As a first evolutionary step Selective-Ion-Flow-Tube – Mass Spectrometry (SIFT-MS) solved some of the key limitations of the flowing afterglow technique from the 1960s which can be considered as the roots of our current PTR-MS technology. In SIFT-MS a mass filter is installed for selecting the reagent ions, which results in high reagent ion purity but also makes the construction of the instrument challenging and, most importantly, limits the number of available reagent ions because of the transmission efficiency of the mass filter, which can by rules of physics never reach 100%.
However, similar to flowing afterglow also SIFT-MS was not invented as an analytical technique but rather for measuring kinetics in a lab environment. That is, when using SIFT-MS for trace gas analysis, in addition to its limited sensitivity, there are several other drawbacks:
All of the above-mentioned shortcomings have been overcome with the introduction of PTR-MS in the 1990s:
In summary, after decades of research, real-time trace gas analysis culminated in the development of the extremely sensitive, robust and easy-to-use PTR-MS technology. There was, however, one more step to perfection: The coupling of PTR with Time-Of-Flight (TOF) mass spectrometers.
So far, commercially available SIFT-MS instruments employ quadrupole mass filters for product ion analysis. In PTR-MS, on the other hand, TOF mass analyzers have been introduced in commercial instrumentation already in 2009. Although these first generation instruments had a lower sensitivity than the quadrupole based PTR-QMS devices at that time, the introduction of TOF mass analyzers in PTR-MS presented several convincing advantages:
To summarize, this is how SIFT-MS compares to the advanced IONICON PTR-TOFMS technology:
|
SIFT-MS |
IONICON PTR-TOFMS |
Measurement time for mass spectrum up to m/z 400 at low concentrations |
tens of minutes |
well below one second |
Measurement of mass spectra |
one nominal m/z after the other |
instantaneous |
Separation of isobars via exact mass |
no |
yes |
Compound identification via exact mass |
no |
yes |
Quantification of compounds not defined prior to the start of the experiment |
no |
yes |
Generation of NH4+ reagent ions without the need of hazardous ammonia |
no |
yes |
Adjustment of the E/N in the reaction region |
no |
yes |
Are you interested in more details, graphs, tables and references? Download this Whitepaper: Evolution from SIFT to PTR-MS and PTR-TOF
Ever since the PTR-MS technology was commercialized by IONICON in 1998, the interest in the emerging “gold standard” for real-time organic trace gas analysis has been growing quickly. In the last two decades more than 400 instruments were sold by IONICON. Analytical scientists and process engineers from all over the world have quickly adopted these new instruments which are continuously becoming more popular, performant and versatile as our R&D activities go on.
PTR-MS compared to the less advanced SIFT-MS technology also has much more impact in the literature which a 2019 Web of Science query reveals. In terms of published peer-reviewed academic journal articles PTR-MS scores 3x the amount of papers than those dealing with SIFT-MS, roughly 1,500 compared to 500, respectively. Moreover citations of these articles confirm the trend. PTR-MS related publications have been cited an astonishing 40,000+ times whereas SIFT-MS has less than 13,000 citations in the same timeframe.
This confirms that while SIFT-MS being one of the many evolutionary steps in the development of real-time trace gas analyzers and especially for the more mature PTR-MS technology in particular, SIFT-MS never played such an important role and also has not been able to attract the same level of attention as PTR-MS in the last two decades.