Anal. Chem. 1980, 52, 1281-1283
Table IV. Despin Mechanical Assembly Oil Samples sample no.a R-11-2 s-8-8 R-7-6
R-8-6 R-8-3 H-3-3
sample composition, % amount oil A oil B oil C oil D Pb, ng 0
0
0 0 0
12 25 0
0
0 100
0
0 88 75 100 100 0
100 O O
0 0 0
b b b 6 5 5
These numbers refer to the locations in the despin mechanical assembly from which the samples were removed, e.g., R is a bearing reservoir, S the shaft, and H the housing of the apparatus. Not measured. a
those samples for which the amount of lead was measured, it was always approximately 5 ng. More than 100 of such samples were analyzed to ascertain the transport properties of the lubricant during the life test of the DMA. The results of these analyses will be reported in a subsequent publication.
ACKNOWLEDGMENT I t is a pleasure to acknowledge the assistance of T. J. Murphy and I. L. Barnes of The National Bureau of Standards during the early stages of this program. We also thank N. Marquez, who carried out the IMMA analyses. This work was supported by the U.S. Air Force under Space Divison Contract F04701-79-C-0080.
LITERATURE CITED
-
sponge technique. With one oil chosen as the “known”, the amount of the other oil was calculated using the abundance ratios measured by the IMMA. The results of several of these experiments are provided in Table 111. Results accurate to within f 6 % are obtainable with this method of quantitative analysis. Samples collected from the despin mechanical assembly (DMA) were analyzed by means of this technique, and some of the results are presented in Table IV. T h e samples were collected from various locations in the DMA and while some contain only one oil, others contain a mixture of several. In
1281
(1) Feidman, C. Anal. Chem. 1974, 46, 1606. (2) Simmons, W. J.; Loneragen, J. F. Anal. Chem. 1975, 47, 566. (3) Toda, S.; Fuwa, K.; Bodiaender, P.; Valiee, 6. L. Spectrosc. Lett. 1976, 9. 225. (4) Nesbitt, R. S.; Wessel, J. E.;Wolten, C;. M.; Jones, P. F. J . Forensic Sci. 1977, 22, 286. (5) Barnes, I. L.; Murphy, T. J.; Gramlich, J. W.; Shields, W. R. Ami. Chem. 1973. 45. 1881. (6) Issaq, H. J.: Zielinski, W. L. Anal. Chem. 1974, 4 6 , 1328. (7) Kuehner. E. C.: Alvarez. R.: Paulsen, P. J.; MurDhv. T. R. Anal. Chem. 1972, 4 4 , 2050 (8) Wachi, F M I Giimartin, D E Rev . S a Instrum 1977, 48, 703
RECEIVED for review January 14, 1980. Accepted April 16, 1980.
Simultaneous Determination of Iron, Cadmium, Zinc, Copper, Nickel, Lead, and Uranium in Seawater by Stable Isotope Dilution Spark Source Mass Spectrometry A. P. Mykytiuk, D. S. Russell,” and R. E. Sturgeon Analytical Chemistry Section, Division of Chemistry, National Research Council of Canada, Montreal Road, Ottawa, K I A OR9, Canada
Trace concentrations (ng/mL) of Fe, Cd, Zn, Cu, Ni, Pb, U, and Co have been determined in seawater by stable isotope dilution spark source mass spectrometry. The seawater samples were preconcentrated on the ion exchanger Chelex-100 and the concentrate was evaporated on a graphite or silver electrode. The results are compared with those obtained by graphite furnace atomic absorption spectrometry and inductively coupled plasma emission spectrometry. The technique avoids the use of calibration standards and is capable of producing results in cases where the analyte is only partially recovered.
