Anal. Chem. 1995,67, 1553- 1557
Ondine Standard Addition Method with ICPMS Using Flow Injection Diane Beauchemin Queen's University, Department of Chemistry, Kingston, Ontario K7L 3N6, Canada
A single-line flow injection manifold was used to perform the standard addition method on-line with ICPMS. The proposed approach requires injection of the sample into two different carriers (Le., a blank and a standard with a greater concentrationthan that of the sample), as well as injection of the standard in the blank carrier. The method takes into account the change in sensitivity induced by the sample upon its injection into the standard carrier. In the best conditions, one replicate multielemental analysis by the on-line standard addition method was accomplished in 200 s (using 100-pLinjections). Furthermore, the setup induced only limited dispersion (i.e., the dispersion coefficient was between 1.2 and 2) and therefore provided sensitivity similar to that expectedh m direct continuous nebulization of the sample. Although precision was poorer than that which is typical of, for instance, &mal calibration, the proposed method adequatelycompensatedfor the effect of a 0.01 M K matrix, allowing the accurate determinationof V, Co, Ni, Cu, Zn, Mo, Cd, Sb, and Pb in this matrix. The method was also successfullyapplied to the direct determination of Mo in seawater. Although inductively coupled plasma mass spectrometry QCPMS) was commercially introduced about a decade ago, its use is rapidly expanding' because it combines the multielemental detection of its ICP optical emission spectroscopy (ICPOES) counterpart with the very low detection limits and isotopic detection capability of mass spectrometry? However, it is more susceptible to matrix effects than ICPOES, because sampling ions from the plasma is not as passive as measuring emitted light.334 The stable isotope dilution technique, which involves spiking each sample with stable isotopes of each analyte, compensatesfor both matrix effects and drift,providing results of great accuracy and preci~ion.~ However, it requires two isotopes free of spectroscopic interferences for each analyte and therefore cannot be used for the determination of monoisotopic element^.^ The standard addition method (SAM) also efficiently compensates for matrix effects, but it is applicable to all elements and does not require the purchase of expensive enriched stable isotopes. On the other hand, it is relatively labor-intensive since, similarly to the stable isotope dilution technique, it involves the addition of standards to each sample. An increase in sample throughput with a possible decrease in contaminations would (1) Hieftje, G. M.;Vickers, G. H. Anal. Chim. Acta 1989,216,l-24. (2)Beauchemin, D.Trends Anal. Chem. 1991,10, 71-77. (3) Olesik, J. W.Anal. Chem. 1991,63,12A-2lA (4)Beauchemin, D.Spectroscopy 1992,7, 12-18. 0003-2700/95/0367-1553$9.00/0 Q 1995 American Chemical Society
likely result if the SAM was automatically carried out on-line in a closed system. Flow injection (FI)methods have been successfully used for this purpose in ICPOES and flame atomic spectrometry. A review by Tyson5 discusses the various ways in which S A M has been implemented. These can be summarized using selected examples: a merging zone manifold, either without6 or with' a zone sampling manifold (the latter being used to vary the concentration of the standard); a reversed-FI manifold where standards are injected into the sample, which is then used as the carrier;8-10a zone penetration manifold where the sample is sandwiched between water at the front and a standard at the rear;" and a manifold allowing matrix addition to the sample so that the sample matches multielemental standards.12 In the latter case, the concentration of matrix elements in the sample must first be determined. More recently, a zone sampling manifold was used whereby an aliquot of standard is sampled and mixed with a separately injected sample in a merging zone manif01d.l~ The simplest (yet nonetheless very efficient) of the above methods is the reversed-FI manifold originally described by 'ryson.14 Its most important feature is that it involves interpolation to find the sample concentration instead of extrapolation as required by the conventional SAM, which unavoidably introduces greater uncertainties. Although Israel