Anal. Chem. 1997, 69, 791-793
Measurement of Platinum Isotope Amount Ratios by Negative Thermal Ionization Mass Spectrometry Using a Thermionic Quadrupole Mass Spectrometer Ce´line S. J. Briche, Philip D. P. Taylor,* and Paul De Bie`vre
Institute for Reference Materials and Measurements, European Commission, JRC, B-2440 Geel, Belgium
A thermal ionization mass spectrometric procedure was developed for the measurement of platinum. Single- and double-filament techniques were investigated. Negative Pt ions were produced in the source of a thermionic quadrupole mass spectrometer equipped with a secondary electron multiplier operated in ion counting mode. A high-purity platinum aqueous solution of natural isotopic composition was used as sample on a rhenium filament. This method allows the natural amount ratio of the major isotopes n(194Pt)/n(195Pt) to be measured with a repeatability of 0.3%. Since the appearance of platinum-containing catalytic converters in cars there is an increased need for accurate measurement of platinum in the automobile industry as well as in the environment. In both these fields, there is a need for platinum certified reference materials.1 For the certification of both elemental and isotopic reference materials, thermal ionization mass spectrometry (TIMS), usually in combination with isotope dilution analysis, is an excellent tool, in view of its superior precision on the measurement of isotope amount ratios (also called colloquially “isotope ratios”) and its potential for improved accuracy. Isotope dilution mass spectrometry has been described by the Comite´ Consultatif pour la Quantite´ de Matie`re (CCQM) as a primary method of measurement and will therefore play an important role in providing certified values traceable to the SI (Syste`me International) system. Procedures for measuring platinum isotopic composition have been very scarce. In 1956, White et al. used a magnetic sector mass spectrometer and developed a method that produced positive thermal Pt ions.2 Obtaining high ion currents using positive Pt ions is problematic because of the high first ionization potential of Pt at ∼9.0 eV. This was later acknowledged by IUPAC as the best isotopic composition measurement of platinum until 1995.3 Other more recent studies have shown that it is possible to produce negative thermal Pt ions (electron affinity EA ) 2.13 eV) and platinum oxide ions from a heated platinum filament.4,5 In this work, a new method for isotopic measurement of negative Pt (1) Bettmer, J.; Buscher, W.; Cammann, K. Fresenius J. Anal. Chem. 1996, 354, 521-528. (2) White, F. A.; Collins, T. L.; Rourke, F. M. Phys. Rev. 1956, 101, 17861791. (3) Taylor, P. D. P.; Valkiers, S.; De Bie`vre, P.; Flegel, U.; Kruck, Th. Proceedings of the Second Alfred Q. Nier Symposium on Inorganic Mass Spectrometry, Durango, CO, May 10-12, 1994. (4) Heumann, K. G. In Inorganic Mass Spectrometry; Adams, F., Gijbels, R., Van Grieken, R., Eds.; Wiley & Sons Inc.: New York, 1988; Chapter 7. S0003-2700(96)00736-6 CCC: $14.00
© 1997 American Chemical Society
ions by TIMS has been developed. An ion beam of Pt- is produced by depositing a platinum sample solution on a heated rhenium filament. The measurements are performed on a thermionic quadrupole mass spectrometer. EXPERIMENTAL SECTION Reagents. For the ionization enhancement, analytical grade lanthanum and barium salts (Merck, Darmstad, Germany) were used: La(NO3)3‚6H2O was dissolved in Milli-Q (Millipore) water and Ba(OH)2‚8H2O in acetic acid (Merck). A silica gel solution was prepared from Aerosil 3000 hydrolyzed methylsilane (Degussa, Frankfurt, Germany).6 The boric acid solution used with the silica gel was prepared from the reference material IRMM 011 (IRMM, Geel, Belgium), which was dissolved in subboiled water. The platinum solution was obtained by dissolving a piece of platinum ribbon (Johnson Matthey, Karlsruhe, Germany) in aqua regia.7 Instrumentation. The measurements were performed using a thermionic quadrupole mass spectrometer THQ (Finnigan MAT, Bremen, Germany). It was equipped with a single Faraday cup in line with the quadrupole axis and a secondary electron multiplier (SEM) mounted 90° off-axis (type 217, Balzers AG, Liechtenstein) coupled to an ion counter (Kontron electronicUniversal counter/timer 6006, Kontron Messtechnik GmbH, Eching, Germany). The dead time for the ion counter was 25 ns. Samples were introduced in the mass spectrometer via a turret taking 13 samples that accepts single, double, and triple filaments. In order to reduce the background due to electrons, two small magnets were positioned symetrically on the outside of the flange supporting the ion source. Filament Loading. The filaments were made from a 99.99% rhenium ribbon, width 0.7 mm, thickness 0.025 mm (Goodfellow, Cambridge, UK). The rhenium ribbon was cleaned by heating it for ∼1 h in 5% HNO3, rinsing it several times with subboiled water and drying it in a clean bench. The filaments were degassed at 4.0 A for 20 min in a vacuum of ∼10-6 mbar before their loading. The loading procedures on double and single filaments were as described hereafter. Double-Filament Technique. On the ionization filament, either 60 µg of Ba or 30 µg of La was loaded and dried by applying a current of 0.5 A to the filament. The current was then increased (5) Creaser, R. A.; Papanastassiou, D. A.; Wasserburg, G. J. Geochim. Cosmochim. Acta 1991, 55, 397-401. (6) Kynaston, A. Institute for Reference Materials and Measurements; Geel, personal communication, 1995. (7) Van Nevel, L. Institute for Reference Materials and Measurements; Geel, personal communication, 1995.
