Anal. Chem. 1981, 53, 223-227
Table VII. Comparison of Different Methods from Noisy Simulated Data Corresponding to Mixed First- and m-Order (Input Conditions: k,' = k,' = 1 )"
-noise = 0.5b 12,'
k,'
m = 1.5 1.003 0.996 0.979 1.028 m=2 1.002 0.996 0.981 1.030 1.001 0.999 1.008 0.963
this work Toby this work Toby ShankC Kelter and Camd
noise =
5b
kl'
km'
1.027 0.718
0,960 1,365
1.018 0.868 0.981
0.963 1.215 1.047
-
Noise = ([AJ, [AJN)/[A], = 0 . 8 0 ; N = 200 points. lOOe,,. Shank's method is based on plotting y = 1/ [A]jf vs. x = l/[A]j. Although the variances on the two a
variables are of the same order of magnitude and are not constant, these rate constants are obtained by using the
ordinary least-squares treatment, ignoring the weighting factors which would be needed for rigorous calculations. No significant differences from the graphical procedure were observed. From ref 8 (180 points). ~
~~
~
carried out calculations for m = 1.5 and m = 2 by using the two methods (input values: kl' = k,'= 1). As shown in Table VII, Toby's method is unsuitable for accurate determination of rate constants from very noisy data and can even lead to significant errors for a noise level as low as 0.5%. This is due to the very small variations with K of the curvature of the
223
drawings obtained by plotting the left-hand term of eq 3 vs. time and consequently to unsignificant variations in the correlation coefficient r used to test the linearity (e.g., the following r values were observed for different arbitrary K input values with m = 2 and emax = 0.005: K = 1.1, r = 0.999933; K = 1.05, r = 0.9999345; K = 1.0, r = 0.9999344). Moreover, taking 2 as a goodness-of-fit parameter does not improve the method. For m = 2, the results obtained here have also been compared with those deduced by using the methods of Shank and Kelter and Cam,which deal with this case only. As also shown in Table VII, accuracies of the calculations are of the same range of magnitude.
LITERATURE CITED (1) Toby. 6. H.; Toby, F. S.; Toby, S. Int. J. Chem. Kinet. 1078, 70, 417-422. 12) . . Linschitz. H.: Sarkanen. K. J. Am. Chem. SOC. 1058. 80. 4826-4832. (3) Fletcher, R.; Powell, M. J. D. Comput. J. 1083, 6 , 163-168. (4) Gorman, D. S.; Connoly, J. S. Int. J . Chem. Kinet. 1073, 5 , ~ - .7.7-4- -8.9 (5) Shank, N. E.; Dorfman, L. M. J. Chem. Phys. 1070, 52, 4441-4447. (6) Guggenheim, E. A. Philos. Mag. 1026, 2 , 538-543. (7) Kelter, P. B.;Carr, J. D. Anal. Chem. 1070, 57, 1828-1834. (8) Swinbourne, E. S. J. Chem. Soc. 1960, 2371-2372. (9) Shank, N. E. Int. J. Chem. Kinet. 1073, 5 , 577-582. (10) Niebergall, P. J.; Suglta, E. T. J. Pharm. Scl. 1068, 57, 1805-1808. (11) Draper, N.; Smlth, H. "Applled Regression Analysis"; Wlley: New York, 1966; pp 95-99. (12) Ellerton, R. W.; Wayne, E. W. Anal. Chem. 1080, 52, 773-774.
RECEIVED for review July 7,1980. Accepted October 28,1980.
Preconcentration of Trace Metals in Environmental and Biological Samples by Cation Exchange Resin Filters for X-ray Spectrometry H. Kingston* and P. A. Pella U.S. Depatfment of Commerce, National Bureau of Standards, Washington, D.C. 20234
A preconcentration method is descrlbed for the X-ray spectrometric analysis of several trace elements In seawater, NBS-SRM 1648 urban particulate, NBS-SRM 1632 trace elements in coal, and nickel in urlne at concentrations as low as 1 ppb. The elements in the coal and urban partlculate samples were loaded quantitatively on catlon exchange resin fllters and subsequently analyzed by energy-dispersive X-ray fluorescence spectrometry using secondary targets for monochromatic excitation of the fllter sample. Prior to the analysis of seawater and urine the trace elements from the matrlx were separated wlth a chelatlng resin. Comparison of the results obtained with NBS-SRM certificate values and/or those of other workers indicated agreement withln &lo%. Detectlon and quantitation llmlts for thls preconcentratlon method are also presented.
