GC 6850 peak tailing

[SmitaKamal]

Column 30mx0.53ux1u
Cappillary column
Splitless inlet
Pressure 8.48
Temp 220C

Aerage velocity 179 cm/ sec

Carrier gas H2 flow 40
Makeup gas N2 35
Air 400

Injection volume1 ul.

Standard in water.
Standard: Ethylene oxide and Ethylene chlorohydrins. Diluted to get 100-1ppm

Concern: gauscian peaks for all concentrations but peak tailing is >2.0 and it should be <1.8. Interesting fact is peak for EO is > 2 and peak for ECH is <1.2. Both come out insame injection at ~ 0.407sec and 3.25 mins.

Installed new column. Baseline is perfect. Blank water samples noooooooo peaks. Changed inlet (900ul) septa.

Any suggestion to reduce peak tailing.

[Peter Apps]

A retention time of 0.4 seconds is impossible - you are probably looking at a pressure pulse caused by the water injection. Even if it is 0.4 min it is so close to the dead time of the column that the peak could be distorted by flow disruptions. You need more retention - what is your temperature programme, and how hot is the inlet ?

Peter

[dblux_]

...
Aerage velocity 179 cm/ sec
...

It's a hurricane.

[SmitaKamal]

Actually this is how it worked for 5 years and then the software crashed. We got Openlab CDS now and new autosampler tray so I am trying to revalidate it with the old method. Person who did initial validation is no longer working in the same company.

I ran pure EO mix in mehtanol. Got the peak around the same time. Also, boiling point of EO is around 58C.

Inlet temp; 220C


Oven programme
50C for 1min
Ramps at 25 temp 130C for 1 min and then at 50 temp at 200C for 0.50 min

[rb6banjo]

I agree with Peter. This is not a good method. You need to start at an oven temperature where you are reasonably sure your analytes condense on the column after injection. 8.5 psig at 50 °C gives about 163 cm/s and a void volume of about 0.3 minutes. For your column and carrier gas, the optimum linear velocity is about 40-50 cm/s (2.4 psig). For EO, if it's me, I drop the oven temperature to 40 °C as well (2.3 psig inlet pressure).

You are just blasting your analytes through the column. As dblux_ said, you're trying to do chromatography in a hurricane. I'm a little surprised that you get any separation at all with that flow rate.

By the way, Agilent has a great little "flow calculator" app for your smartphone in the iTunes store. You can plug in your column dimensions and the pressures to calculate the parameters you need for optimum conditions. Just search for Agilent and you'll find it.

[Peter Apps]

Try as a I might I cannot think of any way that a change of software and a new autosampler tray can cause peak tailing, so something else must have changed since the method last worked properly.

Can you confirm the retention time of EO (what you gave before really is impossible), and what is the retention time of the methanol solvent ?

If the rationale behind the very high flow rate is to get the analytes out quicker, you would achieve the same more elegantly by cutting the column in half.

Peter

[SmitaKamal]

I am not sure how I can attach chromatogram to this post?

[rb6banjo]

What they are saying is possible. I cranked up the pressure on one of my GC's and injected methane (30 m x 0.53 mm x 1.0 µm wax column, helium carrier gas). The flow calculator says the RT should be 0.604 minutes. Here's what I measure:



For H2, 8.5 psig on the same column should yield a time for unretained of about 0.3 minutes.

SimtaKamal, your flow is too high.

[Peter Apps]

What they are saying is possible. I cranked up the pressure on one of my GC's and injected methane (30 m x 0.53 mm x 1.0 µm wax column, helium carrier gas). The flow calculator says the RT should be 0.604 minutes. Here's what I measure:



For H2, 8.5 psig on the same column should yield a time for unretained of about 0.3 minutes.

SimtaKamal, your flow is too high.

OK, now show me a peak eluting in 0.4 sec ! which is what is in the OP.

Peter

[rb6banjo]

Got me there. I overlooked that and figured they meant 0.4 minutes.

[SmitaKamal]

blame it to my fat fingers....it is indeed 0.4MINUTES...


How can I add my printscreen to the post rb6banjo how did you do that.

I tried reducing inlet pressure to 2.3 and oven temp to 40C and then 45C but I am getting shoulder peaks.

[rb6banjo]

There are directions for doing this in the very first post to this forum (embedding chromatograms and reports). You need to subscribe to a photosharing site (http://www.photobucket.com is what I use) so that you can embed the link into your posts. It's pretty easy.

Could it be that your mixture is more complicated that you think and those "shoulders" are coeluting materials?

[SmitaKamal]

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Thanks a bunch Rb6 banjo. Here is the chromatogram.

[rb6banjo]

You are welcome. How is the peaks shape on something that's not active but is retained? Something like butane or pentane? I'm wondering if you might have some activity in your injector. If you have access to a cheap plastic cigarette lighter, you can shoot the vapor from that and see the peak shapes for the types of molecules I mentioned.

[James_Ball]

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Thanks a bunch Rb6 banjo. Here is the chromatogram.

I am wondering if the peak at 2.4 minutes is actually Ethylene Oxide.

By the way, what is the stationary phase on your column?