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Measurement of “Difficult” Elements and Isotopes Using a Hexapole ICPMS

Measurement of “Difficult” Elements and Isotopes Using a Hexapole ICPMS. Zenon Palacz, Simon Meffan-Main. Micromass U.K. Ltd. “Difficult” Isotopes. Argides (Ar, ArN, ArO, ArC) are Isobaric with transition metals and Calcium. 40 Ar- 40 Ca ArN- 54 Fe ArO- 56 Fe ArC- 52 Cr.

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Measurement of “Difficult” Elements and Isotopes Using a Hexapole ICPMS

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  1. Measurement of “Difficult” Elements and Isotopes Using a Hexapole ICPMS Zenon Palacz, Simon Meffan-Main. Micromass U.K. Ltd

  2. “Difficult” Isotopes • Argides (Ar, ArN, ArO, ArC) are Isobaric with transition metals and Calcium. • 40Ar-40Ca • ArN-54Fe • ArO-56Fe • ArC-52Cr

  3. Minimum Resolution Required to Remove Argides IsotopeInterferentRP 40Ca 40Ar 192,500 80Se 40Ar2 9,692 75As 40Ar35Cl 7,771 56Fe 40Ar16O 2,501 54Fe 40Ar14N 2,087 52Cr 40Ar12C 2,211

  4. High Resolution Instruments. • Maximum resolution 10-15,000. • Increased resolution reduces transmission. (<5% at maximum resolution). • High resolution produces peak shapes without flat tops. Compromises precision of isotope ratios when peak jumping. • Single collector systems, so peak jumping necessary.

  5. ICP Multicollectors • Low resolution (400). • Large peak flat and multicollection produce <10ppm precisions. • Cannot resolve argides to do multicollection at high resolution. • Can have one collector (axial) with adjustable slits to go up to 3000 resolution.

  6. HEXAPOLE ICPMS • Hexapole collision cell removes argides, by colliding the argide molecules from the plasma, with an inert gas in the cell at room temperature. • Charge transfer with H in the hexapole can also neutralise Ar+. • Collisions in the cell reduces ion energy spread from plasma <1 volt, to allow single focusing magnetic sector.

  7. Micromass IsoProbe

  8. Sensitivity 11 10 In Pb 10 10 Nd U Rh Hf Cs cps/ppm Tb Th Sr Os Co Mg 3-4 eV 9 10 5-6 eV Ionization Potential 6-7 eV Fe 7-8 eV Li 8-9 eV 8 10 200 0 50 100 150 250 Mass

  9. Measurement of Pb Isotope Ratios

  10. Measurement of Pb Isotope Ratios

  11. ArN from N2 Sweep Gas in MCN 6000. No H in Collision Cell.

  12. Mass Scan Across Cr and Fe in Water. Ar + H in Cell.

  13. Mass Bias • ICP-MS has little or no time dependent mass fractionation, unlike TIMS. • ICP-MS has a mass dependent mass bias which increases with decreasing mass. • Elements of similar mass have similar mass bias. • Fractionation correction with a different element is possible.

  14. IsoProbe Mass Bias Response 0.4 0.35 B With B Without B 0.3 y = 7.1389x -1.2497 y = 8.5478 x -1.2874 R2 = 0.9812 R2 = 0.9739 0.25 Mass Bias/amu 0.2 0.15 0.1 Ca 0.05 Sr, Zr & Mo Cr & Fe Ag Nd Hf, W, Re & Os Pb & Tl U Cu & Zn 0 0 50 100 150 200 Mass

  15. IsoProbe Mass Bias Response • All elements fall on the same mass response curve. • If Argon or Argides were present this relationship would not occur. • The mass bias response is not effected by the type of gas in the collision cell. It must be created by fractionation across sample/skimmer cones

  16. Cu and Fe Isotope Data 56Fe ion beam 8e-11A Baselines were measured at +/-0.5amu wrt to Fe. Block 54Fe/56Fe %1se 57Fe/56Fe %1se 63Cu/65Cu %1se 1 0.056649 0.0014 0.025859 0.005 2.0613223 0.0008 2 0.056661 0.0016 0.025885 0.0084 2.061517 0.0014 3 0.056663 0.003 0.025899 0.0171 2.0617356 0.0016 4 0.056672 0.0016 0.025935 0.007 2.0620406 0.0009 5 0.056674 0.0024 0.025951 0.013 2.0620413 0.0011 6 0.056675 0.0015 0.025974 0.0074 2.0620578 0.0013 7 0.05667 0.0025 0.025995 0.015 2.0620453 0.0014 8 0.056667 0.0019 0.02593 0.017 2.0618316 0.0011 mean 0.056666 0.025929 2.0618239 1SD 8.67E-06 4.58E-05 0.0002807 %1SD 0.015293 0.176492 0.0136147

  17. Mass Bias Correction • It is possible to correct for mass bias of one element with a different element. • Pb-Tl • U-Tl • Fe-Cu • Accuracy requires precise understanding of the isotope ratio of the normalising element, and no isobaric interferences.

  18. Mass Bias Correction for Fe • Exponential correction • 54Fe/56Fen = 54Fe/56Fem * beta • beta=(54/56)^(ln((63Cu/65Cu)ref/(63Cu/65Cu)mes)*(ln(63/65))) • 63Cu/65Cu ref = 2.2795

  19. Cu Normalised Fe Ratios 54Fe/56Fe 57Fe/56Fe 1 0.0636849 0.024427 2 0.0636912 0.0244525 3 0.0636859 0.0244675 4 0.0636854 0.0245038 5 0.0636872 0.0245187 6 0.0636878 0.0245406 7 0.0636821 0.0245602 8 0.0636873 0.024497 mean 0.0636865 0.0244959 1SD 2.627E-06 4.497E-05 %1SD 0.0041245 0.1835974

  20. 54Fe/56Fe Ratio Measurements Using Different Techniques. 0.06385 TIMS External normalization Taylor et.al 1992 ISOPROBE HEX-MULTICOLLECTOR 0.06375 Cu NORMALIZED 63/65 2.2795 0.06365 TIMS DOUBLE SPIKE NTIMS Walczyk (1997) 0.06355 0 1 2 3 4 5

  21. 52Cr/53Cr vs Spike/Std concentration (Errors 2 sigma St. Dev.) 8.39 5ppb Cr standard 8.38 80 ml 8.37 155 ml R2 = 0.998 8.36 235 ml 8.35 52Cr/53Cr 8.34 5ppb Cr standard with increasing amounts of 0.63ppb 53Cr spike. 8.33 8.32 8.31 786 ml 8.3 8.29 0.000000001 0.00000001 0.0000001 0.000001 0.00001 0.0001 0.001 Log Spike/Std concentration ratio

  22. Conclusions • Argides are removed by the hexapole. • Argon is removed to allow 40Ca measurements. • High precision Fe isotope ratios can be obtained at low resolution by normalisation with Cu.

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