1568
Anal. Chem. 1987, 59, 1568-1575
(15) Jewett, K. L.; Brinckmen, F. E.; I n LlqoM Chromatography Defectors; VMrey, 7. M.. Ed.; Marcel Dekker: New York. 1983; pp 205-241. (16) Jones, D. R., IV; Manahan, S. E. Anal. Chem. 1976, 48, 502. (17) Jones, D. R., IV; Tung, H. C.; Manahan, S . E. Anal. Chem. 1978, 48, 7. (18) Jones, D. R., IV; Tung, H. C.; Manahan, S. E. Anal. Chem. 1976, 4 8 , 1897. (19) Reed, D. J. Anal. Chem. 1975, 47, 186. (20) Ricci, G. R., Shepard, L. S., Colovos, G.; Hester, N. E. Anal. Chem. 1981, 53, 611. (21) Grabinski, A. A. Anal. Chem. 1981, 53, 968. (22) Yashida, K.; Haraguchi, H. Anal. Chem. 1984, 56, 258. (23) Hausbr, 0. W.; Taylor, L. T. Ana/. Chem. 1981, 53, 1223. (24) Men, P. C.; Quimbv, B. D.: Barnes, R. M.: Elliot, W. G. Anal. Chim. Acta 1978, 101, 99. (25) Gast, C. H.; Kraak, J. C.; Hoppe, H.;Maessen, F. J. M. J. J. Chromatogr. 1979, 185, 549.
(26) Men, P. C.; Bigley, I.E.: J . Chromatogr. 1977, 9 4 , 29. (27) Wen, P. C.: Bigley, I.E.; Waiters, F. H. J . Chromatogr. 1978, 100, 555. (28) Spat W. D.; Lynn, J. G.; Andersen, J. L.; Vaider. J. G.; Gurley, L. R . Anal. Chem. 1986. 58, 1340. (29) Urasa, I . T. W.D. Thesis, Colorado State University, Fort Collins, Coiorado, 1977. (30) Urasa, I. T. Anal. Chem. 1984, 56, 904. (31) Tan, L. K.; Dutrlzac, J. E. Anal. Chem. 1985, 5 7 , 1027. (32) Tan, L. K.; Dutrizac. J. E. Anal. Chem. 1985, 5 7 , 2615.
RECEIVED for review September 12,1986. Accepted February 25,1987. This research was supported by a grant from the U S . National Science Foundation, Grant No. RII 83-05293, under the RIM1 program.
Stable Isotope Dilution Analysis of Hydrologic Samples by Inductively Coupled Plasma Mass Spectrometry John R. Garbarino* and Howard E. Taylor
US.Geological Survey, Box 25046,MS 407,Denver Federal Center, Denver, Colorado 80225
Inducthrely coupled plasma mass spectrometry Is employed In the detemrlnatbn of NI, Cu, Sr, Cd, 6a, TI, and Pb In nonsallne, natural water samples by stable Isotope dllutlon analyrb. Hydrokglc samples were directly analyzed wlthout any unusual pretreatment. Interference effects related to overtapplng Lsobars, formallon of metal oxkle and multlply charged Ions, and matrlx compodtlon were Identtfled and sultaMe meffhocle of correction evaluated. A comparablllty study Showedtm dr@Mmldmope dlktkn analyak was only marglnaliy better than 88quentld multlelement Isotope dllutlon analyds. Accuracy and precision of the slngle-element method were detemrfned on the bad8 of results obtalned for standard reterence materials. The Instrumental technique was shown to be malty wlted for programs assoclated wlth certificatkn of standard reference materials.
Historically, stable isotope dilution mass spectrometric analysis has been regarded as a definitive method. Determinations are based on a simple ratio measurement relating an unknown number of atoms to a known number of atoms in a fashion similar to the method of standard additions and having added advantages usually associated with the use of an internal standard, where the internal standard is an enriched isotope of the element itself. Applications using stable isotopes were initiated by Schoenheimer e t al. (1) and Rittenberg and Foster (2,3).The theory was developed further by Gest et al. (4), and general mathematical expressions were derived later by Hintenberger (5). The only requirement for stable isotope dilution analysis is that the analyte must have a t least two naturally occurring isotopes; over 80% of the elements satisfy this requirement and a number of those not meeting this criteria can be determined by using an artificial isotope that has sufficiently long half-life that losses from decay are negligible over the duration of the determination. The instrumentation employed in the past for isotope ratio measurements has varied with respect to the type of mass analyzer, ionization source, and detection system. Mass analyzers were either high-resolution magnetic and energy-sector instruments or low-resolution magnetic sector instruments. Ions were generated by using either surface (6-11),electron
impact (12-14),spark (15,16),or field desorption (17-19) ionization sources. Ions were detected by using either a Faraday cage, galvanometer, electron multiplier, or photographic plate. Each of the previously mentioned ion sources has been successfully applied to stable isotope dilution analysis. Although each has advantages in specific applications, they generally require considerable sample preparation and extended analysis times. Selected ion sources also are limited by the form of the samples that can be analyzed, either solid, liquid, or gas phases, and to particular elements having suitable ionization potentials. Expensive dual-sector mass spectrometers, using highquality surface ionization or electron impact ion sources, can achieve the most precise isotope-ratio measurements. Highprecision measurements with relative external standard deviations of between 0.1% and 0.001% are achieved routinely. Such precision is needed for geological-age determinations as well as for the measurement of natural isotope variations. However, precisions on the order of 0.1% to 1%give satisfactory analytical data for many other applications, including isotope dilution analysis using less expensive instrumentation. In 1980, inductively coupled argon plasma was combined with a quadrupole mass spectrometer to form a technique capable of producing isotope abundance information directly from a sample in solution (20).Since this initial application, the technique has advanced to a stage where commercially manufactured instruments are available. The inductively coupled plasma is a very efficient ion source because the ionization potential of the argon plasma is such that only a few elements cannot be ionized. Spectral interferences are associated with the formation of molecular and doubly charged ion species whose mass-to-charge ratio (rnlz) coincides with isotopes of interest (21). Ion intensities are also influenced by the composition of the sample matrix. Implementation of the inductively coupled plasma source with mass spectrometry has enhanced the scope of isotope dilution analysis, in addition to reducing the requisite time and labor. Previously mentioned ion sources primarily emphasized solids analysis or required extensive chemical separations or derivatizations. The versatility of the inductively coupled plasma source is evidenced by the range of sample types that are
This article not subject to US. Copyright. Published 1987 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 11, JUNE 1, 1987
Table I. ICP/MS Instrumentation inductively coupled plasma mass spectrometer: SCIEX ELAN 250 ion optics: SCIEXs second generation; positive ion mode detector: Galileo Channeltron torch quartz extended geometry interface: sampler orifice, nickel, 0.45 in. diameter skimmer orifice, nickel, 0.35 in. diameter pressure: base 3 x Torr operating 2 X Torr nebulizer: pneumatic concentric Meinhard Type A mass flow controller: Matheson Model 8209 recirculating cooler: FTS Systems Model RC-100; temperature,
