Purge and Trap attached to GC/FID

Discussions about sample preparation: extraction, cleanup, derivatization, etc.

15 posts Page 1 of 1
Hello all!

I have been working on getting this system up and running to capture GRO (Fraction 1). I have ran some standards and am getting a good calibration curve but I cannot get a retention time marker to pass. The marker is a purchased solution of 2000 ug/mL diluted to 50ug/mL and it contains 3 compounds n-hexane, toluene, and n-decane. In order for this standard to pass hexane and decane must have area counts within 30% of toluene and right now I am getting low recoveries for decane and peak tailing. Is there anyone that can give me an idea of what to do?

What I have done:
Changed column
increased dector temp by 50 degrees
increased inlet temp by 50 degrees
replaced ferrules
tightend purge and trap vessels
Matt

Mr. Barteau,

What column, what oven conditions, what flow rates? EPC or manual pneumatics?

Best regards.

Column - DB1, 30 meter, 0.53mm Mega bore, 1.00 film thickness

Oven conditions = initial temp of 45 degrees, held for 2 mins. then a ramp of 5 degrees a min until 150 degrees. then a ramp of 15 degrees a min to 240 degrees.

System equipped with Manual pneumatics

Flow rates:
Column Head pressure @ 20ml/ min
flow from concentrator vent 40 ml/min
total flow = through column 300ml/min
Matt
GC is attached to a Tekmar 3000 series concentrator as well and these are its settings

use "J" trap

gc start DeStart
cryrofocuser off
gc cycle time

cryo stdby 100
cryro focus temp -150
inject time 0.75
cryo inj temp 180

desorb preheat 220
desorb time 6.00
desorb temp 225
sample drain on

bake time 10
bake temp 225
bgb on delay 0
mcs bake temp 300

line temp 150
valve temp 150
20xx line 150
20xx valve 150

mcs line temp 40
purge redy temp 39
purge temp 0
turbo cool temp -20

sample heater off
prepurge time 3.00
preheat time 5.00
sample temp 40

purge time 11
dry purge time 4.00
mcs des temp 50


Also ran a few Retention time markers and then reran the same vessel to see if anything had been left behind and it seems that all of the decane in the solution isnt comming out where as all of the hexane and toluene are. Could this be due to a short purge time???
Matt

Mr. Barteau,

There are two things I would focus on to begin with. The first is what appears to be too low a desorb temperature for that trap. Having looked at a Supelco app note, I would try something like 250 desorb and 260 bake.

The second thing, and the one I really think is the problem, is the flow rate. First, you say you have a total flow of 300 mL/min. If that is column plus split flow you have two problems. One, you are desorbing with 300 when it switches (if this is the traditional carrier to split inlet configuration) and the trap will not like such a high flow. Second, you are very likely starving the column for flow at the upper reaches of temperature which often causes the tailing of the heavies you are seeing. I would suggest cutting the split flow down to something on the order of 40 mL/min. However, since you are using a megabore, a direct connection is possible in which case I need you to expound on your stated flow rates.

Best regards.

Hello

Thank you for your help! I have been on vacation for the previous week and am just getting back to this.

I tried increasing the temperatures as you suggested but with similar results

I have tried cleaning the sample pathway and that also does not seem to have had any effect on the situation.

When you asked me to expound on the flow rates that I am getting, what ones do you need?

Right now there is 40ml/min comming out of the concentrator vent, and the hydrogen is set to 40 psi
helium to 80 psi
and air at 80 psi

The total flow though the column is approx 300ml/min

what flows should I be concerned with?

Also the column is directly attached to the inlet

Thanks again for all of your help!
Matt

First of all how is the P&T and GC connected? Have you cut the helium line going to the injection port and installed the P&T in series? This id how I have my 6890/ Stratum installed.
Secondly the 300 ml/min seems way too high to me. Even if the 300 is total flow to the injector still seems high. For the column you are using I would think a flow of 8-10 ml/min would be good.
Can you inject an unretained compound directly into the injection port and see how much time is needed to get to the detector? For your column to get 8 ml/min would be roughly 53cm/sec, and 64 cm/sec for 10 ml/min.

Mr. Barteau,

Sorry to beat a dead horse. When you say column flow is 300 my eyes bug out. However, I suspect that you are measuring column flow with detector gases so I am not too panicked (yet.) Otherwise you would not get a GRO chromatogram so much as a total hydrocarbons chromatogram. If you turn off the detector gases (assuming an FID here) what is the flow out the FID exhaust? Then, what is the measured flow out the split port if you are using one?

The second thing, upon review, I am intrigued by is the cryofocus temp. I would try making the standby temp 175 for a couple of runs to see what impact that has on your chromatography.

Best regards.

Hey! Thank you for the suggestions. I was measuring the flow of all gasses and you were correct to assume I am using an FID. To do the measurement, I cooled off the detector and measured the rate of just the carrier gas. This gave me a flow of 46.2 ml/min going through the column.
I believe that this is a splitless injection port and it also bypasses the septum.
I will try a few runs with the cryro focus higher as you suggested and i'll post the results.

Thanks for all your help!
Matt
Matt

That's still a ton of flow for that column. Do you use make up flow at the detector? If yes and that flow is 30 ml/min that still leaves 16 ml/min which I feel is too fast.

Hey!

I have adjusted the flow rate to 7.45ml/min in the column and the makeup gas flow is set to 29.9ml/min and I am passing my criteria, barely but it is passing!! Thank you for all of your help!
The only problem that still remains is my compounds are tailing do you have any insight on how to make this stop?

I have also noted that heating my solution a little over room temperature allows me to recover more decane.

Thanks again

Matt
Matt

When is the last time you changed liner, septa, gold seal, and split vent trap? Did you measure how far the column extends past the ferrule on the inlet side? How far?

Changed the liner last week. The column extends up into the inlet 2mm. The septum doesnt get punctured but I changed that within the last week because I did some direct injections into the column. The splitvent trap is another story, I did think that this particular instrument had one as there are no split lines. simply one line comming from the trap to the inlet of the GC. If there is one I havent no record of it ever being changed.
Matt

Mr. Barteau,

Can you post a picture of your inlet and how it is connected to your purge and trap? I repsectfully disagree with Aldehyde, a mega-bore column can handle 40 mL's/min since they were designed to replace packed columns in applications just like this which suggests to me that something else may still be causing the problem.

Best regards.

2mm insertion depth is way off. The best is 5 mm from the sealing end ( narrow) of the ferrule.
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