Anal. Chem. 1994,66, 1027-1031
Comparative Strategies for Correction of Interferences in Isotope Dilution Mass Spectrometric Determination of Vanadium Jack D. Fassett' and Eiiyn S. Beary National Institute of Standards and Technology, Technology Administration, Department of Commerce, Gaithersburg, Maryland 20899 Xiaoxiong Xiongt and Larry J. Moore Eastern Analytical Inc., 335 Paint Branch Drive, College Park, Maryland 20742
Vanadium has been determined inSRM 1573a, TomatoLeaves, by isotope dilution mass spectrometryusing thermal ionization (TIMS) and resonance ionization (RIMS). The capabilities of the two techniques to compensate for interferences from chromium and titanium are compared. For thermal ionization, corrections to the m / e 50 signal of 13-60% were made for interferences. The resulting V concentration had a relative standard deviation of 1.2%. Regression analysis of the data identified second-ordercorrections that resulted in a betweensample precision of 0.46%. Resonance ionization had a demonstrated specificity of 20001 against Cr and Ti interferences. The resultingratio measurementswere less precise than the thermal ionization,but the vanadium ratio could be measured directly with no corrections necessary for interferences. The concentration(f1u measurement uncertainty)for V determined was 835.4 f 3.8 ppb by TIMS with second-order interference corrections, 824 f 10 ppb by TIMS as initially measured, and 828 f 15 ppb as measured by RIMS. Isotope dilution mass spectrometry (IDMS) is a highly accurate analytical technique based on the measurement of elemental isotope ratios.' Natural isotopic ratios are altered by the addition of calibrated amounts of a separated, stable, "spike" isotope. After equilibration of the spike with the sample, the ratio defines the concentration and quantitative separation of the analyte element is not required for this quantitative concentration measurement. The technique is inherently accurate because potential sources of systematic error are understood and controllable. One of the major sources of potential systematic error in isotope ratio measurement is mass spectrometric isobaric interference. Isobaric interferences can result from elemental interference, molecular ion interference, or multiply charged ions. There are a variety of experimental procedures to control these interferences. First, potential interference can be targeted during chemical treatment and purification of the analyte.2 Second, mass spectrometer operation can be optimized to minimize instrumental sources of interference. The selectivity of the ionization process can be tailored for the analyte. And, third, interferences can be monitored and corrections applied to measured isotopic ratios. (1) Fassett, J. D.; Paulsen, P. J. Anal. Chem. 1989, 61, 643A449A. (2) Beary, E. S.; Paulsen, P. J. Anal. Chem. 1993, 65, 1602-1608. 0003-2700/94/0366-1027$04.50/0 0 1994 American Chemical Society
The accurate measurement of vanadium isotopic ratios provides a significant test of our ability to control interferences. Vanadium is an element with two stable isotopes, but is principally slV (51V/50V= 400). The 50Vspike is available to an enrichment of only -36% and this minor isotope of V is interfered with by isotopes of both titanium and chromium. A procedure has been developed in our laboratory for determination of V by isotope dilution and thermal ionization mass spectrometry The interferences limit the capability of this method, especially in its application at the parts-per-billion level, or at the natural levels in serum and foodstuff. The various experimental strategies for controlling the interferences of chromium and titanium in the measurement of 50V/51Vare evaluated in this paper. These experiments were done in the course of determining the V content in tomato leaves, SRM 1573a,which is in thecertification process. Spiked samples were split and isotopic ratios were made by both thermal ionization and resonance ionization mass spectrometers (RIMS).
EXPERIMENTAL SECTION Reagents and Laboratory Ware. High purity reagents, produced by subboiling distillation at NIST and stored in Teflon (FEP) bottles, were used in this study. (Certain commercial equipment, instruments, or materials are identified in this paper in order to adequately specify the experimental procedure. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.) All sample handling was performed in a clean room equipped with laminar flow hoods which met class 10 specifications. Chelex- 100resin (Bio-Rad Laboratories, 200400 mesh) was used to prepare one column. Cation-exchange resin (Bio-Rad Laboratories, AG50x8, 100-200 mesh) was used to prepare a second column. In each case -6-mL of cleaned and conditioned resin was placed in the acid-cleaned polypropylene column. The enriched sOVspike is the same as used in our previous work. The measured isotopic s0V/51V has not changed (3) Fassett, J. D.; Kingston, H. M. Anal. Chem. 1985, 57, 2474-2478.
