I haven't used RT index beofore, but I can answer you derivitisation question. No once it's openned it needs to be used almost immediately.
MSTFA reacts with -OH groups primarily on alcohols, but it will react with water in the atmosphere. Best option is to use a fresh ample everytime.
Thank you !
I have gone ahead and marked gchaplain answer as correct, if further help or discussion is needed , please feel free to post again.
Since the very first beginning of untarget analysis by GC-MS, there was a need for enhanced compound confirmation, by parameters other than Retention Time (RT) and Mass Spectra, specially for complex samples.
In the 60' a couple of researchers (Kovats; Van den Dool and Kratz) evaluated and developed linear equations to assign each peak to a Linear Retention Index (LRI).
The strategy now a days is to run an alkane standard (C8-C40) prior to your batch of samples.
Once you got the RTs from your alkanes peaks in your standard and the RTs of your compounds in your sample, you can correlate them by those equations.
The good thing is that there is no Standard preparation: just buy it from your preferred retailer and inject in your GC-MS!
Keep that in mind that once you run the standards, you can't change any chromatographic parameters that may shift your RTs!!!! Otherwise you'll get wrong LRIs values!!
I hope that this very short explanation helped you. If you have further questions feel free to contact me.
Please also check the following links for further information on LRI:
About the derivatizing agent issue: you do can use the MSTFA lefts on the following 2 days or so, but ONLY if you store them in a crimp sealed vial and under a very dried and vacuumed place! You may store them in a vacuum desiccator with silica or any other drying agent.
I wouldn't recommend to store opened ampules in the fridge (gchaplain has explained it already), and also wouldn't recommend to use the MSTFA lefts 3 days after it was opened (even in dry and vacuum storage).