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A simple, low cost, multielement atomic absorption spectrometer with a tungsten coil atomizer
Spectrochimica Acta Part B (1998) 1507–1516
A simple, low cost, multielement atomic absorption spectrometer with a tungsten coil atomizer Karl A. Wagner, Keith E. Levine, Bradley T. Jones* Department of Chemistry, Wake Forest University, Winston-Salem, NC 27109, USA Received 10 January 1998; accepted 7 April 1998
Abstract An inexpensive, multielement atomic absorption spectrometer utilizing a tungsten coil atomizer has been developed. The novel optical arrangement employs three 608 beam combiners to blend the spectral output from four light sources such as electrodeless discharge lamps, or hollow cathode lamps, and then direct that output over an atomizer. This instrument uses an inexpensive tungsten coil atomizer that is extracted from a standard 150 W projector bulb. The temperature of the coil is computer-controlled by changing the voltage across the coil. A low voltage is first used to dry the sample then a higher voltage is used to atomize the sample. Simultaneous detection of the analyte absorption signals is accomplished using a charge-coupled device. The elements of interest in this study were Cd, Pb, and Cu. Near-line background correction was used to correct for nonspecific analyte absorption. q 1998 Elsevier Science B.V. All rights reserved Keywords: Tungsten coil; Atomic absorption spectrometry; Multielement; Near-line background correction
1. Introduction The potential health risks associated with exposure to toxic metals are great. Health problems, including skeletal abnormalities, kidney damage, mental deficiencies, and cancer, have all been documented to occur from metal poisoning [1]. Increased regulation will put an even greater demand on the capabilities of analytical instrumentation for lower detection limits while still keeping the cost of analysis reasonable. Currently, the most common analytical methods for trace metal determinations are flame atomic absorption spectrometry (FAAS), graphite furnace atomic absorption spectrometry (GFAAS), and inductively coupled plasma (ICP) emission spectrometry. Of the three methods listed, GFAAS is the most * Corresponding author
sensitive technique, often producing detection limits in the low to sub-picogram range for most metals [2–8]. On the other hand, FAAS and ICP typically produce detection limits that are three orders of magnitude higher than GFAAS [7,9,10]. More recently, ICP with axial viewing and ICP–mass spectrometry have produced detection limits approaching that of GFAAS, but at the cost of greater sample volume requirements and much more expensive instrumentation [11,12]. Despite the production of superior detection limits, GFAAS does suffer from a few limitations [6]. Specifically, the instrumentation is bulky and complex, due in part to the power requirements of the furnace. The large mass and high resistance of graphite tubes necessitate a power supply capable of generating power in excess of 2 kW in order to quickly heat the furnace [13]. Furthermore, GFAAS is typically a
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Fig. 1. Schematic diagram of the multielement W coil atomic absorption spectrometer.
single element technique, as are all AA techniques, unless a special optical arrangement (or multi-element source) is used. However, these limitations are often overlooked due to the technique’s low detection limits, small sample requirements, and modest instrumentation cost. In certain circumstances, the use of a tungsten (W) coil atomizer does offer a few advantages over the typical graphite furnace atomizer. Specifically, these advantages are decreases in instrument cost and size, which are mostly due to the smaller power supply requirements of the W coil atomizer. In 1972, Williams and Piepmeier were the first to suggest using a W coil extracted from a mass-produced projector bulb [14]. Since that time, there have been many other W coil articles, most of which used tightly wound spiral coils that were extracted from a 150 W Osram halogen projector bulb [15–21]. The shape of
Fig. 2. 608 beam combiner (Oriel part # 38111).
the 150 W coil makes it possible to easily deliver up to 20 ml of an aqueous solution onto the filament. Additionally, a small, inexpensive, 150 W power supply can be used with a W coil, as opposed to the costly power supply capable of producing at least 2 kW, as is needed for a graphite furnace atomizer. The small size of the power supply makes the production of a portable W coil AAS possible [21]. A 150 W power supply is sufficient due to the coil’s low mass and resistance [16]. Tungsten coils reportedly can reach temperatures exceeding 3000 K, which is greater than the atomization temperature for most metals using GFAAS [15]. As an additional benefit, a W coil can be heated at a rate of up to 30 K ms −1, compared to 2–4 K ms −1 for a graphite tube atomizer [17]. As mentioned earlier, by nature, almost all AA techniques are single element techniques. Typically, a different source is needed for each element to be determined, which greatly increases analysis time and cost. In addition, because AAS is a single element technique, larger quantities of sample are needed for multiple determinations of the same sample. Multielement HCLs can be used for some applications but the correct combination of elements may not always be available. Fairly recently, there have been two papers published on commercially available multielement GF instruments [22,23]. Perkin Elmer (PE) manufactures a multielement GFAAS (model # SIMAA 6000) that is capable of simultaneously determining up to six elements. The optical system consists of a high dispersion echelle monochromator in combination with a two-dimensional photodiode array. A prism is used to disperse the light in the second plane onto the array. With this system, only four lamps are held in the lamp turret at one time. Therefore, in order to determine six elements, two multielement lamps must be used. The instrument uses a rather complicated and expensive optical arrangement in order to combine the four source beams and then disperse and detect the radiation in two planes. This added cost and complexity may be appropriate for a GF instrument but it would not be for an inexpensive W coil instrument. Furthermore, an optical arrangement like that described on the PE SIMAA 6000 would prohibit the design of a portable instrument. Another area of focus for multielement AA has
K. A. Wagner et al./Spectrochimica Acta Part B (1998) 1507–1516
been with continuum sources, such as high intensity xenon arc lamps. This arrangement solves the problem of aligning multiple sources through a single atomization source, but there are many inherent difficulties with this technique. The main disadvantage of continuum source AAS, in relation to this project, is that instrument cost is greatly increased, compared to conventional AA because a high-resolution monochromator is needed to provide the required spectral dispersion. Furthermore, the addition of a high dispersion monochromator would also greatly increase instrument size. Once again, increased instrument cost and size would not be appropriate on an inexpensive, portable W coil AA spectrometer.
