Fast and Precise Method
for Pb Isotope Ratio Determination
in Complex Matrices using GC-MC-ICPMS: Application to Crude Oil, Kerogen,
and Asphaltene Samples
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Abstract
A new method to determine Pb isotope ratio without ion-exchange-matrix
separation is proposed. After acid digestion, Pb was ethylated to
Et<sub>4</sub>Pb, separated from the digested solution (black shale,
asphaltene, crude oil and kerogen) by extraction in isooctane, and
then injected into a gas chromatograph coupled to a multicollector
inductively coupled plasma mass spectrometer. Seven isotopes (<sup>202</sup>Hg, <sup>203</sup>Tl, <sup>204</sup>Pb, <sup>205</sup>Tl, <sup>206</sup>Pb, <sup>207</sup>Pb, <sup>208</sup>Pb) were monitored simultaneously
with peak duration of 23 s. GC elution was operated under wet plasma
conditions where a thallium standard solution was introduced to the
mass spectrometer for mass bias correction. The total time of the
procedure (sample preparation and analysis, after acid digestion)
was reduced by a factor of 15 compared to conventional-continuous
sample introduction. Data treatment was carried out using the linear
regression slope method. Mass bias was corrected using the double
correction method (first thallium normalization followed by classical
bracketing). For the <sup>208/206</sup>Pb and <sup>207/206</sup>Pb
ratios, precision (2RSD<sub>EXT</sub>, <i>n</i> = 21) was
49 and 69 ppm, and the bias between experimental results and reference
values was better than 0.0033 and 0.0007 ‰, when injecting
1.2 ng of ethylated Pb SRM NIST 981 solution. Results obtained by
this method were validated by comparison with those obtained via conventional-continuous
sample introduction. The applicability of this approach was demonstrated
with the analysis of black shale, asphaltene, crude oil and kerogen
samples