Chromatography Forum

Hello all, I have a question regarding the %RSD of GC standards when using the IS. We are performing the USP method for chlorobutanol assay, the internal standard is 15 mg/mL 2,2,2-trichloroethanol. Chlorobutanol std conc. is 10 mg/mL. USP has a specification for %RSD of chlorobutanol/IS ratio of NMT 0.3%. We are having a hard time achieving the spec, the best time we got 0.36%. The majority of time we got around 1%.

Because it is USP method, there is not much room we can change. We contacted FDA, and was told they can easily achieve the required %RSD (0.04%-0.13%). Could you please provide some suggestion of how to improve the reproducibility? Any suggestion is welcome.

Here is the condition:
Injection port: 260°
Detector: 280°
Column: 135°
Carrier gas: Helium
Flow rate: 1.0 mL/min
Injection volume: 1 µL (use 10 uL syringe)
Injection type: Split injection, split ratio 10:1
Run time: 12 min
column is 0.32-mm × 30-m fused silica, 0.25-µm DB-wax (brand new column)
liner: Restek split liner with wool
Septum purge flow: 3 mL/min
injection speed: 500 uL/sec

Thank you!

This method is linear and commonplace. Unless you are injecting a 'low' concentration or a complex sample matrix you should not have a problem. I would call in a service technician for routine maintenance particularly highlighting the injector and detector.

We need to see a chromatogram - there are instructions on how to post them somewhere on the board - my guess is that you have poor peak shapes and/or noisy baselines that are confusing the integrator.

Peter

Hello

What system you're using, what injector?
Don't forget that each injector system has specification (injection precision) so make sure you are in manufacturer spec.
Apart from that check your inlet and injector (syringe tightness and inlet leak test)

Regards

Tomasz Kubowicz

The peak shape was okay, tailing factor is about 1.0.

Right now, the %RSD is passing after we added more needle wash step and also vial all standards at the same time.