ν
TRACE ORGANIC AN Until recently the major emphasis in trace analysis has been in the deter mination of inorganic substances. However, we are now coming to realize t h a t many of our most pressing prob lems require competence in trace or ganic analysis. These analyses are needed to protect our health and our environment, and to ensure the purity and nutritional value of our food. Re cent Federal legislation recognizing these needs includes: the Federal Water Pollution Control Act (1972), the Federal Environmental Pesticide Control Act (1972), the Safe Drinking Water Act (1974), the Food Nutrition al Labeling Act (1975), the Toxic Sub stances Control Act (1976), the Medi cal Devices Amendment (1976), and the pending refinements to the Clini cal Laboratory Improvement Act (1967). T h e enforcement of such legis lation is ultimately based on the abili ty of analysts to accurately identify and quantify trace levels of organic substances in diverse matrices. When compared to trace inorganic analysis, the current state-of-the-art of trace organic analysis appears inad equate. One has to bear in mind, how ever, t h a t trace inorganic analyses are usually limited to a finite number of elements, whereas the number of or ganic compounds of analytical interest is essentially infinite. A number of dif ficulties in trace organic analysis have limited the achievement of accuracy in this area. Lewis (1) and Beyermann (2) have discussed some of these prob lems. T h e purpose of this report is to assess the current state-of-the-art of trace organic analysis and to discuss the problems associated with perform ing such analyses. T h e first section, which discusses the difficulties in achieving accuracy, is intended as an overview for the analytical chemist.
Analysts actually involved in trace or ganic analysis will find the interlaboratory comparison studies in the sec ond section, which deals with the cur rent state-of-the-art, of more interest. Analysts unfamiliar with trace organic analysis will probably find t h a t the discussion of the current state-of-thea r t reveals a somewhat disturbing pic ture. This report is intended to make the analytical chemist, and chemists utilizing the results of organic analy ses, aware of the limitations of these analyses and of the need for research to improve such measurements.
Difficulties in Achieving Accuracy in Trace Organic Analysis An accurate measurement system is one t h a t produces precise numerical values t h a t are free of, or corrected for, all systematic errors (3). T h e achievement of accuracy must be the major consideration in trace organic analysis. While true accuracy has not been realized in trace organic analysis of natural samples, studies of the intercomparability of laboratory data (relative precision) do exist. Relative determinations, although useful for monitoring changes in levels or identi fying particularly high or low values, suffer from two serious disadvantages. First, these relative values are often method dependent; thus, values deter mined by one method may bear little absolute resemblance to those deter mined by another. Secondly, relative values do not suffice when absolute values are required. For example, leg islated environmental standards are generally expressed in absolute quan tities; therefore, unless analytical methods can be validated for accura cy, regulatory decisions will be based on values derived from methods of de-
428 A · ANALYTICAL CHEMISTRY, VOL. 50, NO. 4, APRIL 1978
This article not subject to U.S. Copyright Published 1978 American Chemical Society
Report
H. S. Hertz, W. E. May, S. A. Wise, and S. N. Chesler Institute for Materials Research, Analytical Chemistry Division National Bureau of Standards, Washington, D.C. 20234
batable quality. A notable exception to standards based on absolute values is the High Volume Method for the measurement of suspended particles in the atmosphere (4). This method requires no "true value"; instead, deci sions are based on the response from a specific method written as part of the regulation. Accuracy can be achieved only by identifying all sources of systematic error and by then removing or correct ing for their effects. The typical trace organic analytical scheme consists of several distinct operations, namely: collection, storage, extraction, con centration, isolation, identification, and quantitation. Each of these indi vidual steps possesses potential sources of error which will now be de scribed. Collection. The collection of the sample is often ignored as a significant source of error. This is a serious falla cy, however, since there are two poten tial sources of error that can occur in the sample collection. To assure meaningful results, the subsample se lected must be representative of the system as a whole. An analysis of a nonrepresentative sample is by defini tion an inaccurate measurement. Sam ple contamination due to careless col lection and handling is the second source of error. At trace levels almost any surface with which the sample comes in contact becomes a possible source of contamination, e.g., extractables from plastic containers, residues on glass or metal collection devices, etc. Storage. Chemical processes that occur in the sample during storage, between the time of sample collection and analysis, can invalidate the ana lytical results. Organic compounds are susceptible to processes such as photo-
