Carrier Gas Selection for Capillary Gas Chromatography

Transcription

1 Carrier Gas Selection for Capillary Gas Chromatography Len Sidisky, Greg Baney, Katherine Stenerson, and James L. Desorcie Supelco, Div. of Sigma-Aldrich, Bellefonte, PA USA T

2 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. 2

3 Introduction Carrier 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. 3

4 Carrier Gases for GC (contd.) Carrier gases are compressible gases that expand with increasing temperature. 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. 4

5 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. 5

6 van Deemter Plot H Average Linear Velocity 6

7 Simplified van Deemter Equation H=A + B/µ + cµ HETP has also been used for H A is the Eddy Diffusion term B is the Molecular Diffusion term C is the Resistance to Mass Transfer Term µ is the carrier gas linear velocity 7

8 Typical Golay Curves 8

9 Rate of Band Broadening Capillary Columns - Golay: H 2D g 2kd f 2 31 k) 2 Dliq r 2 1 6k 11k 2 24D g 1 k 9

10 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. t r = L (k+1) µ where t r = retention time L = column length k = retention factor µ = carrier gas linear velocity 10

11 SUPELCOWAX 10 Isothermal 25 cm/sec. Helium Carrier k (C20)=6.10 CE=86% N= C Time (min) 11

12 SUPELCOWAX 10 Isothermal 50 cm/sec. Helium Carrier k (C20)=6.10 CE=43% N=60633 C Time (min) 12

13 SUPELCOWAX 10 Isothermal 50 cm/sec. Hydrogen Carrier k (C20)=6.03 CE=85% N= C Time (min) 13

14 SUPELCOWAX 10 Isothermal 10 cm/sec. Hydrogen Carrier k (C20)=6.03 CE=50% N=70223 C Time (min) 14

15 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. 15

16 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. 16

17 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. 17

18 Carrier Gas Viscosity (µp) Temperature C Nitrogen Helium Hydrogen Adapted from Ettre & Hinshaw, Basic Relationships of Gas Chromatography, 1993, Advanstar. 18

19 Carrier Gas Viscosity & Temperature Viscosity (µp) Helium Nitrogen Hydrogen Temperature ( C) 19

20 Carrier Gas Viscosity & Temperature (contd.) Linear Velocity (cm/sec) Hydrogen Helium Nitrogen Temperature ( C) 20

21 Carrier Gas Viscosity & Temperature (contd.) Hydrogen Experimentally determined t R & µ Equity C t R = minutes and ū = cm/sec. 300 C t R = minutes and ū = cm/sec. Linear velocity (µ) change was 34.0% over a 260 ºC temperature window 21

22 Bacterial Acid Methyl Esters 25 cm/sec. Hydrogen Equity Time (min) 22

23 Bacterial Acid Methyl Esters 50 cm/sec. Hydrogen Equity Time (min) 23

24 Run Conditions column: Equity-1, 30 m x 0.25 mm I.D. x 0.25 µm oven: 150 C (4 min.), 8 C/min. to 250 C (10 min.) inj.: 250 C det.: FID, 250 C carrier gas: hydrogen as listed injection: 1 µl, 100:1 split liner: 4 mm I.D. cup style sample: 10 mg/ml bacterial acid methyl ester standard in methyl caproate, Cat. No U 24

25 Linear Velocity & Retention Time t R is inversely proportional to ū double ū, cut t R 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. 25

26 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. 26

27 Summary 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. 27