In situ chemical ionization in ion trap mass spectrometry--the beneficial influence of isobutane as a reagent gas.

We report a comparison of the ionization yields provided by the most common reagents (methane, ammonia, methanol, acetonitrile, isobutane) performing in situ chemical ionization with an ion trap mass spectrometer. Four molecules were chosen in the medical field to illustrate experimental results: alprazolam, diazepam, flunitrazepam and acetaminophen. Under usual operational conditions, relative abundances of protonated ions appreciably depend on the reagents. The greatest abundance of MH+ ions was obtained with isobutane while observed intensities for MH+ ions varied from 73% for methanol and ammonia to about 23% for acetonitrile and methane. Results were temptatively rationalized comparing energies of formation of the reagent ions and storage efficiency in the trapping field.