Anal. Chem. 1983, 55,473-476
473
Determination of Chromium in Seawater by Isotope Dilution Gas Chromatography/Mass Spectrometry K. W. Michael Slu," Marven E. Bednas, and Shier S. Berman Division of Chemistry, National Research Council of Canada, Ottawa, Ontario K I A OR9, Canada
Sub-part-per-billion comcentratlons of chromlum In seawater were determined by Isotope dllution gas chromatography/ mass spectrometry (IDGC/MS). The samples were reduced to ensure Cr( I II ) and then extracted and concentrated as Iris( l,l,l-trifluoro-2,4-pentanedlono)chromlum( II I ) (Cr(tfa),) into hexane. The Cr(tfa),+ mass fragments were monltored with a selected ion moriltoring (SIM) mode. Precisions wRhin 5 % were typical.
The determination of low concentrations of chromium (96% 53Cr,Oak Ridge National Laboratory) in a few milliliters of perchloric acid and diluting with DDW. A standard spike solution of 309 wg/L (total) was prepared by further dilution with DDW. The isotopic abundance of this standard was checked by mass spectrometry, and the chromium concentration verified by reverse ID-GC/MS and graphite furnace atomic absorption spectrophotometry (GFAAS). Analytical Procedures. The open ocean water, salinity 35%0, was collected with GO FLO samplers and a stainless-steel hydrowire at a depth of about 1300 m from a site about 15 km southeast of Bermuda. The samples were acidified to pH 1.6 with high-puritynitric acid (30)and stored in precleaned polypropylene carboys. The details of the collection, treatment, transport, and storage of the seawater will appear in a separate paper (32). All sample handling, save for the GCMS analysis, was carried out in a class 100 environment in a clean laboratory equipped with laminar flow benches and fume hoods. All laboratory wares were leached overnight in 1:l nitric acid prior to use and washed with DDW, ACS grade hexane, methanol, or an appropriate combination of them. A 128-fiLportion of the "Cr standard solution was added to 100 mL of seawater and the sample was allowed t o equilibrate overnight at 65 "C in a covered polypropylene beaker. The equilibrated sample was transferred into a 125-mL "Hypo-Vial'' (Chromatographic Specialties) equipped with a PTFE (poly(tetrafluoroethylene))coated spin bar. Eight hundred microliters
0003-2700/83/0355-0473$01.50/0Published 1983 by the American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 3, MARCH 1983
of the sulfur dioxide solution was added and the pH raised t o about 5.2 with high-purity concentrated ammonium hydroxide solution ( 3 1 ) . Two milliliters of the sodium acetate/acetic acid buffer was added. This was followed by 2 mL of the 0.1 M H(tfa) solution. The vial was capped with a PTFE-lined silicone septum crimped on with an aluminum seal. It was then placed on a stirring hot plate, and the contents were stirred vigorously at 75 OC for 2 h. At the end of this period, the vial was cooled under running water and the seal removed. The hexane phase was then carefully transferred with a disposable pipet into a 5-mL "Reacti-Vial" (Chromatographic Specialties). One milliliter of the 0.1 N ammonium hydroxide solution was added and the mixture shaken vigorously for 2 min. The hexane phase was again transferred to another 5-mL Reacti-Vial and evaporated to about 100 pL on a hot plate. The vial was then capped and stored at 4 "C until analysis. One microliter of the hexane solution was introduced into the inlet of the gas chromatograph with a 10-pL syringe (Hamilton No. 701). The Finnigan SIM mode allows four ions to be monitored simultaneously. Two of these were m / e 358 and 359, Le., principally 52Cr(tfa)2+and 53Cr(tfa)2+.The remaining two were other Cr(tfa)2+satellites such as m/e 356 and 360. Determinations were made at least in triplicate. To measure the natural isotopic abundance of chromium and the isotopic abundance of the chromium-53 spike, we modified the above procedure slightly to ensure good accuracies for the lesser chromium isotopes. Two milliliters of chromium solution, about 10 mg/L, was transferred to a 6-mL Hypo-Vial and 1 mL of the sulfur dioxide solution plus 1 mL of the buffer solution were added. These were followed by 1mL of the H(tfa) solution, and the vial was sealed. The vial contents were vigorously stirred (magnetically) at a temperature of 75 OC for 1 h. It was then cooled under running water and uncapped. The hexane phase was removed to a Reacti-Vial and 1 mL of the ammonium hydroxide solution added. The vial was shaken vigorously for 2 min. The hexane phase was then pipetted into a clean vial ready for analysis.
