Nonlinear FID results

[dchandler]

I would very much appreciate some advice. I have spent several weeks collecting data trying to determine the cause of non linearity, with no success. I am using a HP 5890 Series II Plus. The original capillary was replaced with a Rxi-1ms- (.25mm ID) 60m Cat#13356. The relative response coefficients are not linear when injection volumes are changed. For example a 25uL, 50uL, 100uL, and 200 uL will all have variable relative response coefficients( at times the responsive coefficients are not linear even within the same injection size). I am injecting a standard that consist of 3% Pentane, 4% Isobutane (IS), 3% Neon, and a balance of Helium. Flow rates are: 2.2 ml/min Carrier(H2), 30 ml/min Aux(H2), 30 ml/min H2, 400 ml/min Air. I have also tried a Helium and Nitrogen Carrier. I am using a simple oven program in order to bake out the column each run and provide quick run times. Initial Oven temp is 35 C It then ramps at 12 C/min to 300 C and holds for 5 minutes. The retention time of the Pentane is three minutes. I have adjusted air flow rate, carrier flow rate, H2 rate, Inititial Oven temp. The Injector temp is 200 C the Detector temp is 250 C. Please Help.

[aldehyde]

How are you introducing the sample? For those volumes I would expect a gas sampling valve?

[chromatographer1]

If you are using a side port needle syringe you have found your cause.

best wishes,

Rod

[dchandler]

I am using a SGE syringe/SGE Luer Lock for gastight syringes Cat# 24763.

[Peter Apps]

If my calculations are correct you are putting around 0.16 mg of octane into the inlet - if you do not have a very large split ratio you are at the top of the FID linear range.

Also you say that you have hydrogen as make up gas (auxillary) if this is not a typo you have double the required hydrogen flow to the flame.

Depending on how you are doing the injections there is all sorts of potential for poor repeatability and non-linearity due to pressure fluctuations in the inlet.

How are you ensuring that the octane and pentane do not stick to the syringe in varying amounts ?

The FID itself is very well established to be very linear over a very wide range - but only when it is operated correctly. The inlet is always a far more likely suspect.

Peter

[dchandler]

How are you ensuring that the octane and pentane do not stick to the syringe in varying amounts ?

-Should I use an oven to keep the syringe hot?

Also you say that you have hydrogen as make up gas (auxillary) if this is not a typo you have double the required hydrogen flow to the flame.

- Please explain. When using helium an auxillary gas is required, but when using H2 it is not?


"If my calculations are correct you are putting around 0.16 mg of octane into the inlet - if you do not have a very large split ratio you are at the top of the FID linear range."

With a 100 uL injection there is 102 nanograms of pentane.

[Peter Apps]

How are you ensuring that the octane and pentane do not stick to the syringe in varying amounts ?

-Should I use an oven to keep the syringe hot? If you need a sample introduction method that is repeatable enough to detect deviations from linearity in an FID you really need an automated system with the syringe permanently an oven

Also you say that you have hydrogen as make up gas (auxillary) if this is not a typo you have double the required hydrogen flow to the flame.

- Please explain. When using helium an auxillary gas is required, but when using H2 it is not? It is useful to have a make up (auxillary) gas flow to the FID, but it has to be something inert like nitrogen or helium, not hydrogen which is combustible


"If my calculations are correct you are putting around 0.16 mg of octane into the inlet - if you do not have a very large split ratio you are at the top of the FID linear range."

With a 100 uL injection there is 102 nanograms of pentane.

can you show how you calculate that ?

I am intrigued as to why you are investigating the FID's linearity.

Peter

[dchandler]

It is useful to have a make up (auxillary) gas flow to the FID, but it has to be something inert like nitrogen or helium, not hydrogen which is combustible"

There are three ports in the back of the 5890: one is H2(for the FID), one is Air(for the FID), the other is for the carrier gas the auxillary is made up of the carrier gas.

can you show how you calculate that ?

I multipleid the 100uL injection by .03 (3% pentane) The initial partial pressure of pentane in the standard was 46.17 kPa using the gas density at this pressure (1.37 kg/m3) I determined the number of grams in the injection and divided by 40 (for the split).

I am intrigued as to why you are investigating the FID's linearity.

The goal of my project is to determine the amount of products in a FT reaction. It is my understanding in order to do this you must have similar relative response coefficients over a wide range when comparing the different analytes to the internal standard.

[gamarra]

The use of N2 as make up improve the FID responds when you have small amount of change in flows, if you use H2 as make up its worst.


Best regards
Daniel Sagarra
www.flowmeet.com

[dchandler]

Thanks for the suggestions everyone.