decomposition, adsorption, vaporiza tion loss, thermal decomposition, mi crobial action, and chemical reaction; consequently, the choice of sample container and the conditions under which the sample is stored are of criti cal importance. Samples being held for extended periods prior to organic analysis should generally be stored in darkness in glass containers and main tained at subzero (°C) temperatures, since these conditions tend to inhibit the processes listed above. At trace levels the loss of analyte due to ad sorption can result in serious inac curacies. Adsorptive losses to the walls of storage vessels have been recently investigated in this laboratory. Exper iments with polynuclear aromatic hy drocarbons (PAH's) in stirred aqueous solutions at the 1 ppb level indicated that losses of ~80% occurred after only 4 h and that losses increased to ~95% after 40 h. The use of freezedrying for sample preservation results in the loss of volatile organic com pounds. Extraction. Extractions are com monly used to remove the analyte components from the sample matrix. Unfortunately, a large number of nonanalyte components are also extracted from the sample matrix. Organic sol vents have been traditionally used as extracting agents, although some re cent methods utilize an inert gas (57). To assure accurate results, the ex traction efficiency for the removal of each organic compound from the sam ple matrix must be determined. This is a difficult task (often impossible) in the case of environmental analyses where hundreds of organic compounds are frequently analyzed simultaneous ly. It is incorrect to assume (as is com monly done) an extraction efficiency of 100% for the removal of organic
compounds from matrices such as bio logical tissue, sediment, or food prod ucts. Even after extraction, unforeseen chemical reactions involving some of the extracted species may occur. Ad sorptive loss of the analyte can also in troduce errors during the extraction step. Another major source of error in trace organic analysis is the existence of impurities in the extracting solvent. Thus, solvents of high purity are a prerequisite for accuracy. Extraction procedures are rarely so selective that nonanalyte organic compounds are not also extracted; therefore, a sample cleanup is usually required to isolate the components of interest. These cleanup procedures generally involve column liquid chromatography (LC) or thin-layer chromatography (TLC). Concentration. After extraction and chromatographic cleanup, the compounds are usually contained in a large volume of solvent requiring concentration prior to analysis. This sample concentration is usually ac complished by evaporation of the sol vent and often results in severe and nonreproducible loss of the volatile components. These losses are difficult to prevent; however, they can be mini mized by use of gentle evaporation procedures and/or a specialized evapo ration apparatus, such as a KudernaDanish concentrator (8). With this ap paratus, solutions can be concentrated from several hundred milliliters to a few milliliters in one step with only minimal losses of the volatile com pounds. Further concentration to ~50 μίι is possible with a micro-KudernaDanish concentrator. Alternatively, the use of organophilic resins, such as XAD-2 (Rohm and Haas, Philadel phia, Pa. 19105) and TENAX-GC (Applied Science Laboratories, State College, Pa. 16801), for removal and
ANALYTICAL CHEMISTRY, VOL. 50, NO. 4, APRIL 1978 · 429 A
Table I. Identification of Peaks in Figure 1 a b c d
e f g h
ι i k I m η 0
Ρ q r s
Figure 1. GC/MS analysis of petroleum hydrocarbon-containing sediment (A) Total ion chromatogram, (B) m/e 43 mass chromatogram, (C) composite ml Θ 142, 156, and 170 mass chromatograms indicating presence of Ci-, C 2 -, and C3-naphthalenes, respectively
