other manufacturers are expected to be available soon. A bespoke electronics unit is required to supply radio frequency (RF) waveforms and excitation pulses to the four quadrupole rods. No further modifications are required if CID is the chosen fragmentation method. UVPD requires further adaptations to allow a laser beam to be directed along the axis of the quadrupole.
Applications
Analyses of a simple mixture of active pharmaceutical ingredients (APIs) by traditional MS and 2DMS are compared in Figures 2 and 3, respectively. Using traditional MS (Q1 operated in RF-only mode so that all precursors are transmitted), the precursors are easily identified when CID
is inactive but many additional product ion peaks appear when the collision energy is 20 eV. Clearly, interpretation of this data would be challenging if the identities of the compounds were unknown. When 2DMS is applied to the same mixture, peaks corresponding to the intact precursors are seen on the diagonal autocorrelation line. Fragmentation by CID results in the appearance of product ion peaks lying on horizontal lines passing through the precursor peak positions. Interpretation of the data is now straightforward. For example, product ion peaks 1-5, as well as the precursor peak on the same line, can be used to confirm or deduce the presence of ranitidine in the mixture. The empirical formula for each ion can be determined with high confidence as the x-axis coordinate of every peak retains the mass accuracy of the ToF analyser (~2 ppm).
While manual data analysis is possible for simple mixtures, in most cases, automated peak picking is used to generate lists of product ion peaks belonging to each precursor found on the autocorrelation line. The components of the mixture can then be determined using spectral databases
The data generated is
unbiased, complete, and easily interrogated, both manually and by software.
or common software tools. While broad overviews of the entire mass range such as those in Figure 3 are instructive, zoomed-in views can yield a great deal of additional detail. As the chemical noise is also separated in two dimensions, it is possible to identify peaks of interest with intensities varying over five orders of magnitude.
There are numerous application areas and scenarios that are likely to benefit from the application of 2DMS:
Figure 2: A mixture of pharmaceuticals analysed using traditional mass spectrometry (Q1 in RF-only mode). When the components are fragmented by CID, interpretation of the spectrum is very challenging as it is unclear which product ion peaks belong to each of the precursors.
• 2DMS can be used to rapidly identify unknown or unexpected contaminants in applications ranging from food safety to industrial process control, providing early warning of unexpected pesticides or reaction byproducts, for example.
• Rather than replace chromatography, 2DMS can also augment traditional targeted method development. A survey scan using 2DMS provides a list of all precursors present and prominent fragments that can then be used for selected reaction monitoring (SRM).
• A 2DMS spectrum provides a complete historical record of the sample composition. The data can be retrospectively examined for components that were not of interest at the time of analysis but later become significant.
• For complex samples, complete structural elucidation and identification of all components remains a formidable task and is often unnecessary. A 2DMS spectrum has a prodigious peak capacity and is consequently data rich. Statistical methods can be applied to extract nuanced information such as the country of origin or the flavour profile of foods and foodstuffs without assigning every peak in the spectrum.
• Additional fragments may be generated if the UVPD option is implemented. CID generally favours low energy fragmentation routes whereas unusual and/or higher energy pathways can be accessed by UVPD. For some compound classes, these additional fragments provide important structural information, such as the position of double bonds in lipids.
Conclusions
Figure 3: Analysis of the same pharmaceutical mixture using 2DMS. Precursor ion peaks are found on the diagonal autocorrelation line and peaks corresponding to their respective fragments are located along separate horizontal lines. Structural elucidation and precursor identification are now straightforward. For example, peaks 1-5 are all fragments of ranitidine.
2DMS is an emerging tool for mainstream laboratory applications. Existing QToF instruments with modest specifications can be upgraded to perform this powerful DIA technique. Here, the principle of 2DMS has been demonstrated with a simple mixture but much more complex samples can also be analysed. The data generated is unbiased, complete, and easily interrogated, both manually and by software.
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