Retention Index and Alkane Standards

[Karen01]

For teh first part of my career, most of my experience had been LC, so when I had to do GC, I carried over the same practice of running standards for the analyses of interest with every run, and using them to set retention times for each component for every analysis.

Because of that I don't have feel for how long a retention calibration is typically good for on GC...

But now I find myself in a situation where I have to rely on published retention indices for compound ID (and there can be hundreds of compounds!!!) for high probability compound ID, so the "calibration" is done using n-Alkanes to have the calculate the retention indices

In this case the run times are "VERY" long and I get a LOT of samples. So much so that it is hard to keep up, so the less I can run n-the alkane standards the better...

Mu question is what would be the longest reasonable time between running the alkane standards ...

This is R&D support for early stage process development, so there are no specific QC/QA requirements and the chromatograms can be very different with each experiment...

I do have more to go on than retention index, but a good retention index would really help nail things down... I do have an MS on the GC, but MS often does not ID the right isomer even for large peaks, and has a lot of trouble IDing the small peaks correctly... The MS hits ALONG with retention index would make the IDs much more reliable.

Thanks for any info.

- karen

[Peter Apps]

How long is a piece of string ? How often you need to re-run standards depends on how stable your analysis is, which you can get a good estimate of by looking at the history of retention times.

The other thing to take into account is how big a problem it will be if you get an identification wrong because the retentions have drifted.

Peter

[Consumer Products Guy]

Karen01 -
#1. agree that your retention index standard needs to be run each session of the GC use, maybe even at start and end.

#2. agree that MS spectra of alkane peaks are not so definitive.

Because of #2, and also because the fragrance suppliers all used this technique, we aligned with them and used ethyl esters of fatty acids (yes, ethyl esters, not methyl esters) as retention index standards. These could be obtained from suppliers such as Nu Chek Prep. When we really needed a hydrocarbon/alkane standard, we used octacosane, which could be obtained in a quite pure state, and then other paraffins could be determined using octacosane retention time.

[varossf]

Hi Karen, would a faster GC help you out here, like the Intuvo? With OpenLAB 2 control and built-in KI calculations you could use less alkane standards. With the additional RTL (Retention Time Locking) you can calibrate the GC in such a way that it always deliver the exact retention time over and over again.
just my 2-cents.