The application of X-ray fluorescence (XRF) spectrometry to the analysis of environmental and biological samples for trace elements demands considerable sample pretreatment and preconcentration. Ideally, it is desirable to prepare the sample in a form for analysis so that interelement X-ray absorption and/or enhancement effects are negligible and
thereby the lowest possible detection limits for the elements of interest are achieved. Analytes which can be deposited in a thin layer on suitable substrates can meet these requirements. Preconcentration has been accomplished through the use of ion-exchange resin-loaded filter papers (1-I2),or by coprecipitation of the analyte elements with chelating agents followed by collection on membrane filters (13-16). Campbell et al. (I) first characterized ion-exchange resin-loaded filter papers for preconcentrating microgram quantities of cations and anions for X-ray fluorescence analysis. Since the introduction of this technique, it was generally recognized that one major drawback limiting its application was the relatively low loading capacity of the filters. Because of this factor it is useful to separate samples to be analyzed into two categories. The first and more widely studied category includes samples wherein the major constituents do not contribute to the ionic loading of the filters [e.g., trace metals in freshwater (2-5) and reactor water (6),molybdenum, nickel, and vanadium in petroleum (7, a), trace metals in terephthalic acid (9), tin in hydrofluoric acid (IO),and 15 rare earth metals in perchloric, hydrochloric, and nitric acids (11)]. The second category consists of samples wherein the concentration of the major constituents is so high that quantitative retention of the trace metals on the filters is not possible because of the competition
This article not subject to US. Copyrlght. Published 1981 by the American Chemical Society
224
ANALYTICAL CHEMISTRY, VOL. 53, NO. 2, FEBRUARY 1981
for resin sites. In such cases, a prior separation of the trace elements from the major constituents is necessary (12). The trace analysis of seawater presents a particular challenge because the ratio of major cations (Na, K, Ca, and Mg) exceeds the trace element content by a factor of lo9. Applications of Chelex-100 resin for trace metal preconcentration from seawater have been reviewed by Riley and Skirrow (17). Kingston e t al. (18) have recently reported on the separation of eight trace transition elements in seawater from sodium, potassium, calcium, and magnesium followed by graphite furnace atomic absorption spectrometric determination. In the XRF analysis of biological samples such as urine (19, 20), similar matrix problems are apparent. Here the elements of interest can be bound within organic structures including blood cells in a high ionic strength matrix. The present work describes the application of the SA-2 resin-loaded filter technique to the XRF analysis of selected elements in a variety of samples at concentration levels as low as 1ppb. The samples include seawater (Chesapeake Bay), NBS-SRM 1648 urban particulate, NBS-SRM 1632 trace elements in coal, and urine. We have used the Chelex-100 separation scheme developed by one of us (18) together with the SA-2 filters for the X-ray fluorescence analysis of seawater. For removal of trace elements from the urine matrix, the above Chelex-100 separation method was also investigated. To determine the feasibility of this approach, we have measured the nickel concentration in urine specimens collected from workers exposed to nickel in the work place. The urban particulate and coal analyte solutions were loaded directly on the filters with no prior separation. The analytical results reported herein are compared to NBS certificate values and/or those of other workers where certified values are not available. Limits of detection and quantitation were also calculated to characterize the sensitivity of this method using an energy-dispersive X-ray spectrometer with secondary target X-ray excitation.
Flgure 1. Modified filter holder assembly: (a) column, (b) screw closure, (c) 47 mm dlameter filter, (d) filter base, (e)polyethylene insert, (f) Teflon tube, (9) Teflon beaker, (h) bell jar.
