Anal. Chem. 1087, 59, 2810-2813
2810
Determination of Uranium in Environmental Samples Using Inductively Coupled Plasma Mass Spectrometry D. W. Boomer and M. J. Powell* Ministry of T h e Environment, Rexdale, Ontario M 9 W 5L1,Canada
An analytical technique uslng lnductlvely coupled plasma mass spectrometry has been developed to estimate the concentratlon of uranlum in a varlety of environmental samples. The lower ibnlt for quantWatlon Is 0.1 ng/mL. Calibration is linear from the low limit to 1000 ng/mL. Precision, accuracy, and a quality control protocol have been establlshed. A comparleon wlth the conventlonal fluorometric method has been performed.
The maximum allowable concentration for uranium as set in the Ontario Drinking Water Objectives is 20.0 bg/L ( I ) . Studies have shown that concentrations of uranium as low as 500 pg/L have been found to affect the reproductive capability of aquatic organisms (2). There are a number of methods for estimating uranium concentrations in various matrices (3-6). The fluorometric method used in our laboratories (7) had a detection limit of 5.0 pg/L uranium in environmental samples. This technique is slow and subject to interferences. Often, lower limits of detection are required for base-line studies and to form models for the dispersion of uranium from its source. Inductively coupled plasma atomic emission spectrometry (ICP-AES) has been used for the analysis of uranium. However the technique suffers from spectral interferences and it has relatively poor detection limits (8). Inductively coupled plasma mass spectrometry is a relatively new technique for elemental analysis and has superior limits of detection over optical methods (9). Also, this technique has an order of magnitude better detection limit than that obtained for the conventional fluorometric method. Uranium has many stable and unstable isotopes but 238U has the largest percent abundance (99.274%). 238v is free from interference from other elements and it is therefore possible to detect lower concentrations. Sample pretreatment can be a source of indeterminate error and can be time-consuming. The fluorometric method needs more sample workup compared to ICP/MS analysis. A digestion is required in addition to a fusion with sodium fluoride. The sample types of interest are water, soil, vegetation, air particulate, and dustfalls. EXPERIMENTAL SECTION Reagents. All acids used for sample preservation were reagent grade Baker. All calibration standards were made from laboratory prepared stock solutions of lo00 mg/L. Concentrated acids used in the digestion procedures were reagent grade Baker and BDH. Sampling and Pretreatment. Water. Samples are preserved with 1%HNO, and stored in plastic or glass bottles with plastic-lined caps. No preparation or digestion step is used. This could lead to complications which are discussed later in this report. A minimum volume of 10 mL is required for analysis. Air Samples. Air particulate samples are collected using high volume samplers. Air is drawn through a tared glass fiber filter in a covered metal housing at a rate of 40-60 ft3/minute. Suspended air particulate with an aerodynamic diameter between 0.3 and 100 pm is trapped on the filter. The unit operates on a timed 24-h cycle.
The volume of air passing through the filter is calculated and recorded. An aliquot of the Titer is added to a test tube containing 2 mL of 8 N "OB. The digestate is then diluted to 10 mL with double distilled water. Digestion takes place for 2 h at 95 "C. Dustfull. Dust particles are collected by using a dustfall jar containing a plastic bag insert which is exposed for a predetermined time. After collection, the samples are filtered through a course filter to remove leaves, dead insects, etc. The filtrate is evaporated to dryness in a beaker. Fifteen milliliters of 5.3 N HNO, is added to the beaker and then heated for 15 min. The sample is filtered into a 50-mL centrifuge tube and diluted to volume. The filtrate is now ready for analysis. The filter is digested with 15 mL of "0,-HF-H20 (l/l/l). The contents are heated to dryness. Five milliliters of HNO, is added and the solution is taken to dryness. This process is repeated with 1 mL of HN03. Twenty milliliters of 0.8 N HNO, is added and the solution evaporated to 5 mL. The contents are then diluted to 20 ml with double distilled water. The sample is heated overnight at 40 "C and diluted to 20 mL. The insoluble portion is now ready for analysis. Vegetation. Samples are placed in perforated polyethylene bags and refrigerated. The sample is dried overnight at 85 "C and ground into an homogeneous mixture. A 0.5-g aliquot is placed in a test tube then set in a muffle furnace for a period of 6 h at a temperature of 550 "C. The tube is cooled and 2 mL of 8 N HNOBadded. The tube is placed in a digestion block and heated until 1mL of solution remains. The sample is then diluted to 10 mL