rsd failures

[pstaunton]

Running a method that uses heptane/toluene in dichloromethane as a standard, concentration 2.6ppm.

Problem is that every so often the run fails rsd of nmt 4%. Get rsd's usually around 10% from three injections.

I have tried changing everything before a run (septum, liner, oring, ferrules). I only take one injection per vial. Usually the last injection jumps up to an area of ten but it can be random. It's only the rsd of toluene that's important. Retention time is around 6.2min. Also
I have checked for leaks and pretty sure it's fine.

Db5 column
GC 6890.
Carrier gas: helium (7ml/min), splitless.
Inlet temp: 245C
Detector temp:300C
Purge on 1min @ 25ml/min
Program: initial 35C for 8min; 15C/min to 290C, Hold 25min.

0.5ul from 5ul needle.
Injector speed fast.
Liner straight, tapered bottom.

Any tips would be helpful. Please don't ask me to change any parameters above as this method does not allow. Except purge time which can be 1 to 1.5min.

[dap]

Hello. We always use internal standards for GC methods. RSD usually ≤ 0.5 %.

[HPLC chemist]

Try increasing your injection volume to 2 uL (be sure to hand mix vials before injection). This will give you a more consistent %RSD. What is your current peak area?

[pstaunton]

Thanks for reply.

Unfortunately I can't make modifications such as injection volume or internal standards. I understand this limits me to what I can try but any thing that helps is great. Typical peak area is 40 then one will jump to 48.

[HPLC chemist]

40 is 'FAR' too low. Check your preparation and how you calculate your concentrations. 40 sounds like ppb range (ng/mL or ug/L) not 2.6 ppm (2600 ng/mL)!

[Peter Apps]

What is the column diameter ? - 7 ml/min is way too fast for a 0.32 mm or 0.25 mm column, OK for a 0.53 mm.

If you can't change the method then you can't fix the problem, unless something is malfunctioning.

Has the problem always been there, or did the method used to work ? If the latter, what changed just before the problem appeared ?

What is the variability in the retention time ?

What is the analyte(s).

Are you using an autoinjector ? What are the settings - injection speed, number of pumps, number of wash strokes, what wash solvent ?

Peter