Increasing the sensitivity of GC-QTOF screening by using chemical ionization

Code: 19A04MP 

In 2018, the current GC-QQQMS routine screening method (i.e., initial testing procedure) for human doping control was successfully converted into an equivalent and complete GC high resolution acquisition screening method for routine purposes by using low energy electron ionization (EI) GCQTOF. This GC-QTOF screening method is compliant with the WADA requirements and allows the detection of 294 target compounds (and 14 internal standards), including diuretics, stimulants, narcotics, beta-2-agonists, beta-blockers, hormone modulators, anabolic agents and the quantification of 14 endogenous steroids in a single fast run (14.1 min). Because of the full scan high resolution data acquisition ability of TOF technology (and the corresponding retrospective capabilities), this proved to be a big step forward in comparison with the current GC-QQQMS routine screening methods. Taking into account that anti-doping samples can be stored and reanalyzed for up to ten years, the retrospectivity and sensitivity offered by GC-QTOF opened the door to a cleaner sport. Sensitivity is obviously compound depended, but in general the sensitivity of the low energy EI GCQTOF is situated between EI GC-QQQMS and CI GC-QQQMS.

Chemical ionization (CI) is a softer ionization than low energy EI and has the potential to further increase the sensitivity, in parallel with our previous EI/CI work on GC-QQQMS. Combining GC-QTOF with CI is the next logical step and this project aims at exploring, testing and exploiting the potential of CI GC-QTOF in all its aspects. This will result in the development of a high-resolution acquisition screening method with higher sensitivities. Higher sensitivities lead to more flexibility, longer detection times and a more extended list of compounds that can be monitored.

Main Findings

The main objective of this project was to improve the sensitivity of the GC-QTOF by using CI instead of EI, in order to maximize the capabilities of the GC-QTOF. Our experiments show that at high concentrations, CI is indeed more sensitive than EI, as we expected. Unfortunately, at lower concentrations, in general, the signal generated by CI ionization drops substantially faster than with EI ionization, making CI less sensitive than EI at those crucial low concentrations. To make It worthwhile to shift from EI to CI ionization on the GC-QTOF, the sensitivity at the low levels should be at least 5 times higher. Only then, it becomes worthwhile shifting from EI to CI ionization as CI also has some important inherent disadvantages such as the requirement for laborious and frequent instrument maintenance. The performance of CI GC-QTOF is insufficient and was found unsuitable for ITP purposes. At high concentrations, CI indeed outperforms EI, as we expected. Unfortunately, at lower concentrations, in general, the signal generated by CI ionization drops substantially faster than with EI ionization, making CI less suitable than EI at those low concentrations. The reason for this phenomena is up to this day unclear to us and we are at this stage cooperating with the R&D department of Agilent the share our experiences and to examine possible causes and solutions. From a theoretically point of view CI should outperform EI. However, in practice, this is only the case at high concentrations. That is the main cause and issue, preventing the use of CI for an ITP. Due to the gain/loss of sensitivity/specificity when comparing CI GC-QTOF versus EI GC-QTOF, CI might be useful to use as a CP. However, this needs to be checked on a substance to substance basis and depends on sample prep, GC parameters, etc. For example, for some AAS, CI will be a better option, for others it will not.