Chromatography Forum

Hello everyone.

I'm working with an Agilent 7890 GC coupled to an Agilent 7000 Triple Quad MS. I'm validating a method for pesticides (organochlorines and organophosphates). I run one replicate at a time, from the lowest to the highest level, and then the second replicate the same way, and so on. All injections are from the same sample but different vial. My problem is that I don't get good precision injecting that way. I don't know what causes this. I put 600 uL solution in each vial (2 mL capacity), I control the temperature, I check the autotune parameters and everything is ok. I noticed when I plot the results for each replicate I get excelent r2 coefficients with linear fit (>0.999), but when I plot all the replies together the r2 coefficient drops to values such as 0.95. Furthermore, I realised that in general the highest level of the curve has higher variability (this is the latest level injected for each curve replicate). The run time is ~35 min.

What could be causing this? Should I change my injection sequence?

Thanks for your attention.

What is the relative standard deviation of the replicates at each level ?

Peter

Hello,
are you working with internal standard or external?

José L.

I'm missing some information, like
sample, sample concentration, sample solvent, sample vial, installed liner, split-flow injection volume, syringe volume, syringe speed...

Then we can also discuss the infuence of the temperatures in injector and on the column.

Best regards
Klaus

Sorry for not answering, I'm working simultaneously in another project.

Well, I've recently run the calibration curve using internal standard (Aldrin) and that improved the correlation (0.997 for most of the analytes).

Here are the instrument conditions:

GC conditions
Sample solvent: Hexane
Syringe Size 10 μL
Injection Volume 5 μL
Solvent Wash Draw Speed 300 μL/min
Solvent Wash Dispense Speed 6000 μL/min
Sample Wash Draw Speed 300 μL/min
Sample Wash Dispense Speed 6000 μL/min
Injection Dispense Speed 6000 μL/min

Front MM Inlet He
Mode Pulsed Splitless
Heater On 250 °C
Pressure On 30.922 psi
Total Flow On 81.2 mL/min
Septum Purge Flow On 3 mL/min
Temperature Program On
250 °C for 36.867 min
Run Time 36.867 min

Gas Saver On 20 mL/min After 2 min
Injection Pulse Pressure 60 psi Until 2 min
Purge Flow to Split Vent 75 mL/min at 2 min

Calibration range: 0.5 - 25 ug/L
Sample volume in vial (2 mL capacity): 500 uL