I am primarily an HPLC person. I have worked with them for 10 years. But, I have not done that much with GC. I have been asked to see if I can do anything with the current GC methods used on the truck samples. They would like to speed up the run time. Initially I thought a shorter column and a shorter ramp.
I have used the LC forum and have had great help there. I assumed that I could get the same great help here. I appriciate any assistance that you might give me.
By gary hayes on Wednesday, January 16, 2002 - 08:34 am:
You can only use a shorter column, of the same internal diameter, if you have resolution to spare.Otherwise you will need to use a shorter column with a smaller ID.(In the same way you can use a shorter LC column with smaller particle diameter packing). Also, hydrogen carrier gas can be used at higher linear gas velocities without an excessive loss of efficiency.
By Anonymous on Wednesday, January 16, 2002 - 09:28 am:
Thanks Gary. I will try that.
By Anonymous on Wednesday, January 16, 2002 - 12:47 pm:
What Gary says is correct as far as it goes, but we might be able to help more if you provided more details. Gary addressed the question of reducing run time without changing the method, just the column dimensions, which is generally the easiest approach. There are other approaches that can work in certain cases.
Gary apparently assumed you were using helium carrier gas. In this case there would be an improvement by switching to hydrogen. If you're using nitrogen, then helium will be a tremendous improvement and may allow you to increase flow with little loss in resolution.
Think twice before using hydrogen carrier gas, simply because it's explosive. Make sure you're not going to be putting yourself or others in danger.
By Leon on Thursday, January 17, 2002 - 01:04 pm:
Your options depend on whether you need to speed-up an existing method or to develop a new one. In the former case, you can make the following changes that WILL NOT CHANGE THE PEAK ELUTION PATTERN. (In the following text, “high pressure drop” means that the absolute input pressure is more than two times higher than the absolute outlet pressure.)
1. If you can sacrifice the resolution then “cut the column”. The resolution of all peak pairs will change in proportion with the square root of the column length. Depending on the column pressure, the analysis time will change in inverse proportion with the column length (low pressure drop) or in inverse proportion with 3/2 power of the length.
2. If you can reduce the sample capacity of the column without the column overloading then reduce the column length, the internal diameter, and the film thickness in the same proportion. The sample capacity of a column will change in proportion with the cube of the internal diameter, the resolutions of all peak pairs will not change, the analysis time will change in proportion with the square of the internal diameter (low pressure drop) or with the internal diameter (high pressure drop).
3. Make sure that you use the speed-optimized flow rate (SOF) of the carrier gas. (SOF gives the best resolution-speed tradeoff.) For hydrogen, choose SOF as SOF=10*id*mL/min where id is internal diameter (in millimeters) of the column. (For 0.1mm column, SOF of hydrogen is 1mL/min). The helium and the nitrogen SOF are, respectively, 20% and 75% lower than the hydrogen SOF.
4. You can change the carrier gas. The maximum available resolutions of all peak pairs do not depend on the carrier gas. At the SOF for each gas, the analysis times of hydrogen/helium/nitrogen relate as 1/1.25/4 at low pressure drop, and as 1/1.7/3 at high pressure drop.
5. You can combine all previous steps.
One thing is VERY IMPORTANT. For all the above promises to be valid,
NEVER MAKE ANY OF THE ABOVE MENTIONED CHANGES WITHOUT THE APPROPRIATE CHANGE (THE TRANSLATION) IN THE TEMPERATURE PROGRAM!
Use Agilent’s GC Method Translation software to find the translated temperature for all of the above listed changes. To my best knowledge (I do not work for Agilent), the GC Method Translation software can be downloaded free of charge from www.chem.agilent.com/cag/servsup/usersoft/main.html#mxlator.
If you have to develop a new method, your options are hard to enumerate. However, it is beneficial to always start as follows.
1. Use the above described SOF
2. Measure void time, tm, at 100 degree C
3. Start with the following default temperature program (if you need one at all). 1.5*tm at initial temperature (that should preferably be as high as possible, but not too high for resolving the first peaks) then ramp at 10 degree C per void time till final temperature (when, hopefully, all peaks elute). For example, if tm=2min then the default temperature program is: 3 min at initial temperature then 5 degree C per minute till final temperature.
1. L. M. Blumberg, "Theory of Fast Capillary Gas Chromatography. Part 2: Speed of Analysis". J. High Resolut. Chromatogr. 1997, 20, 679-87.
2. L. M. Blumberg, "Theory of Fast Capillary Gas Chromatography. Part 3: Column Performance vs. Gas Flow Rate". J. High Resolut. Chromatogr. 1999, 22, 403-13.
3. L. M. Blumberg and M. S. Klee "Optimal Heating Rate in Gas Chromatography". J. Micro. Sep. 2000, 12, 508-14.
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