2. Experimental 2.1. Instrument design This inexpensive, low power, W coil atomic absorption spectrometer uses a novel optical arrangement to gain multielement capabilities. Although the optical system, depicted in Fig. 1, has been developed to utilize three 608 coarse grating beam combiners (Oriel model No. 38111) and up to four HCLs or EDLs, only the use of three lamps is reported for this study. The one-inch square beam combiner, shown in Fig. 2, has triangular, mirrored facets that are able to reflect light that is incident at 608 to the beam combiner normal. Therefore, the output from two lamps can be combined into a single beam. The coarse groove density of 4 lines per mm prevents diffraction of the incident light. This study used a cadmium EDL and a copper HCL placed on one side of the main optics axis and a lead EDL placed on the opposite side. The collimated spectral output from two lamps is mixed by a beam combiner. Similarly, the output from the second pair of lamps, placed on the other side of the main optics axis, is mixed by a different beam combiner. The two combined beams are then mixed by a third beam combiner, which is centrally located on the main optics axis. The third beam combiner directs the output from the four lamps to a fused silica lens (2.5 cm diameter, 15 cm focal length) that focuses the combined output 2 mm over a tungsten coil atomizer. A fused silica lens (2.5 cm diameter,
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5 cm focal length) collects and focuses the radiation onto the entrance slit of a miniature charge-coupled device (CCD) spectrometer. An interesting aspect of this optical design is that lamp position is independent of wavelength. For this reason, any (or all) of the four elements can be replaced without changing the position of the lamp holders. The overall size of the instrument is approximately 80 cm long by 40 cm in width. These dimensions could be reduced by 25% if the lamps and beam combiners were moved closer to each other. The Oriel catalog claims 76% efficiency at 632.8 nm for a single beam combiner, but the final throughput efficiency for this study was only approximately 10%. There are a few reasons for this discrepancy. Due to the fact that two beam combiners were used in this study, the radiation loss is expected to double. Secondly, the aluminum coating used on the beam combiner surface is a less efficient reflector in the lower UV region compared to the stated efficiency value at 632 nm. Also, due to the steep angle of incidence and the relatively small size of the beam combiner (one-inch square), approximately 15% of the incident radiation missed the beam combiner surface, which degraded the efficiency even further. The W coils are obtained from standard 150 W projector bulbs (OSRAM Part No. BRJ or GE Part No. EVB) from which the glass shrouds have been removed, leaving the bulb bases intact. The W coil is housed in the center of a custom-designed glass cell (75 ml internal volume) that is made by Ace Glass (Vineland, NJ. Ace Glass Part No. 7488-383). The cell, shown in Fig. 3, is filled with a shield gas (10% hydrogen in argon) enclosed by a quartz window at each end in order to protect the coil from oxidation. The quartz windows are held in place with #25 nylon Ace Glass cell window holders. The cell is provided with an outer member 10/18 ground glass joint, placed 608 off vertical, for sample introduction. On the bottom center of the cell is a #25 threaded Ace fitting that holds the cell in place on the coil mount. All Ace fittings are sealed with o-rings in order to maintain the reducing atmosphere of 10% H 2 in Argon that protects against coil oxidation [20]. A gas mixture containing 10% H 2 was chosen because that is a mixture which is readily available as a standard welding gas. The W coil mount, shown in Fig. 4, was laboratory
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Fig. 3. Schematic diagram of atomization cell. Fig. 5. Electrical schematic.
designed from a one-inch diameter aluminum rod approximately two inches long. The coil is mounted in a mass-produced ceramic bulb mount that is 19 mm in diameter (Gray Supply Company, Chicago, IN. Part # ORX6350). Replacement of a worn W coil is relatively simple as the coil remains attached to the original bulb base and a factory-produced bulb mount is used. In fact, the bulbs have two prongs on them that slip into the receptacles on the ceramic base, which simplifies coil replacement. The electrical lines and a 3 mm O.D. gas line enter the coil mount through a hole in the side of the Al rod. The gas and electrical lines are then passed through a hole in the center of the mount where they are connected to the ceramic bulb base. Next, the ceramic bulb base is epoxied in place. In addition, the coil mount cavity (used for the gas and electrical lines)
Fig. 4. Schematic diagram of W coil mount.
is filled with epoxy in order to provide an air tight seal. The cell is held in place on the coil mount with a rubber o-ring. The coil mount, including the cell, is then mounted on a standard 12.7 mm diameter optical post and holder (Newport, Fountain Valley, CA, part # SP4 and VPH-4), which allows for easy coil height adjustment. 2.2. Detection and coil control A miniature CCD spectrometer (Ocean Optics model S2000) is used for simultaneous detection of all elements during the absorption process. The CCD covers a spectral range of 200–400 nm. The detector was originally designed to use a fiber optic for light input; but this set-up was discarded due to poor throughput of the fiber optic below 230 nm. In order to support the detector in the optics path for direct transmission of the light, a detector mount was designed and fabricated in the laboratory. Also, a 10 mm slit was factory installed on the detector to provide the necessary resolution because the fiber optic, which typically acts as the entrance slit, could not be used. The CCD is coupled to a personal computer with a 500-kHz, 12-bit A/D data acquisition system. The coil is powered by a small (9 × 16 cm 2) solid state power supply manufactured by Payne Engineering (model 150928). The power supply is computer controlled by a Computer Boards D/A CIO-DAC02 card.
