Anal. Chem. 1983, 55, 1055-1058
1055
Performance Characteristics of a Continuum-Source Wavelength-Modulated Atomic Absorption Spectrometer J. D. Messman,* M. S. Epsteln, and T. C. Ralns Center for Analytkal Chemistty, National Bureau of Standards, Washington, D.C. 20234
T. C. O’Haver Department of Chemistry, University of Maviand, College Park, Matyland 20742
Characterlstic ooncentratlons, detection Ilmlts, and upper concentration Iiniits for 32 elements have been experimentally determined by (a wavelength-modulated atomic absorptlon spectrometer usiing a continuum source (WM-AAC) and either air-acetylene oir nitrous oxlde-acetylene flames. Characteristic concentiratlons of WM-AAC are about 3-fold poorer than those obtaiined by conventional line-source atomlc absorption (AAL) over the entire range of wavelengths investigated. Detection limlts of WM-AAC are about four times poorer than AAL detection llmlts for those elements wlth analytical lines above 250 nm; WM-AAC detection llmlts are about 20-fold poorer for those elements with analytlcal lines below 250 nm. Upper concentration llmlts are comparable for the two technlques. Factors whlch influence the selectlon of optimal analyllcal wavelengths In WM-AAC are discussed. The determlnatlon of several elements In selected reference materials with ciertified elemental compositlon using elther flame or electrothermal atomizers demonstrated comparable accuracy and preclslon between WM-AAC and backgroundcorrected AAL (AAL-BC).
An atomic absorption spectrometer based on a continuum source and an echelle monochromator modified for wavelength modulation (CE:WM-AA) was first described in 1976 by Zander, O’Haver, and Keliher ( I ) . Modification of the echelle monochromator for wavelength modulation (2) provides dynamic compensation for scatter and broad band absorption and also provides effective discrimination against low-frequency additive noise (e.g., nonanalyte-related flicker cornponents from both the atomizer and source). Such a wavelength-modulated atomic absorption spectrometer using a continuum source (WM-AAC) offers two useful advantages over conventional line-gource atomic absorption (AAL) instrumentation fior spectrochemical analyses: (i) a single primary source provides a rapid and economical means for surveying a large number of elements in a sample without the costly investment in individual hollow cathode or electrodeless discharge lamps normally required, and (ii) the versatile dynamic background correction capabilities inherent in the wavelength moduilation technique ( 3 , 4 )permit more complex samples to be analyzed with minimal time-consuming chemical pretreatment procedures. In recent years, further development of a single-channel WM-AAC system into a fully background-corrected atomic absorption instrument for simultaneous multielement analyses has been demonstrated (5-1 2).
Although charaicteristic concentrations and detection limits have been determined for a few elements using single-channel ( I , 4,13) and multichannel WM-AAC (5,6,12),figure of merit data have not been obtained for a comprehensive list of elements. For those elements which had been characterized in previous WM-AAC studies, analytical lines recommended for conventional AAL were normally used. However, the most
sensitive analytical line for an element may not always be optimal for trace metal determinations in WM-AAC. A less sensitive analytical line for an element situated in a region of the spectrum where the photon flux from the continuum source is more intense may have a higher signal-to-noise ratio (and better detection limit). Indeed this will be the case when the square root of the increase in photon flux from the continuum source is greater than the relative loss in sensitivity when going from the more sensitive to the less sensitive line and when detection is limited by photomultiplier shot noise. Figure 1 readily shows the effect of source intensity on the base line absorbance noise for six elements in the 220-430 nm spectral range, Moreover, the efficiency of the echelle monochromator decreases with decreasing wavelength in the ultraviolet (UV) region and also within each spectral order (14). Consequently the detection limit for an analytical wavelength which appears a t either end of an order will be poorer than if it was positioned in the center of the order. Approximately 100 spectral lines have been investigated in this study by WM-AAC, using an air-acetylene or nitrous oxide-acetylene flame, in an attempt to systematically catalogue figure of merit data for the most useful analytical lines for 32 elements. Characteristic concentrations, detection limits, and upper concentration limits of the optimal analytical lines experimentally determined by WM-AAC are compared to those obtained by conventional AAL. A catalogue of WM-AAC figure of