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New methodical and instrumental developments in laser ablation inductively coupled plasma mass spectrometry
Spectrochimica Acta Part B 57 (2002) 1535–1545
New methodical and instrumental developments in laser ablation inductively coupled plasma mass spectrometry夞 a ¨ E. Hoffmanna,*, C. Ludke , J. Skolea, H. Stephanowitzb, J. Wollbrandtb, W. Beckerc a
¨ Spektrochemie und Angewandte Spektroskopie, Institutsteil Berlin, Albert-Einstein-Str. 9, D-12489 Berlin, Germany Institut fur b ¨ Gesellschaft zur Forderung Angewandter Optik, Optoelektronik, Quantenelektronik und Spektroskopie e.V., Rudower Chaussee 29, D-12489 Berlin, Germany c Becker & Hickl GmbH, Nahmitzer Damm 30, D-12277 Berlin, Germany Received 29 April 2002; accepted 25 July 2002
Abstract Many tasks in bulk analysis, micro analysis and depth profile analysis can be solved advantageously by laser ablation inductively coupled plasma mass spectrometry (Laser ICP-MS) in particular, when both the chemical and elemental distributions in the sample are to be determined. However, the analyst has to take into account that the analytical precision and accuracy of the Laser ICP-MS is influenced decisively by signal standardization, the homogeneity of the samples as well as calibration standards and the mass-spectrometric measuring mode, which is usually sequential when performed with scanning mass spectrometers such as quadrupol- or sector-based instruments. Using the ablated mass as standard, an excellent level of the analytical precision and accuracy (relative standard deviation R.S.D.-0.5%) has been obtained for homogeneous sample materials such as alloys. For inhomogeneous samples, such as pressed pellets, a statistical test is described, which is based upon the auto-correlation function to characterize the sample inhomogeneity. The application of the test allows us to calculate the representative mass for the quantitative analysis at previously defined analytical precision. In the instrumental part of the paper a new type of an ICP—time-of-flight (TOF) mass spectrometer—is described, constructed and built up in our laboratory. For fast signal counting an application-specific integrated circuit (ASIC) was developed, which permits a time resolution of 1 ns. The analytical performance of the TOF when used in combination with an ICP is demonstrated in terms of resolution, ion extraction rate, detection limits and dynamic range. The determination of 39 Kq and 40 Caq at trace level can be realized in a cool plasma condition (high central gas flow) only with a small interference by 40Arq. Detection limits of 23 elements were measured with typical values in the lower nanograms per liter range. The ion extraction rates, measured for a sample mass of 1 ng in terms of counts per second divided by the relative isotope abundance, are one order of magnitude higher than those obtained with a quadrupol-based instrument. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Laser ICP-MS; Signal standardization; Representative sample mass; Time-of-flight mass spectrometer 夞 This paper is published in the special issue of Spectrochimica Acta Part B dedicated to the 50th anniversary of the ISAS. *Corresponding author. E-mail address: [email protected] (E. Hoffmann). 0584-8547/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 5 8 4 - 8 5 4 7 Ž 0 2 . 0 0 1 0 9 - X
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1. Introduction The potentials of laser ICP-MS for both bulk and micro analysis have been reviewed by many authors in the literature w1–15x. Laser sampling can be applied to electrically conducting as well as non-conducting materials. Especially, for nonconducting materials such as glass w16–20x and ceramics w21–23x, and for biological materials w24–30x laser ICP-MS has proven to be a valuable tool. However, there are a number of concerns and problems that have prevented laser ICP-MS from becoming a general technique for analyzing solid materials. Practical concerns include the difficulty of obtaining or making matrix-matched standards that contain all the elements of interest and the accuracy of the resulting calibration w31–39x. The problem of matrix matching arises because the ablation yield varies with material properties such as reflectivity, thermal conductivity and melting and boiling points. To some extent, variations in ablation yield can be corrected by internal standardization using an element present in both the standard and the sample. On the other hand, nonrepresentative sampling is generally difficult to correct since it causes enhancements or reductions in relative analyte signal intensities and sensitivities. In part there are problems in laser ICP-MS because interactions between high-energy laser photons and material are complex w40x and not taken into account to a suitable degree yet. When a laser beam is focussed onto a solid surface, the irradiance in the spot can lead to a variety of effects such as reflexion, heating, desorption and emission of ions and electrons. Essentially, the laser energy is absorbed by electrons more or less strongly bound in the solid-state configuration, so there are differences in physical mechanisms leading to the ablation among the various materials such as metals, insulators and organic polymers. The experimentally observed characteristics of ablation of all the materials are rapid material removal, existence of a threshold and a non-linear dependence of yield on laser intensity. Of particular concern to the analyst is the formation of a plasma, which expands out from the surface, when the power density exceeds some
threshold value, typically of the order of 1=108 W cmy2. Laser ablation into an atmosphere creates a shock wave, which is formed by the piston-like action of the quickly expanding ablated material pushing outward on the backing gas. As the shock wave expands, more gas is swept up by the shock front, and since the laser pulse delivers a finite amount of the energy, the expansion velocity decreases with increasing distance from the target. As a result of the strong deceleration, a high density of target species is maintained for a long time. The high density and high pressure of the backing gas in turn promote volume condensation of the target species leading to small particles. Depending on the internal energy content and the size of the particles, significant cooling by the backing gas may have to occur before the particles can add additional atoms or molecules. Thus, particulates can be created of a size allowing transportation with a gas stream as carrier via long distances with negligible loss. In the inductively coupled plasma the particles are vaporized again. The ions produced in the plasma are subsequently separated using a mass spectrometer. In spite of the analytical efforts of numerous laboratories, many analysts think the laser sampling technique exhibits unsatisfactory reproducibility for many applications. This is, for example, the case for analyzing industrial products, and in the life sciences where a high precision is required as characterized by R.S.D.-0.1 % for the major elements and a few percent for the trace elements. The aim of our methodical and instrumental developments is to achieve such a high level of analytical precision and accuracy for laser ICP-MS. In the methodical part of the paper, results of investigations focussed on signal standardization using the vaporized sample mass are presented. To compensate signal fluctuations caused by inhomogeneities, a minimum mass has to be vaporized to obtain an analysis representative of the sample composition (representative sample mass). The statistical procedure to determine the representative sample mass is described below. ICP-MS is usually performed with scanning mass spectrometers such as quadrupol or sector field instruments. Because only ions of one myz (mass to charge) value can be monitored at any
