S/SP Inlet instead of On-column ?

[Peter Apps]

Peter,

I write "it could happen" because I don't know the nature of your sample and method you are using. To determine how it will affect the retention time of your sample is necessary.
In my experience with samples that I used to analyze, the gain in separation using a ramped temperature inlet fully compensates a shift in retention times.

And how thohry said, the method that he follows requires an on-column inlet, so it was proven that for his kind of sample an on-column inlet works better than a S/SL inlet. Probably because an isothermal temperature in inlet can break down some molecules in sample.

Leonardo.

Leonardo, you made a general claim based on boiling point, i.e. not restricted to the specific samples I am running, so what difference to your general claim do my samples make ?

Do you have any hard evidence (i.e. not your experience) that chromatography is improved by delayed transfer of heavy compounds from a GC inlet into the column ?, or any hard evidence of retarded elutions under such conditions ?

I am harping on about this because separations are what chromatography is about, and your claim of improved separations contradicts accepted practice, which is to start the separation on the column with all the analytes in a narrow band near its inlet end.

Peter

[lynoguchi]

Peter,

An on-column inlet can improve separation, and reduce inlet discrimination when compared to S/SL inlet.
Here is a definition from Agilent for both inlets:

Split/Splitless
The first type of inlet designed for capillary analysis was the split inlet, which is most commonly available now as a combination inlet for both split and splitless injections. This is a vaporizing inlet which vents most of the sample in the split mode and transfers most of it to the column in the splitless mode.
Because it is a vaporizing inlet, it is column protecting but can cause solute discrimination and decomposition. Split injection is used for general analysis, whereas, splitless injection is most frequently used for trace analysis.

Cool on-column
Cool on-column inlets give high accuracy and reproducibility, are sample
protecting, have the least solute discrimination among all the inlets, and work by depositing the sample directly into the column. Unlike the other vaporizing or “hot” sample introduction techniques, the sample is not exposed to high temperatures during injection or transfer to the column. Cool on-column inlets are used for the analysis of samples with a wide boiling-point range or those that are thermally sensitive, and for trace analysis.


In this file you can find more detailed differences between how each inlet can affect a chromatographic result.
https://www.agilent.com/cs/library/user ... 041007.pdf

Leonardo.

[Peter Apps]

Hi Leonardo

I know what the difference is between a split-splitless inlet and a cool-on-column inlet. What I am probing is whether using a temperature programmed inlet results in analytes with different vapour pressures being transferred to the column at different times and whether, if it does happen, this improves the separation.

Bear in mid that there is nothing magical about boiling point; everything which will go through a GC column has a vapor pressure at all temperatures above absolute zero, and that vapour pressure increases smoothly with temperature - an analyte does not sit firmly in the inlet until the temperature reaches its boiling point and then jump onto the column.

Peter

[lynoguchi]

Peter,

Yes the analytes don't "jump" in column, because they already are in column.
With on-column injection the sample are inject directly in column, but it only be vaporized when each compound boiling point temperature is reached.

I know that some fractions can coelute or you can have band broadening, but for some applications it is the only way to assure sample integrity.
For example, FAME analysis has two differents standard methods: EN 14103 - Determination of ester and linolenic acid methyl ester contents and EN 14105 - Determination of free and total glycerol and mono-, di-, triglyceride contents, for first is used a S/SL inlet and for second an MMI inlet with ramped temperature.

Maybe a S/SL for this application will work, but thohry will get better results using an on-column inlet.

Leonardo.

[Peter Apps]

Peter,

but it only be vaporized when each compound boiling point temperature is reached.

Leonardo.

This is a common confusion - but as long as the vapour pressure is above zero a substance will vaporize; it does not have to be at its boiling point. You can prove this for yourself experimentally by leaving a small quantity of water at room temperature - come back later and it will have evaporated. There is nothing magical about boiling point; it is just the temperature at which a substance's vapour pressure happens to equal the ambient pressure, which is usually taken to be atmospheric. At that temperature bubbles of vapour can form in the liquid, and it boils. At the increased pressure of a GC inlet boiling points are increased, but to speak of nanoliters of a substance spread as a thin film onto a surface or dispersed as an aerosol as "boiling" stretches the use of the term beyond breaking point.

And if thohry had an on-column inlet he wouldn't have posted in the first place. His question is not whether an on-column inlet is better for high-boiling wax esters, but whether a split-splitless inlet might work.

[GOM]

Hi

As Peter and James have correctly pointed out

Separation in the inlet is called discrimination, and it is a bad thing.

This is a common confusion - but as long as the vapour pressure is above zero a substance will vaporize; it does not have to be at its boiling point.

And if thohry had an on-column inlet he wouldn't have posted in the first place. His question is not whether an on-column inlet is better for high-boiling wax esters, but whether a split-splitless inlet might work.

Just to repeat - the separation takes place on the column - the injection port is "just" a means of moving the sample onto the column in such away that the sample can be successfully eluted

Perhaps if Thohry could reply with more information about the actual sample?

Kind regards

Ralph

[lynoguchi]

Yes, we're making suppositions based only in a simple change between inlets.
We don't know nothing about the sample.

What is the thermal stability of wax ester samples?
If a S/SL inlet could be used whithout interferences on result, why the methodology recommends an on-column inlet?

Leonardo.

[gcguy]

Interesting comments...

I have used on column injection for many methods but only as a means to prevent decomposition of thermally unstable compounds. Never as a means to improve a separation. Yes, repeatability of peak areas does improve and generally as a technique I like it.
Following on from Peter's point, using a hot split injection will cause the transfer of compounds onto the column in a gaseous state. As the oven starting temperature is generally lower than that of the injector the transferred compound will then condense. As the oven temperature increases the compounds will volatilise in the column and move along the column in the gas phase.
Going back to the original question... In my experience changing a method to the extent of a different injection mode would require revalidation of the method. I would also suggest that as on-column methods are less common than split/splitless there is probably a reason that mode of injection was chosen. Thermal instability is the normal reason for choosing on-column.

GCguy

[Peter Apps]

One of the forum's venerable members; Hans Muller had a saying for this kind of situation; "Don't think, measure !".

As long as a method does not trample roughshod over established practice the best way to find out if it will work is to try it. For the cost of a Uniliner and a few injections a split - splitless injection has got to be worth a try.

Thohry, when you try it be sure to tell us how it goes.

Peter

[James_Ball]

Thinking about the Uniliner, it would probably work best to get one with the hole near the bottom so you can inject splitless then vent to sweep out any high boiling stuff that might be left over, or solvent to prevent the solvent peak from being too large. I have used them with good success for clean samples like drinking waters. You get little to no breakdown of sensitive components too.

[Peter Apps]

Nobody blinks an eye when people do "on-column" injection into a length of uncoated megabore capillary press-fitted to the separating column. The way I see it, Uniliners take this process a few steps further with the added advantage that you can control the temperature of the (very short and fat) "retention gap" independently of the temperature of the column.

Peter