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to 2 A for a few seconds for barium and to 1.5 A for 30 s for lanthanum in order to transform them into their oxide forms. On the evaporation filament, the silica gel technique8,9 was used: 1 µL of a silica gel suspension was loaded on the filament followed by 60 µg of Pt as H2PtCl6 and 12 µg of boric acid. After drying at 0.5 A, the current was increased to 1.0 A for 2 min. Single-Filament Technique. A 30 µg sample of lanthanum was loaded on a filament which was heated to 0.5 A. Then 40-60 µg of platinum as H2PtCl6 was added and left to dry at 1.0 A. After drying, the current was increased to 1.5 A for 30 s. This increase in current was important in order to completely fix the deposit. If the current was too high or too low at this stage, the sample could fall off the filament before or at the beginning of the heating procedure in the mass spectrometer. Measurement Procedure. The measurement procedures were started when a vacuum (2-3) × 10-7 mbar or better had been obtained in the ion source of the mass spectrometer. Double-Filament Technique. Ionization and evaporation filaments were first heated quickly to 1.5 A and then an increase of 0.1 A‚min-1 was applied to the two filaments alternately. The ion beam was monitored at a mass-to-charge ratio m/z ) 195 during the heating and the ion optical system adjustment. The final filament currents were 2.8-3.0 and 2.4-2.5 A for the ionization and evaporation filaments, respectively. Even though the filament currents differed, the temperatures were roughly the same: ∼1600 °C for both filaments. This is because the deposit on each filament differs and, hence, changes the resistivity of the filament. Temperatures were measured with an optical pyrometer (Keller GmbH, type PB 06 AF 3, Ibbenu¨ren, Germany). Single-Filament Arrangement. The current was increased in the same way as that for the double filament. After the optimization of the parameters, the current applied was 2.2-2.6 A, corresponding to a temperature of ∼1400 °C. The data collection started about 45 and 30 min after the heating procedure had started for the double- and single-filament arrangement, respectively. The measurement consisted of gathering 5-10 blocks of 10 scans each, resulting in 45-90 measured ratios. Each peak was measured for 8 s with a delay of 4 s between peaks. The baseline was measured at m/z ) 200. When the isotope amount ratios were calculated, a linear interpolation technique was applied in order to correct for variations in signal intensity. No corrections for mass fractionation were applied in this study. RESULTS AND DISCUSSION The ionization enhancers, Ba (IP ) 5.21 eV) and La (IP ) 5.58 eV) are commonly used in order to reduce the work function of the rhenium filament. They are well-known as electron emitters, thus promoting the production of negative thermal ions. With Ba on the ionization filament, only small ion currents were observed, which were not stable enough to realize ratio measurements. This reduced ion beam stability when Ba was employed is possibly due to a strong electron emission from the ionization filament. An electron cloud is probably formed, which creates a space charge, and therefore the observed Pt- ion current is reduced. This phenomenon occurs at a lower temperature for Ba than for La, thus creating a higher instability with Ba due to (8) Cameron, A. E.; Smith, D. H.; Walker, R. L. Anal. Chem. 1969, 41, 525526. (9) Barnes, I. L.; Murphy T. J.; Gramlich, J. W.; Shields, W. R. Anal. Chem. 1973, 45, 1881-1884.