and Barnesg demonstrated that the number of standards could be reduced to two (i.e., a blank and a standard of greater concentration than that of the sample) in ICPOES because of the wide linear dynamic range of this technique, the reversed-FI method is rather wasteful of sample and would eventually result in clogging of the interface in ICPMS if the sample contains '0.2% of dissolved solids? In a paper by 'Qson and Idris, it was stated that a FI manifold (Le., where the sample is injected into a series of standard solutions, which are used in turn as carrier) also worked,15 but this was never supported by any data (neither in that paper nor in subsequent publications). Israel and Barnes described how this FI manifold (5) Tyson, J. F. Specfrochim. A d a Rev. 1991,14, 169-233. (6)Zagatto, E.A. G.; Jacintho, A 0.;Krug, F. J.; dos Reis, B. F.; Bruns, R E.; Aralijo, M. C . U.Anal. Chim. Acta 1983,145,169-178. (7) Gin& M.F.; Krug, F. J.; Bergamin, Fo. H.; dos Reis, B. F.; Zagatto, E. A. G.; Bruns, R E. J. Anal. At. Spectrom. 1988.3,673-678. (8)Tyson, J. F.; Idris,A B. Analyst 1984,109, 23-26. (9)Israel, Y.;Bames, R M. Anal. Chem. 1984,56,1188-1192. (10)Tyson, J. F.; Adeeyinwo, C. E.; Bysouth, S. R J. Anal. At. Spectrom. 1989, 4, 191-194. (11) Fang, Z.; Harris, J. M.; Ruzicka, J.; Hansen, E. H. Anal. Chem. 1985,57, 1457-1461. (12)Gin& M.F.; Bergamin, F". H.; dos Reis, B. F.;Tuon, R L.Anal. Chim. Acta 1990,234, 207-212. (13)dos Reis, B. F.; Gin6, M. F.; Krug, F. J.; Bergamin, P. H. J Anal. At. Spectrom. 1992,7, 865-868. (14)Tyson, J. F. Anal. PYOC. 1981,18,542-545. (15)Tyson, J. F.; Idris, A B. Analyst 1981,106, 1125-1129.
Analytical Chemistry, Vol. 67, No. 9, May I, 1995 1553
lstandard carrier
-1
1
(3)
i
200
1504
!
!standard
100
o-b
e,,
1
ll \
''I; u, - Ck,\
50
100
where m2 is the ICPMS sensitivity at the tip of the transient signal (Le,,in the presence of the matrix which is slightly diluted through dispersion in the carrier), is the analyte concentration of the sample injected, and D is the dispersion coefficient. Unlike in the method of Israel and Barnes,16the sensitivity for the steadystate signal (ml) was not assumed to be equal to that during the FI transient signal (mz)because when ml is equal to m2, there is indeed no matrix effect. By using the same sensitivity (Le., k in eqs 5 and 7 of ref 16), the final expression (i.e., eq 12) obtained by these authors16is therefore valid only when matrix effects are negligible, in which case the method of standard additions is really not necessary. The reversed-FI manifold14J5works because the sensitivity for the sample carrier (i.e., in the presence of the matrix) does not change as a result of the dilution which occurs upon injection of a standard solution. However, the concentration ratio of interferent to analyte above which the matrix effect is constant must first be determhed, and the interferent concentration in the sample may then have to be increased to ensure a higher rati0!J4J5 On the other hand, if the same interferent-free standard solutions are used in turn as the carrier and the sample is injected, the sensitivity will change upon each sample injection because of the sudden increase in the matrix concentration (which is 0 in the standard). Therefore, despite what Tyson and Idris15stated, the FI manifold cannot work as described if there is a matrix effect, unless matrix-matched standards are used, which would then somewhat defeat the no-need-to-know-the-matrixadvantage of the SAM. However, as demonstrated below, the SAM can be implemented using a FI manifold, with appropriate modfications to the procedure. The negative peak height of the FI peak, Em:mto-std, which is related to the concentration of the sample, is then given by subtracting eq 1 from eq 3, after expressing the former as
150
~
,
200 250 time (s)
~
300
350
4k,
Figure 1. Proposed on-line SAM. A blank standard is first used as carrier, into which a standard (of greater concentration than that of the sample) is injected, followed by injection of the sample. The standard is then used as the carrier, and the sample is injected into it. The dotted lines indicate the linear interpolation, which was performed to find the baseline of each peak. See the text for a detailed description of how the various signals ( I ) are obtained and used to find the concentration of the sample.
