Chromatography Forum

So, I've created a decent generic method to scan through most of my company's raw materials. However, we order denatured alcohol in two flavors: methanol free and the other contains methanol. They want to make sure the methanol free stuff really has no methanol in it.

My current method did not resolve the peaks between methanol and ethanol, so I lowered initial temperature, lowered the temp. ramp a few minutes and lowered the column flow. After this point, though, there's no need for such extreme separation and I'd like to speed up the method. However, I'm unsure about what affects changing the flow would have on peak area. From what I understand, the peaks are determined as the concentration of vapor-phase product in the carrier gas.

So, I'm torn; the flow over the detector is constant, so the concentration here should be unaffected. However, the flow rate of the column changes, so... yea, I am confused. Please, shed some light.

Thanks!

So long as you run your standards (calibrators) and your samples under identical conditions, it should not matter.

There are two kinds of detectors - Mass sensitive detectors and concentration sensitive detectors. I am assuming that you are using FID, which is a mass sensitive detector. In this type, the shape of the peak will change if you change your flow rates, but the area should ideally remain the same. Also, you have to remember that if you have different flow rates for two different runs, there may be slight differences in peak area depending on how much of the analyte enters the head of the column depending on the injector settings (mainly split ratio and liner dimensions).

For example, if you double the flow rate just before a peak elutes, the peak width will become half, but the peak area will remain the same.
Irrespective of what kind of detector you are using, you need to run your standards under the same conditions as Tom suggests.

Cheers,
Suresh.