First of all, here is the spectrum of nitrogen gas:
Now let's look at the spectrum of ethylene produced by electron impact.
It should be immediately obvious why GC-FID has traditionally been the preferred method for analyzing ethylene in head space samples. All the nitrogen is invisible to a FID, whereas it totally saturates the signal of ethylene at m/z 28, and using SIM won't help us here for the same reason.
One solution in this case is to monitor ions only at m/z 26 and 27, which are still very abundant in the ethylene spectrum and totally absent from the nitrogen spectrum. When we look at a compound as concentrated at nitrogen in air, it will still produce a small signal when monitoring 26 and 27, probably because its concentration is so high that a tiny portion of its pool at the extremes of its energy distribution overlaps with those energy windows. But the situation is much improved by eliminating m/z 28. Consider the following two total ion chromatograms.
Here we are monitoring m/z 26-28 of an ethylene standard (20 ppm) in nitrogen. The nitrogen signal at m/z 28 totally overwhelms the signal and the ethylene cannot be detected.
Here however, we can clearly see a minor peak for ethylene when we limit the SIM to m/z 26 and 27. Another improvement implemented included here is to run the purge flow to the split vent from the moment of sample injection rather than waiting 20-60 s as is normally done. This favors the entrance of sample gas into the column. This has nothing to do with mass spectrometer in this case but can be helpful for the analysis of samples that are already in the gas phase, versus a more typical scenario where a liquid sample is vaporized in the injection port liner. In the latter case, we wouldn't want the purge flow to run to the split vent immediately. Furthermore, because all that nitrogen gas in a 1 mL headspace sample is hard on the filament, it is best to use a solvent delay to keep the filament turned off until the nitrogen has passed through the detector, in this case about 2 min. One additional change included below is the use of pulsed splitless injection.
This injection technique requires a slightly more elaborate piece of hardware such as a multi-mode injection module but can be very helpful for gas injection on a GC system. It effectively provides a short pulse of additional carrier gas pressure to the sample during injection which prevents sample loss out the split vent. For comparison, this analysis was done by scanning m/z 26-28 in SIM mode, and we see the background is high even after the nitrogen gas has already passed. That's because there is always some nitrogen in the system, and this produces an unacceptable level of background.
When we now limit monitoring to the m/z 26 & 27 non-base peak ions, we can appreciate the combined effect of injection conditions that favor gas samples and selective detection of ions to favor our signal to noise ration for ethylene. This analysis achieves a good separation of ethylene from nitrogen by running isocratically for 1.5 min at 30 C before ramping to 100 C on this Al2O3 capillary column. The choice of SIM ions minimizes nitrogen background interference. The end result:
This entry intends to provide another example of how mass spectrometry can be applied to the study of plant biological processes using commonly available laboratory instruments, in this case a capillary gas chromatograph and quadrupole mass spectrometer. Even a highly volatile, low abundance gas hormone like ethylene can be reliably and rapidly quantified with GC-MS by combining the right conditions at the injection port and in the analyzer.
Injection port T: 100 C
Liner: Single taper empty
Injection vol: 250-1000 uL
Standard: 20 ppm ethylene in N2
Oven program: 30 C for 1.5 min, then 20 C/min to 100
Column: 0.25 mm x 30 m Al2O3/KCl
Detector: Agilent MSD quadrupole operating in positive mode, SIM m/z 26 and 27 100 msec dwell time