RESULTS AND DISCUSSION Although Cr(II1) is the most abundant naturally occurring form of chromium, the predominant species in seawater is Cr(V1) (33-39). H(tfa) reacts with dissolved chromium to form chromium trifluoroacetylacetonate which is extractable into an organic phase. This reaction, as previously stated, is specific for Cr(II1). Thus the Cr(V1) has to be reduced to the trivalent state for complexation to ensue. This can be done easily with a reducing agent such as sulfur dioxide or sodium sulfite (21). The generally adopted procedure for formation and extraction of the chromium chelate is to extract the sample solution a t about p H 5 with a large excess of H(tfa) dissolved in either hexane or benzene at an elevated temperature for an hour or so. Two features in the present solvent-extraction procedure, namely, magnetic stirring and a high aqueous to organic phase ratio (50:1), are different. Due to the low concentration of chromium in seawater, a high aqueous to organic phase ratio has to be used. This is usually avoided because the large difference in phase volumes tends to make the extraction process inefficient. Also, the reaction between Cr(II1) and H(tfa) is slow, necessitating long reaction times. Magnetic stirring helps in this situation as it promotes contact between fresh portions of the phases. Besides, it is the easiest way of agitating the vial contents a t 75 "C for 2 h. Quantitative extraction of chromium from the aqueous (sample) phase, though desirable, is not a necessary condition in an isotope dilution procedure. The percentage of recovery needs only be high enough to result in sufficient chromium being extracted for precise measurements to be made. However, in trace analysis, this may necessitate almost quantitative recoveries. Two hours of solvent extraction is sufficient to transfer most, if not all, of the chromium into the organic phase.
250
300
500
550 m/e
[ -I 350
400
4 50
Figure 1. Mass spectrum of Cr(tfa),.
Table I. Natural Abundance of Cr(tfa),+ m/ e
% calcd
356 3 57 358 369 360
3.8 0.5 75.2 17.0 3.5
% measd
3.8 0.6
74.9 16.6 4.0
Table 11. Abundance of Cr(tfa),+ for the Chromium53 Spike ml e
% calcd
% measd
3 58 359 3 60 361
3.1 86.5 9.9 0.5
3.3 86.1 9.1 0.9
Lengthening the extraction time does not increase the height or area of the Cr(tfa)3 peak. Raising the extraction temperature does not appear to improve the efficiency of the extraction. This is contrary to earlier reports that a temperature as high as 150 "C is needed for extraction into hexane ( 1 4 , 20). Extraction of chromium occurs a t a wide pH range with an optimum between 5 and 6 (20,21). Since H(tfa) is added in large excess, it is desirable to remove the unreacted portion from the hexane phase. This is accomplished by a quick backwashing step with 0.1 N ammonium hydroxide. A comparison of the Cr(tfa), peak obtained with and without this backwashing step shows no significant destruction of the chelate under the conditions described. A mass spectrum of Cr(tfa), is shown in Figure 1. The isotopic distribution of the Cr(tfa)*+fragment ( m / e 358 and 359 here) is evident. This is readily calculable if the individual elemental abundances are known (40). Assuming the isotopic abundance of I2Cand 13Cto be 98.89 and 1.11%and "Cr, j2Cr, 63Cr,and 54Crto be 4.31,83.16, 9.55, and 2.38%, respectively, and neglecting any isotopic abundances less than 1% , one can obtain a set of calculated abundances for the Cr(tfa)2f ion. These and the measured isotopic abundances (by SIM) are listed in Table I. The agreement between the two sets is excellent. The same calculation can be made for the chromium 53 spike solution by using isotopic abundances given by the supplier: 52Cr,3.44%, 53Cr,96.4%, and 54Cr,0.18%. Table I1 lists the calculated and the measured isotopic abundances for the spike solution. A series of typical SIM chromatograms of a spiked seawater sample is shown in Figure 2. The geometric isomers of chromium trifluoroacetylacetone are not fully resolved under
ANALYTICAL CHEMISTRY, VOL. 55, NO. 3,MARCH 1983 0 475
1 1
360.0
R
X
7’
Table 111. Chromium Concentration in Standards amt of standard, pg/L
amt found,
0.200 0.200 0.170
0.201
i
0.001a
0.196 i 0.001 0.164 i: 0.0008
a Precision expressed as the standard deviation of three injections from the same standard.
