Anal. Chem. 1985, 57,2545-2548
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Standards Prepared by Rotary Evaporation for Analysis of Granular Activated Carbon by Scanning Electron Mic roscopy-En ergy-Dispersive X-ray FIuorescence K a t h e r i n e T. Alben
Wadsworth Center for Laboratories and Research, New York State Department of Health, Albany, New York 12201 Electron excited energy dlsperslve X-ray fluorescence (EDXRF) analysis was lnvestlgated for granular activated carbon (GAC) using standards of powdered Calgon F400 splked at 1-10 mg/g GAC by rotary evaporation of methanollc solutions of the elements of Interest: AI, SI,S, CI, K, Cu, Ti, Mn, Fe, Cu, Zn, and Br. Typical experlmental operatlng condltlons were 20-kV electron beam energy for excitation and 32% llve time for data acqulsltion. Peak lntegratlon was accomplished by dlgltal fllterlng to remove the continuum. Experimental condltlons chosen resulted In satisfactory lnstrumental sensitivity for determlnatlons of S, CI, K, Ca, TI, Mn, Fe, and Cu. Analytical preclslon averaged l 6 % , wlth a range from 10 to 28% for elements at concentrations between 0.5 and 10 mg/g GAC. Detectlon limits based on background count rates and experimental uncertalntles at the 95 % confldence level averaged 0.3 mg/g GAC, wlth a range from 0.23 to 0.58 mg/g for 8, CI, K, Ca, TI, Mn, Fe, and Cu.
Various reports have indicated the potential of energy dispersive X-ray fluorescence methods for multielemental analyses of granular activated carbon (GAC) and similar adsorbent materials encountered in water chemistry and/or treatment processes, including both thin specimens (e.g., filters) and thick specimens (e.g., whole particles and pelletized powders) (1-6). Multielemental analyses of coal particles, whether as polished or pelletized specimens (7-lo),are also of interest since coal is a common feedstock for GAC production. However, quantitative methods of X-ray fluorescence analysis differ according to specimen characteristics, such as thickness, topography, homogeneity, and composition-dependent interactions with the source of excitation (electrons, protons, or X-rays) and/or the X-rays emitted. As a general rule, accurate quantitation of X-ray fluorescence measurements is based on standard reference materials prepared to closely match the matrix and composition of the samples to be analyzed (11). The purpose of this paper is to discuss the use of spiked GAC standards, prepared by rotary evaporation of methanolic solutions of the elements of interest, for quantitative X-ray fluorescence analysis of powdered GAC samples. Given the typical surface area of a commercial GAC, 1100 m2/g for Calgon F400 (12),it can be calculated that deposition of an inorganic on the GAC is favored by a factor of lo5, relative to its deposition on the walls of a vessel used for rotary evaporation. Use of an organic solvent such as methanol should favor transport of an inorganic within the GAC micropores, which are generally regarded as hydrophobic. Analyses of samples from pilot columns at Waterford, NY, and elsewhere indicate that i t would be useful to determine elemental concentrations in the range of 0.1-10 mg/g GAC (13-15). For this paper the primary electron beam (20 keV) of a scanning electron microscope (SEM) is used for excitation of X-ray emission and an energy dispersive X-ray fluorescence spectrometer (EDXRF) i s used for detection. The advantage
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of this instrumentation is its potential for parallel elemental and microscopic analyses. Thus, while this paper describes methods developed for quantitative SEM-EDXRF analysis of powdered GAC specimens, a subsequent paper deals with SEM-EDXRF analyses of mineral inclusions in intact granules (16).For bulk analysis independent of microscopic studies, X-ray tube excited X-ray fluorescence and neutron activation analysis are acknowledged to have the advantage of 100- to 1000-fold lower detection limits based on larger sample sizes and/or reductions in background (2).For microscopic studies, proton-induced X-ray emission is capable of similar detection limits (parts per million), although limited to elements with atomic