EXPERIMENTAL SECTION Reagents. All reagents such as water and perchloric, nitric, hydrochloric, hydrofluoric, and acetic acids were of high purity prepared as described previously (21). High-purity ammonium hydroxide was prepared by bubbling filtered ammonia gas into water to produce a saturated solution. All reagent preparations, sample pretreatment, and sample loading operations on SA-2 filters were performed in a class 100 clean air laboratory. Standard solutions for preparing single element standards on SA-2 filters were prepared with high-purity (at least 99.99%) metals. Storage and stability of solutions of this type in Teflon ware have been described (22). Apparatus. Teflon ware was used in all phases of chemical pretreatment of the sample and precleaned as described (23). SA-2 cation exchange resin-loaded filters, 4.7 cm in diameter, Lot No. 6378-1 (Whatman, Inc., Clifton, NJ), were used. The filtering apparatus used for mounting the SA-2 filters was a modified Bio-Rad holder (Bio-Rad Laboratories, Richmond, CA). The outlet and vacuum sealing connector were removed and fitted with a Teflon (FEP) tube of sufficient length to extend into the collection vessel 3-5 mm from the bottom (see Figure 1). This modification prevented splashing and loss of solution due to dead volume in the lower portion of the collection vessel. X-ray Spectrometer. The energy-dispersive X-ray spectrometer has been described elsewhere (24). Various secondary target emitters were used for excitation of the filter sample and consisted of Ti, Ni, Zn, and Mo. The total count rates were adjusted to 100&3000 counts/s by varying the W-tube parameters (kV, mA). The live time counting intervals ranged from 30 min to 1.4 h per filter sample (i.e., one side). A reduced pressure of 70 Pa (0.5 mmHg) was maintained for the analysis of elements below atomic number 24. Measurement of the X-ray intensities was performed on both sides of the filter sample within selected energy regions distributed symmetrically about the full width at half-maximum of each X-ray line. The gross average X-ray in-
tensity for each analyte was computed from these measurements. Background intensities were determined from precleaned SA-2 filters. To obtain accurate background intensity measurements for manganese in the NBS-SRM urban particulate and coal samples, we used precleaned filters loaded with iron at approximately the same level as in the sample. This was performed because of the large background contribution from the iron X-ray. Sample Preparation. Seawater (Chesapeake Bay). Four 1-L samples of seawater were processed for removal of Na, K, Ca, and Mg from the trace elements as described (18). The trace metals were eluted from the column with 8-10 mL of 2.5 M HNOBinto a 100-mL Teflon beaker. Several drops of 12 M HC1 were added, and the solution was taken to dryness on a hot plate. Ammonium nitrate and ammonium chloride were sublimed from the residues by heating overnight on a hot plate having a surface temperature of 200 "C. To aid in the sublimation of these salts, we tightly wrapped the outside of the beaker in A1 foil to increase the temperature of the sides of the beaker to prevent recrystallization of these salts on the beaker walls. The residue was taken up in a few drops of 1:lO HN03 and the solution heated until only a single drop remained. About 40 mL of water was then added to give a solution whose pH was between 2.0 and 3.0 for loading on the SA-2 filters. Urine. Urine samples were received in sealed polyethylene containers. Four samples containing about 50-100 g of urine each were poured into Teflon beakers, and each tube was rinsed with 5-10 mL of HN03 (1:9) and then with 5-10 mL of 15 M HN03 and rinsed again with 5-10 mL of HN03 (1:9). The washings were quantitatively transferred to each respective sample and the resulting solutions heated on a hot plate until the volumes decreased to about 25 mL. Five milliliters of 15 M HN03 and 5 mL of 12 M HCIOl were added to each of the solutions and then heated to near dryness. This procedure was repeated and the sample was then diluted to approximately 40 mL with water. The excess acid was neutralized with a few drops of concentrated ammonium hydroxide and the pH stabilized at 5.4 0.3 with the
0
I
3
I
I I I
II
I
I
I
I
I
I I
I I I I I
I
I
I
I
II
I-a
I I I
I I I I
*
ANALYTICAL CHEMISTRY, VOL. 53, NO. 2, FEBRUARY I981
225
Table I. XRF Determination of Selected Elements in Seawater after Removal of Major Constituents concentration in ng/g (ppb) sample
Ni
1 2 3 4
1.20 1.19 1.09 1.56 1.26 0.20 1.2 ~ f 0.1 f
av Sa ~
~
NAAC?~
b
Mn 1.97 2.03 2.12 2.04 0.08 2.0 f 0.1 1.9 f O . l d
Zn
cu
4.45 4.18 4.34 5.08 4.51 0.39 4.8 f 0.3 4.8 f O.!jd
1.98 1.80 1.84 2.20 1.96 0.18 2.0 f 0.1 1.8f O.ld
Pb
