Intermittent response for high boiling point organic solvent

[Joshp]

Hey guys,

I am trying to determine concentrations of Toluene, DMF, Xylene and NMP in mixtures of various ketones (MIBK, MEK) and alcohols (IPA, isobutanol) at ppm levels. Some of you probably remember me asking questions previously about headspace equilibrium time.

Anyway I have found good linearity for toluene (0.999), xylene (0.999) and DMF(0.997) for my calibration solutions with the following settings:

Sample amount: 25ul
Column: DB-Wax
Headspace equilibrium temp : 120C
Headspace equilibrium time: 12 mins
Loop temp: 130C
Transfer line: 140C
Inlet: 150C
Temp program: Initial 60C for 1 min hold then:
Rate Final temp Final time
10 150 0.00
60 200 2.00

Detector is set at 250C

Now I know I missed some important settings so if you need any please let me know. The trouble is getting good linearity from NMP (bp = 202C). This method had been given to me by people higher up and I am tweaking it to make it work, essentially I can change what I want (I have changed a lot to get to this point). At the moment I am getting intermittent response from NMP for lower concentration solutions (~250ppm) but I pick it up at ~500ppm. I am thinking I either need to increase my equilibrium time or possibly even higher on the equilibrium temp (I raised it from 80 to 120 to get good results for DMF). Also I have a feeling the final ramp is a little aggressive in timing and ramp gradient?

Any feedback would be highly valued, thank you.

[Peter Apps]

All the headspacers that I have worked with had problems with some spots being cooler than others - and the performance was determined by the temperature of the cool spots. Although you have your main zone temperatures set high, I expect that there might be parts of the plumbing that are significantly cooler. Try putting extra insulation around the valve oven and a wrapping of aluminium foil around the tip of the transfer line and the inlet nut (I assume that you have the transfer line going in through the septum ?) with extra insulation around that.

Peter

[chromatographer1]

I would also raise each zone temperature to 150C and evaluate the results .

best wishes

Rod