[Dolphinsiu]

Why does the air or hydrogen peak appear as a 'negative peak'? Explain the significance of the air peak.

I get stucked when I meet this question in lab datasheet. I do this lab by using Shandon UK3 gas-liquid chromotagraph with thermal conductivity detector and nitrogen gas as carrier gas. Why air and hydrogen will appear on my datasheet. Is someting wrong with the question?

[invisiblegs]

So, why is this peak negative, and what does it mean?  The presence/absence and direction of deflection of any chromatographic peak are determined by the response of the detector.  So, why would air or hydrogen cause the TCD to respond in the opposite manner to that of another analyte?  As far as analytes go, how does air or hydrogen interact with the column that you are using?  Do you expect it to be retained or not?  The answer to this question may be able to tell you something about the volume of the chromatographic system if you know the flow rate of your eluting gas. 

[Dolphinsiu]

My book says that hydrogen gas has the highest thermal conductivity which is the ability of heat transfer from hotter place to colder place. From my notes, dead time due to unretended components and retention time due to the analyte both gives positive peak (upward peak), but my notes haven't mentioned any more in hydrogen/air interaction with column. If hydrogen is unretended/retended component, it is not possible to give positive peak.

[Dolphinsiu]

My attempted answer:

‘Negative’ peak is defined as the signal from the detector is opposite in direction to the signal from the eluting peak. In the injection of an analyte into a GC column, air or water vapor often accompanies the analyte. Thermal conductivity, in which air or hydrogen is higher than nitrogen carrier gas, is defined as the ability of a substance to transfer heat from the hotter place to the colder place. If air or hydrogen gas is eluted to the column, as they are unretended in stationary phase, more heat is transferred to the filament of the detector and resistance increases. Base current decreases and gives rise to ‘negative’ peak.

Do my concept right?

[invisiblegs]

That sounds like a well though answer to me.  You also noted the dead time required for elution of the unretained components.  Given the flow rate, and this dead time, what else could you figure out?