merit data is useful in the event that a spectral line interference is encountered and an alternate line is required to perform a particular analysis. Moreover, in a multichannel system using an echelle polychromator, mechanical restrictions may be encountered in the design of the exit slit cassette which may contain up to 20 exit slits. The overlap of adjacent exit apertures in either the horizontal (wavelength) or vertical (order) direction forces the selection of another analytical line for one of the elements (15). Accuracy and precision for WM-AAC have been evaluated by the determination of several elements in selected reference materials with certified elemental composition. The same reference materials were analyzed by background-corrected line-source atomic absorption (AAL-BC) for comparison purposes. EXPERIMENTAL S E C T I O N Instrumentation. A block diagram of the WM-AAC system is shown in Figure 2. The primary light source is a 300-W high-pressure xenon short-arc continuum lamp (ILC Technology, Sunnyvale, CA) operated at a current of 20 A in the continuous mode. A lamp of this type incorporates an integral parabolic reflector which collimates the light for greater light gathering efficiency. A Spectraspan I11 echelle monochromator (SpectraMetrics, Inc., Andover, MA) was modified for wavelength modulation by mounting a 25 mm X 25 mm X 3 mm Suprasil quartz refractor plate (Amersil, Inc., Sayreville, NJ) on a Model G-300PD optical scanner torque motor (General Scanning, Inc., Watertown, MA) immediately behind the entrance slit. An amplified 80 Hz sinusoidal signal from a function generator (Eico Electronic Instrument Co., Brooklyn, NY) was used to drive the
This article not subject to U S . Copyright. Published 1983 by the American Chemlcal Society
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE 1983
Zn 213 856 nm
Abs I_
Fe 248.327 nm
Mg 285.213 nm
--cu
324.754 nm
Cr
357 869 nm
Ca
422.673 nm
Figure 1. Base line absorbance noise as a function of wavelength in WM-AAC with a xenon short-arc lamp. Time constant Is 1 s.
torque motor. Modulation of the quartz refractor plate over an angle of a few degrees (maximum plate angle of 29O or 0.50 rad for the present system) moves the image of the entrance slit rapidly back and forth across the exit slit. Any spectral curvature within the narrow modulation interval will generate an ac signal which is detected by a Model HR-8 lock-in amplifier (EG&G Princeton Applied Research Corp., Princeton, NJ). The lock-in amplifier receives its reference signal from the function generator and is tuned to detect the second harmonic component (2 F, 160 Hz) of the photosignal. A 28 mm diameter, head-on type photodetector (Hamamatsu Corp., Middlesex, NJ) was used. A R292 (160-650 nm spectral range) photodetector was used for all WM-AAC measurements except for potassium and sodium in which a R372 (185-850 nm spectral range) photodetector was used. Entrance and exit slit widths of 50 pm were used throughout the WM-AAC study. Slit heights of 100 pm were used to reduce order-overlapstray radiation. Optical focusing was accomplished with two quartz lenses (10 cm focal lengths). An electronic analog circuit similar to the one described by Harnly and O’Haver ( 4 ) was used for real-time absorbance readout which continuously corrects multiplicative sensitivity changes generated by nonspecific source attenuation and low-frequency source flicker. The output of the absorbance readout circuit was directed to an analog integrator (16)and to a strip-chart recorder. The photomultiplier signal (vertical input) and wavelength modulation wave form (horizontal input) were combined to give a real-time X-Y oscilloscope display of the absorption spectrum near the analytical line. This permitted direct observation of the analyte absorption profile and any background character within the modulation interval. Either flame or electrothermal atomizers may be used with the WM-AAC system. A HGA-2100 graphite furnace/controller and AS-1 autosampling device (Perkin-Elmer Corp., Norwalk, CT) were used for electrothermal WM-AAC analyses. Either airacetylene or nitrous oxide-acetylene flames were used for flame WM-AAC analyses. The oxidant and fuel flows were monitored and controlled by a gas manifold mixing system (17). A commercial burner assembly (Perkin-Elmer Corp., Norwalk, CT) consisting of a premix burner chamber, flow spoiler, and adjustable pneumatic nebulizer with standard 10-cm and 5-cm single slot burner heads was used for the flame studies. Atomic absorption measurements were also performed on commercial line-source instrumentation (Perkin-Elmer Corp., Norwalk, CT) for comparison purposes. Electrothermal linesource AA measurements were performed on a Model 603 spectrophotometer with a HGA-2200 graphite furnace/controller, temperature ramp accessory, and AS-1 autosampling