Anal. Chem. 1997, 69, 4291-4293
Correspondence
Cancellation of Matrix Effects and Calibration by Isotope Dilution in Isotope-Selective Diode Laser Atomic Absorption Spectrometry H. D. Wizemann† and K. Niemax*,‡
Institute of Physics, Universita¨ t Hohenheim, Garbenstrasse 30, 70599 Stuttgart, Germany, and Institute of Spectrochemistry and Applied Spectroscopy (ISAS), Bunsen-Kirchhoff-Strasse 11, 44139 Dortmund, Germany
High-resolution atomic absorption measurements of lead isotopes are performed in a graphite tube atomizer with integrated low-pressure dc discharge using frequencydoubled laser diode radiation at 405.78 nm. For the first time, calibration by isotope dilution is demonstrated in analytical laser spectroscopy. Furthermore, isotope dilution is proved to be an effective technique for cancellation of chemical as well as physical matrix effects in optical spectrochemistry. Semiconductor laser diodes are very promising tools for the transfer of powerful laser spectroscopic techniques from the laboratory to the routine level.1 One of the predominant properties of etalon-type single-mode laser diodes is the very narrow bandwidth of radiation, typically 0.04 pm in the red and nearinfrared wavelength range.2,3 Such laser line widths are considerably narrower than the line widths of free atoms in thermal atomizers, analytical flames, or plasmas. Therefore, laser diodes are proper tools for high-resolution atomic spectrometry. If atomization takes place at reduced pressure, e.g., at about 1-10 hPa, the contribution of pressure broadening (homogeneous broadening) to the width of an atomic spectral line is generally small compared with the Doppler broadening (inhomogeneous broadening), and Doppler-broadened lines can be resolved completely. Since, in general, the isotope shifts of spectral lines of heavy and light elements are larger than the Doppler widths of the corresponding lines, isotope-selective measurements are possible for these elements. There are numerous papers on isotope measurements of Doppler-broadened lines applying laser diodes, particularly in physics journals. Isotope-selective measurements with laser diodes in analytical chemistry are still rare: to our knowledge, there are only three papers. Two deal with the measurement of 235U and 238U in 235U-enriched and -depleted samples by Doppler-limited diode laser optogalvanic spectroscopy in hollow-cathode discharges,4,5 and the third paper is on 6Li/7Li, which have been measured by wavelength modulation diode laser †
Universita¨t Hohenheim. ‡ Institute of Spectrochemistry and Applied Spectroscopy. (1) Niemax, K.; Zybin, A.; Schnu ¨ rer-Patschan, C.; Groll, H. Anal. Chem. 1996, 68, 351A-356A. (2) Wieman, C.; Hollberg, L. Rev. Sci. Instrum. 1991, 62, 1-20. (3) Franzke, J.; Schnell, A.; Niemax, K. Spectrochim. Acta Rev. 1993, 15, 37995. (4) Barshick, C. M.; Shaw, R. W.; Young, J. P.; Ramsey, J. M. Anal. Chem. 1994, 66, 4154-4158. S0003-2700(97)00272-2 CCC: $14.00
© 1997 American Chemical Society
atomic absorption spectrometry (WM-DLAAS) in a low-pressure graphite furnace.6 Laser diodes can be also successfully used to eliminate the Doppler effect and to perform high-resolution spectroscopy.7 Doppler-free techniques have to be applied if, in particular, isotopes of medium heavy elements with small isotope shifts have to be measured. In an early paper by our group,8 we used counterpropagating beams from two laser diodes in a doubleresonance experiment with thermionic detection to resolve the isotope components of barium lines, normally hidden in the Doppler profile, and measured the isotope ratios. In other experiments performed at ORNL, high-resolution measurements of lanthanum have been made by Shaw et al.,9,10 applying resonance ionization mass spectrometry (RIMS) with laser diodes. In a