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significantly: 50Vis 36.089% and 51Vis 63.91 1% (50V/51V = 0.564 68; standard deviation of a single measurement, fO.OO1 13; number of determinations, 15). The V concentration of the spike was determined by isotope dilution and thermal ionization mass spectrometry vs gravimetrically prepared solutions of natural vanadium. Long-term stability of the vanadium spike solution has been verified over many years. The Re filaments were outgassed at 2000 OC for 30 min in a vacuum under a potential field. A layer of spectroscopic grade graphite was dried onto the filament from an acetone slurry before outgassing. Chemical Preparationand SeparationProcedures. Tomato leaf samples, 1 g, were taken for analysis and for drying. The specified procedure of drying over magnesium perchlorate for 120 h resulted in an average weight loss of 3.3%. The basis weight for each analytical sample was corrected by use of its paired, dried sample. The analytical samples were spiked with 50V and digested in an open beaker on a hot plate using a HN03/HC104/HF mixture. The HC104 promoted the decomposition of the organic matrix and ensured that the V was in the +5 oxidation state necessary for the chromatographic separation. The samples were evaporated to dryness and rinsed with water to remove the HC104. The first separation used the Chelex-100 column. The samples were loaded onto the column in 5.5 g of 2 mol/L ammonium acetate, pH 6.03. Impurities were eluted using 15-20 g of 2 mol/L ammonium acetate, followed by a water rinse. The V was eluted with 15 g of 2 mol/L ammonium hydroxide. These fractions were evaporated to dryness, rinsed with water, and evaporated again. The second separation used the cation-exchange column. The sample residues were redissolved in 4 g of 0.1 mol/L HCl/l% H202 and loaded onto the column. The samples were added in small portions as the eluent was collected. Ten grams of 0.05 mol/L HCl/ 1% H202 was used to elute the vanadium. The sample portions containing the V were evaporated to dryness and converted to the nitrate for the mass spectrometric analysis. Concentration Calculation. The principles of isotope dilution are well established.' The uncertainty in isotope ratio measurement is magnified in the calculation of concentration, and this factor is dependent upon the enrichment of the spike relative to the natural isotopic composition of an element. The minimum error magnification factor for this *OV spike is 1.143 and occurs for an experimental ratio of 50V/51V= 0.037 62. However, counting statistic uncertainty dominates in this case and the samples were deliberately overspiked to account for this fact. The experimental ratios ranged from 0.144 to 0.184, with accompanying error magnifications of 1.36-1.50. It is seen that the increase in error magnification is more than offset by a factor of 2 improvement in counting statistics. An uncertainty analysis which considers the conditions of vanadium measurement has recently been publi~hed.~ Blank Measurements. Two types of blanks were measured in this study using thermal ionization. The mass spectrometric loading blank, which estimates the instrumental background
-
(4) Adriaens, A. G.; Kelly, W. R.; Adam, F. C. Anal. Chem. 1993.65.66C-663.