number 19 (K) or above (17-19),and excluding C1, a key element of interest in water treatment GAC chemistry. Our experience to date indicates that the SEM-EDXRF detection limits are adequate for the elements of interest in GAC specimens. Future work with a broader range of elements (e.g., atomic number above 30) may lead one to choose alternate conditions for X-ray excitation or analysis, but in any case methods developed to prepare spiked GAC standards by rotary evaporation should still apply. EXPERIMENTAL SECTION Materials. Various standards were made from the following reagent-grade chemicals: A1(N03)3.9H20,KSCN, NH2CSNH2, CH3CSNH2, KCN, CaC12.2H20,Tic&, TiBr,, MnC12.4H20, FeC12.4H20, FeC1,v6H20, CuC12.2H20,ZnC12, 1,2,4-trichloroand pentachlorobenzene. benzene, 1,2,4,5-tetrachlorobenzene, The GAC used to make standards was virgin Calgon F400, with the following physical properties (12): source, bituminous coal; sieve classification, 12 x 40; mean particle diameter, 0.9-1.1 mm; total surface area, 105C-1200 m2/g; and pore volume, 0.94 cm3/g. The Calgon F400 was ground for 1 min in a rotary disk mill (Spex Shatterbox) to S50-wm diameter. Preparation of Standards. A weighed amount (2.5 g) of powdered GAC was transferred to a round-bottomed flask (50 mL) and spiked by pipetting an appropriate volume (1-10 mL) of a compound dissolved in methanol to give the desired final concentrations (typically 1-10 mg/g GAC) of the element(s) of interest. More methanol was added as needed to bring the initial volume of the spiked GAC solution to -10 mL. The flask was turned on a Buchler rotary evaporator to wet the GAC, then lowered into a hot water bath (-100 qC) and heated for 10-20 min until the methanol was removed by distillation, leaving the dry spiked GAC. No suitable materials for preparing Si-spiked GAC standards have yet been found. Many commonly available inorganic silicon compounds are not sufficiently soluble in methanol or other low-boiling solvents. GAC standards prepared by rotary evaporation from aqueous solutions, as an alternative, have not been reproducible. Spiked GAC powder (-5 mg) was pressed onto a polished carbon planchet (12.7 mm diameter, 1.6 mm thick), coated with a layer of double faced adhesive; the carbon planchet was similarly attached to an aluminum pin mount (12.7 mm diameter) to facilitate manipulation of the specimen. EDXRF maps of standards did not reveal any elemental inhomogeneities, only the effects of topographic shielding from 50-100 pm GAC particles. In experiments to further improve the quality of GAC specimens for EDXRF analysis, pellets of powdered GAC in. (1.2 cm) in diameter were pressed at 16000 lbs pressure (560 MPa),
0003-2700/85/0357-2545$01.50/0 0 1985 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 13, NOVEMBER 1985
following procedures similar to those for preparation of coal samples (9). Although the pellets were flatter than loosely pressed powders, they had several disadvantages: (i) The powdered GAC had to be mixed with a binder such as methyl cellulose (-4:lO w/w) to keep the pellet intact, but this step effectively diluted the GAC specimen, decreasing the signal for a given time of analysis. (ii) The pellets took time to prepare. (iii) The methyl cellulose used (Mallinckrodt USP) was contaminated with approximately 3.0 mg of C!l/g, Instrumentation. X-ray fluorescence was measured on an ETEC Autoscan SEM combined with a Princeton Gamma-Tech (PGT) LS-15 energy-dispersive X-ray analyzer. The signal collected by the PGT detector (and preamplifier) was amplified (Canberra Model 2021), digitized (Tracor Northern NS 623), and transmitted to a Tracor Northern NS 880 data system interfaced to a PDP 11/05 computer, programmed for multichannel spectral analysis. Quantitative analyses were performed with instrumental operating conditions held constant for standards and samples. Typical operating conditions were as follows: 20-kV electron beam energy; 1.0-nA electron beam current; 1024-channel spectra collected from 0 to 10 kV; 325s live time, 30% dead time; excitation and X-ray takeoff angles, each 45O; Be detector window (7.6 pm X 3 mm X 4 mm), -35 mm from the specimen; sample area analyzed, -1.6 