If different GC systems were used then retention index is useful, but for a same system I think Rt will do the job. Moreover when you are using GC-MS, mass spectra can confirm the identity of the compound, rather than Rt.

Hi Mohsen,

If I understand you correctly you cultures 1 bacterial strain under 8 conditions and now you want to compare these conditions for 35 VOCs (this is quite little for GCMS analysis of a culture, how did you do peak detection, alignment and matching?).

Normally you would just use a kruskall wallis (dependent: multiple groups, independent: not-normally distributed variable; voc concentration) and the ones that have a p-value below 0.05 you can perform post-hoc tests (you can choose this in SPSS). However, since you test multiple vocs you will have to take care of multiple comparison by lowering alpha (p-value treshold).

Several additional questions arise: how many duplicates per culture condition do you have, this effects the effect size you can discover. Can you convert the 8 categories into subcategories? This increases power etc.

Could you provide additional information?

Lieuwe

Dear Dr Bos

Thanks for the answer.

1. I have 2 run per culture.

2. Yes I can subgroup them into 4 categories.

1. I just considered peaks with more than 40000 min.area height and slop of 1000/min. so, my identified compounds based on NIST matching were about 30 t0 40 per sample.