in plasmas and flames is presented as a powerful method for analysis of molecular species. The analytical figures of merit of the technique are demonstrated by the measurement of Cr(VI) and Mn compounds, as well as molecular species including halogen atoms, hydrogen, carbon and sulfur. © 2004 Elsevier B.V. All rights reserved. Keywords: Diode laser atomic absorption spectroscopy; Chromatography; Tunable diode laser spectroscopy
1. Introduction Traces of small molecules in gases can be directly measured in the mid-infrared spectral range by tunable diode laser absorption spectroscopy (TDLAS). This is a versatile and highly selective technique with sensitivities in the sub-part-per-billion concentration range. Generally the tunable lasers are lead salt diode lasers which emit in the region between 3 and 30 m. Unfortunately, lead salt lasers have to be cryogenically cooled to temperatures much below room temperature which makes the instrumentation less convenient for operation. However, this drawback of TDLAS of small molecules will vanish in the near future due to the rapid progress in mid-IR quantum cascade laser development for gas sensing, in particular, for room temperature operation (see [1] or the relevant papers in this special isssue). A completely different approach for quantitative measurements of molecules by tunable diode laser absorption spectroscopy will be discussed in the present paper. It is based on the detection of atoms (elements) in the gas phase by diode laser atomic absorption spectroscopy (DLAAS) 夽 Presented at the Fourth International Conference on Tunable Diode Laser Spectroscopy, Zermatt, Switzerland, July 17, 2003. ∗ Corresponding author. Tel.: +49-231-1392-210; fax: +49-231-1392-120. E-mail address: [email protected] (K. Kunze).
1386-1425/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2003.11.047
after dissociation of the molecules of interest in an appropriate atomizer. In contrast to TDLAS of small molecules the wavelengths of the diode lasers systems applied in atomic absorption spectrometry (AAS) should be in the visible and UV spectral range to match the electronic transitions of the atoms. There are basic physical advantages to use DLAAS for molecular analysis. First, the absorption cross-sections of strong atomic transitions are of the order of 10−11 to 10−12 cm2 . They are much larger than the absorption cross-sections of small molecules in the mid-IR (typically 10−17 to 10−21 cm2 ) resulting in a higher detection sensitivity. Second, the molecule of interest might have more than one atom of a particular element which correspondingly increases the number density in an absorption experiment. Third, there are much less spectral interferences in DLAAS than in TDLAS of small molecules. The search for “spectral windows” in the mid-IR for undisturbed measurement and the reduction of the line widths by measurements under low pressure conditions are complications well known in TDLAS of molecules. On the other hand, DLAAS of molecular species is not a “direct” method as TDLAS is. Different separated steps are required in experiment. The first step is the preparation of the sample. It is followed by the separation step of the different species by an appropriate chromatographic or electrophoretic technique. Subsequently the separated molecules
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Fig. 1. Principal detection scheme of molecular species by atomic spectrometry.
have to be atomized before they can be measured by absorption. Popular atomizers are resistively heated graphite tube furnaces, and for continuous operation, needed in combination with separation techniques, plasmas or flames. The general scheme of element-selective measurements of molecular species is sketched in Fig. 1. It has to be noted that such schemes are well established in analytical chemistry in particular with atomic emission detectors (atomic emission spectrometry (AES) applying plasmas) and inductively coupled plasma-mass spectrometry (ICP-MS). It will be shown below, that it is possible to measure fractional absorption I/I to the order of 10−7 by DLAAS. This means that it is possible to measure approximately 103 atoms cm−3 in a 10 cm long layer since the fractional absorption for small extinctions is given by I/I = σNL (: absorption cross-section; N: atomic number density and L: absorption length) and the absorption cross-section of a strong optical transition is approximately 10−11 cm2 .
2. The analytical figures of merit for DLAAS The laser diodes used in diode laser atomic absorption spectroscopy are commercial, etalon-type devices of different producers. They can be operated in the free running mode without external optical stabilization and they emit light in the region of 625–1600 nm. Recently, also laser diodes in the blue region (375–445 nm) became available. The laser diodes can be typically tuned over 10–20 nm by temperature
(−20 to 50 ◦ C) and current. The mode-hop free range is restricted to several hundred picometer. Wavelength tuning by temperature is a slow process, while current tuning can be very fast. Wavelength modulation frequencies up to 500 kHz can easily be achieved [2]. All etalon-type laser diodes show mode hopes and therefore mostly wavelength gaps, which cannot be covered in a free running mode. This problem can be overcome by the use of external cavities, in which a grating reflects the first-order of the laser light back into the diode in order to build an external resonator. In this way the mode-hop free tuning range is limited up to about 50 GHz, but all wavelengths in the gain profile can be obtained. Further, a very simple but effective feedback method is the mounting of a glass plate in front of the diode laser chip [3]. The wavelength range of diode laser spectroscopy can be extended by second harmonic generation (SHG) using non-linear phase-matched or periodically poled crystals, whereby powers up to about 1 mW can be achieved in the 335–430 nm region, depending on the initial laser power and the crystal material applied. Due to the fact that the wavelength range of laser diodes is still very limited, the number of attainable elements is restricted. Furthermore, only a few strong optical transitions from the ground state of (mainly metallic) elements are in the specific wavelength windows delivered by the laser diodes. Therefore, also transitions from excited levels have to be taken into account. In particular transitions from metastable levels, which are well populated, e.g.
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in low-pressure discharges, can successfully be used. As mentioned above, commonly applied atomization sources are graphite furnace, flames and electrical discharges like microwave-induced plasma (MIP), inductively coupled plasma (ICP) or direct current discharge (dc). It has to be stressed, that those sources have to be powerful enough to dissociate all molecules. For the detection of molecular species all continuous atomization sources (plasmas and flames) can be coupled with different separation techniques like gas chromatography (GC), high-performance liquid chromatography (HPLC) or capillary electrophoresis (CE). In order to suppress various noise sources and to discriminate non-specific background signals different detection techniques applying modulation of the absorption and of the diode laser wavelength have been developed. The usual technique for baseline reduction is wavelength modulation with detection at the second harmonic of the modulation frequency 2f [4,5]. Unfortunately, wavelength modulation of a laser diode is, as a rule, accompanied by residual amplitude modulation (RAM). This leads to a background signal at the registration frequency 2f, which is much smaller than the background signal in the case of 1f detection, but retains the multiplicative structure. Because this background signal is proportional to the laser power and the optical transmittance, the low-frequency noise of the laser radiation at the detector is mixed with the modulation frequency and included in the detection bandwidth.