I t became apparent some time ago that much of the data concerning the concentrations of trace metals in seawater was unreliable. A review ( I ) indicated t h a t the values for some elements had diminished progressively over several decades, suggesting t h a t they were largely a measure of the degree of contamination contributed by sampling and analytical operations. As the quality of reagents and facilities improved, the values have diminished to sub-ng/L levels. T h e growing demand for more adequate analytical d a t a prompted the National Research Council of Canada to establish the Marine Analytical Chemistry Project in 1976. Part of this project is concerned with problems associated with the analysis of heavy metal elements and with the development 0003-2700/80/0352-1281$01 .OO/O
of seawater standards suitable for use in oceanographic laboratories. Since most of the heavy metals occur a t extremely low concentrations, very sensitive techniques are required. These have been reviewed recently by Riley ( 2 ) . ‘“lameless atomic absorption spectrometry has been the most extensively used, largely because of its extreme sensitivity aiid the ease of operation. However a number of other sensitive techniques such as anodic stripping voltammetry, neutron activation, and mass spectrometry have also been useful. Isotope dilution using a thermal mass spectrometer has been used by a number of workers, and Patterson et al. have made very effective use of this technique in t h e determination of lead in ocean waters ( 3 ) . Chow has reviewed the application to seawater ( 4 ) and although the determination of a number of elements has been reported, lithium ( 5 ) , barium (6), lead (31, rubidium (7), uranium (8) and more recently copper (9, IO), this technique has not been extensively applied. The earlier stable isotope dilution mass spectrographic work was accomplished with a thermal ion mass spectrometer which had been specifically designed for isotope abundance measurements. However Leipziger (i‘l) demonstrated t h a t the spark source mass spectrometer (SSMS) could also be used satisfactorily for this purpose. Although it did not possess the excellent precision of the thermal unit, Paulsen (12) pointed out t h a t it did have a number of important advantages. 0 1980 American
Chemical Society
1282
ANALYTICAL CHEMISTRY, VOL. 52, NO. 8, JULY 1980
I n t h e development of a n analytical standard material, t h e most outstanding requirement is that its composition be rigorously and irrefutably established. This is usually achieved by demonstrating that several relatively independent techniques provide values which are i n acceptable agreement. Isotope dilution spark source mass spectrometry (IDSSMS) has been very effectively used for this purpose by Paulsen (12-14) at t h e National Bureau of Standards in t h e certification of a number of standard reference materials. Therefore our laboratory has been using IDSSMS to confirm the analyses of seawater and other marine materials that have been obtained by graphite furnace atomic absorption (GFAAS) and inductively coupled plasma spectrometry (ICP-ES). Since the concentration of the heavy metals is usually very low, a preconcentration step is usually necessary before any of t h e analytical techniques can be applied. T h i s also serves to remove t h e large quantity of sodium and other salts which complicate the analysis. It is usually accomplished by solvent extraction using pyrrolidine N-carbodithioate (APDC) or by ion exchange with a resin such as Chelex-100. Sturgeon has recently compared these methods for use with GFAAS (15).
EXPERIMENTAL Reagents. All reagents were purified before use. Nitric, hydrochloric, and acetic acids were prepared from reagent grade chemicals by sub-boiling distillation from a quartz still and checked for impurities by SSMS (16, Saturat,ed ammonium hydroxide was prepared from anhydrous ammonia by passing the gas through a scrubber containing an EDTA solution and absorbing the purified gas in chilled deionized water (DIW). A 1 N ammonium acet,ate buffer solution was prepared by adding 77 m L of 28% N H 4 0 H to 57 mL of glacial acetic acid, diluting to 1.OL with DIW and adjusting the p H to 5 . 2 . Chelex-100 resin (20&400 mesh, Bio-Rad Laboratories, Richmond, Calif.) was purified by successive batch extractions with 5 M "OB, 4 M HC1, and DIW. It was converted to the ammonium form by equilibration with 1 M NHIOH and rinsed with DIW. The coastal seawater was obtained from the At,lantic Regional Laboratory of the National Research Council of Canada in Halifax, Nova Scotia. The samples were filtered through a 0.45-pm membrane filter, acidified to pH 1.6, and stored in polyethylene bottles. The enriched isotopes which were obtained from the Oak Ridge National Laboratory included 53Cr,"Fe, 61Ni, "Cu, 67Zn,"'Cd, and 'O'Pb. The 235Uwas obtained from the Atomic Energy of Canada Limited, Chalk River, Ontario. All stable isotopes were obtained as metals or oxides (lead as the nitrate) with an isotopic enrichment >95%. Quantities of each were carefully weighed to three significant figures and dissolved in nitric acid (except