Chromatography Forum

I have Varian GC CP-3800 with column temperature set to 40-100 degree C with 20 degree C/min rate, run time 5 min. The retention time for dichloromethane (DCM) have 3 peaks (1.26,1.34 and 1.98). Why are there so many peaks? I used HPLC grade DCM and a clean GC vial. I also cleaned the injector port, glass insert and replace the injector septa. Why is DCM eluting so fast, should I decrease the temperature? The peak at 1.98 is quite significant at 20 mVolt. Any suggestion?

Hla

My suggestion is that you tell us the column length, diameter and film thickness and the carrier gas linear flow rate. Why do you say that the DCM is eluting fast ?. The temperature programme is very unusual, what are you trying to analyse ?

Peter

Methylene chloride elutes very early on most standard columns. That rate of heating is generally employed when you're trying to analyze very heavy materials.

Peter is absolutely correct. The participants on this board are very willing to help but you will get much better advice if you can provide as accurate a description of your problem as possible.

Thanks Peter and rb6anjo. I will get more detail on Monday and I will post the chromatograph as well.

Hla

We are trying to analyze nucleophilic substitution reaction products 3-chloropentane and 3-bromopentane. We have Zebron GC column, ZB-5 30 mL x 0.32 mmID x 0.25 um df
Liquid Phase: 5% Pheny-95% Dimethylpolysiloxane. Carrier gas is helium, flow rate 1ml/min, injection rate 5 microliter/sec, column temperature 40-105 C, hold 2 min, rate 20 C/min, total run 5.25 min. I don't know why there are many peaks for DCM (1.245, 1.275, 1.949 are all big peaks > 20 mV), the products peaks are small 3-chloropentane (2.197), 3-bromopentane (3.0185). The DCM peak at 1.949 is strange, DCM should be 1 peak. I run a black solvent to rule out impurities from sample. My question is
1) The DCM peak is splitting due to fast or slow injection or eluting too fast?
2) Why is there a significant peak at 1.949 for DCM.
3) How do I slow down the elution time? Should I change the column temperature setting? Should I be running Isothermal?
I would like to attach an image.
Its seems I have to create url to insert image on here?

Here is the link to the chromatograph
https://onedrive.live.com/redir?resid=C ... ile%2cpptx

DCM is your solvent? what are your injection parameters and volume?

The solvent is DCM (HPLC grade), injection volume is 1 microliter, solvent plug size=1 microL, needle depth 70%, uptake speed 5 microL/sec, injection rate 5 microL/sec. The reason I asked why there is a drift in DCM peaks was that, the last time I run the sample last year winter 2014 the DCM was at 2.9717. Each quarter the retention time seems to drift, could it be due to leaky carrier gas? Last week I installed, a new column ZB-5, a new septa and cleaned out the injector port. The quart insert in the injector port have some stain on it that I couldn't get rid of, I am waiting for a replacement. We have split injector varian CP-1177. Autosampler 8200.

You do not have problem, your two analytes of interest are nicely seperated from one another and from interferences. So do not fix what is not broken.

Peter

And an impurity peak in your DCM. If you look up some typical retention times for the Zebron column you should be able to guess its identity, or spike a few lab solvents in there. The main peak might be split just due to the interpretation of the integration software, I wouldn't; worry about it. Re. the drifting times, you need to leak check the inlet and check all your gas flow against the method. If all this is fine then the previous long retention times could have been an FID jet blockage - have you changed it at all? The method sounds fine, you can't really slow it down much more, and you won't gain any extra resolution anyway.

I believe DCM has some low mol weight stabilizers. Cyclohexene or amylene are possible.

http://www.sigmaaldrich.com/chemistry/s ... stems.html

Thanks for all the suggestion. I have not drop off from this discussion yet. I've been working actively on the GC but my knowledge on GC is limited. I agreed the peak at 1.949 could be the DCM stabilizer. I used DCM from MACRON cat#4879-10. It said >99.9% pure, the 0.01% must be cyclohexane stabilizer. I am not sure about FID ject blockage. None of the FID unit or injector port have been cleaned or replace for the past 10 years. Should I replace the FID unit? How often one should clean or replace GC parts? Thanks.

The cyclohexene stabilizer works by reacting with generated HCl in the DCM. Products include chloro- and dichlorocyclohexane and chlorocyclohexanol. Once the bottle has been exposed to air and moisture the process starts.
If there are too many interfering peaks try the JTBaker DCM that has methanol/amylene as a stabilizer. We went to that for one of our analyses to get past an interference from dichlorocylcohexane that we were getting from our B&J.

In terms of maintenance I would think there are other threads on here that talk about it. FIDs require little maintenance, and to be honest are often best left alone if your performance is good. Keep a spare jet or two, and glowplug though. Sometimes (at least with Agilents) if someone fits the column tail to the FID, they can overtighten the nut and push graphite up into the jet. This is near-impossible to clean up, best to put a new jet in.

Thanks Johnny and All. Agreed, I won't mess around with FID, just clean the injector port. Now, we have a new issue the instrument won't equilibrate.