778
Anal. Chem. 1987, 59, 778-783
contrary, tends to react with the same number, and even with fewer dye molecules producing fluorescent reactions. As shown above, Nb(V) reacts with ferron in micellar media forming simultaneously the 1:l and the 1:3 complexes, the former being more phosphorescent than the latter. These facts tend to indicate that the more dye molecules around the metal ion, the higher is the probability of nonradiative deactivation (perhaps related to steric hindrance of the complex to fit on the micellar surface in a more planar and immobilized situation). Of course, this topic is rather speculative and awaits much more experimental work. Registry No. CTAB, 57-09-0;SDS, 151-21-3;Nb, 7440-03-1; Brij-35, 9002-92-0; ferron, 547-91-1. LITERATURE C I T E D (1) Solovev, E. A,; Lebedeva, N. A,; Sidenko, Z. S. Zh. Anal. Khim. 1974, 29, 1531-1534. (2) Holzbecher, 2 . ; Hafrnanek, M.; Sobalik, Z. Collect. Czech. Chem. Common. 1918, 4 3 , 3325-3338. (3) Marcantonatos, G. F.: Garnba, G.; Monnier, D. Anal. Chim. Acta 1973, 67,220-224. (4) Kirkbright. G.F.; Thompson, J. V.; West, T. S. Anal. Chem. 1970, 42, 782-784. (5) Aaron, J. J.; Winefordner, J. D. Talanfa 1975, 22, 707-715. (6) Hurtubise. R. J. Anal. Chem. 1983, 5 5 , 669A-678A. (7) Parker, C. A,; Hatchard, C . G. J . Phys. Chem. 1962, 66, 2506-2511. (8) Turro, N. J.; Liu, K-Ch.; Chow, M. F.; Lee, P. Photochem. Phofobiol. 1978, 27, 523-529. (9) Frei, R. W.; Birks, J. W. Eur. Spectrosc. News 1984, 5 7 , 15-20.
(10) Gooijer, G.; Velthorst, N. H.; Frei, R. W. Trends Anal. Chem. 1984, 3 , 259-265. (11) Weinberger, R.; Cline Love, L. J. Appl. Spectrosc. 1985, 39, 5 16-5 19. (12) Scypinski, S.;Cline Love, L. J. Anal. Chem. 1984, 56, 322-327. (13) Cline Love, L. J.; Skrilec, M; Habarta, J. G. Anal. Chem. 1980, 52, 754-759. (14) Cline Love, L. J.; Weinberger. R. Specfrochim. Acta., Parf 6 1983, 386,1421;1433. (15) Diaz Garcia, M. E.; Sanz-Medel, A. Anal. Chem. 1986, 58, 1436- 1440. (16) Sanz-Medei, A.; Cdrnara Rica, C.; Perez-Bustamante. J. A. Anal. Chem. 1980, 52, 1035-1039. (17) Sanz-Medel, A.: Garcia Alonso, J. I.; Blanco Gonzilez, E. Anal. Chem. 1985, 5 7 , 1681-1687. (18) Fennell-Evans, D.;Allen, M.; Ninham, B. W.; Fouda, A. J . Solution Chem. 1984, 13, 87-101. (19) Shinoda, K. Solvent Properties of Surfactant Solutions; Marcel Dekker: New York, 1967; pp 16-20. (20) Dill, K. A.; Koppel, D. E.; Cantor, R. S.;Dill, J. D.; Bendedouch, D.; Chen, S. H. Nature (London) 1984, 309, 42-45. (21) Dlaz Garcja, M. E.; Sanz-Medel, A. Talanfa 1985, 32, 189-193. (22) Diaz Garcia, M. E.: Blanco Gonzllez, E.; Sanz-Medel, A. Microchem. J . 1984, 30, 211-220. (23) Savvin. S. B. CRC Crit. Rev. Anal. Chem. 1979, 55-109.
RECEIVED for review March 26, 1986. Accepted September 22, 1986. Support from the Comision Asesora para la Investigacih Cientifica y TBcnica (CAICYT), Proyecto No. 2837183, is gratefully acknowledged. This work was presented at the European Congress on Molecular Spectroscopy, Madrid (Spain), Sept. 1985 (Abstract No. 260).
Determination of Trace Metals in a River Water Reference Material by Inductively Coupled Plasma Mass Spectrometry Diane Beauchemin,* J. W. McLaren, A. P. Mykytiuk, a n d S . S . Berman
Analytical Chemistry Section, Chemistry Division, National Research Council of Canada, Ottawa, Ontario, Canada K 1 A OR9
The detectlon power of inductlvely coupled plasma mass spectrometry (ICP-MS) and Its capaclty for rapM multlelement analysis were demonsh.ated by the analysis of a rlverlne water reference materlal. Fifteen elements (Na, Mg, K, Ca, AI, V, Cr, Mn, Cu, Zn, Sr, Mo, Sb,Ba, and U) were determined dlrectly while five (As, Co, Ni, Cd, and Pb) requlred a preconcentration prior to analysis, elther by evaporatlon (As) or by chelation by slWcaimmaMUzed 8-hydroxyqulnoflne (Co, NI, Cd, and Pb). Accurate results were obtalned by external calbratlon, standard addnlons, or Isotope dllutlon techniques. However, stable isotope dUutlon generally glves the most accurate and precise results.
