New to GC-MS and have questions

[kst1]

Hi everyone! Just stumbled on to this forum through Google and seems like a great place for me to get started.

I am new to GC-MS work. I am setting up a headspace SPME procedure to detect a compound which is typically present in low ppt levels. So far, I hhave not been able to see peaks around those levels; see peaks around ppb levels. So, I am troubleshooting and streamlining the process at every step.

I am looking at my injection settings and was wondering if I should do splitless or pulsed splitless mode? Also, how to determine when to open the purge valve and flow rate? For pulsed splitless injections, how to determine the pressure applied?

As far as the MS goes, what effect does the voltage have on the peak resolution and overall sensitivity?

Hope I am making some sense. I have many more questions to ask... Would appreciate responses/feedback from people here. Thanks!

[kst1]

Nobody has any ideas or experience on this forum??

[mfiligenzi]

Well, I don't qualify as an SPME expert, but I have run it a few times.

As to the split valve, I'm wondering why you would need to open it at all? SPME has no solvent to vent, so you may want to try leaving the split valve closed (I can't get to my system at the moment, so I can't tell you what I've run in the past). I'd at least consider waiting a couple of minutes before opening it in order to get everything from the fiber onto the column.

Have you optimized your injector temperature?

I can't see any benefit to pulsed splitless for SPME.

As for your sensitivity issues, have you optimized parameters such as the temperature of your sample, the amount of time spent with the fiber in the headspace, the particular fiber used, etc.? I would think that these parameters would be possibly more important than the injector parameters.

Anyway, I'd be curious in knowing how it goes. Best of luck.

Mike

[skunked_once]

Injection parameters are important. Use splitless or have the purge valve off for about 5 minutes at the beginning of the run. With headspace the amount of sample is very small so any loss through the split or purge would be significant. Leave the SPME needle in the injector for several (~5) minutes. The best sensitivity on the MS end is from select ion monitoring (SIM).

[chhubert]

I did have several trials using headspace SPME coupled to GCMS. Splitless injection is always used. Sometimes, the septum purge has to be plugged to prevent the analyte vapor from escaping during desorption. Also use liner of 0.7mm i.d. for better peak shape. Column focussing may be required for volatile organics and so GC column of thicker stationary phase e.g. 0.5-1.0um may be necessary. For SPME, Supelco's web site contains heap of information on selection of different phases for various volatile organics and is a good start for your headspace SPME experiment.

[kst1]

Thank you guys for the info! It all came down to having the purge valve set to open too early. I guess a rookie mistake. Anyhow, now I am able to get down to 1 ppt for my sample which is kind of the area I ant to be. Now, I have to learn how to set up standard curves!! Any ideas?