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10 Common GC Mistakes

1) Not Taking Enough Care at the Inlet

Any leak in GC will cause significant problems with chromatography and the inlet is the most likely area. Don't take the risk of having to troubleshoot a leak, change the septum regularly and make sure the column is correctly installed and leak-free.

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2) Turning the oven on before the carrier gas

If there's one thing that will permanently damage a column more quickly than anything else it's heating the column without any carrier gas flowing. Get into the habit of making sure the carrier gas is flowing before you switch on the oven AND make sure the oven is cool before switching the carrier off. When switching the oven off, set the oven temperature to 35oC before switching the oven off as this will cool much quicker.

3) Not programming in column dimensions

Most modern GCs use an EPC (electronic pneumatic controller) to accurately deliver carrier flows and this relies totally on the correct column dimensions being entered by the operator. Make sure anytime you change the column you enter the correct dimensions into the GC or else your flowrate and split ratios will be wildly inaccurate.

4) Not using optimal flowrate for carrier

Each carrier gas has an optimal linear velocity, which gives the best efficiency (Nitrogen c.12cm/sec, Helium c.35cm/sec and Hydrogen 40 - 50cm/sec).

To obtain this optimal linear velocity the flowrate is matched to the internal diameter of the column in use. e.g. for a 0.32mm ID column using helium the optimal flowrate will be about 2.0ml/min.

If the optimal flowrate is not used, column efficiency can be significantly reduced.

5) Using too large an injection volume

When a liquid sample is injected into the GC inlet, this vapourises into a much larger volume of gas. The size of the vapour is dependant on the solvent being used and the injection volume.

If this volume exceeds the size of the liner a condition called backflash occurs.

Using a higher split ratio does not reduce this affect, you must either reduce the injection volume, use a different solvent or use a larger liner.

 

 

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6) Not specifying constant flow / constant pressure

As the oven temperature increases, the pressure within the column increases. In a constant pressure system, flowrate will decrease to maintain a constant pressure. This increases retention time and decreases efficiency as the flow moves away from the optimal linear velocity.
In constant flow mode, the instrument maintains a constant flow (with optimal linear velocity) by increasing pressure. Retention times are shorter and efficiency maintained.
Constant pressure and constant flow modes will give different retention times and peak shapes, so make sure you specify which mode should be used.

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7) Water on non-polar column

With some sample types it can be un-avoidable to inject the sample in water. Be sure that the column you use is suitable for water injections. As the column phase becomes increasingly non-polar (e.g. a DB-1), it loses the ability to adsorb the water sufficiently and chromatography breaks down. For water injections try to use the most polar phase you can (e.g. Wax) and accept that column life will decreases and bleed increase.

 

8) Not using the correct flame gas ratios

Lighting and maintaining a hydrogen / air flame relies on an optimal flow and ratio for
all the gases in the detector. See the advice in ‘My Detector Flame Won’t Light’.

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9) Not capping the column during storage

The bonded stationary phase in a GC column is easily oxidized, particularly in the presence of UV light. Be sure to cap the ends of the column to seal in inert carrier gas and stop air getting in. Store the column in a box in a dark cupboard to minimize the amount of light energy reaching the column.
This is important to stop the column degenerating on storage.

 

10) Not checking you have enough gas

In many labs, the gas supply is remote from the GC. Although it isn’t always easily accessible, it’s a whole lot less inconvenient than having to repeat a run of samples because either the carrier or detector gasses ran out before the original run was complete.
Make checking the gas supplies one of the first things you do.

 

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