concentration of sample components minimizes volatilization losses (5-7). In either case, impurities present in the solvent are magnified by the con centration step, again emphasizing the need for pure extraction solvents. Isolation. After the extraction and the concentration steps, the analyst can still have a multitude of organic compounds t h a t must be separated for identification and quantitation. T h e isolation of individual components is generally accomplished using gas chromatography (GC), high-perfor mance liquid chromatography (HPLC), or thin-layer chromatogra phy (TLC). These chromatographic techniques can also introduce system atic errors due to irreversible adsorp tion, decomposition, chemical reac tion, or incomplete separation (insuf ficient resolution). Identification. After t h e chromato graphic separation the analyst is con fronted with the identification of the individual compounds. Identifications may be difficult not only because of the large number of organic species t h a t may be present, but also because the compounds of interest may be substances for which the chemical na ture is unknown, such as metabolites and decomposition products of drugs and pesticides. In addition, suitable, pure standards to allow positive iden tification are often unavailable. Should the substance of interest be a specific isomer of a compound, differ entiating among several isomers can 430 A ·
C2-cyclohexane 02-φ C2-
1978
V
C 2 -decalin n-C 1 2 Ce-cyclohexane
C 3 -thiophene rt-Cg
w
C-i-naphthalene
X
C 3 -cyclohexane Propyl-φ
^-naphthalene n-C 1 3
ζ
y
aa 03-φ bb 03-φ & C 4 -thiophene cc C 4 -thiophene dd ee n-C 1 0 03-φ ff C4-cyclohexane gg hh CA-Φ C5-thiophene? ii n-Cn ii 03-ψ
Ethylnaphthalene C 2 -naphthalene n-Ci4 & C 2 -naphthalene C 2 -naphthalene C 2 -naphthalene Ethylnaphthalene? "-Cl5 C 3 -naphthalene "-Cl6 n-C 1 7
CA-Φ
kk
Pristane n-C 1 8
C 5 -cyclohexane
II mm
Phytane n-C 1 9
" Cx = alkane containing χ carbon atoms. Cx- = benzene substituted w χ carbon atoms (e.g., C3-0 could be trimethyl-, propyl-, isopropylbenzene etc.). Peaks labeled 1,2,3,4 are internal standards (methyl-C,,, methyl-C,4 methyl-de, and methyl-Cie, respectively). Identifications followed by "?" ar< not definite due to incompletely resolved spectra.
be a formidable task. Obviously, if a compound cannot be identified, the concept of accuracy for t h a t particular analysis has little or no meaning. Quantitation. T h e final step in the analysis, quantitation, usually in volves the electronic manipulation of a signal arising from the presence of the organic compound in the detector of an analytical instrument. Since er rors in this measurement process are not readily apparent (i.e., hidden in side of a black box), they are often ne glected. To ensure accuracy, detectors must be calibrated using appropriate chemical standards, and measurement algorithms must be verified. T h e measurement of and correction for all systematic errors in the analyti cal procedure is a tedious and often impossible process. One way to cir cumvent much of this problem is by the use of an internal standard t h a t should be added to the sample as soon as possible in the analytical scheme. T h e internal standard is assumed to be susceptible to the same systematic errors during the analytical scheme as the compound being determined. By measuring the relative signals for the internal standard and for the un known organic compound, the concen tration of the unknown can be calcu lated. This supposition is valid only if the internal standard exhibits all the chemical and physical properties of the organic compound being deter mined and is present in approximately the same concentration as the com
ANALYTICAL CHEMISTRY, VOL. 50, NO. 4, APRIL
t u
pound of interest. However, internal standards are never liable to exactly the same systematic errors as the com pound being determined. These errors are minimized in the case where the internal standard differs only in its isotopic composition (e.g., 13 CH4 as an internal standard for the determi nation of 12 CH4). Even an isotopic in ternal standard (called an isotope di luant) cannot accurately mimic the behavior of the organic unknown un less the diluant is associated with the sample matrix in exactly the same manner as the compound to be deter mined and no isotope effect is ob served. This is the crux of the problem in achieving accuracy in trace organic analysis. One can use an isotopically labeled internal standard, b u t the question of whether the internal stan dard is bound in the matrix exactly as the compound of interest is the ulti m a t e limit on accuracy in trace organ ic analysis. (This problem is obviated in inorganic analyses by the use of di gestion procedures t h a t completely destroy the matrix prior to elemental analysis.) Current State-of-the-Art In chemical analyses, accuracy is a measure of how close a determined value comes to the "true value". Cur rently, in trace organic analysis an im portant consideration is often whether or not the correct analyte was mea sured. In the determination of organic compounds present at trace levels in
