Chromatography Forum

Hi,

I am attempting to perform a linearity check using an Agilent GC-MS:7890B/5977B, by injecting 0.2, 0.4, 0.6, 0.8, 1.0 uL/mL solutions of 1-octanol. My R^2 is 0.98.

Other than solution preparation errors, what else could affect linearity on a GC-MS system?

Hi,

I am attempting to perform a linearity check using an Agilent GC-MS:7890B/5977B, by injecting 0.2, 0.4, 0.6, 0.8, 1.0 uL/mL solutions of 1-octanol. My R^2 is 0.98.

Other than solution preparation errors, what else could affect linearity on a GC-MS system?

That is NOT how I'd do linearity. I'd keep the injection volume constant and vary the concentrations of the reference solution (your 1-octanol).

Hi,

I am attempting to perform a linearity check using an Agilent GC-MS:7890B/5977B, by injecting 0.2, 0.4, 0.6, 0.8, 1.0 uL/mL solutions of 1-octanol. My R^2 is 0.98.

Other than solution preparation errors, what else could affect linearity on a GC-MS system?

That is NOT how I'd do linearity. I'd keep the injection volume constant and vary the concentrations of the reference solution (your 1-octanol).

I injected the same volume, but varied the concentrations of the reference solution 0.2 uL/mL - 1.0 uL/mL 1-octanol

OK.

May be time to clean/service the inlet.

What is your injection volume and inlet parameters? Could check agilents vapor volume calculator to see if youre overloading the inlet/liner?

What are the responses for each level? Does it seem off on a low point or high point specifically? I would check precision if youre able, make 5 or six replicates of a few levels and see if those are reproducible. When was the last PM on the instrument?

Hi,

I am attempting to perform a linearity check using an Agilent GC-MS:7890B/5977B, by injecting 0.2, 0.4, 0.6, 0.8, 1.0 uL/mL solutions of 1-octanol. My R^2 is 0.98.

Other than solution preparation errors, what else could affect linearity on a GC-MS system?

ul/ml?

Would that make the solution concentrations .166mg/ml, .332mg/ml, .498mg/ml, .664mg/ml and .830mg/ml?

That makes the low standard 166ppm and the high standard 830ppm

What is the injection solvent?

Are you doing a split injection and what is the size of your column?

With a 1:5 split I have some analytes max out by the time they are at 100ppm, it could be column or detector overload unless you are using a fairly high split injection.

Post the curve, that could help us see what is going on.

The 5977 is really sensitive. I presume the concentrations you chose are based upon internal protocols / SOP. So, start backwards. Inject the most concentrated solution and optimize the chromatography. Strong peak shape and good response without overloading the column or MSD. Once done, inject the lowest concentration and do a quick concentration calculation. If it's close (you be the judge here) then run the study.

Compounds with OH, COOH or NH decompose at the EI ion source metallic surface and thus exhibit non-linear response particularly at sub ng on column amounts
One reference to such non linearity is in cholesterol
http://blog.avivanalytical.com/2014/05/ ... ponse.html
In GC-MS with Cold EI there is no such problems due to its fly-through ion source
With standard EI you need derivatization of the alcohols
You may write me at
amirav@tau.ac.il

Unless you have a really high split ratio you are overloadinbg the column, or the detector, or both.