Impact of pump flow fluctuations on post column online ID-ICP-MS.

In post column online isotope dilution mass spectrometry (IDMS), the stability of the spike mass flow is a key element. Changes in viscosity or fluctuations in the pump rate of the peristaltic pump may affect the results of post column online IDMS measurements. It was shown by simulating random fluctuations and studying the changes in the resulting integrals of the isotope ratio chromatogram of the sample that even small fluctuations, observable when using peristaltic pumps, can influence the result and especially its uncertainty. The use of a balance to continuously monitor the mass flow of the spike during the measurement which we presented in a previous publication allows now to correct the isotope ratio chromatogram for these fluctuations. Subsequently, the simulated effect was verified experimentally for the determination of Se-Met in the human serum reference material BCR 637, where the corrected mass fraction was plainly closer to the mass fraction obtained by species specific IDMS. Additional attention was paid to the fact that there is a time shift between the observation of the fluctuations in the pump rate and the detection of these fluctuations in the ICP-MS.