gc analysis/direct injection problems

[martinamarie]

Hi, I have a gc analysis where the system suitability passes specification and the standard check is fine, but when I inject the spiked placebo (accuracy) the internal standard value is consistently high but the active areas remain consistent with typical standard active areas. This leads to the accuracy failing on the low side.

In previous trials, there was no issue with this analysis

Anyone got any thoughts on what the problem could be

Thanks

MartinaMarie

[Bruce Hamilton]

Assuming that your standards and samples have all been prepared using the same fresh Internal Std solution, and exactly the same procedures as previously, then one likely cause is discrimination in your injector, especially if the Int std is more volatile than the analyte. It could be thermal, or enhancing active surfaces.

Other possibilities include:- your samples contain something that is enhancing the volatility of your Int std ( water? ), the sample viscosity is different, your analyte is degrading due to activity, and Int Std is more stable, your sample contains a compound that co-elutes with the Int Std, etc etc.

My first action would be to obtain a retention sample from earlier runs and run that.

If you are really, really confident that all of your sample preparation is identical to past runs, and the samples are the same, I would focus on the injection system, starting at the syringe and moving to inlet.

Please keep having fun,

Bruce Hamilton

[martinamarie]

Hi Bruce, the standards and samples have all been prepared using the same fresh Internal Std solution, and exactly the same procedures as previous.

I injected internal std solution and I get an area count equivalent to the std area count, but as soon as i spike the placebo with IS the peak area increases.

I will look at the syringe and the inlet and I appreciate the advice

Thanks

MartinaMarie

[Peter Apps]

We need more details on this - what are the compounds, solvents, temperatures etc.

Peter