Chromatography Forum

Hi,

I have been having difficulty with my GC/MS (Agilent 7890A with 5975C MSD) on certain temperature program methods. On methods that begin at 100C, there is a huge peak that comes off at about 800,000 abundance and drifts downward to less than 10,000 after about 8 minutes. On methods that begin at 200C, the baseline is between 2000 and 4000. I don't know what is causing this baseline problem for the lower temperature methods. I have tried baking the column, changing the gold seal, liner and septum. I've also clipped my column, but the peak still remains. Has anyone else had this problem or know what to do?

Here are the parameters for one of the 100C method:
Inlet:
Heater - 250C
Pressure - 10.244 psi
Total flow - 54mL/min
Septum purge flow - 3mL/min

Oven:
Equilibration time: 1 min
Initial: 100C (hold 2 min)
Ramp: 20C/min until 300C (hold 18 min)

Parameters for one of the 200C methods:
Inlet:
Heater - 250C
Pressure - 16.089 psi
Total flow - 54mL/min
Septum purge flow - 3mL/min

Oven:
Equilibration time: 1 min
Initial: 200C (hold 0 min)
Ramp: 20C/min until 280C (hold 8 min)

What are the characteristics of the spectra for each of these scenarios - Hydrocarbon, Column bleed, Something else?

The only ions coming out from the baseline are: 29, 30, 31, 32 (highest abundance), 33, 34, 40, 43 and 44.
My last tune showed: 0.47% H2O, 1.23% N2, and 0.69% O2.

More information is needed. What type of column are you using? Is this a new column? Is this a new method? What have you injected?

The only ions coming out from the baseline are: 29, 30, 31, 32 (highest abundance), 33, 34, 40, 43 and 44.
My last tune showed: 0.47% H2O, 1.23% N2, and 0.69% O2.

These are common for an air leak. You most probably have a leak at the MS interface due to the ferrule not sealing. As the oven temperature rises the leak will seal because the ferrule expands. If you give the interface nut about 1/8 to 1/4 turn it should go away. If it doesn't then the ferrule could be cracked.

We used to have this problem from time to time with runs beginning at 35c and ramping up to 230c at about 50c the background would drop to normal from a very high value.

When you install a graphite/vespel ferrule on the hot interface, they will shrink from the heat. Once they stop shrinking they will stay tight. One trick to use is to place a few ferrules into a small beaker and keep them in the oven, they will pre-shrink due to the temperature cycles of the oven and be ready to go when you install a new one. Also keep the interface temperature near the temperature of the maximum during your oven ramp so you don't get a lot of shrinking and expanding of the nut during each oven cycle.