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Table 11. Isotope Ratios Used for Isotope Dilution Analysis and Isotopically Enriched Starting Material
element nickel copper strontium cadmium barium thallium lead
isotope ratio
enriched isotope ratio
61Ni/60Ni 14.5 65Cu/63Cu 321 sBSr/ssSr 45.5 116Cd/114Cd 45.1 3.40 135Ba/138Ba 203Tl/205Tl 23.0 zo6Pb/208Pb 3325
natural isotope ratio 0.048 0.45 0.12 0.27 0.092 0.42 0.48
starting material Ni metal CUO SrC03 CdO
Ba(N03), T1203
PbC03
12 o c
sample delivery pump: Gilson Minipuls I1 autosampler: ISCO ISIS; rinse time, 2 min; flush time, 1 min applicable; solids, gases, and solutions have been analyzed directly. Solutions have been introduced by pneumatic or ultrasonic nebulization, electrothermal atomization (22,23), and direct-insertion probe methods (24). Solids also have been analyzed by using laser ablation introduction (25). The object of this investigation is the development of a rapid multielement method utilizing the stable isotope dilution technique for the analysis of hydrologic samples using inductively coupled plasma mass spectrometric instrumentation. The subject method was designed to determine accurately Ni, Cu, Sr, Cd, Ba, T1, and Pb in natural waters, primarily surface waters and meteorological precipitation, directly, without prior chemical or physical separations or derivatizations. Detection limits are less than 1ppb for most elements, and isotope ratio measurements can be made with external relative precisions of between 0.2% and 1%. The method was evaluated with National Bureau of Standards (NBS) reference materials and U.S. Geological Survey standard reference water samples (SRWS). A comparison also was made between single-element and multielement stable isotope dilution analyses.
EXPERIMENTAL SECTION Instrumentation. The inductively coupled plasma mass spectrometer system and supporting equipment employed in this study are listed in Table I. The ELAN 250 used SCIEXs second-generation ion optics, an extended geometry torch, and unmodified nickel orifices. Polarity of the ion lenses was set for transmission of positive ions. The ion-sampling zone was just above the yttrium-emission bullet a t a distance of approximately 10 mm from the sampling orifice. Several minor modifications and additions made to the instrumentation improved its operation, as well as increased its analytical efficiency. A recirculating cooler using 30% ethylene glycol coolant at 12 "C replaced the municipal water source. Effective cooling of the interface region, and secondarily the torch coil, necessitated modification of the ELAN'S plumbing system. Coolant for the interface now originates upstream of the cryogenic pump ahead of the pressure regulator; restrictive check valves were also removed. Because of the low municipal water pressure and the availability of high-pressure argon, the sliding interface was modified to operate pneumatically off the argon supply to the nebulizer. A multichannel Matheson mass flow controller was implemented to accurately control the plasma flow rate, auxiliary flow rate, and the nebulizer flow rate. Three flow rate ranges were used: 0-20 L/min for the plasma; 0-2000 mL/min for the auxiliary; and 0-1000 mL/min for the nebulizer. An integral mass flow transducer and control solenoid valve replaced the corresponding solenoid valve, metering valve, and rotameter for each gas. The source of plasma and auxiliary gas was a Dewar of liquid argon regulated at 25 psi. The nebulizer gas was supplied from a separate T-type cylinder regulated at 60 psi and further reduced to nebulizer-operating pressure a t the pressure regulator on the ELAN. An ISCO ISIS autosampler was used to reduce operator interaction. Prerinsed polystyrene culture tubes measuring 17 mm by 100 mm, holding a maximum volume of 15 mL, were used in the 76-position sample tray. Sample contamination was
minimized by using a 3 mm 0.d. quartz sample probe. Reagents. All standards were prepared with ultra-high-purity metals, metal salts, or metal oxides, doubly deionized water, and ultrapure acids. Dissolution primarily was performed in either poly(tetrafluoroethy1ene) or quartz vessels. Nitric acid was the preferred acid for dissolution, although hydrochloric and perchloric acids were required in some cases. Lithium metaborate fusion was used in some instances. Poly(methy1pentene) volumetric flasks with polypropylene caps were used for all dilutions. All standard and stock solutions were stored in poly(tetrafluor0ethylene) bottles. Final acid concentrations are expressed in terms of volume/volume percent. Enriched stable isotopes in various chemical forms were obtained from Oak Ridge National Laboratory, Oak Ridge, TN. The pair of isotopes used in the isotope dilution procedure, their respective isotopic abundances, their corresponding natural isotopic abundances, and the chemical form purchased are listed in Table 11. Primary single-element isotopically enriched stock solutions were gravimetrically prepared and acidified with nitric acid to a final acid content of approximately 1% . The certified isotopic abundance for each solution was verified within experimental error. Secondary working solutions were prepared through appropriate dilutions of the primary stock to give concentrations that were applicable to probable analyte concentration ranges in reference materials and unknown hydrologic samples. Secondary stock solutions were acidified with nitric acid to a final acid content of approximately 0.1%. Primary single-element analyte and internal standard stock solutions used for preliminary determinations based on working curve calibrations were prepared at concentrations of about 100 mg/L in 1%nitric acid. Serial dilutions of the primary analyte stock solutions produced three multielement working standards acidified to a final acid content of 0.1%. The concentration range of the multielement working standards varied on an elemental basis where as follows: 0, 10, 50, and 100 pg/L for Ni, Cd, T1, and Pb; 0,50,250, and 500 pg/L for Cu; and 0,100,500, and 1000 pg/L for Sr and Ba. The concentration range for each element was chosen based on its linear working range and probable concentration range in natural waters. The blank was prepared with doubly deionized water acidified with nitric acid to 0.1%. A multielement internal standard working solution was prepared from the single-element primaries having Ge, Re, and Pr concentrations of 1000, 1000, and 200 pg/L, respectively, in 0.1% nitric acid. An aliquot of the multielement internal standard working solution was added to the blank and each multielement standard to give a final concentration for Ge, Re, and Pr equal to 40,40, and 8 pg/L, respectively. The ion intensities resulting from these concentration levels ensured intensity ratios near unity and minimized possible contamination. Analytical Procedures. Isotope dilution analysis accurately determines an analyte concentration from three different isotope ratio measurements. These isotope ratios correspond to the isotopically enriched standard, the original sample, and the isotopically diluted original sample, here after referred to as the blend. The concentration of the analyte C, in micrograms per liter, is calculated from
