Environ. Sei. Technol. 1994, 28, 538-540
Determining Lead in Sediments by X-ray Fluorescence and the Method of Standard Additions Lynn Vogel Koplitz,'nt Jessica Urbanlk,t Susan Harris,+and Owen Mills*
Department of Chemistry, Loyola University, New Orleans, Louisiana 701 18-6195, and Coordinated Instrumentation Facility, Tulane University, New Orleans, Louisiana 701 18-5698
Introduction To effectively assess current and future water quality in a given location, it is necessary to take the quality of associated sediments into account. The impact of contaminated sediment on aquatic macroinvertebrates and fish populations is also an importantt reason to investigate sediment quality. The present level of total heavy metal concentrations in sediment samples is conventionally determined by acid digestion and subsequent analysis of the resulting solutions using atomic absorption spectrometry (AAS) or inductively coupled plasma (ICP) methods. The alternative method described herein is an inexpensive, fast, safe, nondestructive way to determine total lead in sediments. By combining X-ray fluorescence (XRF) analysis with the method of standard additions, we have developed a relatively uncomplicated technique to determine the lead concentration in a collection of sediment samples. This approach will almost certainly prove to be applicable to other metals of environmental interest as well. Since the sample is analyzed in its solid form, this method avoids the additional time and hazards associated with acid digestion. The most time-consuming steps involve physical preparations such as drying and grinding, which are generally necessary even for conventional methods. This technique could be useful for the semiquantitative determination of many elements in a variety of solid sample types including urban soiIs, samples of geochemical interest, and material from hazardous waste sites to name just a few. It might be used to screen large sample sets, possibly on site. It is particularly useful when appropriate well-characterized reference standards are not available. The method does not use fundamental parameters (1-3), an internal standard ( 4 ) ,or traditional reference standards (1--7),as do most other recently published XRF analytical approaches.
Experimental Section Five grab samples were taken from the Corps of Engineers Canal just east of the lower containment levee of the Bonnet Carre Floodway, approximately 22 mi west of New Orleans. All five sampling locations were within 0.8 mi of the confluence of Engineer's Canal and Bayou LaBranche along the south shore of Lake Pontchartrain. Two other samples were taken in the mouth of the floodway where it meets Lake Pontchartrain, both within 0.25 mi of the boat launch off the lower containment levee. A t each sampling location, a Wildco-Ekman-type dredge was used to collect two grab samples, which were then mixed together and stored in labeled plastic bags. After returning to the lab, excess water was drained from the bags, and
* Author to whom all correspondence should be addressed. + Loyola University. t Tulane University. 538
Environ. Sci. Technol., Vol. 28, No. 3, 1994
then the wet samples were dumped onto Styrofoam plates and allowed to dry on a shelf in a fume hood for 2 weeks. Calibration standards were prepared by adding a known amount of lead as aqueous lead nitrate to a known mass of a matrix made from the field sediment samples. The sediment matrix was prepared by mixing equal amounts of air-dried sediment from the seven collection sites. For example, in order to create a standard having 10 ppm added Pb, one would need to add 10 pg of P b to every 1 g of dry sediment matrix. This task can be accomplished by volumetrically pipetting 10 mL of a 5 ppm P b solution (5 pg of Pb2+/mLof solution) onto 5 g of sediment. Ten standards over the range of 5-200 ppm added Pb were made. By creating a set of calibration standards in this fashion, one can avoid having to do standard additions to every individual sample. This practice can be advantageous when a large number of site samples are involved or when samples are taken from the same sites repeatedly over time since the same calibration standard pellets can be used again later. In practice, air-dried sediment was used, and the actual ppm P b added was later corrected for the mass percent of water in the air-dried sediment. The mass percent water, generally 596, was found by drying -5 g of air-dried sediment in an oven at 110 