Chromatography Forum

Sorry for the rookie question. I searched and did not find an answer.

Usually we use N2 as make up gas. What is the function of make up gas?

Thanks a lot!

As far as i know make up gas is used to increase the sensitivity of the system.

i'm sure someone has got a better explanation why though.

steve

Make up gas is used with capillary GC to better match flow volumes required for detector optimum performance (which usually are similar to the carrier flow that packed columns would supply).

Why can not one carrier gas supply meet the sensitivity or optimum requirement? Usually make up gas (N2) and carrier gas (He) can be different gas. Can we use the same kind of gas for carrier and makeup?

Thanks

Zimanli,

Most of the detector designs out there today are derivatives of detectors that were built for packed columns. Very little has been done to reduce the flow requirements of the detectors for capillary columns, especially for the FID. Part of the reason for this is the fact that the FID is not concentration dependent so the addition of make up does not really affect sensitivity that adversely. I think make up with an FID is mostly about getting the flame in the proper place. I cannot see a reason why you can't run helium as both the carrier and the make up except that helium is more expensive than nitrogen. When column flow is 1 mL and make up flow is between 20-40 mL/min that might be a factor to consider.

Best regards.

You can use helium as a make up gas for an FID.

It is only if you use an FID at the most optimum sensitivity that you will find a difference using nitrogen, and then the difference is not very great.

If fact you have to carefully 'tune' the detector (each individual detector) with the air and hydrogen fuel flows to optimize the response versus noise. In so doing you will find that you will get a small improvement if you use nitrogen makeup instead of helium. I assume this is because helium is an 'insulator' across the electrodes in comparison to nitrogen, but I don't know the actual reason per se. There may be brands of FID where this does not happen so I cannot claim this effect to be universal.

best wishes,

Rod

The Carlo Erba/Fisons/Thermo FID is designed for capillary and requires no make up, just Air, Hydrogen and your carrier. It has however been demonstrated that a Nitrogen rich air (Sold as GC air in cylinders) gives better responses so you can add Nitrogen make up to increase response.

ECD. Some new design ECDs do not need make up as they have micro cells designed for capillary You still add Nitrogen or Argon/methane as this acts to slow the electrons and to enhance their ability to be "collected" by the halogens.

TCD TCD detectors are predominantly used with megabore or packed columns. Most TCDs need at least 10ml/minute flow to work. If you have a lower flow you need to add make up gas that should be the same as your carrier gas. TCDs also need a reference gas as they work on the comparison of the thermal conductivities of the respective flows passed over two hot wires.

Zimanli,

In the GC you are not only looking at the detector and it's gases but also the column. Generally, one gets better results on a capilllary column using helium or hydrogen, but you can not always use these gases with all detectors. The comments posted earlier about flows being set for packed columns are correct, and the reason for the make-up gas is to bring the capillary column flow up to a packed column flow amount. However, for each detector you may have to use a different makeup gas.

1. For TCD, the makeup gas MUST be the same as the carrier gas.

2. For FID, if helium is being used as carrier gas, then either helium or nitrogen may be used. There is a small increase in sensitivity when using nitrogen as makeup gas.

3. For ECD, if helium is being used AT LOW FLOW rates, nitrogen or argon/methane must be used as the makeup gas. If helium is used, it will ionize in the detector and you will get a full scale reading as the signal.

The above is true for HP/Agilent detectors and I assume it is probably true for other manufacturers.

Gasman

Thanks a lot everyone.

You can also use your air for a make-up gas if you do not want a 4th gas in the system. I have been using the air from my zero air generator and had no noticeable loss of sensitivity. It is free and my boss loves it.