Inconsistent GC/MS results

Discussions about GC and other "gas phase" separation techniques.

3 posts Page 1 of 1
Hello all,
I have been working on a calibration curve to analyze cyclohexane, cyclohexanol, and cyclohexanone for the last two weeks. I am using acetophenone as the internal standard for cyclohexane and 1,4-bis(trifluoromethyl)benzene as the ISTD for cyclohexanol and cyclohexanone. Unfortunately, when running the validation I am getting very inconsistent results for each sample. For example, one sample vial is run 3 times and each run has drastically different mol ratios and area integrations. I am using a Shimadzu GC/MS Gas Chromatograph GC-2010 plus. The solvent being used is acetonitrile
Column: BAC PLUS 2, 30m length, .6um thickness, .32mm.
-GC parameters-
oven temp: 40C
Injection temp: 250C
Injection Mode: Split
Split ratio: 50
Carrier Gas: Helium
Pressure: 6.6psi
Total Flow: 132 mL/min
Column Flow: 2.53 mL/min
Linear Velocity: 57.4 cm/sec
Purge Flow: 3 mL/min


I have tried 1 and 2uL injections and have varied the split ratio but that doesn't seem to be helping. Cyclohexanone seems relatively consistent, but cyclohexanol and cyclohexane are not. I am suspecting that there is something wrong with this method that is causing this variation, perhaps something to do with the thermal stability or properties of the compounds that are not working with this method. I have changed the septum, & the syringe was also changed last week. If anyone has any ideas as to what could be causing this variation I would be super grateful for any advice! If there is any other info I can provide that can help please let me know. THANK YOU!!!!
You have polar and nonpolar analytes it seems, need to know polarity of the stationary phase. Some will work with both types of analytes, some don't.

BAC2 looks to be optimized for Ethanol, so it should match closely the polarity of cyclohexanol which has a relative polarity of 0.509 cyclohexanone has a relative polarity of 0.281 and cyclohexane has a relative polarity of 0.006 (very non-polar).

What solvent are you using for this mixture? Solvent polarity can determine how it behaves when injected onto the column. A non-polar solvent injected onto a polar column can form droplets along the column, and the same is true when the column and solvent are reversed.

Those are the first questions I can think to ask for more information.
The past is there to guide us into the future, not to dwell in.
James_Ball wrote:
You have polar and nonpolar analytes it seems, need to know polarity of the stationary phase. Some will work with both types of analytes, some don't.

BAC2 looks to be optimized for Ethanol, so it should match closely the polarity of cyclohexanol which has a relative polarity of 0.509 cyclohexanone has a relative polarity of 0.281 and cyclohexane has a relative polarity of 0.006 (very non-polar).

What solvent are you using for this mixture? Solvent polarity can determine how it behaves when injected onto the column. A non-polar solvent injected onto a polar column can form droplets along the column, and the same is true when the column and solvent are reversed.

Those are the first questions I can think to ask for more information.


Thank you so much for your response! I am using acetonitrile as the solvent.
I have used this solvent previously to analyze these compounds and have not experienced this issue before.
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