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Missing heavies C32 and C28
Posted: Thu Apr 13, 2006 4:25 pm
by pseudotexan
I am running a SIM DIS on a MXT-1. I have excellent response on everything except the very heavies. I have cleaned and/or replaced the entire sample path and can not get them to show up. C 28 is coming out but about 100 times less than expected, C32 is not coming out at all. I am heating the sample to liquid and making a 1uL injection. I have adjusted the split from 1-1 all the way to 10-1 and it didn't help in the problem area. The system is a process control GC and is limited in injection temp (250 C) and detector temp (225 C) I have verification that the application works on the system and I have matched the conditions. What could I be missing
Posted: Thu Apr 13, 2006 10:34 pm
by Consumer Products Guy
Aldrich lists a boiling point of 278 C for octacosane C28......your relatively low temperatures may be your problem.....
Posted: Fri Apr 14, 2006 12:33 pm
by pseudotexan
My oven profile goes to 320 so I should be getting them off of the column
heavies
Posted: Fri Apr 14, 2006 1:32 pm
by chromatographer1
I suspect you either replaced a component in the injection system with a bare metal version of originally a FS or glass coated component, or you have a partially blocked injection valve.
best wishes,
Rod
Posted: Fri Apr 14, 2006 3:17 pm
by Consumer Products Guy
...assuming your C28 and C32 ever GET to the column at those inlet temperatures.....
other issues
Posted: Fri Apr 14, 2006 4:42 pm
by chromatographer1
Certainly you may have a cold spot in your injector and I assume this process analyzer was validated before being shipped to you.
If you are reengineering the analyzer that is a different question.
If you are trying to fix what was working, please state that to be the case.
Otherwise, I suspect like others here that your temperature is too low or you require a 'wash plug' behind your sample to carry your heavies to the column where they be separated and eluted.
best wishes,
Rod
Posted: Sun Apr 16, 2006 4:52 pm
by Victor
I agree with CSG-these components may never be getting near your column. Maybe they are not even leaving the syringe needle.
What sort of GC are you using? I presume it has an autosampler but is it making rapid injections in 0.1 sec or less like the Agilent 6890 (laboratory instrument) autosampler? I don't know anything about the spec of process GC instruments. If not, the sample may be merely distilling out of the syringe needle rather than being mechanically expelled. In the former case the heavies are mostly left behind in the syringe needle. This could then be a classical case of sample discrimination as described by Grob.