'C for 2 h and then cooling in a desiccator and repeating this procedure until a constant mass was reached. The standards were air-dried again after the addition of Pb(NOMaq), then ground for 15 min in a Spex mixer-mill to a talcum powder consistency, and pressed into pellets under 20 000 psi. Individual site samples were similarly ground and pelletized. Pellets were stored in a desiccator. The P b LBemission line at 12.6 KeV was collected for each pellet by a Siemens SRS200 XRF spectrometer with a LiF 100dispersing crystal using both flow and scintillator detectors. The excitation source was a molybdenum X-ray tube operated at 50 kV and 40 mA. All measurements were made under vacuum with 0.40' collimators and no filter or aperture. Signal counting time was 40 s at 27.60, 28.27, and 29.10' 20. A baseline was linearly interpolated between the high and low angle points and subtracted from the peak signal at 28.27' (the 12.6 KeV line). (See ref 8 for further discussion of XRF techniques.) A linear fit of corrected peak intensities in counts per second vs added ppm P b for the calibration standards was extrapolated back to an x-intercept value of -40 or a background level of 40 ppm for the matrix. After this matrix background amount of Pb was determined, it was combined with the known added levels to give total ppm Pb for the standards. These total ppm P b values were then plotted against the corrected XRF intensities to form a linear calibration curve, which was used to find the total ppm P b in the site sample pellets. So far we have observed the linear range of this technique to extend up to at least 392 total ppm Pb. 0013-938X/94/0928-0538$04.50/0
0 1994 American Chemical Society
Table 1. Comparison of Pb Content of Selected Samples (S) and Calibration Standards (C) As Determined by an EPA Contract Lab and Our XRF/Standard Additions Method“
id
EPA contract lab
XRFistandard additions
% deviation from meanb
NIST SRM 2704e
15.2“ 57.1 42.2e 43.6 71.8 163c 164d 19@
43.4 57.1 42.4 44.9 76.8 149 203 152
48.1 0.0 0.2 1.5 3.4 4.5 10.6 13.1
s1 s2 s3 c1 c2 c3 c4
All values are pg of Pb per g of dry sediment (pprn). *lo0 X
bi
-yavl/yav. Analysis performed at 1:lOdilution. Analysis performed at 1:lOO dilution. e Certified value = 161 A 17.
Results and Discussion The procedure described above resulted in a calibration curve with the regression fit linear equation
R2 = 1.000 y = 12.512~ Individual site values ranged from 29 to 60 ppm Pb. Pristine sediments would be expected to contain not more than 20 ppm P b by inference from shale values and average crustal abundances (9, IO). However, lead levels below about 50 ppm in the sediment could not be detected using conventional digestion, dilution routines, and our flame AAS since the resulting solutions were below the instrumental detection limit of 1 mg of Pb/L. NIST SRM 2704 (Buffalo River sediment) was determined to contain 128 ppm P b using this calibration curve, while the reported content is 161 f 17 ppm. Clearly, it is not prudent to apply the calibration curve from this sediment matrix to the NIST standard. But when we started with NIST SRM 2704 itself as the matrix and created five standards with 40-240 ppm P b added, the linear regression fit was
+
R2 = 1.000 y = 10.845~ 1647.8 Extrapolation t o y = 0 gives x = -152 or 152 ppm P b for the sample with no P b added. This value, generated by an appropriate calibration, is within the certified range. Selected samples and standards were sent to an EPA contract laboratory in order to check the accuracy of our results. EPA Method 3050 was used for digestion while Method 7421 was employed for analysis by graphite furnace atomic absorption spectrometry (GFAAS). The results are shown in Table 1. Most of the determinations compare favorably, to within 5 % deviation from their mean, with three exceptions-S1, C4, and NIST SRM 2704. It seems unlikely that S1 would have a lead level so much lower than all of the other samples. This sample and its nearest neighbor were collected from an area bounded by four lanes of Interstate 10. Some lead contamination by vehicular exhaust is to be expected, so a level above 20 ppm P b is likely. The XRF signal from S1 was very similar to that of S3, which apparently has a P b content of about 42 ppm according to both methods. Therefore, we judge the GFAAS result for S1to be suspect. The discrepancy for C4 is certainly due to an error in the contract lab analysis, since we added 162 ppm P b to the air-dried matrix in order to make that calibration standard. The value of 164 ppm total P b cannot be correct since the