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Analytical Chemistry, Vol. 66,No. 7, April 1, 1994
for vanadium, was determined by directly loading 500 pg of the spike solution on a Re filament in the same manner as the samples. The result, 21 pg, agreed with previous measurements. The chemical blank was measured (in duplicate) by adding 5OV spike to quantities of reagents equal to those used to dissolve the samples, evaporating this material to dryness, separating the V in the same manner as a sample, and determining the V by isotope dilution. The chemical blank estimates the contamination that occurs during chemical processing, but also includes the contribution due to the loading blank. The blank levels measured were 107 and 91 pg. Mass Spectrometry. The samples for RIMS and TIMS were prepared in an identical manner. A Re filament was coated with a layer of graphite from a slurry of graphite in acetone. The V sample in 5 p L of dilute HCl was dried upon this graphite-treated filament. In contrast to previous work, no additional graphite was added. The graphite promotes the reduction of vanadium oxides and the volatilization of Vo and V+ specie^.^ This procedure improves the sensitivity and, for RIMS,S the selectivity of the ionization process. A single magnetic sector thermal ionization mass spectrometer of basic NBS design, which possessed a highsensitivity ion counting detector system was used. This instrument and the procedures for making thermal ionization measurements of V have been described previously.3 The vanadium measurements were made at 1350-1400 OC, as measured with an optical pyrometer focused on the back of the filament. The resonance ionization mass spectrometer instrument utilized for this study has been previously described in detail.5 The laser consists of a pulsed (10-Hz), Nd:YAG pumped, tunabledye systemcapableof producing 3 mJof UVradiation when frequency doubled. The mass spectrometer consists of a 60' radius of curvature magnetic sector with a thermal source and an ion multiplier detector. The instrument is fully automated with respect to magnet switching, laser wavelength scanning, and data acquisition. The previously used data quantification system based on the Tektronix 76 12D transient digitizer has been replaced here. A Stanford Research boxcar averager (Model 245) was used with a integrating window which encompassed the time-of-flight of both isotopes. The per pulse output of the boxcar was signal averaged using the mass spectrometer control software written in Basic. The RIMS V measurements were made at 1150-1200 OC asmeasured pyrometrically. The temperature of the filament was adjusted such that the 51V signal levels among samples were reasonably constant (within a factor of 2). The gain of the multiplier was kept constant (1300 V) for all RIMS measurements. RESULTS AND DISCUSSION As sample sizes and corresponding ion currents used to measure isotope ratios become increasingly small, the control of interferences becomes crucial for accurate analysis using IDMS. There are two primary ways to handle potential interferences: chemical and instrumental. Separation and purification of the analyte element by chemical means has (5) Fassett, J. D.;Travis, J. C.;Moore, L. J. In Applicationsof fuser Chemistry andDiagnostics;Harvey, A. B., Ed. Proc. SPIE Int. SOC.Opt. Eng. 1984,482, 36-43.
Table 1. Results tor Vanbdlum In SRM 1573a, Tomato Leaves thermal ionization
corrections (% ) sample v1 v2 v3 v4 v5 V6 v7a V7Aa av
resonance ionization
5'3V/51V
RSD (% )
Ti
Cr
conc (ng/g)
wV/61V
RSD (%)
conc (ng/g)
0.150 94 0.164 43 0.167 74 0.183 62 0.156 77 0.154 33 0.144 11 0.147 89
0.26 0.34 0.06 0.24 0.85 0.26 0.36 0.37
7.2-8.6 21.7-30.8 36.3-39.0 28.9-28.2 23.0-34.2 19.0-21.8 0.0-0.0 34.1-38.9
7.5-4.3 5.8-2.6 4.9-3.0 3.8-2.3 13.5-5.8 5.8-2.6 60.1-33.3 7.7-1.9
832 822 815 828 817 821 843 814 824 10 1.2
0.151 57 0.163 38 0.167 46 0.187 49 0.152 07 0.152 41 0.145 91 0.144 95
1.18 0.86 1.09 1.23 0.98 1.27 1.37 1.50
827 830 816 802 852 835 829 836 828 15 1.8
SD RSD ( % ) a
Sample was split after spiking and separations reversed.