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given time, there is a necessary trade-off between the number of masses that can be measured in any given period of time and the analytical precision that is attainable. This compromise becomes crucial in the analysis of transient signals such as those generated by laser sampling w41x. Because each myz value is measured at a different time, changes in the sample introduction degrade the precision and accuracy of analyses. To overcome the shortcomings and to fully eliminate the effect of ion-flow changes on analytical precision, one must measure all myz values simultaneously. To this end other types of mass analyzers such as time-of-flight (TOF) instruments, have been investigated w42–47x as alternatives to the commonly used quadrupol and sector-field based analyzer. Although ICP-TOF instruments have been commercially available for several years, there is still substantial room for progress in the figures of merit such as sensitivity and resolving power. The performance of the current ICP-TOF systems could be improved with a reduced ion-beam energy and a reduction in the velocity spread, both along and perpendicular to the flight tube. Taking up these questions an ICP-TOF instrument based on an offaxis geometry has been developed in our laboratory. The performance of this mass spectrometer will be demonstrated in Section 2.1. Because it is easy to handle, multielement solutions were used to determine the instrumental figures of merits such as the mass resolution, the dynamic range of the signals, the ion extraction rate and the detection limits. 2. Methodical investigations 2.1. Instrumentation All laser ablation experiments reported here, were performed on the ICP mass spectrometer Elan 6000 (by Perkin–Elmer; Sciex, Toronto, Canada) in combination with the laser sampler Model 320 (Perkin–Elmer, Sciex). The dwell time of the quadrupol mass spectrometer was 20 ms and the repetition frequency of the laser was 10 Hz for each experiment. The laser ablation unit includes a Nd:YAG laser (model Quanta-Ray DCR-11 from Spectra-Physics, Mountain View, CA, USA), a
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stepper motor controlled sample stage and a video camera system allowing the observation of the sample enlarged by a factor up to 80 on a TV screen. The unit was modified in various ways to have a more flexible and user friendly system. The hardware modifications include a special sampling cell which allows the illumination of the sample from all sides and an additional red diode laser helping to address points of interest on the sample surface w48x. Additionally, a generator for the higher harmonics of 532, 355 and 266 nm of the basis wavelength of 1064 nm is installed. Their pulse durations are: at 1064 nm, 10 ns; 532 nm, 8 ns; at 355 nm, 6 ns; and at 266 nm, 5 ns. One hundred pulses were applied to produce a crater. The ablated sample material was transported through a Tygon tube of 5 mm in diameter by an argon stream of 1 l miny1 as a particle carrier. 2.2. Signal standardization In spite of the stability of the laser pulse energy which varies in the order of a few percent from shot to shot, the analytical signals can fluctuate by more than 100%. In principle, averaging over many laser shots should lead to results reproducible with deviations smaller than 1%. However, when micro samples are analyzed or the analysis of the sample has to be spatially resolved the limited number of laser shots causes a low level of the analytical precision. Internal standardization of the analytical signals improves the reproducibility provided that the concentration of the corresponding isotope is constant in the sample from crater to crater. If this is not the case and the analytical and the standard signals are not correlated, internal standardization further reduces the analytical precision (addition rule of the quadratic standard deviations). The same is valid when the standard isotope has a constant concentration in the sample material, but the sampling generates transient signals measured with a mass spectrometer with sequential measuring mode. The fundamental solution of the problem is to ratio the measured ion signal Ii related to the element i against the ablated mass M. Then, the analytical signal Si of the element i is defined by:
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The calibration factors Ki can be determined with calibration standards. When the element concentrations in the calibration standards are C0i the following equation system is valid: C0isKiØ
Ii N
is1.....N
8KjØJj js1
Fig. 1. Relative standard deviation. Samples: three different alloys; the number behind a element symbol indicates the content in percent. Laser parameters: single crater; number of pulses, 300; pulse energy, 200 mJ; and wavelength, 1064 nm.
Sis
Ii M
is1.....N
The prerequisite for the procedure is the determination of all elements of significant concentration. The concentration Ci of the element i in the sample is defined by: CisKiSi
is1.....N
and normalized by: N
8Cis1 1
with N the number of the elements which are to be considered, and Ki the corresponding calibration factors. M is obtained by the sum of KiIi of all the elements measured :
Including the normalization condition of the element concentrations, the equation system can be solved for the ablated mass M. Fig. 1 shows relative standard deviations resulting from the procedure described above in comparison with results obtained without any standardization. The experimental conditions were: laser wavelength, 1064 nm; pulse energy, 200 mJ; number of pulses, 300; crater diameter, 0.15 mm; and crater mass estimated to be 50 mg. The samples used for measurements were alloys of different compositions consisting of In, Sb, Ga and Bi in the first sample, Pb and Ga in the second sample, and In and Sn in the third sample. To demonstrate the analytical accuracy which was obtained using the standardization procedure described above the results of a diffusion experiment under conditions of weightlessness is presented. In the middle of a wire of an InSn alloy (50 mm long, 1 mm diameter), a 1-mm-thick slice with equal element composition but an increased content of the 113In isotope was inserted. In the space laboratory the wire was heated and kept at the temperature of the melting point over a certain period of time. When the material was molten the
N
8Cis1
is1.....N
1 N
1s8KiØ 1
Ii M
N
Ms8KiØIi 1
Then the standardized signals are: Sis
Ii 8KiØIi
is1.....N
Fig. 2. Abundance of 113In measured in an InSn 15 alloy in comparison with the ones obtained by SIMS. The laser parameters were the same as in Fig. 1.