could be applied to the spectrophotometricdetermination of HC1 and orthophosphate.'6 However, their approach, besides being flawed (as critically discussed in the next section), greatly complicates the analysis of samples with complex matrices since it requires, in addition to the injection of samples, the injection of blanks which are matrix-matched to these samples. The goals of the present work were to design a FI SAM manifold for ICPMS where (1) the sample is injected into the standard, (2) no matrix-matched blank is required for any sample, and (3) sensitivity is preserved. The latter goal stems from the demonstration that by increasing dispersion, hence dilution, matrix effects could be essentially eliminated, at albeit the expense of ~ensitivity.'~ THEORY If a sample was injected into a standard solution carrier having a greater concentration than that of the sample, a negative peak would result as the sample plug would locally dilute the standard (Figure I, at 300 s). The steady-state signal, due to the carrier before and following sample injection is
&,
ctd = mlCtd
which yields
+ Ibak
where c t d is the concentration of the standard solution used as the carrier, It,& is the background, and ml is the ICPMS sensitivity in presence of the standard solution, which is also given by
On the other hand, the signal at the tip of the negative FI peak, I-.~+~td, is given by16 (16) Israel, Y.; Barnes. R M. Analyst 1 9 8 9 , 114, 843-848. (17) Vickers, G. H.; Ross, B. S.; Hieftje, G. M. Appl. Spectrosc. 1 9 8 9 , 4 3 , 13301333.
1554 Analytical Chemistry, Vol. 67, No. 9, May 1, 1995
em)
Equation 5 contains four parameters (mz,ml, D and which cannot be determined from the sole injection of the sample in a standard solution carrier. However, ml can easily be determined by aspirating a blank (matrix-matched to the standard) to find Ib& and then using eq 2, and D can be calculated following injection of the standard solution into the blank through = (etd - Ibak)/Etd
(6)
where e t d is the signal at the top of the FI peak due to injection of the standard into the blank carrier.
Table I.Notations Used To Label Figure I and To Derive Equations
Table 2. Operating Conditions
definition steady-state signal due to the standard background signal (steady-state signal due to the blank) analyte concentration of the standard ICPMS sensitivity in presence of the standard signal at the tip of the peak observed upon injection of the sample into the standard carrier ICPMS sensitivity at the tip of the transient signal observed by injecting the sample into the standard carrier analyte concentration of the sample dispersion coefficient peak height of the negative peak resulting from injecting the sample into the standard carrier peak height of the positive peak resulting from injecting the standard into the blank carrier peak height of the positive peak resulting from injecting the sample into the blank carrier This, however, leaves two unknowns. Another equation is therefore needed to solve the system, which can simply be obtained through injection of the sample into the blank carrier. Indeed, since c t d is then 0, eq 5 simplifies to
(7) where pm:mt,.bk is the peak height of the FI peak due to injection of the sample into the blank carrier. Isolating m2 from eq 7 and substituting the result in eq 5 yields an expression relating the unknown concentration of the sample to determinable parameters:
This expression assumes that m2 is constant when the sample is injected in either the blank or the standard, Le., that the matrix effect is constant over the range of analyte concentrations involved. Since ICPMS is a sensitive technique which is used especially for ultratrace analysis, the analyte concentration will likely be so small in comparison to the matrix that, under these conditions, a constant effect will be encountered. This obviates the need to find the interferentbanalyte concentration ratio above which the effect becomes constant and independent of analyte concentration. Otherwise, the proposed approach would not be successful, since the matrix effects typically observed in ICPMS do not level off as the interferent concentration increases.18Jg The notation used is summarized in Table 1,whereas Figure 1shows the sequence of events: injections of the standard (which will later be used as carrier) and the sample into a blank carrier, and switch to the standard carrier, into which a second injection of the sample is made. Given the wide linear dynamic range of ICPMS, a single standard of higher concentration than that of the sample should be sufficient for accurate analysis (as was shown in ICPOES)? EXPERIMENTAL SECTION Instrumental Conditions. A Perkin-Elmer/SCIEX ELAN 500 ICPMS instrument was used with several modifications. These (18) Olivares, J. A; Houk, R S.Anal. Chem. 1986,58,20-25. (19) Tan, S. H.;Horlick, G. 1.Anal. At. Spectrom. 1987, 2, 745-763.
