S/SP Inlet instead of On-column ?

[thohry]

Hi all,

We are deploying an Agilent method for wax ester analysis. The oven temp is programmed up to 335 oC and it requires on-column inlet.
But our GC has only S/SP inlet available. I am not sure if we can use S/SP Inlet instead of On-column Inlet ?
Could I use the S/SP ?

Thanks for any input.

[Peter Apps]

Possibly if you use a Restek Uniliner.

Peter

[thohry]

Possibly if you use a Restek Uniliner.

Peter

Thank you very much. We'll try it.

[Peter Apps]

Let us know how it goes.

Peter

[lynoguchi]

Hi all,

We are deploying an Agilent method for wax ester analysis. The oven temp is programmed up to 335 oC and it requires on-column inlet.
But our GC has only S/SP inlet available. I am not sure if we can use S/SP Inlet instead of On-column Inlet ?
Could I use the S/SP ?

Thanks for any input.

Yes, you can use a S/SL inlet, but you will get a poor separation for light compounds, because the S/SL works only on isothermal temperatures, usually above the temperature of heavier compound in your sample.
With an On-column inlet you can use a ramp for heating the inlet, beggining from low temperatures (ambient +5) and improving separation of compounds with low Boiling Point.

What method (ASTM) you will follow?

Leonardo.

[Peter Apps]

Hi all,

We are deploying an Agilent method for wax ester analysis. The oven temp is programmed up to 335 oC and it requires on-column inlet.
But our GC has only S/SP inlet available. I am not sure if we can use S/SP Inlet instead of On-column Inlet ?
Could I use the S/SP ?

Thanks for any input.

Yes, you can use a S/SL inlet, but you will get a poor separation for light compounds, because the S/SL works only on isothermal temperatures, usually above the temperature of heavier compound in your sample.
With an On-column inlet you can use a ramp for heating the inlet, beggining from low temperatures (ambient +5) and improving separation of compounds with low Boiling Point.

What method (ASTM) you will follow?

Leonardo.

Leonardo - what effect does the type of injection have on the separation ? The separation happens on the column. Separation in the inlet is called discrimination, and it is a bad thing.

Peter

[lynoguchi]

thohry,

What exactly do you analyze in your sample, the composition?
Are there saturated or unsaturated wax esters molecules in sample?

The COC inlet have max temp of 450 °C and the S/SL inlet 400 °C, to vaporize some molecules maybe a temperature higher than 400 °C is necessary.

With an isothermal injection you can have the break down of unstable molecules, therefore an injection with ramp temperature can avoid this.

Leonardo

[James_Ball]

Hi all,

We are deploying an Agilent method for wax ester analysis. The oven temp is programmed up to 335 oC and it requires on-column inlet.
But our GC has only S/SP inlet available. I am not sure if we can use S/SP Inlet instead of On-column Inlet ?
Could I use the S/SP ?

Thanks for any input.

Yes, you can use a S/SL inlet, but you will get a poor separation for light compounds, because the S/SL works only on isothermal temperatures, usually above the temperature of heavier compound in your sample.
With an On-column inlet you can use a ramp for heating the inlet, beggining from low temperatures (ambient +5) and improving separation of compounds with low Boiling Point.

What method (ASTM) you will follow?

Leonardo.

Leonardo - what effect does the type of injection have on the separation ? The separation happens on the column. Separation in the inlet is called discrimination, and it is a bad thing.

Peter

With cool on column injection it would not be in the inlet, but in the column. The inlet is usually cooled to focus analytes on the head of the column then fast ramped to flash off the solvent and vaporize the analytes.

Hot injection into a uniliner will approximate cool on column but will be a hot on column unless you have the MMI inlet or other PTV inlet. If you begin with the oven temperature below the boiling point of the solvent you could refocus using the solvent effect and more closely mimic cool on column performance, that would also be best done using a retention gap.

[lynoguchi]

Even the sample being injected directly in column, the first 10 cm of column are inside the inlet, therefore the temperature for vaporization are controlled by inlet.
The PTV is a good alternative and has the same principle of COC inlet, the main difference are that you have a liner inside connected to the column.