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The background signal can be cancelled if, additionally, the absorption is modulated and detection is performed at the difference or sum of the wavelength and absorption modulation frequencies [6]. In this case it is possible to achieve detection limits determined by the laser excess noise only, because the background signal is absent. The most successful detection technique is the so-called double-beam double-modulation procedure [7,8], which can also cancel the laser excess noise by applying of a double-beam arrangement. A principal setup is presented in Fig. 2. The beam of a laser diode is splitted in two parts, whereas one beam is directly detected by a photo detector, while the other one is absorbed in any medium (e.g. low-pressure discharge, flame) before detection. Both, the absorption cell and the wavelength of the laser diode are modulated with different frequencies. The logarithmically subtracted signals of both photodetectors were preamplified and detected on a mixed frequency with a lock-in amplifier. The signal S in the double-beam double-modulation techniques is given by S(υ, t) ∝ ln iS / iR = ln aS /aR + ln TS /TR − κ(υ, t)L. Hereby, aS /aR is the splitting of absorption and reference beam splitting ratio, TS,R the corresponding transmittances, κ(υ, t) the absorption coefficient and L the absorption length. The registered signal is proportional to the absorption coefficient κL implying that the measured signal is independent of the applied laser power unless the considered transition
Fig. 2. Principal experimental arrangement of double-beam double-modulation absorption spectrometry [8].
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capability and versatility of this method has permanently been proven by a variety of analytical applications, such as DLAAS in graphite tube furnace and flames [11,12], for element-selective measurements in GC [6,13] and HPLC [14,15], isotope-selective analysis [16–18] and in connection with laser ablation [18,19]. In the following, examples will be given for the analysis of molecular species by DLAAS coupled with separation techniques like GC and HPLC. The efficiency of this procedures is demonstrated by the flexibility to analyze aerosols, liquid, and gaseous samples. Finally it will be shown, that DLAAS is a promising task to build small-sized detectors. In all cases an experimental arrangement similar to that shown in Fig. 2 is used. However, sample preparation and introduction as well as the modulation techniques may vary. 3.1. DLAAS and GC
Fig. 3. Comparison of AAS-signals of Al using a hollow cathode lamp and a diode laser.
is saturated. According to the intentionally accepted 3 criterion, the theoretical detection limit (DL) is three times the shot noise given by DL = 3(2e2 ηωPhν)1/2 with e the elementary charge, η the quantum efficiency of the photo detector, h the Planck’s constant, ω the detection bandwidth, P the laser power and ν the laser frequency. In this way using a laser diode with 100 nW power (typical for single pass SHG in non-linear crystals) an experimental absorption of 10−4 absorption units (AU)) can be reached, while the theoretical shot noise limit is 10−5 AU. This result is already one-order of magnitude better than the typical 10−3 AU, which is usually achieved with the well-known hollow cathode lamp AAS. Increasing the laser power even higher to 2 mW absorptions down to 2·10−7 AU have been measured [7]. Fig. 3 shows the improvement of diode laser AAS in comparison with hollow cathode AAS for aluminum. The signal was obtained by a commercial graphite tube DLAAS instrument. 3. Analytical applications Though DLs have extensively been used in the field of fundamental spectrometry from the beginning of the 1970s onwards, the analytical relevance of DLs for element analysis was not discussed before the late 1980s [9]. The potential applicability of DLAAS for the routine element analysis was first demonstrated elsewhere [10]. Since that time, the
The electron-capture detector (ECD) and flame ionization detector (FID) are widely used in GC analysis. Both are non-element-selective detectors, which have reasonable disadvantages in comparison with element-selective detectors. The latter ones are applied if the analytes of interest cannot be separated from other species or from strongly interfering matrix components. Furthermore, they give direct information on the elemental composition of the separated species. The most popular element-selective detector is based on atomic emission spectrometry (AES) of MIPs. McCormack et al. [20] have characterized this type of detector extensively since its first realization in 1965. Today, the MIP–GC–AES technique is well established as a standard method in analytical chemistry besides the most common used GC–MS. Here, we will demonstrate the capability of DLAAS for GC detection. The eluents are introduced in the lowtemperature microwave discharge where they are atomized. The elements are probed by single-beam double-modulation DLAAS of the electronically excited atoms. Experimental details on the gas chromatograph, the microwave-induced plasma, the laser diodes, and the sample preparation and introduction typically used in such measurements can be found in [6,13,21]. 3.1.1. Characterization of a GC–DLAAS The correct reproduction of the stoichiometrical element ratios of the species can be taken as evidence for complete dissociation. The complete atomization of the analytes by the plasma is not only important for accurate calibration by an internal standard added to the sample, but also for analyte identification. In the latter case, the ratios between the element concentrations (inter-element ratio) have to be measured after calibration of the elements of interest with a suitable standard species with different compound structures. Chromatograms (each analyte 1.2 mg ml−1 ) measured simultaneously on H, Cl, and C excited atoms lines produced in a MIP are shown in Fig. 4 [21]. The stoichiometric ratios were determined separately for C, H, and Cl taking one
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Fig. 4. Chromatograms for C, H, and Cl applying 1.2 mg ml−1 of the analytes: (1) heptane C7 H16 , (2) 1-chloropentane C5 H11 Cl, (3) 1-octene C8 H16 , (4) 1-octyne C8 H14 , (5) chlorobenzene C6 H5 Cl, and (6) 1,4-dichlorobutane C4 H8 Cl2 [21].