where the subscripts e, n, and b identify terms corresponding to the isotopically enriched standard, the sample, and the blend,
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 11, JUNE 1, 1987
Table 111. Data-Acqusition Parameters"
,
I
method
mode
calibration curve isotope dilution isotope dilution
multielement single element multielement
measurement time, s
replicates
0.25
5
0.50
25
0.50
5
a All methods employed low resolution, three measurements per peak, and acquired data through routine operations, elemental analvsis. and auantitative analvsis.
1 )
0.01
I
0.1
1 MOLAR RATIO
10
100
Flgwe 1. Error as a function of molar ratio.
respectively, Re, R,, and Rbare the measured isotope ratios, CR, and ERe equal to the sum of the ratia of every isotope abundance to a chosen reference isotope abundance, A , and A , are atomic weights, W denotes the amount of enriched analyte added in micrograms to the sample to form the blend, and V is volume of the sample in liters. The variation of isotope abundances for isotopically enriched standards with time has been found to be negligible, thereby eliminating the need for constant monitoring. Therefore, only two isotope ratio measurements are required for the determination of each analyte. Accuracy of the isotope dilution technique depends on the analytical sensitivity and precision with which these isotope ratio measurements can be made. Generally, sensitivity is on the order of 1000 ions/s for every microgram per liter of analyte present; variations primarily are the result of differences in isotope abundances. The overall precision of the technique depends on the measurement precision, the isotope abundances of the analyte in the sample and isotopically enriched standard, and the amount of enriched isotope added. Three of these parameters can be controlled to some degree, only the isotopic abundance in the sample cannot. Relative measurement precision is influenced by the measurement time from counting statistics. The isotopically enriched material can be obtained in several difference isotope abundances. A molar ratio of sample analyte to spike enriched isotope, that produces the highest precision or that minimizes the propagation of error, can be calculated by using a method described by De Bievre and Debus (26). This obviously requires a prior estimate of the amount of analyte present in the sample; the method employed to estimate this concentration will be described later. The functional relationship of propagated error vs. the molar ratio can be represented graphically as in Figure 1 using the isotope ratios for Ni, Cu, Sr, Cd, Ba, T1, and Pb shown in Table 11. The graphs show that the error in the isotope ratio measurement is more or less strongly propagated as a function of the molar ratio. The graphs also indicate that an analyte molar ratio of approximately unity produces the least amount of error and a range of molar ratios exists that give nearly this same error, with the width of this range dependent on the values of R, and Re. Therefore, based on these criteria, a molar ratio of approximately unity was used for all analyses. Analytical data were acquired while in the quantitative analysis mode as ion intensities, in ions per second. The data-acquisition parameters used in this study are outlined in Table 111. Measurement time and the number of replicates varied according to the method being used. The measurements were made sequentially, low mass to high mass, for each of the isotopes listed in Table 11. In addition, measurements were made on isotopes contributing to isobaric interferences: Kr on Sr, Sn on Cd, La and Ce on Ba, and CaO and CaOH on Ni and Cu. Under lowresolution conditions, the peak width at 10% peak height is 1amu. Because the tops of the ion peaks are fairly flat under these conditions, three intensity measurements were made: at 0.1 amu to the low-mass side of the nominal mass; at noma1 mass; and
0.1 amu to the high-mass side of the nominal mass. The average of these three values was used in all calculations to minimize any effects from drift in the mass calibration, if any occur. The precision of intensity measurements is affected by the length of time spent counting the ions as they strike the detector. Counting statistics show that the standard deviation of the intensity measurement, in units of ions per second, is equal to the square root of the counting rate divided by the counting period. Therefore increasing the counting time likely will increase the relative precision. However, extensive increases in the measurement time eventually fail to increase the precision. At that point, random errors resulting from instrumental uncertainties became the limiting factor. A second limiting factor on the measurement time is the volume of sample available. Consideration of these factors led to using a 0.5-s measurement time for single-element and multielement isotope dilution determinations. The isotope ratio data were processed in a block that contained 25 replicate determinations for the single-element mode and 5 replicate determinations for the multielement mode. The total measurement time for single-element determinations was about 40 s per isotope compared to about 8 s per isotope for multielement determinations. The means and standard deviations for all isotope ratios were calculated from three blocks of data. The average isotope ratios were used in eq 1, and the standard deviations were used to estimate the experimental error. All ion intensity data were acquired uncorrected and uploaded into another computer where interference corrections, isotope ratios, and isotope dilution calculations were performed. The initial estimate of the analyte concentration in the sample was based on a calibration curve of ion-intensity ratio as a function of concentration. The ion-intensity ratio was the ratio of analyte ion intensity to internal standard ion intensity. The internal standard technique was employed to eliminate problems associated with instrumental drift, in effect extending the time between adjacent calibrations. Three internal standards, Ge, Pr, and Re, were required for the suite of seven andyks. The internal standard to analyte pairing was based roughly on a mass: 72Ge was used as the internal standard for 60Ni,'Wu, %r, and