chromium) and diluted t o produce a stock solution of about 100 ppm. The Cr203was dissolved by prolonged digestion with several milliliters of perchloric acid. The concentrations of the spike solutions were checked both by AAS and by reverse spiking and found t o be within 3% of the calculated values. Apparatus. An Associated Electrical Industries (now Kratos Inc.) MS 702 Spark Source Mass Spectrometer was used to determine the isotopic ratios. Ilford Q-2 photoplates were used and developed with Kodak developer D-19 for 4 min. They were examined on an National Spectrographic Laboratories Microdensitometer model XM102 which has been fitted with an analog system as described by Paulsen et al. (18). High purity pyrolytically coated graphite electrodes (3/,fi inch cylinders, ' I 2inch long) were obtained from Ringsdorrff-Werke GmbH, 53 Bonn-Bad, Godesberg, West Germany. For some special cases, silver electrodes were prepared from 6 9's silver in the form of square bars 3 mm X 10 mm long. The counter electrode was also prepared from high purity silver bar, 10 mm long, curved, and pointed a t one end. The separatory columns were constructed from "French style" globe separatory funnels (200-mL) by pressure fitting a porous frit into the drainage barrel of the funnel (15). Procedure. All sample preparations were carried out in a clean laboratory equipped with laminar flow benches and fume cup-
In.
Table I. Analysis of Seawater Sample A
Concentration, ng/mL IDSSMS, GFAAS, solvent ICPES, ion ion exchange extraction exchange Fe
1.4
Cd Zn cu Ni Pb
0.28 z 1.6 ? 0.7 i: 0.37 * 0.35 i. 0.020 *
co U
i:
0.1' 0.02 0.1 0.1 0.02 0.03
0.003' 3.0 i. 0.3
1.5 i. 0.1 0.24 t 0.04 1 . 9 t 0.2 0.6 i. 0.2 0 . 3 3 t 0.08 0.22 i. 0.04 0.018 * 0.008
ND
a Precision expressed as standard deviation. determined, ' By internal standard.
1.5 t 0.6
N D ~ 1.5 i. 0.4 0.7 2 0.2 0.4 i: 0.1
ND ND ND Not
boards providing a class 100 atmosphere. All laboratory wares were cleaned in 1:l HNOBand rinsed in DIW. Once in use, they were rinsed with DIW between determinations. A master spike solution was prepared from the isotope stock solutions in sufficient quantity to spike a number of seawater samples. The quantity of each isotope was estimated to produce an isotopic ratio of approximately 1 when added to 100 mL of seawater. The seawater sample (100 mL) to which the master spike had been added was allowed to sit overnight in a polypropylene beaker a t 80 "C to produce isotopic equilibrium. The analytes were recovered by the separatory column using Chelex-100 by the procedure developed by Sturgeon et al. (15). The spiked seawater was adjusted to pH 5.2 with 10 mL of ammonium acetate buffer and was shaken with 4 mL of the prepared resin for 3 min in the separatory column. After the resin had settled, the solution was drained off at 1-2 mL min-' and rinsed with 3-5 mL aliquots of DIW. The column and resin were washed 4 times with 10-mL aliquots of ammonium acetate buffer and drained completely. The metals were extracted from the column by shaking the resin vigorously with 5 mL of 5 M H N 0 3 for 3 min. The phases were allowed to separate and the extract was drained from the column and diluted to 10.0 mL with washings from the column using DIW. The solution was evaporated to fumes in a T P X (polymethyl pentene) beaker after 7 mL of 10 M HC1 had been added to decompose the ammonium salts. The residue was dissolved in about 1mL of "OB and carefully evaporated to a drop of about 0.02 mL. This was transferred with a plastic capillary pipet to the end of a graphite electrode. It was evaporated to dryness and heated on a hot-plate to 30&400 O C (to remove organic contaminants). The electrode was mounted in the left electrode holder of the sample chamber and sparked against a curved silver counter electrode as described earlier (16). A spark voltage of 15% (about 15 kV) with a pulse length of 25 p s and a frequency of 100 pulses/s was used. In the case of very low concentrations, the entire surface of the electrode was scanned with the spark to recover the whole residue for one exposure but for higher concentrations multiple exposures of the surface area were usually sufficient. Very low concentrations of lead and cadmium were usually evaporated onto a silver electrode instead of graphite, for reasons discussed later. In this case, the nitrate salts were converted to chlorides before the solution was transferred. The intensity of the lines on the photoplate was measured and the following ion ratios were determined: 114Cd/1''Cd,52Cr/53Cr, 63Cu/65Cu,5fiFe/57Fe,60Ni/61Ni,2osPb/207Pb,66Zn/67Znand 238U/235U.An unspiked sample was also examined to establish the naturally occurring abundance of the lead isotopes to compensate for the isotopic variations found in nature (19). The concentrations of the elements were computed from the following equation ng/mL =
W,K(A, - R,R) lOOOV(BR - A )
where W is the weight of stable isotope spike in pg, Vis the volume of sample in mL, R is the measured "altered" isotope ratio a/b, K is the ratio of the natural atomic weightlatomic weight of the spike. A and B are the natural abundances of isotopes a and b.
