Chromatography Forum

Hello all.

I currently have a method that lasts 12 mins. This 12 min time picks up all compounds i observe.

However i only care about compounds up to 7.5mins and i am trying to shorten the method. I need to run a blank inbetween samples, so i need to basically flush everything after 7.5 mins before it carries over (so flush everything else before the blank is injected.

I want to keep the parameters for this short method exactly the same as the 12 min method (up to 7.5mins) as we Retention time lock standards. I will still be using the 12 min method along side this short method.


What i have tried so far.


After 7.5 mins. Ramping up the temperature for 1 min (this didnt get rid of anything that elutes 7.5 - 12 mins)
I also tried increasing the flow after 7.5 to maybe flush them all out quickly, but this didnt work either.

I dont want to degrade the column to much but cant think of another way to do this.

I hope this make sense! and any advise appreciated.

PS Im using an agilent 6890N oven with 5973 MSD and chemstation.

thanks

The traditional way to do this is to put a back flush column and valve in your system.

This can also be done with a deans switch.

Good luck,

Rod

????????
What you did should work. What do you see in the blank between samples? Do a no injection run ( take the syringe out of the autosampler). What do you see?
How did you ramp the temp after 7.5 min and for how long?
Is the "crud" comming from the column or is your injection port dirty?

Try something like this:
@ 7.6 min do a 70C/min ramp to the end temp used in the past and hold for 0.5 - 1 min.

thanks both for your prompt replies.

At 7.4 mins i want it to be at 300 degrees
I tried ramping it ,at 40C per min for 1 min until 8.5 min (only up until 325 degrees, im thinking this is not hot/long enough. Our coloums can take up to 350 degrees but im worried about degrading the column, its quite a harsh gradient as it is.

I see in the blank the carry over peak that has been cut off as expected

I will try again at 70 degrees/min and hold for slightly longer. and see what happenes thanks

If your existing programme gets to 300C at 7.4 min, and your column maximum is 350C then all that you can do with temperature is to heat at maximum rate from 300 to 350C and hold until everything elutes. I doubt that you will get to where you want to be.

What increases in flow rate did you try ?

You might need to try a shorter or thinner film column, run at a lower temperature (use the Agilent method converter to get the retention times the same) so that the extra bake out has more effect on the late-eluting junk.

Peter

It sounds like you are trying to get rid of some peaks that show before 7.4 min - and they show in the blank? If this is the case, the compunds left the column in the GC run - you saw the peak. How did they get back on the column to be seen in the blank? Several possibilities (mentioned above):

1) contamination of the solvent injected as the blank, perhaps even by the syringe in use. Try pushing the run button on the GC as if you were making a manual injection and see what shows up.

2) There is carryover in the inlet. This would be because of inlet contamination from backflash (too large an injection), too cool an inlet (with compunds slowly migrating from the liner, even after a run is completed), or someother reason. I am assuming liquid injections. Other injection systems have their own peculiarities. Clean the inlet thoroughly and check to be sure that you are not overloading the inlet.

3) You are looking at peaks of column degredation. Once this happens, the only solution is a new column AND an oxygen scrubber at the back of the instrument. Once teh stationary phase begins to degrade, conditioning the column never fixes it. The polymer has begun to unzip - and heating only accelerates the process.

If I got it wrong and you are looking to get rid of long rolling peaks that do not show in the run where you make the injection, but show up in the next run - these are compunds that remained on the column and take a lot of time and/or temperature to elute. Backflush (mentioned above) is a great idea -- or examine the possiblity of using as cool an inlet temperature as possible and a plug of glass wool in the liner. If you can keep the high boiling garbage in the liner, you do not have to figure out how to get it off the column. In the second approach, you need to be sure that the inlet is warm enough to transfer all your analyte, even after several injections. And you have to be sure to change the liner with sufficient frequency.

The benefits of using backflush are other than adding a small amount of dead space in your chromatography system, is the ability to perform the analysis quickly isothermally, repeatedly, without using temperature changes which require time to reequilibrate the oven, flexibility to adjust the time of analysis by eliminating the later eluting peaks or NOT, and to keep the same retention time and separation of the desired analytes.

If you do use a valve rather than a Deans switch, try to keep the valve at the same temperature for a longer operating lifetime of the valve.

best wishes,

Rod

Following Rod's lead....

If you use a valve and you install it in the column oven, be sure and note the upper temperature limit of the valve rotor, which likely is lower than the column limit. Ignore this and you get a ruined rotor. Been there, have the T-shirt.

And the t-shirt is NOT a pretty one.

Rod