My main concern is actually the run time of GCMS (around 1.5 hr per sample) and i do not want to let my samples waiting for longer periods if I do replicated injections, which may affect my results (I will actually do it in batches of 5 samples), aside from that, i have a lot of samples to analyze, and that would take a toll to the machine and also take much of my time.
Fred, injection-to-injection time may be reduced by programming the solvent prerinse earlier while the previous run is being acquired, such that the autosampler performs all the rinsing (solvent and sample) needed prior the next injection. The same can be done with the incubation needed prior injection, especially in biological setup to induce formation of the compound of interest rior injection. This permits the autosampler to collect the final sample aliquot when the instrument becomes ready, injecting almost immediately soon after the completion of the previous run. Another approach is to speed up the passing through of the sample in the column by (i) using a smaller-diameter column (or shorter column) with a thin phase that would require less column consumption of sample and less time for the sample to pass through the entire column, (ii) a gradual speed up of the temperature increase in the program set for the oven, and (iii) their combination.
Fred,
I think you have to calculate what is the reproducibility of methodology, in order to know what is your %RSD. So you could analyze just once each sample, only if your RSD is appropiate ( for example less than 3-5%)
Remember that sensitivity is not the same than reproducibility
Dear Fred,
There are a number of reasons why it is NOT a good idea to do single runs in GC-MS.
Some of them are the poor reproducibility of the MS signal (did you use internal standard? maybe stable isotope dilution assay?), but the main problems come from what you do prior to injection, often multistep procedures using derivatisations, extractions... All of them add potential for analytical uncertainty, which can partly be controlled through internal standards, but not totally.
Try repeated injections / repeated extractions / sample prep to identify your uncertainty. You'll end up with a need for strict control of conditions and still replicates. And btw, without replicates the top Journals will not accept your data.
I do have biological replicates and internal standard, thank you very much for all of your helpful comments!
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