very recent experiment, Doppler-free saturation spectroscopy with intensity and wavelength modulation of a diode laser was used to measure the isotope components of 85,87Rb and 6,7Li in a low-pressure graphite furnace.6 The present paper reports on isotope-selective measurements of lead in aqueous samples. The measurements are performed with a furnace atomic nonthermal excitation spectroscopy (FANES) atomizer,11 an electrothermally heated graphite tube atomizer with integrated low-pressure discharge. FANES combined with optical emission spectrometry is known as a very sensitive technique for aqueous samples.11 However, FANES is also known to be very sensitive to chemical and physical matrix effects if real samples are measured. We will demonstrate that isotope dilution in isotope-selective diode laser spectrometry not only can be used for calibration but also is a perfect method to cancel severe matrix effects in optical spectrochemistry, even in strongly matrixdependent atomizers. EXPERIMENTAL SECTION The experimental arrangement is shown in Figure 1. The wavelength of a commercial laser diode (Sharp LTO 16MFO; (5) Barshick, C. M.; Shaw, R. W.; Young, J. P.; Ramsey, J. M. Anal. Chem. 1995, 67, 3814-3818. (6) Wizemann, H. D.; Niemax, K., submitted for publication. (7) Demtro ¨der, W. Laser Spectroscopy; Springer Verlag: Berlin, 1981. (8) Lawrenz, J.; Obrebski, A.; Niemax, K. Anal. Chem. 1987, 59, 1236-1238. (9) Shaw, R. W.; Young, J. P.; Smith, D. H. Anal. Chem. 1989, 61, 695-697. (10) Shaw, R. W.; Young, J. P.; Smith, D. H.; Bonanno, A. S.; Dale, J. M. Phys. Rev. A 1990, 41, 2566-2573. (11) Falk, H.; Hoffmann, E.; Lu ¨ dke, Ch. Prog. Anal. Spectrosc. 1988, 11, 417480.
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Figure 1. Experimental arrangement.
power, 30 mW; fundamental wavelength at room temperature, 810 nm; power supply, LDC400 by Profile) was tuned by temperature to 811.56 nm. The radiation was frequency-doubled in a LiIO3 crystal to match the wavelength of the 6p2 3P2-6p7s 3P10 transition at 405.78 nm in lead. Typically, about 20 nW of blue radiation was generated from about 20 mW fundamental radiation focused into the crystal. While the fundamental radiation was blocked by a 405 nm bandpass filter, the blue light modulated by a mechanical chopper (modulation frequency, 10 kHz) was used for absorption measurements of 6p2 3P2 metastable Pb atoms produced by the discharge in the FANES system. The radiation was detected by a compact, battery-powered Hamamatsu photomultiplier (H5783-03). The signal was amplified by a lock-in amplifier (Stanford Instruments RS 830DSP) and stored by a personal computer for further processing. We used an original FANES system, a loan from the Institute of Spectrochemistry and Applied Spectroscopy (ISAS-LSMU) in Berlin. The FANES source is described in detail in a review.11 The FANES source was operated with argon as a buffer gas at a pressure of 10 hPa. Lead samples were prepared from a 0.5 M HNO3 Merck stock solution with the addition of 0.1 M H3PO4 for stabilization of the samples. The concentration range of the Pb samples varied from 0.5 to 100 µg/mL. Amounts of 10 µL were injected to the FANES atomizer. Solutions of PbCO3 (Chemotrade) with 99.82% 206Pb were used for isotope dilution. NaCl was added to the aqueous lead samples for the investigation of matrix effects. The NaCl concentration varied between 0.12% and 0.5%. Such high salt concentrations affect the plasma discharge in the FANES source and reduce the population densities of excited states, in our case, the metastable 6p2 3P2 state of Pb.
Figure 2. The 100 ms scan of the analytical relevant part of the Pb 405.78 nm absorption line measured in the FANES cell. The positions and relative strengths of the isotope components of Pb are given below.
