MOST SUITABLE COLUMN FOR ANALYSING C1 TO C9 HYDROCARBONS

[christoffer]

Hello everyone,
I am very new to GC and I have a gas bottle containing a standard gas mixture composed of C1 TO C7 ALKANES, ACETYLENE AND TOLUENE (1000PPM EACH) in nitrogen (BALANCE GAS). We want to develop a method for GC analysis of this standard gas mixture. At some point, we want to introduce CO2 into the gas standard and also analyse it.

I need to know the best column for developing the GC analysis for BOTH this standard gas mixture and the standard gas mixture + CO2 using an AGILENT 7890B GC in our lab.

I learnt that CO2 and water vapour is BAD for GC columns and I intend to use the method developed for the analysis a gas sample which we expect to contain C1 TO C9 HYDROCARBONS (not just alkanes) + CO2 + WATER VAPOUR.



WHICH COLUMN IS SUITABLE FOR THIS KIND OF ANALYSIS AND CAN HANDLE THE INTERFERENCE FROM CO2 AND WATER VAPOUR?
DOES ANYONE HERE HAVE AN EXPERIENCE OR HAS CARRIED OUT A SIMILAR ANALYSIS?

I am grateful in advance for all your responses.

Thanks,
Christopher

[rb6banjo]

Christopher--

Welcome to the forum. First of all, those are myths. Your columns will take water and CO2 just like any other analyte -without adverse effects to the stationary phase. The big problem with water is that it's not very soluble in the nonpolar phases (probably the most popular) and because of that, you don't get much help condensing your analytes out of the injector in the solvent at the head of the column. Your analytes will be somewhat smeared because the water doesn't lay down nicely at the head of the cool column. This will likely appear to you as tailing of your analyte peaks IF it's excessive. Water vapor will not be a problem. CO2 is definitely not a problem.

Water injections can wreak havoc with some detectors. It may extinguish a flame detector for instance if you inject too much. If it's a CO2 sample, you will get a massive response in a pulsed-discharge helium-ionization detector, etc. If your sample is a gas to start, CO2 and water vapor will not be a problem for you. What is/are your detectors?

If you know your analytes, you should start at one of the chromatography house websites and look for application notes that show analysis for something similar to your sample. That'll give you a good starting point as to what column to choose. My favorites include:

www.restek.com
www.sigma-aldrich.com
www.agilent.com

One of those will likely give you someplace to start. Good luck!

[muGC]

For hydrocarbons, a DB-1 equivalent is your best bet.

[GasMan]

Christopher,

It would appear that all newcomers to GC have been told that all you need is the correct column and you will get the results that you want. In some cases this is true, but for gas analysis in most situations this is not the case. I build many analyzers for gas analysis, and some may need three detectors and up to seven columns installed, just so that the customer gets the results that they want.

Besides the column, there is also the question of detector(s). The most common detector in GC is the Flame Ionization Detector (FID), but this will only detect compounds that will burn. You say that you will in future wish to measure CO2. You cannot do this with the FID, you will need a Thermal Conductivity Detector (TCD). The TCD will measure all compounds except for the carrier gas, but it does not have the sensitivity of the FID.

To help you out, the forum will need the following information.

1. GC model (you have supplied this, thank you)
2. Inlets and detectors installed on the GC.
3. How sample is being introduced.
4. List of compounds to be analyzed with their max and minimum values ( 0 is not an acceptable lower limit).
5. List of ALL compounds that could be present in the sample.
6. Is time of analysis important.
7. How is your data being handled.

In situations where you asking for help in resolving a problem, besides the above, the following information is always needed.

1. Columns used with flow rates
2. All GC conditions such as flow rates and temperatures.

Gasman

[christoffer]

Hello everyone, thanks for all your replies. I am very grateful. Let me provide more information about the analysis I need to do.

1) we do NOT wish to measure CO2, in fact, we need to get rid of it
2) At the moment, we just have a gas standard comprising all hydrocarbons at 1000ppm
methane
Ethane
Propane
Butane
Pentane
Hexane
Jeptane
Acetylene
Toluene
Nitrogen balance (99%)

3) we syringe out 10ml of the gas standard and inject it in the gas sampling valve of our Agilent 7890B GC. THE VALVE switches after 10s of LOAD TIME to allow sample through while holding it for another 60s (INJECT TIME) before allowing Helium carrier gas to continue flowing. Injection source is set as : valve/immediate start

4) detectors used are FID (250degC, 400ml/min air flow, 40ml/min hydrogen flow, 20ml/min make up nitrogen flow) and TCD (250degC, make up helium at 5ml/min, reference flow of 10ml/min)

5) time is not important at the moment as we are still developing the method

6) HP 1 column is used (30m by 0.32mm by 5micrometre). We selected constant pressure (10.323psi)

7)INLET: split inlet and 100:1 split ratio, heater @290degC

oven temperature: starts at 35degC and hold for 12mins, ramp at 10degC/min to 170degC

9)Gc column flow is 2.11ml/min, average velocity is 35cm/sec, hold up time is 1.4286min

Please let me know what ever information you require .

Thanks
Chris