Ion Mobility Spectroscopy (IMS) has gained widespread acceptance in many applications for detecting and identifying contaminant molecules.
IMS is an ionization-based time-of-flight technique performed at atmospheric pressure. The heart of IMS is the sample cell. A continuous ambient air sample is drawn over a semi-permeable membrane. The membrane serves to protect the interior of the cell from particles and moisture, provide a degree of contaminant selectivity, and allow various levels of sensitivity based on ambient contamination conditions. The molecules of interest permeate through the membrane, and are picked up by purified dry instrument air which sweeps across the membrane and delivers the sample to the reaction region. There, the sample is ionized by low-level beta energy emitted by a sealed nickel-63 radiation source.
The ionized sample drifts through the cell under the influence of an electrostatic field. A shutter grid is biased electrically to either block the ions or allow them to pass through. This shutter grid is pulsed to periodically allow the ions into the drift region. There, they begin to separate out based on their size and shape while flowing counter to a drift gas flow, which is introduced at the end of the drift tube. The smaller ions move faster than larger ions through the drift tube and arrive at the detector. A collector (Faraday plate) located at the end of the tube detects the arrival of the ions and produces a current. This current is amplified to produce a time-of-flight spectrum. A microprocessor evaluates the spectrum for the target compound and determines the concentration based on the peak height. Because of the specificity of the membrane, enhanced ionization, and time-of-flight, there is the highest degree of certainty that the analyzer is measuring only the compound of interest, even in the presence of other interferents.
From: “Ion Mobility Theory and Applications”
