HP 5890 II question regarding holding oven temperature

[Wendy]

Hello,

I am using HP 5890 II GC-5972 MS to analyze TMS derivative of ergosterol. The solvent is hexane. The GC oven temperature program is:
80 C (1min) , 50 C/min , 300 C (16 mins).

I have been using this for several months, but I found recently that the oven temperature can not raise at a rate of 50 C/min after it was close to 170 C. The Rate LED was blinking and the NOT READY LED was on. The oven temperature could be 30 degrees lower than the set point (The oven temperature eventually reached 300 C). As a result, I can not detect the ergosterol, the chromatography showed a baseline.

I called Agilent technician and he told me that the GC should be able to handle 50 C/min. I did some trail run and I can reach 30 C/min but not 40 C/min or 50 C/min. Could it be some parts failure? Any suggestions will be appreciated.

[Schmitty]

Do you still have that Agilent technician's name? He's your best bet. Are you sure you need to run at 50°C/min? Maybe a more consistent gradient between blank/spike runs is necessary to show a difference/peak.

My first guess as to why the instrument can not heat at 50°C/min is that the element is on it's way out. Is the baffle/door shut all the way on the back?

[Victor]

Why are you starting the oven temperature at 80 degrees? Are there other more volatile componets in the sample that you also want to measure? If you start the oven at a higher temperature and use a less severe temperature program, you may get better results. I would expect that you will cold trap your ergosterol derivative on the front of the column without needing any solvent effects.

[Wendy]

Thank you, Victor and Schmitty.

I used this temperature program based on a published paper. A Shimadzu GCMS-QP2010 was used by the author.

I only want to measure the ergosterol and 7-dehydrocholesterol (as internal standard) in the sample.

The Agilent technician (only remember his first name Mike) asked me to check the flaps ( I guess that's what Schmitty meant by baffle/door) and it was shut all the way, at least observed by eyes.

I know other people used 170 to 290 C with 20 C/min. But the response was very poor. I can not get the instrument back to good performance since the supply helium gas was run out during a sequence. I have cleaned the ion source box, replaced the liner, conditioned the column. But now the oven can not heat at 50C/min and I can not see the peaks well using the 170-290 programm. I am not sure if there is something wrong with the instrument although it seems fine on other sample analysis.

[Schmitty]

I can not get the instrument back to good performance since the supply helium gas was run out during a sequence.

So it was working before? Then the column went *dry*. Now it doesn't work?

Did you observe the temperature during previous runs, to see if the oven could keep up with the program? Maybe your column is shot, and now you are just noticing that the oven can't keep up?

[Wendy]

Yes, the instrument was working and then it went 'dry'. Since then, it was not reliable. I got two or three successful analysis; but most of the times, it gave me a baseline.

I did not pay much attention to the temperature during the previous runs.

[Maybe your column is shot].

Is there any way to verify it? I conditioned the column and run hexane through after it went "Dry".

[Schmitty]

Do you have a GC/MS test mixture? That is a place to start. You could also try analyzing for something that you know should work with that column/detector. It might be in your best interest (time/money) to just toss the column and buy new.

[Consumer Products Guy]

Wendy - personally, I've never even tried greater than 30C/min on my 5890s or 6890s. Agree with others, maybe start at a higher temperature, make sure the back flap is COMPLETELY closed while running/heating (feel for air flow with your hand). And you may need to try a new column since you ran out of carrier at temperature.

[gcguy]

On a 5890 you can use the keypad to monitor the power usage in % of the oven. When it hits 100% that is the maxium oven heat up rate. We recently did some testing on all our GC ovens using a thermocouple and a data logger to check what the heat up rates of various instruments were (bored on a Friday afternoon job).
As expected there is a big difference in the heat up rates at differing temps.
50-150 C was the max heat up rate and at 250-300 C the rate was about half.

The instruments used were 5890s, 6850s, 6890s and PE Autosystems.

GCguy