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auto-correlation function in the following way w34x: The analytical signals Iij (i denotes the analytical element) of a number of laser craters ( j refers to the crater) produced at equal distances as shown in Fig. 3 are centered and normalized by Iim, which is the average value of all Iij: Fig. 3. Laser craters powder, doped with graphite powder and pulses; 100 mJ pulse
with equal distances. Sample: cellulose standard solutions, dried, mixed with pressed to pellets. Laser parameters: 50 energy; and wavelength, 1064 nm.
diffusion of the isotope 113In from the inserted slice into the wire material took place. Fig. 2 shows the measured abundance of the 113In isotope compared with results obtained by SIMS. The agreement of the values is within the interval of 0.5%. 2.3. Analytical representative sample mass In bulk analysis of inhomogeneous samples, precision and accuracy depend on the quantity of the material ablated by the laser beam. ‘Trial and error’ is a time and cost consuming method to determine the minimum mass, which has to be analyzed to achieve the required reproducibility. Consequently, we use the statistical method of the
Sijs
Iij y1 Iim
is1...N js1...n
The auto-correlation function of the element i is given by: n
ciŽt.s8Sij=Sijqt
is1...N
1
The number of craters whose vaporized mass is representative of the sample can be defined by the number t0 of craters for which the auto-correlation function is decreased down to 37% of the maximum value (at ts0) w49x. ciŽt0.sey1CiŽ0. When mc is the average crater mass (determined by weighing the sample without and with craters, then the last one was subtracted from the first one and the mass difference was divided by the number of pulses), the analytically representative sample mass m0 can be calculated from: m0st0Ømc In Fig. 4 are shown two normalized autocorrelation functions related to measurements of two soil standards (BCR 142, BCR 146; Community Bureau of References, Brussels). Obviously, the standard BCR 142 is more homogeneous than BCR 146. Table 1 Analytical representative sample masses (r.s.m.) determined with the auto-correlation function (relative signal deviation is below 10 % when the r.s.m. is ablated)
Fig. 4. Normalized auto-correlation functions of isotope ratio 208 Pby45Sc. Samples: soil standards BCR 142 and BCR 146, doped with Sc standard solution, dried, ground and pressed to pellets. Laser parameters are the same as those in Fig. 3. t0 equals the representative number of craters, and crater diameters0.4 mm. Distance between two craters is 2 mm.
Element
Real soil (g)
BCR 142 standard (g)
Brass (g)
Steel (g)
Wood (pine) (g)
Ni Zn Pb Cr Cu
3.0 3.0 6.0 1.0 0.2
0.2 0.2 0.6 0.2 0.1
0.1 0.1 0.1 0.1 0.05
0.08 – – 0.05 0.05
0.008 0.005 0.005 – 0.005
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to the R.S.D. of the single isotope measurement s1 according to the relationship: sNss1ØyN
Fig. 5. Schematic of the newly developed TOF mass spectrometer.
Table 1 contains analytical representative sample masses of selected sample materials.
A TOF mass spectrometer for use in combination with an ICP has been developed which is capable to measure complete mass spectra with high precision, sensitivity and resolution independent of the number of masses studied. The goals of the instrument development were to make the most efficient use of the ions produced by the ICP source, to realize adequate resolving power, and to retain the speed and mass range for which TOF mass spectrometry is well known. A scheme of the TOF mass spectrometer is shown in Fig. 5. The ion beam extracted from the ICP via a two-stage pumping system is focussed into the repeller space by a special ion optic. After acceleration by the voltage pulse of the repeller and the grid arrangement, each ion flies with a velocity depending on its mass. The ions are reflected by an ion mirror and detected with a micro channel plate. An ion blanker is installed preventing unwelcome ions such as 40Arq, 16Oq and 32Oq 2 from reaching the detector. The ability to measure full spectra at tens of kiloHertz repetition rates places extraordinary demands on the data-acquisition system. An ASIC (MSA-1000 by Becker & Hickl, Berlin, Germany) was developed offering multichannel counting capabilities for low-level signals over a wide dynamic range. The scheme of the module is shown in Fig. 6. It uses a 32 bit shift register for fast data acquisition and a fully parallel 256 bit
3. Instrument development: ICP-TOF mass spectrometer Most of the commercially available mass spectrometers connected with an ICP work with a sequential measurement mode allowing only single mass determination step by step. Consequently, the measurement precision is reduced with the increasing number of the isotopes measured in a given time; this is particularly disadvantageous in studies of transient signals as they occur in laser ablation. The RSD sN when measuring N isotopes is related
Fig. 6. The ion counting module.
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Fig. 7. The analog measuring module.
memory structure for direct accumulation. The measured signal is segmented into 1 ns intervals (by the 1 GHz clock) in which the detection of a pulse (width down to 700 ps) is markedyregistered in the shift register. Every 32 ns the contents of the shift register is stored in a temporary register. While the shift register is used for data acquisition again and without delay, the contents of the temporary register, i.e. the previous 32 ns section of the detector signal, is stored in the memory, where a signal can be stored up to length of 128 ms. Corresponding measurement cycles (e.g. due to subsequent ion clouds) can be accumulated in the memory up to a predefined number of runsysweeps if the shift register operation is restarted by equivalent trigger pulses (at discriminator D2). In this way any dead time is avoided. For stronger signals, i.e. those where more than one pulse per nanosecond may occur, a fast multichannel analog system was developed, which adds another three orders of magnitude to the dynamic range. It operates at a time resolution of 10 ns per channel and its analog-to-digital conversion with accumulation (ADA-100 by Becker Hickl, Berlin, Germany) provides 8 bit resolution of signals between 0.1 and 1 V; up to 255 measurement cycles can be accumulated. The dual structure of the device presented in Fig. 7 is used alternately to avoid any dead time. The two acquisition modules operate in parallel and are integrated in a Pentium III PC.
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Observation of the entire atomic range at each extraction event places great demands on the dynamic range. Therefore, the full spectrum is measured simultaneously by ion counting and by analog registration. In Fig. 8 are shown mass spectra measured in the counting (a) and the analog (b) mode. The dynamic range selected of the signal measurement at the shortest measuring time of 35 ms is approximately three orders of magnitude, and increases up to nine orders of magnitudes at 10 s. To study the instrumental parameters of the ICPTOFMS instrument a multielement standard (ICP standard IV with 23 elements by Merck, Germany) was used, diluted with HNO3 (0.2% in deionized water). The relative extraction rate E in counts per second, per 10 ml sprayed multielement solution of a concentration of 1 ppm of each element and divided by the relative isotope abundance of each isotope measured is shown in Fig. 9. Results are given for two flow rates of the nebulizer gas: 1.3 and 1.0 l hy1 (‘hot’ and ‘cool’ plasma conditions, respectively) and compared with results obtained with the ICP-quadrupol mass spectrometer (nebulizer gas flow rate 1.0 l hy1; optimized according to recommended rule). The plasma generator power was 1 kW both for the TOF mass spectrometer and for the quadrupol mass spectrometer. The measuring time was 3 min for each measurement. It can be seen that the extraction rate for TOF measurements is one order of magnitude higher than those for the quadrupol mass spectrometer. The operating parameters of the TOF mass spectrometer and the quadrupol mass spectrometer are summarized in Table 2. A comparison of the mass resolution Mym (M is the mass number, m is the half-width of a mass peak) measured with the ICP-TOF mass spectrometer and the quadrupol mass spectrometer is represented in Fig. 10. The mass resolution of the ICP-TOFMS instrument is improved by a factor of two and nearly constant over all mass numbers. The detection limits measured with multielement solutions containing 23 elements and calculated according to the 3s criterion are given in Fig. 11. They also are compared with the values obtained with the quadrupol mass spectrometer.
