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1. Instrumentation

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1. Instrumentation

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  1. An evaluation of the isotope ratio performance of an axial TOF - ICP - mass spectrometerF. 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.

  2. 1. Instrumentation LECO Renaissance TOF-ICPMS instrument Detector Vacuum Stages Flight Tube 3 2 1 ICP Torch Ion Mirror Sampler Skimmer Extraction Acceleration

  3. 1. Instrumentation (2) 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

  4. 2. Data processing Accurate background correction  application of 1 mV offset value

  5. 2. Data processing (2) 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

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

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

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

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

  10. 5. Isotope ratio precision for transient signals (RSD% for n = 5) - FI signals, FWHM = 6 s Isotope ratio precision at 500 µg/L still comparable to the best values ever reported for quadrupole-based ICPMS

  11. 6. Mass discrimination 1% per mass unit at mid-mass (Cd)  comparable to quadrupole-based or sector field instrumentation 7. Accuracy The 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.

  12. 7. Accuracy (2) 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.

  13. 8. Conclusions 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.

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