RESULTS AND DISCUSSION The absorption spectrum of the 405.78 nm Pb line is shown in Figure 2. The spectrum was obtained by a rapid scan of the laser wavelength during the atomization phase of a 1000 µg/mL sample in the FANES system and shows the part of the line relevant for isotope-selective DLAAS of Pb. The scan time was 100 ms. The wavelength was tuned by diode current. The baseline of the spectrum increased, since the wavelength was tuned by diode current. The isotope shifts of the 405.78 nm line are known precisely.12 The positions and the relative intensities of isotope components are shown in the lower part of Figure 2. The shifts (in optical frequency) of 204Pb-206Pb, 206Pb-208Pb, and 207Pb-208Pb are 2.162(4), 2.429(5), and 1.516(3) GHz, respectively. Note that
the 207Pb-208Pb isotope shift refers to the center of gravity of the hyperfine structure of the 207Pb line. The relative intensities of the F ) 5/2 f F′ ) 3/2, F ) 3/2 f F′ ) 1/2, and F ) 3/2 f F′ ) 3/ hyperfine components of a J ) 2 f J′ ) 1 transition (nuclear 2 quantum number of 207Pb, I ) 1/2) are 100, 55.6, and 11.1, respectively.13 The F ) 3/2 f F′ ) 1/2 hyperfine structure (hfs) component of 207Pb, the faint line of 204Pb, and the 206Pb line are resolved spectroscopically. As can be seen in the lower part of Figure 2, the 208Pb line interfers with the F ) 5/2 f F′ ) 3/2 hfs component of 207Pb. The peak absorption of the 208Pb line is about 4 times larger than the F ) 5/2 f F′ ) 3/2 component of 207Pb. The shift between the 208Pb and the 5/2-3/2 207Pb line is about 250 MHz. Since the third 207Pb component, the F ) 3/2 f F′ ) 3/ transition, is the weakest 207Pb line, it is of no analytical 2 relevance. It is placed at shorter wavelength, slightly outside the spectral range shown, and shows approximately the same absorption as the 204Pb component (about 1.4% abundance). The transient absorption signals of 208Pb, including 207Pb, 206Pb, and the F ) 3/2 f F′ ) 1/2 component of 207Pb, are shown in Figure 3. The measurements were made sequentially, tuning the laser wavelength to the peaks of the relevant isotope components. Figure 3a gives the DLAAS signals of a 10 µg/mL sample prepared from the Merck stock solution, while Figure 3b was measured with the 10 µg/mL sample spiked with 5 µg/mL 206Pb. The signals shown in Figure 3c were obtained with the spiked sample and 0.12% NaCl. Note that the signals in Figure 3c are amplified by a factor of 5, indicating a severe matrix effect. However, within the limits of experimental uncertainty, the ratios of the isotope peak signals are identical to those in Figure 3b. Within the limits of experimental uncertainty, the Pb concentrations in the samples could be verified by isotope dilution. Accurate data were also obtained with samples which produced very strong matrix effects. The signals of 206Pb and 207Pb (F ) 3/ f F′ ) 1/ ) were measured in the spiked and unspiked sample, 2 2 giving the absolute concentrations of both isotopes in the samples prepared from the stock solution. The 208Pb concentration cannot
(12) Hu ¨ ffer, W. Ph.D Thesis, University of Ko ¨ln, 1982.
(13) Kopfermann, H. Nuclear Moments; Academic Press: New York, 1958.
4292 Analytical Chemistry, Vol. 69, No. 20, October 15, 1997
Figure 3. Transient absorption signals of 206Pb, 207Pb, and 208Pb measured sequentially by isotope-selective DLAAS in the FANES atomizer. (a) Isotope signals of 10 µg/mL Pb prepared from stock solution; (b) isotope signals of 10 µg/mL Pb spiked with 5 µg/mL 206Pb; (c) isotope signals of 206Pb-enriched 10 µg/mL Pb samples in the presence of 0.12% NaCl. The signals in (c) are amplified by a factor of 5.