mm2 (magnification 75X), corresponding to -50 pg GAC. A prickly gold standard was used to check energy calibration routinely at 2.12 and 9.71 kV; the zero offset and gain of the EDX spectrometer were readjusted as needed. Data Processing. Data reduction was accomplished with Tracor Northern software designed for the NS 880, specifically Flextran XML (20, 21). Thus the following energy windows (kiloelectronvolts)were defined for each element, to permit peak integration (a) and digital filtering of the underlying continuum (b): A1 (a) 1.35-1.55 (b) 1.24-1.64; Si (a) 1.63-1.89 (b) 1.52-1.98; S (a) 2.19-2.45 (b) 2.10-3.01; C1 (a) 2.46-2.72 (b) 2.37-3.01; K (a) 3.19-3.46 (b) 3.01-4.05; Ca (a) 3.50-3.82 (b) 3.01-3.89; Ti (a) 4.28-4.67 (b) 4.10-5.32; Mn (a) 5.70-6.05 (b) 5.43-6.15; Fe (a) 6.20-6.60 (b) 5.10-7.35; Cu (a) 7.86-8.23 (b) 7.50-9.41; Zn (a) 8.42-8.84 (b) 7.50-9.22; Br (a) 11.70-12.10 (b) 11.52-12.42. Flexan IB, which uses simple linear interpolation of the continuum beneath each peak for background subtraction, was also investigated. A useful test of these integration procedures was analysis of the carbon planchet spectrum, which contains only continuum and instrumental background (traces of copper, which were eliminated for certain combinations of SEM column liner and objective lens aperture). Flextran XML was found to accurately resolve peak areas from background and to integrate to zero when an element was not present: in particular, this was true for analysis of Al, Si, S, and C1 which emit in a region of rapidly changing continuum. In this respect, Flexan IB was unsatisfactory.
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RESULTS AND DISCUSSION Elemental Range of Analysis; Response Factors. Average values for intercepts a. (mg/g) and slopes a1 (mg/g) of standard curves for data fit to C(mg/g) = a. a& are given in Table I, since they are the basis for quantitation; k is the k ratio (0 Ik I1) generated by Flextran XML. However the physical significance of al, as an absolute measure of instrumental sensitivity, is lost through normalization of an unknown spectrum to an arbitrarily chosen standard spectrum for which k = 1.000 by definition. In principal a. should converge to a negative valye corresponding to the residual concentration of an element in unspiked virgin Calgon F400. Likewise, if a nominal 10 mg/g spiked Calgon F400 sample is used as the standard, al should converge to a constant (10 - ao) 1 10.00. Individual values for a, in Table I differ according to actual concentrations of the standards used to calculate k ratios. Experimental values for response factors Rf (mg/g)/ (counts/s) are also given in Table I since they are a measure of abgolute sensitivity. Values for Rf were calculated from Rf = a , / ( A , / t )where t is the analysis time (325 s) and A, is the absolute peak areas (counts) of the Flextran XML reference spectrum corresponding to k = 1. Values for Rf reveal
+
Table I. Coefficients of Standard Curves and Response Factors for Elements Spiked into Calgon F400 Rf, (mg/g)/
element
do, mg/g
a,, mg/g
(counts/s)
@
Ala Alb S
-6.0 -5.2 -10.7 -0.007 -0.004 -1.47 -1.35 -0.64 -0.07 -2.04 -1.89 -0.39 -1.9
15.2 14.6 27.6 10.3 10.2 16.8 7.32 11.3 9.90 8.14 10.5 10.4 5.36
4.01 1.82 0.230 0.159 0.157 0.131 0.093 0.090 0.118 0.120 0.262 0.318 1.81
0.98 0.99 0.97 0.997 0.996 0.99 0.996 0.99 0.998 0.98 0.99 0.99 0.997
c1c
Cld K Ca Ti Mn Fe cu Zne Brf
Ng 1 1
3 21
35 2
30 2 1 2 1 1 1
"20-kV electron beam energy for excitation. 10-kV electron beam energy for excitation. cBased on CaC1, standards only. Based on all C1-containing compounds listed in the Experimental Section. eIntercept includes contribution from Cu K, peak, which must be subtracted out. fRange of standards was high, 6.6-66 mg of Br/g GAC; therefore the accuracy of the 1.9 mg/g intercept is limited. EThe coefficient of correlation r fanges between -1 and +1, where a value of +1 implies a perfect linear fit to the experimental data. Values for do, d,, and 7 are calculated as the average from N separately determined standard curves. Generally each standard curve was based on analyses of samples spiked at five different concentration levels between 0 and 10 mg/g.