ANALYTICAL CHEMISTRY, VOL. 52, NO. 8, JULY 1980
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Table 11. Analysis of Seawater Sample B
IDSSMS, ion exchange Fe Cd
Zn cu Ni Pb co
U a
3.4 i 0 . 3 = 0.07 i- 0.01 1.9 i. 0.1 0.61 + 0.04 0.43 * 0.03 0.11 i. 0.02 0.028 i- O . O 0 l c 2.6
0.2
Precision expressed as standard deviation.
Concentration, ng/mL GFAAS solvent extraction ion exchange 3.2 i. 0.2
0.06 i- 0.01 1.8 i- 0.1 0.5 i- 0.1 0.46 i. 0.03 0.06 i. 0.02 0.015 i. 0.003 ND
Not determined.
A , and B , are the abundances of isotopes a and b in the spike. The blanks were determined by carrying the spiked buffer solution through the entire procedure.
RESULTS AND DISCUSSION T h e results obtained on two seawater samples are shown in Tables I a n d I1 where they compare favorably with those obtained by GFAAS a n d ICPES. T h e preconcentration for all three techniques was achieved by Chelex-100 ion exchange. However, since solvent extraction with APDC is the most commonly accepted method, the values obtained using it with GFAAS are also included for comparison. T h e analyses on the above samples required about two months. T h e spread in the results may also include minor changes in composition resulting from slight deterioration of the samples. Although cobalt is monoisotopic, preventing the direct use of isotope dilution, it can be determined using nickel as a n internal standard as Paulsen (13) has demonstrated. Both cobalt and nickel have been found to be essentially quantitatively recovered under the conditions used. Attempts were made to determine manganese using iron as a n internal standard. However the recovery of manganese was so erratic that this was not successful. T h e graphite electrode was used because it was compatible with both nitric and hydrochloric acid solutions of the analyte. However, i t provided a n order of magnitude less sensitivity than the silver electrode, probably because some of the sample became absorbed a n d obscured by the rougher graphite surface. Therefore the silver electrodes were useful when the concentration was below 0.1 p p b as in the case of cadmium a n d lead. Since they were attacked by nitrates, the sample solutions were converted to chlorides before transference. Unfortunately its use could not be extended to the lighter elements. Sufficient sodium and magnesium salts were present to produce mixed ions which caused interference with 59C0, 60Ni, and "Cu. Since this is not a serious problem in nitrate salts, the use of gold electrodes would probably solve t h e problem. Since the electrode contributes most of the spectrum, it is essential that it be very pure. Preconcentration by solvent extraction using APDC was also tried but it was found that traces of sodium chloride and other salts survived the extraction. Although these were very small, they produced sufficient powdery residue on the electrode to cause serious losses. While this could probably be remedied by a second extraction, it appeared to be easier t o use ion exchange. One of the advantages of the isotope dilution technique is that the quantitative recovery of the analytes is not required. Since it is only their isotope ratios t h a t are being measured, i t is necessary only to recover sufficient analyte to make an adequate measurement. Therefore, when this technique is used in conjunction with GFAAS, it is possible to determine the efficiency of the preconcentration step. This is particularly
3.4 i. 0.4 0.053* 0.007 2.0
0.1 0.51 i 0.03 f
0.45 r 0.05 0.10i. 0.01 0.018i. 0.008 ND
ICPES, ion exchange 3.2 i 0.2
N D ~ 1.6 i- 0.2 0.73 i. 0.06 0.38 * 0.02 ND ND ND
By internal standard.
