Carrier Gas Selection for Capillary Gas Chromatography

T411126

www.sigma-aldrich.com

Len Sidisky, Greg Baney, Katherine Stenerson, and James L. Desorcie

Supelco, Div. of Sigma-Aldrich, Bellefonte, PA 16823 USA

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Abstract

Nitrogen, helium and hydrogen are the most commonly used carrier gases for capillary gas chromatographic analyses. We will compare the performance of these gases focusing on their affects on the speed of analysis, selectivity, resolution, sensitivity, and safety of use. Theoretical and practical examples will be presented.

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IntroductionCarrier Gases for GC

The carrier gas (mobile phase) for gas chromatography should be an inert gas that does not react with the sample components. The GC carrier gas should contribute minimally to the partitioning process. This differs from the mobile phase in liquid chromatography. In GC the carrier gas is simply stated as just a carrier to transport the vaporized solute molecules through the column during the partitioning process.

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Carrier Gases for GC (contd.)

Carrier gases are compressible gases that expand with increasingtemperature. This results in a change in the gas viscosity.The selection and linear velocity of the carrier gas will affect resolution and retention times. Carrier gases should be inert to the stationary phase and free of detectable contaminants.

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Efficiency Effects of Carrier Gases

Carrier gas linear velocity plays a significant role in the resulting efficiency of a chromatographic system. The optimal carrier gas linear velocity is characteristic for each gas. van Deemter curves and Golay Plots are used to demonstrate optimal carrier gas linear velocities. van Deemter plots are used for packed columns since the A term for eddy diffusion is present.Golay plots are used for capillary columns (open tubular) and the A term is dropped.

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van Deemter Plot

H

Average Linear Velocity

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Simplified van Deemter Equation

H=A + B/µ + cµHETP has also been used for H

A is the Eddy Diffusion termB is the Molecular Diffusion term

C is the Resistance to Mass Transfer Termµ is the carrier gas linear velocity

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Typical Golay Curves

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Rate of Band Broadening

Capillary Columns - Golay:

H 2Dg

2 k df

2

3 1 k) 2 Dliq

r2 1 6k 11k 2 24Dg 1 k

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Retention Time Effects

Analyses can be either isothermal or temperature programmed runs in GC. The carrier gas linear veloctiy can influence the overall time of analysis and the efficiency of the separations. The following equation demonstrates the affect of linear velocity on retention time.

tr = L (k+1)µ

wheretr = retention timeL = column lengthk = retention factor

µ = carrier gas linear velocity

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SUPELCOWAX™ 10 Isothermal 25 cm/sec. Helium Carrier

0 2 4 6 8 10 12 14 16Time (min)

k’(C20)=6.10

CE=86% N=119924

C20

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SUPELCOWAX 10 Isothermal 50 cm/sec. Helium Carrier

1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0Time (min)

k’(C20)=6.10

CE=43% N=60633

C20

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SUPELCOWAX 10 Isothermal 50 cm/sec. Hydrogen Carrier

1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0Time (min)

k’(C20)=6.03

CE=85% N=114344

C20

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SUPELCOWAX 10 Isothermal 10 cm/sec. Hydrogen Carrier

0 10 20 30Time (min)

k’(C20)=6.03

CE=50% N=70223

C20

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The four figures demonstrate the affect of carrier gas linear velocity on the time of analysis and the efficiency of the chromatography. Helium carrier gas is near optimum linear velocity at 25 cm/sec. as shown in the first figure. The analysis time is approximately 14 minutes and the % coating efficiency (CE) is 86%. When we double the helium carrier gas velocity to 50 cm/sec. we now decrease the analysis time to about 7 minutes and also see a significant reduction in the column efficiency to about 43%. This is due to operating the carrier gas on the far right side of the Golay plot. This demonstrates the mass transfer term affect on the column performance. A similar effect is demonstrated for the hydrogen carrier gas analysis. Hydrogen is optimum near 50 cm/sec. as shown in the third figure. When we lower the hydrogen carrier gas linear velocity to approximately 10 cm/sec. we see the corresponding increase in the retention time and a large drop off in the efficiency of the column as shown in the fourth chromatogram. This demonstrates the effect of operating on the far left side of the Golay plot where the molecular diffusion term is the major factor.