with double distilled water. Soil. The sample is dried overnight at 85 "C and ground into an homogeneous mixture. A 1-gsoil sample is placed into a beaker and 10 mL of concentrated HNO, added. The solution is heated to dryness and 5 mL of concentrated HNO, is added. The uranium is redissolved in 5 mL of 8 N HNO, and diluted to 25 mL with distilled water. NBS Reference Material Digestion. Bomb Digestion. Samples are dried overnight at 85 "C. A 0.2-g portion of dried material is weighed into the Teflon insert of a Parr acid digestion bomb. Nitric acid (2.5 mL) is added and the digestion bomb closed. The bombs are transferred to an oven and heated at a temperature of 160 "C. Digestion is carried out overnight. Samples are allowed to cool and are diluted to 15 mL with double distilled water. HF-H2S0,-HN03 Digestion. A 0.5-g sample is weighed into a 100-mL teflon crucible. One milliliter of sulfuric and 2 mL of hydrofluoric acid are added to the sample. Heat the mixture at about 260 "C until the volume is reduced to less than 0.5 mL. If the sample appears to have an organic residue, then 1 mL of nitric acid is added and the process is repeated. Instrumental Parameters. The ICP/MS system is an ELAN Model 250. The ion source consists of a modified Plasma Therm Model 2500 control box with a conventional 27 MHz rf generator. The mass spectrometer contains a quadrupole mass filter capable of a mass range to m / z 300 with a pulse counting channel electron multiplier for ion detection. ICPIMS Determination. The plasma is ignited and the instrument allowed to equilibrate for a 30-min time period. The plasma and ion lenses were set to conditions previously determined by a univariate search. The forward power was set at 1200 W with the plasma flow, auxiliary flow, and nebulizer pressure set at 13 L/min, 1.0 L/min, and 39 psi, respectively. The focusing lenses B, El, P, and S2 are set at +5.3 V, -12.5 V, -18.0 V, and -7.6 V, respectively. The m / z 238 ion was monitored for 2 s with five replicates of this measurement carried out for each determination.
0003-2700/87/0359-2810$01.50/00 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 23, DECEMBER 1, 1987
-~
12
I
I
11
34.5
31.5
2811
4
I
1 03
0.54
0.78
102
1 28
15
Audllary Flomte (1 p m I
199.0
210.0
220.0
230.0
240.0
250.0
260.0
270.0
281.0
Ma88
Flgure 1. Scan of m / z 200-290 vs ionsls using 100 pg/L aqueous uranium standard (worst case condition).
-1
Flgure 3. Percent oxMe ratio (U+/UO+) vs auxiliary flow rate (Llmin) aspirating a 100 ng/L aqueous uranium standard.
Table I. Spike Recovery: High vs Low Oxide Conditions, NBS Reference Materials
I
39.5
rep 1645 1633
25.5
1633a 1573 1575
.I
__ 26
29.8
33.8
37.4
Nebulizer Preaaurr IP 8
41 2
45
I)
Flguro 2. Percent oxMe ratlo (U+/UO+) vs nebulizer pressure (psi) using a 100 pg/L aqueous uranium standard (Meinhard nebulizer C3).
RESULTS AND DISCUSSION A major advantage to ICP-MS is sensitivity, but to optimize sensitivity alone is not adequate for good analytical performance. Previous studies (IO) have shown that high oxide formation can occur if the instrument is optimized for sensitivity only. We have observed that the presence of high levels of oxide formation is often coincident with sample matrix interference (signal suppression or enhancement). We take advantage of this and monitor the reduction of oxide interference with changing operation parameters. Selection of Optimum Operating Conditions. Different operating conditions can lead to varying oxide formation. Figure 1shows the effect of a high oxide forming plasma. A 100 ng/mL aqueous standard of uranium was aspirated and a scan over the mass ranges of m/z 200 and mf z 290 was initiated. UO+ and UOz+are observable at m/z 254 and m/z 270, respectively. The extensive oxide formation of uranium could partly be due to the strong M-O bond (dissociation energy 6.03 eV) that is believed to survive in the plasma (11). The extent to which the oxide ratio forms may depend on the ion sampling characteristics of the supersonic nozzle and how plasma conditions such as power and gas flow rate affect these characteristics. In an effort to observe varying oxide formation, plasma conditions were changed and the intensity for U+ and UO+ was monitored for a 100 ng/mL aqueous uranium standard. Figures 2 and 3 show the effects of varying these parameters on percent oxide formation. Forward power has a substantial effect on oxide formation. As the forward power is increased, the percent oxide that is formed decreases in an approximate linear curve. Nebulizer flow rate also affects the amount of oxide formed. As flow rate increases (pressure is monitored) the oxide formation becomes more extensive as shown by Figure 2. The effects of auxiliary flow rate on oxide formation are less pronounced as can be seen by Figure 3. Less than a 2% change in oxide formation is seen until the flow rate is increased to greater than 1.0 L/min.
