Chromatography Forum

Dear All,

I am running a method for detection of methanol on an Agilent 6890 GC-FID. The details of the methods are as following:

Column: 30m, 0.32mm ID, 10μm Rt-QPLOTâ„¢
2.0μL split injection
Oven program: 120°C to 220°C @ 5°C/min. (hold 30 min.)
Injector: split @250°C
Carrier gas: hydrogen (constant flow mode)
Column flow rate: 1.1cc/min.
Split ratio: 70:1
Detector: FID @270°C

I noticed that sometimes the peak of methanol when I run a standard up to 1000 mg/l is not detected. My LOD is about 50 mg/l.

The samples I am analysis are shampoo products, and I prepare these samples just diluting them in water (10 fold).

I am using a guard column.

The sample matrix is not very clean and this cause the liner to get dirty very easily, so I have to replace it quite often.

I was wondering whether you could help to understand the reason why the peak of methanol is sometimes missing when I inject standards.

Thanks

Either the dirty liner or the contaminated column are absorbing the methanol, or the standards are not prepared without losing the methanol which can happen easily during the preparation.

I can only assume that your technique of sampling with a syringe is good as well as sealing the vials with the standard solution.

best wishes,

Rodney George

For purpose of troubleshooting it could be helpful to just make a dilute solution of methanol in water, without the rest of the matrix, to see if you get your normal peak without matrix effects. If no peak do inlet maintenance and try again, if no peak remove your guard column and see if you get a peak with just the column. If this works go backward: put a new guard column on, if the peak is still there go back to injecting your normal standard/sample matrix.

Is the guard column connected with a glass press fit connector?

the standard are prepared in water. I did inlet maintenance as well. I notice that the needle of the autosampler siringe starts to get slightly stuck after a few injections of samples, so I was thinking that maybe the siringe does not inject the right volume of standard solution.
However if the syringe get completely stuck and the needle cannot be withdrawn the autosampler shows an error and stop the run, but this has not happened.

Syringe problems are common, I would try another syringe first because its quick, easy, and very obvious when its the fix.

And with a gooey matrix like shampoo use heaps of post injection washes, probably water would be best in your case. Maybe even water; then acetone; then water again, just make sure you don't carry over the acetone.
And you will need a long viscosity delay with aqueous injections.