Reproducibility Issues

[mosesde]

Hi All,

I'm having some serious reproducibility issues with an established test method for the % purity of toluene. The nominal method conditions are as follows:

Column: Stabilwax, 30m, 0.25mm, 0.5µm thickness
Injector: Split, 200C, Split ratio 1:20, 1.0µl injection, pre-dwell time 0.01, post dwell time 0.01.
Detector: FID, 300C, Combustible H2 @ 35 mL/min, Combustible air @ 400mL/min, no make-up gas
Oven: 40C for 0 mins, 32C/min to 200C for 5 mins
Carrier Gas: Hydrogen at 1.6 mL/min

I have run the method on multiple Agilent 6890Ns with no success. Could incomplete volitalization of toluene in the injector port be possible? Would the liner type be a concern? I've used 4mm straight liners with and without glass wool in addition to a gooseneck and saw the same problems. Any ideas would be greatly appreciated!

Thanks very much

[Don_Hilton]

Step 1. How have you confirmed that the GC is working and can see a compund elute from the column?

[chromatographer1]

Sounds like someone forget to change the hydrogen cylinder and it is empty.

Or did they forget to turn on the computer to the data acquisition system?

best wishes,

Rodney George

[krickos]

Hi

With established method do you mean it has worked in the past? and how does the issues appear? (large peak area difference from injection to injection, specific peaks only or?).Have you tried a new column? More details is needed to pinpoint the problem.

We use something similar on our 6890s no issues, but note that the split ratio is higher and column dimension is larger as with injetor temperature.

Column: Ultra-2 ("like DB-5 or equivalent"), 25m, 0.32mm, 0.52µm thickness
Injector: Split, 250C, Split ratio ~1:50, 1.0µl injection, No dwell times
Detector: FID, 300C, Combustible H2 @ 35 mL/min, Combustible air @ 400mL/min, 25ml/min makeup N2
Oven: 40C for 2 mins, 30C/min to 180C for 2 mins
Carrier Gas: He ~2,2ml/min

Liner: standard 4mm straightliner with wool

(and yes I know that that all xylenes impurities wont separate under the above conditions but is not necessary for our purpose)

[mosesde]

Thanks for the replies!

The peak is very visible which would eliminate any question of compound elution, lack of carrier gas (tank is full), or data acquisition system problems. By reprodicibility issues, I mean the area counts are just inconsistent from injection to injection. This method is run frequently at a different facility within our company with no issues, however they use a Thermo Quest Trace 2000 and we're using an Agilent 6890N. The column I'm using is brand new, but I will likely try a new one to see if it helps. One thing I have noticed is that there appears to be some peak fronting which may suggest column overload. I will also try using a high split ratio as in your method krickos, to see if introducing less sample to the column will help with peak area consistency.

Thanks again for the responses,

Dave

[chromatographer1]

There can be so many small issues which can cause reproducibility problems.

Non reproducible splitting of the sample would seem to be the problem. Now, what could be the cause?

1. Column installation and the depth of the tip into the injector.

2. Slight inclination of the column in the injector, not a perfect 90° mounting of the ferrule in the injector nut.

3. Too fast an injection 'burst' of a volatile sample, use the slower speed setting for the 6890's autosampler, or inject manually more slowly.

4. What if the vials are not kept cool in the rack, but are warmed as they wait to be injected? Volatile impurities could be pressurizing the vial and changing the amount of sample loaded into the syringe.

5. I have even seen the tip of the needle cause issues if the liquid is 'sprayed' into several streams instead of one.

6. loss of volatile impurities relative to the primary peak can occur if the vials are not sealed or sampled reproducibly.

I suspect #4 is the most likely.

Good luck.

ROd

[mosesde]

Thanks for the ideas Rod. Definitely good points to consider.

Have a good weekend,

Dave

[Rouan]

Hi Dave,

You tried it on diffrent instruments? They all give the same problem? Try changing the injection technique. Dwell times can give problems with evaporation of the needle not being consistent.