I am trying to identify some pheromone compounds, though facing a problem with GC-MS. At first i injected the crude extract of bugs and identified some targeted compounds by looking at their mass spectral information and by matching them with NIST database. The sample one which was injected at November 3rd 2021, later same sample injected on November 18th using same column with same method (time difference 15 days), there is little delay between same compounds peaks in TIC (total ion chromatography), and can't assume the logical reason. Have a look at attached Fig. Where i tried a standard compound at earlier its Rt was (17.75) after a week or more it was injected again and popped up at (17.93 Rt) only the concentration was different.
How to make it stabilized, so you don't need to inject the crude extracts again and again to compare with standard compounds.
A more considered answer to your question would need require you to provide a comprehensive technical description of your sample, experimental and chromatographic conditions. Plus some indication of the system usage between the two injections.
Nevertheless, it is seems pretty clear that something significant has happened to your column in the time between injections. You have gone from a well shaped GC peak to one with a horrendous tail. If all the experimental details for the two injections are the same then my first thought is that the column has been seriously degraded in some way. Two immediate possibilities are (a) an accumulation of debris (change the liner and septum and remove the first 20cm from the start of the column) and (b) an expired gas filter or an air leak causing oxidative damage to the bonded phase.
There are many reasons why the shift in retention time and tailing can happen. I completely agree with Charles. Other than that you can try to
- check for leaks
- condition the column
- check the total flow and pressure of your carrier gas
- check the GC column cut and positioning
- try to use another column
Also it would be useful to check did other compounds change their RT or form. It can help with troubleshooting.
Also, you mentioned that concentration was different. At higher concentrations a shift in RT and tailing can happen (it's probably not the case, but anyway).
I would highly recommend reading troubleshooting materials for you GC model.
These slight shifts in retention time are very common, some reasons are aging of the column, more dirty/clean liner, etc: you should not worry about that. If you want a more stable parameter for compund identification (apart from mass spectrum obviously), you should use relative retention time (RRT) instead of retention time (RT). RRT can be calculated as the ratio between the RT of your analyte and the RT of another compound that you can easily and certainly identify (best option is an isotope-labeled compound, but you might use a halogenated or any other).
Charles H Hocart Valerio Paolini Aset Muratuly Your answers and suggestions are appreciated, let me try the mentioned suggestions and methods, at present isotope labeled compound will might work for me. Thanks
- Badges
- Science topic