November 25, 2020, 08:46:00 AM
Forum Rules: Read This Before Posting


Topic: A few questions about gas chromatography  (Read 9351 times)

0 Members and 1 Guest are viewing this topic.

Offline CrimpJiggler

  • Full Member
  • ****
  • Posts: 113
  • Mole Snacks: +5/-3
A few questions about gas chromatography
« on: December 29, 2011, 03:45:11 AM »
1.) I can understand why highly inert gases like N2 and He are used as carrier gases in GC but why is H2 used? Is H2 unreactive in the absence of oxygen or something?

2.) In my lecture notes, it states that when using a non polar capillary GC stationary phase such as squalane, compounds will elute in order of increasing boiling point. I don't really get it. Boiling point is directly proportional to polarity so shouldn't the higher boiling point compounds elute first since they don't interact with the non polar stationary phase as much? Also, the greater the number of atoms in the molecule, the higher the boiling point (due to greater London dispersion forces) so using aliphatic hydrocarbons as an example, wouldn't decane spend more time in the stationary phase than pentane seeing as it exhibits greater London dispersion forces?

Offline marquis

  • Full Member
  • ****
  • Posts: 336
  • Mole Snacks: +32/-3
Re: A few questions about gas chromatography
« Reply #1 on: December 29, 2011, 02:05:14 PM »
Hydrogen is usually considered the best carrier gas for use in GLC (gas liquid chromatography). The flammability leads to safety issues, which is why hydrogen is not used often.  Some manufacturers use pressure sensing technology to sense a leak and automatically shut the gas flow off in case of a hydrogen leak.  Helium is non-reactive, so it is the most common carrier gas.  Others like Argon, etc, are usually used only when some special condition requires it.

Some professionals will refer to GC as a "modified boiling point separation technique".  Usually, a GC method makes use of a temperature gradient method.  You start the run at a low temperature (say 40C) and ramp the temperature up over the course of the run (sometimes to as high as 350C).  When a method uses this kind of temperature gradient, the boiling point is the most important characteristic for separating the individual compounds. 

Sometimes, GC will be run in an isothermal mode (same temperature for the whole run).  Then the boiling point rules don't apply.

Hope this helps.

 

Offline Stepan

  • Chemist
  • Full Member
  • *
  • Posts: 358
  • Mole Snacks: +39/-4
  • Gender: Male
  • Air Chemistry Man
    • Supplier of air sampling equipment and services
Re: A few questions about gas chromatography
« Reply #2 on: December 31, 2011, 12:41:39 PM »
1.) I can understand why highly inert gases like N2 and He are used as carrier gases in GC but why is H2 used? Is H2 unreactive in the absence of oxygen or something?

2.) In my lecture notes, it states that when using a non polar capillary GC stationary phase such as squalane, compounds will elute in order of increasing boiling point. I don't really get it. Boiling point is directly proportional to polarity so shouldn't the higher boiling point compounds elute first since they don't interact with the non polar stationary phase as much? Also, the greater the number of atoms in the molecule, the higher the boiling point (due to greater London dispersion forces) so using aliphatic hydrocarbons as an example, wouldn't decane spend more time in the stationary phase than pentane seeing as it exhibits greater London dispersion forces?

Hydrogen can reduce some unsaturated compounds. The metal surface of the instrument will work as a catalyst. Also, many detectors are built to be used with inert gas as a mobile phase. In this setup using hydrogen would be in conflict with GC hardware.

The second question is too big to be answered on this forum. It usually discussed in the first or second chapter of any GC book.

Offline Train

  • Regular Member
  • ***
  • Posts: 68
  • Mole Snacks: +2/-0
Re: A few questions about gas chromatography
« Reply #3 on: December 31, 2011, 10:57:50 PM »
Viscosity of hydrogen < viscosity of helium < viscosity of nitrogen

When you add in factors of cost and safety, everything should become clear.
:)

Offline Stepan

  • Chemist
  • Full Member
  • *
  • Posts: 358
  • Mole Snacks: +39/-4
  • Gender: Male
  • Air Chemistry Man
    • Supplier of air sampling equipment and services
Re: A few questions about gas chromatography
« Reply #4 on: January 01, 2012, 04:50:22 AM »
Viscosity of hydrogen < viscosity of helium < viscosity of nitrogen

When you add in factors of cost and safety, everything should become clear.
:)

What is wrong with H2  viscosity? It is used in every GC anyway. Cost of Hydrogen is 60% less than Helium. Additional safety concern is insignificant, as H2 is used on all GC FID and PID anyway.

Offline Train

  • Regular Member
  • ***
  • Posts: 68
  • Mole Snacks: +2/-0
Re: A few questions about gas chromatography
« Reply #5 on: January 01, 2012, 02:11:59 PM »
What is wrong with H2  viscosity?

Hydrogen has the best viscosity.  It's not as widely used because of perceived safety concerns.  Nitrogen has the worst viscosity, but is (I think) much cheaper than helium.

Offline voidSetup

  • Full Member
  • ****
  • Posts: 112
  • Mole Snacks: +5/-1
  • Gender: Male
Re: A few questions about gas chromatography
« Reply #6 on: January 22, 2012, 10:47:01 AM »
Hydrogen is good because it  is much cheaper than helium (world supplies are dwindling), and if you look at the Van deemter curve, hydrogen is most efficient at a higher velocity than nitrogen or helium.  This is good when time is an issue and you want to reduce run times.  In terms of safety, a hydrogen generator with DI water can produce hydrogen as it is needed so there is never a buildup of the gas.


Offline JGK

  • Chemist
  • Full Member
  • *
  • Posts: 738
  • Mole Snacks: +66/-19
  • Gender: Male
Re: A few questions about gas chromatography
« Reply #7 on: January 23, 2012, 02:20:57 PM »
The use of hydrogen as a carrier gas is on the increase again, primarily due to the price of Helium rising quite dramatically and the shortage of the supply (our supplier was forced to reduce availability of replacement helium cylinders last year due to supply restrictions) will probably accelerate this.

Our lab estimates a cost saving of ~40% on gas costs if we were to switch over.
Experience is something you don't get until just after you need it.

Sponsored Links