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Linear Velocity/Flow Control

Linear velocity or flow can either be run in a constant pressure or constant flow mode.This is important in temperature programmed analyses. In the constant pressure mode, the column head pressure is set at a temperature and the pressure remains constant throughout the analysis. This can result in linear velocity changes especially over a wide temperature programming range. In the constant flow mode, the column head pressure will change throughout a temperature programmed analysis in order to keep the carrier gas flow at a constant flow rate.

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Carrier Gas Viscosity

Carrier gas viscosity is a temperature dependent parameter.As temperature increases, the viscosity of the gas increases.When using a constant pressure mode for carrier gas and temperature programming, the viscosity of the gas will increase and the average linear velocity will decrease.

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Carrier Gas Viscosity (µP)

127.84282.45259.46240131.5290.43267.14260

135.15298.42274.81280138.81306.41282.49300

120.53266.48244.11200113.23250.50228.76160105.02234.53213.4112094.95210.57190.386091.30202.59182.714087.64194.60175.032083.99186.62167.360

HydrogenHeliumNitrogenTemperature °C

Adapted from Ettre & Hinshaw, Basic Relationships of Gas Chromatography, 1993, Advanstar.

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Carrier Gas Viscosity & Temperature

050

100150200250300350

0 50 100 150 200 250 300 350Temperature (°C)

Vis

cosi

ty (

µP

)

Helium

Nitrogen

Hydrogen

20

0

10

20

30

40

50

60

0 100 200 300 400Temperature (°C)

Line

ar V

elo

city

(cm

/sec

)

Hydrogen

Helium

Nitrogen

Carrier Gas Viscosity & Temperature (contd.)

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• Hydrogen Experimentally determined tR & µ• Equity®-1 • 40 °C tR = 1.025 minutes and ū = 48.78 cm/sec. • 300 °C tR = 1.553 minutes and ū = 32.20 cm/sec.

• Linear velocity (µ) change was 34.0% over a 260 ºC temperature window

Carrier Gas Viscosity & Temperature (contd.)

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Bacterial Acid Methyl Esters25 cm/sec. Hydrogen Equity-1

0 10 20Time (min)

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Bacterial Acid Methyl Esters50 cm/sec. Hydrogen Equity-1

0 2 4 6 8 10 12 14 16 18Time (min)

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Run Conditions

column: Equity-1, 30 m x 0.25 mm I.D. x 0.25 µmoven: 150 °C (4 min.), 8 °C/min. to 250 °C (10 min.)

inj.: 250 °Cdet.: FID, 250 °C

carrier gas: hydrogen as listedinjection: 1 µL, 100:1 split

liner: 4 mm I.D. cup stylesample: 10 mg/mL bacterial acid methyl ester standard in methyl caproate, Cat. No. 47080-U

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tR is inversely proportional to ū double ū, cut tR in half for an isothermal analysis.Retention time in temperature programmed analyses is primarily dependent on programming rate.Viscosity impacts the linear velocity of the carrier gas over a wide temperature programming range and can change peak elution patterns. Constant flow mode can be used to minimize the changes in carrier gas viscosity for temperature programmed analyses.

Linear Velocity & Retention Time

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Safety Concerns with Carrier Gases

Safety concerns with nitrogen and helium are minimal.Both are compressed gases and can cause asphyxiation if rapidly released in a small confined area. Hydrogen is combustible over a concentration range of 4% to 74.2% by volume.Combustion can occur due to rapid expansion of the gas from a high pressure cylinder.Hydrogen is a highly diffusive gas in air.Hydrogen generators and EPC typically have automatic built-in shut down devices when a leak is detected.

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Carrier gases in gas chromatography are used to move the solutes through the column. Helium, hydrogen and nitrogen are the most widely used gases.Nitrogen provides the best efficiency but is extremely slow.Helium provides good efficiency and analysis times but is an expensive choice for a carrier gas. Hydrogen provides the fastest analysis times over a broad linear velocity range. Temperature programming changes the viscosity of a carrier gas resulting in a decrease in linear velocity/flow over the programmed range when run in a constant pressure mode.Hydrogen is the best choice for capillary GC due to diffusivity and a broad working range as long as safety concerns and proper controls are in place.

Summary