analyte as internal standard. It could be shown, that in an argon as well as in a helium MIP the measured inter-element ratios for the three elements were correct. Only for chlorobenzene a small deviation was observed. Furthermore, the correlation between the population density of metastable plasma atoms and the total dissociation energy of the organic analytes were investigated. It was pointed out, that the relative depopulation of Ar as well as Kr metastable atoms in the discharges was proportional to the dissociation energy of the specific eluate. 3.1.2. Chlorinated hydrocarbons in oil There is a growing pressure on industrial societies to avoid plastic material waste or to find acceptable solutions for disposal. The best solution is the recycling of plastic material. Since the market for simple recycling products such as filling material or pieces for building industry is limited, plastic material is converted to oil. The oil products derived in the first step of plastic material recycling are further processed to basic materials of higher quality in chemical reactors. However, before this refinement, the samples have to be analyzed. Halogenated hydrocarbons, in particular volatile chlorinated hydrocarbons, have to be measured,
since chlorine is known to be a poison for catalysis and also attacks the reactor walls. Even in low concentrations it limits significantly the lifetime of the catalysts. Therefore, a sensitive GC analysis of the strongly changing composition in the primary oil for the refinement procedure is necessary. The widely used ECD is obviously not sufficient for the variety of species in the sample. But even the commercial element-specific AES detector seems to be unsuited as it is shown in Fig. 5 [13]. A sample of 10% recycling oil from plastic material solved in toluene was detected on the C and Cl line of the AES. For comparison, a chlorine-specific chromatogram obtained with the GC–DLAAS in a MIP is plotted in the lower part of Fig. 5. Several chlorinated hydrocarbons in the oil sample could be detected, which could not be seen at all in the GC–AES chromatogram. A precise evaluation of the chromatograms gave an improvement factor of the signal-to-noise ratio of 20–30. 3.2. DLAAS and HPLC If samples, which are difficult to volatilize, have to be analyzed electrophoretic techniques or liquid chromatography
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Fig. 5. Chromatograms of an oil sample from plastic material measured with a commercial AES detector on (a) the C 247.86 nm line and (b) the Cl 479.45 nm line, and (c) chromatogram of the same sample as in (a) and (b), however, measured in an MIP by double-modulation DLAAS [13].
is used for separation. Two examples of metal speciation by HPLC and DLAAS in a flame will be given. The relevant experimental details on the HPLC-system, the analytical flame as continuous atomizer, the diode lasers, and the sample preparation used in such experiments can be found [14,25]. 3.2.1. Cr(VI) in aerosols and water In order to assess environmental risks of bio- and anthropogenic organo-metallics and toxic oxidation states of certain transition metals, a species-selective analysis of complex matrices is indispensable. With regard to the so-called redox speciation of metals, the bioavailability and toxicity often depends on the present oxidation state and not on the absolute element concentrations. Therefore, a valid analysis generally requires a chromatographic separation of the oxidation states before element-selective detection. In the case of chromium, the oxidation states Cr(III) and Cr(VI) are important. Cr(III) is an essential trace element in human metabolism while Cr(VI) species are suspected of causing irreversible damage of organic tissue. The toxic properties of Cr(VI) species, predominantly occurring in the form CrO4 −2 and Cr2 O7 −2 are due to their ability to penetrate the cell velum of erythrocytes via anionic channels,
which are impermeable to the cationic Cr(III) compound complex [22]. Within the scope of this project [8,14], the Cr content of samples taken from tap water and ambient aerosol was determined. The metallic species of the sample are separated in a HPLC module, nebulized in an air/acetylene flame, and measured by DLAAS. Ion pair chromatography was applied for the separation of the analyzed samples. The separation required a preparation procedure, which consisted of the addition of acetic acid to a certain pH value, and tetrabutylammonium acetate for the ion-pairing process. Sample volumes of 1 ml were applied to the HPLC system. A detailed description of the full procedure has been published by Berndt and Müller [23]. The specific absorption of Cr was measured at the wavelength 427.480 nm (transition 3d5 4s7 S3 → 3d5 4p7 P3 ). The radiation of a DL emitting at 854.960 nm, was frequency doubled using a phase matched KNbO3 crystal resulting in an output power of 3 W. The absorption signals were registered by means of a double-beam wavenlength-modulation detection scheme in order to compensate the noise, arising from the frequency conversion. The detection limit of Cr(VI) was approximately 0.1 ng ml−1 , which corresponds to a fractional absorption of approximately 10−4 . This value was just slightly above the theoretical shot noise level taking into account the experimental laser power. The detection limit is so low, that Cr(VI) could even be detected in ordinary tap water [14]. Another example, which demonstrates the analytical power of HPLC–DLAAS in a flame, is given in Fig. 6. It shows the measurements of aerosols samples collected in the neighborhood of ISAS in Dortmund. The dashed trace is the chromatogram of the collected and chemically solved aerosol particles while the full trace represents a measurement of the same solution sample spiked with 1 ng ml−1 Cr(VI), which corresponds to a concentration approximately 1.5 ng Cr(VI) in 1 m3 of air.
Fig. 6. Cr-specific chromatogram of solved ambient aerosol particles determined by using double-beam single-modulation DLAAS in an air/acetylene flame [8].
K. Kunze et al. / Spectrochimica Acta Part A 60 (2004) 3393–3401
3.2.2. Fuel analysis Controversy has surrounded the potential toxicity of the fuel additive methylcyclopentadienyl manganese tricarbonyl (MMT) since it was first introduced as an octane-boosting, anti-knocking agent in Canada in 1977. Initially denied approval for widespread use in the US, MMT was only approved for limited use in leaded gasoline from 1977 to 1995. In 1995, the manufacturer of MMT challenged the denial in court and won, thereby opening the door for MMT to be marketed for use in unleaded gasoline as well. Although manganese is considered as nutritionally essential trace element, it is known that very high levels of inhaled manganese induce neurobehavioral and respiratory effects [15,24]. However, the influence of MMT as a fuel additive on the health risk is not well known because of limitations in data. Nevertheless, MMT is known to be toxic in contrary to other inorganic forms of manganese. The experimental setup [25] was similar to the one used for Cr(VI) determination. The 806.378 nm output of a laser diode was frequency doubled with a LiIO3 crystal to achieve 170 nW of blue light at 403.189 nm (Mn line), which is a weak absorption line. In contrast to the other setup only single-beam single-modulation was used but with logarithmic detection on the fourth harmonic of the wavelength modulated signal. The samples were introduced into the air/acetylene flame with hydraulic high-pressure nebulization (HHPN) using a HPLC pump. Either a continuous introduction of the sample could be applied or a column was inserted for HPLC. A typical chromatogram of 50 ng ml−1 of inorganic manganese, cyclopentadienyl manganese tricarbonyl (CMT), and MMT each, solved in a 65:35 methanol/pH 4 buffer, showing the different retention time of the species is plotted in Fig. 7. The detection limit of the total manganese amount was 1 ng ml−1 , which is worse than other spectroscopic techniques. But it has to be emphasized that these experimental results can be improved by the use of higher laser
Inorganic Manganese
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CMT
Absorption Signal, rel. units
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0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0.0
0.5
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Fig. 7. Chromatogram of inorganic manganese, CMT, and MMT (concentrations: 50 ng ml−1 each) by HPLC–DLAAS in an air/acetylene flame [25].