K. A. Wagner et al./Spectrochimica Acta Part B (1998) 1507–1516 Table 1 Perkin Elmer Optima 3000 DV operating parameters Parameter of interest
Value
R.f. power Auxiliary Ar gas flow Nebulizer flow Plasma flow Sample flow rate Wash time Sample read delay time Processing mode Background Replicates
1360 W 0.5 l min −1 0.7 l min −1 15 l min −1 1.60 ml min −1 30 s 50 s area manual point selection 3
Fig. 5 depicts the electrical diagram for the project. A 6.8 Q, 1000 W resistor is put in series with the power supply to step down the power. The electrical system is straightforward and works as follows: first, a low voltage (between 0 and 5 V) is sent to the power supply, via the D/A card, to produce a low power setting to dry the coil. After the drying cycle is complete, the power is increased for the atomization process. At that time, the D/A card sends a voltage to the spectrometer for the data acquisition trigger. An Excel macro is used for data manipulation. A typical atomization cycle is completed as follows: (1) a 20 ml sample is deposited onto the coil with a micro-pipette; (2) 2.4 A (0.40 W) is applied to the coil for 200 s in order to dry the sample; (3) the current is increased to 6.8 A (24 W) (approximately 2500 K) for the atomization cycle. At this point, the spectrometer is automatically triggered and data is collected every 100 ms for approximately 3 s. The coil is held at the atomization current for 10 s to ensure no residue is left on the coil. A constant flow of 10% hydrogen in argon (300 ml min −1) was used as a purge gas for all work. Using these conditions, the coil had a lifetime of approximately 150 shots. It was determined that lower gas flow rates resulted in the analyte atoms having longer residence times in the radiation path. The chosen purge gas flow rate of 300 ml min −1 was sufficient to protect the coil from oxidation while still providing reasonable sensitivity. 2.3. Sample preparation Spex Plasma Standards, 1000 mg l −1 were used to prepare the reference solutions. Dilutions were made with distilled, de-ionized water. Two NIST standard
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reference materials were analyzed: SRM #1566a ‘Oyster Tissue’ and SRM #1645 ‘River Sediment’. As a test of accuracy and precision, the technique was compared to a commercial ICP instrument. The SRMs were first microwave digested for ICP–AES analysis and then analyzed with the W coil instrument. The Oyster Tissue was prepared as follows. A 0.4612 g sample was weighed into a Kohlrausch flask and then treated with 10 ml of trace metal grade nitric acid at room temperature for 1 h. Next, the sample underwent open vessel microwave digestion in a CEM, MDS-2100 microwave. The sample was brought to near dryness and then a second 10 ml aliquot of nitric acid and 5 ml of Ultrapure J.T. Baker 30% hydrogen peroxide were added. The sample was brought to near dryness again, diluted to 100.0 ml with distilled, de-ionized water, filtered through a 0.45 mm filter, and finally stored in a clean centrifuge vial. The River Sediment sample was digested in a closed vessel by a CEM MDS-2100 instrument. This matrix decomposition method was selected in order to achieve acceptable elemental recoveries for the sample. A 0.2125 g sample was weighed directly into the digestion vessel and treated with a 5 ml aliquot of trace metal grade nitric acid at room temperature for 1 h. The sample was then sealed for a closed vessel microwave digestion procedure. The procedure was considered to be complete when a temperature of 1758C was maintained for 30 min. In order to achieve this objective without rupturing the digestion vessel, the River Sediment sample was heated gradually in four discrete steps. The closed vessel was allowed to cool to room temperature at the end of each step and vented to release decomposition gases. The steps employed for this digestion procedure were: 1258C for 20 min, 1258C for 60 min, 1508C for 60 min, and 1758C for 30 min. At the end of the digestion, the sample was quantitatively transferred to a 100 ml volumetric flask and diluted with distilled, de-ionized water to volume. The sample was finally passed through a 0.45 mm filter and stored in a clean centrifuge vial. For comparative purposes, the SRMs were also analyzed with an Optima 3000 DV axial viewed ICP optical emission spectrometer (Perkin Elmer Corp.) equipped with a segmented-array CCD. The ICP was equipped with an AS-90 autosampler. A summary of
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Fig. 6. Hollow cathode lamp emission profiles.
the ICP parameters employed for this investigation is presented in Table 1. As stated earlier, one of the largest benefits of a W coil atomizer is the low power requirements. This would make it possible to produce a low cost, portable instrument that would be appropriate for an on-site screening instrument. This is something that would not be possible with a GF instrument due to its size, weight, and power requirements [21]. Understandably, a portable instrument would be limited to samples with less complex matrices not needing digestion, such as water samples or samples that had been cleaned with an extraction step.
3. Results and discussion 3.1. Background correction Due to its simplicity and the fact that it does not add
extra cost, near-line background correction is a logical choice to be used with a W coil atomizer. Furthermore, because no additional optical components, such as additional sources, choppers, polarizers, magnets, etc., are needed for the near-line method, it would be possible to use this correction method on a low cost, portable instrument. Also worth noting, the source signals are not modulated with the near-line method, as is the case with the Zeeman and continuum methods, therefore the duty cycle is nearly doubled. For the near-line background correction method, a second emission line from the light source is used to correct for nonspecific analyte absorption. A potential failing with the near-line method is that there may not always be a non-absorbing line close to the analytical line. For accurate correction, the background line should be as close as possible to the analytical line because background absorption is often wavelength specific. The only requirement for the background correction line is that it should have very little or no
Table 2 Analytical and background wavelengths
Cadmium Lead Copper
Analytical line (nm)
Background line (nm)
Dl (nm)
228.8 283.3 324.8
Cd 226.5 Pb 280.2 Cd 326.1
2.3 3.1 1.3
K. A. Wagner et al./Spectrochimica Acta Part B (1998) 1507–1516
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Fig. 7. Three-dimensional cadmium HCL emission profile collected during the atomization of a 20 ml aliquot containing 20 mg l −1 Cd.
sensitivity toward analyte absorption. Meeting these requirements was not a problem with the optical design of the current study because the radiation from three separate line sources was combined into a single beam, which passed over the atomizer then to the detector. The emission profiles for the lamps are shown in Fig. 6. Table 2 lists the analytical and background wavelengths used for each element. Originally, another drawback of near-line background correction was that the analytical and correction lines had to be measured sequentially. This method of measurement often introduced error because experimental conditions could change over the data collection cycle. This was not a problem in the current study since a CCD spectrometer was used. In light of this, the analyte and background correction lines could be measured simultaneously. In short, although the near-line method may not be as accurate or as universally applicable as the Zeeman or continuum correction methods, near-line is a viable option for a W coil atomizer since this type of atomizer would most likely be used on a low cost, portable, screening device. The equations used for calculating the background-corrected absorbance are not given here as they have been discussed previously [21]. Figs 7 and 8 are data plots from the atomization of a 20 ml aliquot of a 20 mg l −1 Cd standard solution. Fig. 7 is a three-dimensional cadmium HCL emission profile and Fig. 8 is a plot of the uncorrected absorbances at the analytical and background lines. The figures clearly show analyte absorption at the 228.8 nm
analytical line, but not at the background line (226.5 nm). The unusual, double humped absorption profile of Cd occurred at all atomization temperatures. This feature did not seem to degrade accuracy or precision. 3.2. System optimization The selected atomization power of 24 W was a slight compromise due to the difference in the volatilities of the three elements. Fig. 9 shows the various absorption profiles that result from this selected atomization power. Use of a lower atomization power resulted in poor absorption profiles for the less volatile elements. During analysis, a constant purge gas (10% hydrogen in argon) flow rate of 300 ml min −1 was used. In general, analyte residence times increased with lower flow rates, which had the effect of producing larger absorption signals. 300 ml min −1 was a high enough flow rate to provide satisfactory coil protection while still allowing reasonable analyte residence times in the radiation path. As mentioned earlier, the combined source beams were focused 2 mm over the W coil. Optimization of coil height in relation to the combined source beams has also been discussed previously [21]. Signal-to-noise (S/N) data for each element was collected for 25 consecutive episodes at varying signal levels. Source intensities were altered by placing different combinations of neutral density filters in the optical path. The slopes of the log mean signal versus
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Fig. 8. Uncorrected absorbance profiles collected at the 228.8 nm Cd analytical line (l) and the 226.5 nm background correction line (B). The data was collected during the atomization of a 20 ml aliquot containing 20 mg l −1 Cd.
log S/N were all approximately 0.5, indicating that the system was primarily shot noise limited. Because shot noise is proportional to (S) 1/2, this suggests that the S=N ratio could be improved by increasing light throughput.