792 Analytical Chemistry, Vol. 69, No. 4, February 15, 1997
Table 1. Repeatability of the Platinum Isotope Amount Ratio n(194Pt)/n(195Pt) Using the Single-Filament Technique filament
n(194Pt)/n(195Pt)
std dev
RSD (%)
1 2 3 4 5 6 7 8 9 10 Mean
0.9894 0.9866 0.9896 0.9849 0.9858 0.9877 0.9828 0.9805 0.9838 0.9843 0.9855
0.0033 0.0021 0.0022 0.0039 0.0018 0.0024 0.0028 0.0021 0.0036 0.0020 0.0029
0.34 0.21 0.23 0.40 0.18 0.25 0.29 0.22 0.36 0.20 0.29
the high temperature reached (1600 °C). This effect of space charging also favors a high background due to electrons that are not filtered by the quadrupole. With La, ion beams with a current of 3 × 10-14 A (200 000 ions‚s-1) at m/z ) 195 were observed at temperatures of ∼1600 °C for both ionization and evaporation filaments without any significant difference. With the magnets close to the ion source, the mean background was 5 × 10-14 A (25 000 ions‚s-1), a factor of 2 less from the settings without magnets. The ion current was decreasing with time but it could be measured during 30 min (45 ratios measured). Due to this decrease, the relative standard deviation of the measurement was ∼1% for the isotope amount ratio n(194Pt)/n(195Pt) equal to 0.99. The silica gel and boric acid are usually added to the filament in order to create a “glass” on the filament surface that produces a more homogeneous evaporation of the sample. In this study, their addition helped to get a signal but it also increased the background considerably. The double-filament arrangement was used as it allowed evaporation and ionization to occur at different temperatures. This study shows that this is not necessary to dissociate these two processes for platinum. For the single-filament arrangement, a temperature of 13501450 °C was required to get an ion beam intensity of 3 × 10-14 to 5 × 10-14 A (200 000-300 000 ions‚s-1) for the isotope 195Pt. At this temperature and with the magnets installed close to the ion source, the ion beam was stable for at least 2 h with a background below 2 × 10-16 A (1000 ions‚s-1). Without the magnets, the background reached a current of ∼5 × 10-15 A (30 000-35 000 ions‚s-1). It is expected that the use of a magnetic sector mass spectrometer will virtually eliminate the background due to electrons. At higher temperatures, the stability of the ion current diminished. The stability obtained with the single filament was better. The lower temperature needed with the single-filament technique to obtain a certain ion current reduces the mass fractionation in the ion source. Measurements were performed for the ratio n(194Pt)/n(195Pt) during 1 h (90 ratios measured). For an individual filament, the relative standard deviation on the ratio was between 0.18 and 0.40%. The measurement of 10 filaments gave an average ratio, n(194Pt)/n(195Pt), equal to 0.9855 ( 0.0029, which represents an external repeatability of the measurement equal to 0.29% (results are summarized in Table 1). The different setups are summarized in Table 2. The singlefilament arrangement is easier to handle than the double one and allows one to get stable ion beams to measure ratios to less than 1% relative uncertainty (repeatability). A multiratio measurement was performed in order to compare the results for different ratios. The ratios n(194Pt)/n(195Pt),
Table 2. Comparison of the Different Parameters in This Study with the Work of White et al.2 ion ionization temp detected enhancer (°C) White et al.2 double filament double filament single filament
Pt+ PtPtPt-
no data Ba La La
1800 1600 1600 1400
background (A) (with magnets) ion beam no data 5 × 10-15 5 × 10-15 2 × 10-16
no data unstable not stable stable
n(196Pt)/n(195Pt), and n(198Pt)/n(195Pt) were measured 45 times each during 1 h. For an individual filament, the results were 0.9887 ( 0.0029 (0.29%), 0.7320 ( 0.0026 (0.36%), and 0.2071 ( 0.0013 (0.64%), respectively. The reasonable standard deviation, obtained for ratios different from unity, is important for the isotope dilution technique. In different studies, ion currents from Pt- and the oxides PtO-, PtO2-, and PtO3- were observed from a heated platinum filament at temperatures above 750 °C.4,5 During this study however (H2PtCl6 deposited with La(NO3)3 on a Re filament), the species 195Pt16O- (m/z ) 211), 195Pt16O - (m/z ) 227), and 195Pt16O - (m/z 2 3 ) 243) were monitored during the heating procedure, but no ion beam was detected for these species at temperatures below 1500 °C.
CONCLUSIONS This work describes a simple method to produce negative platinum ions with stable and reasonably large ion currents. The procedure can be easily adapted for isotope dilution analysis of platinum. For those cases where a precision of 1% is sufficient, the THQ mass spectrometer is well suited for this kind of work. This procedure will, with small modifications, also be adapted to a magnetic sector mass spectrometer to perform certification measurements for isotopic reference materials of platinum. These reference materials, which are under preparation, will allow correction for mass fractionation and allow one to carry out IDMS measurements. ACKNOWLEDGMENT C.S.J.B. thanks the European Union for the “Human Capital and Mobility” fellowship awarded. This work was carried out under the 4th Framework programme of the European Union.
Received for review July 23, 1996. Accepted October 22, 1996.X AC9607368 X
Abstract published in Advance ACS Abstracts, December 15, 1996.
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