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Selective Reagent Ionization - Mass Spectrometry

SRI-MS is an IONICON exclusive technology allowing to automatically switch between different reagent ions for chemical ionization direct injection mass spectrometry.

Currently the the range of selective reagent ions includes H3O+, O2+, NO+, NH4+, Kr+ or Xe+.


SRI-MS is a proprietary where the user can select from different reagent ions for chemical ionization direct injection mass spectrometry, only available from IONICON.

Our expertise in ion chemistry, being as the world's leading PTR-MS company, lead to the development of new ionization technologies currently including H3O+, O2+, NO+, NH4+, Kr+ or Xe+ as selective reagent ions (US Pat. 9,188,564, EP 2606505 A1).

SRI-MS instruments can be ordered with your individual choice of reagent ions but in generel the basic configuration offered is a PTR-MS instrument (using H3O+ as reagent ion) with optional SRI and SRI+ configurations. Available for all new or as upgrades of existing PTR-MS systems.

With SRI you can choose between NO+, O2+ and NH4+ as additional reagent ions. Using NO+ enables separating many isomeric compounds and thus allowing for detecting and quantifying them independently. O2+ possesses an ionization energy (IE) of 12.1 eV, which is considerably higher than the IE of NO (9.3 eV) and can therefore ionize substance classes that are not detectable via CTR from NO+ or even PTR ionization from H3O+. However, the IE is still below the IE of common air compounds and therefore no diluting buffer gas is needed. NH4+ reagent ions are particularly beneficial in complex mass spectra because of higher sensitivity (less fragmentation) and higher selectivity (higher proton affinity). We developed a unique method for fast switching to/from NH4+ reagent ions without adding ammonia (patent pending).

For SRI+ the available reagent ions are extended to Xe+ and Kr+. Xenon has an IE of 12.1 eV, i.e. the IE is very similar to the IE of O2. Therefore this reagent ion can be used as a substitute for O2+ in environments where no oxygen cylinders are allowed (e.g. due to fire protection or ex-proof laws). Krypton, on the other hand, has an IE of 14.0 eV, i.e. well above all reagent ions utilized in PTR-MS instruments so far. According to the National Institute of Standards and Technology (NIST) there are only 18 compounds possessing a higher IE than Kr, thus it can be said that Kr+ is transforming an IONICON PTR-MS instrument into a universal trace gas analyzer.

What are the advantages of SRI-MS?

While most of the astonishing advantages of PTR-MS are preserved when operating the instrument in SRI mode, several additional benefits arise:

Isomeric separation

Aldehydes and ketones, carboxylic acids and esters, carboxylic acids and alcohols and many more isomers can be separated (i.e. they will appear on different nominal masses) with NO+ as reagent ion.

Highest level of selectivity

Via fast switching of the reagent ions (for many reagent ions the switching process takes less than 10 s) different reaction pathways can be triggered. By comparing and interpreting these pathways substance identification becomes nearly unambiguous.

Virtually unlimited number of detectable substances

Ethylene, Acetylene, halocarbons and moreover CH4, CO, CO2, NO2, SO2 etc. cannot be ionized with proton tranfer from hydronium. All of these substances become detectable and quantifiable with an IONICON SRI-MS instrument using the appropriate reagent ion.

Simple quantification

As IONICON PTR-MS instruments show a linear response over many orders of magnitude, quantification is made especially easy. For SRI-MS we recommend the use of a gas standard containing the compounds of interest in order to subsequently calculate absolute concentrations from the real sample.

Very high reagent ion yields and sensitivity

No matter if H3O+, O2+, NO+, NH4+, Kr+ or Xe+ is utilized as a reagent ion, IONICON instruments never require the use of a signal diminishing mass filter between the ion source and the drift tube. This results in outstandingly high reagent ion yields and subsequently to market-leading sensitivities and detection limits, IONICON is and has always been known for.

Learn more about the IONICON technologies here.