3.3. Analytical figures of merit Table 3 lists the various figures of merit for Cd, Pb, and Cu collected using the above stated conditions. As usual, the total absorbance was calculated by summing
Fig. 9. Absorption profiles that resulted from the atomization of a 20 ml aliquot containing the following: 20 mg l −1 Cd (l), 100 mg l −1 Pb (B), and 80 mg l −1 Cu (O).
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K. A. Wagner et al./Spectrochimica Acta Part B (1998) 1507–1516 Table 3 Figures of merit for Cd, Pb, and Cu. All of the detection limits (DL) are reported for 3j
W coil characteristic mass (pg) GFAAS characteristic mass (pg) a W coil instrument DL (pg) GF instrument DL (pg) a ICP method DL (mg l −1) b W coil LDR (decades) W coil method DL (mg g −1 of oyster tissue) W coil method DL (mg g −1 of river sediment) a b
Cadmium
Lead
Copper
20 0.5 2 0.3 0.4 2.5 0.02 0.05
600 5 120 3 7 2 below DL 3
100 3 20 2 0.9 2 0.2 0.4
Values were taken from Ref. [24]. Actual values obtained with a Perkin Elmer Optima 3000 DV ICP.
the absorbance values over the given episodes. For each case, a 20 ml sample aliquot was deposited onto the coil and three replicates were made. For the purpose of comparison, detection limits (DL) are given for GFAAS (literature values) and the Perkin Elmer Optima ICP (observed values). The instrument DLs reported for GF are superior to those obtained with the W coil atomizer. The most likely reason for this disparity is the difference in the internal volumes of the two atomizers. As mentioned earlier, the internal volume of the W coil atomization cell is 75 ml while the internal volume of a standard PE graphite tube is approximately 0.8 ml. And, of course, smaller atomizer internal volumes permit less diffusion of the atom cloud, which results in better sensitivities. Table 4 represents the results from the NIST standard reference materials (SRM) that were analyzed in order to test the accuracy and precision of the instrumental design and also the W coil atomizer. Lead was not determined in ‘Oyster Tissue’ because the concentration was below the limit of quantitation. The three metals were determined in the SRMs with the standard addition method. Both accuracy and precision were very reasonable and comparable to
values obtained with a Perkin Elmer Optima 3000 DV ICP.
4. Conclusion An inexpensive, multi-element W coil atomic absorption spectrometer has been developed that is capable of combining the radiation from four separate sources. An interesting aspect of the optical design is that the position of the four lamps is independent of wavelength. In other words, any four HCLs or EDLs can be used. Furthermore, even considering the 200 s dry time, the multi-element capabilities of this instrument should decrease the time and cost needed for analysis. The addition of an autosampler could also decrease operating cost and increase precision; although, this addition would likely not be possible on a portable instrument due to the added size and weight. There is also a significant cost and space savings compared to graphite furnace. These savings are due primarily to the reduction in power supply requirements needed for the W coil atomizer. Another aspect worth noting
Table 4 Analysis of NIST Standard Reference Materials SRM Oyster Tissue #1566a
River Sediment #1645
Cadmium Lead Copper Cadmium Lead Copper
Certified values (mg g −1)
W coil values (%RSD)
ICP values (%RSD)
4.15 6 0.38 0.317 6 0.014 66.3 6 4.3 10.2 6 1.5 714 6 28 109 6 19
4.32(3.0) below DL 64.4(1.2) 11.6(2.8) 710(3.7) 128(4.1)
4.3(4.5) below DL 63.9(3.5) 10.1(2.5) 693(2.2) 100(4.3)
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is that a W coil atomizer does not need cooling water, as is the case with a graphite atomizer. Although near-line background correction is not as universally applicable as the Zeeman or continuum correction methods, near-line does offer a few important advantages when considering the fact that a W coil AA spectrometer would likely be used as a low cost screening device that could be made portable. Namely, the added cost, bulk, and complexity of a second radiation source, a magnet, or extra optical components are not needed for the near-line method.
Acknowledgements This work has been supported by grants from NSF (STTR-DMI-9523190; GOALI-CHE-9710218) and Leeman Labs, Inc. The authors would also like to thank R.J. Reynolds Tobacco Company for the use of the Perkin Elmer, Optima ICP.
References [1] U. Forstner, G. Wittmann, Metal Pollution in the Environment, Springer-Verlag, New York, 1979. [2] B.V. L’vov, Anal. Chem. 63 (1991) 924A. [3] A. Walsh, Anal. Chem. 62 (1991) 933A. [4] S.R. Koirtyohann, Anal. Chem. 63 (1991) 1024A. [5] W. Slavin, Anal. Chem. 63 (1991) 1033A.
