A new carder gas medium for differential or field asymmetric waveform on mobility spectrometry (FAIMS) is disclosed. Separations in this medium generally follow those in He/N.sub.2 mixtures known in the art, but are faster and/or exhibit previously unachievable resolving power, peak capacity, and species resolution. The new medium is resistant to electrical breakdown, readily available, and less expensive than current buffers involving substantial He fractions. Performance gains apply broadly across analyte classes, including peptides and other larger biomolecules, and to ions of various charge states. Improvements are particularly large for larger and singly charged ions with lower mobility in gases.
STATEMENT REGARDING RIGHTS TO INVENTION MADE UNDER FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT
 This invention was made with Government support under Contract DE-AC05-76RLO1830 awarded by the U.S. Department of Energy. The Government has certain rights in the invention.