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Fig. 8. Spectra measured in the counting (a) and analog (b) mode.
Fig. 9. Relative extraction rate, (a) relative abundance of isotope.
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Table 2 Operating parameters (ICP-TOF mass spectrometer; ICP-quadrupol mass spectrometer) TOF mass spectrometer Repetition rate Repeller time Repeller voltage Drift tube Length Diameter Voltage Solid state generator (power) Plasma gas (flow rate) Auxiliary gas (flow rate) Nebulizer gas (flow rate) Quadrupol mass spectrometer Tube generator (power) Plasma gas (flow rate) Auxiliary gas (flow rate) Nebulizer gas (flow rate)
Maximum 30 kHz 5 ms 700 V 600 mm 60 mm 1830 V 1 kW 15 l miny1 Ar 1.4 l miny1 Ar 1.0, 1.3 l miny1 Ar 1 kW 15 l miny1 Ar 1.6 l miny1 Ar 1.0 l miny1 Ar
4. Summary An improvement in precision and accuracy of the analysis with laser ICP-MS can be achieved by using the ablated sample mass for the signal standardization. The necessary number of craters in a heterogeneous sample for a given R.S.D. can be determined by evaluation of the measurements using an auto-correlation function. The ICP-
Fig. 10. Comparison of the mass resolution for the TOF and the quadrupol instrument.
TOFMS instrument developed provides an analytical performance exceeding that of their quadrupol counterparts. The extraction rate from the plasma jet is one order of magnitude better in comparison with quadrupol systems, and the mass resolution is improved by a factor of two at all mass numbers. The rapid data acquisition achieved with the specially developed ASIC-chip permits the design of a geometrically small instrument: the drift tube of our instrument is 0.6 m long. The ICP-TOFMS instrument has a particular potential for transient
Fig. 11. Detection limits calculated according to the 3s criterion.
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signal analysis, thus extending the capabilities of established elemental methods. In a time interval of 10 ms—corresponding to the dwell-time for one mass in a quadrupol instrument—the TOF can deliver 300 spectra. Therefore, a further improvement of the analytical performance of laser ICPMS should be expected by the general application of TOF mass spectrometers. Regrettably, however, satisfactory calibration standards are not yet available. Acknowledgments The financial support given by the Senatsver¨ Wissenschaft, Forschung und Kultur waltung fur des Landes Berlin and the Bundesministerium ¨ fur Bildung und Forschung is gratefully acknowledged. References w1x A.L. Gray, Solid sample introduction by laser ablation for inductively coupled plasma source mass spectrometry, Analyst 110 (1985) 551–556. w2x P. Arrowsmith, Laser ablation of solids for elemental analysis by inductively coupled plasma mass spectrometry, Anal. Chem. 59 (1987) 1437–1444. w3x P. Arrowsmith, S.K. Hughes, Entrainment and transport of laser ablated plumes for subsequent elemental analysis, Appl. Spectrosc. 42 (1988) 1231–1239. w4x S.A. Darke, J.F. Tyson, Interaction of laser radiation with solid materials and its significance to analytical spectrometry, JAAS 8 (1993) 145–163. w5x N. Imai, Quantitative analysis of original and powered rocks and mineral inclusions by laser ablation inductively coupled plasma mass spectrometry, Anal. Chim. Acta 235 (1990) 381–391. w6x T. Mochizuki, A. Sakashita, H. Iwata, Y. Ishibashi, N. Gunji, Selective vaporization in laser solid sampling for inductively coupled plasma atomic emission and mass spectrometry of steel, Anal. Sci. 7 (1991) 479–481. w7x S.A. Darke, S.E. Long, C.J. Pickford, J.F. Tyson, A study of laser ablation and slurry nebulisation sample introduction for the analysis of geochemical materials by inductively coupled plasma spectrometry, Fresenius J. Anal. Chem. 337 (1990) 284–289. w8x J.W. Hager, Relative elemental responses for laser ablation-inductively coupled plasma mass spectrometry, Anal. Chem. 61 (1989) 1243–1248. w9x W.T. Perkins, R. Fuge, N.J.G. Pearce, Quantitative analysis of trace elements in carbonates using laser ablation inductively coupled plasma mass spectrometry, JAAS 6 (1991) 445–449.
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E. Hoffmann et al. / Spectrochimica Acta Part B 57 (2002) 1535–1545 w25x E. Hoffmann, H. Stephanowitz, J. Skole, Investigation of the migration of elements in tree rings by laser-ICPMS, Fresenius J. Anal. Chem. 355 (1996) 690–693. w26x E. Hoffmann, C. Ludke, ¨ H. Scholze, H. Stephanowitz, Analytical investigation of tree rings by laser ablation ICP-MS, Fresenius J. Anal. Chem. 350 (1994) 253–259. w27x T. Prohaska, C. Stadlbauer, R. Wimmer, G. Stingeder, Ch. Latkoczy, E. Hoffmann, H. Stephanowitz, Investigation of element variability in tree rings of young Norway spruce by laser-ablation-ICP-MS, Sci. Total Environ. 219 (1998) 29–39. w28x E. Hoffmann, H. Stephanowitz, E. Ullrich, J. Skole, C. ¨ Ludke, B. Hoffmann, Investigation of mercury migration in human teeth using spatially resolved analysis by laser ablation-ICP-MS, JAAS 15 (2000) 663–667. w29x E. Hoffmann, C. Ludke, ¨ J. Skole, H. Stephanowitz, E. Ullrich, D. Colditz, Spatial determination of elements in green leaves of oak trees (Quercus robur) by laser ablation-ICP-MS, Fresenius J. Anal. Chem. 367 (2000) 579–585. w30x S. Wang, R. Brown, D.J. Gray, Application of laser ablation-ICPMS to the spatially resolved micro-analysis of biological tissue, Appl. Spectrosc. 48 (1994) 1321–1325. w31x A. Raith, W.T. Perkins, N.J.G. Pearce, T.E. Jeffries, Environmental monitoring on shellfish using UV laser ablation ICP-MS, Fresenius J. Anal. Chem. 355 (1996) 789–792. w32x H. Scholze, E. Hoffmann, C. Ludke, ¨ A. Platalla, Analysis of leaves by using the laser ICP-MS with isotope dissolution method, Fresenius J. Anal. Chem. 355 (1996) 892–894. w33x H.P. Longerich, S.E. Jackson, D. Gunther, ¨ Laser ablation inductively coupled plasma mass spectrometric transient signal data acquisition and analyte concentration calculation, JAAS 11 (1996) 899–904. w34x E. Hoffmann, C. Ludke, ¨ H. Scholze, Is laser-ablationICP-MS an alternative to solution analysis of solid samples?, Fresenius J. Anal. Chem. 359 (1997) 394–398. w35x A. Moisette, T.J. Shepherd, S.R. Chenery, Calibration strategies for elemental analysis of individual aqueous fluid inclusions by laser ablation inductively coupled plasma mass spectrometry, JAAS 11 (1996) 177–185. w36x H.P. Longerich, D. Gunther, ¨ S.E. Jackson, Elemental fractionation in laser ablation inductively coupled plasma mass spectrometry, Fresenius J. Anal. Chem. 355 (1996) 538–542.
