Accuracy of reduced mobilities and measurement of instrumental parameters in ion mobility spectrometry

Abstract

Ion mobility spectrometry (IMS) separates gas-phase ions drifting under an electric field according to their size to charge ratio. We used electrospray ionization-drift tube IMS coupled to quadrupole mass spectrometry to obtain the mobilities of common amino acids, amines, valinol, atenolol, and the chemical standards tetramethylammonium ion (TMA), tetraethylammonium ion (TEA), tetrapropylammonium ion (TPA), and tetrabutylammonium (TBA) ions, 2,4-lutidine and 2,6-di-tert-butyl pyridine (DTBP). The mobilities were obtained in pure nitrogen or when shift reagents (SR) such as ammonia, 2-butanol, ethyl lactate, methanol, methyl 2-chloropropionate, nitrobenzene, 1-phenyl ethanol, trifluoromethyl benzyl alcohol, and water were introduced in the buffer gas. We found important differences in the buffer gas temperature between different regions of the drift tube and differences between the buffer gas and drift tube temperatures, which is normally used instead of the buffer gas temperature in reduced mobility calculations. Therefore, we used the buffer gas temperature instead of the drift tube temperature and a calibration method with two types of chemical standards, finding excellent precision, reproducibilities from 0.3 to 0.6% for reduced mobilities (K0) of the chemical standards during nine months. Repeatability during this period was 0.17% for the drift times of all the analytes. We also show that the changes in instrumental parameters such as temperature, pressure and voltage that produce important variations in drift times are small; for this, we recommend to calculate K0 from calibration with chemical standards instead of replacing instrumental parameters in the IMS fundamental equations.