I have an HP 6890/5973 GC/MSD, and I inject SPExtracts of biological fluids for analysis of drugs (both prescription and illicit). My column is 30m, 0.25µ, 0.25mm, HP-5MS, flow 1mL/min. I notice that most of the drugs get BETTER response when split 1:5 than when run splitless (ex--amphetamine, 4.4 min retention time, run splitless shows mostly adduct with a small amphetamine peak. When run 1:5 split, it shows large parent and small adduct; trazodone, with retention time of 18 min, becomes ~10x larger when run split).
Does anyone have any ideas about why this happens? I can't explain it and my supervisor wants an explaination!
By Ron on Tuesday, November 14, 2000 - 10:22 am:
What time is the purge valve set to come on? The 6890 default to a 0.0 minute purge on time when performing splitless injections, thus when you chose splitless you actually inject in split mode unless you change the purge valve on time. As long as you purge valve on time is set to 0.5 min or longer you should get better sensitivity in split mode.
By Lisa on Tuesday, November 14, 2000 - 11:33 am:
When running splitless, the purge was set to 50 mL/min to start at 1.0 min. The majority of peaks in my test mixture do, in fact, get smaller when I go to split mode--except the early eluters and the late eluters. And those that do get smaller are not 5X smaller as it would seem they should be when I split 1:5. They're reduced to ~70% of the original size. I don't think that I am overloading the column in splitless--but I could be.
By H W Mueller on Wednesday, November 15, 2000 - 11:57 pm:
Why work with split mode when you don´t have to? Split mode is not a robust method, thats why many people have gone through great length to develop temperature controled injectors (cold injection). Besides all the usual problems, some of which have been discussed here, we had a severe discrimination of fatty acid methyl esters in split mode (J Chrom, 228, 75 (1982)) which increased steadily parallel to increasing deposits, in the injector, of co-extractants (triglycerides, phospholipids) out of plasma/serum.
By Jason Ellis on Thursday, November 16, 2000 - 09:38 am:
I believe that the issue is related to residence time in the injector. In split mode you have greatly reduced the residence time in the injector for your analytes. This reduction in residence time can improve response (sometimes dramatically) for thermally labile or active analytes. In other words, they have much less time in the inlet to "get into trouble" (so to speak). You may see an improvement in your splitless method by using a shorter purge activation time as well. I'd try using a 0.25 or 0.50 min purge time to see what that does.
Another thing to consider is that split ratios should never be considered linear. In other words, if you double a split ratio you shouldn't expect half the response. The split that you effectively see depends upon analyte volatility and behavior in the inlet much more than the "theoretical" split ratio itself.
Active analytes typically perform better in splitless injections when single or double taper liners are used. These tapers "shield" the analytes from interaction with cold spots and active sites at the top and bottom of the inlet.
How much sample are you injecting? If you are backflashing in the inlet then that certainly could be causing these problems. Those problems would be a lot worse in splitless mode than split.
By lisa on Thursday, November 16, 2000 - 10:48 am:
What you say about residence time in the inlet makes a lot of sense. We inject 1µL samples (solvent=ethyl acetate or methylene chloride), and currently we are using a straight, non-tapered inlet. I am going to hunt around and try to find a tapered one, and also try changing the purge time.
If I am injecting pretty dirty samples, would it be better to go back to splitless, or stay in split (assuming that sensitivity isn't a major problem in split)?
By Jason Ellis on Monday, November 20, 2000 - 07:11 am:
If sensitivity isn't an issue then I'd stay with split injections. This will introduce less contaminant material to the head of the column, however it obviously will not eliminate the need for routine front-end maintenance on the system.
By lisa on Monday, November 20, 2000 - 10:45 am:
Thank you for replying! I get the required sensitivity, and I really wasn't looking forward to re-working up methods that are currently adequate :)!
By beppe on Friday, November 24, 2000 - 06:39 am:
Splitless mode is very powerfull, but is a little bit tricky to use; do not forget there are two possible modes : "cold trapping" or "solvent effect".
For cold trapping, the analytes ebulition point should be at least 150°C greater than the solvent one. The injector temp should be higher than the highest analyte ebulition point and the oven temp during injection and purge activation time should be somewhere between solvent and analytes EP, so that analytes are trapped in the column; elution will really start with oven gradient.
If you don't have these 150°C between solvent and analytes, use sovent effect : you have to set your oven temp ~10°C below your solvent EB so that part of the solvent condense in the column, making a liquid trap for the analytes; here again oven gradient will start the elution (it may be useful to have a short speep ramp to volatilize solvent before the "true" gradient ramp).
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