One powerful tool to considerably increase selectivity is variation of the reduced electric field strength E/N in die PTR reaction region.
Aprea et al. published an early study on a broad variety of esters in water (and water/ethanol solutions) where they investigated the product ion ratios of these key aroma inducing compounds as a function of E/N for each ester. They concluded that the E/N data can "extend the possibility to distinguish between isobaric and isomeric compounds improving the discrimination capability of PTR-MS."
However, nowadays the reaction regions of most high sensitivity PTR-MS instruments are equipped with RF driven focusing devices. Some manufacturers apply RF fields along the whole PTR reaction region in order to suppress ion losses to walls, whereas other manufacturers replace large parts of the reaction region with RF ion funnels. Although this might increase the sensitivity of some compounds, the E/N cannot be calculated anymore, controlling the E/N becomes very difficult to impossible and even "unusual" fragmentation of molecules has been reported by groups using these devices. Consequently, this not only complicates compound identification, but also restrains substance quantification without previous calibration. Eventually, measurement results obtained with these instruments cannot be compared among each other, which considerably limits the value of published data.
IONICON exclusively offers the TRU-E/N technology (patented; EP3309817B1, US10074531B2) which enables full control of E/N even with the most sensitive PTR-MS instruments. Thus, in addition to ultra-high sensitivity IONICON TRU-E/N instruments provide:
- Improved compound identification via E/N dependent product ion ratios
- Simple substance quantification
Comparability of measurement results obtained with different PTR-MS instruments