Il4Cd; 141Prwas used for 138Ba;and 18'Re was used for 206T1 and 20sPb. Selection of the internal standards was influenced by their negligible concentration in the original samples, their availability in high-purity form, and their proximity to the analyte mass being measured. Long-term drift in calibration was negligible over more than a 2-h period when internal standards were employed. Ion-intensity data were acquired in the quantitative elemental analysis mode for all the calibration standards. A total of 14 measurements, corresponding to each of the seven analyte ions, five isobaric ions, and three internal standard ions, were determined sequentially by using a measurement time of 0.5 s and a total of five replicate. These data were uploaded into commercial spreadsheet software in another computer where the mean intensity ratios were calculated, and the calibration equation was generated by using second-order polynomial regression. Representative calibration curves obtaitled by using this procedures are shown in Figure 2. Detection limits were calculated, as defined by Skogerboe and Grant (27) and IUPAC (28), from ion intensities using a 95% confidence level; these limits are given in Table IV. The detection limits approximate the concentrations attainable by isotope dilution; however they are more than a factor of 10 better than the detection limits based on ion intensity ratios. Sample Preparation. Concentrations of the secondary enriched isotope standards were determined periodically by reverse
ANALYTICAL CHEMISTRY, VOL. 59, NO. 11, JUNE 1, 1987
1571
Table V. Optimum Operating Parameters Quadrupole Ion Optics lenses" einzels ~
isotopes
barrel
stop
@"i,63C~
+4.3 +4.6 +1.7 +5.5 +5.3 +4.3
-6.1 -4.8 -9.5 -1.0 -4.8 -5.2
@Sr
l14Cd '"Ba 206T1 208pb compromiseb CONCENTRATIONC~~~L)
Flgure 2. Calibration curves using an internal standard.
Table IV. Detection Limits ion
detection limit," pg/L
ion
detection limit: pg/L
"Ni 63Cu @Sr l14Cd
0.2 0.1 0.03 0.2
'%Ba 206T1 2oePb
0.03 0.1 0.3
a Based on 95% confidence level and the standard deviation of the blank intensity.
isotope dilution analysis using dilutions of the primary singleelement stock solutions. The expression used to calculate these concentrations was derived from eq 1by solving the expression in terms of W / V and letting C equal the concentration of the diluted single-element solution. All samples were analyzed directly, without prior sample preparation, using the calibration curve method described above. Results from these determinations then were used to calculate the amount of enriched isotope needed for a 15-mL aliquot of sample to produce a molar ratio approximately equal to unity for each analyte. The corresponding volume of enriched isotope for each analyte then was spiked into the sample using a calibrated digital pipet. The same procedure was followed for each sample. After all the samples were augmented by the addition of the enriched isotopes, the samples were loaded into the autosampler and analyzed automatically. Instrument Optimization. Ion signal strength is influenced by several factors: (1) voltage on the electron multiplier and deflector; (2) operating voltages of the ion lenses in the quadrupole mass analyzer; and (3) operating conditions of the ion source, in this case the inductively coupled argon plasma. The voltage on the electron multilier was adjusted to approximately -4100 V dc, that corresponded to the plateau region of the ion signal vs. multiplier voltage curve. The deflector was adjusted to about +3600 V dc for maximum signal at 63Cu. A modified simplex optimizationprocedure was used to optimize operating parameters for the mass analyzer and the inductively coupled plasma in separate experiments. Two separate optimization experiments were conducted because a single experiment optimizing all the important factors took substilntially longer time thereby increasing the effects from instrumental drift. Interactions between factors of both optimization experiments were not investigated, for example, the effects of the plasma conditions on the optimum lens voltages. The simplex algorithm used in this procedure was based on the King modification (29) of the Nelder and Mead algorithm (30) as described by Leary et al. (31). The response optimized was the signal-to-background ratio (S/B), where the signal was measured at the analyte isotope of interest and the background was measured at a mass in close proximity to the analyte isotope. The voltages applied to four ion lenses were chosen as factors in the simplex optimization, primarily because of their effect on the analyte signal and their easy adjustability. The ion lenses optimized were the barrel lens, stop lens, first and third einzel lenses (electrically connected), and the plate lens. Voltages on the second einzel lens, shield lens, and exit lens were held constants; these settings were -130 V dc for the second einzel lens, -5 V dc for the shield lens, and -130 V dc for the exit lens. The
-16 -17 -14 -10 -6.5 -13
~~
plate -18 -14 -16 -7.9 -14 -14
Inductively Coupled Plasma incident power: 1.2 kW plasma flow rate: 13 L/min auxilliary flow rate: 1.4 L/min -nebulizer pressure: 37 psi (0.55 L/min) sample delivery rate to nebulizer: 1.7 mL/min "All lens voltages in dc. bCompromise settings equal the lens voltage mean of all isotopes. simplex boundary for each lens optimized equaled the entire voltage range of that lens. The S/B ratios corresponding to mass measurements at 63/69 for Cu, 88/89 for Sr, 114/119 for Cd, 138/141 for Ba, and 208/209 for Pb were optimized on a single-element basis: Ni and Cu were assumed to have similar optimum values, because of the close proximity of their masses as were T1 and Pb. The optimum voltages obtained from these experiments were used for all single-element isotope dilution analyses and are tabulated in Table V for all seven analytes. When multielement isotope dilution was used, compromise lens voltages, as shown in Table V, were calculated simply by averaging the optimum lens voltages for the entire suite of isotopes. Compromise conditions sacrificed the optimum sensitivity for the suite of isotopes for the best possible sensitivity obtainable at a common set of operating voltages. The factors optimized for the inductively coupled plasma were the incident power, plasma-argon flow rate, auxiliary argon flow rate, nebulizer pressure, and sample delivery rate to the nebulizer. Simplex boundaries for these factors were based on operationally compatible flow rates, pressures, and pump rates. The simplex optimization was conducted on a single-element basis as before; lens voltages were held constant at their optimum values. The resulting optimum values for the five factors for each of the seven analytes were essentially identical, within experimental error; these optimum values for these factors also are listed in Table V.
