I prepared three replicates of methanolic extract for volatile compounds identification through GC-MS.. for first replicate, GC-MS was done within a week but for other two it was done after a month... The identified compouds are 50% different among the replicates even after using same standard and same GC settings... what could be the probable reasons..
Any suggestions or answers would be appreciable.
Hi Pooja I'm afraid the answer is: quite likely. Some compounds are very unstable in storage and so I could imaging seeing the level of difference you mention between replicates, especially after a month of storage.
Sorry I cannot be more specific but it really depends on the storage stability of individual compounds.
Good luck!
Hi Pooja
It really depends upon the compounds you are looking for, some of the legacy SVOCs like PCBs are very stable others can degrade. A good thing to check is the storage recommendations for that compound, and the usage life and check if your parameters exceeded these.
If you have used internal standards it would be good to check the responses of these as if your loss is through volatilization these compounds may also show variation either at greater concentration if they are not very volatile, but you have solvent loss, or less if they are similar to the compounds of interest. It does as Simon noted depend on how you stored the samples. Other factors in storage to consider is light as some compounds are photosensative.
A good check for the instrumental side would be to inject the same sample in triplicate to ensure this is stable.
If all this comes to nothing it may be that your extraction method is not reproducible not all methods are stable for all compounds and you may need to extract another triplicate and run at the same time to validate this.
Hope this helps a bit
G
Material extracted may be/may not be analyzed immediately. The time lapses do affect the chemical composition of the extracts. It is practically difficult to get the samples of extract analyzed as soon as they are obtained. But these problems doesn't matter much as long as the experimental protocol/storage conditions and time of storage are recorded and mentioned meticulously.
They provide a variety of constructive suggestions. Extraction and storage of some compounds should be at a relatively lower temperature in case of their valation. Furthermore, some compounds are easily oxidated into other substances during storage.
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