Perspective Metz, Baker, Schymanski et al.
Figure 2. Anthropogenic chemicals and endogenous metabolite concentrations in blood. A wide dynamic range of concentrations of chemicals and metabolites are observed in human blood, making the comprehensive measurement of these in an exposomics approach extremely challenging. Reproduced with permission from [44].
DTIMS increases the dynamic range of existing LC-MS methods In DTIMS platforms, ions travel through a drift tube under the influence of a weak attractive electric field while colliding with a stationary buffer gas (Figure 3A). In this way, ions with small CCSs spend less time inside the drift tube than other equally charged ions with larger CCS values [83,84], as described in the fol- lowing equation:
where e is the elementary charge, z is the charge on the ion, N the gas density number (mol/m3
), K is the
T is absolute temperature. While the m/z of an ion strongly influences the time it spends in the drift tube, the CCS value directly reflects the mobility of each ion and correlates with the shape of the molecule based
mobility of the ion, μ is the reduced mass of the col- lision gas and ion, kb
is the Boltzmann constant and 33 Bioanalysis (2017) Bioanalysis (2017) 9(1), 81–98(1)
on its ion-neutral interaction potential [85]. As the m/z ratios of ions can be measured following the DTIMS separation using a mass spectrometer, this allows CCSs to be directly determined for each ion via the funda- mental zero-field (i.e., Mason Schamp equation [86]). In general, this orthogonality between the separation mechanisms of IMS and MS is key to the utilization of this combination of techniques, particularly for the separation of isobaric compounds. The ability to resolve isomers that are difficult
to distinguish using LC-MS alone is an inherent strength of DTIMS, particularly in small molecule analysis since many metabolites and other chemicals have the same molecular formula but play very dif- ferent roles in biological systems. Indeed, DTIMS has been used to separate some important classes of isomers [87–91]. Stephen et al. used library CCS val- ues to confirm the identities of the isomeric tramadol and desvenlafaxine in wastewater samples [92], which have the same molecular formula and hence the same
future science group
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