Chromatography Forum

Hello all, I'm new to the forum and hope this will become a place of great knowledge for me. I'm having a problem with the Agilent 7890A I work with. I'm running a DB-Wax 30m x 0.320mm capillary column, helium as the carrier gas with an FID. Problem I'm having is that multiple injections of the same vial from the autosampler are getting vastly different values. Currently I'm using it to do determination of Ethylene Chlorohydrin in water samples after sterilization. My company has been doing this test on an old 6890 for years before it died and I convinced them to buy the new instrument. It just doesn't make any sense to me, how doing multiple injections from the same vial can yield such varying results. Does anyone have any insight or do you need more information. I understand chromatography theory but I am not a GC expert.

I had some trouble with reproducibility when my 7890B was installed last summer. I mostly use SPME for my sample analyses. The root of it was in the septum purge. There are 2 settings. Are you running with the "standard" or the other one? "Standard" is a constant flow like what was always done in the past. When I entered my method, I didn't pay close attention to that part and I was running in that "non-standard" mode for a while at first. The irreproducibility was maddening! Works well now!!

Nope I'm running "Standard" Septum Purge

..................... do you need more information.

Welcome to the forum.

We need to know everything about your method - the sample concentration, what is it dissolved in, injection volume, inlet temperature, flow rates and split/splitless conditions, temperature programme, detector temperature and gas flows.

Peter

What type of inlet? Split/splitless, MMI, or ?

Is correct liner installed, check that quartz wool, if used, is fully inside liner.

So at the start of every run, we run a calibration curve of three standards, 2 injections on each. This lot of standards is 26.02 ug/mL, 130.1 ug/mL, and 520.4ug/mL of Ethylene Chlorohydrin. It's a liquid dissolved in DI Water. Injection volume is 1 uL. Temperatures are 150C for inlet, detector is 250C, oven is 90C for 2 minutes and increasing 25C/min to 180C then hold for 1 minute. Flow is 10 mL/min helium, 30 mL/min hydrogen. Splitless.

So after the calibration curve, I have an RSD of 5% i need to hit which is fine, I run 10 samples and then we run a drift check. This is where is becomes weird because the drift check is the same stock as the middle calibration standard, 130.1 ug/mL, pulled from the same stock. Yet it will come out significantly higher than the cal standard, along with having the 2 injections varying greatly, areas of 450 and 495. My drift check has to fall within 5% of the calibration standard. I run a blank water sample between my samples and my standard to make sure there is no carryover. I'm at a loss of what to do.

Edit: Yes the quartz wool is installed correctly.

I'm assuming you're using a Split/Splitless injector, correct?

What's your inlet pressure? Water is a difficult solvent to use in GC as its vapour volume is massive in comparison to other solvents. Works but you have to take care.

Download Agilent's Vapour Volume Calculator and input your GC parameters and type of liner. See if your vapour volume exceeds the liner volume. If so, that's likely where you're getting some if not all of your variation.

If that's all good, check the column insertion depth (critical) and your syringe (actuate the plunger manually to see whether it's consistently drawing liquid properly).

Can you post a chromatogram? What phase thickness is your Wax column? Are you certain the column is not overloaded? If not, try a split injection and see if things improve.

I know, a lot of questions, but the more info you give us the easier it will be to pinpoint what's going on.

1 ul of water into an inlet at 150C gives a huge vapour cloud with a pressure pulse that pushes sample vapour in all directions.

"Flow is 10 mL/min helium, 30 mL/min hydrogen. Splitless." 10 ml/min down a 30m 0.32 mm column ?! What is the inlet pressure ? I presume the hydrogen is going to the detector, if not I'm really puzzled.

With the column at 90C a 1ul plug of water might condense to liquid, and it will do so erratically. If you can, increase the starting temperature - what else is in the sample that has to be separated from the chlorohydrin ?

The middle standard puts 130 ng of chlorohydrin onto the column. You could reduce injection volume, or run split 10:1 and still have decent sized peaks.

You might also be getting progressive saturation of active sites as you run samples - allowing more of the final standard to get to the detector.

Peter

I agree with the above statements. If you are injecting water you would be best served to inject no more than 0.5 ul into an injector at about 110 to 120C. Use a 4 mm liner as the internal volume is 900 ul. A pulsed injection may help. I have been told ( by Agilent) not to take more than 2 injections / vial as a partial vacuum causes inconsistent syringe filling.

So I ran the Vapor Volume Calculator and it said it was fine, I tried posting a picture but I couldn't figure out how to without posting it to a image hosting website which is not allowed at work. So the specifics, using water, injection liner 19251-60540, 1 uL injection volume, inlet temperature of 150C, inlet pressure of 35.202 psi, gave me an estimated volume of 566uL at 57% capacity.

So I ran the Vapor Volume Calculator and it said it was fine, I tried posting a picture but I couldn't figure out how to without posting it to a image hosting website which is not allowed at work. So the specifics, using water, injection liner 19251-60540, 1 uL injection volume, inlet temperature of 150C, inlet pressure of 35.202 psi, gave me an estimated volume of 566uL at 57% capacity.

Why are you using such a high carrier flow rate and inlet pressure ? I cannot think of anything that it will do better than a flow rate of 2 ml/min.

Peter

Honestly I can't tell you why we are running such a high flow. I work off of Standard Operating Procedures and the current version says to run at such a high flow rate. It was also validated at that flow rate so that can not be changed.

HELLOW

YOUR INLET MODE IS SPLITLESS .
AND YOUR COLUMN IS CAPLLIRY

WHAT DO YOU MEAN BY ( FLOW IS 10ML/MIN OF HELLIUM )!!!

I think it is too high flow rate .
I don't think it is acceptable for capillary column.
if you run like this flow and your system accept it ( I don't think that )
100% will lead to a big bleed in your column . and gradually you will lose your column .