Checking Carrier Gas Flow

[Ghost Writer]

I have recently been getting essentially the same peak heights using different split ratios for the same sample (100:1 vs. 20:1). This has not been a problem in the past. It was suggested that I check the flow of the carrier gas at different PSIG to assure he splitter is working. How do I do this? Thank you in advance for your guidance.

[Consumer Products Guy]

Ghost - your flow through the column is pretty much determined by your head pressure and your column dimensions. The split flow can be measured usng a flow meter, and the ratio determined. A utility like Agilent's (see their web page) can calculate the carrier flow for your column dimensions and pressure.

[chhubert]

What is the detector used for your GC? To check the flow rate of carrier gas, you have to calculate it by knowing the retention time of unretained analyte. If the detector used is FID, ECD, or MSD, you could simply inject methane or butane (from gas lighter), DCM (obtained from headspace; do not inject DCM directly into the GCECD), or air respectively. The linear velocity is then calculated by dividing column length with retention time of unretained analyte. Volumetric flow rate is simply multiplying cross sectional area of the column to the linear flow rate.
To measure the split vent flow, simply connect the flowmeter to the outlet of the split vent of the GC. Make sure not to connect it to the septum purge outlet.
Hope this helps.

[Ghost Writer]

I appreciate your input on my gas output. I have a Clarus 500 FYI which I failed to mention in the original post. My objective is to just see if there is a flow difference between two split ratios to assess the functionality of my splitter. Do splitters go bad like this where the split ratios don't change?

[CE Instruments]

All GC systems with Electronic pressure and flow control on carrier and split must remove the solvent and compounds from the carrier gas in the split lines to avoid damage to the controller. It is possible your filter is now suficiently blocked that it limits the split flow and you have the same flow regardless of the one set . Measure the split flow this must be the first step to diagnose the problem.

[Peter Apps]

In addition to a splitter malfunction, uniform peak heights for different quantities of analyte can also be caused by detector saturation, the signal being above the range of the electronics, or by column overloading.

Do the areas of the peaks change while their heights remain the same ?

What quantity of analyte are you putting onto the column at the different split ratios ?, what is the analyte, and what is the stationary phase in the column ?

What happens if you change the detector range or signal attenuation settings ?

Peter