"real world" matrices, accurate qualitative results are often as difficult to obtain as accurate quantitative results. Several of the state-of-the-art techniques utilized in qualitative and quantitative measurement of tracelevel organic compounds are discussed below. Compound Identification. A number of different techniques are currently used for compound identification. A common method for identification is comparison of the chromatographic retention time (retention volu m e in LC, R{ in T L C ) of a standard with t h a t of the unknown under identical chromatographic conditions. However, incorrect identification may result using this method, since several compounds may coelute. Greater confidence with respect to qualitative accuracy can be achieved by utilizing: selectivity in the isolation and cleanup step, chromatography on two or more columns (or systems) utilizing different separation mechanisms, and highly selective chromatographic detectors. If more definitive qualitative information is necessary or if a standard for comparison is unavailable, more sophisticated means of identification are required. Spectroscopic analysis provides detailed information about an organic compound. Infrared (IR) and nuclear magnetic resonance spectroscopy (NMR) are widely used in qualitative organic analysis. However, even with Fourier transform systems to enhance signal-to-noise ratios, detectability limits still preclude the use of these spectroscopic techniques for most trace analyses. Ultraviolet spectroscopy (UV) has sufficient sensitivity, but it is only of limited value in deducing the molecular structure of an unknown compound. T h e mass spectrometer provides sufficient sensitivity for trace analysis, and it is easily interfaced to a gas chromatograph. Gas chromatography/ mass spectrometry (GC/MS) can provide qualitative information with nanogram amounts of single compounds present in the sample. Combined gas chromatography/mass spectrometry is currently the most powerful technique for the identification of trace levels of organic compounds. In addition to providing a mass spectrum of each peak eluting from the GC, G C / M S data can be plotted in the form of mass chromatograms as an additional interpretive aid. Such computer-generated, mass-specific gas chromatograms can be used in locating particular compounds or classes of compounds containing an indicative mass in their mass spectra. These capabilities of the G C / M S technique are illustrated in Figure 1 for a sedim e n t sample with a low-level petroleum hydrocarbon burden. T h e total ion
Figure 2. Left: liquid chromatograms of a sediment extract using UV and fluorescence detection. Right: fluorescence emission spectra obtained on peaks a (chrysene) and b-g (alkylated chrysenes), with wavelengths (nm) of maxima indicated
chromatogram (reconstructed gas chromatogram) for the sediment sample is presented in Figure 1A (peaks identified in Table I). A peak a t mass 43 (or more correctly mass-to-charge ratio, m/e, 43) is indicative of aliphatic hydrocarbons. In Figure I B the mass chromatogram for m/e 43 helps to identify the homologous series, which could not have been otherwise visually identified in the complex chromatogram. By plotting mass chromatograms at other m/e ratios, additional components can be resolved easily from the complex chromatogram. An example is shown in Figure I C which contains the composite mass chromatograms for m/e 142,156, and 170, the molecular ions indicating the presence of Ci-, C2-, and C3-substituted naphthalenes, respectively. A detailed description of the use of the mass spectrometer as a GC detector is presented in a recent review by Fenselau (9). Several laboratories are now developing liquid chromatography/ mass spectrometry instrumentation (10-12). T h e combination of LC and M S opens up new horizons in trace organic analysis for the identification of polar, less volatile, or thermally labile compounds. For certain classes of compounds, fluorescence emission spectroscopy has also proved to be a powerful qualitative tool. We have employed an HPLC/fluorescence technique to analyze trace levels (Mg/kg) of polynuclear aromatic hydrocarbons (PAH's) in marine sediments and tissue extracts. Fluorescence spectroscopy offers se-
