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This is a place to keep information about all forms of chromatography, and in particular high-resolution (gas) chromatography.

Carrier Gas Calculations Supplement for GC Connections March 2012

This is the supplemental information to How to Restore an Idle GC Column to Operating Condition that appears in the October, 2011 issue of LC/GC North America ^[1].

Relationship of pressure drop and average linear carrier gas velocity

Figure 1 in the article shows how the pressure drop required to achieve an average linear carrier gas velocity of 40 cm·s^-1 varies with column length and inner diameter at a fixed 120 °C column temperature.

Figure 1: Inlet pressure as a function of column length for a series of column inner diameters. Carrier gas: helium at 120 °C. Average linear velocity: 40 cm/s. Column outlet at atmospheric pressure.

The plots in Figure 1 were calculated from theoretical relationships between column dimensions and the operating parameters of velocity and pressure, as shwon below in Equations 1–4. These equations or related forms are used by GC electronic pneumatic controllers to relate pressure drop, Δp, and the average carrier gas linear velocity with column temperature and the carrier gas type. The pressure drop is equal to the difference between the inlet and outlet absolute pressures:

(1)

The pressure drop required to produce a specific average carrier gas linear velocity, , through a column with a certain carrier gas and temperature can be calculated from:

(2)

Here, d_c is the column inner diameter, L is the column length, η is the carrier gas viscosity at the column temperature, and j' is a pressure correction factor:

(3)

This factor, termed “j-prime,” adjusts for the influence of carrier gas compressibility on the flow of gas through the column, specifically its influence on the average velocity. Don't confuse it with the “j” factor which relates the average velocity to the outlet velocity on the basis of the volumetric expansion of gas as it transits the column from the inlet to the outlet pressure. The variable P is the pressure ratio of the column:

(4)

Since the required pressure drop is involved in two terms on the left-hand side of equation (2), one way it can be solved in specific cases is by iteration using a spreadsheet or other algorithmic tool.

Headspace GC Theory Supplement for GC Connections October 2011

The supplemental information to the "GC Connections" installment on Headspace GC that appears in the October, 2011 issue of LC/GC North America ^[2] has been moved to its own page.

Supplemental Vacuum Outlet Flow Theory for GC Connections January 2011

The supplementary material to Hydrogen Carrier Gas and Vacuum Compensation that appeared the January, 2011, "GC Connections" column in LC/GC North America.^[3] has been moved to its own page.

Tables of Chromatography Discussion Groups and Web Sites

The table of Discussion Groups originally was published on-line as "Get Involved!" on the Chromatography Online website as well as in the print version of LC/GC North America in the September 2009 issue^[4].

The tables have been moved to their own page.

References

1. ↑ John Hinshaw, LC/GC North America, 30 (3), in publication (2012).
2. ↑ John Hinshaw, LC/GC North America, 29 (10), 914–924 (2011).
3. ↑ John Hinshaw, LC/GC North America, 29 (1), (2011).
4. ↑ John Hinshaw, LC/GC North America, 27 (9), pp. 822–827 (2009).

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