Table 11. Composition of Calgon F400 (mg/g) element A1 S
c1
K Ca Ti Mn Fe cu
SEM-EDXRF intercepta averageb 5.2 10.6 f 3.6 n.d. 1.47 f 0.72 1.35 f 0.14 0.64 f 0.17 n.d. 1.6 0.2 n.d.
*
(3.8 7.5 f 1.2 n.d. 0.50 f 0.14 1.10 i 0.11 0.50 f 0.08 n.d. 1.7 f 0.2 n.d.
lit. resultsc 12.3 8.3 0.3 1.6 0.74 0.01
5.0 0.03
"Flextran XML data: intercept ho from Table I minus effective background from carbon planchet blank; mean f standard deviation. Flextran XML data: average of 32 samples minus average background from carbon planchet spectra; mean f standard deviation. cManufacturer's data (22). dn.d. = no positive peak area was found; this is in contrast to the case for A1 where the calculated concentration 3.5 was less than the detection limit, 3.8 (cf. Table 111).
that EDXRF analysis is optimal for midrange elements such as Ca and Ti. Goldstein et al. have discussed in theoretical terms the rationale for adjustments in instrumental operating conditions to optimize analyses for specific elements (11). In our case the use of a 20-keV electron beam for excitation represents a compromise between performing a single analysis for a wide range of elements in GAC samples and performing multiple analyses with experimental conditions optimized for individual elements. Quantative Analysis of Calgon F400. Elemental concentrations calculated for virgin Calgon F400 GAC are given in Table 11. The concentrations were calculated from (i) intercepts of standard curves and (ii) analyses of a large number (n = 37) of replicate virgin Calgon F400 samples, using coefficients of standard curves to convert peak areas to concentrations. This is essentially equivalent to quantitation by the method of standard additions. No detectable concentrations were found for C1 or Mn, or for Cu (once instrumental background was taken into account). A1 was detected, but
ANALYTICAL CHEMISTRY, VOL. 57, NO. 13, NOVEMBER 1985
2547
Table 111. EDXRF Detection Limits
element
Nb, counts
Nb/t, COUntS/S
BEC," mg/g
A1 S
6 955 21 125
K Ca Ti Mn Fe cu Zn Br
26 325 26 650 35 100 34 125 25 350 24 700 16250 15925 12 675
21.4 65 81
85.8 15.0 12.7 10.7
c1
82
108 105 78 76 50 49 39
10'
ob'
1.6 1.4 1.3 0.91 0.91 0.91 0.91
10.0
9.4 9.2 9.1 13.1 15.6 70.6
1.1 1.1
lo2 rde
Cd: mg/g
4.43 3.88 3.60 2.52 2.52 2.52 2.51 3.05 3.05
3.80 0.58 0.46 0.27
0.25 0.24 0.23 0.28 0.40
'BEC is the background equivalent concentration, as defined in ref 4 by BEC = (Nb/t)Rf; Nb/t is the background count rate for Nb counts accumulated in t = 325 s. b b = [xl-ln(Nbl- ii$,)'/(n(n - 1)&)]"' is the experimental relative standard deviation of the average background Nb based on n = 33 separate determinations. r d is the absolute value of the signal-to-background ratio at the detection limit, calculated for paired observations and 95% confidence limits (2 = 1.96): r d = 21/2Zu= 2.77~.Cd is the detection limit calculated from Cd = rdBEC.