RESULTS AND DISCUSSION Interferences. Interferences arising from isobars, oxide formation, multiply charged ions, and matrix composition were evaluated with respect to their effects on analyses by isotope dilution and standard-curve methods. Many of the interferences are spectral in nature, resulting in either additive or multiplicative effects. Isobaric interferences are influenced by the lack of mass resolution in the quadrupole system. Nominal resolution of the quadrupole mass spectrometer is approximately 1 amu; therefore, many isobaric interferences can occur. When these interferences occur, corrections must be made to maintain the accuracy of the determinations. The formation of molecular oxides effectively decreases the analyte signal, by the reduction in the population of analyte ions, and probably arises during the sampling process, either by regiond cooling of the plasma a t the sampler orifice or by rapid ion/molecule reactions in the supersonic-expansion region behind the sampler (32). Similar effects result from doubly charged ionic species. Doubly charged ions occur for elements having second ionization potentials that are less than the first ionization potential of argon. Finally, evidence has indicated that matrix composition can affect analyte-ion intensities. Significant signal suppressions have been reported with a high correlation to both mass and ionization potential (33). Ionization suppression has been identified in this report for
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 11, JUNE
1 , 1987
Table VI. Identified Ion Interferences isotope
isobar
nickel-61 44CaOH+ -60 43CaOH+ UCaO' copper-65 48CaOH+ -63 46CaOH+ strontium-86 86Kr+ -88 a cadmium-116 1*6Sn+ -114 l14Sn+ barium-135 a -138 138La+,
A
oxide
doubly charged
a a
factor: n4Cd. 0.Omx ll8Sn
a
a
%SrO+ Y+0+ a a 135Ba0+
138Ba0+,
0 A'%d in absence of Sn
135Ba2+ 13sBa2+ 138La2+ 138Ce2+
OCorrection using derived function:
n8Cd-0.617~naSn.145
138~~0+
138Ce+
OCcrrection usingnatural abundance factor:
weO+ thallium-203 a -205 a lead-206 a -208 a a
"'Cd inabsence of Sn
a
a
a
a
a a
a a
"%d.0.593xll~Sn
I
Figure 3.
various constituents that can occur in natural water at relatively high concentrations. Isobaric ions, metal oxide ions, and doubly charged ions that have been identified as interferents to the analytes in this study are listed in Table VI. The analyte isotopes were chosen on the basis of their abundance and freedom from interferences. Nickel-60 was preferred over nickel-58 because molecular ion interferences occurring at m / z 58 were sufficiently high to affect Ni determinations at microgram per liter levels. The most abundant isotopes were chosen for the remaining analytes. Corrections for isobaric interferences using data system software are based on the subtraction of the isobar's contribution to the analyte signal. The contribution is calculated by measuring the isobar intensity at one of its uninterfered isotopes and then converting the resulting intensity to one corresponding to the analyte isotope, using a ratio of the theoretical natural abundances as the conversion factor. Isobaric corrections are limited to those corresponding to elemental isotopes and do not include corrections for isobars associated with molecular ions. Interferences resulting from elemental isobars, Sn, La, and Ce, and molecular isobars, CaO and CaOH, can be significant without correction. Interference from Kr on Sr was negligible because Kr is normally found a t very low concentrations in bulk liquid argon. The method of correction outlined above for elemental isobars was evaluated by performing experiments where the analytes were determined in the presence of varying concentrations of interferent. Uncorrected data were acquired, using the data system, followed by manual calculation of the interferent's contribution to the analyte signal using the exact procedure that normally is performed automatically by the data system. Isotope abundances of the isobaric elements also were measured and compared to theoretical natural isotope abundances, since the data system used conversion factors that are based on the ratio of these assumed natural isotope abundances and since accurate corrections ultimately depend on these factors. An example of the evaluation is provided here for Cd in the presence of Sn; similar results were obtained for Ba in the presence of La and Ce, even though the procedure was complicated by the effects from signal suppression and the nonlinearity of the derived correction function. Uncorrected ion intensities were measured for l14Cd, "Td, and 118Snfor a reagent blank, and for solutions containing 50 Ng/L Cd and Sn concentrations of 0,50,100, and 500 pg/L. Tin isotope abundance measurements produced conversion factors slightly different from those based on assumed natural abundances; 114Sn/11sSntheoretical abundance ratio is 0.0271,
200
500
TIN CONClENTRATION(pg/L)
None identified.
Isobaric corrections on cadmium ion intensity.
