Differential Mobility Analysis

Ion-mobility spectrometry (IMS) is an analytical technique used to separate and identify ionized molecules in the gas phase based on their mobility in a carrier buffer gas. Though heavily employed for military or security purposes, such as detecting drugs and explosives, the technique also has many laboratory analytical applications, including the analysis of both small and large biomolecules. IMS instruments are extremely sensitive stand-alone devices, but are often coupled with mass spectrometry, gas chromatography or high-performance liquid chromatography in order to achieve a multi-dimensional separation. They come in various sizes, ranging from a few millimeters to several meters depending on the specific application, and are capable of operating under a broad range of conditions. IMS instruments such as microscale high-field asymmetric waveform ion mobility spectrometry can be palm-portable for use in a range of applications including volatile organic compound (VOC) monitoring, biological sample analysis, medical diagnosis and food quality monitoring. Systems operated at higher pressure (i.e. atmospheric conditions, 1 atm or 1013 hPa) are often accompanied by elevated temperature (above 100 °C), while lower pressure systems (1-20 hPa) do not require heating. IMS was first developed primarily by Earl W. McDaniel of Georgia Institute of Technology in the 1950s and 1960s when he used drift cells with low applied electric fields to study gas phase ion mobilities and reactions. In the following decades, he coupled his new technique with a magnetic-sector mass spectrometer, with others also utilizing his techniques in new ways. IMS cells have since been attached to many other mass spectrometers, gas chromatographs and high-performance liquid chromatography setups. IMS is a widely used technique, and improvements and other uses are continually being developed. Perhaps ion mobility spectrometry's greatest strength is the speed at which separations occur—typically on the order of tens of milliseconds. This feature combined with its ease of use, relatively high sensitivity, and highly compact design have allowed IMS as a commercial product to be used as a routine tool for the field detection of explosives, drugs, and chemical weapons. Major manufacturers of IMS screening devices used in airports are Morpho and Smiths Detection.Smiths purchased Morpho Detection in 2017 and subsequently had to legally divest ownership of the Trace side of the business which was sold on to Rapiscan Systems in mid 2017. The products are listed under ETD Itemisers. The latest model is a non radiation 4DX. In the pharmaceutical industry IMS is used in cleaning validations, demonstrating that reaction vessels are sufficiently clean to proceed with the next batch of pharmaceutical product. IMS is much faster and more accurate than HPLC and total organic carbon methods previously used. IMS is also used for analyzing the composition of drugs produced, thereby finding a place in quality assurance and control. As a research tool ion mobility is becoming more widely used in the analysis of biological materials, specifically, proteomics and metabolomics. For example, IMS-MS using MALDI as the ionization method has helped make advances in proteomics, providing faster high-resolution separations of protein pieces in analysis. Moreover, it is a really promising tool for glycomics, as rotationally averaged collision cross section (CCS) values can be obtained. CCS values are important distinguishing characteristics of ions in the gas phase, and in addition to the empirical determinations it can also be calculated computationally when the 3D structure of the molecule is known. This way, adding CCS values of glycans and their fragments to databases will increase structural identification confidence and accuracy. Outside of laboratory purposes, IMS has found great usage as a detection tool for hazardous substances. More than 10,000 IMS devices are in use worldwide in airports, and the US Army has more than 50,000 IMS devices. In industrial settings, uses of IMS include checking equipment cleanliness and detecting emission contents, such as determining the amount of hydrochloric and hydrofluoric acid in a stack gas from a process. It is also applied in industrial purposes to detect harmful substances in air. In metabolomics the IMS is used to detect lung cancer, Chronic obstructive pulmonary disease, sarcoidosis, potential rejections after lung transplantation and relations to bacteria within the lung (see breath gas analysis). The physical quantity ion mobility K is defined as the proportionality factor between an ion's drift velocity vd in a gas and an electric field of strength E.