Hi

could you give me informaton about normaly
variation in retention time in Gas cromatography.

give me values, number of inyection and conditions please.

Thanks.

Is very easy. I hope many answers

There are two different components of variability in GC retention times: 1.) a "fixed" or finite variability generally caused by the variation in the exact moment of injection vs when the data system is started; 2.) a "proportional" variation in retention time, caused by variations in instrument control such as temperature fluctuations, carrier flow changes and the like, that tend to cause variations in retention time that are proportional to the retention time of each peak.

For early eluting peaks, the "finite" source of variation predominates. If you are carefull to inject reproducibly and start the measurement of retention time reproducibly, this variation should be no more than a few seconds. I would consider a 6 second variation a lot.

For later eluting peaks, I would get suspicious of some mechnical problem with the GC if the variation in retention time exceeds 1 or 2% of the retention time. Most likely a leak, but could also be caused by poor temperature control or a bad carrier gas regulator/flow controller.

I looked at this in some detail a few years ago. I had a HP5880 hooked up to a P-E automatic thermal desorber. The "proportional" variation in r.t. mentioned above I tried to estimate independently of other factors. I had a 50m capillary, fixed sample size, slow program, mild temperatures and no solvent of course ie. close to ideal conditions. The short-term proportional variation due to pressure/temperature/clock errors was vanishingly small and could not have exceeded 0.02 secs over a 20min run (0.002%). Next the random contribution of the injection was +/- 0.06 secs s.d. (0.005%), but this could be more if a dilution solvent is involved. Then comes the much larger variation due to sample size, say +/- 1 second if you stay within the sample capacity of your column, much more if you don't. Finally the long-term drift over days - anyone's guess, say +/- 2 seconds, which is how I end up normally using no less than a 0.04 min r.t. window (0.2%). If you want a manufacturer to specify all this, don't blame them if they say "it depends". Incidentally I got all these figures on equipment manufactured in 1981.

It depends is the only real answer. There are a large number of variables, not the least of which is the skill and knowlege level of the operator. Split injections will tend to be more reproducible than splitless, especially as the septum is punctured and begins to leak. The septum does not reseal immediately when the syringe is withdrawn and there is a small variation in head pressure in splitless mode. A high split masks that effect. Modern electronic flow control systems with atmospheric pressure compensation are very good at maintaining a consistent flow rate from sample to sample.

Some hard numbers from some testing I did to evaluate the Merlin microseal for our system.

725 injections
6 component light aromatic hydrocarbon sample
1 uL injection using autosampler
15 sec injection port dwell time
100:1 split ratio
20m x 0.18mm column
3.5 minute GC run time
Retention time RSDs were all less than 0.25%, or 0.30 second. Maximum retention time deviation from the mean for a compound was 1.5 sec.

Interestingly, I got very similar results from a conventional septum over a series of 400 injections. I had to stop for another project, but one of the major factors affecting reproducability appears to be the sealing of the injection port, especially the septum, during injection.

I will add a disclaimer here. I have done this for a long time and have had a lot of experience on various GCs. Someone could come in and set this system up differently and obtain different results, either better or worse.

My problem is, that there is a loss of resolution of later eluting peaks. The earlier eluting peaks are quite normal. Solution of this problem?

I want to know, how the retentiont time can help me to know what kind of compounds i have in my samples if i am using a FID detector.

i have online gc that analyzes light naphta,
recently i noticed that every time i start the analzer after long period of shutdown , the retention times are very late.and i had to increase the carrier pressure.????would u tell me a remedy??