important in the analysis of seawater where the recovery is very difficult to determine by other techniques since the concentration of t h e unrecovered analyte is so low. In using this technique, one must assume that isotopic equilibrium has been achieved with the analyte regardless of' the species in which it may exist. Values for chromium obtained by IDSSMS and GFAAS indicated that under the described conditions it was only 5 2 0 % recovered. Unfortunately high and erratic blanks prevented a satisfactory determination of its concentration. T h e thermal ion mass spectrometer was specifically developed for the measurement of isotope abundances and is capable of excellent precision. Although the spark source mass spectrometer lacks some of this precision, it has proved itself to be very useful in stable isotope dilution work. I t has a number of advantages including greater versatility, relatively uniform sensitivity, and better applicability t o a wide range of elements. Although the electrical detection system provides much better precision than the photoplate, this limitation can be remedied to some extent by adjusting the ion ratios to unity and enables the determination of a number of elements simultaneously.
ACKNOWLEDGMENT T h e authors thank J. W. McLaren of this laboratory for ICP-ES analyses.
LITERATURE CITED (1) Wong, C. S.;Cretney, W. J.; Piuze, J.; Christensen, P.; Berrang, P. G. 8th NBS Materials Research Symposium, Gaithersburg, Md., 1976. (2) Riley, J. P. "Chemical Oceanography", Riley, J. P.. Skirrow, G., Eds.; London, 1975;Vol. 3, p 193. (3) Chow, T. J.; Patterson, C. C. Earfh Plant. Sci. Lett. 1966, 1 , 397. (4) Chow, T. J. J . Water Pollut. Control Fed. 1966, 4 0 , 399. (5) Chow, T. J.; Goldberg, E. D. J . Marine Res. 1962, 20, 163. (6) Chow. T. J.; Goldberg, E. D. -him. C o s m h i m . ,4cta 1961, 20, 192.
-
(. 7.) Smith. R. C.: Pillai. K. C.: Chow. T. J.: Folsom. T. R. Limnol. Oceanoar.
1965, 10, 226. (8) Wilson, J. D.; Webster, R . K.; Milner, G. W. C.; Bariiett. G. A,; Smales, A. A. Anal. Cbim. Acta 1960, 2 3 , 505. (9) Abe, T.; Itoh. K.; Murozumi, M. Bunseki Kiki 1977 15(2),65. (10) Harvev. B. R. Anal. Chem. 1978. 5 0 . 1866. (lli Leipziger, F. D. Anal. Cbem.~l965;:;7, 171. (12) Paulsen, P. J.; Alvarez, R.; Kelleher, D. Spectrochim Acta, Parts 1969, 2 4 , 535. (13) Alvarez, R.; Paulsen, P. J.; Kelleher, D. E. Anal. Chem. 1969, 4 1 , 955. (14) Paulsen, P. J.; Alvarez, R.; Mueller, C. W. Anal. Cbem. 1970, 42, 673. (15) Sturgeon, R. E.; Berrnan, S. S.;Desaulniers, A.; Russell. D. S. Talanfa, 1960, 27, 85. (16) Mykytiuk, A . ; Russell, D. S.; Boyko, V . Anal. Chem. 1976, 4 8 , 1462. (17) Kuehner, E. C.; Alvarez, R.; Paulsen, P J.; Murphy T. J. Anal. Chem. 1972, 4 4 , 2050. (18) Paulsen, P. J.; Branch, P. E. NaN. Bur. Stand. ( U . S ) , Tech. Note 437, Sept. 1966. (19) Catanzaro, E.J.; Murphy, T. J.; Shields, W. R.; Garner, E. L. J . Res. NaN. Bur. Stand., Sect. A 1968, 72. 261.
RECEIVED for review February 20, .L980.Accepted April 21, 1980. National Research Council of Canada publication number, NRCC 18330.