lectivity since it is possible to vary both the excitation wavelength and the wavelength at which the emission is observed, thus providing additional spectrometric information. Fluorescence excitation and emission spectra provide useful qualitative information. T h e qualitative capabilities of H P L C with fluorescence detection are illustrated in Figure 2. T h e liquid chromatograms in Figure 2 were recorded simultaneously by ultraviolet (UV) absorption and fluorescence emission. Fluorescence emission spectra obtained for each peak and chromatographic retention d a t a were the basis for tentative compound identification. In addition to fluorescence emission spectroscopy, there are several other promising approaches to selectivity in H P L C detection. A detailed discussion of some of these newer liquid chromatographic detection techniques has been presented in a recent review by Wise and May (13). Selective detectors for gas chromatography have been discussed by David (14). Each of the identification techniques discussed above has advantages and limitations; therefore, a combination of techniques is often required to achieve positive identification of organic compounds. Analyte Quantitation. Most trace organic analyses involve a chromatographic technique for the final quantitative determination. Gas chromatography is still the most widely used technique in trace organic analysis. However, recent advances in H P L C
ANALYTICAL CHEMISTRY, VOL. 50, NO. 4, APRIL 1978 · 431 A
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Table II. Chromatographic Analysis of Mg/g-Level Solutions NBSav rel error for S compounds,
All labs combined av rel error for 5 compounds, %
Sample
Method
NO. of labs reporting
1st Hexane solution Of PAH's
LC GC
4 6
1.8 2.2
9.6 11.0
2nd Hexane solution ol PAH's
LC GC
4 6
2.1 2.0
12.3 10.4
Aqueous solution of phenols
LC GC
4 6
1.8 3.5
12.2 16.3
technology have made H P L C compa rable to GC in speed, convenience, and efficiency (15). GC and H P L C com plement each other in t h a t each tech nique is better suited for different classes of compounds. T o begin assessing the accuracy cur rently obtainable in trace organic analyses, we have conducted a series of collaborative studies on samples containing environmentally signifi cant molecules. P u r e specimens of the compounds selected for analysis were sent to the participating laboratories. In addition, each laboratory received a solution containing all of these com pounds at concentrations ranging from 1 to 100 μg/mL. At these concen trations there is sufficient sample present in a few microliters of solution to permit direct instrumental analysis. These studies were thus assessing the accuracy and precision currently achieved in the final step (the quanti tative measurement step) of the ana lytical scheme presented above. T h e first study involved a solution of five polynuclear aromatic hydrocarbons (PAH's) in hexane. T h e second study
%
was similar to the first, b u t the com pounds were present at different con centrations. In a third study aqueous solutions containing five phenols were analyzed. Table II contains a summa ry of the results obtained in the three studies. Between the first and second study there was a meeting of represen tatives of the participating laborato ries to discuss the results, methods used, and problems encountered with the first sample. This discussion of methods and problems did not im prove the overall results for the second P A H study. Table III contains, in somewhat more detail, a summary of the results obtained in the study on phenols. T h e intra laboratory relative standard de viation for a single compound for these analyses varied between ±0.2 and ±6%, b u t the overall mterlaboratory relative standard deviation was greater than ±20% for three of the five phenolic compounds. Tables II and III indicate t h a t a substantial error is often introduced in the step t h a t or ganic analysts take for granted: the chromatographic quantitation.
Table III. Study of Quantitative Analysis of Phenols in Water Phenol
p-Cresol
o-Cresol2-Naphthol
2,4,6Trlmethylphenol
10
9
9
9
10
Gravimetric value (μβ/g)
21.0
54.3
34.3
71.4
38.8
Av value reported
18.6
49.0
32.1
66.9
37.4
Max value reported
26.0
57.5
36.0
106.3
56.1
Mln value reported
11.7
35.5
25.7
12.9
17.0
No. of labs reporting
±4.4(24%)a ±7.2(15%) ±3.5(11%) ±24.2(36%) SD -11% -6% -6% % Deviation of -10% a v from gravimetric value " Numbers in parentheses represent relative standard deviations.
432 A · ANALYTICAL CHEMISTRY, VOL. 50, NO. 4, APRIL 1978
±10.8(29%) -4%
Table IV. 1973 AOAC Collaborati ive Assays of Vitamin A Av IU/lb
Range IU/lb
Breakfast cereal
36 000
24 800-42 000
Feed (trace carotenolds)
19 000
A r~ τ / " \ / " \
Sample
Λ4
Coeff var, %
No. of labs*
13
21 (20)
8
21(18)
ο/Λ/Λ
Feed (normal carotenolds)
6 800
15 700-21 300 4 100-8 500
15
21(17)
Feed (trace carotenolds)
6 800
5 100-8 800
13
21 (16)
Liquid feed supplement
17 200
12 400-21 200
12
19(17)
Beverage powder
29 600
25 100-36 300
10
16(15)
* Values in parentheses represent number of laboratori es whose results were usee I in calculating the data listed; the remainder were outside statistical limits.
