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- Posts: 1
- Joined: Tue Feb 17, 2015 3:37 pm
My company has been analysing cholesterol levels in our products for many years without issue. Recently we have been encountering an increasing number of system suitability failures. %RSD for multiple injections of the first standard have been too high and the standard agreements between bracketing standards has been failing tolerance.
The standard solutions contain cholesterol and the internal standard 5-alpha-cholestane. For the multiple injections of the first standard, the very first injection is typically rather different from those which follow. Repeated injection of the same standard results in the instrument reaching a steady stand whereby the response factor is consistant. If the instrument does not inject a standard for any significant period of time (>1 hour) the reponse of the next standard injection will be markedly different. This happens even if sample solutions have been injected in the interim period.
To me, this behaviour seems to indicate active sites in the GC inlet. To attempt to eliminate any active sites as much as possible, the inlet weldment has been replaced with a new silanised part. Consumables such as gold seals and inlet liners have been changed very frequently. Still the problem remains.
Perhaps I should have added earlier that these are splitless injections. Any advice that any of you can offer will be gratefully received.
The standard solutions contain cholesterol and the internal standard 5-alpha-cholestane. For the multiple injections of the first standard, the very first injection is typically rather different from those which follow. Repeated injection of the same standard results in the instrument reaching a steady stand whereby the response factor is consistant. If the instrument does not inject a standard for any significant period of time (>1 hour) the reponse of the next standard injection will be markedly different. This happens even if sample solutions have been injected in the interim period.
To me, this behaviour seems to indicate active sites in the GC inlet. To attempt to eliminate any active sites as much as possible, the inlet weldment has been replaced with a new silanised part. Consumables such as gold seals and inlet liners have been changed very frequently. Still the problem remains.
Perhaps I should have added earlier that these are splitless injections. Any advice that any of you can offer will be gratefully received.
