Aviv Amirav, Tal Alon and Alexander B. Fialkov, Tel Aviv University and Aviv Analytical
- Reduced GC separation.
- Largely reduced peak height and signal, far more than could be guessed from the slightly deteriorated tailing peak shape. As shown in the figure above, slight peak tailing results in about six times loss of peak height.
- Increased noise. The lost signal is converted into a long (and in many cases very long) tail that is transformed into noise as can be seen in many RSIM traces after the elution of a tailing peak.
- Increased RSD. The analysis reproducibility is adversely affected and the RSD is significantly increased even due to a slightly tailing peak.
- Sample identification via library search is adversely affected by peak tailing and particularly from the increase of ion source temperature that is needed to reduce the peak tailing.
- While the peak tailing can be reduced via the increase of the ion source temperature such temperature increase exponentially reduces the relative abundance of the molecular ion, reduce the sensitivity in RSIM on the molecular ions and increases the chances of sample degradation at the ion source.
- A few classes of compounds such as free fatty acids require laborious derivatization for their GC-MS analysis as otherwise their analysis becomes impractical, and these derivatization procedures aren't always practical themselves.
In the upper middle insert figure we show the effect of ion source peak tailing on n-C24H50 in which the change of temperature from 100ºC to 250ºC increases the peak height by about a factor of six. At 100ºC the peak shows a "small" tail but like an iceberg this small tail hides a factor of six reduced peak height which is not easily recognizable via the peak shape.
As demonstrated in Figures 4 and 5 above, the Cold EI peak shapes are fully symmetric, as originated via the GC column and without any ion source Peak Tail. Thus, Cold EI is characterized by the following peak tailing related advantages over standard EI.
- Improved GC separation via the elimination of peak tailing.
- Largely increased peak height and signal, and the harder the compound for analysis the greater is the sensitivity gain with Cold EI over standard EI.
- Lower noise. The elimination of peak tailing results in having lower noise and the Cold EI noise is further reduced via the elimination of vacuum background. Combined with the signal gain the obtained signal to noise ratio can be a few orders of magnitude better than in standard EI (over a factor of 100 for Cholesterol).
- Improved RSD.
- Full elimination of any intra ion source degradation.
- Sample identification via library search is improved and combined with the Tal-Aviv Molecule Identifier software it is the best, compared with any other GC-MS.
- The range of compounds amenable for analysis is increased with Cold EI for a few reasons such as the ability to use short column and high column flow rate and in addition via the ability to analyze polar compounds such as free fatty acids without derivatization.