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T h e second stage in assessing the accuracy obtainable in trace organic analysis is the analysis of spiked natu ral samples. Table IV contains a sum mary of the results of a 1973 Associa tion of Official Analytical Chemists (AOAC) collaborative study in which vitamin A was added to six different food products and assayed by stan dard methods in several laboratories (16). Interlaboratory averages, overall ranges, coefficients of variation, and the number of laboratories reporting data are listed. T h e interlaboratory coefficients of variation were consid ered acceptable for vitamin A analysis, and the averages were reasonably close to the amounts of vitamin A added. T h e results reported, however, do not represent all laboratories t h a t participated, since data from some laboratories were eliminated as out liers in the statistical evaluation (see Table IV). Furthermore, the wide ranges of results indicate the difficulty experienced in achieving accurate trace organic analyses. T h e third and final stage of assess ing the current state-of-the-art of trace organic analysis is through inter laboratory comparison on natural ("real-world") samples. In the analysis of "real-world" samples, it is currently impossible to assess absolute accuracy, since extractions are often incomplete and since the matrix cannot be com pletely destroyed to release the analytes totally as in trace element analy ses. However, the interlaboratory pre cision gives an indication of the need for improved methodology in trace or ganic analysis. Partial results of a col laborative study on a natural sample are summarized in Table V (17). Lab oratories were supplied an oyster tis sue homogenate and were asked to an alyze for organochlorine compounds. Homogeneity studies of the samples, carried out at the International Labo ratory of Marine Radioactivity, showed less t h a n ±15% relative stan dard deviations for all compounds. T h u s , the range of values reported in Table V is not the result of sample inhomogeneity but indicates t h a t the state-of-the-art of trace organic analy-
CIRCLE 6 ON READER SERVICE CARD 4 3 4 A · ANALYTICAL CHEMISTRY, VOL. 5 0 , NO. 4, APRIL
1978
Table V. Organochlorine Compound Concentrations in Oyster Tissue Homogenate [Values Reported in ng/g (ppb) Dry Weight] σ BHC·
No. labs reporting Av value Max value Mln value SD
yBHC "
10
21
14
10 66
22 139
15 52
0.15
0.93
0.5
±20 (200%)"
±32 (140%)
±13 (87%)
•BHC = 1,2,3.4,5,6-hexachlorocyclohexane. Numbers in parentheses represent relative standard deviations. 6
sis of "real-world" samples, such as this oyster homogenate, requires con siderable improvement. Conclusions T o perform accurate and precise an alytical measurements, there are two prerequisites: suitable analytical methods and standards for quality control or calibration purposes. In most current problem areas in trace organic analysis, methods are still in adequate, a n d primary trace level standards in "real-world" matrices are nonexistent. A major goal of trace or ganic analysis at the National Bureau of S t a n d a r d s is t h e development of appropriate Standard Reference Ma terials (SRM's) for trace level organics in natural matrices. Cali (3, 18) re cently discussed SRM's and t h e re quirements for certification of such materials. T h e definition of an S R M requires the use of one of three modes of measurement to assure t h e accuracy of the value(s) of the S R M property(ies). These modes are: (a) defin itive methods, (b) reference methods, or (c) two or more independent and reliable methods. T h e current stateof-the-art of trace organic analysis does not permit the issuance of an S R M based on this definition. Defini tive methods in this area do not exist.
