Time-resolved Mass Spectrometry Systems

The main goal of data treatment in conventional mass spectrometry (MS) is to facilitate identification and quantification of analytes. The focus of time-resolved mass spectrometry (TRMS) is to track variations of identities and quantities of analytes and products over time. In many chemical reactions, the concentrations of reactants decrease and those of products increase with time. In more complex reactions, reaction intermediates exist and their concentrations may increase and decrease within certain periods of time. The evolution of reaction intermediates is distinct from that of reactants and products. TRMS provides an insight into the progress of reactions by identifying molecules based on their mass-to-charge (m/z) ratios. It also determines concentrations of molecules based on signal intensities of ions. Thus, it is important for TRMS to interpret MS data on highly complex and dynamic systems correctly. This chapter will first introduce definitions of various technical terms, and then discuss how to predict molecular compositions of complex mixtures based on the information contained in mass spectra. 

Experimental design of time-resolved mass spectrometry (TRMS) systems can greatly affect temporal resolution in the analysis of dynamic samples (see also Section 4.2). The two popular MS approaches - laser desorption/ionization (LDI)-MS and electrospray ionization (ESI)-MS - are suitable for studies of enzymatic reactions [8]. 

Time-resolved Mass Spectrometry in Systems and Synthetic Biology... 

Figure 13.5 Spatiotemporal effects of a bioautocatalytic chemical wave revealed by time-resolved mass spectrometry, (a) Investigation of a chemical wave due to "passive" transduction and a bienzymatic amplification system. (A) Experimental setup incorporating a horizontal drift cell and mass spectrometer. (B) Schematic representation of chemical wave propagation in the drift cell due to the passive and the enzyme-accelerated transduction. (b) Transduction of labeled and unlabeled ATP along the drift cell. Concentration of the C. g-ATP trigger 0 M (A) and 5 x 10 M (B). Exponential smoothing with a time constant of 4.1 s has been applied, and followed by normalization (scaling to the maximal value). The dashed line denotes the time lapse between half-maxima of the normalized curves (0.5 level) corresponding to the passive and accelerated chemical transduction 93 and 740 s in the case of the 10 and 5 x 10 iW trigger solutions, respectively [6], Adapted from Ting, H., Urban, P.L (2014) Spatiotemporal Effects of a Bioautocatalytic Chemical Wave revealed by Time-resolved Mass Spectrometry. RSCAdv. 4 2103-2108 with permission from the Royal Society of Chemistry...

The concept of peak capacity is rather universal in instrumental analytical chemistry. For example, one can resolve components in time as in column chromatography or space, similar to the planar separation systems however, the concept transcends chromatography. Mass spectrometry, for example, a powerful detection method, which is often the detector of choice for complex samples after separation by chromatography, is a separation system itself. Mass spectrometry can separate samples in time when the mass filter is scanned, for example, when the mass-to-charge ratio is scanned in a quadrupole detector. The sample can also be separated in time with a time-of-flight (TOF) mass detector so that the arrival time is related to the mass-to-charge ratio. 

Capillary electrophoresis (CE) is a powerful separation technique. It is especially useful for separation of ionic compounds and chiral mixtures. Mass spectrometry has been coupled with CE to provide a powerful platform for separation and detection of complex mixtures such as combinatorial libraries. However, the full potential of CE in the application of routine analysis of samples has yet to be realized. This is in part due to perceived difficulty in the use of the CE technique compared to the more mature techniques of HPLC and even SFC. Dunayevskiy et al. [136] analyzed a library of 171 theoretically disubstituted xanthene derivatives with a CE/ESI-MS system. The method allowed the purity and makeup of the library to be determined 160 of the expected compounds were found to be present, and 12 side products were also detected in the mixture. Due to the ability of CE to separate analytes on the basis of charge, most of the xanthene derivatives could be resolved by simple CE-MS procedures even though 124 of the 171 theoretical compounds were isobaric with at least one other molecule in the mixture. Any remaining unresolved peaks were resolved by MS/MS experiments. The method shows promise for the analysis of small combinatorial libraries with fewer than 1000 components. Boutin et al. [137] used CE-MS along with NMR and MS/MS to characterize combinatorial peptide libraries that contain 3 variable positions. The CE-MS method was used to provide a rapid and routine method for initial assessment of the construction of the library. Simms et al. [138] developed a micellar electrokinetic chromatography method for the analysis of combinatorial libraries with an open-tube capillary and UV detection. The quick analysis time of the method made it suitable for the analysis of combinatorial library samples. CE-MS was also used in the analysis... 

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