Carrier gas viscosity

Increasing the speed of analysis has always been an important goal for GC separations. All other parameters being equal, the time of GC separations can be decreased in a number of ways (1) shorten the column (2) increase the carrier gas flow rate (3) reduce the column film thickness (4) reduce the carrier gas viscosity (5) increase the column diameter and/or (6) heat the column more quickly. The trade-off for increased speed, however, is reduced sample capacity, higher detection limits, and/or worse separation efficiency. 

The carrier gas viscosity is given as r, L the column length, pQ the outlet pressure, P the ratio of inlet pressure to outlet pressure and dc the column diameter. Analysts should take care to be sure that the methods used for determining the flow rate are consistent from in-strument-to-instrument and from method-to-method. Otherwise it will be difficult to compare any data that have the flow rate, gas hold-up time or linear carrier gas velocity as a component. 

Column gas velocity is dependent upon the column length, carrier gas viscosity, pressure drop through the column and the column permeability. Column permeability is expressed by the specific permeability coefficient, B0, which may be calculated by the following expression. 

Figure 2.2 Efficiency as a function of the linear velocity of the carrier gas — viscosities of carrier gases. These are typical Van Deemter curves showing that hydrogen, of the three gases studied under the same conditions, allows a faster separation, conveying a greater flexibility in terms of the flow rate, which is very useful for temperature programming. Note the increase in the viscosity of these gases with the temperature and that helium is more viscous than nitrogen at the same temperature.

The test is performed under optimized conditions of carrier gas flow and temperature program rates, which are adjusted for column length and carrier gas viscosity. Table 2.16. To obtain a correct value for the gas hold-up time for thick-film columns (df > 0.7 p,m) it should be measured at 100°C (methane is considerably retained at... 

In GCxGC, the operates in isothermal mode, and estimation of the retention at different temperatures should be easy, although the requirement of an accurate value for is a problem, as described in Section 3.3. But the column usually operates in programmed temperature mode. The continuous changes in temperature and in carrier gas viscosity present additional problems. Several approaches have been followed in order to estimate GCxGC retention. 

Effect of Carrier-Gas Viscosity on Linear Velocity Phase Ratio... 

FIGURE 3.30 Effect of temperature on carrier-gas viscosity. (Data for curves generated from viscosity-temperature relationships in Reference 83.)... 

The unretained peak time can be related to the pressure drop, column dimensions, and carrier-gas viscosity by combining the—hopefully—familiar relationship between column length, unretained peak time, and average carrier-gas linear velocity tM = Liu with liquation 4.10 to yield... 

The carrier-gas viscosity ri increases with rising temperatures according to the following relationship ... 

Viscosity. Carrier gas viscosity increases with increasing temperature. 

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