Chromatography Forum

Hello,

I've been having issues with inconsistent signal response on my GC MS/MS. I'm looking at a set of 23 terpenes where I am injecting a five point curve ranging from 0.25 ppm to 6.25 ppm via headspace. When running the curve, the response on some of the heavier terpenes is varied and not at all linear. in fact, I get better signal responses for some of the lower concentrations for these heavy terpenes than I do with the higher concentrations. I'm sort of at a loss on what's causing this inconsistency and was hoping someone had some experience or pointers to help me puzzle this out.

My method is being run on a Thermo Fischer Trace 1310 Gc with a TSQ 9000 MS and a Triplus RSH autosampler. My inlet temperature is 225C with a 40:1 split. The column flow starts at 0.8mL/min but then ramps up to 1.5mL/min at a rate of 0.3mL/min after 10 mins. My oven program goes from 75C to 140C at a rate of 20C/min. 140C is held for 9 mins and then it ramps up to 280C at a rate of 22C/min and that is held for 1 min. My ion source is a EI source and its temp is 300. The Ms transfer line temp is 280C.

The source of the problem is most likely to be the headspcae sampling - which you tell us nothing about except what hardware you use. What is the matrix of the standards, what are the temperatures, pressues, volume and flow rate settings of the headspace sampler?

The source of the problem is most likely to be the headspcae sampling - which you tell us nothing about except what hardware you use. What is the matrix of the standards, what are the temperatures, pressues, volume and flow rate settings of the headspace sampler?

My apologies. The matrix is just pure solvent. The solvent is Isopropyl alcohol. The samples are incubated in an oven at 160C for 30 minuntes. The autosampler I have is a little weird. The oven is an open air, insulated box that the vials are put in by having the robot arm slide back a cover and drop them in. There is no pressurization of headspace vials going on during incubation, only heating. Post incubation in this oven, a heated syringe in the robot arm comes down, withdraws the aliquot for the injection, and injects it directly into the inlet. As such, there is no flow going through the auto sampler (beyond the nitrogen used to purge the needle. That's set to 20mL a min.) as it doesn't really go into a sample loop persay. The volumes vary, but they get no higher than 100uL per vial and as low as 12.5uL. For the injection volume, it's 800uL.

For a better visual on the auto sampler I've attached a picture. The robot arm is the rectangular box on the far left. The incubator is the small box below and little to the right of it.

You are using a syringe based headspace autosampler not a pressure based loop autosampler, your parameters are probably too aggressive. The boiling point of your solvent is 83C, so I would suggest to reduce the temperature to a temperature below that. Make sure you have the syringe temperature slightly above the agitator temperature to avoid condensation. I understand you inject 800uL sample. Do I assume correctly, that you inject into a split(?), make sure you have at least a 10 or 20:1 split, for reasonable peak shape and better RSD. Lastly, it may be a good idea to reduce the injection speed, so that you don't over pressurize the inlet during injection.
A guide can be found here, I admit written for the legacy and xt PAL, but the principles are the same and the parameters are identical. Headspace Analysis -- Getting Started toward the bottom of the web site: AutosamplerGuys.com Good luck!