% recovery of 100 ppb plasma plasma conditions conditions high oxide low oxide
47 85 70
55 88
94 99 92 98 106
‘1645, river sediment; 1633, urban particulate; 1633a, coal fly ash; 1573, tomato leaves; 1575, pine needles. Other workers have reported oxide levels for many elements. The effects of changing various instrumental operating parameters have been documented (12,I3). In many cases the reduction of oxide is necessary to reduce the spectral interference caused from the oxide of one analyte on the mass of interest of another analyte. Since this does not create complications for this method, our efforts were entirely for the purpose of reducing some of the interferences caused by various sample matrices. As mentioned earlier oxide formation is coincidental with matrix effects. An experiment was conducted in which the instrument parameters used above to obtain highf low oxide were incorporated into a spike recovery test on some NBS matrices. Table I shows the difference in spike recovery of a 100 ng/mL spike of U. Results indicate that low oxide conditions produce a better spike recovery. The adjustments that reduce oxide formation have the effect of placing a “hotter” region of the plasma at the sampling orifice of the spectrometer. Metal oxide species dissociate more readily and oxide signals will be reduced. Data from the effects of varying parameters were used to estimate the best operating conditions for oxide reduction while maintaining reasonable signal strength. To estimate the sampling depth at these conditions, a 1000 ppm Y solution was aspirated. The distance from the tip of the initial radiation zone and the sampling orifice was 5 mm. Figure 4 shows a scan of 100 ng/mL solution of uranium at the selected conditions. These conditions do not show complete elimination of the oxide. A lower level of formation was possible, but the cost, in terms of sensitivity, would have been too great. Variations in the ratio of analyte/oxide can cause errors. An experiment was conducted to determine the stability of this ratio. A 100 ng/mL aqueous solution of uranium was aspirated over a period of 2 h. The intensities of uranium and its oxide were monitored. The ratio of U+/UO+ was calculated and the deviation of the ratio did not change by more than 4%. Variations in the ratio of oxide over an extended time period for different matrices are quite possible; however subsequent accuracy data for NBS reference materials and controls indicate the contrary (see tables).
ANALYTICAL CHEMISTRY, VOL. 59, NO. 23,DECEMBER 1, 1987
2812
Table 11. Uranium in Standard Reference Materials (SRM), d g
I
6
5
"' SRMo
4 3
2
1 0 199.0
210.0
220.0
230.0
240.0
250.0
260.0
270.0
281.0
Mass
Figure 4. A typical scan of a 100 pg/L aqueous uranium standard (compromise conditions). 10
1645 1633 1633a 1571 1573 1570 1575
amount found HF:HN03 HF:HN03 digestion bomb 1.012, 1.020 4.913, 4.777 9.890, 10.258 0.034, 0.026 0.050, 0.046 0.040, 0.037 0.024, 0.036
5.63, 5.37 0.058, 0.063 0.021, 0.020
NBS certified value 1.1 t 0.05 5.5 f 0.10 10.2 f 0.10 0.029 f 0.005 0.061 f 0.003 0.046 f 0.009 0.020 f 0.004
1645, river sediment; 1633, urban particulate: 1633a, coal fly ash; 1571, orchard leaves; 1573, tomato leaves: 1570, spinach: 1575, pine needles.
' Table 111. Regression of Fluorometer on ICP/MS
loB106-
intercept slope corr coeff std error of estimate
10'-
io310210
% 68
-10
18
94
120
42
Tlme (sed
Figure 5. Effect of wash time for varying analyte concentration vs
ionsis.
