Chromatography Forum

Dear All,

This is my first post so please be gentle!

I'm currently working in a QC lab testing the menthol content of various sugar and sorbitol based lozenges by GC.
The method for testing has been in use for decades, and is thoroughly validated... The problem is, that in general day to day use, we seem to get lots of out of trend results.

Does anyone have any idea where we are going wrong?

GC Conditions
Column: DB Wax 30m x 0.25mm x 1um
Carrier Gas: Hydrogen @ 1.25ml/min
Split Ratio: 50:1
Injector Temp: 250C
Detector: FID @ 260C
Oven Temp: 156C
Inj Vol: 1ul

Sample Preparation
lozenge is dissolved in 13ml of water and 1ml 50% v/v HCl, then extracted with internal standard solution (0.01g/10ml Heptan-1-ol in Butanone).

Any ideas on different solvents to use in the ISS would be greatly appreciated!

Many thanks

I am not sure what you mean by "out of trend" - if it means that the results you get on real samples are out of specification for the menthol content of the sample, but your QC injections etc all say that the method is working correctly, then the lozenges do not have the correct amount of menthol in them, and the problem is with production not with analysis.

If it means that you are QC control charting with standard injections, reference samples or whatever, and your QC results are out of the acceptable range, then you have to troubleshoot the analysis.

There is nothing obviously wrong with the method as written. How often do you clean inlets, change septa, change columns etc ?

Peter

I wonder how tightly controlled the extraction is? Are you getting varying partitioning during the extraction. I take it is a two phase extraction. I wonder if the istd is partially water soluable and if the inorganic concentration changes in the aqueous layer this will effect the solubility.

I wonder if you inlet is getting dirty with the sugar?

I suppose you will have to suck it and see! (sorry)

GCguy

Your post confuses me.

You state that the method is thoroughly validated and you are getting results which seem unsatisfactory.

How could be both be true unless you are not following the method or your matrix has changed in which case you need a new method.

As Peter has noted perhaps you should be performing more maintenance on your equipment.

Isopropanol, hexane, and ether are alternatives as extraction solvents.

Are you extracting something that might coelute with menthol in your analysis and causing 'out of spec' numbers? Are your results high or low?

best wishes,

Rod