Inductively coupled plasma mass spectrometry (ICP-MS) is a powerful technique many features of which have been summarized in three recent review articles (1-3). Essentially, it combines the high detection power of mass spectrometry with the capability of simultaneous elemental analysis of solutions. Furthermore, it enables very rapid isotope ratio determinations which in turn makes possible stable isotope dilution techniques. However, the application of ICP-MS to routine analysis has been somewhat hampered by its greater susceptibility to ionization interferences than inductively coupled plasma atomic emission spectrometry (ICP-AES) (3, 4) and by the problem of isobaric interferences from molecular species arising from either the solvent used in sample preparation (5,6)or from the sample itself (7-9). This is why most 0003-2700/87/0359-0778$0 1.50/0
of the analyses performed to date have required some pretreatment of the sample, either a special dissolution (e.g., ref 10) for solid samples and/or a separation with preconcentration (6, 11). A few reports describe the direct analysis of water samples (12-14). Taylor and Garbarino (12) reported the determination of metals in a standard reference water sample by using stable isotope dilution. Date and Gray (13) illustrated their system performance with the determination of 13 trace elements in three international standard reference water samples. Boomer and Powell (14) discussed a method for estimating the concentration of Al, Mn, Fe and Zn in acid precipitation. In all these cases, the water analyzed did not contain high salt concentrations; thus, no great problems of ionization interferences were encountered. Recently, the development of a river water reference material in this laboratory, with the acronym SLRS-l, provided an opportunity to assess the performance of ICP-MS when determining many trace metals directly in the presence of a complex matrix. The certificate for SLRS-1 gives the total concentrations of 21 elements. ICP-MS was used for the determination of 20 of them: 15 directly in the water itself, and 5 after a preconcentration. EXPERIMENTAL SECTION Instrumentation. The inductively coupled plasma mass spectrometer used for this work was the ELAN 250 from SCIEX Division of MDS Health Group, Ltd. (Thornhill, ON, Canada). Three modifications were made to the originally supplied in0 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 5, MARCH 1, 1987
strument. A mass flow controller (Model 5850, Brooks Instrument Division, Emerson Electric, Hatfield, PA) was added to the nebulizer gas line and a peristaltic pump (Minipuls 11, Gilson Medical Electronics, Inc., Middleton, WI) was put on the sample delivery tube to provide a constant sample delivery rate of 1.1 mL/min. Also, a conventional ICP-AES torch was used instead of the approximately 15-mm-longer one that was provided with the instrument. The operating conditions used throughout this work were the following: an rf power of 1.2 kW (reflected power 6 W) was applied to the plasma and flow rates of 2.0,0.9, and 14 L/min were used for, respectively, the auxiliary,the nebulizer, and the plasma gases. In these conditions, the sampling height (or sampling depth) is about 10 mm, which is the distance between the tip of the sampler and the initial radiation zone (15) as measured when aspirating a 1000-mg/L Y solution (16). As for the mass spectrometer settings, the Bessel box stop was set at -7.0 V (-5.8 V for the determination of As which was done after a major hardware modification), the Bessel box barrel at +2.95 V (+2.85 V for the determination of As); Einzel lenses 1 and 3 were in the range -10.0 to -16.0 V. Einzel lens 2 was at -130 V and the Bessel box plates were at -11.3 V with an ac rod offset of 0 V. The sampler and skimmer were both made of nickel with orifice diameters of 1.14 and 0.89 mm, respectively. Throughout the work, the interface running pressure was in the range 0.8-1 torr with a mass spectrometer running pressure in the range 2.0-4.4 X torr. The measurement parameters used were the following: the resolution was set manually so that the peak width was 0.8 u at 10% of peak height (which corresponds to a peak width adjustment setting of 480). The sequential measurement mode (where the measurements are done by spending the entire measurement time at one mass before going to the next) was used for all the elements except As for which the multichannel mode was used. In this latter case, the measurements were done by peak hopping rapidly from one mass to the other, staying only a short time (called a "dwell time") of 25 ms at each mass, until the total measurement time was reached. The scanning mode was either elemental (for the calibrations and standard additions) or isotopic (for the measurement of isotope ratios and isotope dilution analysis). A range of 0.1-2.0 s (1.0 s for As) was used as the measurement time, the shortest time being used for the acquisition of spectra, and the longest for quantitative determinations. Either one or three measurements per peak were taken, in the latter case, one measurement being done at the central mass while the two others were done at i0.05 u from the assumed peak center. Finally, from 10 to 50 (5 for As) repeats were made, with a counting precision of 0.5% and a threshold of 1 ion/s. Reagents. All reagents for synthesis of the silica-immobilized 8-hydroxyquinoline (I-8-HOQ) used for preconcentration were purified prior to use as described previously (17). All acids were purified by subboiling distillation in a quartz still. The enriched isotopes used for the stable isotope dilution analysis