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w37x H. Scholze, H. Stephanowitz, E. Hoffmann, J. Skole, Element analysis of inhomogeneous samples by laser ablation ICP mass spectrometry, Fresenius J. Anal. Chem. 350 (1994) 247–252. w38x H. Cousin, B. Magyar, Precision and accuracy of laser ablation-ICP-MS analysis of rare earth elements with external calibration, Mikrochim. Acta 113 (1994) 313–323. w39x E.F. Cromwell, P. Arrowsmith, Fractionation effects in laser ablation inductively coupled plasma mass spectrometry, Appl. Spectrosc. 49 (1995) 1652–1660. w40x J.C. Miller, Laser Ablation, Principles and Applications, Springer-Verlag, Berlin, Heidelberg, New York, London, Paris, Tokyo, Hong Kong, Barcelona, Budapest, 1994. w41x J.F. Holland, C.G. Enke, J. Allison, J.T. Stults, J.D. Pinkston, B. Newcome, J.T. Watson, Fast gas chromatography-mass spectrometry, Anal. Chem. 55 (1983) 997A–1010A. w42x P.P. Mahoney, S.J. Ray, G.M. Hieftje, Time-of-flight mass spectrometry for elemental analysis, Appl. Spectrosc. 51 (1997) 16A–28A. w43x X. Tian, H. Emteborg, F.C. Adams, Analytical performance of axial inductively coupled plasma time of flight mass spectrometry (ICP-TOFMS), JAAS 12 (1999) 1807–1814. w44x D.P. Myers, G. Li, P.P. Mahoney, G.M. Hieftje, An inductively coupled plasma-time-of-flight mass spectrometer (ICP-TOFMS) for elemental analysis. Part III: analytical performance of the TOFMS with improved ion optics and vacuum chamber, J. Am. Soc. Mass. Spec. 6 (1995) 411–420. w45x D.P. Myers, G.M. Hieftje, Preliminary design considerations and characteristics of an inductively coupled plasma-time-of-flight mass spectrometer, Microchim. J. 48 (1993) 259–277. w46x P.P. Mahoney, G. Li, G.M. Hieftje, Laser ablationinductively coupled plasma mass spectrometry with a time-of-flight mass analyser, JAAS 11 (1996) 401–405. w47x D.P. Myers, G. Li, P. Yang, G.M. Hieftje, An inductively coupled plasma-time-of-flight mass spectrometer (ICPTOFMS) for elemental analysis. Part I: optimization and characteristics, J. Am. Soc. Mass. Spectrom. 5 (1994) 1008–1016. w48x P. Richner, D. Evans, Automated laser ablation sampling (ALAS) for ICP-MS, At. Spectrosc. 14 (1993) 157–161. w49x F.H. Lange, Signale und Systeme, Bd. 3, VEB Verlag Technik, Berlin, 1973.
New methodical and instrumental developments in laser ablation inductively coupled plasma mass spectrometry夞 a ¨ E. Hoffmanna,*, C. Ludke , J. Skolea, H. Stephanowitzb, J. Wollbrandtb, W. Beckerc a
¨ Spektrochemie und Angewandte Spektroskopie, Institutsteil Berlin, Albert-Einstein-Str. 9, D-12489 Berlin, Germany Institut fur b ¨ Gesellschaft zur Forderung Angewandter Optik, Optoelektronik, Quantenelektronik und Spektroskopie e.V., Rudower Chaussee 29, D-12489 Berlin, Germany c Becker & Hickl GmbH, Nahmitzer Damm 30, D-12277 Berlin, Germany Received 29 April 2002; accepted 25 July 2002
Abstract Many tasks in bulk analysis, micro analysis and depth profile analysis can be solved advantageously by laser ablation inductively coupled plasma mass spectrometry (Laser ICP-MS) in particular, when both the chemical and elemental distributions in the sample are to be determined. However, the analyst has to take into account that the analytical precision and accuracy of the Laser ICP-MS is influenced decisively by signal standardization, the homogeneity of the samples as well as calibration standards and the mass-spectrometric measuring mode, which is usually sequential when performed with scanning mass spectrometers such as quadrupol- or sector-based instruments. Using the ablated mass as standard, an excellent level of the analytical precision and accuracy (relative standard deviation R.S.D.-0.5%) has been obtained for homogeneous sample materials such as alloys. For inhomogeneous samples, such as pressed pellets, a statistical test is described, which is based upon the auto-correlation function to characterize the sample inhomogeneity. The application of the test allows us to calculate the representative mass for the quantitative analysis at previously defined analytical precision. In the instrumental part of the paper a new type of an ICP—time-of-flight (TOF) mass spectrometer—is described, constructed and built up in our laboratory. For fast signal counting an application-specific integrated circuit (ASIC) was developed, which permits a time resolution of 1 ns. The analytical performance of the TOF when used in combination with an ICP is demonstrated in terms of resolution, ion extraction rate, detection limits and dynamic range. The determination of 39 Kq and 40 Caq at trace level can be realized in a cool plasma condition (high central gas flow) only with a small interference by 40Arq. Detection limits of 23 elements were measured with typical values in the lower nanograms per liter range. The ion extraction rates, measured for a sample mass of 1 ng in terms of counts per second divided by the relative isotope abundance, are one order of magnitude higher than those obtained with a quadrupol-based instrument. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Laser ICP-MS; Signal standardization; Representative sample mass; Time-of-flight mass spectrometer 夞 This paper is published in the special issue of Spectrochimica Acta Part B dedicated to the 50th anniversary of the ISAS. *Corresponding author. E-mail address: [email protected] (E. Hoffmann). 0584-8547/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 5 8 4 - 8 5 4 7 Ž 0 2 . 0 0 1 0 9 - X
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1. Introduction The potentials of laser ICP-MS for both bulk and micro analysis have been reviewed by many authors in the literature w1–15x. Laser sampling can be applied to electrically conducting as well as non-conducting materials. Especially, for nonconducting materials such as glass w16–20x and ceramics w21–23x, and for biological materials w24–30x laser ICP-MS has proven to be a valuable tool. However, there are a number of concerns and problems that have prevented laser ICP-MS from becoming a general technique for analyzing solid materials. Practical concerns include the difficulty of obtaining or making matrix-matched standards that contain all the elements of interest and the accuracy of the resulting calibration w31–39x. The problem of matrix matching arises because the ablation yield varies with material properties such as reflectivity, thermal conductivity and melting and boiling points. To some extent, variations in ablation yield can be corrected by internal standardization using an element present in both the standard and the sample. On the other hand, nonrepresentative sampling is generally difficult to correct since it causes enhancements or reductions in relative analyte signal intensities and sensitivities. In part there are problems in laser ICP-MS because interactions between high-energy laser photons and material are complex w40x and not taken into account to a suitable degree yet. When a laser beam is focussed onto a solid surface, the irradiance in the spot can lead to a variety of effects such as reflexion, heating, desorption and emission of ions and electrons. Essentially, the laser energy is absorbed by electrons more or less strongly bound in the solid-state configuration, so there are differences in physical mechanisms leading to the ablation among the various materials such as metals, insulators and organic polymers. The experimentally observed characteristics of ablation of all the materials are rapid material removal, existence of a threshold and a non-linear dependence of yield on laser intensity. Of particular concern to the analyst is the formation of a plasma, which expands out from the surface, when the power density exceeds some
