Chromatography Forum

Hi, I'm a postgraduate student and about to use the GC-MS. I have a few questions which I hope could be answered here:

1. What are the alternatives to SPME to collect volatiles from samples e.g plant leaves
2. What is the lifespan of a SPME needle? Duration in time or how many samplings?
3. Why octane and dectane are usually used as internal standards? Advantage/disadvantage?

Thank you and hope to hear some feed backs soon

One alternative I looked at was single drop microextraction. Basically you use a 10ul syringe and dangle a drop of a solvent (octanol, ethylene glycol, I had some success with neobee) and expose it to the headspace, then you withdawl it back into the syringe and inject it into the gc as a liquids injection or I saw a Pitcon presentation where they used a nonvolatile ionic liquid and dangled it in the injector as well. I didn't get the sensitivity of SPME plus now you have a huge solvent peak not in the void volume of the GC chromatogram (that was why I picked neobee, gylcerol tricaprylate and tricaprin come off way at the end.) I also found it hard to setup so that I can stir the sample and not have the droplet fall off the needle.

SPME fibers are typically good for 200 or so injections. Mine typically last a quarter to a third of a year or so. I buy a 3 pack one a year. The 2cm carb/dvb/pdms fiber I used has gone up to ~ $175 per fiber,

There is also static headspace which a portion of the air above the sample in the vial is injected. It has much less sensitivity but for quantitation I can generate a SIM method on the GC/MS.

Dynamic headspace aka purge and trap where the sample is stripped with gas and the analytes are traped onto a large trap like tennax, then heated to desorb them into the GC. Very sensative but the traps are biased typically strongly against polar compounds, the high desorb flow means you either need a high split (40:1) or you need to cryotrap.

Stir Bar Sorbative extraction. Requires expensive equipment and the stir bars are typically either polar (ethylene glycol) or nonpolar (PDMS),

Thermal Desorption. Also needs expensive equipment usually a CIS inlet with liquid nitrogen.