How does my sample prep change between GC/MS and LC/MS/MS?

LC/MS/MS seems to be the new gold standard in sample analysis nowadays. However, some labs (like forensic labs) are a little slower to get LC/MS/MS instruments and are still analyzing samples using GC/MS. The majority of published articles in peer-reviewed journals are for LC/MS/MS so those working in laboratories with GC/MS instrumentation may not know how to change the sample preparation to better fit GC/MS.

First and foremost, GC/MS methods tend to need a higher sample volume than LC/MS/MS methods. GC/MS is a less sensitive instrument, which means more sample is needed to achieve lower limits of quantitation (LOQs). Many LC/MS/MS methods only require 50-200 uL of sample. GC/MS methods usually require 1-3 mL of sample.

Because of the higher sample volume needed for GC/MS analysis, higher mass sorbent beds in the extraction cartridge are usually needed as well. The larger sorbent bed allows for a higher amount of the compounds of interest to bind to the sorbent bed. Because of the larger sorbent bed, higher volumes of wash solvent and elution solvent are needed as well.

GC/MS also can’t be used for super volatile compounds because of the temperature of the injection port. For many methods, the injection port can be anywhere from 150C to 300C. Many compounds can’t hold up to these temperatures. Because of this, derivatization becomes necessary for many compounds when analyzing by GC/MS. Very simply, derivatization involves adding a structure onto your compound of interest that acts as a stabilizer. This adds time to your sample preparation as an incubation period is necessary in order to make sure that the compounds in your sample are fully derivatized.

When reconstituting samples for LC/MS/MS analysis after the extraction and dry down are completed, the mobile phase starting conditions are usually used as a reconstitution solvent. For example, if the LC gradient in a method starts as 90% aqueous mobile phase A and 10% organic mobile phase B, the reconstitution solvent would be that same 90:10 mix. In GC/MS analysis, the reconstitution solvent needs to be 100% organic, usually ethyl acetate or acetonitrile. The reconstitution volume is usually much lower in GC/MS than in LC/MS/MS. Since GC/MS is a less sensitive instrument, the less solvent used to reconstitute the sample, the more concentrated the compounds are as they are injected into the GC/MS.

These are some of the biggest differences between sample preparation methods for GC/MS and LC/MS/MS analysis. As can be seen above, the differences between methods are minimal, which makes it a fairly simple process to develop a new method for one or the other instrument type. If any questions come up during this process, reach out and ask us!

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