Chromatography Forum

Dears,
What is the difference between Mol% and Vol% in Gas Chromatogrphy analysis and how can we convert them from one to another.
Thanks

Is wt% and Mol% same?

If all gases were ideal then the two values would be the same.

However, the ideal gas laws are just that IDEAL not real. The volume and density of a gas deviates from the ideal based on the actual pressure and temperature when injected.

Mole percent is based on the weight of each of the gas components per volume and volume % is based on the volume of each of the gas components.

You might remember from high school chemistry that mixing 50cc of water and 50cc of alcohol do not produce 100cc of the 50/50 liquid mix. The same is true for gases.

To determine the mole % the weight of each gas has to be determined in a fixed volume at a certain temperature. Each gas will be present as a fraction of a mole. To determine the volume % the same is done but volume not weight is used. Different gases will not behave at all times in an ideal manner.

Thank You,
In our on-line GC we have a LSV (liquid Sample Valve)which vaporizes the sample before injection. The GC measures all composition consists on 1,3 Butadiene (40% Mol%), 1-Butene (20% Mol%), Butene-2 (20% Mol%)and other C5 in ballance (20% Mol%). The GC is well calibrated on Liquid Cylinder but on process it does not read 100% results. Whereas Laboratory also uses GC with Mol% and gives 100% results.
We are trying to see the reason. Can you please advise.

You wrote:

"we have a LSV (liquid Sample Valve)which vaporizes the sample before injection."

You have a VSV (vapor sample valve) then that actually injects the vaporized liquid. You have a vaporizing heater in your sample system to convert your liquid sample into vapor for injection. But that is besides the point.

Your instruments (lab and ON-LINE) must be calibrated correctly. In order to calculate mole %, all components of liquid sample MUST be measured accurately, the calibration for each component must be accurate. This is one cause for non-100% measurements. Presence of 13BD polymers in a stream will cause this, water or air in your stream will cause this.

I wonder if your lab GC is really measuring all the components that are present. If you are measuring C5s as a group, are you certain you have an accurate factor to measure (account for ) them.

How far are you measuring away from 100%? 0.1%? 10%?

Have you talked to your vendor about calibration? Do you have anyone who knows how to calibrate your process analyzer?

Sorry I cannot be of more help. I suggest you talk to your vendor.

Hi i think that your lab is missimg some light ends (C3 & other C4), and generaly they calculate the heavier com'p as group, so you can't have 100% in both cases. other cause to difference may be in sampling procedure. if lab samples liquid and injects to gc through gas sampling valve or syringe, they loose equilibrium of the sample.