[6] A. Varma, Handbook of Furnace Atomic Absorption Spectrometry, CRC Press, Boca Raton, FL, 1990. [7] W. Slavin, Anal. Chem. 58 (1986) 589A. [8] D. Littlejohn, J.N. Egila, R.M. Gosland, Anal. Chim. Acta. 250 (1991) 71. [9] A. Varma, Handbook of Inductively Coupled Plasma Atomic Emission Spectrometry, CRC Press, Boston, MA, 1991. [10] S.J. Haswell (Ed.), Atomic Absorption Spectrometry: Theory, Design, and Applications, Elsevier, New York, 1991. [11] A. Montaser, D.W. Golighly (Eds.), Inductively Coupled Plasmas in Analytical Atomic Spectrometry, VCH Publishers, New York, 1992. [12] R.S. Houk, S.C. Shum, D.R. Wiederin, Anal. Chim. Acta. 250 (1991) 61. [13] E.G. Codding, J.D. Ingle, A.J. Stratton, Anal. Chem. 52 (1980) 2133. [14] M. Williams, E.H. Piepmeier, Anal. Chem. 44 (1972) 1342. [15] H. Berndt, G. Schaldach, J. Anal. At. Spectrom. 3 (1988) 709. [16] V. Karanassios, P. Drouin, G.G. Reynolds, Spectrochim. Acta Part B 50 (1995) 415. [17] M.F. Gine, F.J. Krug, S.F. Reis, J.A. Nobrega, H. Berndt, J. Anal. Atom. Spectrom. 8 (1993) 243. [18] M.M. Silva, R.B. Silva, F.J. Krug, J.A. Nobrega, H. Berndt, J. Anal. At. Spectrom. 9 (1994) 861. [19] M.M. Silva, F.J. Krug, J.A. Nobrega, P.V. Oliveira, Spectrochim. Acta Part B 50 (1995) 1469. [20] P.J. Parsons, H. Qiao, K.M. Aldous, E. Mills, W. Slavin, Spectrochim. Acta Part B 50 (1995) 1475. [21] B. T Jones, C.L. Sanford, S.E. Thomas,, Appl. Spectroscopy 50 (1996) 174. [22] J.G. Sen Gupta, J.L. Bouvier, Talanta 42 (1995) 269. [23] M. Hoenig, A. Cilissen, Spectrochim. Acta Part B 52 (1997) 1443. [24] J. Ingle, S. Chrouch, Spectrochemical Analysis, Prentice Hall, Englewood Cliffs, NJ, 1988.
A simple, low cost, multielement atomic absorption spectrometer with a tungsten coil atomizer Karl A. Wagner, Keith E. Levine, Bradley T. Jones* Department of Chemistry, Wake Forest University, Winston-Salem, NC 27109, USA Received 10 January 1998; accepted 7 April 1998
Abstract An inexpensive, multielement atomic absorption spectrometer utilizing a tungsten coil atomizer has been developed. The novel optical arrangement employs three 608 beam combiners to blend the spectral output from four light sources such as electrodeless discharge lamps, or hollow cathode lamps, and then direct that output over an atomizer. This instrument uses an inexpensive tungsten coil atomizer that is extracted from a standard 150 W projector bulb. The temperature of the coil is computer-controlled by changing the voltage across the coil. A low voltage is first used to dry the sample then a higher voltage is used to atomize the sample. Simultaneous detection of the analyte absorption signals is accomplished using a charge-coupled device. The elements of interest in this study were Cd, Pb, and Cu. Near-line background correction was used to correct for nonspecific analyte absorption. q 1998 Elsevier Science B.V. All rights reserved Keywords: Tungsten coil; Atomic absorption spectrometry; Multielement; Near-line background correction
1. Introduction The potential health risks associated with exposure to toxic metals are great. Health problems, including skeletal abnormalities, kidney damage, mental deficiencies, and cancer, have all been documented to occur from metal poisoning [1]. Increased regulation will put an even greater demand on the capabilities of analytical instrumentation for lower detection limits while still keeping the cost of analysis reasonable. Currently, the most common analytical methods for trace metal determinations are flame atomic absorption spectrometry (FAAS), graphite furnace atomic absorption spectrometry (GFAAS), and inductively coupled plasma (ICP) emission spectrometry. Of the three methods listed, GFAAS is the most * Corresponding author
sensitive technique, often producing detection limits in the low to sub-picogram range for most metals [2–8]. On the other hand, FAAS and ICP typically produce detection limits that are three orders of magnitude higher than GFAAS [7,9,10]. More recently, ICP with axial viewing and ICP–mass spectrometry have produced detection limits approaching that of GFAAS, but at the cost of greater sample volume requirements and much more expensive instrumentation [11,12]. Despite the production of superior detection limits, GFAAS does suffer from a few limitations [6]. Specifically, the instrumentation is bulky and complex, due in part to the power requirements of the furnace. The large mass and high resistance of graphite tubes necessitate a power supply capable of generating power in excess of 2 kW in order to quickly heat the furnace [13]. Furthermore, GFAAS is typically a
0584-8547/98/$19.00 q 1998 Elsevier Science B.V. All rights reserved PII S 0 58 4- 8 54 7 (9 8 )0 0 12 7 -X
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Fig. 1. Schematic diagram of the multielement W coil atomic absorption spectrometer.
single element technique, as are all AA techniques, unless a special optical arrangement (or multi-element source) is used. However, these limitations are often overlooked due to the technique’s low detection limits, small sample requirements, and modest instrumentation cost. In certain circumstances, the use of a tungsten (W) coil atomizer does offer a few advantages over the typical graphite furnace atomizer. Specifically, these advantages are decreases in instrument cost and size, which are mostly due to the smaller power supply requirements of the W coil atomizer. In 1972, Williams and Piepmeier were the first to suggest using a W coil extracted from a mass-produced projector bulb [14]. Since that time, there have been many other W coil articles, most of which used tightly wound spiral coils that were extracted from a 150 W Osram halogen projector bulb [15–21]. The shape of
Fig. 2. 608 beam combiner (Oriel part # 38111).
the 150 W coil makes it possible to easily deliver up to 20 ml of an aqueous solution onto the filament. Additionally, a small, inexpensive, 150 W power supply can be used with a W coil, as opposed to the costly power supply capable of producing at least 2 kW, as is needed for a graphite furnace atomizer. The small size of the power supply makes the production of a portable W coil AAS possible [21]. A 150 W power supply is sufficient due to the coil’s low mass and resistance [16]. Tungsten coils reportedly can reach temperatures exceeding 3000 K, which is greater than the atomization temperature for most metals using GFAAS [15]. As an additional benefit, a W coil can be heated at a rate of up to 30 K ms −1, compared to 2–4 K ms −1 for a graphite tube atomizer [17]. As mentioned earlier, by nature, almost all AA techniques are single element techniques. Typically, a different source is needed for each element to be determined, which greatly increases analysis time and cost. In addition, because AAS is a single element technique, larger quantities of sample are needed for multiple determinations of the same sample. Multielement HCLs can be used for some applications but the correct combination of elements may not always be available. Fairly recently, there have been two papers published on commercially available multielement GF instruments [22,23]. Perkin Elmer (PE) manufactures a multielement GFAAS (model # SIMAA 6000) that is capable of simultaneously determining up to six elements. The optical system consists of a high dispersion echelle monochromator in combination with a two-dimensional photodiode array. A prism is used to disperse the light in the second plane onto the array. With this system, only four lamps are held in the lamp turret at one time. Therefore, in order to determine six elements, two multielement lamps must be used. The instrument uses a rather complicated and expensive optical arrangement in order to combine the four source beams and then disperse and detect the radiation in two planes. This added cost and complexity may be appropriate for a GF instrument but it would not be for an inexpensive W coil instrument. Furthermore, an optical arrangement like that described on the PE SIMAA 6000 would prohibit the design of a portable instrument. Another area of focus for multielement AA has
K. A. Wagner et al./Spectrochimica Acta Part B (1998) 1507–1516
been with continuum sources, such as high intensity xenon arc lamps. This arrangement solves the problem of aligning multiple sources through a single atomization source, but there are many inherent difficulties with this technique. The main disadvantage of continuum source AAS, in relation to this project, is that instrument cost is greatly increased, compared to conventional AA because a high-resolution monochromator is needed to provide the required spectral dispersion. Furthermore, the addition of a high dispersion monochromator would also greatly increase instrument size. Once again, increased instrument cost and size would not be appropriate on an inexpensive, portable W coil AA spectrometer.
