Extra Column Dispersion - Dispersion in Contemporary GC Columns > Page 6


Employing equation (1) the volume standard deviation for three popular, contemporary, LC columns eluting solutes of different (k') values can be calculated. The type of packing need not be defined and may be silica, bonded silica or polymeric in character, but all will be assumed to be spherical in form. In fact, the particle diameter of the stationary phase controls the magnitude of column dispersion and not its chemical character. A value of 0.6 is taken for (e) which is generally accepted (2). The properties of the three columns are shown in table 1 including the peak volume, (4sv), which has more significance to the practicing chromatographer

It is seen that the dead volume peak widths are very narrow (assessed as (4sv(k'=0)) and range from about 5 ml for the peak from the microbore column to about 15 ml for the 3 mm I.D. and 4.6 mm I.D. columns.


Dispersion in Contemporary GC Columns

The same mathematical arguments apply to GC columns as to LC columns except that, as the mobile phase is compressible, the pressure correction factor must be applied.


Consequently, for a GC packed column, from equation (1)


In addition, for a capillary column, which has no packing, the column volume will be . where (rc) is the radius of the capillary column. Furthermore, the approximate value of (Hmin.) (see Dispersion in Chromatography Columns ) will be given by,

It will be seen in due course that extra column dispersion is not nearly so much a problem in GC as it is in LC. There are a number of reasons for this, among which are the high diffusivities of solutes in a gas relative to those in a liquid; the mass sensitivity of the flame ionization detector (FID) which is the most universally used detector in GC (i.e. the detector responds to the mass of solute passing through it per unit time and not to the solute concentration); and the split injection system which is commonly used to place the sample on a capillary column in GC. These points will be discussed as they arise during the course of this book.