Preparing a calibration gas standard

[shanbfh]

Hello GC experts,

I need some guidance about methane measurements on GC. what is an ideal method to make a calibration gas standard in the lab using the gas cylinder/gas bottle. we have a synthetic biogas bottle (50:50 v/v).

earlier, we used to make the calibration standard by filling the 120ml serum bottle with synthetic biogas through one needle and the air was let out with another needle (this needle was immersed under water). well there was no mass flow meter, but we used to fill the bottle with gas for about 3 minutes roughly. my new boss is not very pleased with this method. . . does anyone have any suggestions? if so, could you share it please?

will appreciate your hep.

thanks

[Don_Hilton]

I assume that the problem with the method of filling the serum bottle is that you don't know how much air is really left in the bottle. If you fill a Tedlar (R) bag with gas, you can start with negligible air in the bag and fill to a considerable volume. You can withdraw gas from the bag into a syringe - and given the negligible weight of the bag, the system is close enough to 1 atmosphere for most of us. You can use a bag wtih an valve in it and a syringe with a valve to avoid changes by diffusion.

And, if you are mixing standards, you can fill a bag from syringes with known volumes of gas.

[shanbfh]

Thank you very much Don! I will try this method for sure!

[cooperwood]

hi...when you fill the tank and any bag with the liquid there is some gas a present in the tank or bag.To remove these gas you can use syringe through the valve.

[Peter Apps]

If all you are doing is trying to fill a serum bottle with the gas from the cylinder, presumably so that you can subsequently take a sample from the bottle with a syringe, then by far the easiest way to accomplish the same thing is to put a septum port on the gas bottle.

Peter

[AICMM]

shanbfh,

You have a TCD, right? So you should be able to see whatever impurities you have in your inverted serum bottle standard. Plus, it is a very linear detector. So, if you don't see interfering (contaminating/diluting) peaks, what is the new bosses' problem with this technique?

I also agree with Peter about syringe adapter, available from someone like Supelco or Alltech.

The argument does not hold with an FID.

Best regards,

AICMM

[rb6banjo]

Your original method is perfectly valid. If you do GC analysis, you must have a flow meter around your lab somewhere. You can calculate the time required to get to the end. Assume perfect mixing of the gases in the receiving vessel.

V = volume of the receiving vessel
F = flow rate of the syn gas coming in and leaving the vessel
A = the amount of air in the vessel at any time t.

dA/dt = IN - OUT

No air is coming in so IN = 0.

OUT = A*F/V

So, dA/dt = - A*F/V

Solving this equation:

A = Ao*exp(-F*t/V)

where Ao is the amount of air present in the vessel at t=0 (let's call it 100% air at t=0). For V = 0.12 L, you need a flow rate of 0.27 L/min to completely change over (99.9%) the interior of the vessel in 3 minutes. If you can wait 10 minutes, you only need about 0.080 L/min (80 mL/min).

I do pyrolysis GCMS in a vial all the time. I seal a very small bit of polymer in a headspace vial and hook it up to a split vent of a GC (helium carrier gas) to exchange the air in the vial for helium (inert atmosphere). The flow rate out the split vent is 0.0065 L/min. For a 0.020 L vial, it takes about 15 minutes to change it out.

You can look at it from the other direction. Let S = the amount of syn gas in the vial at any time t.

dS/dt = IN - OUT

For this one,

IN = F*100/V (So = 100% syn gas coming in)
OUT = F*S/V

dS/dt = IN - OUT = F*100/V - F*S/V

If you solve this equation, you get

S = So*(1-exp(-F*t/V))

Either way, you get the same answer. You need to have a flow of 0.27 L/min to get 99.9% syn gas in the vial in 3 minutes.