were purchased from the Oak Ridge National Laboratory. They included 53Cr,62Ni,65Cu,67Zn,ffiSr,@ M ''o, W!d, lZ3Sb,and 20'Pb. 235U was the National Bureau of Standards SRM U-930. All these stable isotopes were put into solution as described previously (7). The riverine water reference material SLRS-1 was gathered in the St. Lawrence River at the 2-3 m level, several kilometers upstream from Quebec City (Canada) and about 30-40 km upriver from the saltwater mixing zone. The water was filtered through 0.45-pm porosity filters during collection and immediately acidified with ultrapure nitric acid to pH 1.6. It was later refiltered through 0.2-pm porosity filters, blended, and bottled in 2-L quantities in polyethylene. (Complete information on the procurement of the riverine water reference material SLRS-1 and other marine reference materials can be obtained from S. Berman, Marine Analytical Chemistry Standards Program, Chemistry Division, M-12, National Research Council of Canada, Ottawa, ON, Canada K1A OR9.) Concentration Procedures. All sample preparations were carried out in a clean laboratory providing a better than class 100 working environment. The concentration procedure for As determination consisted of evaporating 100-mL aliquots of water (spiked or unspiked) to dryness, followed by a redissolution of the residue in 5 mL of 0.1 M HN03 Three such 100-mL aliquots were used: one unspiked, one spiked with 80 ng of As, and one
779
Table I. Spikes Added to a 2.0-L Sample of Riverine Water, Prior to Preconcentration, for Its Isotope Dilution Analysis isotope
spike, pg
isotope
spike, pg
53Cr
0.497 0.682 2.65 0.775 222.9O
IWMo
0.269 0.00575 0.189 0.0848 2.37
62Ni 65Cu
61Zn %Sr
W d lZ3Sb zOIPb 235U
"This spike was made to a 100-mL aliquot of river water, which was later analyzed directly by isotope dilution. spiked with 160 ng of As. The whole procedure was repeated twice. For all the other elements (a different procedure was used for As because it is not concentrated by I-gHOQ), the preconcentration was done on I-8-HOQ (as described in ref 17) because this method involves not only a concentration of a number of trace metals but also their separation from the univalent major ions and to some extent the divalent ions such as Ca and Mg. Briefly, 300- or 500-mL aliquots of riverine water were passed through columns of I-8-HOQ. A preelution wash was made with deionized distilled water and the trace metals were stripped with 10 mL of a 1 M HC1/0.1 M HN03 acid mixture. For the standard additions analysis, three 300-mL aliquots of river water were used an unspiked sample, a sample spiked with approximately the same analyte concentrations as in the riverine water, and a third sample with twice these quantities. An elution blank for the method was also prepared by submitting deionized distilled water to the same treatment as the riverine water. The whole procedure was repeated 12-16 times. For the measurements of isotope ratios, six 300-mL aliquots of river water were used. The resulting eluates were evaporated to dryness in quartz beakers and the residues redissolved in 0.5 mL of concentrated HN03 and 10 mL of deionized distilled water. These solutions were then transferred to plastic bottles and diluted to approximately 15 mL. Blanks were also prepared from 0.5 mL of concentrated HN03 and 10 mL of deionized distilled water in quartz beakers, following the same procedure as above, starting with the evaporation to dryness. For isotope dilution analysis, a 2.0-L sample of the riverine water was spiked as described in Table I. The spiked sample was warmed overnight under a heat lamp and then left to stand for 3 days to ensure equilibration of the isotopes. Three 500-mL aliquots were then passed through the I-8-HOQ columns. The 10-mL eluates were evaporated to dryness and the residues redissoved in 5% "OB. Three column blanks were also prepared by passing 100-mL aliquots of deionized distilled water (after spike addition and pH adjustment to 8.0) through the I-8-HOQ columns. The resulting 10-mL eluates were then evaporated to dryness and the residues redissolved in 5% HN03. The whole procedure was repeated twice. Analysis Procedure. Initialization. The instrument was initialized with a 100 pg/L Li, Rh, and U solution by varying the ion lens voltages so as to maximize the lo3Rhsignal and make the 7Li and 238Usignals equal to one another. This was done each time the sampler and skimmer were replaced and/or cleaned. Thus, the ranges of operating voltages listed earlier cover the values used during the whole certification process (Le., several months). The plasma operating conditions were chosen to obtain a compromise between high sensitivity and low oxide levels, as was described in another work (18). Qualitative and Semiquantitative Analysis. Spectra of both the riverine water and a 30-fold trace metal concentrate obtained by the I-SHOQ separation procedure were taken in the mass range 23-238 u to get an idea of what could be determined directly in the water and what would require a concentration prior to analysis. For many analytes, the spectra of some standard solutions (in 0.1 M "OB) were also acquired in order to make an estimate of the analyte concentrations. Finally, to check the extent of isobaric interferences caused by molecular species, spectra of standard solutions of potential interferents and of the analytes, both at their expected concentrations, were taken. Analysis with External Calibrations. Calibration was accomplished by using a blank and a standard solution the concentration of which was close to the estimated one for the analyte considered.
