Note: Buffer gas temperature inhomogeneities and design of drift-tube ion mobility spectrometers: Warnings for real-world applications by non-specialists.

Ion mobility spectrometry (IMS) separates gas phase ions moving under an electric field according to their size-to-charge ratio. IMS is the method of choice to detect illegal drugs and explosives in customs and airports making accurate determination of reduced ion mobilities (K) important for national security. An ion mobility spectrometer with electrospray ionization coupled to a quadrupole mass spectrometer was used to study uncertainties in buffer gas temperatures during mobility experiments. Differences up to 16°C were found in the buffer gas temperatures in different regions of the drift tube and up to 42°C between the buffer gas and the drift tube temperatures. The drift tube temperature is used as an approximation to the buffer gas temperature for the calculation of K because the buffer gas temperature is hard to measure. This is leading to uncertainties in the determination of K values. Inaccurate determination of K values yields false positives that delay the cargo and passengers in customs and airports. Therefore, recommendations are issued for building mobility tubes to assure a homogeneous temperature of the buffer gas. Because the temperature and other instrumental parameters are difficult to measure in IMS, chemical standards should always be used when calculating K. The difference of 42°C between the drift tube and buffer gas temperatures found in these experiments produces a 10.5% error in the calculation of K. This large inaccuracy in K shows the importance of a correct temperature measurement in IMS.