Perspective Metz, Baker, Schymanski et al.
reliable molecular feature alignment (i.e., identity confirmation) across multiple samples. Furthermore, measurements from different instruments in different laboratories have also been compared and their val- ues normally agree within <2% error [96], with recent DTIMS instruments yielding values with reproduc- ibility precision of <1% [93,94]. The high reproduc- ibility and speed of DTIMS allows it to be easily nested between LC and MS to provide additional separation power and dynamic range of detection in measuring the exposome (Figure 3B). The resulting LC-IMS-MS approach can either provide greater cov- erage of the sample for the same LC separation length (i.e., by increasing the dynamic range of the measure- ment) [97], or allows for decreasing the LC separation length while maintaining the same measurement coverage (Figure 3C). For example, incorporating DTIMS into a LC-MS-based proteomics method allowed shortening of the LC gradient from 100 to 15.5 min with no loss in coverage of 20 standard peptides spiked into a mouse plasma protein digest over a range of 1 ng/ml to 10 μ/ml [98]. Similarly, Stephen et al. incorporated DTIMS into 1D and 2D LC-MS methods, supporting the identity confirma- tion of 22 and 53 different compounds, r espectively, in highly complex wastewater samples [92].
DTIMS separations are ultra-high throughput While using chromatographic separations prior to the IMS stage will continue to play a role in exposome stud- ies, DTIMS-MS alone lends itself extremely well to coping with the high-throughput demands of measur- ing large population cohorts. Interfacing DTIMS with high-resolution MS, such as TOF [99] permits simulta- neous separation of sample constituents and acquisi- tion of structural information and high-accuracy MS data, and when coupled with quadrupole-TOF MS, also allows for acquisition of tandem mass spectra in targeted, data-dependent or data-independent modes. IMS separations can also be coupled with trapping MS instruments such as Orbitrap and Fourier Transform Ion Cyclotron Resonance. However, in these cases the millisecond IMS separations would be faster than the second timescale sampling of the mass spectrometers, and so MS data acquisition manipulations would be required. Thus, TOF-based instruments that sample on the microsecond timescale are more practical to retain the IMS measurement throughput. In DTIMS- TOF MS analysis, a single separation typically occurs in 10–100 ms and the TOF MS pulser samples each peak approximately every 100 μs, so DTIMS separa- tions can be reconstructed with sufficient points-per- (IMS)-peak to allow for reliable discrimination of features in 3D space (drift time, m/z, abundance).
35 Bioanalysis (2017) Bioanalysis (2017) 9(1), 81–98(1)
DTIMS transients are normally summed for at least 0.5 s to acquire enough signal for reproducible IMS peaks and repeatable determination of peak apices, which is essential for accurate determination of CCS. DTIMS separations do not require derivatization
and can be combined with different ionization sources for analysis of distinct compound types such as polar metabolites (ESI in positive or negative ionization modes) and as well as compounds for which other ionization modes are required (e.g., APCI in positive or negative modes for analysis of polyaromatic hydro- carbons), allowing for multiple analyses to obtain high coverage of the exposome. Without chromatographic or other preseparation prior to ESI, ionization suppres- sion due to the presence of highly abundant chemicals has previously limited the utility of DTIMS-MS for analysis of complex mixtures. However, a front-end automated SPE system was recently coupled directly to DTIMS-MS and different cartridge chemistries were utilized to provide a means to reduce ion suppression in analysis of complex mixtures. This automated SPE- IMS-MS system provided a 10-s sample-to-sample duty cycle and a theoretical maximum throughput of >8000 injections per day [100,101]. This is 2–3 orders of magnitude higher throughput than conventional GC-MS or LC-MS methods and makes the study of large patient cohorts practical. When necessary, DTIMS-MS can also be coupled with GC or LC to address prohibitively complex samples and reduce front-end analysis times [98].
Incorporating CCS into molecular identification workflows The implementation of IMS in exposomics studies will lead to increasingly more frequent observations of previously undetected chemicals and metabolites. LC-IMS-MS will provide increased overall measure- ment dynamic range, resulting in detections of lower abundance molecules, and the throughput of IMS-MS alone will provide the opportunity to analyze many thousands of longitudinal samples over lifetimes of exposure, capturing evidence of transitory accumula- tions of chemicals or metabolites. The volume of data corresponding to these new chemical observations will almost certainly outpace the development of reference data to enable their confident identification. Thus, the question remains as to how these molecules can then be identified in exposomics studies. We here explore the possibility of using compu-
tationally predicted CCS to assist in the identifica- tions of newly observed anthropogenic chemicals and metabolites, analogous to previous and ongoing efforts to predict retention times or tandem mass spectra dur- ing analysis of small molecules [29,30,102,103]. In this
future science group
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