780
ANALYTICAL CHEMISTRY, VOL. 59, NO. 5, MARCH 1, 1987
The matrix of the blank and the standard solution was either 0.1 M HNOB(when the river water was run directly)or a 1M HC1/0.1 M "0, acid mixture (when the preconcentrates used in the standard additions analysis were run). The blank and the standard solution were run, followed by the sample (which was repetitively run 3-5 times), and finally the standard solution was run again to allow a correction for intensity drift. The blank intensity was subtracted from both the standard and the sample intensities. The two standard intensities were then used to do a linear regression of the standard intensities with time in order to get the intensity of the standard solution at the time the sample was run. This intensity was finally used to calculate the analyte concentration in the sample. In most cases, the major isotope of each analyte was used. However, for some elements (whose concentrations were high), either another isotope (48Ca,2SMg) and/or a dilution of the water (100-fold for Na determination) had to be used. Also, for K determination, the highest resolution was used in order to minimize the contribution from 4oAron 39K. Standard Additions Analysis. Each analysis was done by using the intensities obtained by subtracting the blank intensity from that of the three solutions (one unspiked and two spiked as described earlier). No correction was made for drift. In all cases, the major isotope of each analyte was used. For the preconcentrated samples, the elution blank concentration was also subtracted from the concentration found for the samples. Isotopic Ratio Measurements. Prior to isotope dilution analysis, a series of isotopic ratio measurements was made on the river water, some unspiked concentrates, and standard solutions (in 0.1 M "OB) to determine which isotopic ratios were free from isobaric interferences from molecular species. (The software of the instrument corrects automatically for elemental isobaric interferences.) It had been noticed previously (7) that any difference between the measured ratios and the expected ones (Le., that of standard solutions) indicated the presence of isobaric interference(s). The measurements were done in low resolution (which corresponds to a peak width of 1.1 u at 10% of peak height). Isotope Dilution Analysis. Two different sets of analyses were done: one directly on the water and one on 50-fold concentrates. In the first case, four to six separate measurements were made while in the second case, three were done. In all cases, a blank consisting of 5% HN03 (used to acidify the water) was subtracted. It was also rerun every three measurements to check for any drift. As for the isotope ratio measurements, the low resolution was used. The analyte concentrations were calculated by using the following formula:
128.0
130.0
132.0
134.0
136.0
138.0
140.0
142.0
144.0
m/z
Figure 1. ICP mass spectra illustrating the qualitative and semiquantitative analysis of the riverine water reference material SLRS-1: -, 0.1 M "0,; -., riverine water: - - -, 40 Fg/L Ba; - -, 100 gg/L Ce .
'""I .
600500 h
-
.
5s 400-1
1
.
,.,
/
,:
.
! \
100
0 1.0
Figure 2. ICP mass spectra showing how the efficiency of a preconcentration procedure can be qualitatively evaluated: -, 0.1 M "0,; - - -, riverine water; -, 30-fold concentrated riverine water.
where C is the analyte concentration in the sample (kg/L), M , is the mass of the stable isotope spike (pg), V is the volume of sample (L), A is the natural abundance of the reference isotope, B is the natural abundance of the spike isotope, A, is the abundance of the reference isotope in the spike, B, is the abundance of the spike isotope in the spike, K is the ratio of the natural and spike atomic weights, and R is the measured ratio (reference isotope/spike isotope), corrected for mass discrimination where needed (as in ref 7 ) , after the spike addition. RESULTS AND DISCUSSION Qualitative and Semiquantitative Analysis. A series of spectra of both the water and a 30-fold concentrate gave a great deal of information on the content of the riverine water reference material SLRS-1. For instance, 18 elements (Na, Mg, Al, K, Ca, V, Cr, Mn, Ni, Cu, Zn, Br, Sr, Rb, Mo, Sb, Ba, and Ce) could be readily seen in the river water itself, whereas eight others (Co, As, Zr, Nb, Cd, Sn, Pb, and U) were better observed after a concentration. Figure 1 demonstrates the semiquantitative determination of Ba in the riverine water. As can be seen, running a Ba standard solution provided an easy, rapid, and good estimate of the concentration. The same figure also shows the presence of Ce (at 140 and 142 u), La (at 139 u), and P r (at 141 u). Comparing spectra of the river water and a concentrate obtained by the I-8-HOQseparation procedure could also give
m/z
Flgure 3. ICP mass spectra showing the selective preconcentration 30-fold of trace metals as opposed to Ca: -, 0.1 M "0,; concentrated riverine water; -- -, riverine water; - -, 20 mg/L Ca: ---, 100 pg/L V, Cr, Mn, Co, and Ni.