traditionally been the way that IDMS using thermal ionization is done. Although chemical purification of the analyte considerably reduces the sources of potential interferences, this process is done in TIMS because of the sensitivity of thermal ion sources to impurities. Impurities can significantly affect signal intensities and isotopic fractionation, which is reflected in the reproducibility of the measurement process. There are disadvantages and difficulties associated with the requirement to make chemical separations which are amplified as the amount of analyte becomes increasingly small. The first concern is contamination. All reagents and laboratory handling have the potential for introducing contamination, or blank, to the sample. For instance, previous IDMS measurements of V showed that the V content of reagents was in the picogram per gram range, and it was estimated that the reagents used for dissolution and separations added 74 pg of V to the sample. The blank levels measured here areconsistent with these previous estimates. The second concern is separation efficiency. Losses of analyte cannot be tolerated in separations of small amounts of analyte. Such losses result in reduced ion signals and sensitivity. Although such losses by themselves do not affect accuracy in isotope dilution because the isotopic ratio defines the concentration, the effect of blank is magnified, which can result in significant, potentially erroneous corrections. Previous work reported from this laboratory illustrated results achieved when a cation-exchange separation was followed by a Chelex-100 ~eparation.~ Since the blank for that procedure at that time was 3 ng for V, a separation using Chelex- 100 alone was first tried, unsuccessfully. The cationexchange separation was then included. Thus, the separation procedure used here reversed the order of the two chromatographic separations, which coincidentally resulted in a significantly smaller blank than the previous work. However, as a point of comparison, one sample was split and separated both ways: Chelex/cation and cation/Chelex. Both methods of separation left considerable residual Cr and Ti as a challenge for the mass spectrometric measurement. Thermal Ionization Mass Spectrometry. Thermal ionization results are summarized in Table 1. Corrections for 'OTi and 'OCr were made by measuring 49Tiand 52Crand using the known natural isotopic abundances of these elements, '@Ti/ 49Ti= 0.954 and 50Cr/'2Cr = 0.051 858, to determine these
Table 2. Typlcal Thermal Ionlzatlon Run (Sample V2) av signals (ions/s) corrections ( 5% )
set
51V
sototal
4Ti
1 11689 2663 606 2 21735 5086 1341 3 17025 4124 1260 4 14843 3684 1193
6aCr %tal/61V
3008 4384 2545 1905
0.22756 0.23340 0.24178 0.24818
Ti
Cr
wV/61V
21.7 25.2 29.1 30.8
5.8 4.5 3.2 2.6
0,16477 0.16422 0.16375 0.16499 ~
interferences.6 Although 48Ti, which is 13.6 times more abundant than 49Ti, could have provided a more precise estimate of the 'OTi interference, it was avoided because of the potential interference of this isotope from 48Ca. The corrections were determined by measurement of the Ti and Cr beam intensities before and after the V ratio measurement and interpolation of these signals by assuming linear change with time. The observed mass spectrometric behavior of these two elements is considerably different. The optimum temperature for Cr emission is at a lower temperature than V. A Cr signal, if present, tends to decrease in time relative to V, while a Ti signal, if present, tends to increase in time relative to V. A typical mass spectrometric run illustrating the ion signal levels and the ratios measured and corrected is illustrated in Table 2. Resonance Ionization Mass Spectrometry. The RIMS of V has been demonstrated previously in this laboratory.' As noted therein, thermal vaporization of V results in the population of the low-energy excited levels according to the Boltzman distribution.8 Although this distribution of atom population fractionally reduces the vanadium that is accessible with a single resonance ionization scheme, the number of possible schemes increases because of the larger availability of initial states. The fact that we have chosen a scheme different from that was used previously illustrates this flexibility. The RIMS schemeused for V is a two-photon, single-color one (1 1) which has been used in our laboratory for a wide range of elemenkg We have examined the spectroscopy for
+
(6) DeBievre, P.; Gallet, M.; Holden, N. E.; Barnes, I. L. J . Phys. Chcm. Re$ Dura 1984, 13, 809-891. (7) Mayo, S.;Fassett, J. D.; Kingston, H. M.; Walker, R. J. Anal. Chem. 1990, 62, 24C-244. (8) Fassett, J. D.; Moore, L. J.; Travis, J . C.; Lytle, F. E. Inf. J. Mass Specrrom. Ion Phys. 1983, 54, 201-216. (9) Moore, L.J.; Fassett, J. D.; Travis, J. C. Anal. Chcm. 1984,56, 277C-2775.
