measurement of “difficult” elements and isotopes using a hexapole icpms zenon palacz, simon...
TRANSCRIPT
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 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.
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 50 100 150 200
Mass
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 & Fe
Cu & Zn
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
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 0.0008
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
Mass Bias Correction
It is possible to correct for mass bias of one element with a different element.
Pb-Tl
U-Tl
Fe-CuAccuracy 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 Ratios
54Fe/56Fe 57Fe/56Fe1 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/5
3C
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.