-
*e.,
information on the preconcentration procedure. For example, Figure 2 shows the mass spectra in the range 90-100 u. The spectrum of the river water contains seven peaks which can be attributed to Mo. After concentration, those peaks completely disappeared and new peaks a t 90, 91, 92, 94 and 96 u indicated the presence of Zr while a peak at 93 u indicated Nb. Thus the I-8-HOQ does not concentrate Mo, presumably because it is in an anionic form. Similarly, Figure 3 indicates
ANALYTICAL CHEMISTRY, VOL. 59, NO. 5, MARCH 1, 1987 I
400 i
I
4
Table 11. Isotopic Ratiosa Measured in a Riverine Water Concentrate element isotopic ratio in a concentrateb Cr Ni
cu Zn Cd Pb' m/z
Figure 4. ICP mass spectra illustrating the interferences of Ci02 on 0.1 M "0,; ---, 25 pg/L Zn; 1 M HCI. some Zn isotopes: -,
.-,
IdT---
I
$ 10' 6
c;r
s b
E Id 10'
'OD
40.0
43.0
46.0
49.0
a 0
55.0
58.0
610
64.0
67.0
4 2
Figure 5. ICP mass spectra illustratingthe CaO and CaOH interferences on Co,Ni, Cu, and Zn: -, 0.1 M HNO,; e-, 20 mg/L Ca; ---, 1 pg/L Co, NI, Cu, and Zn.
that while calcium (peaks at 44 and 48 u) is not completely rejected by the I-8-HOQ separation/preconcentration procedure, the Ca content of the concentrate is still less than that of the original sample. Furthermore, the great difference in peak height between the water and its concentrate at 54,56, and 57 u verified the contamination problem of the concentration procedure for Fe. The iron is released from the silica at pH 8.0 (17). Finally, the fact that the increase in intensity at 51,52, and 53 u was greater than by a factor of 30 showed isobaric interferences arising from the eluting acid mixture used. As encountered previously (7,18),HCl forms molecular species which can interfere: 35C1160,35C1160H,and 37C1160 fall at the same masses as 51V,52Cr,and 53Cr,respectively. A spectrum of the eluting acid mixture compared to that of a standard solution of the analyte can very readily show the extent of the interferences. This is illustrated in Figure 4 where the spectrum of a 250 pg/L Zn solution is compared to that of the 1 M HClI0.1 M H N 0 3 acid mixture. The Zn peaks at 66 and 68 u are free from interference whereas those at 67 and 70 have interferences from 35C11602and 35C12,respectively. Similarly, Figure 5 demonstrates the interferences of molecular Ca species CaO and CaOH (8). It is obvious that Ni (58,60,61, and 62 u), Co (59 u), Cu (63 u), and Zn (64 u) will be affected. However, in the cases of Cu and Zn, the interferences are not as strong as shown since it was later found that the Ca standard solution used also contained some Cu and Zn. But for both Ni and Co the extent of the interferences is such that a direct determination of these elements in the river water is impossible. However, as was noted earlier, a preconcentration and separation with I-8-HOQ would solve the problem as it would selectively increase the concentrations
781
53/52 61/58 62/58 61/60 62/60 65/63 67/66 68/66 111/114 204/208 206/208 207/208
0.1147 f 0.0022 0.0232 f 0.0004 0.0520 f 0.0005 0.0645 f 0.0014 0.1446 f 0.0025 0.4726 f 0.0044 0.1737 f 0.0014 0.687 f 0.019 0.4418 f 0.0004 0.0319 f 0.0054 0.508 f 0.016 0.418 f 0.010
in a 100 pg/L standard 0.1176 f 0.0005 0.0181 f 0.0001 0.0591 f 0.0002 0.0449 f 0.0003 0.1463 f 0.0004 0.469 f 0.007 0.2057 f 0.0021 0.7028 f 0.0065 0.445 f 0.006 0.0279 f 0.0006 0.464 f 0.006 0.423 f 0.002
Precision expressed as the standard deviation (n = 3). A 30fold concentrate was used (containing approximately 12, 30, 108, 39, 0.6, and 3 pg/L of Cr, Ni, Cu, Zn, Cd, and Pb, respectively). 'Pb of the standard solution was from the NBS SRM 981.