Analytical Chemistry, Vol. 66, No. 7, April 1, 1994
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.....-
I
Table 4. Selectlvlty of RIMS vs TIMS RIMS (volts)
48
TI 51
- v 52 ...........
mass
Cr
TIMS (ions/s)
295.52 nm
9600 8360 1500
4.9 C0.002 300 >385
Sample V3 51v
48Ti 52Cr
0
>. - 3
51v
3
48Ti 52Cr
a C
13400 5650 2340
0.77 Sample V6 4.0 0.006 0.042 74.1
14.1
2350 1760
M
t;;2
0 295.50
295.65
295.80
296.85
297.00
Wavelength (nm) Figure 1. Resonance ionization spectra of V, Cr, and Ti for the wavelengthregion applied in this paper. All measurementswere made from e slngle fllament wlth a mlxture of these elements dried upon It, heated to 3.0 A. The gain of the detector was increased (1.4 keV) for the Ti and Cr scans from the gain for V (1.0 keV).
Table 3. Spectroscopic Informetlon for Resonance Ionlrallon of V, TI, and Cr
element
Ei (cm-1)
Ti
0.00 170.13 386.87 386.87 137.38 323.42 323.42 553.02 0.00 0.00
V
Cr
Ji 2 3 4 4 2l/2
3l/2 3112 4l/2 3 3
Ef(cm-1)
Jt
33680.16 33980.68 34078.61 34205.00 339 76.02 341 28.04 341 67.84 343 74.81 33671.55 33816.06
3 2 3 4 l'/z 3l/2 2l/2 3l/2 3 4
A
(nm)
296.91 295.76 296.81 295.70 295.52 295.82 295.47 295.67 296.99 295.72
V, as well as for Cr and Ti, the elements that have specific isobaric interferences. Figure 1 shows the spectra taken by loading microgram quantities of each element on a filament and scanning the laser wavelength under similar conditions while the major isotopes of each element (48Ti, 51V, 52Cr) were monitored. The peaks are all ascribed to 1 + 1 resonance ionization schemes1° as shown in Table 3. The wavelength used for V at 295.5nm corresponds to a region where chromium and titanium interferences are negligible. The higher wavelength region was also scanned to identify a RIMS peak for Ti (296.9 nm) that was used with the Cr peak at 295.7 nm to assess the selectivity achieved using the resonance ionization. The RIMS selectivity was estimated for the samples by measuring the signals at m/e 48, 50, 51, and 52 at the V resonance wavelength and then measuring the m/e 48 and 52 signals at the resonance wavelengths of Ti and Cr, respectively. The results are shown in Table 4 for two of the samples. In sample V3, Ti and Cr were detected and selectivity factors of 2000 were calculated for the resonance ionization process. Sample V6, in contrast, had lower levels of both Cr and Ti
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(10) Moore, C. E. Narl. Stand. Ref: Data Ser. (US., Natl. Bur. Stand.) 1971, NSRDS-NBS 35.
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and the selectivity was high enough that no signal was observed for these elements at the V resonance wavelength. The ratio of TIMS vs RIMS signals for the three elements is not consistent between the two samples and probably reflects the different running conditions, especially the higher temperature required to achieve thermal ionization, as well as the different relative amounts of the elements in the two samples. Isotope effects must be considered in any RIMS measurements. The processes leading to laser-induced isotopic effects have been discussed in several recent studies.llJ2 Measured ratios can be affected by (1) the laser wavelength, bandwidth, and its resetability; (2) the optical transition(s); (3) the degree of transition saturation and laser power; and (4) the polarization state of the laser. In addition to laserinduced effects, we have also been concerned with the linearity of the detection system used in our RIMS instrument. Thus, we have measured the ratios for V under closely matched conditions. In order to normalize the ratio measurements between the RIMS and the TIMS instruments, we prepared a series of mixes of spike and natural vanadium whose ratios were similar to the samples. The result was a RIMS/TIMS factor equal to 0.981 f 0.013 ( N = 6) for the 50V/51Vratio, and this factor was used to correct the RIMS sample ratios. This factor (and its reproducibility) could be ascribed to a laser-induced isotope effect or just as easily to detector discrimination or some other instrumentation effect. The RIMS results for the tomato leaf samples are included in Table 1. Two groups of five sets of ratios were taken for each sample. The temperature was slightly increased between each group and the instrument refocused. An entire run took 60 min. The among-set precision shown in the table reflects the pulse-to-pulse variability of the RIMS process. Second-Order Interference Corrections for TIMS. The TIMS results in Table 1 have a strong correlation between the Ti and Cr corrections and the calculated concentration. This correlation is best illustrated by the split sample V7, which should have the same ratio, irrespective of the order of separations, and irrespective of the amounts of Cr and Ti present. Thus, the data indicate that the corrections were not wholly adequate in compensating for the interferences. The main source of this residual error is the use of the reference values for the 50Ti/49Tiand 50Cr/52Cr,especially the highly uncertain 50Ti/49Tireference value. Even if these reference values were well-known, instrumental isotopic fractionation (11) Wunderlich, R. K.; Wasserburg, G. J.; Hutcheon, I. D.; Blake, G. A. Anal. Chem. 1993,65, 1411-1418. (12) Wunderlich, R. K.; Hutcheon, I. D.; Wasscrburg, G. J.; Blake, G. A. In?. J . Mass Spectrom. Ion Processes 1992, 115, 123-155.
