Hello Bedigama,

A ±0.5 min is acceptable between  standard and sample analysis when replicates are run and there is a good consistency each time. You can also  use Nucheck 37  fatty acid FAME mix for the identification of RTs .There is possibility  this can happen if there is a drop in pressure of carrier gas. 

I absolutely agree with Simon.  You need to check whether you have variation in retention times of your standard compounds when you do repeat runs and do you also get variation in the retention times of your sample compounds on repeating the run?  If the standard and sample retention times are persistently different by a similar amount every time - regardless of the actual retention times - they are probably different compounds.  However, if you are getting a lot of variation in retention time when you do repeat runs then your system needs to be checked for leaks, gas flow/pressure variation etc.  A variation of 1 minute between runs is too large. 

I also agree with Simon and Valerie that a +/- 0.5 min change is too much variation.

I have a question about your question which states: "A ±0.5 min is acceptable between standard and sample analysis when replicates are run and there is a good consistency each time."

Are your standards  of a consistent retention time from run to run, and your samples of a consistent retention time from run to run but 0.5 min different from the standards?  This can sometimes happen when there is an early eluting and large amount of a contaminant in the samples that overloads the analytical column and prevents individual analytes, including fatty acids, partitioning in and out of the stationary phase. 

I suggest looking at your sample and standard chromatograms to determine if there are interfering peaks that could shift retention times.

Another step I would take is to co-inject standards with samples and investigate retention times. This is the standard addition process.  If the sample compounds are fatty acids, then their peak areas should be increased by the amount of the standard addition and there should be one peak.  If two peaks occur with standard addition, then clearly there are two separate compounds and the fatty acids in the sample are not the fatty acids in standards.

Yes, there is a good constancy between the  replicates.  What I mean here. If I am  running  a standard for example   lauric, acid  each time I  should be getting same RT time. But when I running  a sample containing lauric acid a difference of 0.5 min is acceptable between standard lauric acid and lauric acid in sample.

This doesn't mean in the replicate  there should be more than 0.5min difference .If lauric acid in sample is eluting at 31.5 min , it should be eluting at 31.5 min every time,where as standard may elute at 31.5 or 32.0 min. Again each time standard should  be eluting exactly at the same RT. Yes you can also check  RT by spiking or co eluting  but  you won't see much difference due  to very small conc in the sample unless it is a different fatty acid.