AREA DEREASING DURING THE RUN

[AMIT KUMAR JAIN]

Dear All

We are analysing the three solvents. Our observation is area of the one solvent is decreasing after the run, and finally that peak disappeared.
Can any one help us to ractifying the cause.

Solvets are : Tetra deccan,Penta deccan,Hexa deccan.

Problem is with Hexadeccan.

Regards

AkJ

[zokitano]

Dear Amit,

It would be easier for us to reply when we'd know the GC parameters that you use: column, temperature, injector settings, detector etc.
Did you mean that the third peak appears only in the first run, followed by subsequent decreasing of its area in the next runs, and finally it disappears?

Regards

[chromatographer1]

I am assuming .......................... err, guessing:

You are doing a solvent analysis of a solution of a matrix.

One of the solvents is ether or methanol, or some other volatile solvent.

In which case:

The solvent that is disappearing is disappearing not from the column separation but from the solution you are sampling due to its volatility in the vial (container) you are holding it in.

How did I do with this guessing game, AMIT KUMAR JAIN ?

best wishes,

Rod

[zokitano]

Dear chromatographer1,

Would you agree that if hexadecane disappears from the sample solution due to its volatility it would cause reduction of the sample solution volume, and that would increase the concentration of the other -decanes in the solution. So, if this is a case in the same time when one observes disappearing of hexadecane from the chromatograms, should also observe increase of the tetra- and pentadecane peak area (presuming that equal injection volumes are subsequently injected in the GC)?

Regards

[chromatographer1]

Dear Zokitano

No, I would not.

best wishes,

Rod

[chromatographer1]

Sorry I seem to have not received the original post correctly.

I was missing the last lines of the post.

So I was a bit confused then about the details which AMIT KUMAR JAIN provided but that I DID NOT SEE.

Terribly sorry for my rudeness following.

The three alkanes should behave similarly. If the nC16 is missing or lost it could be an issue with a backflush valve and a column leak.

If a single dimensional system is being used then I would guess an inlet heater is not working correctly and the inlet is cooler than may be indicated.

Nothing else comes to mind except precipitation or partition of the hexadecane out of solution that is being injected if it is true that the retention times are found to be consistent.

Is this reproducible and what are the sample preparation parameters?

best wishes,

Rod

[zokitano]

Dear Rod,

apology accepted
I supposed before that you have quickly overlooked some facts about the problem.
By the way, those three alkanes have high boiling points (b.p of hexadecane is 287C, b.p of pentadecane is 270C and b.p of tetradecane is 253C) thus will be less volatile.

Best wishes,
Zoran

[Peter Apps]

I did not get any details either, AKJ, could you post again with the GC setup etc.

Peter

[Bruce Hamilton]

Although it's probably the most obvious, I'm not sure that I'd go with a cold injector, unless run time is short and sample injection volume and solvent are likely to provide cooling. I'd alsmost suspect the final oven temperature is too low, but....

My next guess would be for the concentration of nC16 alkane to decrease in the sample solution, possibly due to insolubility in solvent at autosampler temperatures, or absorption onto vial surfaces, or use of an immiscible syringe wash solvent.

We really need much more detail, especially instrument conditions, sample preparation, concentrations and solvents, wash solvents, etc.

This also sounds like a study exercise, as it's very difficult to lose nC16 alkane without proportional losses of nC15 alkane, especially when compared to nC14 alkane, given their similar volatile properties.

Bruce Hamilton