be measured directly because of the spectral overlap of the Doppler-broadened 208Pb line with the Doppler profile of the 207Pb hfs component. However, the intensity of the 207Pb isotope component, F ) 5/2 f F′ ) 3/2, is known since the F ) 3/2 f F′ ) 1/2 component has been measured and the intensity ratio of the two hfs lines is 100:55.6. Therefore, the contribution to the 208Pb signal can be calculated if the wavelength difference of the 207Pb (F ) 5/ f F′ ) 3/ ) and the 208Pb lines (about 250 MHz) 2 2 and the Doppler line shapes are taken into account. The Doppler widths of the Pb isotope components were about 1.3 GHz. For 204Pb, which was not measured, 1.4% abundance was assumed. The standard deviation for isotope signals was found to be always smaller than +5%, typically 4%, taking into account six or seven independent measurements. In particular, we found the following abundances: 51.0 ( 2.6% for 208Pb, 27.3 ( 1.4% for 206Pb, and 20.3 ( 1.0% for 207Pb. Within the limits of uncertainty, our data were in agreement with values obtained by ICPMS measurements. The ICPMS data were 51.6 ( 0.5%, 25.7 ( 0.3%, 21.3 ( 0.2%, and 1.4 ( 0.01% for 208Pb, 206Pb, 207Pb, and 204Pb, respectively. For the future, we plan simultaneous absorption measurements of lead isotopes with independent laser diodes. Such measurements will significantly reduce the error bars. The detection limits of 206Pb and 208Pb by DLAAS in the FANES system were found to be about 0.06 µg/mL. The detection limit of 207Pb was about 2 times higher. These values correspond to a fraction absorbed of 4.5 × 10-3. Such absorption data are relatively poor. They are shot noise limited due to the low power of the frequency-doubled radiation. The potential of DLAAS is (14) Zybin, A.; Schnu ¨ rer-Patschan, C.; Niemax, K. J. Anal. At. Spectrom. 1995, 10, 563-567. (15) Liger, V.; Zybin, A.; Kuritsyn, Yu.; Niemax, K. Spectrochim. Acta, in press.
much better if laser powers of, for example, 2 mW instead of 20 nW are applied. In such a case, one can expect an improvement of at least 3 orders of magnitude, taking into account experimental data obtained recently by WM-DLAAS in modulated low-pressure plasmas.14 An even more promising method for extremely sensitive absorption measurements was demonstrated recently.15 It is a combination of a double-beam absorption arrangement with logarithmic detection and WM-DLAAS of a modulated absorption source. The detection limit of this new technique was found to be equivalent to about 10-7 absorption, applying a time constant of 1 s. At the moment, powers higher than 2 mW are delivered by commercial laser diodes only in the red and infrared spectral ranges, or by sophisticated intracavity frequency-doubling arrangements.2 However, it can be expected that a new generation of laser diodes of the GaN or ZnS type with operation wavelengths in the blue spectral range and powers in the milliwatt range will be available in the near future.1 CONCLUSION For the first time, calibration by isotope dilution has been performed in atomic spectrometry. Furthermore, it was demonstrated by addition of salt to the aqueous samples that isotope dilution is also an effective technique for optical spectrochemistry to minimize severe matrix effects. We like to stress the general applicability of isotope dilution in optical spectrochemistry. Measurements can be performed not only in absorption but also in fluorescence (laser-induced fluorescence) or by applying resonance ionization spectroscopy where isotopes are stepwise ionized with two or more lasers. Preconditions of the application of isotope dilution in laser spectrochemistry are (a) narrow laser line widths, as delivered by laser diodes, (b) sample atomization at low pressure, to keep pressure broadening small, and (c) sufficiently large isotope shifts if Doppler-limited spectroscopy is applied. If the isotope shifts are smaller than the Doppler width of the spectral line, Doppler-free laser spectroscopic techniques have to be applied. Isotope-selective diode laser spectrometry of elements with small isotope shifts applying Doppler-free techniques is currently under way in our laboratory. ACKNOWLEDGMENT The authors thank Drs. E. Hoffmann and C. Lu¨dke (Institute of Spectrochemistry and Applied Spectroscopy, Berlin, Germany) for the loan of the FANES system and Drs. D. Stu¨wer and N. Jakubowski (Institute of Spectrochemistry and Applied Spectroscopy, Dortmund, Germany) for the ICPMS measurements of the Pb isotope ratios in the stock solution. Received for review March 11, 1997. Accepted July 6, 1997.X AC970272T X
Abstract published in Advance ACS Abstracts, August 15, 1997.
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