Plasma Conditions torch low flow (PlasmaTherm) rf power 1.2 kW argon plasma gas flow rate 12 L min-1 2 L min-1 argon auxiliary gas flow rate 0.88-0.93 L min-l argon aerosol carrier gas flow rate Flow Injection Conditions carrier flow rate 1.8 mL min-l sample loop size 100 PI. tubing i.d. to the nebulizer 0.3 mm 150 cm (approximate) tubing length to the nebulizer Mass Spectrometer Settings Bessel box stop 2v Bessel box barrel -20 v Einzel lenses 1and 3 (El) -5 v -6 V Bessel box end lenses (P) Ni sampler orifice diameter 1.14 mm Ni skimmer orifice diameter 0.89 mm
include a mass flow controller (Model 1259B, MKS Instruments, Andover, MA) on the nebulizer gas line, a PlasmaTherm torch to conserve argon, and a homemade x-y-z translation stage to position the torch box. A singleline FI manifold was used, where a peristaltic pump (Minipuls 11, Gilson Medical Electronics, Middleton, WI) controlled the carrier flow rate to a sample injection valve (Model 5020, Rheodyne, Cotati, CA) which was electronically actuated by a switchingmodule (Universal, Anachem, Luton, U.K.). Teflon tubing connected the valve to the Meinhard (C-3) nebulizer. The operating conditions used (unless otherwise indicated) are summarized in Table 2. Data acquisition was made via the graphics program of ELAN 5000 software, in the peak hop scanning mode. A 5@ms dwell time was selected with either 3 points/spectral peak and 3 sweepdreading or 1 point/spectral peak and 1 sweepheading. In all cases, the resolution (i.e., the peak width at 10%peak height) was 1.0 u. The operating conditions of Table 2 were obtained, while continuously aspirating a 100 pg L-' solution of Li, Rh, and Pb, by optimizing the position of the torch box with respect to the interface and then the aerosol carrier flow rate to maximize sensitivity. The lens settings were then adjusted (if needed) so as to equalize the Li and Pb signals with little sacrifice in the Rh signal. Reagents. Multielemental solutions (10-100 pg L-l) in 1% HN03 were prepared using high-purity HN03 (Ultrex I1 from J. T. Baker Inc, Phillipsburg, NJ), 1000 mg L-l (or 10 000 mg L-l for the K matrix) monoelemental standard solutions (Spex Industries, Edison, NJ), and deionized distilled water (Milli-QPlus System, Millipore, Mississauga, Canada). A 20 pg L-' solution in 0.01 M K was used as a sample, as well as the open ocean seawater reference material for trace metals NASS4 (National Research Council of Canada, Ottawa, Canada). Procedure. To perform the proposed SAM, a 1%HN03 solution (i.e., the blank) was first used as carrier, into which a standard was injected, followed by, after thorough rinsing of the loop, an injection of the sample. The standard was then used as the carrier into which the sample was injected. To minimize the total rinsing time and thereby maximize the sample throughput, the next replicate standard addition was made by reversing the order of the injections, i.e., the sample was again injected into the standard carrier, then into the blank carrier, immediately Analytical Chemistry, Vol. 67, No. 9,May 1, 1995
1555
120,
I
I
I
Table 3. Concentration Found ( n = 5 ) for a 20 pg L-’/O.Ol M K Sample Using a SO pg L-‘ Standard in 10/0 HNOJ (Using 3 Sweepfleading and 3 PointolSpectrai Peak).
analyte 51v
Wr 55Mn
I
59c0
62Ni
x u MZn 98M0
Il4Cd
lZISb 20sPb
1
500
time (s)
Figure 2. Two replicate standard additions performed so as to maximize the sample throughput: 0-350 s shows the same sequence of injections as in Figure 1, whereas 350-700 s shows the reversed sequence of injections to minimize rinsing time. The multielement standard was 50pg L-I in 1% HN03, the blank standard 1% HNO3, and the sample 20 p g L-I in 0.01 M K. In all cases, 100pL injections were made and delivered to the nebulizer through 0.3mm4.d. Teflon tubing. The data acquisition parameters were as follow: 50-ms dwell time, 3 sweepsheading, and 3 points/spectral peak. Top, 51V+; bottom, 62Ni+.
