Switching Purge Gas from He to.....

[mickygd]

Just wondering if there are any issues w/ Nitrogen? how about air? H2?

Which works best... believe it or not we're paying almost the same for N2 as He? ($15 per tank difference)

thanks

[chromatographer1]

Helium is a better carrier gas. It is more inert than nitrogen and analytes elute faster than with nitrogen. Helium has a wider range of flow for superior chromatography than does nitrogen.

Hydrogen is not as inert and some do not like to use it for Mass Spectroscopy. It has an even wider range of flow as a carrier gas for chromatography. Some have safety concerns with it.

Air is generally not used as a carrier gas although there is no reason why it could not be used for gas solid chromatography purposes, especially if a FID is the detector choice.

I hope I have given you the answers you were seeking. Your questions were not entirely clear.

best wishes,

Rod

[Alchemist5]

I also like to refer to the van Deemter plot for gas efficiencies. Depending on your pressure of the system once set up you can determine which gas would be best to use and possibly the cheapest.

[lmb]

Helium has a wider range of flow for superior chromatography than does nitrogen
[Hydrogen] has an even wider range of flow as a carrier gas for chromatography

Rod,

I agree that ABSOLUTE range of acceptable flows in THE SAME COLUM is the widest for hydrogen and the narrowest for nitrogen. However, there could be another take on the implications.

According to Golay equation for a column plate height, the loss in a column efficiency for any gas depends on the ratio of actual flow to optimal flow. In other words, the loss depends on RELATIVE (NOT ABSOLUTE) difference between actual flow and optimal flow. For example, if flow of any particular gas is twice as high compared to optimal flow of that gas then efficiency of the same column would be the same whether the gas is hydrogen, helium or nitrogen.

One can conclude that
1. It is the RELATIVE (NOT ABSOLUTE) range of acceptable flows that mostly affects a column optimization.
2. Because there are very few (if any) practical reasons to choose the relative acceptable range of flows differently for different gas, there are very few (if any) practical reasons to consider this factor at all in choosing the carrier gas.

Example.
Optimal flow in 0.25 mm column is estimated as
2.5 mL/min for hydrogen
2 mL/min for helium
0.63 mL/min for nitrogen
Suppose that, based on some set of considerations, helium flow of 1.5 mL/min to 3 mL/min is considered to be acceptable. This translates into the range of 1.9 mL/min to 3.8 mL/min for hydrogen, and 0.47 mL/min to 0.94 mL/min for nitrogen. Typically, it would be acceptable to choose the range of flows for each gas according to this list, and to scale the flow rate of each gas accordingly. As a result, the plate number in the column would be the same regardless of the gas.

All in all, it is reasonable to suggest that carrier gas should be chosen for its affordability, speed of analysis, safety, etc., but (as a rule) not for the range of its flow rates.

lmb

[chromatographer1]

lmb

What you stated would be true if the plots of HETP were the same for all three gases, BUT

they are not.

One can get the same or better chromatography using nitrogen as a carrier. But it is nowhere as easy as it is with hydrogen.

Sometimes mere numbers of plates are NOT the prime consideration, rather, it is the time the analysis requires to reach completion.

Time is money. Money is rare. Thus time is rare and that is my reason for not getting my work done on time. At least that is my story and I am holding to it.



best wishes,

Rod

[AICMM]

Mickygd,

Purge gas or carrier gas? Discussion takes a very different bent depending on your response.

Best regards.

[lmb]

Rod,

I agree 100% that hydrogen is the fastest carrier gas and should be used whenever possible.

However, that was not the subject of my previous posting where I tried to stress that there is no theoretical justification and practical need to consider the range of acceptable flow rates in selecting a gas. And I still believe that my explanation of the reasons for that was correct because the plots of plate height vs. normalized flow rate, F/Fopt (F is flow rate, Fopt is optimal flow rate for a given gas), are the same for all gases.

In your last posting, you brought up another issue.

One can get the same or better chromatography using nitrogen as a carrier [gas]

It is widely believed that, under optimal conditions, nitrogen can generate more plates than hydrogen and helium can in the same column. However, this perception contradicts to theory and is not supported by experimental evidence.

lmb