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power or the selection of a stronger absorption line. For example, the new blue diode lasers [26], which offer almost 50 times more power and therefore reduce the shot noise will certainly results in an improved detection limit of almost one-order of magnitude. Nevertheless, even in these conditions the experiment allows measurements of inorganic and organic manganese compounds at concentration levels present in environmental and toxicological studies [27]. It was also demonstrated, that MMT could be detected in spiked gasoline, human urine, and tap water samples in good agreement with the added concentration. 3.3. Miniaturization Over the last few years, there have been worldwide efforts to develop miniaturized instrumentations for chemical analysis. The great demand for small and powerful analytical systems comes, in particular, from application fields such as biotechnology, biomedicine and process analysis. Biotechnology analytical measurements need a lot of time since many samples have to be measured. Therefore, high throughput analysis of very large number of samples would be desirable to obtain as much information as possible in an appropriate time. Miniaturization of analytical devices and methods would promote the use of many instruments for parallel measurements in the laboratory. Laser diodes are already tiny devices but one has to look for small-sized atomization sources in order to replace “big atomization sources” such as ICPs, MIPs or flames. Several research groups worldwide are developing microplasmas [28] in order to implement them in lab-on-the-chip systems. One of those discharges is the dielectric barrier discharge (DBD), which is described in detail in [29–31]. Shortly the linear dielectric barrier discharge consists of two parallel aluminum electrodes (50 mm length, 0.7 mm width) covered by a glass type dielectric (εr = 6) layer of 20 m with an interelectrodic space of 1 mm. The discharge is operating at reduced pressure (10–100 mbar) in argon as well as helium with a gas flow of 10–1000 ml min−1 . The applied voltage has a rectangular shape with a frequency of 5–20 kHz and an amplitude of 750 Vpp . The discharge shows a transient behavior. The halfwidth of the current peak is about 10 s and the discharge is ignited over the whole length of the electrodes. DLAAS is not only a powerful tool for analytical sciences but also for plasma diagnostics. For the investigation of this discharge an optical set-up was prepared, which allows the detection of atomic species with a high spatial and temporal resolution of 40 m and less than few nanoseconds, respectively. In this way the highest concentration of excited argon atoms was determined to be always near the temporary cathode around 12 s after the start of the current pulse. The absolute density of both resonance and metastable states were 2×1012 and 1013 atoms cm−3 , respectively. The gas temperature and the electron density, calculated from the Doppler
absorption signal [µV]
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This detector was coupled with gas chromatography and S as well as the halogens F, Cl, Br, and I resulted from the decomposition in the DBD were measured sequentially. An example is given in Fig. 9. The obtained detection limit for Cl was about 10 pg s−1 . This is in the same order of magnitude as obtained with conventional non-miniaturized MIP.
(CCl2F2) (CHClF2 ) (CClF3 )
1000
4. Conclusions
100
1
10
Cl concentration [ppm]
Fig. 8. Calibration curve of three freon gases in a linear dielectric barrier discharge [29].
and Lorentzian broadened absorption line profiles, reach at the maximum of the current peak maximum values of about 1000 K and 1015 cm−3 , respectively. During the remaining time of the discharge cycle and in other positions the gas temperature is close to room temperature and the electron density is much lower and could not be determined [30]. The concentration of almost all power in a small volume of the DBD leads to effective dissociation, which was demonstrated by the detection of fluorine and chlorine arising from the atomization of different freon gases. Fig. 8 shows the calibration curve for three different freon gases at high concentrations and the manifest, that the absorption signals match with the stoichiometric coefficients. The detection limits of Cl and F in He were about 400 and 2 ppbv , respectively.
DLAAS is a powerful technique for the detection of molecular species with detection limits in the ppb–ppt range. Nevertheless this technique is neither accepted nor established by the analytical community because the ICP–MS is a strong competitor, which can be found in almost all analytical laboratories. This device offers the possibility to detect nearly all elements with detection limits of ppb–ppt. However, ICP–MS instruments are bulky and expensive and have high operation costs. Furthermore, ICP–MS systems are laboratory instruments, and it is difficult to operate them outside of laboratories, for example, for on-line process control and for in situ measurements of environmental samples. Here, is the strength of diode laser-based instruments. They are robust and compact, and can be easily operated outside of laboratories without loosing their analytical figures of merit.
Acknowledgements Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged.
References
Fig. 9. Normalized chromatograms of fluorobenzene (C6 H5 F), 1,2-dichloropropane (C3 H6 Cl2 ), 1-bromobutane (C4 H9 Br), 1-chloropentane (C5 H11 Cl), 1-iodobutane (C4 H9 I), 2,5-dimethylthiophene (C6 H8 S), 1bromo-4-chlorobutane (C4 H8 BrCl) in hexane monitoring the absorption lines of F (685 nm), Cl (837 nm), Br (827 nm), I (906 nm), and S (921 nm). The concentration of each species is 300 nl ml−1 [31].
