baseline goes up and comes down within 15-20min of sample

[rajdeeproy]

Dear all

Last time i had a problem and i did something it worked well but i feel only for few days and peter and bruce gave me good suggestion.
The base line now goes up from 15 min to 20min and then goes down and again i see some broad peaks at the end of the run.

please suggest some thing , my previous post named "base line wandering during temperature programme has every thing in details of the method.

i will try to post some latest image

[rajdeeproy]

here is the link for the chromatogram

http://i9.tinypic.com/2zf678j.jpg

[Bruce Hamilton]

Apologies for the delayed response. It still looks like at least one cause is very late-eluting rubbish from samples, and there could be more ( leak or thermal effect). Assuming no leaks.

Try:-
1. Long condition at near max isothermal temp of column, watch to see if baseline gets smoother. Inject a sample, and run two to three blanks after it, or put long 120min final temp hold. If no peaks, then
2. Cool and remove and reinstall column, taking care to ensure ends are clean and fully inseted into detector, and that none of the column is touching the walls of the oven. If still problem, then
3. Check all gas flows to detector, esp air and H2 to ensure they are consistent and correct. Ensure the detector body is well seated and grounded, and there isn't a lot of silica on the collector.

You can try all of the above at once , but I'd focus on where the souce of any baseline pyramids or mounds could be, and samples are the most likely, either much earlier ones or muck in current ones. The much earlier ones could have contaminated your injector, so you could clean that as well.

Baseline plateaus are more associated with detector sources, or strange events like columns touching walls, Be systematic, and focus on identifying each problem. I'd start with eliminating the samples and blanks, as the plateaus may just be from muck from old samples/injector.

Good luck,

Bruce Hamilton

[rajdeeproy]

Dear Bruce
thank you for those useful suggestion. I would start working on it and update u soon.
i have seen some part of the column touching the base of the oven. I will start trouble shooting with this first.

best regard
rajdeep

[rajdeeproy]

Dear Bruce and Peter

When i keep the column for conditioning after analysis at night I see few negative peaks in the base line. Is there any problem with the detector?


please send me ur suggestion

[Peter Apps]

The last chromatogram that you posted shows some ugly baseline deviations, but they do not interfere with the peaks that you are interested in, and so thay are not really a problem.

An ECD is selective, not specific. Your samples contain a large variety of components that the ECD responds to very weakly, these responses can be positive or negative. Since they are due to large quanties of substance they can overload the column, which is why the baseline wanders instead of showing small peaks.

If you ran the same samples with a non-selctive detector such as an FID you would see huge peaks where you see baseline deviations on the ECD.

As long as you have you rpeaks of interest on a reasonably flat basline and you can get the lilimits od detection and quantitation that you need, do not worry about the baseline later on in the chromatogram.

Peter

[JI2002]

It looks like that the sample preparation method is not compatible with the analysis method.

When you extract seawater samples with pentane by liquid-liquid extraction, you have all kinds of chemicals in the pentane extract---VOCs including the two target compounds chloroform and carbon tetrachloride, semi VOCs, some of the chemicals are aromatic hydrocarbons, some are aliphatic hydrocarcons, C36 for example. DB624 is for VOCs with maximum of oven temp of 260C. In order to elute all the compounds out of the column, you need a final oven temp of about 300C. There are two options:

A) Use a column with maximum oven temp of about 350C (DB5 maybe?). Set the inj temp to 250C, oven final temp of 300C and DET temp of 310C. You can ramp up your oven temp at 40C/min after the two target compounds come out.
B) Use SPME or possibly purge and Trap system as your sample preparation method, you can still use the same column and oven temp program for the separation.

This is irrelevent to your problem: Unless you are using FID also, hydrocarbon trap is not necessary for GC-ECD, but oxygen trap is very important to protect ECD.

[Bruce Hamilton]

Oops, apologies, I should have checked the chromatogram more carefully.

As it's an ECD, I'd go with the above suggestion, either cleanup your sample or change to a column that can be cleaned faster. Changing the column could be best option, as your overnight experience indicates that the other peaks are removed.

I'm not a fan of co-elution of poorly-detected peaks, as the possibility ( Murphy's law ) exists they will compromise your analysis at the most inconvenient time. A longer run time near the max temp may also bring an awful lot of peace of mind, even if the baseline isn't perfect.

Good luck,

Bruce Hamilton

[rajdeeproy]

Dear Bruce

The system which i am using is compatibale with megabore column (Varian CP 3800) and packed column which is connected to gas sampling valve but this GSC is only for FID.

The system configurtion is so typical that i dont have a capillary column option. I am not sure that i can get any connectors which can convert megabore unit to capillary.

Now i have raised the oven ramp to 200 degree and conditiong overnight last day The problem is minimised a lot. I have done at 10 injection and i havent got those broad pekas. The run time is now 62 mihn and 200 degree it is kept for 25 min.

I will post few chromatogram.

best regards
rajdeep

[rajdeeproy]

Here are the latest chromatogram


http://i9.tinypic.com/2zrmdlg.jpg

http://i9.tinypic.com/2r46cs1.jpg

http://i10.tinypic.com/4d2xch1.jpg

[Bruce Hamilton]

OK, if you can remove the noisy peaks by conditioning, then I'd go with that option, and not change the column or instrument settings too much.

You might like to try and increase the head pressure to speed up the analysis, and perhaps also determine the optimum final temperature time.

Companies like SGE sell connectors that allow you to combine capillary and megaore columns. However, if you now have the resolution with your existing column, I'd stick with that. Going to different column dimensions can also require changes to injector and detector settings.

I'd just take the minimum energy path to get the data you require, and only make major changes if you can't obtain clean chromatograms.

Bruce Hamilton