Chromatography Forum

Can anyone offer some assistance.
Im attempting to quantitate Isostearly Alcohol, Cetyl Alcohol, and Unreacted Dodecenes using tetradecane as the internal standard. My problem/s are
Reproducibility and even detection are all over the place

Anyone have any suggestions or idea?
thx

You wrote, Mr. Law_son:

"...quantitate Isostearly Alcohol, Cetyl Alcohol, and Unreacted Dodecenes using tetradecane as the internal standard. My problem/s are
Reproducibility and even detection are all over the place ......."

My first suggestion is to stop being shy and tell us the matrix, the column, the injection system/technique, the carrier gas, and the detector of the Gc you are using.

Then perhaps the forum members will have enough information to help you.

best wishes,

Chromatographer1

Without getting too specific the sample matrix is a mixture of alkylmethyl siloxane copolyol (~60%), isostearly alcohol and 1-dodecene (~10%).
The column is a 25mX0.25mm id glass cap. with 100% dimethylpolysiloxane (0.4um) packing
0.5um direct injection (autoinjector) into a 270dC injector (glass wool packed liner) with a 25ml/min split.
Oven Program is 105dC for 10min, ramp at 15dC to 320dC, and hold for 10min.
FID detector at 300dC
The column is constant pressure at 15psi and carrier gas is Helium on a PE autosystem xl GC

We are in the process of qualifying a vendor for a European submission so we have to hold close to the given method, but do have a little wiggle room.

Hope this is enough information to get me some feedback, any and all is very much appreciated.

Thx,
Law_son

...The column is constant pressure at 15psi (~2ml/min) and carrier gas is Helium ..."

Carrier gas is really Nitrogen.

We have tried cutting the split flow altogether and injecting a spiked standard (~10%). Also increased the flow, first round to 7ml/min and the second to a suggestion of 25m/min, but still no consistent retention or area counts.
The instrument is due for a PM on Wed. so we are holding until then.
In the meantime, anyone else have any suggestions?

Regards,
~Law_son

It appears that a split of 100:1 is being used, and the sample is a heavy molecular weight composition. A reasonable injection volume is being used. My thoughts:

One, the sample is too thick to injection reproducibly. true or false?

Two, the packed liner is not working properly, perhaps due to the sample matrix or due to the installation of the column. true or false?

My suggestions:

Dilute the sample prior to injection in a volatile solvent, hexane? methyl formate? methylene chloride? to improve the uniformity of the vaporization of the sample in the injection port, after changing the injection liner with a new, clean, and freshly packed replacement.

Be certain you are not dehydrating the alcohol as at 270°C that is a possibility. I have done it given the right conditions.

Good luck, I hope it is only a partially blocked splitter trap or a leak elsewhere in the system.

Chromatographer1