Gas Chromatography Detector

[qca2003]

I would like to know some opinions on the following question :
1. When We should be used the following detector and what is the difference ?
- FID = Flame Ionization Detector
- TCD = Thermal Conductivity Detector
- FPD = Flame Photometric Detector
- FTD = Flame Thermionic Detector
- ECD = Electron Capture Detector

2. How to select the suitable carrier gas for Gas Chromatography ?

3. What is the difference between Correctected area method, absolute method and internal standard methode for gas chromatography ?

Best regards.

 

[Baldor]

I have some experience with FIDs, TCDs, and gas chromatographs. Also photo ionization detectors(PIDs). Most of them were designed for the natural gas industry.
The FIDs were mainly used in open environments for leak detection. They burned, within a closed cell, a mix of hydrogen and some inert gas, with a sample pump to bring in outside air through a probe/hose. Any hydrocarbons present in the sample would ionize as they burned, giving you an indication. They were fairly sensitive(accurate in the 10 PPM range), but would easily get contaminated with petroleum distillates.
PIDs worked in a similar way, only the ionization of the sample would occur with ultraviolet light. Many of these models were intrinsically safe, allowing use in a confined environment. The disadvantage is that they had to be calibrated for the specific hydrocarbon you're looking for.

TCDs had a certain resistance, which changed according to whatever gas moved over it. I remember thinking (it's been 10 years) that they weren't too accurate, but no ionization occured so the sample remained unaltered.

The chromatographs I used were designed specifically for checking the composition and concentration of odorants in natural gas lines. They worked by forcing the gas through a permiable column, with the detector at the other end. The size of the molecule determined how long it took to pass over the detector. We knew the approx. time it took for methane to pass, thereby excluding it from our result "window". I'd suggest to you to select a carrier gas that won't overlap any of the molecules you want to monitor.