2. Experimental 2.1. Instrument design This inexpensive, low power, W coil atomic absorption spectrometer uses a novel optical arrangement to gain multielement capabilities. Although the optical system, depicted in Fig. 1, has been developed to utilize three 608 coarse grating beam combiners (Oriel model No. 38111) and up to four HCLs or EDLs, only the use of three lamps is reported for this study. The one-inch square beam combiner, shown in Fig. 2, has triangular, mirrored facets that are able to reflect light that is incident at 608 to the beam combiner normal. Therefore, the output from two lamps can be combined into a single beam. The coarse groove density of 4 lines per mm prevents diffraction of the incident light. This study used a cadmium EDL and a copper HCL placed on one side of the main optics axis and a lead EDL placed on the opposite side. The collimated spectral output from two lamps is mixed by a beam combiner. Similarly, the output from the second pair of lamps, placed on the other side of the main optics axis, is mixed by a different beam combiner. The two combined beams are then mixed by a third beam combiner, which is centrally located on the main optics axis. The third beam combiner directs the output from the four lamps to a fused silica lens (2.5 cm diameter, 15 cm focal length) that focuses the combined output 2 mm over a tungsten coil atomizer. A fused silica lens (2.5 cm diameter,
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5 cm focal length) collects and focuses the radiation onto the entrance slit of a miniature charge-coupled device (CCD) spectrometer. An interesting aspect of this optical design is that lamp position is independent of wavelength. For this reason, any (or all) of the four elements can be replaced without changing the position of the lamp holders. The overall size of the instrument is approximately 80 cm long by 40 cm in width. These dimensions could be reduced by 25% if the lamps and beam combiners were moved closer to each other. The Oriel catalog claims 76% efficiency at 632.8 nm for a single beam combiner, but the final throughput efficiency for this study was only approximately 10%. There are a few reasons for this discrepancy. Due to the fact that two beam combiners were used in this study, the radiation loss is expected to double. Secondly, the aluminum coating used on the beam combiner surface is a less efficient reflector in the lower UV region compared to the stated efficiency value at 632 nm. Also, due to the steep angle of incidence and the relatively small size of the beam combiner (one-inch square), approximately 15% of the incident radiation missed the beam combiner surface, which degraded the efficiency even further. The W coils are obtained from standard 150 W projector bulbs (OSRAM Part No. BRJ or GE Part No. EVB) from which the glass shrouds have been removed, leaving the bulb bases intact. The W coil is housed in the center of a custom-designed glass cell (75 ml internal volume) that is made by Ace Glass (Vineland, NJ. Ace Glass Part No. 7488-383). The cell, shown in Fig. 3, is filled with a shield gas (10% hydrogen in argon) enclosed by a quartz window at each end in order to protect the coil from oxidation. The quartz windows are held in place with #25 nylon Ace Glass cell window holders. The cell is provided with an outer member 10/18 ground glass joint, placed 608 off vertical, for sample introduction. On the bottom center of the cell is a #25 threaded Ace fitting that holds the cell in place on the coil mount. All Ace fittings are sealed with o-rings in order to maintain the reducing atmosphere of 10% H 2 in Argon that protects against coil oxidation [20]. A gas mixture containing 10% H 2 was chosen because that is a mixture which is readily available as a standard welding gas. The W coil mount, shown in Fig. 4, was laboratory
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Fig. 3. Schematic diagram of atomization cell. Fig. 5. Electrical schematic.
designed from a one-inch diameter aluminum rod approximately two inches long. The coil is mounted in a mass-produced ceramic bulb mount that is 19 mm in diameter (Gray Supply Company, Chicago, IN. Part # ORX6350). Replacement of a worn W coil is relatively simple as the coil remains attached to the original bulb base and a factory-produced bulb mount is used. In fact, the bulbs have two prongs on them that slip into the receptacles on the ceramic base, which simplifies coil replacement. The electrical lines and a 3 mm O.D. gas line enter the coil mount through a hole in the side of the Al rod. The gas and electrical lines are then passed through a hole in the center of the mount where they are connected to the ceramic bulb base. Next, the ceramic bulb base is epoxied in place. In addition, the coil mount cavity (used for the gas and electrical lines)
Fig. 4. Schematic diagram of W coil mount.
is filled with epoxy in order to provide an air tight seal. The cell is held in place on the coil mount with a rubber o-ring. The coil mount, including the cell, is then mounted on a standard 12.7 mm diameter optical post and holder (Newport, Fountain Valley, CA, part # SP4 and VPH-4), which allows for easy coil height adjustment. 2.2. Detection and coil control A miniature CCD spectrometer (Ocean Optics model S2000) is used for simultaneous detection of all elements during the absorption process. The CCD covers a spectral range of 200–400 nm. The detector was originally designed to use a fiber optic for light input; but this set-up was discarded due to poor throughput of the fiber optic below 230 nm. In order to support the detector in the optics path for direct transmission of the light, a detector mount was designed and fabricated in the laboratory. Also, a 10 mm slit was factory installed on the detector to provide the necessary resolution because the fiber optic, which typically acts as the entrance slit, could not be used. The CCD is coupled to a personal computer with a 500-kHz, 12-bit A/D data acquisition system. The coil is powered by a small (9 × 16 cm 2) solid state power supply manufactured by Payne Engineering (model 150928). The power supply is computer controlled by a Computer Boards D/A CIO-DAC02 card.
