Chromatography Forum

I am trying to analyze a terpene hydrocarbon Myrcene with LCMS ESI mode. I have standard and I see uv peak and also mass peak but the Molecular weight its showing is 312 (ESI+ 313 and ESI- 311) . Myrcene molecular weight is 136 (C10H16) why i am seeing this molecular weight? My mobile phase has 0.1% formic acid.

I doubt myrcene can be ionized in ESI mode. Possibly in APCI, but even not sure. GC-MS would be more suitable for such molecule.

The peak at 313/311 you detect is probably a contaminant from the system.

Myrcene with three double bonds is very susceptible to dimerization. In fact polymerization is going on all the time and can be followed daily by observing the change in the specific gravity. Shelf life in presence of air is measured in days. Even under Nitrogen and kept in the refrigerator the process is on going albeit much slower. Flash distillation through a fractionating column at about 80mm vacuum will produce high grade material but also a tenacious residue that must be quickly diluted with a similar material (limonene) before it sets up into plastic mass.

I'd reiterate what Gaetan wrote: you are not going to see this compound in ESI. Firstly it has no ionisable groups whatsoever. Secondly it is volatile and would almost certainly be dried off and pumped away like the solvents.

thanks all