Inlet Temperature, explanation

Discussions about GC and other "gas phase" separation techniques.

4 posts Page 1 of 1
Good afternoon, colleagues!
I would like to ask you a possibly very simple question, but this question is very interesting to me. One of my students asked it to me. I will be glad if you can explain to me. Thanks in advance!

The question is next.
I would like to understand how to correctly calculate the Inlet temperature. I know that most experts often it is considered as +10-20C from the highest boiling compound in the mixture. Ok. But if, for example, I have lambda-cyhalothrin in my mix, which is often analyzed using GC. But, according to the reference material BP = 498.9C. How such a molecule evaporates at 250 degrees? Obviously, solvents change this parameter (boiling point) and it is enough for us to use 250-280 degrees on the evaporator to turn lambda-cyhalothrin molecules into vapor (gas).

Perhaps you can help me or somehow explain the principle of choosing the temperature for the Inlet (depending on the properties of the analyzed substances), and also perhaps there is some calculation (change) of the boiling point in solvent.

Thank you!

With best regards,
When the path is over and passed successfully, it's nice to remember even the mistakes ...
I don't have the calculations that would explain it handy, but I believe it is based on partial pressure laws. When you have an analyte it will have part of its mass in both the liquid (or solid) phase and a partial amount of it in the gas phase at the same time, and that amount is dependent on its partial pressure at a given temperature and pressure of the system. Since most injections are in nanograms or smaller, if say, 10% of the mass would enter the gas phase at 250C and 7psi inlet pressure, that would place 10% in gas phase at the moment of injection. Partial pressures are based on an equilibrium of the analyte between their gas concentration and the liquid concentration, so that percentage of the analyte is evaporating until it reaches the equilibrium point in the gas phase. Now if you are constantly flowing the carrier gas through the inlet, into the column and/or out the split vent, then the analyte is constantly going into the gaseous phase trying to reach equilibrium at the partial pressure concentration, but since the gas is constantly sweeping it away it will never reach that amount and be always evaporating.

If the carrier gas in the inlet was static, then you would end up with a liquid analyte droplet and some portion of it in the carrier gas equal to the partial pressure of the analyte at the inlet temperature. But carrier gas is flowing so that doesn't happen.
The past is there to guide us into the future, not to dwell in.
Thank you, James! Thanks a lot!
I have understood it!

Great support!
When the path is over and passed successfully, it's nice to remember even the mistakes ...
All liquids evaporate at temperatures below their boiling points. The hotter an inlet is the less gas volume has to flow through it to completely evaporate an analyte, which gives a narrower starting band on the column, and a sharper peak. BUT, when analyses are temperature programmed the width of the starting band is dramatically reduced by stationary phase focussing; the analyte is captured by the stationary phase, and begins to migrate (and spread out) only when the column temperature increases. Consequently, the boiling point plus 10 - 20 degrees rule really only applies to isothermal analyses. For temperature programmed analyses the inlet can conveniently be 10 to 20 degrees hotter than the maximum of the temperature programme.

Peter
Peter Apps
4 posts Page 1 of 1

Who is online

In total there are 2 users online :: 1 registered, 0 hidden and 1 guest (based on users active over the past 5 minutes)
Most users ever online was 1117 on Mon Jan 31, 2022 2:50 pm

Users browsing this forum: boulderco and 1 guest

Latest Blog Posts from Separation Science

Separation Science offers free learning from the experts covering methods, applications, webinars, eSeminars, videos, tutorials for users of liquid chromatography, gas chromatography, mass spectrometry, sample preparation and related analytical techniques.

Subscribe to our eNewsletter with daily, weekly or monthly updates: Food & Beverage, Environmental, (Bio)Pharmaceutical, Bioclinical, Liquid Chromatography, Gas Chromatography and Mass Spectrometry.

Liquid Chromatography

Gas Chromatography

Mass Spectrometry