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.

40Ar-40Ca ArN-54Fe ArO-56Fe ArC-52Cr

Minimum Resolution Required to Remove Argides

Isotope Interferent RP 40Ca 40Ar 192,500 80Se 40Ar2 9,692

75As 40Ar35Cl 7,771 56Fe 40Ar16O 2,501 54Fe 40Ar14N 2,087 52Cr 40Ar12C 2,211

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.

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.

HEXAPOLE ICPMSHexapole 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.

Micromass IsoProbe

Sensitivity

10 8

10 9

10 10

10 11

250 150 100 50 0

Li

Mg

Fe

Co Sr

Rh Cs

Nd

Tb

Pb

Os

Hf

Th

U

In

cps/

ppm

3-4 eV 5-6 eV 6-7 eV 7-8 eV 8-9 eV

200

Ionization Potential

Mass

Measurement of Pb Isotope Ratios

Measurement of Pb Isotope Ratios

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

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

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.

IsoProbe Mass Bias Response

Without By = 8.5478 x -1.2874

R2 = 0.9739

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 50 100 150 200Mass

Mas

s B

ias/

amu

Ca

Sr, Zr & MoAg Nd Hf, W, Re & Os Pb & Tl U

B With By = 7.1389x -1.2497

R2= 0.9812

Cr & FeCu & Zn

IsoProbe Mass Bias ResponseAll 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

Cu and Fe Isotope Data56Fe 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 0.00082 0.056661 0.0016 0.025885 0.0084 2.061517 0.00143 0.056663 0.003 0.025899 0.0171 2.0617356 0.00164 0.056672 0.0016 0.025935 0.007 2.0620406 0.00095 0.056674 0.0024 0.025951 0.013 2.0620413 0.00116 0.056675 0.0015 0.025974 0.0074 2.0620578 0.00137 0.05667 0.0025 0.025995 0.015 2.0620453 0.00148 0.056667 0.0019 0.02593 0.017 2.0618316 0.0011

mean 0.056666 0.025929 2.06182391SD 8.67E-06 4.58E-05 0.0002807%1SD 0.015293 0.176492 0.0136147

Mass Bias Correction It is possible to correct for mass bias of one

element with a different element.Pb-TlU-TlFe-Cu

Accuracy requires precise understanding of the isotope ratio of the normalising element, and no isobaric interferences.

Mass Bias Correction for Fe

Exponential correction 54Fe/56Fen = 54Fe/56Fem * betabeta=(54/56)^(ln((63Cu/65Cu)ref/(63Cu/

65Cu)mes)*(ln(63/65)))63Cu/65Cu ref = 2.2795

Cu Normalised Fe Ratios54Fe/56Fe 57Fe/56Fe

1 0.0636849 0.0244272 0.0636912 0.02445253 0.0636859 0.02446754 0.0636854 0.02450385 0.0636872 0.02451876 0.0636878 0.02454067 0.0636821 0.02456028 0.0636873 0.024497

mean 0.0636865 0.02449591SD 2.627E-06 4.497E-05%1SD 0.0041245 0.1835974

54Fe/56Fe Ratio Measurements Using Different Techniques.

0.06355

0.06365

0.06375

0.06385

0 1 2 3 4 5

TIMS External normalizationTaylor et.al 1992

NTIMSWalczyk (1997)

TIMSDOUBLE SPIKE

ISOPROBEHEX-MULTICOLLECTORCu NORMALIZED63/65 2.2795

52Cr/53Cr vs Spike/Std concentration (Errors 2 sigma St. Dev.)

R2 = 0.998

8.29

8.3

8.31

8.32

8.33

8.34

8.35

8.36

8.37

8.38

8.39

0.000000001 0.00000001 0.0000001 0.000001 0.00001 0.0001 0.001

Log Spike/Std concentration ratio

52C

r/53C

r

5ppb Cr standard

5ppb Cr standard with increasing amounts of 0.63ppb 53Cr spike.

80 l

155 l

235 l

786 l

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.