Choice of Carrier Gas

Capillary-column efficiency is dependent on the carrier gas used, the length and inner diameter of the column, the retention factor of the particular solute selected for the calculation of the number of theoretical plates, and the film thickness of stationary phase. Profiles ofH versus u for three carrier gases with a thin-film capillary column are displayed in Figure 3.27. Although the lowest minimum and, therefore, the greatest efficiency are obtained with nitrogen, speed of analysis must be sacrificed, as is shown in Figure 3.28 (93). The increasing portion of the curve is steeper for nitrogen in Figure 3.27, which necessitates working at or near uopt; otherwise, loss in efficiency (and resolution) quickly results. On the other hand, if one is willing to accept a slight loss in the number of theoretical plates, a more favorable analysis time is possible with helium and hydrogen as carrier gases, becauseu_optoccurs at a higher linear velocity. Moreover, the mass transfer contribution or rising portion of a curve is less steep with helium or hydrogen, which permits working over a wider range of linear velocities without substantial sacrifice in resolution. This advantage becomes evident in comparing the capillary separation of the components in calmus oil with nitrogen and hydrogen as carrier gases in Figure 3.29.

TABLE 3.17 Column Efficiency as a Function of Inner Diameter and Retention Factor

Inner Diameter

(mm) k h_min

Maximum Plates per Meter,N

Effective Plates per Meter,

N_eff

1 0.061 16,393 4,098

2 0.073 13,697 6,027

5 0.084 11,905 6,667

0.10 10 0.090 11,111 9,222

20 0.093 10,752 9,784

50 0.095 10,526 10,105

1 0.153 6,536 1,634

2 0.182 5,495 2,442

5 0.210 4,762 3,307

0.25 10 0.224 4,464 3,689

20 0.231 4,329 3,925

50 0.236 4,237 4,073

1 0.196 5,102 1,276

2 0.232 4,310 1,896

5 0.269 3,717 2,082

0.32 10 0.286 3,497 2,903

20 0.296 3,378 3,074

50 0.302 3,311 3,179

1 0.325 3,076 769

2 0.384 2,604 1,146

5 0.445 2,247 1,258

0.53 10 0.474 2,110 1,751

20 0.490 2,041 1,857

50 0.500 2,000 1,920

In comparing these carrier gases, another benefit becomes apparent at linear velocities corresponding to equal values of plate height. With the lighter carrier gases solutes can be eluted at lower column temperatures during temperature programming with narrower band profiles, since higher linear velocities can be used. Thus, either helium or hydrogen is recommended over nitrogen and indeed these gases are used today as carrier gases for capillary gas chromatography. One advantage of using hydrogen is that plate number varies less for hydrogen than for helium as linear velocity increases. The use of hydrogen for any application in the laboratory always requires safety precautions in the event of a leak. Precautionary measures should be taken for the safe discharge of hydrogen from the split vent in the split-injection mode.

3.10.2.1 Measurement of Linear Velocity

The flowrate through a capillary column whose inner diameter is less than 0.53 mm is difficult to measure accurately and reproducibly by a conventional

FIGURE 3.27 Profiles of HETP versus linear velocity for the carrier gases: helium, hydrogen, and nitrogen; courtesy of Agilent Technologies.

FIGURE 3.28 Effect of carrier gas on separation at optimum linear velocities (repro- duced from Reference 93: D. W. Grant, in Capillary Gas Chromatography, copyright 1996, John Wiley & Sons Limited; reproduced with permission).

FIGURE 3.29 Chromatograms of the separation of calmus oil using (a) hydrogen as carrier gas, 4.2 mL/min at a programming rate of 4.0^◦C/min and (b) nitrogen as carrier gas, 2.0 mL/min programming rate of 1.6^◦C/min on a 40-m×0.3-mm-i.d. capillary column (0.12-µm film) (reproduced from Reference 97 and reprinted with permission from Dr.

Alfred Huethig Publishers).

soap-bubble flowmeter. Instead, the flow of carrier gas through a capillary col- umn is usually expressed as linear velocity rather than as a volumetric flowrate.

Linear velocity may be calculated by injecting a volatile, nonretained solute and noting its retention time tM (seconds). For a capillary column of length L in centimeters, we obtain

u(cm/s)= L

t_M (3.33)

For example, the linear velocity of carrier gas through a 30-m column where methane has a retention time of 2 min is 3000 cm/120 s or 25 cm/s. If desired, the volumetric flowrateF (mL/min) can be computed from the relationship

F (mL/min)=60πr²u (3.34) where r is the radius of the column in centimeters. An injection of methane is convenient to use with a FID to determine tM and/or a headspace injection

of methylene chloride and acetonitrile can be made with an ECD and NPD, respectively. Nitrogen and oxygen (air) may be used with a MS while ethylene or acetylene vapors can be injected with a PID. Recommended linear velocities and flowrates of helium and hydrogen for capillary columns of various inner diameters are listed in Table 3.18.

3.10.2.2 Effect of Carrier-Gas Viscosity on Linear Velocity

Chromatographic separations using capillary columns are achieved under constant pressure conditions, as opposed to packed columns, which are usually operated in a flow-controlled mode. The magnitude of the pressure drop across a capil- lary column necessary to produce a given linear velocity is a function of the particular carrier gas and length/inner diameter of the column. The relationship between viscosity and temperature for any gas is linear, as shown in Figure 3.30 for helium, hydrogen, and nitrogen. In gas chromatography, as column temper- ature increases, linear velocity decreases because of increased viscosity of the carrier gas. Thus, initially higher linear velocities are established for temperature- programmed analyses than for isothermal separations. If we compare columns of identical dimensions and operate them at the same inlet pressure and temperature, the linear velocity will be highest for hydrogen and lowest for helium. Therefore, whenever a change in the type of carrier gas is made in the laboratory, linear velocities should actually be measured and one should not reconnect the pressure regulator using the same delivery pressure.