poor reproducibility by multiples injections

[tinsang]

Dear all,
I'm in the process of developing a method for assay determination of Tetrahydrocannabinol (THC). The realative standard deviation is poor. I've tried with and without dwell time (pre: 0,02 min; post: 0,05 min.) but can't seem to get the RSD lower than 0,5% and the peak area increased by multiples injections. Here is some information:
Agilent 7890B
Column: HP-5MS-UI 30mx0,25mmx0,25µm
Carriergas:He; Flow: 1 ml/min.
Inlet: 250°C Splitratio:1/50
Det.(FID): 250°C
Oven: Initial: 80°C,hold time: 1min.; Rate: 30°C/min. to 280°C; hold: 25 min.
Standard solution: 1mg THC/ml Methanol, no interne standard.
1. Result: Dwell time: 0 min.
Peak area of THC: 394,397,398,403,400,400,396,399,400,401,409,408.
RSD: 0,9%

2. Result: Dwell time: pre 0,02 min. and post: 0,05min.
Peak area of THC:486,490,496,497,500,505,506,513,544,543,549,550,548,552.
RSD:5%

Suggestion? Using interne standard (e.g. Prazepam)?
Thanks.

[tkubowicz]

Hello

Try to inject 5 x std from 5 different vials (not from the same one) so you can confirm that it is not a problem with solvent evaporation (perhaps that's why peak area is going higher)

Then I'd check inlet for leaks - you have spit 50 so it is possible that small leak (split vent trap, septa, o-ring, copper tubing) can cause problem with injection precision.

Also (very first checking) check your syringe if it is not leaking.

Regards

Tomasz Kubowicz