Leonardo.

[Peter Apps]

Yes, you can use a S/SL inlet, but you will get a poor separation for light compounds, because the S/SL works only on isothermal temperatures, usually above the temperature of heavier compound in your sample.
With an On-column inlet you can use a ramp for heating the inlet, beggining from low temperatures (ambient +5) and improving separation of compounds with low Boiling Point.

What method (ASTM) you will follow?

Leonardo.

Leonardo - what effect does the type of injection have on the separation ? The separation happens on the column. Separation in the inlet is called discrimination, and it is a bad thing.

Peter

With cool on column injection it would not be in the inlet, but in the column. The inlet is usually cooled to focus analytes on the head of the column then fast ramped to flash off the solvent and vaporize the analytes.

Hot injection into a uniliner will approximate cool on column but will be a hot on column unless you have the MMI inlet or other PTV inlet. If you begin with the oven temperature below the boiling point of the solvent you could refocus using the solvent effect and more closely mimic cool on column performance, that would also be best done using a retention gap.

Leoardo, what you write is true, but I still do not understand how slowly heating the inlet (even if the top 10 cm of column is in there) improves the separation. The various inlet designs and operations are aimed at putting all the analytes together in one spot at the top of the column. An inlet that puts the analytes onto different places at the same time, or in the same place at different times will interfere with the separation carried out by the column. The simplest way to achieve simultaneous spot loading is a hot vaporizing split injection, all the other refinements are aimed at accommodating different kinds of samples without sacrificing separation or resolution. Different inlets and inlet operating modes affect resolution by affecting the width of the starting bands, but they do not affect the separation of peak maxima.

Peter

[lynoguchi]

Peter,

You're correct, the main separation is made by the column. But the inlet can improve the separation, let me give an example.
If your sample has a mix of compounds with Boiling Points from 50°C to 300°C when you inject a sample at same temperature, e.g. 350°C, all compounds will be vaporized and enter in column at same time, then if you have compounds with similar interaction with the column you can have an coelution or split peaks. Applying a ramp temperature in inlet, the compounds will be vaporized accordly with each BP, then only when certain temperature is reached the sample will be "injected", improving the separation.

This type of inlet is used often with complex matrix samples and molecules with number of carbons >15. For this type of sample is necessary shorter columns (< 30 m) that has less theorical plates, then a ramped inlet temperature (usually with 20°C higher than actual oven temperature) helps to separate the compounds.
These inlets allows too a cryogenic injection, and for samples as crude oil you get a much better separation for light compounds (< C10).

I'm using my experience in hydrocarbons and biodiesel (FAME) analysis that I think is similar to wax ester.

Leoardo.

[Peter Apps]

Hi Leonardo

I work with complex samples with a wide boiling point range, so anything that improves resolution catches my interest. The effect that you claim will yield retarded elution of late eluting compounds - is there any evidence for longer relative retentions of heavier compounds.

Peter

[lynoguchi]

Peter,

Yes it could happen. For heavier compounds you can have tailing peaks.
But to minimize this, is possible to use high heating rates after a certain temperature for both, inlet and oven.

Leonardo.

[Peter Apps]

Peter,

Yes it could happen. For heavier compounds you can have tailing peaks.
But to minimize this, is possible to use high heating rates after a certain temperature for both, inlet and oven.

Leonardo.

Hi Leonardo

In the scenario you claim, retarded elution must happen - if slow departure of heavier compounds from the inlet due to slow vs flash heating improves separation as you claim, then it can only do it by making late-eluting compounds elute even later.

Consequently the test of whether slow inlet heating improves separations by delivering heavier compounds to the column later than light ones is whether the retentions of the heavier compounds are retarded relative to the light ones with slow heating compared to flash evaporation. "I could happen" is not evidence, I need to know whether it does happen.

I contend that if the inlet is delivering analytes to the columns at times that differ enough to affect the spearation then the overall quality of the chromatography will be compromized by band broadening. There will also probably be a problem with inconsistent retentions from run to run.

Peter

[lynoguchi]

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.