Highly sensitive emitter for strontium isotope analysis of picogram-level samples by thermal ionization mass spectrometry

Abstract

A method for strontium isotope analysis of picogram-level samples using highly sensitive silicotungstic acid emitter is presented by a thermal ionization mass spectrometry. The emitter has merits of extremely high sensitivity, low cost, simple operation, etc. It is an important innovation of the strontium isotope analysis of the picogram-level samples. Compared with a sample consumption of 1-50 ng of conventional emitter, the present invention only needs 30-200 pg to obtain satisfying measurement accuracy. The present invention greatly improves test sensitivity, and has broad application prospects in future.

Claims

1. A method for strontium isotope analysis of a picogram-level sample, comprising a step of: using silicotungstic acid and phosphoric acid as a highly sensitive emitter for a thermal ionization mass spectrometry.

2. The method, as recited in claim 1, specifically comprising steps of: I) loading the phosphoric acid onto a Re filament with high purity; after the phosphoric acid is evaporated to dryness, loading a silicotungstic acid emitter onto the Re filament with the high-purity; after the silicotungstic acid emitter is evaporated to dryness, loading the sample onto the Re filament and evaporating to dryness; increasing a filament current until the Re filament turns dark red for 4-6 seconds, then tuning the filament current to zero; and II) measuring the sample by the thermal ionization mass spectrometer.

3. The method, as recited in claim 1, wherein a method for preparing and purifying the silicotungstic acid comprises steps of: 1) weighing silicotungstic acid powder into a Teflon® vial, and adding high-purity water to dissolve the silicotungstic acid powder; 2) prewashing an AG50W-X12 cation resin column with 30 mL 6M hydrochloric acid and 20 mL deionized water in turn; and 3) passing silicotungstic acid solution through the AG50 cation resin column containing 1.5 mL of AG50W-X12 resin; wherein a trace amount of strontium and rubidium in the silicotungstic acid are eliminated by column purification technique in order to reduce emitter loading blank of emitter and eliminate potential .sup.87Rb isobaric interference of the emitter.

4. The method, as recited in claim 2, wherein a method for preparing and purifying the silicotungstic acid comprises steps of: 1) weighing silicotungstic acid powder into a Teflon® vial, and adding high purity water to dissolve the silicotungstic acid powder; 2) prewashing a AG50W-X12 cation resin column with 30 mL 6M hydrochloric acid and 20 mL high purity water in turn; and 3) passing silicotungstic acid solution through the AG50W-X12 cation resin column containing 1.5 mL of AG50W-X12 cation resin; wherein a trace amount of strontium and rubidium in the silicotungstic acid are eliminated by column purification technique in order to reduce loading blank of emitter and eliminate potential .sup.87Rb isobaric interference of the emitter.

5. The method, as recited in claim 2, wherein 1 μL 0.8M phosphoric acid and 1 μL silicotungstic acid are used as the emitter.

6. The method, as recited in claim 3, wherein in the step 1), 110±1 mg silicotungstic acid powder is weighed, and 5 mL high-purity water is added; then the silicotungstic acid solution is further purified by the AG50W-X12 cation resin column.

7. The method, as recited in claim 4, wherein in the step 1), 110±1 mg silicotungstic acid powder is weighed, and 5 mL high-purity water is added; then the silicotungstic acid solution is further purified by the AG50W-X12 cation resin column.

8. The method, as recited in claim 3, wherein a particle size of the silicotungstic acid powder is better than 200 mesh, and a purity of the silicotungstic acid powder is higher than 99.9%.

9. The method, as recited in claim 4, wherein a particle size of the silicotungstic acid powder is better than 200 mesh, and a purity of the silicotungstic acid powder is higher than 99.9%.

10. The method, as recited in claim 3, wherein the silicotungstic acid emitter is loaded onto the Re filament with the high purity.

11. The method, as recited in claim 4, wherein the silicotungstic acid emitter is loaded onto the Re filament with the high purity.