-
2,FeC13 nCIZ
V
. a
0
m m
E8
z
0
t
Lz
+ E 4 V
z
0 CONCENTRATION (mg/g GAC) 4
0 0
0
Flgure 1, Precision (relative standard deviations) of EDXRF analyses of GAC. Dashed lines indicate f one standard deviation about mean precision (14), calculated excluding points for K, Ti; data for these elements at these concentrations are only marglnally accessible by the method described. Elements are identified as: A, S; 0, CI; 0 , K; 0,Ca; m, Ti; 8,Fe; 9, Cu.
Figure 2. Standard curves for CI in GAC spiked with various inorganic compounds. Error bars indicate f one standard deviation about the mean value of the k ratio.
in the spectra of all samples analyzed its calculated concentration was below the detection limits calculated for a 95% confidence level (see Table I11 and accompanying discussion). Without a well-characterized GAC standard it is difficult to interpret the accuracy of the EDXRF results in Table 11. Only the manufacturer's data from a composite of 1977 Calgon F400 production lots are available for comparison; those data are not held to be representative of any particular sample or lot, and experimental uncertainties are not reported (22). For the Calgon F400 used to prepare EDXRF standards, we have determined Fe by flame atomic absorption (AA)spectrometry. GAC samples (0.5 g) were ashed in a muffle furnace (550 "C, 1 h) and/or digested in concentrated nitric acid with 7% hydrogen peroxide, then filtered and diluted to 100-mL final volume in deionized HzO. The AA analyses indicated a mean value of 1.6 f 0.3 mg Fe/g GAC which is in good agreement with the EDXRF results in Table 11. Precision for Analysis of GAC Samples and Blanks. Data for precision are plotted in Figure 1for various elements as a function of concentration. Relative standard deviations were calculated for approximately 30 replicate samples, individually prepared and analyzed over a 6-month period. In general the measurements were made on virgin Calgon F400 and therefore include its variation in composition. For C1 and Ca additional results are given, since a similarly large number of analyses were performed over the same time period on Calgon F400 samples spiked with both elements a t varying concentrations. Results for Cu represent the precision of instrumental background for each effective GAC concentration, calculated by using standard curve coefficients for CuGAC concentrations. No results are given for Mn, Zn, and
Br; their concentrations in virgin Calgon F400 are below EDXRF detection limits by the methods described in this paper, and relatively few analyses have been performed on GAC samples spiked with these elements. Reproducibilities of 10 to 28% were obtained for elements at concentrations of 0.5-10 mg/g GAC; the average precision was 16% (Figure 1). Larger uncertainties of -28% were obtained for C1 and K at 0.5-1.0 mg/g GAC. For C1 spiking, a particularly wide range of compounds is available. The C1 curves of GAC standards spiked with a variety of inorganic C1 compounds (Figure 2) showed relatively good agreement: relative standard deviations of the k ratio were 16%)10%)10%)and 12% at 1,2,5, and 10 mg/g concentrations, respectively. The exception was for standards spiked with Tic&, which is more volatile than the other inorganics. The curves for GAC standards prepared with a homologous series of chlorinated benzene compounds and calcium chloride (Figure 3) also showed relatively good agreement: relative standard deviations of the lz ratio were 23%, 11%)6%, and 5% at 1,2,5, and 10 mg/g concentrations, respectively. Qualitatively, the precision of these various C1 calibration curves is the same as that presented in Figure 1. The average precision for continuum fluorescence in all regions of interest was 6.2 1.5%, based on the sample background measured for carbon planchet spectra. Detection Limits. EDXRF detection limits attained under the operating conditions described in this paper are given in Table 111. The methods for calculating EDXRF detection limits, determined primarily by background count rates and analytical precision, are taken from the literature ( 4 , 11, 23-26). As expected (26))detection limits in Table I11 based
K-RATIO
*
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 13, NOVEMBER 1985
Ti, Mn, Fe, and Cu in amounts ranging from 1 to 10 mg/g GAC: standard reference materials can be prepared by rotary evaporation of methanolic solutions containing the elements to be deposited on the GAC.