threshold value, typically of the order of 1=108 W cmy2. Laser ablation into an atmosphere creates a shock wave, which is formed by the piston-like action of the quickly expanding ablated material pushing outward on the backing gas. As the shock wave expands, more gas is swept up by the shock front, and since the laser pulse delivers a finite amount of the energy, the expansion velocity decreases with increasing distance from the target. As a result of the strong deceleration, a high density of target species is maintained for a long time. The high density and high pressure of the backing gas in turn promote volume condensation of the target species leading to small particles. Depending on the internal energy content and the size of the particles, significant cooling by the backing gas may have to occur before the particles can add additional atoms or molecules. Thus, particulates can be created of a size allowing transportation with a gas stream as carrier via long distances with negligible loss. In the inductively coupled plasma the particles are vaporized again. The ions produced in the plasma are subsequently separated using a mass spectrometer. In spite of the analytical efforts of numerous laboratories, many analysts think the laser sampling technique exhibits unsatisfactory reproducibility for many applications. This is, for example, the case for analyzing industrial products, and in the life sciences where a high precision is required as characterized by R.S.D.-0.1 % for the major elements and a few percent for the trace elements. The aim of our methodical and instrumental developments is to achieve such a high level of analytical precision and accuracy for laser ICP-MS. In the methodical part of the paper, results of investigations focussed on signal standardization using the vaporized sample mass are presented. To compensate signal fluctuations caused by inhomogeneities, a minimum mass has to be vaporized to obtain an analysis representative of the sample composition (representative sample mass). The statistical procedure to determine the representative sample mass is described below. ICP-MS is usually performed with scanning mass spectrometers such as quadrupol or sector field instruments. Because only ions of one myz (mass to charge) value can be monitored at any
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given time, there is a necessary trade-off between the number of masses that can be measured in any given period of time and the analytical precision that is attainable. This compromise becomes crucial in the analysis of transient signals such as those generated by laser sampling w41x. Because each myz value is measured at a different time, changes in the sample introduction degrade the precision and accuracy of analyses. To overcome the shortcomings and to fully eliminate the effect of ion-flow changes on analytical precision, one must measure all myz values simultaneously. To this end other types of mass analyzers such as time-of-flight (TOF) instruments, have been investigated w42–47x as alternatives to the commonly used quadrupol and sector-field based analyzer. Although ICP-TOF instruments have been commercially available for several years, there is still substantial room for progress in the figures of merit such as sensitivity and resolving power. The performance of the current ICP-TOF systems could be improved with a reduced ion-beam energy and a reduction in the velocity spread, both along and perpendicular to the flight tube. Taking up these questions an ICP-TOF instrument based on an offaxis geometry has been developed in our laboratory. The performance of this mass spectrometer will be demonstrated in Section 2.1. Because it is easy to handle, multielement solutions were used to determine the instrumental figures of merits such as the mass resolution, the dynamic range of the signals, the ion extraction rate and the detection limits. 2. Methodical investigations 2.1. Instrumentation All laser ablation experiments reported here, were performed on the ICP mass spectrometer Elan 6000 (by Perkin–Elmer; Sciex, Toronto, Canada) in combination with the laser sampler Model 320 (Perkin–Elmer, Sciex). The dwell time of the quadrupol mass spectrometer was 20 ms and the repetition frequency of the laser was 10 Hz for each experiment. The laser ablation unit includes a Nd:YAG laser (model Quanta-Ray DCR-11 from Spectra-Physics, Mountain View, CA, USA), a
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stepper motor controlled sample stage and a video camera system allowing the observation of the sample enlarged by a factor up to 80 on a TV screen. The unit was modified in various ways to have a more flexible and user friendly system. The hardware modifications include a special sampling cell which allows the illumination of the sample from all sides and an additional red diode laser helping to address points of interest on the sample surface w48x. Additionally, a generator for the higher harmonics of 532, 355 and 266 nm of the basis wavelength of 1064 nm is installed. Their pulse durations are: at 1064 nm, 10 ns; 532 nm, 8 ns; at 355 nm, 6 ns; and at 266 nm, 5 ns. One hundred pulses were applied to produce a crater. The ablated sample material was transported through a Tygon tube of 5 mm in diameter by an argon stream of 1 l miny1 as a particle carrier. 2.2. Signal standardization In spite of the stability of the laser pulse energy which varies in the order of a few percent from shot to shot, the analytical signals can fluctuate by more than 100%. In principle, averaging over many laser shots should lead to results reproducible with deviations smaller than 1%. However, when micro samples are analyzed or the analysis of the sample has to be spatially resolved the limited number of laser shots causes a low level of the analytical precision. Internal standardization of the analytical signals improves the reproducibility provided that the concentration of the corresponding isotope is constant in the sample from crater to crater. If this is not the case and the analytical and the standard signals are not correlated, internal standardization further reduces the analytical precision (addition rule of the quadratic standard deviations). The same is valid when the standard isotope has a constant concentration in the sample material, but the sampling generates transient signals measured with a mass spectrometer with sequential measuring mode. The fundamental solution of the problem is to ratio the measured ion signal Ii related to the element i against the ablated mass M. Then, the analytical signal Si of the element i is defined by:
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The calibration factors Ki can be determined with calibration standards. When the element concentrations in the calibration standards are C0i the following equation system is valid: C0isKiØ
Ii N
is1.....N
8KjØJj js1
Fig. 1. Relative standard deviation. Samples: three different alloys; the number behind a element symbol indicates the content in percent. Laser parameters: single crater; number of pulses, 300; pulse energy, 200 mJ; and wavelength, 1064 nm.