of Co and Ni with respect to Ca, making the interferences negligible compared to the analyte signals. Isotopic Ratio Measurements. A series of isotopic ratios was measured in a river water concentrate and the values were compared with those of a lOO-pg/L standard solution. The results are shown in Table 11. As can be seen, most of the ratios were in reasonably good agreement with those of the standard solution although some of them may have been more imprecise because of the very low intensities involved. In all cases, the ratios could be reproduced from day to day with a relative standard deviation never greater than 3% (for three replicates). The only element for which significant differences were observed is Ni. Indeed, of the four isotopic ratios considered, only 62/60 came out as expected which means that the 4Ca160 interference on 60Ni(26.095% natural abundance (19)) is negligible, following a 30-50 fold preconcentration. However, the 61/58 and 61/60 ratios were higher than expected because of the interference of 44Ca160Hon 61Ni (1.134% natural abundance (19)) which is a much less abundant isotope than 60Ni. As for the lower value obtained for the 62/58 ratio, it came from an isobaric interference with 58Fefor which no correction was made. Thus, the only usable Ni isotopic ratio in this case is 62/60. (It should be noted that because P b isotopic abundances vary in nature, the comparison of ratios of samples and standards is not as useful a diagnostic for isobaric interferences as it is for the other elements in Table 11. However, the use of a P b standard of known isotopic composition (NBS SRM 981) allows a check for mass discrimination effects to be made.) Isotopic ratio measurements were also made directly on the river water for other elements (Sr, Mo, Sb, and U). All the ratios agreed with those of standard solutions. As a result, the following pairs of isotopes were chosen for the isotope dilution analysis: 52Cr/53Cr,@"i/@Ni, 63Cu/65Cu,66Zn/67Zn, 8sSr/86Sr,g8Mo/1wMo,l14Cd/'11Cd, 121Sb/123Sb, 208Pb/207Pb, and 238U/235U. Analysis of the Riverine Water SLRS-1. The results obtained by using the three different calibration methods are summarized in Tables I11 and IV, together with the accepted values, for determinations done directly in the water and after a preconcentration, respectively. Each accepted value was obtained by using the criterion that good agreement between at least two independent methods must be achieved. These methods were anodic stripping voltammetry (for Ni, Cu, and As), direct determination by graphite furnace atomic absorption spectrometry (GFAAS) (for Al, Cr, Mn, Fe, Ni, Cu, Zn, Mo, and Ba), direct determination by flame atomic absorption spectrometry (FAAS) (for Na, Mg, K, Ca, and Sr),
782
ANALYTICAL CHEMISTRY, VOL. 59, NO. 5, MARCH 1, 1987
Table 111. Trace Metal Concentrations (in p g / L ) Determined Directly in the Riverine Water Reference Material SLRS-1 element Na" Mp" K" Ca" A1 V Cr Mn cu
external calibration
standard additions
10.31 f 0.42b 5.77 f 0.09 1.52 f 0.03 27.09 f 0.16
24.7 f 3.0 26.6 f 4.1 0.70 f 0.05
0.66 f 0.09
0.40 f 0.03 1.81 f 0.15 3.70 f 0.06 1.35 f 0.21 135.3 f 0.6 0.80 f 0.03 0.68 f 0.07
Zn Sr Mo Sb Ba Pb
isotope dilution
143.6 f 0.6 0.77 f 0.03 0.56 f 0.03 22.70 f 0.42
21.19 f 0.47 0.17 f 0.08 0.28 f 0.03
U
accepted value 10.4 f 0.6c 5.99 f 0.28 1.30 f 0.20 25.1 f 0.9 23.5 f 1.2 0.66 f 0.09 0.36 f 0.04 1.77 f 0.23 3.58 f 0.30 1.34 f 0.20 136 f 3 0.78 f 0.04 0.63 f 0.05 22.2 f 1.7 0.106 f 0.011 0.28 f 0.03
aConcentrations in mg/L. *Precision expressed as the standard deviation (n = 3-6). cThe uncertainties for the accepted values are 95% tolerance limits-not standard deviations.
Table IV. Trace Metal Concentrations (in ,ug/L) in the Riverine Water Reference Material SLRS-1after a 20-50-Fold Preconcentration element V Cr Mn co Ni cu Zn As Cd Pb U
standard additions 0.77 f 0.06" 0.33 f 0.05 1.74 f 0.15 0.041 f 0.007 0.87 f 0.10 4.23 f 0.50 1.39 f 0.28 0.54 f 0.02 0.106 f 0.012
isotope dilution 0.244 f 0.004 1.08 f 0.02 3.90 f 0.03 0.013 f 0.002 0.109 f 0.005 0.292 f 0.002
accepted value 0.66 f 0.09 0.36 f 0.04 1.77 f 0.23 0.043 0.010 1.07 f 0.06 3.58 f 0.30 1.34 f 0.20 0.55 f 0.08 0.015 f 0.002 0.106 f 0.011 0.28 f 0.03
*
"Precision expressed as the standard deviation (n = 3-6).