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In addition, a major obstacle to the development of an organic S R M is the lack of a means of matrix-free analy sis. At present the analyst involved in trace organic analysis must satisfy himself with assuring the accuracy of his measurements on "synthetic" sam ples and with improving the precision of his measurements on "real-world" samples through the use of internal standards and careful monitoring of the analytical system blanks. T h e intricacies of trace organic analysis are only now beginning to in trigue the analytical chemist. It is an ticipated t h a t the next decade will bring the challenges of trace analysis to t h e organic analyst, in the same fashion as the last decade saw these challenges being met in inorganic trace analysis. T o provide a forum for an exchange of ideas and problems in Trace organic analysis, the National Bureau of S t a n d a r d s is sponsoring its Ninth Materials Research Symposium on April 10-13,1978. T h e topic of the symposium is"Trace Organic Analysis: A New Frontier in Analytical Chemis try". Acknowledgment
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T h e authors gratefully acknowledge R. S. Schaffer and W. H. Kirchhoff for critical reading of the manuscript. References (1) R. G. Lewis, in "Accuracy in Trace Analysis: Sampling, Sample Handling, Analysis", NBS Special Publ. 422, ρ 9, GPO, Washington, D.C., 1976. (2) K. Beyermann, Angew. Chem. Int. Ed., 13, 224 (1974). (3) J. P. Cali and W. P. Reed, NBS Special Publ. 422, ρ 41, GPO, Washington, D.C., 1976.
(4) "National Primary and Secondary Am bient Air Quality Standards", Fed. Regist., 36, No. 8A (1971). (5) A. Zlatkis, H. Lichtenstein, and A. Tishbee, Chromatographic!, 6, 67 (1973). (6) W. E. May, S. N. Chester, S. P. Cram, B. H. Gump, H. S. Hertz, D. P. Enagonio, and S. M. Dyszel, J. Chromatogr. Sci., 13, 535 (1975). (7) T. A. Bellar and J. J. Lichtenberg, Water Treat. Êxam., 23,34 (1974). (8) F. A. Gunther, R. C. Blinn, M. J. Kolbezen, J. H. Berkley, W. D. Harris, and H. S. Simon, Anal. Chem., 23,1835 (1951). (9) C. Fenselau, ibid., 49, 563A (1977). (10) R.P.W. Scott, C G. Scott, M. Monroe, and J. Hess, Jr., J. Chromatogr., 99, 395 (1974). (11) P. J. Arpino, B. G. Dawkins, and F. W. McLafferty, J. Chromatogr. Sci., 12, 574 (1974). (12) E. C Horning, D. C. Carroll, I. Dzidic, K. D. Haegele, M. G. Horning, and R. N. Stillwell, ibid., ρ 725. (13) S. A. Wise and W. E. May, Res. Dev., 28,54 (1977). (14) D. J. David, "Gas Chromatographic Detectors", Wiley-Interscience, New York, N.Y., 1974. (15) R. E. Majors, J. Chromatogr. Sci., 15, 334 (1977). (16) Reprinted from E. DeRitter, Cereal Foods World, 20, 34 (1975) with permis sion of American Assoc, of Cereal Chem istry, Inc. (17) Printed with permission of D. Elder, from Progress Report No. 1, "Intercalibration of Organochlorine Compound Measurements in Marine Environmental Samples", International Laboratory of Marine Radioactivity, Monaco. (18) J. P. Cali, Anal. Chem., 48,802A (1977).
Some of the research reported herein was sup ported by the Division of Biomedical and Envi ronmental Research of the Energy Research De velopment Administration. Identification of any commercial product does not imply endorsement by the National Bureau of Standards, nor does it imply that the particular product is necessarily the best available for that purpose.
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From left to right: Willie E. May, S t e p h e n N . Chesler, S t e p h e n A. Wise, and Harry S. Hertz. T h e authors are research chemists in the Trace Organic Analy sis Group of the Analytical Chemistry Division at the National Bureau of Stan dards. Their current research interests are in the area of environmental analyti cal chemistry, with special emphasis on toxic organic compounds related to off shore oil development, coal liquefaction, and oil shale processing. Research in these areas currently focuses on development of improved instrumentation and methodology for trace organic analysis in organic mass spectrometry, liquid chromatography, and gas chromatography.
CIRCLE 208 ON READER SERVICE CARD 436 A . ANALYTICAL CHEMISTRY, VOL. 50, NO. 4, APRIL 1978