K. A. Wagner et al./Spectrochimica Acta Part B (1998) 1507–1516 Table 1 Perkin Elmer Optima 3000 DV operating parameters Parameter of interest
Value
R.f. power Auxiliary Ar gas flow Nebulizer flow Plasma flow Sample flow rate Wash time Sample read delay time Processing mode Background Replicates
1360 W 0.5 l min −1 0.7 l min −1 15 l min −1 1.60 ml min −1 30 s 50 s area manual point selection 3
Fig. 5 depicts the electrical diagram for the project. A 6.8 Q, 1000 W resistor is put in series with the power supply to step down the power. The electrical system is straightforward and works as follows: first, a low voltage (between 0 and 5 V) is sent to the power supply, via the D/A card, to produce a low power setting to dry the coil. After the drying cycle is complete, the power is increased for the atomization process. At that time, the D/A card sends a voltage to the spectrometer for the data acquisition trigger. An Excel macro is used for data manipulation. A typical atomization cycle is completed as follows: (1) a 20 ml sample is deposited onto the coil with a micro-pipette; (2) 2.4 A (0.40 W) is applied to the coil for 200 s in order to dry the sample; (3) the current is increased to 6.8 A (24 W) (approximately 2500 K) for the atomization cycle. At this point, the spectrometer is automatically triggered and data is collected every 100 ms for approximately 3 s. The coil is held at the atomization current for 10 s to ensure no residue is left on the coil. A constant flow of 10% hydrogen in argon (300 ml min −1) was used as a purge gas for all work. Using these conditions, the coil had a lifetime of approximately 150 shots. It was determined that lower gas flow rates resulted in the analyte atoms having longer residence times in the radiation path. The chosen purge gas flow rate of 300 ml min −1 was sufficient to protect the coil from oxidation while still providing reasonable sensitivity. 2.3. Sample preparation Spex Plasma Standards, 1000 mg l −1 were used to prepare the reference solutions. Dilutions were made with distilled, de-ionized water. Two NIST standard
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reference materials were analyzed: SRM #1566a ‘Oyster Tissue’ and SRM #1645 ‘River Sediment’. As a test of accuracy and precision, the technique was compared to a commercial ICP instrument. The SRMs were first microwave digested for ICP–AES analysis and then analyzed with the W coil instrument. The Oyster Tissue was prepared as follows. A 0.4612 g sample was weighed into a Kohlrausch flask and then treated with 10 ml of trace metal grade nitric acid at room temperature for 1 h. Next, the sample underwent open vessel microwave digestion in a CEM, MDS-2100 microwave. The sample was brought to near dryness and then a second 10 ml aliquot of nitric acid and 5 ml of Ultrapure J.T. Baker 30% hydrogen peroxide were added. The sample was brought to near dryness again, diluted to 100.0 ml with distilled, de-ionized water, filtered through a 0.45 mm filter, and finally stored in a clean centrifuge vial. The River Sediment sample was digested in a closed vessel by a CEM MDS-2100 instrument. This matrix decomposition method was selected in order to achieve acceptable elemental recoveries for the sample. A 0.2125 g sample was weighed directly into the digestion vessel and treated with a 5 ml aliquot of trace metal grade nitric acid at room temperature for 1 h. The sample was then sealed for a closed vessel microwave digestion procedure. The procedure was considered to be complete when a temperature of 1758C was maintained for 30 min. In order to achieve this objective without rupturing the digestion vessel, the River Sediment sample was heated gradually in four discrete steps. The closed vessel was allowed to cool to room temperature at the end of each step and vented to release decomposition gases. The steps employed for this digestion procedure were: 1258C for 20 min, 1258C for 60 min, 1508C for 60 min, and 1758C for 30 min. At the end of the digestion, the sample was quantitatively transferred to a 100 ml volumetric flask and diluted with distilled, de-ionized water to volume. The sample was finally passed through a 0.45 mm filter and stored in a clean centrifuge vial. For comparative purposes, the SRMs were also analyzed with an Optima 3000 DV axial viewed ICP optical emission spectrometer (Perkin Elmer Corp.) equipped with a segmented-array CCD. The ICP was equipped with an AS-90 autosampler. A summary of
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Fig. 6. Hollow cathode lamp emission profiles.
the ICP parameters employed for this investigation is presented in Table 1. As stated earlier, one of the largest benefits of a W coil atomizer is the low power requirements. This would make it possible to produce a low cost, portable instrument that would be appropriate for an on-site screening instrument. This is something that would not be possible with a GF instrument due to its size, weight, and power requirements [21]. Understandably, a portable instrument would be limited to samples with less complex matrices not needing digestion, such as water samples or samples that had been cleaned with an extraction step.
3. Results and discussion 3.1. Background correction Due to its simplicity and the fact that it does not add
extra cost, near-line background correction is a logical choice to be used with a W coil atomizer. Furthermore, because no additional optical components, such as additional sources, choppers, polarizers, magnets, etc., are needed for the near-line method, it would be possible to use this correction method on a low cost, portable instrument. Also worth noting, the source signals are not modulated with the near-line method, as is the case with the Zeeman and continuum methods, therefore the duty cycle is nearly doubled. For the near-line background correction method, a second emission line from the light source is used to correct for nonspecific analyte absorption. A potential failing with the near-line method is that there may not always be a non-absorbing line close to the analytical line. For accurate correction, the background line should be as close as possible to the analytical line because background absorption is often wavelength specific. The only requirement for the background correction line is that it should have very little or no
Table 2 Analytical and background wavelengths
Cadmium Lead Copper
Analytical line (nm)
Background line (nm)
Dl (nm)
228.8 283.3 324.8
Cd 226.5 Pb 280.2 Cd 326.1
2.3 3.1 1.3
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Fig. 7. Three-dimensional cadmium HCL emission profile collected during the atomization of a 20 ml aliquot containing 20 mg l −1 Cd.