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Element selective detection of molecular species applying chromatographic techniques and diode laser atomic absorption spectrometry夽 K. Kunze∗ , A. Zybin, J. Koch, J. Franzke, M. Miclea, K. Niemax Institute of Spectrochemistry and Applied Spectroscopy (ISAS), Bunsen-Kirchhoff-Strasse 11, 44139 Dortmund, Germany Received 30 September 2003; accepted 3 November 2003
Abstract Tunable diode laser atomic absorption spectroscopy (DLAAS) combined with separation techniques and atomization in plasmas and flames is presented as a powerful method for analysis of molecular species. The analytical figures of merit of the technique are demonstrated by the measurement of Cr(VI) and Mn compounds, as well as molecular species including halogen atoms, hydrogen, carbon and sulfur. © 2004 Elsevier B.V. All rights reserved. Keywords: Diode laser atomic absorption spectroscopy; Chromatography; Tunable diode laser spectroscopy
1. Introduction Traces of small molecules in gases can be directly measured in the mid-infrared spectral range by tunable diode laser absorption spectroscopy (TDLAS). This is a versatile and highly selective technique with sensitivities in the sub-part-per-billion concentration range. Generally the tunable lasers are lead salt diode lasers which emit in the region between 3 and 30 m. Unfortunately, lead salt lasers have to be cryogenically cooled to temperatures much below room temperature which makes the instrumentation less convenient for operation. However, this drawback of TDLAS of small molecules will vanish in the near future due to the rapid progress in mid-IR quantum cascade laser development for gas sensing, in particular, for room temperature operation (see [1] or the relevant papers in this special isssue). A completely different approach for quantitative measurements of molecules by tunable diode laser absorption spectroscopy will be discussed in the present paper. It is based on the detection of atoms (elements) in the gas phase by diode laser atomic absorption spectroscopy (DLAAS) 夽 Presented at the Fourth International Conference on Tunable Diode Laser Spectroscopy, Zermatt, Switzerland, July 17, 2003. ∗ Corresponding author. Tel.: +49-231-1392-210; fax: +49-231-1392-120. E-mail address: [email protected] (K. Kunze).
1386-1425/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2003.11.047
after dissociation of the molecules of interest in an appropriate atomizer. In contrast to TDLAS of small molecules the wavelengths of the diode lasers systems applied in atomic absorption spectrometry (AAS) should be in the visible and UV spectral range to match the electronic transitions of the atoms. There are basic physical advantages to use DLAAS for molecular analysis. First, the absorption cross-sections of strong atomic transitions are of the order of 10−11 to 10−12 cm2 . They are much larger than the absorption cross-sections of small molecules in the mid-IR (typically 10−17 to 10−21 cm2 ) resulting in a higher detection sensitivity. Second, the molecule of interest might have more than one atom of a particular element which correspondingly increases the number density in an absorption experiment. Third, there are much less spectral interferences in DLAAS than in TDLAS of small molecules. The search for “spectral windows” in the mid-IR for undisturbed measurement and the reduction of the line widths by measurements under low pressure conditions are complications well known in TDLAS of molecules. On the other hand, DLAAS of molecular species is not a “direct” method as TDLAS is. Different separated steps are required in experiment. The first step is the preparation of the sample. It is followed by the separation step of the different species by an appropriate chromatographic or electrophoretic technique. Subsequently the separated molecules
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Fig. 1. Principal detection scheme of molecular species by atomic spectrometry.
have to be atomized before they can be measured by absorption. Popular atomizers are resistively heated graphite tube furnaces, and for continuous operation, needed in combination with separation techniques, plasmas or flames. The general scheme of element-selective measurements of molecular species is sketched in Fig. 1. It has to be noted that such schemes are well established in analytical chemistry in particular with atomic emission detectors (atomic emission spectrometry (AES) applying plasmas) and inductively coupled plasma-mass spectrometry (ICP-MS). It will be shown below, that it is possible to measure fractional absorption I/I to the order of 10−7 by DLAAS. This means that it is possible to measure approximately 103 atoms cm−3 in a 10 cm long layer since the fractional absorption for small extinctions is given by I/I = σNL (: absorption cross-section; N: atomic number density and L: absorption length) and the absorption cross-section of a strong optical transition is approximately 10−11 cm2 .
2. The analytical figures of merit for DLAAS The laser diodes used in diode laser atomic absorption spectroscopy are commercial, etalon-type devices of different producers. They can be operated in the free running mode without external optical stabilization and they emit light in the region of 625–1600 nm. Recently, also laser diodes in the blue region (375–445 nm) became available. The laser diodes can be typically tuned over 10–20 nm by temperature
(−20 to 50 ◦ C) and current. The mode-hop free range is restricted to several hundred picometer. Wavelength tuning by temperature is a slow process, while current tuning can be very fast. Wavelength modulation frequencies up to 500 kHz can easily be achieved [2]. All etalon-type laser diodes show mode hopes and therefore mostly wavelength gaps, which cannot be covered in a free running mode. This problem can be overcome by the use of external cavities, in which a grating reflects the first-order of the laser light back into the diode in order to build an external resonator. In this way the mode-hop free tuning range is limited up to about 50 GHz, but all wavelengths in the gain profile can be obtained. Further, a very simple but effective feedback method is the mounting of a glass plate in front of the diode laser chip [3]. The wavelength range of diode laser spectroscopy can be extended by second harmonic generation (SHG) using non-linear phase-matched or periodically poled crystals, whereby powers up to about 1 mW can be achieved in the 335–430 nm region, depending on the initial laser power and the crystal material applied. Due to the fact that the wavelength range of laser diodes is still very limited, the number of attainable elements is restricted. Furthermore, only a few strong optical transitions from the ground state of (mainly metallic) elements are in the specific wavelength windows delivered by the laser diodes. Therefore, also transitions from excited levels have to be taken into account. In particular transitions from metastable levels, which are well populated, e.g.