Memory Effects. It was observed that a substantial memory effect exists for uranium. The cause of this problem is still under investigation. The effect may possibly be due to residual ions trapped in the boundary layer around the sampling orifice or on the walls of the glass introduction system. X-ray microprobe analysis of the skimmer orifice has shown that U does indeed deposit on this surface (14).The extent of this effect seems to be directly proportional to analyte concentration. This causes a problem for samples with concentrations higher than 50 pg/L. Figure 5 shows the effect of analyte concentration on wash time. The use of an autosampler at this time is not practical because setting the autosampler rinse time for worst case conditions would result in inefficient operation. Development of a feedback loop to vary wash time depending on analyte concentration by direct computer control may be a possible solution. Detection Limits. Theoretical detection limits are calculated by dividing 3 x the standard deviation of the background count rate into the slope of a twepoint standard curve (the first point being the blank and designated as zero concentration) obtained with a 100 pg/L aqueous solution of uranium. Previous work (10) has shown that a practical limit of determination may be obtained by multiplying this result by a fador of at least 5. The practical limit of determination obtained in this manner was 0.1 ng/mL for aqueous solutions. Precision. The precision of the method was estimated by repeat analyses of NBS standards and was between 1% and 3% relative standard deviation within run and better than 6% between run (Table IV). Linear Dynamic Range. Linear dynamic range was established over 4 orders of magnitude from the detection limit to 1000 pg/L with a correlation coefficient better than 0.99. Above 1000 pg/L some nonlinearity was observed. Accuracy. An estimate of the accuracy of the method was obtained by estimating the concentration of NBS certified standards and by conducting a correlation with samples that had also been analyzed by the accepted fluorometric method.
dustfalls
Hi-Vols
soils
vegetation
-0.012 1.016 0.997 0.037
0.007 1.068 0.998 0.015
-0.005 1.048 0.999 0.010
0.133 0.926 0.996 0.414
Table I1 lists concentrations of a number of NBS certified standards determined by ICP-MS. Except for the urban particulate, tomato leaves, and pine needles all NBS materials agree to well within 10% of the certified values. The negative bias is due to the digestion procedure. A HF-HN03 Teflon bomb digestion was performed on the reference materials for which we obtained biased results and the results were in close agreement with certified values. Table I11 shows comparison data between ICP-MS and the fluorometric method. The statistics represent a regression of fluorometric data on ICP-MS. All data are within 95% confidence intervals. Spike recovery was determined on a sample from each matrix with typical spike concentrations representing each matrix (example, uranium concentrations in soil range typically from 10 to 100 pg/L, spikes covering this range were applied). We do not carry out any digestion on the water samples, so there is concern about particulate matter contained in these samples. Adsorption of insoluble uranium on the particulate matter is possible and unreliable results may occur. Comparison data between ICP-MS and the fluorometric method were not possible because the detection power of the fluorometer used was not sufficient to obtain reliable results. A comparison of digested and undigested water samples wm conducted by using ICP-MS. The results showed negligible variation but the study should be carried out on a wider variety of samples before making a final conclusion. Sample Run Setup a n d Quality Control. The instrument was standardized by using a 10 pg/L uranium standard for water samples and a 100 pg/L standard for the other four matrices. A sample composite spiked with 10.6 pg/L of uranium for water samples and 53.3 pg/L for the other matrices was used as a quality control check solution to monitor instrument drift. This solution was analyzed every 10 samples. A value exceeding f10% of the original value would indicate excessive drift and restandardization would be initiated and affected samples would be rerun. A blank solution is also analyzed every 10 samples and is used to monitor intercept drift. Duplicate samples are also determined, with the duplicates separated in the sample run, as a secondary check for drift. As well as instrument control, the digestion must have a control to check the consistency of the digestion procedure.
Anal. Chem. 1907, 59, 2813-2816
complete sample degradation. Development work is being carried out to produce a digestion procedure that is both practical and economical. Memory effects are a problem but can be overcome by increasing the wash time for the higher concentrationsamples. Finally, one must weigh the economics in terms of instrument cost to the advantages such as sensitivity and speed of analysis. Registry No. U, 7440-61-1; HzO, 7732-18-5.