Sis
Ii M
is1.....N
The prerequisite for the procedure is the determination of all elements of significant concentration. The concentration Ci of the element i in the sample is defined by: CisKiSi
is1.....N
and normalized by: N
8Cis1 1
with N the number of the elements which are to be considered, and Ki the corresponding calibration factors. M is obtained by the sum of KiIi of all the elements measured :
Including the normalization condition of the element concentrations, the equation system can be solved for the ablated mass M. Fig. 1 shows relative standard deviations resulting from the procedure described above in comparison with results obtained without any standardization. The experimental conditions were: laser wavelength, 1064 nm; pulse energy, 200 mJ; number of pulses, 300; crater diameter, 0.15 mm; and crater mass estimated to be 50 mg. The samples used for measurements were alloys of different compositions consisting of In, Sb, Ga and Bi in the first sample, Pb and Ga in the second sample, and In and Sn in the third sample. To demonstrate the analytical accuracy which was obtained using the standardization procedure described above the results of a diffusion experiment under conditions of weightlessness is presented. In the middle of a wire of an InSn alloy (50 mm long, 1 mm diameter), a 1-mm-thick slice with equal element composition but an increased content of the 113In isotope was inserted. In the space laboratory the wire was heated and kept at the temperature of the melting point over a certain period of time. When the material was molten the
N
8Cis1
is1.....N
1 N
1s8KiØ 1
Ii M
N
Ms8KiØIi 1
Then the standardized signals are: Sis
Ii 8KiØIi
is1.....N
Fig. 2. Abundance of 113In measured in an InSn 15 alloy in comparison with the ones obtained by SIMS. The laser parameters were the same as in Fig. 1.
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auto-correlation function in the following way w34x: The analytical signals Iij (i denotes the analytical element) of a number of laser craters ( j refers to the crater) produced at equal distances as shown in Fig. 3 are centered and normalized by Iim, which is the average value of all Iij: Fig. 3. Laser craters powder, doped with graphite powder and pulses; 100 mJ pulse
with equal distances. Sample: cellulose standard solutions, dried, mixed with pressed to pellets. Laser parameters: 50 energy; and wavelength, 1064 nm.
diffusion of the isotope 113In from the inserted slice into the wire material took place. Fig. 2 shows the measured abundance of the 113In isotope compared with results obtained by SIMS. The agreement of the values is within the interval of 0.5%. 2.3. Analytical representative sample mass In bulk analysis of inhomogeneous samples, precision and accuracy depend on the quantity of the material ablated by the laser beam. ‘Trial and error’ is a time and cost consuming method to determine the minimum mass, which has to be analyzed to achieve the required reproducibility. Consequently, we use the statistical method of the
Sijs
Iij y1 Iim
is1...N js1...n
The auto-correlation function of the element i is given by: n
ciŽt.s8Sij=Sijqt
is1...N
1
The number of craters whose vaporized mass is representative of the sample can be defined by the number t0 of craters for which the auto-correlation function is decreased down to 37% of the maximum value (at ts0) w49x. ciŽt0.sey1CiŽ0. When mc is the average crater mass (determined by weighing the sample without and with craters, then the last one was subtracted from the first one and the mass difference was divided by the number of pulses), the analytically representative sample mass m0 can be calculated from: m0st0Ømc In Fig. 4 are shown two normalized autocorrelation functions related to measurements of two soil standards (BCR 142, BCR 146; Community Bureau of References, Brussels). Obviously, the standard BCR 142 is more homogeneous than BCR 146. Table 1 Analytical representative sample masses (r.s.m.) determined with the auto-correlation function (relative signal deviation is below 10 % when the r.s.m. is ablated)
Fig. 4. Normalized auto-correlation functions of isotope ratio 208 Pby45Sc. Samples: soil standards BCR 142 and BCR 146, doped with Sc standard solution, dried, ground and pressed to pellets. Laser parameters are the same as those in Fig. 3. t0 equals the representative number of craters, and crater diameters0.4 mm. Distance between two craters is 2 mm.
Element
Real soil (g)
BCR 142 standard (g)
Brass (g)
Steel (g)
Wood (pine) (g)
Ni Zn Pb Cr Cu
3.0 3.0 6.0 1.0 0.2
0.2 0.2 0.6 0.2 0.1
0.1 0.1 0.1 0.1 0.05
0.08 – – 0.05 0.05
0.008 0.005 0.005 – 0.005
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to the R.S.D. of the single isotope measurement s1 according to the relationship: sNss1ØyN
Fig. 5. Schematic of the newly developed TOF mass spectrometer.
Table 1 contains analytical representative sample masses of selected sample materials.
