25
GCCOLUMNS |
CAPILLARYCOLUMNS
Selecting a GC Column
*
β
= r/2df (r=internal radius of tubing; df= phase film thickness)
Table III
Phase ratio (
β
) values for commoncolumndimensions.*
Film Thickness (df) /
β
Value
Column ID 0.10µm 0.25µm 0.50µm 1.0µm 1.5µm 3.0µm 5.0µm
0.18mm
450
180
90
45
30
15
9
0.25mm
625
250
125
63
42
21
13
0.32mm
800
320
160
80
53
27
16
0.53mm
1325
530
265
128
88
43
27
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Efficiency,N
Column efficiency (N) is the column lengthdividedby theheight equivalent to a theo-
retical plate (HETP). The effective theoretical plates are affectedby howwell the phase
has been coated onto the columnwalls and ismeasured by how narrow the peaks are
when they are eluted at the end of the column. Therefore, the higher the column
efficiency (N), the better resolutionpower the columnwill have.
Capillary columns aremade in various lengths, typically in standard lengths of 10, 15,
30, 60, and 105 meters. Longer columns provide more resolving power, but increase
analysis time.Doubling the column length increases resolution by approximately 41%
(note: thecolumn length isunder the square root function).However,under isothermal
conditions, itwilldoubleanalysis time. In temperature-programmedanalyses, retention
times are more dependent on temperature than column length, with a marginal
increase (approx. 10-20%) in analysis timeupondoubling the column length.
Conclusion
Abasicunderstandingof the resolutionequationallows analysts tomakemoreeffective
column choices. Phase choice is influenced primarily by selectivity, which can be
approximated by considering phase and analyte structures, as well as by referencing
retention indices or existing applications. Column retention (capacity) and efficiency
alsoaffect separationsand should influencedecisionsoncolumn internaldiameter, film
thickness, and length. By considering these factors, analysts can simplify the column
selectionprocess and increase labproductivitybyoptimizing separations.
Film thickness (µ) has adirect effect on the retention and elution temperature for each
sample component. Extremely volatile compounds should be analyzed on thick-film
columns to increase the time the compounds spend in the stationary phase, allowing
them to separate.Highmolecularweight compoundsmust beanalyzedon thinner film
columns.This reduces the lengthof time theanalytes stay in thecolumn,andminimizes
bleedat requiredhigher elution temperatures.Film thickness alsoaffects theamount of
material that can be injected onto the column without overloading. A thicker film
column canbeused for higher concentration samples, such as purity analysis.
Film thicknessdirectlyaffectsphase ratio (
β
),which is an important considerationwhen
changing internal diameter.When internal diameter increases, film thickness (df)must
increase in order to provide similar resolution and retention. Table III shows values for
commondimensionsof columns.Similarvalues indicatesimilarelution fordifferent IDs.
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