RESULTS AND DISCUSSION
Multielemental Determination in 0.01 M K. Figure 2 illustrates one cycle of two replicate standard additions for the analysis of a 20 pg L-’ multielemental solution in 0.01 M K, using 0.3-mm4.d. Teflon tubing between the sample injection valve and the nebulizer. Under these conditions, the dispersion was limited, with an average D for all elements of 1.7 & 0.2 (mean f standard (20) Savitzky, A; Golay, M. J. E. Anal. Chem. 1964,36,1627-1639.
1556 Analytical Chemistry, Vol. 67,No. 9, May 7, 7995
calibration
21.0 f 7.7 19.2 f 5.7 29 f 12 20.0 f 4.2 19.9 f 6.3 18.0 f 6.9 19.8f 6.6 25.4 f 2.8 17.3f 5.3 21.1 f 7.6 15.1f 4.7
17.2 f 3.7 16.5 f 2.7 19.8f 2.5 14.0f 1.3 11.7 f 0.3 11.9 f 0.6 12.9 f 1.5 20.8 i 1.8 14.1 f 2.1 13.8 f 3.6 10.5 f 1.5
Uncertainties are expressed as standard deviations. Table 4. Concentration Found ( n = 6, Except Where Otherwise Indicated) for a 20 pg L-I/O.Ol M K Sample Using a I00 pg L-I Standard in 1% HNOJ (Using 1 Sweepmeading and I PoinUSpectral Peak).
analyte 51v
52Cr
55Mn
followed by an injection of the standard into the blank carrier. The whole cycle of two replicate standard additions (see Figure 2) was then repeated once or twice, yielding a total of 4-6 replicates. To fill the loop with standard or sample, a disposable syringe was used to suck the solution into the loop (thereby minimizing possible contaminations from the syringe itself). Data Processing. The raw count rates were transferred to a 486 personal computer and processed using in-house QBASIC software. In order to find the peak height of each peak, the count rate at the greatest inflection was determined as well as the corresponding baseline, which was found by linear interpolation between two points on both sides of the peak (as shown by the dotted lines in Figure 1). The baseline of the sample peak in the blank carrier was also used as and that of the sample peak in for the calculation of ml. A concenthe standard camer as tration was computed using eq 8 for each replicate standard addition. The average and standard deviation were then calculated. (In some cases, the first replicate had to be rejected as an outlier, presumably because the sample introduction system had not been rinsed properly.) When only 1 sweep/reading and 1 point/spectral peak were measured, the data were first smoothed using a seven-pointSavitzky-Golay moving windowmprior to peak height determinations. For comparison and assessment of the matrix effect, a one-point calibration was performed using &, and the analyte concentration corresponding to was calculated.
this method
59c0
@Ni
ecu
%h 98M~ l14Cd
lzlSb 208Pb (I
this method
calibration
21.8 f 2.4b 23.9 f 7.7 27.8 f 5.@ 20.3 f 4.1 20.9 f 8.7b 20.2 f 6.1b 20.2 f 4.1b 18.8 f 3.2 20.2 f 5.9 20.2 f 6.8 20.1 f 3.1
16.1 f 0.9 15.5 f 1.0 21.2f 1.2 15.4 f 0.6 12.5 f 1.1 13.6 f 0.7 14.3 f 1.0 13.3 1.0 16.9 f 1.0 13.4 f 1.5 14.1 f 0.5
Uncertainties are expressed as standard deviations. n
= 5.
