Mouthwash analysis by GC

[Sharif]

hi,

i analyzed mouthwash by GC. Major component is Eucalyptol oil, Menthol, Methyl Salicylate and Thymol. I prepare standard by dissolved the material in 100% alchohol. In the mouthwash Alcohol:water ratio is 20:80. i use AT-1column, 0.32mm ID, 1Micrometer film thickness. operating condition- Injection port-150 degree celcius, FID- 250, Column Initially 50 for 10minute, 4degree/min to 200 degree for 10 minute. injection volume 1micro liter. split ratio 21. I use autosampler. i have the problem from the sample i couldn't get 100% result. Result below 80%. Assume that i prapare the product in the laboratory. there was no process loss. Anyone tell me whats the possible cause? i also infore that result was reproducible. Anyone tell me limit of % RSD acceptable for GC analysis And Head space analysis.
thanking you
Sharif

[chromatographer1]

You may have backflashed the 80% water preparation in the injector port and caused a loss of sample. Try increasing the split ratio to 100 or 200:1 and see if that changes the recovery.

That is the most simple solution to the issue.

best wishes,

Rod

[Victor]

Why are you making up standards in 100% ethanol when your sample is in 20% ethanol?

[chromatographer1]

Headspace analysis routinely gives an RSD of 2% or less if done well or if the analytes are suitable for headspace analysis.

best wishes,

Rod

[Sharif]

hi rod,

thanks a lot for your reply. i prepared standard in 100% ethanol, because if i use 20% ethanol then the solution become cloudy. in the mouthwash there was a technique to get a clear solution or there may be used an excipient. i will try different split ration and inform the result.

Thanking you

Sharif

[Sharif]

hi victor,

i reply why i prapare the standard in 100% ethanol. please read it and if u have any solution please inform me.

Thanking you

Sharif

[JGK]

Surely, the standards for headspace analysis should be in the same matrix as your samples.

If you are relying on partition coefficients between the liquid and gaseous phases they will be different for 100% and 20% ethanol solutions. Consequently, your results will be inaccurate.

If you can't use 20% ethanol to prepare STDs, what is the lowest % you can use?

and If you have enough sensitivity in the method, can you raise the ethanol content of the samples to this level?