Chromatography Forum

Hello,
I am a student of power engineering. (master degree)

In my university there is a micro gas chromatograph Agilent 4900 with three columns installed: cPsil, Moulsive and PPQ; the carrier gas is Argon.
In this period we are conducting a tests on natural gas (91% CH4, C2H6 3%, 2% N2, 1% CO2, 2% 1% C3H8 C4H10), but we have problems of repeatability.
The gas chromatograph is connected by pipes and connectors 1/16 inch Swagelok to network of natural gas, the ambient temperature is not controlled and varies between 22 and 30 °C, and there is an environment pressure in the room where the gas chromatograph is installed
I would like to know if is necessary, compared to the calibration conditions (24 °C, ambient pressure), to perform the conversions to the measurements obtained.
I would also like to know if it is appropriate to have a flow meter on the line input of natural gas or something similar.

You should stop the flow of sample before injecting the natural gas if you wish consistent results.

You should also calibrate the GC at the conditions at which you wish to test a mixture. Different temperatures in the room I assume will change the amount of gas injected. So calibrate before and after you run samples and see if the results are consistent. If your first std and last std differ then you can expect your samples between them to be incorrect.

best wishes,

ROd

How can I stop the sample flow going into the gas chromatograph?
The injection time is controlled by computer that acts on a small pneumatic valve.
The typical injection time used are of the order of 50-250 milliseconds.
Could you show me a typical configuration connection of the gas chromatograph to the natural gas network

best wishes,

Andrea

How can I stop the sample flow going into the gas chromatograph?

Use a sample shut off valve.

The injection time is controlled by computer that acts on a small pneumatic valve.

Software selectable injection volume, 1 μL to 10 μL is the specification of the 4900. Do you require sample flow to operate this valve, or can it injection with no flow through the sample system?

If your temperature and pressure of the natural gas are varying, what would this do to the mole amount of gas you inject? (Ever hear of the universal gas laws?)

In order to calibrate the Natural gas you require volume reproducible injections at the SAME PRESSURE since the calibration is based on an external std. (IF your pressure is 50% higher you have 50% more gas in your injection. The results will be off by 50%.

Field analyzers of natural gas recalibrate when the atmospheric pressure changes (a cold front comes through), sometimes even more than once daily.

best wishes,

Rod

Micro-GC's seem to be very susceptible to temperature changes in the room. I have two and if the temperature changes 10 degrees or so I will need to re calibrate simply because my total analysis will be about >95% or <105%. I usually need to re calibrate on a daily basis soon before my injections.

"You should also calibrate the GC at the conditions at which you wish to test a mixture. Different temperatures in the room I assume will change the amount of gas injected. So calibrate before and after you run samples and see if the results are consistent. If your first std and last std differ then you can expect your samples between them to be incorrect."

We redirect the flow through a vent, and when we need to inject redirect the flow back through the GC for about 20 min prior to injection.

good luck

If you need % only then variability of P, V and T should not affect results.
Is your system primed well? Use IS if you can. I used manual injections for LPG (hydrocarbons only) works fine.

The GC won't accept high inlet smple pressures so ake sure you're not exceeding the limit in the manual, I can't remember what it is but it's not far above atmospheric pressure. As Rod says you need consistent P and T of the sample stream. The GC draws the sampel in itself so if you have a large flow then maybe use a T so the GC can draw what it needs and the rest vents out the other side of the T?

The gas chromatograph is connected to the process by a T so, as Johnny says, the GC can draw what it needs and the rest vents out the other side of the T, but it is really very difficult to close to 100% with the sum of all gases, GC usually closes the sum from 97% to 103%.
It's also impossible to control the room temperature where GC is installed and sample temperature, so according to the universal law of gases, the same sample time changes the volume aspirated from the GC.

So, after checking the calibration, it is appropriate to normalize the data analysis on a sample from 103% to 100%?

In commerce, NO. In research where price and money are not directly concerned, maybe.

best wishes,

Rod

You could carry out replicate analysis to improve your numbers (e.g. average of three runs per sample). You are quantifying using the single-point external standard method?

Ok, thanks to all, the measures on natural gas are now consistent.
I would like to know the filament temperature of TCD sensor in order to add data to the thesis