matrix probably had 40 ppm and must have had more than 2 ppm P b in it originally. Our value for NIST SRM 2704 is in agreement with the certified value, while theirs is not. Over a period of 2 months, four different data sets were collected on the same samples and standards. P b levels determined for the samples varied by no more than f 6 % for all samples with the largest absolute variation being h3.1 ppm for the 60 ppm sample. Based on this amount of variation in the XRF signal for the 60 ppm sample and on preliminary data for sediments from a different site containing about 20 ppm Pb, we can estimate the detection limit for the method to be lower than 10ppm but probably not lower than 5 ppm Pb. Similar preliminary experiments analyzing for Br (K, = 9.60 KeV) and Zn (K, = 6.96 KeV) in the same samples also appear to be successful. Calibration standards for Zn were prepared in the same fashion as has been described for P b by using ZnClz solutions. A well-correlated linear calibration curve resulted and gave Zn values ranging from 30 to 450 ppm in the seven site samples. An alternative preparation scheme was tried for Br. Instead of adding appropriate volumes of solutions to each weighed amount of sediment matrix, we first made the most concentrated Br calibration standard by adding a known amount of NaBr(aq) to a known mass of air-dried sediment matrix. Appropriate amounts of air-dried highest concentration sediment standard were then “diluted” with portions of the air-dried sediment matrix in order to create a range of added Br standards. This alternative scheme seems to have worked just as well since a linear calibration curve resulted and yielded reasonable Br values (27-94 ppm) in the site samples. We are currently collecting and analyzing a much larger sample set as well as exploring other possible elements for analysis, including Cd, in addition to Br and Zn. It may also be feasible to include several elements in the same calibration set as long as interelement interferences (2,8) can be avoided. For example, the L, line for Pb overlaps with the K, line of As and a similar interference exists between Cr and V.
Conclusions Semiquantitative analyses for total lead in sediments can be performed successfully using calibration standards prepared by the method of standard additions along with XRF of the solids themselves. This approach has decided advantages over other analytical techniques, which require that the analyte be in solution. Most notably, these advantages include shorter turn-around time and improved safety since acid digestion is obviated. It is also a nondestructive technique, which allows prepared samples and standards to remain available for future investigations. Creating calibration standards as described herein may be especially useful when analyzing by XRF if other suitable well-characterized reference standards are not available or if the fundamental parameters method is inaccessibleor somehow unsatisfactory. As demonstrated, this method can be used for a suite of similar samples so that it is not necessary to make a set of standards for each site sample. Certainly, the standard additions/XRF approach can be considered as an alternative method of analysis for environmental samples. Environ. Sci. Technoi., Vol. 28, No. 3,
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Acknowledgments This work was supported by the Tulane/Xavier DOEEM grant and the Center for Bioenvironmental Research. We also thank George C. Flowers for suggesting the field area, Eddie Landrum for instruction in sampling techniques, and Remy Gross for help with sample collection.
Literature Cited (1) (2) (3) (4)
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(5) Heckel, J.; Brumme, M.; Weinert, A.; Irmer, K. X-Ray Spectrom. 1991, 20, 287. (6) Webb, P. C.; Potts, P. J.; Watson, J. S. Geostand. Newsl. 1990, 14, 361. (7) Harding, A. R.; Walsh, J. P. Adv. X-Ray Anal. 1990, 33, 647. (8) Bertin, E. P. Principles and Practice of X-Ray Spectrometric Analysis; Plenum Press: New York, 1975. (9) Turekian, K. K.; Wedepohl, K. H.Bull. Geol. Soc.Am. 1961, 72, 175. (10) Taylor, S. R.; McLennon, S. M. The Continental Crust: Its Composition and Evolution; Blackwell Scientific: Palo Alto, 1985.
Received for review September 29, 1993. Revised manuscript received December 9, 1993. Accepted December 22, 1993.
