Hello All, Not certain if this is the proper location for such a question, so please feel free to indicate the correct location if I am in error.
Issue: Difficulty with GC Chromatography and irreproducible injections - Specifically, the chromatography is inconsistent between injections of sample OR standards. I am working with Menthol analyte in various formulation matrices including sprays (non-propellant) and gels. When preparing a run, I consistently have 1 or 2 injections that have dramatically increased area counts as to what should be expected. This is not necessarily tied to standard preparation (analyte in DMF) or sample preparation (includes formulation matrix) as the issue is random during the injections
I'll see these types of area counts:
Check Std A - 686
Check Std A-2 - 878
Blank (DMF) - 0
Std (n=5)
1) 913
2) 725
3) 681
4) 679
5) 680
RSD of n=5 - 14%
And in contrast, the repeatability preparations for the samples will have a 1-2% RSD of the n=5 injections.
I've also had runs where the %RSD of the std injections is less than 1%, but then the sample repeatability injections have 9%+ RSD.
I am not fully understanding how this can be so inconsistent if all conditions are the same for all of these injections. I do see a bit of tailing which doesn't typically go above 2.5-3, but i notice that some chromatograms have more tailing than the others. I'm mainly looking for any tips as to understanding if this is an issue with column interaction, system settings, etc.
Schema:
Standard Prep: 50 mg Menthol / 50 mL (Dilution with N,N-Dimethylformamide)
Sample Preparation: 125 mg Sample (Corresponding to ~ 1mg/mL Menthol) into a 50 mL flask, diluted to neck with DMF - mixed for 30 minutes on wrist action - diluted to volume and mixed.
Chromatographic Conditions:
Chromatographic Conditions (GC):
8.1 GC Front Inlet : Split mode
Liner : Agilent 5183-4711
Temperature : 250°C
Pressure : 33 psi
Split Ratio : 20:1
Split Flow : 66 mL/min
Total Flow : 72.3 mL/min
GC Column : PhenomenexZB-WAXPlus 30m x 0.25mm 0.25ID
Pressure : 33 psi
Flow : 3.3 mL/min
Velocity : 67.5 cm/sec
Mode : Constant Flow
GC Oven : Temperature Program
Initial Temperature : 100 °C
Initial Time : 3 min
Rate 1 : 20 °C/min
Final Temperature : 200 °C
Hold Time : 3 min
Rate 2 : 15 °C/min
Final Temperature : 230 °C
Total Run Time : 13 min
Detector : FID
Temperature : 250°C
H2 Flow : 30 mL/min
Air Flow : 400 mL/min
Mode : Constant make-up
Make-up [He] : 25.0 mL/min
Front Injector : 10 µL syringe
Injection Volume : 1 µL
