General GC Questions

[sonicboom12345]

Hello all,

Pardon me for asking another question so soon after the first, but I am trying to bring myself up to speed on some of the theory of GC.

1. What extent does polarity of the stationary phase play in GC separation times? The way I was schooled in chromatography was very much HPLC-centric. Polarity was of prime consideration, and boiling point was not even discussed. However, come to find boiling point is actually the biggest factor in the retention time of analytes! So how much of a factor is polarity in GC?

2. What is the physical interpretation for the effect of oven temperature on separation? My understanding is that when the ambient temperature of the oven is below the boiling point of the analyte, the analyte will remain condensed at the head of the column and not travel whatsoever. Is this actually the case? Once the oven temperature has exceeded the boiling point of the analyte, does it affect the frequency with which the analyte is retained by the stationary phase/the number of plates for that analyte?

3. What is the typical injection volume for GC?

4. What are the advantages of a PTV inlet? Obviously it is used to control inlet temperature so you inject cool and then ramp up, but how does this assist the analysis? Is it only to prevent themolysis of sensitive molecules, or is there something more?

Thank you!

[chromatographer1]

You asked: questions that can be found in any good introduction book on GC. To ask anyone to copy down pages and pages from a book so you don't have to go to the library seems a bit extreme to me, however:

1. What extent does polarity of the stationary phase play in GC separation times? The way I was schooled in chromatography was very much HPLC-centric. Polarity was of prime consideration, and boiling point was not even discussed. However, come to find boiling point is actually the biggest factor in the retention time of analytes! So how much of a factor is polarity in GC?

It is a BIG factor. GC is a separation technique that uses a differential of partitioning of an analyte between a moving gas phase and a stationary liquid (or solid) phase. The attraction of the analyte with the stationary phase can be affected by dipole (polar molecular interactions). This attraction decreases the portion of time an analyte is in the gas (moving) phase and thus delays its exit from the column.

2. What is the physical interpretation for the effect of oven temperature on separation? My understanding is that when the ambient temperature of the oven is below the boiling point of the analyte, the analyte will remain condensed at the head of the column and not travel whatsoever. Is this actually the case?

NO, it is not the case. Because any analyte has some vapor pressure at any temperature above absolute zero. Water vapor for example will be present in air even though the temperature of the air is below the boiling point of water.

Once the oven temperature has exceeded the boiling point of the analyte, does it affect the frequency with which the analyte is retained by the stationary phase/the number of plates for that analyte?

The temperature affects the PARTITION of the gas liquid phases of the analyte. Even though a liquid's temperature may be above the boiling point of an analyte, the analyte may be present within the liquid phase. The partition may be almost completely in the vapor phase, but SOME of the analyte will stay in the liquid phase.

If chromatography worked as you have expressed it, then analytes would exit a column without broadening of the plug, coming out like bullets when a certain temperature was reached. And of course, this is not how things happen.


3. What is the typical injection volume for GC? Any volume which when vaporized does not exceed the volume of the injection liner. The volume of liquid SOLVENT is usually 0.1 to 2.0 microliters

4. What are the advantages of a PTV inlet? Obviously it is used to control inlet temperature so you inject cool and then ramp up, but how does this assist the analysis?

A much larger than normal amount of liquid is allowed to enter the column and the solvent is permitted to evaporate slowly, without expanding backwards into the pneumatics of the GC and permitting the less volatile components in solution to remain in a narrow band at the head of the column. After the solvent is evaporated away, additional heating allows the narrow band to move down the column and to separate the retained analytes from each other.

Is it only to prevent themolysis of sensitive molecules, or is there something more?

This can be the case, but another reason is to put MORE SAMPLE onto a column where otherwise due to the limitations of the volume of the injection liner the liner could not hold the amount of sample injected causing all sorts of problems.

My comments have been terse and lacking in detail concerning your questions due to the fact that the answers to your questions are not one sentence answers. But they are a start.

Please go to the library and read a book. Or search the many good descriptions on line from different universities about how GC works.

best wishes,

Rod

[sonicboom12345]

Thank you for your reply.

I am sorry if my questions fall beneath your intellectual pay grade. Suffice it to say, I am not asking people to “copy pages out of a book.” Rather, I am asking for enlightenment on a few paltry questions, which I assumed the friendly and intelligent members here would be able to rattle off without problem.

I have indeed read many a book on chromatography. Unfortunately, all of the books I have been given throughout my career have brushed over rudimentary details such as these in favor of lengthy discussions on mathematical formulae, calculations for partition constants and flow rates, extrapolations of the Van Deemter equation, etc.

Furthermore, I have never met a book to which I can speak, ask questions of, and beg clarification.

But thank you for your answers nevertheless, however “terse” they may have been.

[chromatographer1]

I encourage you to find a good book on chromqtography. What you have found so far is too theoretical in nature. There are excellent web site on the internet that you will find useful.

Your questions are good ones, but require much better answers than space allows here. Pursue your quest for information. You deserve better answers than I can give.

I hope you find what you deserve and need.

best wishes,

Rod