Table IV. Chromium Concentration in Seawater (Mg/L)
Y‘-
ID-SSMS
ID-GC/MS
358.0
0.177
0.19 a
i:
i
O.OOga
0.17 0.18
0.01
i
0.03
i:
0.01
GFAAS
0.19 i: 0.03 NDb
Precision expressed as the standard deviation of three Not determined. -
or more analyses. 356.0
0
108
eo13
300
X
20”
400 sec
Chromatograms of mle 356,358,359,and 360 of a splked seawater sample. Multiplication factor. Figure 2.
the present experimental conditions. For isotope dilution calculations, the areas under both peaks were measured. A simple derivation of the following expression for the ID-GC/MS determination of Cr is detailed in the Appendix
where W is the weight of chromium, A is the isotopic abundance, R is the peak area ratio of mle 3591358 in the spiked sample, superscripts 358 and 359 denote the appropriate mle values, and subscripts !3 and sp represent sample and spike, respectively. Equation 1 is fully compatible with and can be transformed into several expressions in the literature. The various calculation procedures for ID-GC/MS have been recently discussed and compared (41). Substituting abundaince data from Tables I and I1 into eq 1 results in W , = 0.9826W,,(86.7 - 3.3R)/(74.9R - 16.6) (2) I t has been shown from the law of propagation of error that the optimum area ratio of m/e 359 and 358 (ROpJin the spiked sample should be (42, 43) - (A259A 359)1/2/(AS358A 368)1/2 Ropt
-
SP
SP
= (16.6 >< 86.7)1/2/(74.9 X 3.3)1/2 = 2.4 (3) Thus all spikes were made such that R R,,, 2.4. The accuracy of the ID-GC/MS analysis was first checked against chromium standard solutions of similar concentrations to the seawater samples. The results are shown in Table 111. Table IV shows results of two seawater sample analyses. The second sample is the seawater reference material NASS-1. Agreement with data obtained by isotope dilution spark source mass spectrometry (ID-SSMS) (44) and GFAAS (45) was excellent. Initially, blaink runs (substituting seawater with DDW) showed no chromium peak (Figure 3). After some time, the chromium blank increased. This was probably due to chromium trifluoroacetylacetonate, retained on the column from previous injections, eluting in the blank run. By the time
- -
!
358.0
356.0 X 3
ei Figure 3.
100
em
300
400
sec
Blank run.
the study was completed, the blank’s m / e 358 chromatographic peak area reached about 2% of that in a seawater run (about 4 pg). Thus for the present study, the maximum error due to the memory effect was about 2%. The “detection limit” or, better, the minimum quantity that can be analyzed by this ID-GC/MS method is dependent on the maximum percentage error that one can tolerate. For a 10% error, the minimum quantity that can be analyzed is 40 pg of Cr or 0.04 pg of Cr/L of seawater. The latter may be lowered by raising the concentration factor. For example, the hexane phase can be evaporated to 20 p L rather than 100 pL and the minimum quantity would then be 8 ng/L. The memory effect of trifluoroacetylacetonates has been studied in some detail. I t can be reduced by doping a trace amount of trifluoroacetylacetone into the carrier gas stream (46). This method, however, would be difficult, if not impossible, to apply when the mass spectrometer is the gas chromatographic detector. It would appear that ID-GC/MS determination with trifluoroacetylacetone is an accurate and precise way of determining sub-part-per-billion levels of chromium in complex matrices like seawater. Investigations of applying such
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ANALYTICAL CHEMISTRY, VOL. 55, NO.
3, MARCH 1983
methods to other sample types and elements are now under way.
APPENDIX The area ratio of m l e 359 to 358 ( R ) can be written as
+ N,p[Cr(tfa)2+]As,359 Ns[Cr(tfa)2+]A,358+ N,p[Cr(tfa)2+lA,,358 N,[Cr(tfa)z+]A,359
R=
(4)
where N [ ] = the number of [I present, A = the isotopic abundance, subscript s = sample, subscript sp = spike, superscripts 358 and 359 denote m / e value. On rearrangement, eq 4 becomes
Since N[Cr(tfa)2+] 0: N[Cr] One can write
W,
N,[Cr] = -L
a,
where a = the atomic weight of chromium and L = the Avogadro's number. Substituting eq 7 and 8 into eq 6 gives
a, _ asp
(CaiA'), i
(Ca'A'),,
-
0.9826
(10)
i
where ai= the atomic weight of the ith chromium isotope and Ai = the abundance of the ith isotope. Inserting eq 10 into eq 9 results in
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RECEIVED for review August 2, 1982. Accepted November 5, 1982. NRCC 20860.