A TOF mass spectrometer for use in combination with an ICP has been developed which is capable to measure complete mass spectra with high precision, sensitivity and resolution independent of the number of masses studied. The goals of the instrument development were to make the most efficient use of the ions produced by the ICP source, to realize adequate resolving power, and to retain the speed and mass range for which TOF mass spectrometry is well known. A scheme of the TOF mass spectrometer is shown in Fig. 5. The ion beam extracted from the ICP via a two-stage pumping system is focussed into the repeller space by a special ion optic. After acceleration by the voltage pulse of the repeller and the grid arrangement, each ion flies with a velocity depending on its mass. The ions are reflected by an ion mirror and detected with a micro channel plate. An ion blanker is installed preventing unwelcome ions such as 40Arq, 16Oq and 32Oq 2 from reaching the detector. The ability to measure full spectra at tens of kiloHertz repetition rates places extraordinary demands on the data-acquisition system. An ASIC (MSA-1000 by Becker & Hickl, Berlin, Germany) was developed offering multichannel counting capabilities for low-level signals over a wide dynamic range. The scheme of the module is shown in Fig. 6. It uses a 32 bit shift register for fast data acquisition and a fully parallel 256 bit
3. Instrument development: ICP-TOF mass spectrometer Most of the commercially available mass spectrometers connected with an ICP work with a sequential measurement mode allowing only single mass determination step by step. Consequently, the measurement precision is reduced with the increasing number of the isotopes measured in a given time; this is particularly disadvantageous in studies of transient signals as they occur in laser ablation. The RSD sN when measuring N isotopes is related
Fig. 6. The ion counting module.
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Fig. 7. The analog measuring module.
memory structure for direct accumulation. The measured signal is segmented into 1 ns intervals (by the 1 GHz clock) in which the detection of a pulse (width down to 700 ps) is markedyregistered in the shift register. Every 32 ns the contents of the shift register is stored in a temporary register. While the shift register is used for data acquisition again and without delay, the contents of the temporary register, i.e. the previous 32 ns section of the detector signal, is stored in the memory, where a signal can be stored up to length of 128 ms. Corresponding measurement cycles (e.g. due to subsequent ion clouds) can be accumulated in the memory up to a predefined number of runsysweeps if the shift register operation is restarted by equivalent trigger pulses (at discriminator D2). In this way any dead time is avoided. For stronger signals, i.e. those where more than one pulse per nanosecond may occur, a fast multichannel analog system was developed, which adds another three orders of magnitude to the dynamic range. It operates at a time resolution of 10 ns per channel and its analog-to-digital conversion with accumulation (ADA-100 by Becker Hickl, Berlin, Germany) provides 8 bit resolution of signals between 0.1 and 1 V; up to 255 measurement cycles can be accumulated. The dual structure of the device presented in Fig. 7 is used alternately to avoid any dead time. The two acquisition modules operate in parallel and are integrated in a Pentium III PC.
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Observation of the entire atomic range at each extraction event places great demands on the dynamic range. Therefore, the full spectrum is measured simultaneously by ion counting and by analog registration. In Fig. 8 are shown mass spectra measured in the counting (a) and the analog (b) mode. The dynamic range selected of the signal measurement at the shortest measuring time of 35 ms is approximately three orders of magnitude, and increases up to nine orders of magnitudes at 10 s. To study the instrumental parameters of the ICPTOFMS instrument a multielement standard (ICP standard IV with 23 elements by Merck, Germany) was used, diluted with HNO3 (0.2% in deionized water). The relative extraction rate E in counts per second, per 10 ml sprayed multielement solution of a concentration of 1 ppm of each element and divided by the relative isotope abundance of each isotope measured is shown in Fig. 9. Results are given for two flow rates of the nebulizer gas: 1.3 and 1.0 l hy1 (‘hot’ and ‘cool’ plasma conditions, respectively) and compared with results obtained with the ICP-quadrupol mass spectrometer (nebulizer gas flow rate 1.0 l hy1; optimized according to recommended rule). The plasma generator power was 1 kW both for the TOF mass spectrometer and for the quadrupol mass spectrometer. The measuring time was 3 min for each measurement. It can be seen that the extraction rate for TOF measurements is one order of magnitude higher than those for the quadrupol mass spectrometer. The operating parameters of the TOF mass spectrometer and the quadrupol mass spectrometer are summarized in Table 2. A comparison of the mass resolution Mym (M is the mass number, m is the half-width of a mass peak) measured with the ICP-TOF mass spectrometer and the quadrupol mass spectrometer is represented in Fig. 10. The mass resolution of the ICP-TOFMS instrument is improved by a factor of two and nearly constant over all mass numbers. The detection limits measured with multielement solutions containing 23 elements and calculated according to the 3s criterion are given in Fig. 11. They also are compared with the values obtained with the quadrupol mass spectrometer.
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Fig. 8. Spectra measured in the counting (a) and analog (b) mode.
Fig. 9. Relative extraction rate, (a) relative abundance of isotope.
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Table 2 Operating parameters (ICP-TOF mass spectrometer; ICP-quadrupol mass spectrometer) TOF mass spectrometer Repetition rate Repeller time Repeller voltage Drift tube Length Diameter Voltage Solid state generator (power) Plasma gas (flow rate) Auxiliary gas (flow rate) Nebulizer gas (flow rate) Quadrupol mass spectrometer Tube generator (power) Plasma gas (flow rate) Auxiliary gas (flow rate) Nebulizer gas (flow rate)
Maximum 30 kHz 5 ms 700 V 600 mm 60 mm 1830 V 1 kW 15 l miny1 Ar 1.4 l miny1 Ar 1.0, 1.3 l miny1 Ar 1 kW 15 l miny1 Ar 1.6 l miny1 Ar 1.0 l miny1 Ar
4. Summary An improvement in precision and accuracy of the analysis with laser ICP-MS can be achieved by using the ablated sample mass for the signal standardization. The necessary number of craters in a heterogeneous sample for a given R.S.D. can be determined by evaluation of the measurements using an auto-correlation function. The ICP-
Fig. 10. Comparison of the mass resolution for the TOF and the quadrupol instrument.
TOFMS instrument developed provides an analytical performance exceeding that of their quadrupol counterparts. The extraction rate from the plasma jet is one order of magnitude better in comparison with quadrupol systems, and the mass resolution is improved by a factor of two at all mass numbers. The rapid data acquisition achieved with the specially developed ASIC-chip permits the design of a geometrically small instrument: the drift tube of our instrument is 0.6 m long. The ICP-TOFMS instrument has a particular potential for transient
Fig. 11. Detection limits calculated according to the 3s criterion.
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