Selective Reagent Ion/Proton transfer reaction – time-of-flight – mass spectrometry (SRI/PTR-ToF-MS)
Proton transfer reaction is a soft chemical ionization technique. Hydronium ions (H3O+) are generated from water vapour by a hollow cathode discharge. Volatile organic compounds (VOCs) entering the instrument will react with H3O+ when the proton affinity of the VOC is greater than the proton affinity of water (691 kJ/mol). Examples of molecular classes with a higher proton affinity are: alcohols, aldehydes, ketones, carboxylic acids, aromatic compounds, nitriles, amines, amides, thiols and sulphides.
The product ions are then separated and detected based on their time-of-flight. Due to the high mass resolving power and mass accuracy, ions can be assigned a molecular formula and can be used for putative identification.
Figure: The SRI/PTR-ToF-MS in the TDLab
Our PTR-ToF-MS system is equipped with a selective reagent ion (SRI) option. This means we can use alternative reagent ions (NO+ and O2+), allowing us to ionize a broader range of VOCs as well as to distinguish between different molecular classes based on product ions. Using the appropriate reagent ion for a VOC of interest, detection limits can be in the part-per-billion volume (ppbV) or even down to part-per-trillion volume (pptV) range.
|
Reagent ion |
Benefits |
|
H3O+ |
Analysis of a wide range of VOCs: alcohols, aldehydes, ketones, carboxylic acids, aromatic compounds, nitriles, amines, amides, thiols and sulphides. |
|
NO+ |
Similar to H3O+. Advantage of distinguishing between aldehydes and ketones, carboxylic acids and esters, carboxylic acids and alcohols |
|
O2+ |
Analysis of halocarbons, H2S, ethylene, acetylene, chloroform, bromomethane |
Example applications of SRI/PTR-ToF-MS
- (Real-time) exhaled breath analysis
- Monitoring VOC production by bacterial cultures
- Aroma analysis
- Biomarker investigation for poultry disease