Repeatability problems Thermo Ultra Fast Trace

[Uniqema]

Hello,

I am experiencing problems with the repeatability of peak areas when injecting an alkane (split 1:300) in the Thermo UF Trace. This varies by even more than 50%.

The equipment runs under EZChrom and I am thinking that one of the injection options is not correct... Does anyone know what the best dwell (pre and post) times and injection dept are when using the Ultra Fast module with split injection? We have a Merlin Micro seal installed, instead of a regular septum. We are analysing at constant flow (0,5 ml/min) and detection is with FID.
The problem clearly has something to do with something inside the injector, since I visually checked if the injector was indeed injecing 1 µl...

Martien

[Tony]

Hi Martien,
We don't use the ultrafast module but do use a Thermo GC ultra with Fast GC (0.1 mm ID 10 m long column) for FAME analysis. We also use a large split ratio (1:275) with H2 as carrier gas. With any fast or ultrafast analysis, ususally a large split ratio is needed because even with small injection volumes, the column capacity can be easily exceeded. This then translates to rapid gas velocities required through the liner. If your sample is injected but incompletely vaporized before it reaches the split point, then irreproduceable results will ensue. To prevent this problem, we found the following conditions were optimal:

1) Use a liner containing glass wool. This ensures complete sample vaporization before the split point is reached. The captive focusliner designs by SGE are good because you are doing a lot of injections in fast or ultrafast GC, and the wool needs to be kept in place after repeated contact with the needle during injections.

2) Use a 5mm liner - contrary to popular opinion, you don't need a very narrow liner for fast or ultrafast GC if you are already pumping gas through the liner at high velocities to maintain a high split flow. It is only if you are doing splitless injections with low gas velocities into narrow bore columns that you need a narrow liner. A 5 mm liner with wool will allow sufficient time for sample vaporization while also being quick enough to transfer the sample onto the column quickly enough to prevent peak broadening.

3) Use a rapid injection technique with no air volumes, and no pre- or post- injection delays. Pump the syringe with sample before injection to ensure complete filling. The wool in the liner will take care of complete vaporization. We have found with FAME analysis that a delay (e.g. using the traditional "hot needle" technique) can lead to split peaks early in the chromatogram becasue, with eluted peaks a second or less wide, a 2 or 3 second delay during injection can lead to effectively 2 peaks separated by 2 or 3 seconds! This is because a samll portion of the sample in the needle can load onto the column in the delay before the plunger is depressed. In normal GC with peaks 6 seconds or so wide, this effect is not seen.

I hope this helps.

thanks
Tony

[Tony]

One other thing - if you are using the merlin microseal, it is important that you are ussing the large diameter needles specific for this, and not the ususal narrow bore needles for rubber septa, otherwise wuou will get significant leaks during injection...

[GOM]

Tony,

Some useful information there for the rest of us.

Thanks,

Ralph