Seeing peak towards the end of the run

Hi oshin-sebastian,

If you do a blank run (no injection) do you still see the peak at the end? It's possible that your bakeout wasn't long enough.

Hi Abdur,

I bakeout out for almost 12 hours. The image I attached is of a blank run. I realized that when i do blank run at constant temperature, i don't see this problem. When using a temperature ramp, the baseline drift towards the end of the run.

Best regards,

Oshin

Hi Abdur,

Yes, with consecutive runs the baseline shift increases and also peaks also move forward a little.

Regards,

Oshin

Hi Abdur,

Yes, with consecutive runs the baseline shift increases and also peaks also move forward a little.

Regards,

Oshin

It sounds like you column is degrading. As mentioned previously that in the presence of air in the system this can happen more rapidly.

What column are you using? Do you know the maximum operating temperature of the column? And also what temperature and it's ramp is method set to towards the end of the run?

If the temperature exceeds or is close to the maximum operating temperature of the column it will bleed and stationary phase will leach out making RT shorter on following runs.

It's worth noting that C-H bonds break down on average at 320 degrees Celsius, so any bake out above this or close to this value with decompose parts of say a C18 column.

Hi there,

Yeah, the column was definitely degrading. I was using CP-Sil PONA CB 100m capillary column. The maximum operating temperature was 250C and I was operating close to that temperature (230C) for very long time. My ramp rate was also quite aggressive towards the end of the run. 50C (hold for 5 min) -> 20C/min to130 (hold for 3 min) -> 50C/min to 230 (hold for 3 min). The last aggressive ramp rate was put in order to make sure all high boiler were flushed out.  As I mentioned, the GC is connected online to a reactor and I need to make at least 3-4 measurements per hour, so 15-20 min max for a GC run.

Now I am using a new column from Restek (Rtx-PONA 100m) with a maximum temperature of 350C. My max operating temperature of 230C should be fine for this column I guess. But it's still a struggle to get rid of all the high boilers (Ethylcyclopentane, octane, heptane and aromatics so on) with in a 15 min run. I would appreciate if you have some solutions.

Best regards,

Oshin

Hi Oshin,

You may want to consider adding backflush to your system.  That way the late eluters never make it to your column.