Considerations for carrier gases in Gas Chromatography · © 2012 Sigma-Aldrich Co. All rights reserved. sigma-aldrich.com/analytical Considerations for carrier gases in Gas Chromatography

© 2012 Sigma-Aldrich Co. All rights reserved.

sigma-aldrich.com/analytical

Considerations for carrier gases in Gas Chromatography

Dr. Frank Michel

© 2012 Sigma-Aldrich Co. All rights reserved. 2


Carrier gases in GC

• Inert

• Free from impurities

– Hydrocarbons

– Water

– Oxygen

• No impact on partitioning between mobil and stationary phase


Ususal carrier gases in GC

• Nitrogen

• Helium

• Hydrogen

•Dr. Koni Grob: 90 % of GC users go for helium as carrier gas


5

Trivia about Helium

6%

7% 13%

20%26%

28%

MRT/NMR

AerospaceMilitary

Welding

SemiconductorFiber optics

* http://www.scientificamerican.com/blog/post.cfm?id=the-coming-shortage-of-helium-2010-06-30http://www.livescience.com/11137-phht-helium-prices-balloon-world-runs.html


Trivia about Helium

40

45

50

55

60

65

70

75

80

0%

2%

4%

6%

8%

10%

12%

14%

16%

18%

Price [$] Raw Helium, USA

Annual Price increase [%]

Source: Bureau of Land Managementwww.blm.gov


Trivia about Helium

•Prof. Dr. Robert C. Richardson: Helium sources will beexhausted in 25 years*

• Prices should be increased by factor of 20

• If possible: Substitution of Helium

• Recycling of Helium in closed systems

•Share in atmosphere: ~ 5 ppb

•Creation in billions of years by radioactive decay of uraniumand thorium

7

* http://www.scientificamerican.com/blog/post.cfm?id=the-coming-shortage-of-helium-201http://www.livescience.com/technology/helium-reserve-shortage-expensive-party-balloon


Safety considerations with nitrogen and helium

•Low security worries for nitrogen and helium.

• Gase under pressure

• Suffocation hazard possible, if the gases are released in short periodof time in small rooms


Safety considerations with hydrogen

•Explosive in concentration range of 4% to 74.2% (v/v).

•Ignition by

• Fast expansion from high pressure (cylinder)

• Spark

• Heat

•Enrichment of hydrogen in GC ofen with subsequent explosion?


Safety considerations with hydrogen

•Hydrogen is highly diffusive.

•EPCs (Electronic Pressure Control) do have an emergencyswitch-off (detection of a leakage)

•Hydrogen generators create only as much hydrogen as neededfor the GC and are switched off automatically in case of a leakage


Hydrogen generators

•Further gas generators:

• Zero-Air (for FID)

• Nitrogen (as carrier gas)


Hydrogen generators

•Generation of hydrogen by electrolysis

•Purity: 99,9999% at flow rates up to 1200 mL/min

• No variation from cylinder to cylinder

• For use as carrier gas or FID

• No instrument switch-off due to cylinder exchange

• Leak Control

• Less than 100 mL H2 in the lab


Cost of gas supplyDoes the investment of a gas generator pay off?

•Prerequisite: GC(s) run 24h on seven days per week

Mean Volume of a 40L gas cylinder

Cost/a =Flow (mL/min) * 1440 (min/d) * 365 (d/a) * CostCyl. (€)

9000 (L) * 1000 (mL/L)


Carrier gases in GC

•Most important parameter for carrier gases:

• Linear velocity (ū) or

• Flow rate (F)

•Impacts analysis time and chromatographic resolution


•Determination of flow rate by flowmeters or by injection of a not-retained compound

•

•Determination of linear velocity by injection of a not-retainedcompound

– r = column radius in cm

– L = column length in cm

– tm = Retention time of a not-retained peak in seconds

Flow rate (F) & linear velocity (ū)

•πr2L•F = tM

Lū =

tM





•Carrier gas velocity determines efficiency/performance

•Optimal velocity ic characteristic for each carrier gas

•Golay curves for determination of ideal carrier gas velocity


Efficiency in GC

With N = Number of theoretical platestr = Retention time of Peakwb1/2 = Peak width in half height


Golay curves for different carrier gases

H = L/N

•H = B/ū + C ū


20

Properties of different carrier gases

Carrier gas Viskosity, 50 OC

[kg/s m]

Diffusion of butane

100 OC [m2/s]

Hydrogen 9.4 6 x 10-6

Helium 20.8 5.5 x 10-6

Nitrogen 18.8 1.5 x 10-6

Ullmanns Enzyklopedia ofTechnical Chemistry, Vol. 5


Equation for retention time

•Relevant for isothermal separations

•With

•tR = Retention time

•L = Column length

•k = Retention factor

•ū = linear carrier gas velocity

•L(k+1)•tR =

ū


Equity-1 Isothermal, 25 cm/s Helium

0 10 20

Time (min)

k’(C13)=6.61

CE=85% N=119307

C13


Equity-1 Isothermal, 50 cm/s Helium

0 2 4 6 8

Time (min)

k’(C13)=6.61

CE=51% N=71035

C13


Supelcowax 10 Isothermal, 50 cm/s Hydrogen

1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

Time (min)

k’(C20)=6.03

CE=85% N=114344

C20


Supelcowax 10 Isothermal, 10 cm/s Hydrogen

0 10 20 30

Time (min)

k’(C20)=6.03

CE=50% N=70223

C20


Gas supply and purification

•Gas cylinders

• Highly pure gases quite expensive

• Regular exchange of cylinders

• Need for space

•In-house-supply

• Purity of gas getting to the GC?

•Gas generators


Way of the gas to the GC

•Pure gas from central supply = pure gas inGC?

•Potential sources of contamination:

• Leaks in tubings and connectors

• Non-suited material for tubings - diffusion

•Typical Impurities

• Moisture

• Oxygen

• Hydrocarbons

27

→→→→ Foul GC columns


Purification of carrier gases

Supelcarb-HC 120mL

Molsieve120mL OMI-2Gas Supelpure-O

120mL

Gas HC H2O O2Control/

Indication

High Capacity Purifier OMI-2Gas Supelcarb-HC 120mL

Gas Combined Purifier OMI-2


Purification of carrier gases

HC O2H2OO2/H2O/

Indication


Supply of several GCs


Summary

•Carrier gases move analytes through the column.

•Helium, hydrogem and nitrogen most often used.

•Nitrogen provides highest efficiency (only subtle differences tothe other gases), but run time is long.

•Helium provides good efficiency and short analysis times, but isexpensive (and will become even more expensive).

•Hydrogen provides shortest analysis times over a broad rangeof linear velocities.

•Purification of carrier gases essential for maintaining of high quality in GC.


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Considerations for carrier gases in Gas Chromatography · © 2012 Sigma-Aldrich Co. All rights reserved. sigma-aldrich.com/analytical Considerations for carrier gases in Gas Chromatography