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in low-pressure discharges, can successfully be used. As mentioned above, commonly applied atomization sources are graphite furnace, flames and electrical discharges like microwave-induced plasma (MIP), inductively coupled plasma (ICP) or direct current discharge (dc). It has to be stressed, that those sources have to be powerful enough to dissociate all molecules. For the detection of molecular species all continuous atomization sources (plasmas and flames) can be coupled with different separation techniques like gas chromatography (GC), high-performance liquid chromatography (HPLC) or capillary electrophoresis (CE). In order to suppress various noise sources and to discriminate non-specific background signals different detection techniques applying modulation of the absorption and of the diode laser wavelength have been developed. The usual technique for baseline reduction is wavelength modulation with detection at the second harmonic of the modulation frequency 2f [4,5]. Unfortunately, wavelength modulation of a laser diode is, as a rule, accompanied by residual amplitude modulation (RAM). This leads to a background signal at the registration frequency 2f, which is much smaller than the background signal in the case of 1f detection, but retains the multiplicative structure. Because this background signal is proportional to the laser power and the optical transmittance, the low-frequency noise of the laser radiation at the detector is mixed with the modulation frequency and included in the detection bandwidth.
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The background signal can be cancelled if, additionally, the absorption is modulated and detection is performed at the difference or sum of the wavelength and absorption modulation frequencies [6]. In this case it is possible to achieve detection limits determined by the laser excess noise only, because the background signal is absent. The most successful detection technique is the so-called double-beam double-modulation procedure [7,8], which can also cancel the laser excess noise by applying of a double-beam arrangement. A principal setup is presented in Fig. 2. The beam of a laser diode is splitted in two parts, whereas one beam is directly detected by a photo detector, while the other one is absorbed in any medium (e.g. low-pressure discharge, flame) before detection. Both, the absorption cell and the wavelength of the laser diode are modulated with different frequencies. The logarithmically subtracted signals of both photodetectors were preamplified and detected on a mixed frequency with a lock-in amplifier. The signal S in the double-beam double-modulation techniques is given by S(υ, t) ∝ ln iS / iR = ln aS /aR + ln TS /TR − κ(υ, t)L. Hereby, aS /aR is the splitting of absorption and reference beam splitting ratio, TS,R the corresponding transmittances, κ(υ, t) the absorption coefficient and L the absorption length. The registered signal is proportional to the absorption coefficient κL implying that the measured signal is independent of the applied laser power unless the considered transition
Fig. 2. Principal experimental arrangement of double-beam double-modulation absorption spectrometry [8].
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capability and versatility of this method has permanently been proven by a variety of analytical applications, such as DLAAS in graphite tube furnace and flames [11,12], for element-selective measurements in GC [6,13] and HPLC [14,15], isotope-selective analysis [16–18] and in connection with laser ablation [18,19]. In the following, examples will be given for the analysis of molecular species by DLAAS coupled with separation techniques like GC and HPLC. The efficiency of this procedures is demonstrated by the flexibility to analyze aerosols, liquid, and gaseous samples. Finally it will be shown, that DLAAS is a promising task to build small-sized detectors. In all cases an experimental arrangement similar to that shown in Fig. 2 is used. However, sample preparation and introduction as well as the modulation techniques may vary. 3.1. DLAAS and GC
Fig. 3. Comparison of AAS-signals of Al using a hollow cathode lamp and a diode laser.
is saturated. According to the intentionally accepted 3 criterion, the theoretical detection limit (DL) is three times the shot noise given by DL = 3(2e2 ηωPhν)1/2 with e the elementary charge, η the quantum efficiency of the photo detector, h the Planck’s constant, ω the detection bandwidth, P the laser power and ν the laser frequency. In this way using a laser diode with 100 nW power (typical for single pass SHG in non-linear crystals) an experimental absorption of 10−4 absorption units (AU)) can be reached, while the theoretical shot noise limit is 10−5 AU. This result is already one-order of magnitude better than the typical 10−3 AU, which is usually achieved with the well-known hollow cathode lamp AAS. Increasing the laser power even higher to 2 mW absorptions down to 2·10−7 AU have been measured [7]. Fig. 3 shows the improvement of diode laser AAS in comparison with hollow cathode AAS for aluminum. The signal was obtained by a commercial graphite tube DLAAS instrument. 3. Analytical applications Though DLs have extensively been used in the field of fundamental spectrometry from the beginning of the 1970s onwards, the analytical relevance of DLs for element analysis was not discussed before the late 1980s [9]. The potential applicability of DLAAS for the routine element analysis was first demonstrated elsewhere [10]. Since that time, the
The electron-capture detector (ECD) and flame ionization detector (FID) are widely used in GC analysis. Both are non-element-selective detectors, which have reasonable disadvantages in comparison with element-selective detectors. The latter ones are applied if the analytes of interest cannot be separated from other species or from strongly interfering matrix components. Furthermore, they give direct information on the elemental composition of the separated species. The most popular element-selective detector is based on atomic emission spectrometry (AES) of MIPs. McCormack et al. [20] have characterized this type of detector extensively since its first realization in 1965. Today, the MIP–GC–AES technique is well established as a standard method in analytical chemistry besides the most common used GC–MS. Here, we will demonstrate the capability of DLAAS for GC detection. The eluents are introduced in the lowtemperature microwave discharge where they are atomized. The elements are probed by single-beam double-modulation DLAAS of the electronically excited atoms. Experimental details on the gas chromatograph, the microwave-induced plasma, the laser diodes, and the sample preparation and introduction typically used in such measurements can be found in [6,13,21]. 3.1.1. Characterization of a GC–DLAAS The correct reproduction of the stoichiometrical element ratios of the species can be taken as evidence for complete dissociation. The complete atomization of the analytes by the plasma is not only important for accurate calibration by an internal standard added to the sample, but also for analyte identification. In the latter case, the ratios between the element concentrations (inter-element ratio) have to be measured after calibration of the elements of interest with a suitable standard species with different compound structures. Chromatograms (each analyte 1.2 mg ml−1 ) measured simultaneously on H, Cl, and C excited atoms lines produced in a MIP are shown in Fig. 4 [21]. The stoichiometric ratios were determined separately for C, H, and Cl taking one
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Fig. 4. Chromatograms for C, H, and Cl applying 1.2 mg ml−1 of the analytes: (1) heptane C7 H16 , (2) 1-chloropentane C5 H11 Cl, (3) 1-octene C8 H16 , (4) 1-octyne C8 H14 , (5) chlorobenzene C6 H5 Cl, and (6) 1,4-dichlorobutane C4 H8 Cl2 [21].
