carrier gas

[lmb]

To thohry,

1. Yes, in Golay equation for capillary columns (van Deemter is for packed columns), longitudinal diffusion contributes more than something into the HETP. It greatly affects the value of HETP at a given flow, and it totally controls the optimal flow at which HETP is the smallest. However, the diffusion does not control the value of the smallest HETP. It looks like we reached the end of the road with this topic.

2. Yes, there is a strong reason to use speed-optimizing flow rate (SOF) that is about 40% (square root of two) higher than efficiency-optimizing flow rate (EOF). For ANY carrier gas, HETP at SOF is about 6% higher than at EOF. As a result, resolution of ALL peak pairs at SOF is 3% lower than it is at EOF. I am not sure if this difference in resolution is practically measurable. More importantly,

THE DIFFERENCE IS THE SAME FOR ALL GASES

As I mentioned in my last posting, item 2 is a different subject. It has already been addressed in an earlier discussion. Please check http://www.sepsci.com/chromforum/viewto ... 161edb7479.

[thohry]

To thohry,

1. Yes, in Golay equation for capillary columns (van Deemter is for packed columns), longitudinal diffusion contributes more than something into the HETP. It greatly affects the value of HETP at a given flow, and it totally controls the optimal flow at which HETP is the smallest. However, the diffusion does not control the value of the smallest HETP. It looks like we reached the end of the road with this topic.

2. Yes, there is a strong reason to use speed-optimizing flow rate (SOF) that is about 40% (square root of two) higher than efficiency-optimizing flow rate (EOF). For ANY carrier gas, HETP at SOF is about 6% higher than at EOF. As a result, resolution of ALL peak pairs at SOF is 3% lower than it is at EOF. I am not sure if this difference in resolution is practically measurable. More importantly,

THE DIFFERENCE IS THE SAME FOR ALL GASES

As I mentioned in my last posting, item 2 is a different subject. It has already been addressed in an earlier discussion. Please check http://www.sepsci.com/chromforum/viewto ... 161edb7479.

1. In a number of documents I've read (both theoretical and pratical) the smallest HEPT achieved belongs to N2. I myself have tested it. We have measured the van deemter curve and got the opt HEPT of N2 rather similar to other gases (rarely get it lower). The thing here may be the narrowness of the peak and we used rather large increments of flowrate and we did not get the true opt HEPT. This also explains the difficulty of getting the opt velocity in case of N2.

2. You are right that we often operate at higher speeds of optimum. However, in this aspect, N2 has disadvantage: at the same increment of velocity, efficiency is lost more in the case of N2, let alone N2 posseses very much slower opt. flowrate compared to He or H2.

[HW Mueller]

Since my active work with GC ended a few yeaars ago I thought a brush-up is in order. Here is a short summary:

I can not say it better than

Konrad Grob, Kantonales Laboratory, Zurich, in

http://www.restek.com/advantage/adv_1998_02_koni.asp

Here a quote from this link:
“Nitrogen cannot seriously be considered as a carrier gas option, because diffusion speeds of the solute molecules are roughly four times lower than in hydrogen or helium, rendering the separation process exceedingly slow. Helium is the best alternative if hydrogen cannot be used, but hydrogen enables faster chromatography whenever inlet pressure exceeds roughly 0.7 bar, with a rapidly expanding difference when the required inlet pressure increases. Hydrogen is almost a must for high-temperature work such as triglyceride analysis, and analysis with long columns such as fatty acid methyl ester analysis on 100m columns.â€

[Russ]

Since fools rush in where wise men fear to tread, here I go: Somewhere in the deep recesses of what is left of my brain I seem to recall that there was the possibility of some analytes reacting with hydrogen carrier. Is that correct, or should I go back to my padded room and contemplate my navel? I suppose if you are simply using a concentration detector, and any reaction is quatitative, it doesn't matter as you would simply be performing a derivatization on the analyte.

[GasMan]

You are correct Russ, that some analytes can react with the hydrogen carrier gas. I would not recommend using hydrogen as carrier gas if you are looking at low concentrations of oxygen. I have seen cases where the oxygen will react with the hydrogen on hot metal surfaces, given low results for oxygen.

Most of this discussion has centered around 'efficiency' of the column with different carrier gases, but one must also take into account the type of detector and the analytes.

Gasman

[thohry]

Thanks Russ and Gasman. That's very good information. Sometimes we just consider H2 as an innert gas but infact it's not.

[gcguy]

I have also seen unsaturated hydrocarbons undergoing partial hydrogenation using H2 as the carrier. Two peaks with hydrogen, one with helium.

[Russ]

I'm right about something?
Please don't tell anyone where I work about this. I have a reputation to protect.

[thohry]

I'm right about something?
Please don't tell anyone where I work about this. I have a reputation to protect.

I thought you said somethings about the reactiveness of H2 when being used as carrier gas. If you think it's not true, I am wrong.

[AICMM]

I would not advise hydrogen as a carrier for high temp simulated distillation or triglyceride analysis since the upper temperatures in combination with hydrogen would be very likely to cause unwanted side reactions

Best regards.