ACKNOWLEDGMENT W. Samsonoff and I. Green are gratefully acknowledged for maintaining the scanning electron microscope facility. Staff of the NYS Department of Health Routine Inorganic Analytical Laboratory are thanked for comparative atomic absorption analyses of iron, in Calgon F400 digestates.
"O
y
K-
RAT I O Y
-
Figure 3. Standard curves for CI in GAC spiked with three chlorinated benzene compounds and calcium chloride. Error bars indicate f one standard deviation about the mean value of the k ratio. on the experimental precision average 1.7 times larger than detection limits based on the theoretical precision, calculated assuming a Gaussian distribution of background X-ray emission. Reductions in EDXRF detection limits depend on improving analytical precision and increasing the signal-tobackground ratio. An obvious but not necessarily desirable approach is to increase the sampling time t. Since the standard deviation for X-ray emission is proportional to NI12/N,a 2-fold reduction in the standard deviation requires a 4-fold increase in t. In this context it is worth considering the effect of the electron beam on the GAC samples. Biologic specimens,which are rich in low-atomic-number elements such as C and 0, are susceptible to electron beam damage and mass loss a t exposures of 1-20 mC/cm2 (27). For comparison, bulk analyses of the GAC samples described in this paper are performed on sample areas of 1.6 mm2, with a 1-nA electron beam current and a sampling period (325 s, 30% dead time) corresponding to 464 s real time. Thus the exposure is
-caul - - (1 x
10-9 ~ ) ( 4 6 48)
= 0.3 X C/cm2 cm2 (0.0164 cm2) which is well below the threshold for beam damage. Nonetheless an increase in sampling time is regarded as a last resort. An interesting comparison can also be made between the EDXRF C1 standard curves and analyses to determine the TOX content of water samples (28-30). One well-developed TOX procedure uses 40 mg of GAC to adsorb halogenated organics from 100-mL water samples which may contain 5-500 pg of TOX/L. The entire GAC sample is pyrohydrolyzed to reduce organically bound halogen to chloride ion, which is detected by a halogen-specific microcoulometer (31). From an analytical viewpoint the operating range for analysis corresponds to 0.5-50 pg of C1, equivalent to 0.0125-1.25 mg of Cl/g of GAC. The analyses in this paper are based on approximately 100 mg of GAC, of which 50 pg is actually analyzed by the SEM-EDXRF spectrometer. The working range for EDXRF analysis is typically 50-500 ng of C1, for a standard curve from 1 to 10 mg of Cl/g of GAC. Thus while SEM-EDXRF analyses for C1 are appropriate for direct analysis of GAC samples, they are not competitive with pyrohydrolysis-microcoulometry in the direct analysis of water samples. The primary advantage of SEM-EDXRF analysis is as a means of directly determining the multielemental composition of GAC samples.
Registry No. S, 7704-34-9; C1, 7782-50-5; K, 7440-09-7; Ca, 7440-70-2; Ti, 7440-32-6; Mn, 7439-96-5; Fe, 7439-89-6; Cu, 7440-50-8; carbon, 7440-44-0.