system. Flame line-source AA measurements were performed on a Model 5000 spectrophotometer using air-acetylene or nitrous oxideacetylene flames. Individual hollow cathode lamps or electrodeless discharge lamps were used for line-source AA measurements. Both line-source AA instruments were equipped with a continuumsource accessory for optional background-corrected AA measurements (AAL-BC). Analysis conditions used for each element were those recommended by the manufacturer. Standardization and Sample Preparation. Working standards were prepared by serial dilution of aqueous stock solutions as described by Dean and Rains (18). The standards were acidified to match the acid type and concentration of those found in the final sample digest. The single standard addition method (19) was used to check for matrix suppression or enhancement
of the analyte signal as determined from the analytical calibration curve. Both WM-AAC and AAL-BC were used to characterize two Standard Reference Materials (SRMs) issued by the National Bureau of Standards (NBS). NBS-SRM 1643a (Trace Elements in Water) was diluted appropriately and analyzed directly with electrothermal atomization. NBS-SRM 1635 (Trace Elements in Subbituminous Coal) was digested with a HN03/HF/HC104 mixture and analyzed for copper by electrothermal atomization. Two different Quality Assurance Standards (Trace Elements in Simulated 8olid Waste Leachates) were also characterized; both sets were diluted appropriately and analyzed directly using flame atomization. In addition, both WM-AAC and AAL-BC were employed in a collaborative effort with the Technical Research Institute and the Center for Standard Reference Materials (Sao Paulo, Brazil) to assist in the characterization of two of their Standard Reference Materials (IPT Standards). IPT Standard 10A (Bronze) and IPT Standard 40 (Brass) were digested with a HC1/HN03 mixture and then analyzed using flame atomization. In all standard and sample preparation procedures, ultrapure acids (NBS subboiling-distilled acids) were used to minimize contamination (20).
RESULTS AND DISCUSSION Characteristic concentrations, detection limits, and upper concentration limits were experimentally determined for 32 elements by flame WM-AAC. The results are shown in Table I. For comparison purposes, the data obtained by AAL (not background corrected) for the same 32 elements are included in the table. Characteristic concentration is defined as the analyte concentration required to produce an absorbance of 0.0044. The detection limit is defined as the analyte concentration which produces a signal of a magnitude three times that of the base line noise (i.e., S / N = 3). The base line noise is calculated as the standard deviation of 20 5-5 integrations while nebulizing a blank solution into the flame. The upper concentration limit is a figure of merit which gives insight into not only the degree of linearity of the calibration curve but also the shape of the curve a t the higher analyte concentrations where bending toward the concentration axis normally occurs. In this study, the upper concentration limit is arbitrarily defined as that analyte concentration at which the slope of the log-log plot (log absorbance vs. log concentration) is equal t o 0.5. Calibration curves were prepared (series of 2, 5, and 10 per concentration decade) up to 100 mg/L for each element. Table I reveals that WM-AAC characteristic concentrations are approximately 3-fold poorer than those by AAL over the entire 200-800 nm spectral region. Above 250 nm, the WMAAC detection limits are approximately 4-fold poorer than AAL detection limits using flame atomization. Below 250 nm, however, the WM-AAC detection limits are approximately 20 times poorer thah AAL detection limits due to the decreasing photon flux from the continuum source (see Figure 1). Because of this limitation, results of preliminary studies involving the determination of As and Se by the present WM-AAC system have not been very promising. However, WM-AAC detection limits can be improved for four elements (Pb, Pd, Ni, and Sn) whose most sensitive analytical lines lie below 250 nm by using less sensitive analytical lines located in more favorable regions of the spectrum where the continuum source is more intense. The analytical working range of WM-AAC and AAL covers 2-4 orders of magnitude. The upper concentration limits of WM-AAC and AAL are approximately equivalent for each of the elements investigated which suggests a similarity in the shapes of the respective calibration plots in the higher analyte concentration region. However, WM-AAC calibration plots are linear over a shorter concentration range compared to AAL because of poorer detection limits. Upper concentration limits frequently are greater than 100 mg/L for the less sensitive elements, using either WM-AAC or AAL.
ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE 1983
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Table I. Flame Concentration Limits (mg/L) in Aqueous Solution by WM-AAC and AAL
element $b
Au Bab
Be Bi
Flame a AA NA AA NA NA AA
wavelength, nm
char concn
WM-AAC detection limit
328.068 309.27 1 (309.284)d 396.152 242.795 553.548 234.861 223.061 (22 2.8 2 5 422.673 228.802 240.725 357.869 324.754 248.327 766.490 670.784 285.213 279.482 313.259 588.995 (589.5 9 2 ) d 232.003 352.454 216.999 283.306 244.791 247.642 340.458 265.945 343.489 217.581 231.147 251.611 224.605 286.333 460.733 214.281 365.350 276.789 318.540 (318.398)d (318. 341)d 213.856
0.24 1.5
0.02 0.5
1.8
0.2 0.5 0.07 0.04
0.79 0.54 0.060 0.71
1
AAL detection limit
upper limit
char concn
20
0.076 0.79
0.009
0.1
> 100
0.24 0.24 0.024 0.40
0.04 0.03 0.003 0.05
>loo >loo 10 >loo
0.003 0.003 0.02 0.009 0.005 0.02 0.005 0.004 0.0004 0.007 0.1
10
0.024 0.031 0.15 0.068 0.081 0.085 0.044 0.040 0.0051 0.049 0.62 0.018
0.001
5
50
0.14
0.02
50
0.17 0.44 0.24 0.19
0.04 0.05 0.03 0.03
1.5 0.21 0.54
0.1 0.03 0.07
2.1 2 .o 3.1 0.061 0.34 1.6 0.56
0.5 1 0.006 0.07 0.2 0.05 0.2
>loo >loo 50 >loo >loo >loo >loo >loo >loo >loo 50 >loo >loo >loo >loo
0.002
10
>loo >loo >loo >loo 10 >loo
upper limit 20
)Id
Mo Na
NA AA AA AA AA AA AA AA AA AA NA AA
Ni
AA
Cab
Cd co
cr
cu Fe K C
Li
Mg Mn
Pb
AA
Pd
AA
Pt Rh Sb
AA AA AA
Si
NA NA
Sn Srb
V
NA AA NA AA NA
Zn
AA
Te Ti
TI
0.056 0.11 0.34 0.14 0.20 0.23 0.10 0.071 0.010 0.080 2.4 0.034
0.008 0.9 0.2 0.02 0.03
0.29
0.3
1.1
0.1
0.44 0.88 0.72 0.96 2.3 2.4 1.4 2.3 6.9 5.1 6.7 0.15 1.4
0.6 0.2 0.6 0.8 0.4 0.9 0.2 2 2 2 7 2 0.02 2
8.1 1.1
0.3
3.4
1
0.048
0.06
1.1
0.1
0.02 0.008 0.003 0.03 0.4 0.005
1
20 10 50 20 50 50 20 20 2 20
>loo
>loo >loo >loo
> 100 > 100 > 100
>loo >loo >loo > 100 >loo >loo >loo 50
> 100 > 100
>loo >loo
10
1.1
0.016
AA, air-acetylene; NA, nitrous oxidle-acetylene. 1000 mg K/L ionization buffer. Unresolved on AAL instrument using manufacturer’s recommended slit setting.
a
The flame sensitivity ratios for different lines of an element are expected to be identical with those for an electrothermal atomizer. This was experimentally verified by determining that the sensitivity ratios of four nickel lines measured with air-acetylene and nitrous oxide-acetylene flames were the same as the sensitivity ratios obtained with a HGA-2100 electrothermal atomizer. It is importaint to alppreciate that the optimal analytical lines for WM-AAC listed in Table I resulted from studies using air-acetylene or nitrous oxide-acetylene flames. From source intensity considerations alone, the same detection limit ratios for different linles of an element are expected to be identical with those when an electrothermal atomizer is employed. However, selection of optimal analytical lines (e.g., for Al, Bi, and Cr) may also be influenced by overlapping rotational absorption lines of molecular species (e.g., OH, CN, etc.) within the modulation interval. The magnitude of this interference (and hence, degradation in detection limit) will depend on the type of atomization cell employed. The A1309.271-nm line is the most sensitive analytical line for aluminum and is recommended for conventional AAL measurements. However, in WM-AAC using a nitrous ox-
1
20 10
20 50 50 50 10 10
5 20
>loo
500 mg Cs/L ionization buffer.