hydride generation atomic absorption spectrometry (AAS) (for Sb), immobilized ligand separation followed by GFAAS determination (for Co, Ni, Zn, Cd, and Pb), immobilized ligand separation followed by isotope dilution ICP-MS determination (see Table IV), isotope dilution spark source mass spectrometry (for Cr, Fe, and Sr), instrumental neutron activation analysis (for Al, Na, K, Ca, V, Mn, Sr, and Ba), ICP-AES (for Mg, Ca, Sr, and Ba), ICP-MS (see Table 111),isotope dilution ICP-MS (see Table 111),chelation-solvent extraction separation followed by GFAAS determination (for Mn, Co, Ni, Cu, and Zn), ultraviolet photolysis followed by hydride generation AAS (for As), acid decomposition followed by hydride generation AAS (for As), and acid decomposition followed by gas chromatography (for As). Direct Determinations. Table I11 shows that 15 elements could be determined directly in the riverine water. A simple external calibration gave accurate results for Na, Mg, V, Mn, Mo, and Ba. In the case of K, Ca, Sr, and Sb, the differences were later found to come from errors in the concentration of the standard solutions used for calibration. The methods of standard additions provided better accuracy for Ca and was the only way of getting a result for A1 because of erratic background fluctuation at the A1 peak. Furthermore, A1 and Mn are monoisotopic so that isotopic dilution cannot be used for these metals. Finally, the isotope dilution technique was used whenever possible (i.e., when stable isotopes were available) since it gives the most accurate results. This is particularly obvious when considering the results for Sr and Sb as opposed to those obtained with an external calibration.
Cr and U, for which there are no external calibration values, show good agreement with the accepted values. The P b intensities, without concentration, were too low to yield reliable values. Determination after a Preconcentration. The concentration of 11elements was determined after a 20- to 50-fold preconcentration and the results are summarized in Table IV. Of these 11 elements, 4 (Co, Ni, As, and Cd) had not been determined previously during the direct analysis of the water. As mentioned before, Co and Ni had to be preconcentrated in order to reduce some isobaric interferences from Ca molecular species, and the low concentrations of As, Cd, and P b necessitated the use of a concentration prior to analysis. The results obtained by standard additions were acceptable for most elements except Ni and Cu. One element, As, is worth further discussion as it was at first felt that the concentration found was high, other instrumental techniques having initially reported lower values. Some spectra of the preconcentrate were thus taken to verify the absence of any chloride species, as @@C1 interferes directly with 75As.The spectra did not show any visible C1, C10, or ArCl peaks, which suggested that the other instrumental techniques were not detecting the total arsenic content of the riverine water. When a more vigorous digestion procedure (including a preoxidation with peroxide and UV photolysis to ensure that all potentially volatile organoarsenic compounds were converted to nonvolatile salts) was used prior to analysis, the other methods yielded As concentrationsin good agreement with the ICP-MS result (20). With isotope dilution, only Cr of the five elements determined, yielded significantly different results from the accepted value. An isobaric interference was thought improbable as the isotopic ratio was checked previously and was found to be the same as that measured with an aqueous standard solution. Furthermore, the fact that the direct isotope dilution analysis of the same sample of river water gave an accurate result indicated that there was no mistake in the spike concentration used. It would rather appear that, even after having been allowed several days to equilibrate, the spike isotope did not attain full equilibrium with the Cr already present in the water, leading to a different behavior during the preconcentration procedure and to a lower result. (There is previous evidence in this laboratory of lack of equilibration during isotope dilution experiments.) Overall, it appears that, except for the elements for which it was necessary (i.e., Co, Ni, As, Cd, and Pb), the preconcentration did not really improve the accuracy in any case. It did however improve the precision
Anal. Chem. 1907, 5 9 , 783-786
LITERATURE CITED
for the elements a t very low concentration (e.g., P b and U). CONCLUSIONS This work has shown that, although analyses with external calibrations are usually more subject to errors arising from ionization interferences (caused by concomitant elements) in ICP-MS, unless the matrix of the standard solutions used to calibrate closely matches that of the samples, they can nevertheless be used rather efficiently in many cases, provided that an adequate correction for drift is made. The methods of standard additions provides, in general, more accurate results than external calibrations as it compensates for ionization interferences. Finally, the isotope dilution technique gives by far the most accurate and precise results, but it requires two isotopes free of isobaric interferences, and equilibration of the spike isotope with the analyte must be achieved in order to assure their identical behavior during any sample treatment preceding the analysis. ACKNOWLEDGMENT The authors thank Ralph Sturgeon and Scott Willie for preparing the riverine water concentrates used in the standard additions analyses. Registry No. H20, 7732-18-5; Na, 7440-23-5; Mg, 7439-95-4; K, 7440-09-7; Ca, 7440-70-2; Al, 7429-90-5; V, 7440-62-2; Cr, 7440-47-3; Mn, 7439-96-5; Cu, 7440-50-8; Zn, 7440-66-6; Sr, 7440-24-6; Mo, 7439-98-7; Sb, 7440-36-0; Ba, 7440-39-3; U, 7440-61-1; As, 7440-38-2; Co, 7440-48-4; Ni, 7440-02-0; Cd, 7440-43-9; Pb, 7439-92-1.
