new to GC and have some issues so need help

[bogamisaad]

Hello

I'm doing some research using C12 as a solvent and benzothiophene (BZT) as a sulfur compound. I have in my lab Agilent 6890N GC and 5973N MSD. The GC has two detectors FID for hydrocarbons and FPD for sulfur. The two columns are: Agilent 19091Z-205 350C max HP-1 capillary (50m*200microm*0.5 microm).

Right now I'm trying to do a calibration for FPD by injecting different concentrations of BZT 1-10wt/wt % dissolved in C12. First few runs with low concentration produced nice sharp and symmetric FPD peak. After that I'm getting broad peak and sometimes three broad peaks! The way I'm injecting is to my reactor which operates at 250C and the gas sample is sucked by vacuum to a 1000 sulfur coated sampling cylinder. Then through the transfer line to my GC. All temperatures for sample cylinder and transfer line are 250C. The gas used in reactor and sample cylinder is Argon so the sample is always diluted in Argon. Also, I noticed the peaks for C12 and BZT are attached in MSD i.e. not separated.

I'm listing my method parameters here:
For MSD: Acquistion mode scan, threshold 150 counts, sampling rate 2, mas range 5-500, scan/sec 2.97
For GC: mode split, gas He, temp 270C, prssure 31.1, split ratio 25. Ramping for the oven starts at 35C (set point) hold for 10 mins, then 8C/min to 70C no hold, then15C/min to 300C no hold.

I know my message is too long! but I appreciate help

[Don_Hilton]

You have a fairly complex system - and parts are unclear in your description. There are a number of possibilities for the problems you see, many of them with how the sample makes it to the GC.

One thing that may help is to inject samples of your mixture directly into the GC inlet with the GC having only the normal gas flows seen in a GC (all extra plumbing turned off). This would be a matter of making liquid injections with a microliter syringe.

BE sure your sample arrives in the GC in short time period and a small volume. Sampling from a gas stream often involves something like a samling loop, but I don't see a description of how you are managing the transfer from the reactor to the GC inlet.

[bogamisaad]

Thanks for the reply!

Yes I forgot to mention the sampling loop (2 CC) after the sample cylinder from which the sample goes to the GC. The sample takes 5 seconds in the reactor and then evacuated to GC through sample cylinder and sampling loop.

I did not get your point of direct injection. Can I inject liquid sample directly to GC!

Thanks again

[AICMM]

bogamisaad,

Regarding the BZT and C12 elution, have you considered switching to hexadecane? Would probably put your BZT cleanly in front. Other possibility is to switch to something more polar (perhaps a BP-90) but your operating temperature is problematic.

I too am confused by your description of set-up. The GC-MS and both detectors are connected to 50 meter columns? Is there a splitter in there somewhere? Everything so far is product from the reactor? How do you move from the cylinder to the GC (with more argon?)

Finally, if your high concentrations look poor, try increasing your split ratio or decreasing your loop size and see what effect it has.

Best regards,

AICMM

[Don_Hilton]

On the question about injecting liquid into a GC. Unless the gas sampling valve has been installed in a way that the inlet septum had to be removed, there should be an inlet on the top of the GC, sealed with a septum. It is a common procedure to inject small quantities of liquids into the GC by the use of a microliter syringe. Typical volume would be a microliter or less. For a mixture like the one you have, the split ratio would have to be fairly high to avoid overloading the column.

[kostas]

I would also suggest decreasing the loop size. I use 250 ul loops for a 4 mol% organics stream with 8:1 split and have good sensitivity even for products in very small concentrations (ppm).

[aldehyde]

On the question about injecting liquid into a GC. Unless the gas sampling valve has been installed in a way that the inlet septum had to be removed, there should be an inlet on the top of the GC, sealed with a septum. It is a common procedure to inject small quantities of liquids into the GC by the use of a microliter syringe. Typical volume would be a microliter or less. For a mixture like the one you have, the split ratio would have to be fairly high to avoid overloading the column.

And if you do have a septa you should check it every few months even if you're not injecting through it. It is heated and one half of the septa is exposed to the atmosphere, eventually oxygen and heat will break it down creating leaks and allowing the now crumbly septa pieces to fall into the inlet and give you nice contamination peaks. See this frequently with otherwise well taken care of systems which use headspace/purge and trap.

[larkl]

kostas is right. 2 ml injection with 25:1 split is too much sample for % level determinations. I'd either increase the split to maybe 200 or change the sample loop to 250uL or 500uL.