analyte as internal standard. It could be shown, that in an argon as well as in a helium MIP the measured inter-element ratios for the three elements were correct. Only for chlorobenzene a small deviation was observed. Furthermore, the correlation between the population density of metastable plasma atoms and the total dissociation energy of the organic analytes were investigated. It was pointed out, that the relative depopulation of Ar as well as Kr metastable atoms in the discharges was proportional to the dissociation energy of the specific eluate. 3.1.2. Chlorinated hydrocarbons in oil There is a growing pressure on industrial societies to avoid plastic material waste or to find acceptable solutions for disposal. The best solution is the recycling of plastic material. Since the market for simple recycling products such as filling material or pieces for building industry is limited, plastic material is converted to oil. The oil products derived in the first step of plastic material recycling are further processed to basic materials of higher quality in chemical reactors. However, before this refinement, the samples have to be analyzed. Halogenated hydrocarbons, in particular volatile chlorinated hydrocarbons, have to be measured,
since chlorine is known to be a poison for catalysis and also attacks the reactor walls. Even in low concentrations it limits significantly the lifetime of the catalysts. Therefore, a sensitive GC analysis of the strongly changing composition in the primary oil for the refinement procedure is necessary. The widely used ECD is obviously not sufficient for the variety of species in the sample. But even the commercial element-specific AES detector seems to be unsuited as it is shown in Fig. 5 [13]. A sample of 10% recycling oil from plastic material solved in toluene was detected on the C and Cl line of the AES. For comparison, a chlorine-specific chromatogram obtained with the GC–DLAAS in a MIP is plotted in the lower part of Fig. 5. Several chlorinated hydrocarbons in the oil sample could be detected, which could not be seen at all in the GC–AES chromatogram. A precise evaluation of the chromatograms gave an improvement factor of the signal-to-noise ratio of 20–30. 3.2. DLAAS and HPLC If samples, which are difficult to volatilize, have to be analyzed electrophoretic techniques or liquid chromatography
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Fig. 5. Chromatograms of an oil sample from plastic material measured with a commercial AES detector on (a) the C 247.86 nm line and (b) the Cl 479.45 nm line, and (c) chromatogram of the same sample as in (a) and (b), however, measured in an MIP by double-modulation DLAAS [13].
is used for separation. Two examples of metal speciation by HPLC and DLAAS in a flame will be given. The relevant experimental details on the HPLC-system, the analytical flame as continuous atomizer, the diode lasers, and the sample preparation used in such experiments can be found [14,25]. 3.2.1. Cr(VI) in aerosols and water In order to assess environmental risks of bio- and anthropogenic organo-metallics and toxic oxidation states of certain transition metals, a species-selective analysis of complex matrices is indispensable. With regard to the so-called redox speciation of metals, the bioavailability and toxicity often depends on the present oxidation state and not on the absolute element concentrations. Therefore, a valid analysis generally requires a chromatographic separation of the oxidation states before element-selective detection. In the case of chromium, the oxidation states Cr(III) and Cr(VI) are important. Cr(III) is an essential trace element in human metabolism while Cr(VI) species are suspected of causing irreversible damage of organic tissue. The toxic properties of Cr(VI) species, predominantly occurring in the form CrO4 −2 and Cr2 O7 −2 are due to their ability to penetrate the cell velum of erythrocytes via anionic channels,
which are impermeable to the cationic Cr(III) compound complex [22]. Within the scope of this project [8,14], the Cr content of samples taken from tap water and ambient aerosol was determined. The metallic species of the sample are separated in a HPLC module, nebulized in an air/acetylene flame, and measured by DLAAS. Ion pair chromatography was applied for the separation of the analyzed samples. The separation required a preparation procedure, which consisted of the addition of acetic acid to a certain pH value, and tetrabutylammonium acetate for the ion-pairing process. Sample volumes of 1 ml were applied to the HPLC system. A detailed description of the full procedure has been published by Berndt and Müller [23]. The specific absorption of Cr was measured at the wavelength 427.480 nm (transition 3d5 4s7 S3 → 3d5 4p7 P3 ). The radiation of a DL emitting at 854.960 nm, was frequency doubled using a phase matched KNbO3 crystal resulting in an output power of 3 W. The absorption signals were registered by means of a double-beam wavenlength-modulation detection scheme in order to compensate the noise, arising from the frequency conversion. The detection limit of Cr(VI) was approximately 0.1 ng ml−1 , which corresponds to a fractional absorption of approximately 10−4 . This value was just slightly above the theoretical shot noise level taking into account the experimental laser power. The detection limit is so low, that Cr(VI) could even be detected in ordinary tap water [14]. Another example, which demonstrates the analytical power of HPLC–DLAAS in a flame, is given in Fig. 6. It shows the measurements of aerosols samples collected in the neighborhood of ISAS in Dortmund. The dashed trace is the chromatogram of the collected and chemically solved aerosol particles while the full trace represents a measurement of the same solution sample spiked with 1 ng ml−1 Cr(VI), which corresponds to a concentration approximately 1.5 ng Cr(VI) in 1 m3 of air.
Fig. 6. Cr-specific chromatogram of solved ambient aerosol particles determined by using double-beam single-modulation DLAAS in an air/acetylene flame [8].