703
(1) Douglas, Donald, J.; Houk, Robert S. Prog. Anal. At. Spectrosc. 1985, 8 , 1-18. (2) Gray, Alan L. Spectrochim. Acta, Part 8 1985, 4 0 8 , 1525-1537. (3) Houk, Robert S. Anal. Chem. 1986, 5 8 , 97A-105A. (4) Olivares, J. A,; Houk, R. S. Anal. Chem. 1988, 5 8 , 20-25. (5) Tan, Samantha, H.; Horllck, Gary Appl. Spectrosc. 1988, 4 0 , 445-460. (6) McLaren, J. W.; Mykytiuk, A. P.; Willie, S. N.; Berman. S. S. Anal. Chem. 1985, 5 7 , 2907-2911. (7) McLaren, J. W.:Beauchemin, Diane; Berman, S. S. Anal. Chem., in press. (8) Vaughan, M. A,; Horlick, Gary Appl. Spectrosc. 1988, 4 0 , 434-445. (9) McLeod, C. W.; Date, A. R.; Cheung, Y. Y. Spectrochim. Acta, Part 6 1986, 4 1 6 , 169-174. (10) Doherty, William; Vander Voet, Anthony Can. J . Spectrosc. 1985, 30(6),135-141. (11) Gregoire, D. C. SSC Workshop on Applications of ICP-MS; Toronto, Canada, 1985. (12) Taylor, Howard, E.; Garbarino, John R. Winter Conference on Plasma Spectrochemistry; Hawaii, 1986. (13) Date, Alan R.; Gray, Alan L. Spectrochim. Acta, Part 8 1985, 4 0 6 , 115-122. (14) Boomer, D. W.; Powell, M. J. Can. J . Spectrosc. 1986, 3 7 , 104-109. (15) Koirtyohann, S. R.; Jones, J. S.; Jester C. P.; Yates, D. A. Spectrochim. Acta, Part8 1981, 368,49-59. (16) Beauchemin, Diane: McLaren, James ICP I n f . News/. 1985, 11(7). 44 1-446. (17) Sturgeon, R. E.; Berman, S. S.; Wiiiie, S. N.; Desaulniers, J. A. H. Anal. Chem. 1981, 5 3 , 2337-2340. (18) McLaren, J. W.; Beauchemin, Diane; Berman, S. S. J . Anal. At. Spectrom ., In press. (19) CRC Handbook of Chemistry and Physics, 58th ed.: Weast, Robert C.; Ed.; The Chemical Rubber Co.; Cleveland, OH, 1977; pp 8271-8354. (20) Sturgeon, R. E., Ottawa, ON,Canada, 1986, unpublished results.
RECEIVED for review August 29, 1986. Accepted November 7, 1986.
Optical Cells with Partially Reflecting Windows as Nonlinear Absorbance Amplifiers Purnendu K. Dasgupta* and Jae-Seong Rhee Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-4260
An etalon, where the dlelectrlc spacer Is a weakly absorbing solution, forma a nonllnear absorbance ampllfler. The observed net absorbance Is equal to -log [10-3A’4(l R,)/( 1 10-A’2Rw)]where R , Is the reflectance of the windows and A Is the absorbance of the same solution in a conventional cell. Reasonable agreement between theory and experiment Is observed for a coherent source. For conventional sources, the observed amplification factor is much higher due to beam dlvergence, multlpath effect, and multiple beam interference.
-
-
The measurement of smaller and smaller optical absorbance values is a continued preoccupation of the trace analyst. In recent years, the approaches to this problem have included exploitation of the thermal lensing effect ( I ) , utilization of long path capillary cells (2),or reflective helical cells (3). For the modified cell designs mentioned above, it has been shown (3)that the nonlinear absorbance amplification effect (i.e., the fact that the effective path length of the cell, 6, increases with decreasing value of tC, t being the molar absorptivity and C the molar concentration) is due to a multipath effect originating from finite beam divergence. While nonlinear absorbance amplification, which not only facilitates measure-
ments of low absorbance but extends the attainable dynamic range as well, has obvious utilitarian consequences ( 2 ) ,the necessary cell designs or configurations are essentially inapplicable to situations which potentially stand to benefit the most from the ability to measure very low absorbance values. As an example, the utility of optical absorbance detection in open-tubular liquid chromatography, which employs column diameters of the order of 15 km (4)are limited because dispersion considerations dictate that the only permissible cell geometry involves the column itself; i.e., the incident beam is orthogonal to the column long axis and the physical path length is equal to the column diameter. Increasing the physical path length by using an angle of incidence different from orthogonal is of limited utility because of increasing light conduit action of the column wall; this phenomenon will also limit the utility of multipass schemes, e.g., in a White cell (5), aside from increasing the cell volume. Fabry-Perot interferometry as well as laser resonant cavities make use of the well-known optical device, etalon, in which a dielectric is bounded by two partially reflecting surfaces (6). Normally, the dielectric is transparent to the optical region of interest. The results of replacing the nonabsorbing dielectric in an etalon with an absorbing solution (or conversely, the results of replacing the normally transparent cell windows
0003-2700/87/0359-0783$01.50/00 1987 American Chemical Society
