Chromatography Forum

Hi!

I am a fairly young student who has just basically been exposed to the world of Gas chromatography

I have a few basic questions:

If a GC is equipped with a thermal conductivity detector and the temperature is increased from lets say 60 to 70, would the observed peaks on the integrator be sharper and take less space to be drawn? Is the reason behind this because if the temperature is higher, then the sample you inserted evaporates faster?

Vice versa, if temperature is decreased from 60 to 50, then the peaks drawn would be fatter right? For example if water was in the sample inserted, the peak of water would be very fat, since the boiling temperature is so high?

Also what is the general effect of increasing and decreasing sample sizes? From lets say .6 microlitre to .4 microlitre, or .6 microlitre to .8 microlitre?

Thank you guys! Please try not to make fun of me for these basic questions

aischwertz wrote:

"If a GC is equipped with a thermal conductivity detector and the temperature is increased from lets say 60 to 70, would the observed peaks on the integrator be sharper and take less space to be drawn? Is the reason behind this because if the temperature is higher, then the sample you inserted evaporates faster?"

Answer:

The area of the peak should be almost exactly the same, the peak while 'sharper', ie more narrow, should be 'taller' and this the area counts will differ by very little (nothing in this world is perfect, ok?)

The reason is the partition between the liquid phase and gas phase changes for the analyte as the temperature rises. The analyte remains in the liquid phase for a shorter period of time so with a contant flow of carrier gas, the concentration of the analyte increases at its maximum (ie, the peak is taller) and the time it takes to elute from the column is shorter (the retention time and peak width decrease).


"Vice versa, if temperature is decreased from 60 to 50, then the peaks drawn would be fatter right? "

Answer: yes, and they would be shorter but with almost the same area counts.

"For example if water was in the sample inserted, the peak of water would be very fat, since the boiling temperature is so high?"

Answer: Perhaps, but 100°C is not that high a boiling point for an analyte

"Also what is the general effect of increasing and decreasing sample sizes? From lets say .6 microlitre to .4 microlitre, or .6 microlitre to .8 microlitre? "

Answer: In general, the peaks are taller or shorter, not wider or thinner. Until you reach the point where the analyte is saturated in the liquid phase or the gas phase will you find the peaks getting broader. This assumes that the initial peak width caused by the injection and evaporation of the analyte remains constant.

I hope this helps.

Rodney George

evaporates faster?

First, I doubt that anyone here would make fun of you.

Also, the other thing... about "evaporating faster"

The inlet should be a constant temperature in most cases nowadays, and it's at a point where anything put into the inlet instantly vaporizes.