GC-MS problem (methanol is converted to formaldehyd?)

[Chacer]

Newest data

[Peter Apps]

The first thing that strikes me is the inlet pressure - 97.5 KPa pulsed to 241 KPa will give you very fast gas flow rates, and the high pulsed splitless pressure will increase the probability that some solvent condenses at the head of the column and causes peak splitting.

Unless there is a good chromatographic reason for having the pressures this high (and "we have always done it like that" does not qualify as a good chromatographic reason !) I would lower the inlet pressure to give a volume flow through the column of about 1.5 ml/min (on the GC you can set either constant volume flow or constant linear flow, setting the linear flow to a constant 50 cm/s will be the easiest). Then, for troubleshooting purposes at least, leave out the pressure pulse.

All your rention times will be much longer of course.

Peter