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From SIFT-MS to PTR-MS and PTR-TOF

An Evolution that Resulted in Perfection

From SIFT to advanced PTR-MS technology and ultimate performance PTR-TOF trace gas analyzers

Back to the basics: SIFT-MS

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:

  • The constant need of a carrier gas (typically helium) for the flow tube in SIFT-MS results in limited mobility of the instrument, and additional running costs.
  • The flow created by the carrier gas considerably dilutes the sample and therefore lowers the sensitivity even further.
  • The sequence "reagent ion source – mass filter – flow tube – mass filter" results in a huge demand on the vacuum system, as the needed pressures in the respective parts differ by orders of magnitude and thus a bulky pumping system is required.
  • Because the reaction region in a SIFT instrument does not involve any electric drift field the chemical ionization process cannot be controlled via the reduced electric field strength (E/N). This is a major drawback as far as selectivity is concerned.

 

A giant leap in evolution: PTR-MS

All of the above-mentioned shortcomings have been overcome with the introduction of PTR-MS in the 1990s:

  • Because of the sophisticated design of the reagent ion source no mass filter is necessary to guarantee high purity levels of reagent ions. That is, the reagent ions are directly injected into the reaction region, which enables extremely high sensitivities.
  • The air containing the trace compounds to be analyzed acts as a buffer gas, thus no additional carrier gas is necessary, which prevents any dilution effects.
  • As the reagent ion source and the reaction region are at comparable pressure levels (100 hPa) the vacuum system can be kept relatively simple.
  • These two advantages further result in a compact and maintenance-friendly pumping system.
  • The reduced electric field strength E/N is well-defined and can be tuned from virtually fragmentation-free to forced fragmentation for improved selectivity.

 

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.

The final step to perfection: PTR-TOFMS

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:

  • In contrast to the nominal mass resolution of quadrupole mass filters TOF analyzers can resolve isobaric compounds, which is an essential requirement for the analysis of complex samples.
  • In the same time quadrupole based analyzers acquire a single m/z, a PTR-TOFMS instrument acquires a full mass spectrum - in high resolution.
  • All m/z are recorded in a spectrum, i.e. the information on all product ions, their isotopes and fragments is available for analysis and discovery, whereas in quadrupole based systems only selected m/z are recorded.
  • The speed of acquisition in PTR-TOFMS is not dependent on the number of compounds monitored.

 

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

IONICON PTR-MS is trusted - all over the world

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.