-
ide-acetylene flame, an OH rotational absorption line overlaps the A1 309.271-nm line profile and generates a second harmonic signal whose flicker noise component degrades the detection limit for the A1 309.271-nm line. A %fold improvement in detection limit may be achieved by using the less sensitive A1 396.152-nm line which is free from the OH spectral interference. The greater intensity of the continuum source at this wavelength also contributes to the improvement in detection limit. The OH interference on the A1309.271-nm line does not occur when an electrothermal atomizer purged with an inert gas is used. The A1 396.152-nm line is still recommended for electrothermal WM-AAC measurements based on source intensity considerations although the relative improvement over the 309.271-nm line will be less than it is in the nirous oxide-acetylene flame. A similar situation occurs for Bi in an air-acetylene flame. From source intensity considerations, the Bi 306.772-nm line would be expected to have a better detection limit than the more sensitive 223.061-nm line. However, a spectral interference due to overlap of an OH rotational absorption line with the Bi 306.772-nm line degrades the detection limit at that analytical line. The magnitude of the OH absorption can be
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE 1983 LOCK-IN
Flgure 2. Block diagram of the WM-AAC system: ID,Irk diaphragm: L, and L,, lenses: M, and M,, mirrors: S,,entrance slit; S,, exit slit: QP, quartz plate: EG, echelle grating; P, prism.
reduced by using a fuel-rich flame which causes a slight loss in sensitivity but an overall 2-fold improvement in detection limit. However, the detection limit can be improved only to the point where detection limits for the 223.061-nm line and 306.772-nm line are equivalent (1mg/L). The OH interference on the Bi 306.772-nm line would not be expected to occur in an electrothermal atomizer purged with an inert gas. Therefore, although the Bi 223.061-nm line is optimal for flame WM-AAC measurements, the Bi 306.772-nm line is optimal in WM-AAC using electrothermal atomization. A third case involves selection of the optimal analytical line for Cr in WM-AAC. In a nitrous oxide-acetylene flame, the detection limit for the Cr 357.869-nm line is degraded due to overlap with a CN rotational absorption line. The magnitude of this spectral interference is very dependent upon flame stoichiometry and requires careful control of oxidant and fuel flow rates. The CN interference can be avoided simply by using the slightly less sensitive Cr 359.349-nm line resulting in modest improvement in detection limit. For Cr determinations in an air-acetylene flame, the 357.869-nm line still provides the best detection limit. The Cr 357.869-nm line will also have the best detection limit when an electrothermal atomizer is used assuming argon is used as the inert purge gas since CN absorption will occur when a nitrogen purge gas is used. The most significant factor limiting detection in the present WM-AAC system, particularly at wavelengths below 250 nm, is the intensity of absorbable radiation reaching the photodetector. Detection limits can be improved by using a more intense primary light source or a spectrometer with greater light throughput. It has been shown (21)that a 300-W xenon short-arc lamp of the same type used in this work is as intense as or more intense than conventional hollow cathode lamps over a 193-590 nm wavelength range. However, the light throughput of the echelle monochromator commonly used in continuum-source AA (AAC) work is much lower than that of the monochromators conventionally used in line-source AA (AAL) instruments primarily due to the much smaller slit height of the echelle monochromator. Harnly (IO)has shown experimentally that signal-to-noise ratios may be improved by using the largest available slit heights on the echelle monochromator (500 pm). It has also been demonstrated both theoretically (22) and experimentally (9) that the use of a square-wave modulation wave form rather than the sine wave form used in this work leads to an improvement in detection limits. The relative accuracies of WM-AAC and AAL-BC were assessed by determining several elements in reference materials with certified elemental composition using flame atomization; Fe, Ni, Pb, Sb, Sn, and Zn in I P T Standard 10A (Bronze) and Pb, Sn, and Zn in I P T Standard 40 (Brass). Electrothermal atomization was used in both techniques for the determination of Ag, Cr, Cu, Fe, and Ni in NBS-SRM 1643a (Trace Elements in Water) and Cu in NBS-SRM 1635