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3.2.2. Fuel analysis Controversy has surrounded the potential toxicity of the fuel additive methylcyclopentadienyl manganese tricarbonyl (MMT) since it was first introduced as an octane-boosting, anti-knocking agent in Canada in 1977. Initially denied approval for widespread use in the US, MMT was only approved for limited use in leaded gasoline from 1977 to 1995. In 1995, the manufacturer of MMT challenged the denial in court and won, thereby opening the door for MMT to be marketed for use in unleaded gasoline as well. Although manganese is considered as nutritionally essential trace element, it is known that very high levels of inhaled manganese induce neurobehavioral and respiratory effects [15,24]. However, the influence of MMT as a fuel additive on the health risk is not well known because of limitations in data. Nevertheless, MMT is known to be toxic in contrary to other inorganic forms of manganese. The experimental setup [25] was similar to the one used for Cr(VI) determination. The 806.378 nm output of a laser diode was frequency doubled with a LiIO3 crystal to achieve 170 nW of blue light at 403.189 nm (Mn line), which is a weak absorption line. In contrast to the other setup only single-beam single-modulation was used but with logarithmic detection on the fourth harmonic of the wavelength modulated signal. The samples were introduced into the air/acetylene flame with hydraulic high-pressure nebulization (HHPN) using a HPLC pump. Either a continuous introduction of the sample could be applied or a column was inserted for HPLC. A typical chromatogram of 50 ng ml−1 of inorganic manganese, cyclopentadienyl manganese tricarbonyl (CMT), and MMT each, solved in a 65:35 methanol/pH 4 buffer, showing the different retention time of the species is plotted in Fig. 7. The detection limit of the total manganese amount was 1 ng ml−1 , which is worse than other spectroscopic techniques. But it has to be emphasized that these experimental results can be improved by the use of higher laser
Inorganic Manganese
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CMT
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Fig. 7. Chromatogram of inorganic manganese, CMT, and MMT (concentrations: 50 ng ml−1 each) by HPLC–DLAAS in an air/acetylene flame [25].
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power or the selection of a stronger absorption line. For example, the new blue diode lasers [26], which offer almost 50 times more power and therefore reduce the shot noise will certainly results in an improved detection limit of almost one-order of magnitude. Nevertheless, even in these conditions the experiment allows measurements of inorganic and organic manganese compounds at concentration levels present in environmental and toxicological studies [27]. It was also demonstrated, that MMT could be detected in spiked gasoline, human urine, and tap water samples in good agreement with the added concentration. 3.3. Miniaturization Over the last few years, there have been worldwide efforts to develop miniaturized instrumentations for chemical analysis. The great demand for small and powerful analytical systems comes, in particular, from application fields such as biotechnology, biomedicine and process analysis. Biotechnology analytical measurements need a lot of time since many samples have to be measured. Therefore, high throughput analysis of very large number of samples would be desirable to obtain as much information as possible in an appropriate time. Miniaturization of analytical devices and methods would promote the use of many instruments for parallel measurements in the laboratory. Laser diodes are already tiny devices but one has to look for small-sized atomization sources in order to replace “big atomization sources” such as ICPs, MIPs or flames. Several research groups worldwide are developing microplasmas [28] in order to implement them in lab-on-the-chip systems. One of those discharges is the dielectric barrier discharge (DBD), which is described in detail in [29–31]. Shortly the linear dielectric barrier discharge consists of two parallel aluminum electrodes (50 mm length, 0.7 mm width) covered by a glass type dielectric (εr = 6) layer of 20 m with an interelectrodic space of 1 mm. The discharge is operating at reduced pressure (10–100 mbar) in argon as well as helium with a gas flow of 10–1000 ml min−1 . The applied voltage has a rectangular shape with a frequency of 5–20 kHz and an amplitude of 750 Vpp . The discharge shows a transient behavior. The halfwidth of the current peak is about 10 s and the discharge is ignited over the whole length of the electrodes. DLAAS is not only a powerful tool for analytical sciences but also for plasma diagnostics. For the investigation of this discharge an optical set-up was prepared, which allows the detection of atomic species with a high spatial and temporal resolution of 40 m and less than few nanoseconds, respectively. In this way the highest concentration of excited argon atoms was determined to be always near the temporary cathode around 12 s after the start of the current pulse. The absolute density of both resonance and metastable states were 2×1012 and 1013 atoms cm−3 , respectively. The gas temperature and the electron density, calculated from the Doppler
absorption signal [µV]
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This detector was coupled with gas chromatography and S as well as the halogens F, Cl, Br, and I resulted from the decomposition in the DBD were measured sequentially. An example is given in Fig. 9. The obtained detection limit for Cl was about 10 pg s−1 . This is in the same order of magnitude as obtained with conventional non-miniaturized MIP.
(CCl2F2) (CHClF2 ) (CClF3 )
1000
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100
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10
Cl concentration [ppm]
Fig. 8. Calibration curve of three freon gases in a linear dielectric barrier discharge [29].
and Lorentzian broadened absorption line profiles, reach at the maximum of the current peak maximum values of about 1000 K and 1015 cm−3 , respectively. During the remaining time of the discharge cycle and in other positions the gas temperature is close to room temperature and the electron density is much lower and could not be determined [30]. The concentration of almost all power in a small volume of the DBD leads to effective dissociation, which was demonstrated by the detection of fluorine and chlorine arising from the atomization of different freon gases. Fig. 8 shows the calibration curve for three different freon gases at high concentrations and the manifest, that the absorption signals match with the stoichiometric coefficients. The detection limits of Cl and F in He were about 400 and 2 ppbv , respectively.
DLAAS is a powerful technique for the detection of molecular species with detection limits in the ppb–ppt range. Nevertheless this technique is neither accepted nor established by the analytical community because the ICP–MS is a strong competitor, which can be found in almost all analytical laboratories. This device offers the possibility to detect nearly all elements with detection limits of ppb–ppt. However, ICP–MS instruments are bulky and expensive and have high operation costs. Furthermore, ICP–MS systems are laboratory instruments, and it is difficult to operate them outside of laboratories, for example, for on-line process control and for in situ measurements of environmental samples. Here, is the strength of diode laser-based instruments. They are robust and compact, and can be easily operated outside of laboratories without loosing their analytical figures of merit.
Acknowledgements Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged.
References
Fig. 9. Normalized chromatograms of fluorobenzene (C6 H5 F), 1,2-dichloropropane (C3 H6 Cl2 ), 1-bromobutane (C4 H9 Br), 1-chloropentane (C5 H11 Cl), 1-iodobutane (C4 H9 I), 2,5-dimethylthiophene (C6 H8 S), 1bromo-4-chlorobutane (C4 H8 BrCl) in hexane monitoring the absorption lines of F (685 nm), Cl (837 nm), Br (827 nm), I (906 nm), and S (921 nm). The concentration of each species is 300 nl ml−1 [31].
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