Table 5. Regresalon Analysis of V TIMS Data Accordlng to Equatlon 1 param value SD RSD (%) dependency
a b C
835.4 -0.5464 0.1198
3.8 0.1209 0.0994
0.46 22 83
0.9243 0.8926 0.6939
would result in measured values that differed systematically from these reference values. In principle, these secondary effects could be calibrated, but the complexity of the process, the limited time available, and generally small signals would constrain much gain in accuracy. The correlation between the concentration and the interference corrections was evaluated by regression of the data according to the following equation: C, = a
+ b(Ti,) + c(Cr,)
(1)
where C,,,is the calculated concentration and Tic and Crc are the percent interference corrections applied. The adjustable parameters (I, b, and c can be interpreted as the true V concentration and instrumental fractionation factors for the Ti and Cr ratios used to make the corrections. A commercial software package that applied the Marquardt-Levenberg algorithm was used to make this regre~si0n.l~ The results are summarized in Table 5 . This regression analysis is possible only because of the good precision with which the isotope ratios can be measured using thermal ionization and the fact that the apparent variability of vanadium in this SRM is small. This second-order correction resulted in a reduction of the measurement RSD (13) Sigmaplot@,Jandel Scientific, 65 Koch Rd., Cork Madera, CA.
from 1.2% to 0.46% and a shift in average value from 824 to 835 ng/g (1 -3%). The instrumental fractionation factors calculated by the regression of the data of this experiment can be used to estimate the best 5@Ti/49Ti and S°Cr/52Crratios to apply to the data. The correction factors to apply to the reference ratios were 1.0477 f 0.0106 for the Ti and 0.9895 f 0.0087 for the Cr, indicating ratios of 50Ti/49Ti= 0.999 f 0.10 and 50Cr/52Crof 0.5132 f 0.0063. It should be noted that simultaneous corrections for the Cr and Ti were offsetting, depending on their relative magnitudes.
CONCLUSIONS This paper illustrates various strategies that can be applied when interferences are significant in isotope ratio measurement. All strategies have disadvantages that must be considered. Separations tend to be time consuming and have the potential for contamination and analyte loss. A specific ionization technique such as RIMS must be carefully calibrated and does not have the demonstrated precision that can be achieved using thermal ionization. In thermal ionization, corrections can be determined for elemental interferences, but this process requires an increase of time spent monitoring masses other than the analyte with concomitant loss of precision and sensitivity. Corrections add uncertainty to the measurement and are a potential source of systematic error, as illustrated here. As the magnitude of the correction increases, the uncertainties also increase. Nonetheless, we have demonstrated accurate measurement of vanadium is possible at sub-ppm levels with comparable amounts of elemental Cr and Ti present using both the specific ionization technique of RIMS and the nonspecific ionization technique of TIMS. Received for review October 1, 1993. Accepted December 17, 1993."
* Abstract
published in Advance ACS Absrracfs, February 1, 1994.
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