and its experimental abundance ratio was 0.0258; l16Sn/'lsSn theoretical abundance ratio is 0.593, and its experimental abundance ratio was 0.602. Deviations of this magnitude have significant effects on the accuracy of isotope dilution determinations. A graph of corrected "4Cd and "6Cd ion intensities as a function of Sn concentration is shown in Figure 3. The original correction method, using the conversion factor based on natural abundances to calculate the contribution of Sn isotopes to Cd signals, is shown to be insufficient for the Sn concentration range of the experiment. The lI4Cd intensity is undercorrected a t low Sn concentrations, and the l16Cd intensity is undercorrected throughout the entire Sn concentration range studied. Corrections based on this method would result in low Cd concentrations by either the calibration-curve method or the isotope dilution method. These discrepancies were eliminated by deriving a simple mathematical expression, based on the actual Sn contributions to the Cd isotope intensities using linear-regressionanalysis. The resulting expressions have correction terms that are similar to those based on theoretical isotope abundances of the original correction method; however, the correction also requires an additional offset term. Background was eliminated as a possible explanation for the differences exhibited by the two correction mzthods; for example, lllCd ion intensities remained stable at Sn-free levels while in the presence of up to 500 Mg/L Sn. Corrections based on these expressions provided corrected Cd isotope intensities that were within the experimental precision of Cd isotope intensities in the absence of Sn. Derivation of a similar expression in terms of isotope intensity ratios, as in the internal standard method, works equally well, and it minimizes recalculation of the correction terms by eliminating influences from instrumental drift. A correction procedure for interferences associated with CaO and CaOH was developed and evaluated in a fashion similar to the elemental isobaric interferences. The magnitude of oxide and hydroxide interferences depends on the degree of formation of these species, the Ca isotopic abundances, and the concentration of Ca. The percentage of CaO formed, calculated as CaO+/Ca+, was approximately 0.1. The low relative abundances of &Ca and %a reduced the need for CaO and CaOH corrections on e3Cu and W u for Ca concentrations normally found in natural waters; at 100 mg/L Ca, 48CaOH+ would result in an apparent T u + intensity equivalent to about 50 pptr (parts per trillion). However, the relative abundances of 43Ca and 44Caare sufficiently high to require corrections for CaO an CaOH on 60Niand 6'Ni. A linear relationship of apparent Ni ion intensity at mlz 60 and 61 as a function of 43Ca+intensity was implemented as a means of correcting for
ANALYTICAL CHEMISTRY, VOL. 59, NO. 11, JUNE 1, 1987
Table VII. Evaluation of Derived Isobaric Corrections on Barium, Cadmium, and Nickel
element concn, gg f L barium
concn, pg f L
interferent
isotope ratio
lanthanum
'%Baf lSBa
Table VIII. Comparison of Barium Isotope Dilution Analysis with and without Corrections for Metal Oxide and Doubly Charged Ion Formation
concn,O rglL
reference material
with correctionsn
17.2 17.3 17.1 16.9
NBS 1643b SRWS 67 SRWS 93 SRWS 71 SRWS 73 SRWS 81 SRWS 69
40.6 0.50 22.6 f 0.24b 65.4 f 0.98 72.4 f 0.51 15.7 f O . N b 22.2 0.16b 34.1 f 0.26
and cerium
17
0
17
50 100 250
1.283 1.284 1.296 1.295
tin
W d f l14Cd
17 17 cadmium 12 12 12 12
0
50 100
250
nickel
calciumb
21 21 21
10 50
21 21
0 100
200
0.9712 0.9620 0.9611 0.9620
Calculated from isotope dilution analysis. milligrams per liter.
without correctionsu
*
41.8 f 1.2 22.7 f 0.22b 64.8 f 0.52 71.2 f 0.74 15.4 0.12b 22.8 f 0.43b 33.9 0.11
* *
*
Standard deviations are the confidence intervals at lu. Dilution of original sample. (I
12.1
12.4 12.6 12.4
a
E %
61Ni/@"i
0.9817 1.024 1.069 1.119 1.120
1573
20.9 20.0 19.0 18.1 18.0
Concentration in
f
lonlntenatyinababnceof Na
+-----O
5
c
"
V
"
o=S,
+--
5 - +----$
%
- - - - - - -+
3 E
v
8z
-
4 , - - - -5 + +
CaO and CaOH contributions to Ni isotope ratio determinations. The derived correction procedures were evaluated by using synthetic test solutions containing trace concentrations of Ba, Cd, and Ni augmented with varying concentrations of Sn, La, Ce, and Ca. Implementation of the derived correction functions produced the results tabulated in Table VII. The correction procedure for CaO and CaOH interference on Ni failed to perform adequately, especially a t Ca concentrations greater than 50 mg/L. A negative bias of 14% was evident in the presence of 200 mg/L Ca. The bias resulted from an unexplained shift in 43Ca+ intensity for the augmented standards; drift was eliminated as a possible cause by using an internal standard. The correction procedure for Cd and Ba gave results that were within 3% of results obtained in the absence of interferents for Sn and Ba concentrations from 50 to 250 pg/L. Where analysis time and ease of operation are important considerations, choosing isobarically uninterfered analyte isotopes is recommended, if the resulting loss in sensitivity is acceptable. Metal oxide ions associated with the suite of analytes were identified for only Sr and Ba (Table VI). Metal oxide formation only affects the overall sensitivity of these analytes; therefore, no corrections likely will be necessary when measuring isotope ratios, since every isotope abundance is equally affected. When the calibration-curve method is used, the analyte signals for both standards and samples are affected equally. Doubly charged metal ions were identified for Ni, Sr, and Ba, in agreement with their second ionization potentials. These species likely will exhibit effects similar to those for the metal oxides. The extent of metal oxide and doubly charged ion formation primarily is dependent on the operating conditions of the plasma. Under the plasma conditions and orifice diameters employed in this study, the extent of metal oxide and doubly charged ion formation was assessed by calculating ratios of pertinent ions. For example, the BaO+/Ba+ ratio was approximately 0.004, or about 1 order of magnitude lower than that reported by Douglas and French (32), and the ratio of ssSr2+/ssSr+was approximately 0.02; values for Sr were very similar. An experiment was conducted to verify that the formation of these species did not affect the accuracy of isotope dilution analysis. Ion intensities were measured a t mlz 69, corre-
w
+ 2
I
I
I
I
1
10
100
500
1574
ANALYTICAL CHEMISTRY, VOL. 59, NO. 11, JUNE 1, 1987
Table IX. Correlation Statistics for Multielement vs. Single-Element Isotope Dilution Analysis
ion
slopen
intercept
RZb
concn range, rglL
@Ni 63Cu
0.92 1.0 1.0 0.97 0.92 0.86 0.93
-0.3 -1 1 0.5 3 0.3 2
0.994 0.999 0.999 0.995 0.997 0.917 0.988
6-60 20-950 60-250 3-40 20-180 1-8 8-40
@Sr lI4Cd lasBa 205~1
208Pb (I
Based on linear regression analysis.
