An evaluation of the isotope ratio performance of an axial TOF - ICP - mass spectrometer

F. Vanhaecke, L. Moens, R. Dams, L. Allen* and S. Georgitis*University of Ghent, Laboratory of Analytical Chemistry, Proeftuinstraat 86, B-9000 Gent, Belgium.* LECO Corporation, 3000 Lakeview Ave., St. Joseph, MI 49085 USA.

1. Instrumentation

LECO Renaissance TOF-ICPMS instrument

123

Acceleration

Extraction

Skimmer

Sampler

Detector

Ion Mirror

ICP Torch

Vacuum StagesFlight Tube

simultaneous extraction of ions from ICP automatic correction for instabilities

in ion source & during ion sampling process

on-axis acceleration

up to 30000 full spectra / s

modified ETP discrete dynode electron multiplier use of analog detection mode to avoid pulse counting saturation no correction for detector dead time required

Meinhard type C concentric nebulizer& double pass cyclonic spray chamber (PTFE) free aspiration of sample solution

1. Instrumentation (2)

203 204 205 206 207 208 209 210

m/z

0

0.5

1

1.5

2

2.5

3

analog signal intensity (mV)

20 microgram/L Pb

2. Data processingAccurate background correction application of 1 mV offset value

use of signal intensity at m/z = 210 as internal

reference (no ionic species present)

correction for signal drift & instrument instability improvement of both precision & accuracy

Isotope ratio precision

expressed as RSD% for n = 10

2. Data processing (2)

averageblank,

nrepsample,averageblank,nrepsample, (210)signal

(210)signal(M)signalM)(signal

3. Influence of acquisition time per replicate & analyte concentration on precision

figure A

0.5 s 1 s 5 s 10 s 30 s 60 s 120 s 300 s

acquisition time per replicate

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

RS

D%

fo

r 2

06

Pb

/20

7P

b n

= 1

0

500 microgram/L

50 microgram/L

500 microgram/L 0.33 0.13 0.089 0.042 0.023 0.022 0.022 0.033

50 microgram/L 0.7 0.42 0.14 0.18 0.09 0.17 0.071 0.059

figure B

50 ug/L 100 ug/L 250 ug/L 500 ug/L

analyte concentration (microgram/L)

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

RS

D%

fo

r 2

06

Pb

/20

7P

b n

= 1

0

30s / replicate

10s / replicate

30s / replicate 0.11 0.051 0.038 0.037

10s / replicate 0.13 0.12 0.09 0.047

3. Influence of acquisition time per replicate & analyte concentration on precision(2)

Despite use of analog mode,

trends in agreement with Poisson counting statistics average improvement 50 500 g/L = 3.5 10 = 3.2 (A) average improvement 1 30 s = 5.2 30 = 5.5 (A) average improvement 10 30 s = 1.8 3 = 1.7 (B)

No further improvement on increasing acquisition time > 30 s !!

3. Influence of acquisition time per replicate & analyte concentration on precision (3)

4. Typical isotope ratio precision at 30 s acquisition time / replicate (RSD% for n = 10)

50 g/L 500 g/L

63Cu/65Cu 0.12 0.03164Zn/66Zn 0.15 0.05666Zn/68Zn 0.21 0.06386Sr/87Sr 0.30 0.04486Sr/88Sr 0.26 0.038

107Ag/109Ag 0.040 0.033135Ba/138Ba 0.28 0.069137Ba/138Ba 0.14 0.051204Pb/208Pb 2.5 0.069206Pb/207Pb 0.090 0.023206Pb/208Pb 0.075 0.041

5. Isotope ratio precision for transient signals (RSD% for n = 5) - FI signals, FWHM = 6 s

50 g/L 500 g/L

63Cu/65Cu 0.21 0.1264Zn/66Zn 0.66 0.07286Sr/87Sr 0.47 0.1586Sr/88Sr 2.47 0.23

107Ag/109Ag 0.23 0.043137Ba/138Ba 0.31 0.10206Pb/207Pb 0.36 0.12206Pb/208Pb 0.48 0.044207Pb/208Pb 0.48 0.10

Isotope ratio precision at 500 µg/L still comparable to the best values ever reported for quadrupole-based ICPMS

6. Mass discrimination

1% per mass unit at mid-mass (Cd) comparable to quadrupole-based or sector field

instrumentation

7. AccuracyThe accuracy attainable was evaluated by comparing Pb isotopic results for a Merck Titrisol ('natural') Pb standard solution obtained by (1) TOF-ICPMS and (2) single collector TIMS.

For TOF-ICPMS, 'NIST 981 - Common lead isotopic standard' was used as an external standard.

TOF-ICPMS TIMS

1.3

1.325

1.35

1.375206Pb/207Pb

TOF-ICPMS TIMS0.5

0.525

0.55

0.575208Pb/206Pb

1.3295 ± 0.0056

1.3230 ± 0.0055

1.877 ± 0.0201.8779 ± 0.0032

For TOF-ICPMS, the result is the average of 3 subsequent determinations,. Each determination consisted of 5 replicate measurements (30 s) of the 'sample' solution and 5 replicate measurements of the NIST standard solution. For TIMS, the result is the average of 5 determinations. For both techniques, the uncertainties indicated are 95% confidence intervals.

7. Accuracy (2)

technique isotope ratio precision* additional comments

TIMS 0.01 - 0.05% isolation of analyte element required

quadrupole-based ICPMS 0.1 - 0.5%

sector field ICPMS 0.05% (LR) HR capabilities

multi-collector ICPMS 0.01 - 0.05% LR only – expensive

TOF-ICPMS 0.05% 'simultaneous' measurement of several ratios

* : at sufficiently high signal intensities and for isotope ratios close to unity

At sufficiently high signal intensities and for a moderate acquisition time, axial TOF-ICPMS yields a typical isotope ratio precision 0.05% RSD. This precision can be obtained for many ratios simultaneously. The mass discrimination is comparable to that observed with quadrupole-based and sector field instruments and when appropriately corrected for, accurate results can be obtained.

8. Conclusions