Table IV. Between-Run Quality Control Data (April 1986 to August 1986) control
n
X
% re1 std dev
Instrument Control (pg/L) “An0
“B-b
70 29
53.3 10.6
6 4
Digestion Controls Hi-Vol(a) Hi-Vol(b) dustfall soil(a)
10 11 8 7
soil(b) vegetation(a) vegetation(b1 water spikes
16 16 8
7
983.0 11.8 21.4 17.0 24.1 0.44 10.6 198.0
2813
LITERATURE CITED
9 20 11 21 15 18 19 7
Roberts, K.; Hunslnger, R.; Dart, J. Ontario Ministry of the Environment, Revised 1983. Posten Hanf, Simmons Wafer, Air, Soil Pollut. 1984, 22, 269-296. Hues, A. C.; Henicksman, A. L.; Ashley, W. H.; Romero, D. “Fluorometric Determination of Uranium in Natural Waters”; Los Alamos Scientific Lab Report LA-6683-MS; Los Aiamos, NM. Mar 1977; 15 p. Centanni, F. A.; Ross, A. M.; DeSesa, M. A. Anal. Chem. 1956, 28, 1651. Sax, N. I. Dangerous Properties of Industrial Materials, 4th ed. Van Nostrand Rlenhoid: New York, 1975; Section I , p 19. Micheison, C. E. “The Properties of Fluoride Methods for Use in The Fluorometric Method of Uranium Analysis”; AEC Research and Deveiopment Report HW-36631, July 1955. Jarreil-Ash Fluorometer Model 27-000 Manual and Methods Handbook; Thermo Jarrell-Ash: Waltham, MA, June 1981, pt 004131. Winefordner, J. D. Spectrochemical Methods of Analysis ; Wiiey-Interscience: New York, 1971; Voi. 9. Douglas, D. J.; Quan, E. S. K.; Smith, R. G. Spectrochim. Acta, Part 6 1983. 388.39-48. Boomer, D. W.; Powell, M. J. Paper presented at C.I.C. Conference, Kingston, Ont., abs. no. ANf-AL-3, June 1985. Houk, R. S.; Oiivares, J. A. Anal. Chem. 1985, 5 7 , 2674-2679. Horiick, G.; Vaughan. M. A. Appl. Spectrosc. 1986, 4 0 , 434-444. Horiick, G.;Tan, S. H. Appl. Spectrosc. 1988, 4 0 , 445-460. Boomer D. W.; Wainwright, J., unpublished data, Ontario Ministry of the Environment, 1987.
Solution used for Hi-Vols, dustfalls, soils, and vegetation. Solution used for water samples. Table IV shows the deviation of both instrument control and the digestion control over a 5-month period. As can be seen the instrument deviation is minimal compared to the deviation of the digestion control. This suggests that errors are due to incomplete digestion. This conclusion is corroborated by the results obtained by using different digestion procedures for the analysis of NBS reference materials. Unfortunately due to the high volume of samples received a Teflon bomb digestion for this method is not economically practical. CONCLUSION Good accuracy and precision of samples can be obtained in this technique if care is taken in the digestion step to ensure
RECEIVED for review December, 22,1986. Resubmitted May 18, 1987. Accepted August 3, 1987.
CORRESPONDENCE In Situ Photoreduced Silver Nitrate as a Substrate for Surface-Enhanced Raman Spectroscopy Sir: Surface-enhanced Raman scattering (SEW) has been used to study a wide range of ions and molecules adsorbed at metal (usually Ag or Au) surfaces ( I , 2). SERS has extended the utility of Raman spectroscopy as an analytical tool by allowing spectra of very dilute aqueous samples (as low as lo4 M) and spectra of compounds that fluoresce in the visiblewavelength region to be obtained. Both metal colloids and roughened metal electrodes are commonly used as substrates. These substrates can be difficult or inconvenient to prepare and are not stable over long periods of time. The limited number of analytical applications of SERS reported to date is in part attributable to these difficulties. We show here that SER spectra can be obtained easily from colloids generated in situ by laser irradiation. For brevity, we will refer to such substrates as “photocolloids”. Silver metal particles are formed when light impinges upon silver salts; in fact, this is the underlying principle of photography (3, 4 ) . Gao et al. (5) were the first to explore the possibility of combining a photographic process with Raman spectroscopy when they reported enhanced Raman spectra of pyridine and dye-1555 (l,l~-diethyl-2,2/-selenacyanine) 0003-2700/87/0359-2813$01.50/0
adsorbed on silver particles at the surface of suspended aqueous silver chloride grains. In their colloids, the silver was apparently formed prior to laser irradiation. Silver ions in silver halide crystals are reduced by electron transfer from halide ions (3, 4). In this paper, we present evidence that high-quality SER spectra can be obtained from AgN03 solutions without first chemically reducing the silver to form Ag colloid particles. This technique exploits the well-known process whereby aqueous silver ions are photoreduced in the presence of organic compounds (6). EXPERIMENTAL SECTION The following chemicals were used as received: &NO3 (Aldrich, Gold Label, 99.9999%), AgC10, (Aldrich, 99%), NaBH4 (Aldrich, 99+%), pyridine (Baker,spectrophotometricgrade), sodium citrate (Fisher, certified grade), (+)-biotin (Aldrich, 99%), and acetone-1,3-dicarboxylicacid (Aldrich,90-95%). Pyridine N-oxide (Aldrich,95%) was vacuum sublimed prior to use. Doubly distilled water was used throughout. Aqueous stock solutions of pyridine, citrate, and biotin were prepared in concentrations of 0.1, 1.0, and 0.1 M, respectively. Conventional silver colloids were prepared by dissolving ap-
0 1987 American Chemical Society