Correlation coefficient.
concentrations greater than approximately 20 mg/L. When matrix concentrations reach 500 mg/L, the decrease in ion intensities can be as high as 10%. The degree of signal suppression correlates with the mass of the analyte ion; however, a relationship with analyte ionization potential is not indicated. Signal suppression of this type affects analyses based on the calibration curve method; the best approach for its elimination would be standard additions. However, since the calibration curve method only is being used to estimate analyte concentrations and the concentration levels of the matrix constituents in the reference material are generally were less than 100 mg/L, the method of standard additions was not used. Matrix composition has no effect on isotope dilution determinations, because the isotope ratios for the sample and the blend are affected in the same fashion. Changes in background-ion intensities as a function of matrix composition were evaluated by using identical solutions (as described in the preceding paragraph). Background was measured at m / z of 72,103,141,169, and 209. Background intensities in the presence of less than 500 mg/L of matrix constituent equaled reagent-blank levels, generally less than 50 ions/s. Accuracy and Precision. The accuracy of analyses by isotope dilution was evaluated by using results obtained for 2 NBS and 48 SRWS reference materials. The NBS reference materials were SRM 1643a and SRM 1643b; both contain trace elements in water. The SRWS reference materials are composed of natural waters, primarily surface waters or meteorological precipitation, that have had original trace-element
concentrations augmented by known concentrations of various elements. Single-element and multielement isotope dilution analyses were compared on the basis of results obtained for a series of reference materials. Linear-regression analysis produced the coefficients of multielement results as a function of single-element results (Table IX). These correlation statistics show good agreement between the two methods. Some of the differences reflected in these statistics arise from the use of compromise-operating conditions, the deviations a t relatively low concentrations, the narrow concentration range of the reference standards for selected analytes, corrections specific to the multielement method as a result of contamination in the isotopically enriched standards, and the loss of precision in multielement data from the decrease in analysis time. Accuracy of the multielement method would be sufficient for most applications; however, since the highest attainable accuracy was obtained using single-element isotope dilution, results obtained by using this method were compared to published results. The accuracy of single-element isotope dilution analysis is supported by the results compiled in Table X. Results for only eight of the reference materials analyzed are reported here: two NBS reference materials and six SRWS reference materials, five based on natural surface water and one based on meteorological precipitation. Results for the NBS and SRWS reference material generally coincide with the certified mean, within the stated precision interval. The published means and standard deviations for the SRWS are based on round-robin analyses performed by independent laboratories, usually more than 25, using various instrumental techniques (34). Isotope dilution determinations generally agreed with the SRWS most-probable mean values, within the corresponding precision interval. The uncertainty interval for the majority of analytes is quite large because the statistics are based on interlaboratory and multimethod data; however, the most-probable value is shown to be fairly accurate. The NBS and SRWS contained only trace concentrations of La, Ce, and Sn usually making the isobaric corrections unnecessary; however the concentration for Ca ranged from 240 pg/L to 150 mg/L for the eight reference materials reported thereby requiring a correction. The external relative precision of triplicate isotope dilution determinations also is shown in Table X. The range of relative
Table X. Single-Element Isotope Dilution Analyses for Selected Reference Materials" P Ni Cu
Sr Cd Ba
T1 Pb
NBS 1643a ID
56 f 3 18 f 2 239 f 5 10 f 1 47f2 b 27f 1
58.1 f 0.5 20.1 f 0.8 239 f 3 11.9 f 0.1 46f 1 b 27.8 f 0.3
NBS 1643b
cu Sr Cd Ba T1 Pb
SRWS 69
ID
P
ID
P
ID
50 f 3 22.3 f 0.4 227 f 6 20 i 1 45 f 2 8.1 f 0.2 24.1 f 0.7
55 f 1 24 f 1 235 f 2 21.9 f 0.3 42 f 1 8.0 f 0.02 24.1 f 0.2
22 f 8 443 f 20
19.3 f 0.1 444 f 4 b 11.5 f 0.2 718' b 38 f 1
18 f 7 297 f 18 612 f 52 0.8 f 1 43 f 22
17.9 f 0.02 297 f 2 642c 0.51 f 0.01 33.9 i 0.1 0.87 f 0.01 19.1 f 0.6
SRWS 81
Ni
SRWS 53
P
b 12 f 4 760 f 70 b 41 f 10
SRWS 89
2d
23 f 16
P9
SRWS 91
P
ID
P
ID
P
ID
9f6 29i 4 439 f 24 9fl 239 f 26 4 f Id 4f2
6.3 f 0.1 25.5 f 0.4 459 i 1 8.7 f 0.2 226' 4.9 f 0.2 3.2 f 0.2
25 f 9 18 f 4 1739 f 57 15 f 3 70 i 12 4 fId 19 f 2 1
37 f 1 11.9 f 0.2 18OOc 15.5 h 0.1 64 f 1 5.7 f 0.1 0.8 f 0.1
20 f 7 940 f 40 119 f 7 35 f 4 41 f 11 3d 17 f 8
24.9 f 0.2 954c 126 f 2 36.8 f 0.1 37f 1 3.28 f 0.01 18.8 f 0.6
P b 4 f l
b 1.1 f 0.3 b 1.5 f 0.3d 6f7
ID
b 3.79 f 0.05 b 0.87 f 0.05 b 1.4 f 0.1 1.4 f 0.3
"Concentration units are pg/L, precision interval for published (P) and isotope dilution analysis (ID) results is the overall uncertainty of the mean and the standard deviation at la, respectively. Not determined. Dilution factor applied. dPublished SRWS data based on less than five measurements.
ANALYTICAL CHEMISTRY, VOL. 59, NO. 11, JUNE 1, 1987
standard deviations for each analyte over the concentration range of the reference standards in Table IX was as follows: for Ni, 0.8-3%; for Cu, 0.6-4%; for Sr, 0.2-2%; for Cd, 0.32%; for Ba, 0.3-3%; for Tl, 0.2-7%; and for Pb, 1-21%. The highest relative standard deviations corresponded to the lowest mean concentrations.