12. The method, as recited in claim 3, wherein a purity of the rhenium filament is higher than 99.8%.

13. The method, as recited in claim 4, wherein a purity of the rhenium filament is higher than 99.8%.

14. The method, as recited in claim 3, wherein a temperature of the rhenium filament is 1360-1430° C.

15. The method, as recited in claim 4, wherein a temperature of the rhenium filament is 1360-1430° C.

16. A highly sensitive emitter for strontium isotope analysis of a picogram-level sample by a thermal ionization mass spectrometry, comprising silicotungstic acid and phosphoric acid.

Description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

(1) Referring to embodiments, the present invention will be further illustrated.

(2) In the following embodiments, sources of the selected raw materials are:

(3) premium grade pure silicotungstic acid (purity: 99.9%, Aladdin Chemistry Reagents Inc. (Shanghai, China)

(4) ultrapure water (Millipore-Q element ultrapure water system, resistance >18.2MΩ/cm)

(5) strontium isotope standard solution NIST 987 (U.S National Institute of Standards and Technology)

Embodiment 1

(6) 1. Emitter Preparation:

(7) 1) weighing 110±1 mg silicotungstic acid powder, and adding 5 mL ultrapure water;

(8) 2) after the silicotungstic acid powder is dissolved, passing silicotungstic acid solution through an AG50W-X12 cation resin column containing 1.5 mL to eliminate potential background interferences from emitter; and

(9) 3) preparing 0.8M phosphoric acid.

(10) 2. Loading Sample for TIMS Analysis

(11) I) First, 1 μL of 0.8 M phosphoric acid and 1 μL of silicotungstic acid solution were loaded onto a degassed Re filament and dried at 1.1 A in sequence. Then, 1 μL of NIST987 solution containing 200 pg of Sr was loaded and dried at 1.1 A onto the Re filament. Finally, the filament current was slowly increased (˜30 s) to 2.0 A and maintained at this current for 30 s. Then the current was increased slowly until phosphoric acid was fumed away. Following this, the filament current was slowly heated to a dull red glow at c.a 2.3 A for 4-6 seconds, then adjusting the filament current to zero; and

(12) II) All samples are measured by Triton Plus thermal ionization mass spectrometer. The .sup.87Sr/.sup.86Sr ratio data were normalized to .sup.88Sr/.sup.86Sr=8.375209 for mass fractionation correction using the exponential law Every measurement run consisted of 300 cycles of data. The analytical result of 200 pg sample size of NIST987 is shown in Table 1.

(13) TABLE-US-00001 TABLE 1 analysis result of 200 pg standard sample NIST 987 Test number .sup.87Sr/.sup.86Sr 2 SE 1 0.710233 0.000015 2 0.710254 0.000014 3 0.710248 0.000015 4 0.71024 0.000015 5 0.710265 0.000014 6 0.710248 0.000015 7 0.710234 0.000015 8 0.710255 0.000014 Mean ± 2 SD 0.710247 0.000022

Embodiment 2

(14) Embodiment 2 is substantially the same as the embodiment 1, except that 100 pg NIST 987 is loaded to test sensitivity and accuracy of the emitter. The analytical result of 100 pg sample size of NIST987 is shown in Table 2.

(15) TABLE-US-00002 TABLE 2 analysis result of 100 pg standard sample NIST 987 Test number .sup.87Sr/.sup.86Sr 2 SE  9 0.710248 0.000020 10 0.710237 0.000021 11 0.710268 0.000022 12 0.710248 0.000020 13 0.710245 0.000021 14 0.710214 0.000022 15 0.710265 0.000021 16 0.710239 0.000021 Mean ± 2 SD 0.710246 0.000034

Embodiment 3

(16) Embodiment 3 is substantially the same as the embodiment 1, except that 50 pg NIST 987 is loaded to test sensitivity and accuracy of the emitter. The analytical result of 50 pg sample size of NIST987 is shown in Table 3.

(17) TABLE-US-00003 TABLE 3 analysis result of 50 pg standard sample NIST 987 Test number .sup.87Sr/.sup.86Sr 2 SE 17 0.710222 0.000041 18 0.710171 0.000039 19 0.710193 0.000041 20 0.710224 0.000038 21 0.710281 0.000041 22 0.710236 0.000039 23 0.710213 0.000041 24 0.710223 0.000041 Mean ± 2 SD 0.710220 0.000064

Embodiment 4

(18) Embodiment 4 is substantially the same as the embodiment 1, except that 30 pg NIST 987 is loaded to test sensitivity and accuracy of the emitte. The analytical result of 50 pg sample size of NIST987 is shown in Table 4.