deviation). This yielded an analyte concentration close to the expected value for most of the elements, as shown in Table 3. In the case of Mn and Mo, peak height calibration yielded more accurate results. As might have been predicted, the precision with the proposed method was degraded compared to that with the one-point calibration because of the larger propagated errors associated with the determination of a larger number of parameters. However, such a poor precision was not expected. Upon closer inspection of the data, a possible reason was found: when 3 sweepdreading and 3 points/spectral peak were used for data acquisition, a measurement at all the isotopes selected (listed in Tables 3 and 4) was made every 8 s. It was therefore possible for the tip of the FI peaks to be “missed during the acquisition, resulting in great fluctuations in peak height, the associated concentration, and D. The very triangular shape of the peaks in Figure 2 further supported this hypothesis. The experiment was therefore repeated (about a month later, with a new 0.3-mm4.d. Teflon tubing since the original one was used in other FI manifolds), using 1 sweep/reading and 1point/ spectral peak, which yielded a measurement every 0.8 s. Under these conditions, smooth and well-defined peaks resulted, as shown in Figure 3. (The sensitivity was also much higher than that in Figure 2 as a result of the thorough maintenance that the instrument underwent between the two experiments.) This signjiicantlyimproved the precision associated with D (which was 1.24 f 0.021, as well as that of the results by the two methods, as
To minimize salt deposits on the interface, a 5@pL sample injection loop was used. The proposed method was carried out using a 20 pg L-' standard and yielded a concentrationof 8.64 & 0.49 pg L-I (n = 4), which is in excellent agreement with the certjfied value (8.84 f 0.60, the latter uncertainty being a 95% confidence limit). In contrast, peak height calibration gave a rather inaccurate and particularly imprecise result (7.1 f 3.0 pg L-l) because of the huge fluctuations induced by this complex matrix.
1'2 L;!
"'"1
I I1
1
CONCLUSION
"0
50
100
150
200
250 time (s)
300
350
400
450
Figurq 3. Two replicate standard additions as in Figure 2, except that only 1 sweepheading and 1 poinffspectral peak were measured. Top, 51V+; bottom, @Ni+.
can be seen in Table 4 (note that n = 6 in most cases, in contrast to Table 3, where n = 5). As a result, the accuracy by the proposed method was also improved in general. Indeed, the expected concentrationwas essentially obtained across the mass range, whereas calibration yielded a systematically low result. A notable exception is Mn, for which the proposed method yielded too high a result, as under the other data acquisition conditions, while peak height calibration gave acceptable results. More investigations will be required to find the source of this discrep ancy. One possibility is the greater source of fluctuations, which results from the relatively high background at m/z 55 (for Mn) and, to a lesser extent, at m/z 52 (for Cr), compared to the other isotopes monitored, which is exacerbated by the greater number of measurements required for the determination. Another explanation may come from the spikes (possibly from tiny air bubbles) which were often observed in the signal profile for Mn and which may have induced a change in sensitivity immediately following the measurement of Ib& but before that of &. Determination of Mo in Seawater. The proposed method (using 1 sweepheading and 1 point/spectral peak) was tested by applying it to the direct determination of Mo in N M . This element was selected because it does not suffer from spectroscopic interferences from the polyatomic species expected between various elements from the seawater matrix (Na, C1, Ca, etc.) and other elements in the plasma (Ar,0, etc.). In addition, it is the only element whose concentration was high enough to be detected with the relatively old ICPMS instrument used, despite the suppression effect which could be expected due to the seawater matrix.
This work demonstrated that the SAM can be carried out onl i e to ICPMS, in a closed system, using a simple FI setup and two standard solutions: a blank and a solution of greater concentration than that of the sample. The proposed method has several advantages compared to others: (1) it minimizes sample consumption compared to the reversed-FI method14 since the sample is not used as the carrier, (2) it does not require a blank which is matrix-matched to the sample for any sample, and (3) sensitivity decreases by a factor of 0.5 at the most compared to what it would be if the sample were aspirated continuously. Furthermore, the determination of D and ml during each replicate standard addition compensates for drift. The major drawback of the method is a degradation in precision compared to peak height calibration, which results from error propagation of the greater number of parameters being determined to find the concentration. Future work will be devoted to automating the process using a FIAS200 flow injection system in order to eliminate the unavoidable fluctuations associated with manual injections and hopefully significantly improve precision. The efficiency of the proposed method will also be thoroughly demonstrated by applying it to a greater variety of samples. ACKNOWLEDGMENT
The author is grateful to the Natural Sciences and Engineering Research Council of Canada for financial support (through Grant No. OGP0039487), to the Perkin-Elmer Cow. for the donation of the ELAN-5000 software, and to Christiaan Schrag for carrying out some of the preliminary measurementswhich led to this work. Received for review October 7,1994. Accepted February
7,1995.@ AC9409952 ~~
@Abstractpublished in Advance ACS Abstracts, March 1, 1995.
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