LITERATURE CITED (1) Vanderborght, B. M.; VanGrieken, R. E. Anal. Chem. 1977, 49, 311. (2) Vanderborght, B. M.; VanGrleken, R. E. Anal. Chim. Acta 1977, 89, 399 Vanderborght, B. M.; VanGrleken, R. E. I n t . J. Envlron. Anal. Chem. 1978. 5 . 221. Kingston, HT; Pella, P. Anal. Chem. 1981, 53, 223. Weber, W.; Pirbazari, M.; Long, J.; Barton, D. I n "Activated Carbon McGuire, M., Eds.; Ann Arbor Science: Ann Ads6rption"; Suffet, I., Arbor, MI, 1980; Vol. I. pp 317-336. Berry, V.; Annamaiai, V. Microbeam Anal. 1981, 171. Huggins, F.; Kosmack, D.; Huffman, G.; See, R. Scannlng Nectron Microsc. 1980, 1 , 531. Moza, A.; Strickler, D.; Austin, L. Scanning Electron Mlcrosc. 180, 4 , 91. Prather, J.; Guin, J.; Tarres, A. I n "Analytical Methods for Coal and Coal Products"; Karr, Clarence, Ed.; Academic Press: New York, 1979, Voi. 111, pp 357-369. Raymond, R. "Microbeam Analysis"; Newbury, D., Ed.; San Francisco Press: San Francisco, CA, 1979; p 105. Goldstein, J.; Newbury, D.; Echlln, P.; Joy, D.; Fiorl, E.; Lifshln, E. "Scanning Electron Microscopy and X-Ray Microanalysis"; Plenum Press: New York, 1981; Chapters 7-9. Calgon Corp "Filtrasorb 300 and 400 Granular Activated Carbons for Potable Water Treatment"; Activated Carbon Product Bulletin 20-68; Calgon Corp.: Pittsburgh, PA, 1976. Kuhn, W.; Sontheimer, H. Vom Wasser 1973, 41, 65. Quinn, J.; Snoeyink, V. J.-Am. Water Works Assoc. 1980, 72, 483. Aiben, K.; Shpirt, E.; Perrins, N. "Treatment of Water by Granular Activated Carbon"; McGulre, M., Suffet, I. H., Eds.; American Chemical Society: Washington, DC, 1983; Adv. Chem. Ser. No. 202, pp 407-424. Alben, K. T.; Jacobs, I.S. Carbon, in press. Chen, J. R.; Kneis, H.; Martin, B.; Noblllng, R.; Traxei, K.; Chao, E. C. T.; Minkin, J. A. Nucl. Instrm. Methods 1981, 181, 151. Svendsen, L. G.; Hertel, N.; Sorensen, G. Nucl. Insfrm. Methods 1981, 191, 414. Hall, G. S.; Roach, N.; Naumann, M.; Cong, H. Nucl. Instrum. Methods 1984, 8 3 , 431. McCarthy, J.; Schamber, F. "Least Squares Fit with Digital Filter: A Status Report: NBS Special Publication 604, Proceedings of the Workshop on Energy Dispersive X-ray Spectrometry"; National Bureau of Standards: Gaithersburg, MD, April 23-25, 1979. Wodke, N. F.; Schamber, F. "Super ML Operation and Program Description"; Tracor Northern Inc.: Middletown, WI, Jan IO, 1979. Calgon Corp. "Ash and Ash Constituent Analyses of Filtrasorb Carbons"; Calgon Corp: Pittsburgh, PA. August 1978. Currie, L. A. Anal. Chem. 1968, 4 0 , 593. Currie, L. A. I n "X-Ray Fluorescence Analysis of Environmental Samples"; Dzubay, T. G., Ed.; Ann Arbor Science: Ann Arbor, MI, 1977; pp 289-305. Bauer, E. L. "A Statistical Manual for Chemists"; Academic Press; New York. 1971: o 190. Goldstein, 'J. et ai. "A Statistical Manual for Chemists"; Academic Press: New York. 1971; pp 432-433, 435-436. Hecker, J.; Hutchinson, T. I n "Microprobe Analysis of Biological Systems"; Hutchinson, T., Somlyo, A. P., Eds.; Academic Press: New York, 1981, pp 83-100. Kuhn, W. Vom Wasser 1974, 43, 327. Dressman, R.; McFarren. E.; Symons, J. Proc.--AWWA Water Qual. Technol. Conf. 1977 1978, Paper 3A-5. Dressman, R.; Najar, B.; Redzikowski, R. R o c .-A WWA Water Qual. Technol. Conf. 1980 Paper 2A-5. "Total Organic Halide: Interim Method 450.1"; US Environmental Protection agency: Cincinnatl, OH, November 1980.
CONCLUSION Linear standard curves have been obtained for quantitative SEM-EDXRF analysis of GAC specimens for S, C1, K, Ca,
RECEIVED for review March 1, 1984. Resubmitted June 10, 1985. Accepted June 10, 1985.
