Chromatography Forum

I am having difficulty in calibrating n-dodecane as a gas. I am using a Agilent 6890, FID with HP Plot Q column, and a 100 microliter sample loop.

What I do is in a electropolished SS 300ml vessel, I inject n-dodecane either pure or diluted in dichloromethane into the vessel with a 5 ul syringe. I then fill the vessel with high purity helium around 20 psi. The vessel is connected to the sample loop with treated or fused silica tubing, where all the tubing connected to the injection port is heated to 150 deg. C.
The fuel conc. is usually around 3-400ppm. Now the problem arises is that if I inject into the GC 3 ppm the calibration will be different if I make a new mixture and inject 400ppm into the GC. Basically the calibrations are not linear.

Is there a better way of doing this calibration. According to my experimental data, my calibrations for n-dodecane are off by 60-80%.

Would anyone know if there are limitations on the gas injection of a heavy fuel on the PLOT-Q or is the sample loop for the column? Could the column be damaged or too old?

It seems you have proved that at 20 psi and room temperature you have dewing of the C12 so that vaporization of the C12 is incomplete.

Perhaps you should ask a commercial company to determine if they can make a blend of C12 in helium at 400 ppm.

best wishes,

Rodney George

I believe that the vaporization of dodecane is fine, the vessel I inject the diluted dodecane is above 150 deg C into a vessel that has a pressure of 0.200 torr. At this pressure and temp., dodecane boils. I really think is has something to do with the FID or columns.

I know that I am not injecting more than I think I am. Let say I make a 400 ppm mixture (in helium) of dodecane in the heated vessel. I then dilute the mixture to 100ppm and inject that into the FID. I then dilute the 100ppm mixture to 25 ppm and inject as well into the FID. The response I got for the 100ppm and 25ppm was linear. But when I make another 400 ppm mixture and don't dilute but inject into the GC, the response I get is not linear with the earlier calibration, but the response I get is higher.

The problem I am having is that if my calibration is 2.22e-05 ppm/area_inj, the calibration according to the experimental data should be 2.65e-05 ppm/area_inj. The problem is that when I make a new vessel mixture to be calibrated, the calibration is 1.75e-05 or 1.5e-05.

I am also using another FID on a different GC which uses a DB-17ms column. On this 2nd GC there is another FID connected but to a liq injection port. When I inject gas (dodecane on this 2nd GC) the peak is very bad, very. When its 100ppm, there is a second peak and it tails a lot.
Today I injected an C8-C20 alkane mix (40mg/L) into the liq. injection port, 10:1 split. The peaks where perfect. Both columns on this 2nd GC are the same, DB17-MS.

Compounds as heavy as dodecane partition strongly onto surfaces, giving non-linear relationships of quantity in the container, which is what you control, and concentration in the headspace, which is what you measure.

My guess would be that the only way to do this will be in a dynamic system with a diffusion standard and accurately known gas flows.

Peter

I agree with Peter. The only way to get a linear response is with a dinamic blending system. The way you are trying has to many verables - volume, pressure, temperature, amount added. Also as the presure in the container decrease with sampling your headspace concentration increases.

FID is linear upto 10X7. If you calibrate FID 1000 PPM, the calibration hold good for 0.1 ppb also. There is no need of calibration in different concentration level. Calibrate with only highest concentration. Problem must in preparation of different concentration, pressure, not with the GC. Best is buy a standard with highest concentration and calibrate the instrument.

Hi Yuva

The problem with your scheme: calibrate at one point and run a line through zero on the basis of folklore about detector linearity, is that an analytical method involves much more than a detector, and all of the other steps can influence what detector signal you get for a given quantity of analyte in a sample.

Peter

I inject a mix of hydrocarbons on a daily basis ranging from C5 to C30 sometimes much higher, regardless, a Db-5 (5% phenyl) is the column we use on a regular basis.

I inject a mix of hydrocarbons on a daily basis ranging from C5 to C30 sometimes much higher, regardless, a Db-5 (5% phenyl) is the column we use on a regular basis.

How does this help the original poster, who has a problem calibrating gas phase standards ?

Peter

Thank for all the responses.

Today I made a small mixture of n-dodecane/n-decane/nonane/octane/1-heptene/toluene in Methylene Chloride. There was about 20k-40k ug/ml of each substance. I used this mixture and injected 2 ul into a vessel (165 deg C) and used helium to get about 40-70ppm of each. Injected into the GC-FID (plot Q column). My responses were different today then a week ago. They were closer what I need.

Does anyone know a syringe with a plunger, a 5 ul syringe, when you take say 2 ul of the solution, how do you use the syringe. Do you just go up to the level desired or do you do up and down few times and then go a bit over the desired level and once you take the syringe out of the vial, you push squeeze to the desired level.

I am trying to determine where the errors are in preparations of the calibrations mixtures. I would assume making a mixture in another solution helps, but don't see why there is a difference in the response on the FID's versus using a pure solution.

Buy a Chaney adapter for your syringe for reproducible injections or transfers with a syringe.

I have no idea who sells them today but Hamilton might be a place to start looking.

best wishes,

Rod

does anyone know how you would get a gas calibration done of a heavy hydrocarbon, lets say a c16h34.

does anyone use FID response factors. I was thinking of using that fid column (plot Q) and switch it to the liq. injection port and see the response factor of dodecane versus decane.

It would require high temperature carrier gas flow and a heated permeation tube.

I would also try to ascertain equilibration by using another column, a thin film column, not a plot.

Good luck.

Rod

C12 is getting right into the range where, like peter said, it's going to stick to the wall of most vessels you put it in.

Hamilton's gas tight syringes might be your best bet, with an attempt to minimize the time spent in the syringe.

Truly, though, the only way to get into the ppm range with a gas dilution is to do it like Praxair, BOTCO, and some of the other big gas companies do... estimate how much they need to put into the container and then measure how close they got... This is why it takes so long to get custom gas standards made and why they are so expensive.

They don't ever assume they got the correct volume-volume dilution and then use it as a standard.

Make sure to clean out your syringe after each use with something like methylene chloride and then bake it out for a good 15 minutes at 225 or so.

I inject a mix of hydrocarbons on a daily basis ranging from C5 to C30 sometimes much higher, regardless, a Db-5 (5% phenyl) is the column we use on a regular basis.

How does this help the original poster, who has a problem calibrating gas phase standards ?

Peter

Well, it sort of does help... the Plot Q columns are meant for light gases... a great way to separate things like CO and N2 (which are impossible to seperate on most columns and have the exact same mass --- a problem on machines like an MS, and one of the reasons the plot Q was developed).

From the horses mouth:

he HP-PLOT Q is a bonded polystyrene-divinylbenzene (DVB) based column that has been specially developed for the separation of targeted apolar and polar compounds including:

* Hydrocarbon (natural gas, refinery gas, ethylene, propylene, all C1-C3 isomers)
* CO2, methane, air/CO, and water
* Polar solvents (methanol, acetone, methylene chloride, alcohols, ketones, aldehydes, esters)]
* Sulfur compounds (H2S, mercaptans, COS)

Similar Phases:
CP PoraPLOT Q, CP PoraPLOT Q-HT, GS-Q, Rt-QPLOT, SupelQ PLOT

Like they say, light compounds, not C12. Simulated distillation, like what I assume willnatalie is talking about is often done on the 5% phenyl columns, and the same column would indeed be much better suited for the OP's purpose.

So in a way, he is helping, even though he's not being very clear about it.