negative pieks on TCD

[tacohofman]

Hello,
we have a problem and can't jet solve it.
there was a failed reducer on the carrier gas, so we changed it and now there is a problem with a negative spike on the baseline @ TCD. we tried jet a lot of thinks like: change column, change carrier pressure, with no succes..

are there anny other sugestions??

i've forgot to tell that we are working with online process analysers so it's a littlebit different than lab. GC's but the principle is the same..

[Don_Hilton]

What carrier gas are you using for the chromatography? A thermal conductivity detector depends on difference between thermal conductivity and the analyte to produce a signal - and some combinations do produce negative peaks.

Are you using the same carrier gas as you were before? And were there peaks where you are now seing negative peaks? Is the negative peak at the void volume of the column? How does a calibration standard look?

[GasMan]

Are we talking about negative spikes or negative peaks? Are they always the same size?

Does that happen during a run, and if so is it always at the same time?

Are you using valves on this system?

Gasman

[tacohofman]

What carrier gas are you using for the chromatography? A thermal conductivity detector depends on difference between thermal conductivity and the analyte to produce a signal - and some combinations do produce negative peaks.

Are you using the same carrier gas as you were before? And were there peaks where you are now seing negative peaks? Is the negative peak at the void volume of the column? How does a calibration standard look?

where using hydrogen and yes we use the same carrier gas as before, the negative peak always com back at the same time at same place, its just where i should measure the oxygen peak. cal.std is perfect, but when i get back to the process stream, i get the negative peak

[tacohofman]

Are we talking about negative spikes or negative peaks? Are they always the same size?

Does that happen during a run, and if so is it always at the same time?

Are you using valves on this system?

Gasman

sorry, 1 negative peak just where i should measure oxygen the peak is always the same size.we are using slider valves on the system..

[vballest]

Perhaps you still have a leak in the carrier system. What you just described suggests there is a higher Oxygen impurity in the carrier gas than in the cal gas. Try purging you carrier gas regulator at the bottle, it may be trapped.

[tacohofman]

Perhaps you still have a leak in the carrier system. What you just described suggests there is a higher Oxygen impurity in the carrier gas than in the cal gas. Try purging you carrier gas regulator at the bottle, it may be trapped.

well, what u suggest, shouldn´t we have large oxygen peaks than in state of negative peaks?

[JI2002]

First, please change your font color to black.

second, vballest made a good point: If the carrier gas, not the sample, is contaminated with oxygen, then you are going to see a negative peak at the retention time of oxygen if the sample is free of oxygen. If the sample is contaminated with oxygen, you'll see a larger oxygen peak. In your initial post, you mentioned a failed reducer in the carrier gas was changed. Did you have the negative peak before the failed reducer? If not, then that is where the oxygen contaminates the carrier gas.

[tacohofman]

First, please change your font color to black.

second, vballest made a good point: If the carrier gas, not the sample, is contaminated with oxygen, then you are going to see a negative peak at the retention time of oxygen if the sample is free of oxygen. If the sample is contaminated with oxygen, you'll see a larger oxygen peak. In your initial post, you mentioned a failed reducer in the carrier gas was changed. Did you have the negative peak before the failed reducer? If not, then that is where the oxygen contaminates the carrier gas.

Ok thnx, i´m going to test this on monday i´ll let you know if the suggestion is right

[tacohofman]

First, please change your font color to black.

second, vballest made a good point: If the carrier gas, not the sample, is contaminated with oxygen, then you are going to see a negative peak at the retention time of oxygen if the sample is free of oxygen. If the sample is contaminated with oxygen, you'll see a larger oxygen peak. In your initial post, you mentioned a failed reducer in the carrier gas was changed. Did you have the negative peak before the failed reducer? If not, then that is where the oxygen contaminates the carrier gas.

Ok thnx, i´m going to test this on monday i´ll let you know if the suggestion is right

whats the problem with the green font colour anyway..?

[GasMan]

You do not state what type of TCD you are using. Most TCD's compare the 'analytical' side gas stream with a reference gas stream. If your TCD is of this type, do not ignore the reference side, as anything appearing on this side will give you a negative signal.

Gasman