CONCLUSIONS Isotope dilution analysis using inductively coupled plasma mass spectrometry is capable of rapidly providing accurate analytical results for the determination of trace metals in natural, nonmline waters. Solutions can be analyzed directly without any special pretreatment procedures, such as preconcentration, chemical separations, or derivatizations. Data presented here indicate the accuracy of the method compares favorably with established techniques and the sensitivity and detection limits exceed most commonly employed instrumental techniques. Interferences resulting from isobars, the formation of metal oxide and doubly charged ions, and matrix composition have been identified, and correction methods have been implemented when required. The method employed for correcting elemental isobaric interferences arising from Ce, La, and Sn was shown to be accurate, while a similar method for correcting molecular ion interferences from CaO and CaOH was shown to be inaccurate a t Ca concentrations greater than 50 mg/L. The scope of the technique was limited here only by the availability of enriched isotope materials; determinations of additional analytes are possible. The instrumentation and technique provide a relatively inexpensive system capable of performing accurate and precise determinations that are ideally suited for programs associated with certification of standard reference materials.
ACKNOWLEDGMENT We thank D. W. Golightly for providing a copy of the modified simplex optimization software used in this study. Registry No. Ni, 7440-02-0; Cu, 7440-50-8; Sr, 7440-246; Cd, 7440-43-9; Ba, 7440-39-3; T1, 7440-28-0; Pb, 7439-92-1; H20, 7732-18-5.
1575
(5) Hintenberger, H. I n Electromagnetlcally Separated Isotopes and Mass Spectroscopy;Smith, M. L., Ed.; Butterworths: London, 1965; p 177. Moore, L. J.; Machlan, L. A. Anal. Chem. 1972, 44, 2291. a r m , E. L.; Machlan, L. A.; Gramlich, J. W.; Moore, L. J.; Murphy, T. J.; Barnes, I. L. NBS Spec. Pubi. 1976, No. 422, 951. (8) Gramllch, J. W.; Machlan, L. A.; Murphy, T. J.; Moore, L. J. Proc. Trace Substances in Environmental Heanh, X I 1977, 376. Kaaaku (9) Murozumi. M.; Nakamwa. S.;Igarashi, T.; Yoshda, K. Nlmon .. Kaishl 1981, 122, 132. (IO) Murozumi, M.; Igarashi, T.; Nakamura, S.Nippon Kagaku Kaishi 1982, 123. . - - , 54. - ..
(11) Heumann, K. G.; Kastenmayer, P.; Zelninger. H. 2. Anal. them. 1981, 306, 173. (12) Kownatski, R.; Peters, F.; Reil, G. H.; Maab, G. Biomed. Mass Spectrom. 1980. 7 . 540. (13) Reamer, D. C.; Veillon, C. Anal. Chem. 1981, 5 3 , 2166. (14) Nielsen, T.; Egsgaard, H.; Larsen, E. Anal. Chim. Acta 1981, 124, 1. (15) Alvarez, R.; Paulsen, P. J.; Keiieher. D. E. Anal. Chem. 1969, 41, 955. (16) Pauisen, P. J.; Alvarez, R.; Muelier, C. W. Anal. Chem. 1970, 42, 673. (17) Schulten, H. R.; Bahr, U.; Lehmann, W. D. Mickrochim. Acta 1979, 191, 198. (18) Schulten, H. R. Int. J . Mass Specttom. Ion Phys. 1979, 32, 283. (19) Bahr, U.; Schutten, H. R.; Achenback, C.; Ziskoven, R. Z . Anal. Chem. 1982, 312, 307. (20) Houk, R. S.;Fassel, V. A.; Fiesch, G. D.; Svec, H. J.; Gray, A. L.; Taylor, C. E. Anal. Chem. 1980, 5 2 , 2233. (21) Tan, S. H.; Horiick, G. Appl. Spectrosc. 1986, 4 0 , 445. (22) Gray, A. L.; Date, A. R. I n t . J. Mass Spectrom. Ion Phys. 1983, 4 6 , 7. (23) Park, C. J.; French, J. B. Presented at the 1I t h Annual Meeting of the Federation of Analytical Chemistry and Spectroscopy Society, Phiiadelphia, 1984, Paper 500. (24) Boomer, D. W.; Poweii, M.; Sing, R. L. A,; Saiin, E. D. Anal. Chem. 1986. 58, 975. (25) Gray, A. L. Analyst (London) 1985, 110, 551. (26) De Bievre, P. J.; Debus, G. H. Nucl. Instrum. Methods 1965, 32, 224. (27) Skogerboe, R. K.; Grant, C. L. Spectrosc. Lett. 1970, 3 , 215. (28) International Union of Pure and Applied Chemlstry Appi. Spectrosc. 1977, 3 1 , 345. (29) King, P. G. Ph.D. Dissertation, Emery University, Atlanta, GA, 1974. (30) Neider, J. A.; Mead, R. Comput. J. 1965, 7 , 308. (31) Leary, J. J.; Btookes, A. E.; Dorrzapf, A. F., Jr.; Golightly, D. W. Appi. Spectrosc. 1982. 3 6 , 37. (32) Douglas, D. J.; French, J. B. Spectrochim. Acta, Part B 1986, 418, 197. (33) Horllck, G.; Tan, S. H.; Vaughan, M. A,; Lam, J.; Shao, Y. Presented at the Pittsburgh Conference, Atlantic City, NJ, 1986; Paper No. 266. (34) Janzer, V. J. The Use of Natural Waters as U . S . Geological Survey Reference Samples ; Quality Assurance for Environmental Measurements, ASTM STP 867; Taylor, J. K., Stanley, T. W., Eds.; American Society for Testing and Materials: Philadelphia, PA, 1985, pp 319-333.
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RECEIVED for review October 6,1986. Accepted February 25, 1987. The use of trade names is for descriptive purposes only and does not imply endorsement by the U.S. Geological Survey.
