Isobars, having the same nominal mass, but different sum formula: These compounds can be distinguished with one of our PTR-TOFMS series instruments, if the mass resolution is sufficient for the mass difference between the isobars.
- One substance fragments into another substance of interest, or two different substances braking up into the same fragment. As an example, if you have a linear chain of a hydrocarbon, e.g. Octane C8H18 and Nonane C9H20 they are only detectable with O2+. Due to the fact that ionization with O2+ is not as soft as ionization with H3O+ there will be more fragmentation. This may lead to both substances fragmenting into smaller hydrocarbon chains (C2Hx, C3Hx, C4Hx,...) which makes it very difficult to distinguish between these compounds. This could be also a problem for e.g. Tetrachloroethane C2Cl4H2 and Methylenechloride, which could both easily fragment into CHCl2.
- Different isomers (same sum formula) in a sample (e.g. 1-propanol and 2-propanol): they will be summed up on the same nominal mass, the detection limit and the calculated concentration is an aggregate of both substances. Recent IONICON developments allow to use a fastGC add-on with a PTR-MS which enables chemical separation. Ask us for more information.
- Compounds having the same sum formula, but a different structure: e.g. 2-Heptenal and Cycloheptanone (C7H12O) where the 2-Heptenal is an aldehyde and Cycloheptanone is a ketone. These two substances can be distinguished using NO+ because aldehydes and ketones undergo different reactions with NO+. Therefore the substances are detected on different masses.
- We can stimulate or suppress fragmentation by increasing or decreasing the energy in our PTR drift tube (E/N). This is a unique feature to PTR-MS and allows to separate several substances based on their ion branching ratios, i.e. the different patterns of fragmentation in relation to the E/N, which often are not alike. We make use of this method in our Automated Measurement and Evaluation (AME) air monitoring software.