(19) TABLE-US-00004 TABLE 4 analysis result of 30 pg standard sample NIST 987 Test number .sup.87Sr/.sup.86Sr 2 SE 25 0.710305 0.000052 26 0.710218 0.000051 27 0.710155 0.000053 28 0.710165 0.000051 29 0.710202 0.000053 30 0.710225 0.000051 31 0.710179 0.000052 32 0.710217 0.000052 Mean ± 2 SD 0.710208 0.000094

(20) Tables 1 to 4 list the analytical results for different loading size (200 pg, 100 pg, 50 pg, 30 pg) of the standard sample NIST 987 using the silicotungstic acid emitter. In case of 200 pg and 100 pg loading size, internal precision and external precision of the .sup.87Sr/.sup.86Sr ratio was better than ±0.000022 (2 SE) and ±0.000035 (2 SD), respectively. As indicated in Table 1 and Table 2, the average value of 200 pg and 100 pg loads show excellent agreement with the reference value (.sup.87Sr/.sup.86Sr=0.710250±0.000030). As shown in Table 3 and Table 4, the external precision (2SD) and internal precision (2SE) of .sup.87Sr/.sup.86Sr is about ±0.000064 and ±0.000042 for 50 pg loads, respectively, about ±0.000052 and ±0.000094, respectively. As shown in Tables 1, 2, 3 and 4, for NIST987 at 200 pg, 100 pg, 50 pg and 30 pg loads, the external precision obtained by 8 parallel analyses was better than ±0.000022, ±0.000034, ±0.000064 and ±0.000094, respectively.

(21) It can be seen from above data that internal precision of .sup.87Sr/.sup.86Sr isotopic ratios is better than ±0.000052 even with 30 pg loads. These data fully demonstrate that the proposed silicotungstic acid emitter has extremely high sensitivity and high accuracy for Sr isotope analysis.

(22) To clarify the sensitizing effect of the silicotungstic acid emitter on picogram-level Sr samples, Table 5 lists the emission time and emission intensity of different sample loads. .sup.88Sr has the highest isotope abundance in the Sr isotope system, so .sup.88Sr signal intensity is used as a scale for sensitivity evaluation of silicotungstic acid. For example, in case of 200 pg loads, the intensity of .sup.88Sr can be kept for 2100-3200 mV over 30 minutes. Usually, the actual time of sample collection only takes 22 minutes and each run consists of 300 cycles of 4 s integration time to yield a good internal precision (<0.002%, 2RSE). In previous study, Chen et al. (Earth Science-Journal of China University of Geoscience, 2005, 30: 639-645) measured Sr isotope ratios of 500 pg loads using TaF.sub.5 emitter, only about 2000 mV of .sup.88Sr signal and a short emission time were obtained that yielded an analytical internal precision (±0.003%, 2 RSE) for 500 pg sample loads. As shown in Table 2, the same analytical precision can be obtained in 100 pg loads using silicotungstic acid emitter. In contrast with TaF.sub.5 emitter, we reduce 5-fold sample consumption employing using silicotungstic acid emitter. For 100 pg loads with the silicotungstic acid emitter, the intensity of .sup.88Sr was 900-1400 mV and the emission time was longer than 22 minutes. For 50 pg loads with the silicotungstic acid emitter, the intensity of .sup.88Sr was 380-600 mV and the emission time was longer than 22 minutes.

(23) TABLE-US-00005 TABLE 5 Signal intensity of .sup.88Sr for different sample loads using silicotungstic acid emitter Sample size (pg) .sup.88Sr (mV) Emission time (min) 200 2100~3200 >30 100  900~1400 >24 50 380~600 >22 30 250~350 >22

(24) It is to be understood that those skilled in the art will be able to make modifications and changes in accordance with the above description, and all such modifications and changes are intended to be included within the scope of the appended claims.