Effects on Mobility Coefficients

FIG U RE 11.1 Effect of supporting gas atmosphere on collision cross section of six amphetamines in four gases. Although the core ion (MH ) is the same in each gas, adducts and associations are influenced distinctly by each gas. Differences in cross section will lead to differences in mobility. The substances are as follows AM, amphetamine MA, methamphetamine EA, ethylamphetamine MDA, 3,4-methylenedioxy amphetamine MDMA, 3,4-methylenedioxy methamphetamine and MDEA, 3,4-methylenedioxy ethylamphetamine. (From Matz et al.. Investigation of drift gas selectivity inhigh resolution ion mobility spectrometry with mass spectrometry detection, J. Am. Soc. Mass Spectrom. 2002, 300-307. With permission.) 

In a next step, blends or mixtures of these gases were employed in the FAIMS method, and results were surprisingly different from the prediction of Blanc s law. In Blanc s law, the mobility of an ion in a binary mixture of the purified gases is a linear combination of fractional composition times the mobility coefficients in individual gases. This deviation was exploited with planar FAIMS separation of biological molecules. Helium-rich gas mixtures with up to 74% He in nitrogen permitted the separation of phosphopeptides with variant modification sites, and the results were superior in resolution possible with pure gases alone. 

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