sensitivity toward analyte absorption. Meeting these requirements was not a problem with the optical design of the current study because the radiation from three separate line sources was combined into a single beam, which passed over the atomizer then to the detector. The emission profiles for the lamps are shown in Fig. 6. Table 2 lists the analytical and background wavelengths used for each element. Originally, another drawback of near-line background correction was that the analytical and correction lines had to be measured sequentially. This method of measurement often introduced error because experimental conditions could change over the data collection cycle. This was not a problem in the current study since a CCD spectrometer was used. In light of this, the analyte and background correction lines could be measured simultaneously. In short, although the near-line method may not be as accurate or as universally applicable as the Zeeman or continuum correction methods, near-line is a viable option for a W coil atomizer since this type of atomizer would most likely be used on a low cost, portable, screening device. The equations used for calculating the background-corrected absorbance are not given here as they have been discussed previously [21]. Figs 7 and 8 are data plots from the atomization of a 20 ml aliquot of a 20 mg l −1 Cd standard solution. Fig. 7 is a three-dimensional cadmium HCL emission profile and Fig. 8 is a plot of the uncorrected absorbances at the analytical and background lines. The figures clearly show analyte absorption at the 228.8 nm
analytical line, but not at the background line (226.5 nm). The unusual, double humped absorption profile of Cd occurred at all atomization temperatures. This feature did not seem to degrade accuracy or precision. 3.2. System optimization The selected atomization power of 24 W was a slight compromise due to the difference in the volatilities of the three elements. Fig. 9 shows the various absorption profiles that result from this selected atomization power. Use of a lower atomization power resulted in poor absorption profiles for the less volatile elements. During analysis, a constant purge gas (10% hydrogen in argon) flow rate of 300 ml min −1 was used. In general, analyte residence times increased with lower flow rates, which had the effect of producing larger absorption signals. 300 ml min −1 was a high enough flow rate to provide satisfactory coil protection while still allowing reasonable analyte residence times in the radiation path. As mentioned earlier, the combined source beams were focused 2 mm over the W coil. Optimization of coil height in relation to the combined source beams has also been discussed previously [21]. Signal-to-noise (S/N) data for each element was collected for 25 consecutive episodes at varying signal levels. Source intensities were altered by placing different combinations of neutral density filters in the optical path. The slopes of the log mean signal versus
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Fig. 8. Uncorrected absorbance profiles collected at the 228.8 nm Cd analytical line (l) and the 226.5 nm background correction line (B). The data was collected during the atomization of a 20 ml aliquot containing 20 mg l −1 Cd.
log S/N were all approximately 0.5, indicating that the system was primarily shot noise limited. Because shot noise is proportional to (S) 1/2, this suggests that the S=N ratio could be improved by increasing light throughput.
3.3. Analytical figures of merit Table 3 lists the various figures of merit for Cd, Pb, and Cu collected using the above stated conditions. As usual, the total absorbance was calculated by summing
Fig. 9. Absorption profiles that resulted from the atomization of a 20 ml aliquot containing the following: 20 mg l −1 Cd (l), 100 mg l −1 Pb (B), and 80 mg l −1 Cu (O).
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K. A. Wagner et al./Spectrochimica Acta Part B (1998) 1507–1516 Table 3 Figures of merit for Cd, Pb, and Cu. All of the detection limits (DL) are reported for 3j
W coil characteristic mass (pg) GFAAS characteristic mass (pg) a W coil instrument DL (pg) GF instrument DL (pg) a ICP method DL (mg l −1) b W coil LDR (decades) W coil method DL (mg g −1 of oyster tissue) W coil method DL (mg g −1 of river sediment) a b
Cadmium
Lead
Copper
20 0.5 2 0.3 0.4 2.5 0.02 0.05
600 5 120 3 7 2 below DL 3
100 3 20 2 0.9 2 0.2 0.4
Values were taken from Ref. [24]. Actual values obtained with a Perkin Elmer Optima 3000 DV ICP.
the absorbance values over the given episodes. For each case, a 20 ml sample aliquot was deposited onto the coil and three replicates were made. For the purpose of comparison, detection limits (DL) are given for GFAAS (literature values) and the Perkin Elmer Optima ICP (observed values). The instrument DLs reported for GF are superior to those obtained with the W coil atomizer. The most likely reason for this disparity is the difference in the internal volumes of the two atomizers. As mentioned earlier, the internal volume of the W coil atomization cell is 75 ml while the internal volume of a standard PE graphite tube is approximately 0.8 ml. And, of course, smaller atomizer internal volumes permit less diffusion of the atom cloud, which results in better sensitivities. Table 4 represents the results from the NIST standard reference materials (SRM) that were analyzed in order to test the accuracy and precision of the instrumental design and also the W coil atomizer. Lead was not determined in ‘Oyster Tissue’ because the concentration was below the limit of quantitation. The three metals were determined in the SRMs with the standard addition method. Both accuracy and precision were very reasonable and comparable to
values obtained with a Perkin Elmer Optima 3000 DV ICP.
4. Conclusion An inexpensive, multi-element W coil atomic absorption spectrometer has been developed that is capable of combining the radiation from four separate sources. An interesting aspect of the optical design is that the position of the four lamps is independent of wavelength. In other words, any four HCLs or EDLs can be used. Furthermore, even considering the 200 s dry time, the multi-element capabilities of this instrument should decrease the time and cost needed for analysis. The addition of an autosampler could also decrease operating cost and increase precision; although, this addition would likely not be possible on a portable instrument due to the added size and weight. There is also a significant cost and space savings compared to graphite furnace. These savings are due primarily to the reduction in power supply requirements needed for the W coil atomizer. Another aspect worth noting
Table 4 Analysis of NIST Standard Reference Materials SRM Oyster Tissue #1566a
River Sediment #1645
Cadmium Lead Copper Cadmium Lead Copper
Certified values (mg g −1)
W coil values (%RSD)
ICP values (%RSD)
4.15 6 0.38 0.317 6 0.014 66.3 6 4.3 10.2 6 1.5 714 6 28 109 6 19
4.32(3.0) below DL 64.4(1.2) 11.6(2.8) 710(3.7) 128(4.1)
4.3(4.5) below DL 63.9(3.5) 10.1(2.5) 693(2.2) 100(4.3)
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is that a W coil atomizer does not need cooling water, as is the case with a graphite atomizer. Although near-line background correction is not as universally applicable as the Zeeman or continuum correction methods, near-line does offer a few important advantages when considering the fact that a W coil AA spectrometer would likely be used as a low cost screening device that could be made portable. Namely, the added cost, bulk, and complexity of a second radiation source, a magnet, or extra optical components are not needed for the near-line method.
Acknowledgements This work has been supported by grants from NSF (STTR-DMI-9523190; GOALI-CHE-9710218) and Leeman Labs, Inc. The authors would also like to thank R.J. Reynolds Tobacco Company for the use of the Perkin Elmer, Optima ICP.
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