(Trace Element in Subbituminous Coal). For these 15 measurements, the average percent relative accuracies were 1.8% and 2.5% for WM-AAC and AAL-BC, respectively. A comparison of the relative precisions of the two techniques was based upon these data as well as additional flame data from comparative determinations of Ca, Cu, Mg, Mn, and Zn in two different Quality Assurance Standards (Trace Elements in Simulated Solid Waste Leachates). For all 25 determinations (based on n I 6 for each element), the average relative standard deviations were 2.8% and 2.0% for WM-AAC and AAL-BC, respectively. We conclude that there are no significant differences in accuracy and precision between the two techniques. ACKNOWLEDGMENT
J. D. Messman gratefully acknowledges the Inorganic Analytical Research Division of the National Bureau of Standards for the opportunity to conduct this research in collaboration with the University of Maryland. Part of this research was conducted while J. D. Messman was employed by the U S . Geological Survey, Branch of Analytical Laboratories, Reston, VA. Special acknowledgment is made to D. W. Golightly of the U.S.G.S. Optical Spectroscopy Project (Reston, VA) for his support during this collaborative period. LITERATURE C I T E D Zander, A. T.; O'Haver, T. C.; Keliher, P. N. Anal. Chem. 1976, 4 8 , 1 166-1 175. Snelleman, W. Spectrochlm. Acta, Part 6 1968, 236,403-41 1. Zander, A. T.; O'Haver, T. C.; Keliher, P. N. Anal. Chem. 1977, 49, 838-842. Harnly, J. M.; O'Haver, T. C. Anal. Chem. 1977, 49,2187-2193. Harnly, J. M.; O'Haver, T. C.; Golden, B.; Wolf, W. R. Anal. Chem. 1979; 57,2007-2014. Harnly, J. M.; O'Haver, T. C. Anal. Chem. 1981, 53, 1291-1298. Harnly, J. M.; Miller-Ihli, N. J.; O'Haver, T. C. J. Autom. Chem. 1982, 4 , 54-80. Mlller-Ihli, N. J.; O'Haver, T. C.; Harnly, J. M. Anal. Chem. 1982, 5 4 , 799-803. Harnly, J. M. Anal. Chem. 1982, 5 4 , 876-879. Harnly, J. M. Anal. Chem. 1982, 5 4 , 1043-1048. Kane, J. S.;Harnly, J. M. Anal. Chlm. Acta 1982, 739,297-305. Harnly, J. M.; Kane, J. S.;Miiler-Ihli, N. J. Appl. Spectrosc. 1982, 36, 637-643. O'Haver, T. C.; Harnly, J. M.; Zander, A. T. Anal. Chem. 1977, 49, 665-688. Zander, A. T.; Keliher, P. N. Appl. Spectrosc. 1979, 33, 499-502. McCarthy, D. "PiasmaLlne"; SpectraMetrics, Inc.: Andover, MA, 1981; Vol. 2, No. 1. Epstein, M. S. Ph.D. Dissertatlon, University of Maryland, College Park, MD, 1978. Dean, J. A.; Bailey, B. W. "Flame Emission and Atomic Absorption Spectrometry"; Dean, J. A., Rains, T. C., Eds.; Marcel Dekker: New York, 1971; Vol 2, Chapter 1, pp 13-14. Dean, J. A.; Ralns, T. C. "Flame Emission and Atomic Absorption Spectrometry"; Dean, J. A., Rains, T. C., Eds.; Marcel Dekker: New York, 1971; Vol 2, Chapter 13. Keyworth, D . A. "Flame Emlssion and Atomic Absorption Spectrometry"; Dean, J. A,, Rains, T. C., Eds.; Marcel Dekker: New York, 1975; Vol 3, Chapter 18, p 449. Kuehner, E. C.; Alvarez, R.; Paulsen, P. P.; Murphy, T. J. Anal. Chem. 1972, 4 4 , 2050-2058. O'Haver, T. C.; Harnly, J. M.; Zander, A. T. Anal. Chem. 1978, 5 0 , 1218-1221. O'Haver, T. C.; Epstein, M. S.; Zander, A. T. Anal. Chem. 1977, 49, 458-461.
RECEIVED for review September 20,1982. Accepted February 10, 1983. This work was presented in part at the 6th Annual Meeting of the Federation of Analytical Chemistry and Spectroscopy Societies, Philadelphia, PA, Sept 16-21, 1979, and at the 179th National Meeting of the American Chemical Society, Houston, TX, March 23-28, 1980. It is based in part upon a dissertation to be submitted to the Graduate School, University of Maryland, College Park, MD, by J. D. Messman, in partial fulfillment of the requirements for the Ph.D. degree in chemistry. In no instance does identification of commercial products by manufacturers' name or label imply endorsement by the National Bureau of Standards nor does it imply that the particular products or equipment identified are necessarily the best available for that purpose.
