25
GC COLUMNS |
CAPILLARY COLUMNS
Select ing a GC Column
*
β
= r/2df (r=internal radius of tubing; df = phase film thickness)
Table III
Phase ratio (
β
) values for common column dimensions.*
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 length divided by the height equivalent to a theo-
retical plate (HETP). The effective theoretical plates are affected by how well the phase
has been coated onto the column walls and is measured 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 resolution power the column will have.
Capillary columns are made 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: the column length is under the square root function). However, under isothermal
conditions, it will double analysis time. In temperature-programmed analyses, retention
times are more dependent on temperature than column length, with a marginal
increase (approx. 10-20%) in analysis time upon doubling the column length.
Conclusion
A basic understanding of the resolution equation allows analysts to make more effective
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
also affect separations and should influence decisions on column internal diameter, film
thickness, and length. By considering these factors, analysts can simplify the column
selection process and increase lab productivity by optimizing separations.
Film thickness (µ) has a direct 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. High molecular weight compounds must be analyzed on thinner film
columns. This reduces the length of time the analytes stay in the column, and minimizes
bleed at required higher elution temperatures. Film thickness also affects the amount of
material that can be injected onto the column without overloading. A thicker film
column can be used for higher concentration samples, such as purity analysis.
Film thickness directly affects phase ratio (
β
), which is an important consideration when
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
common dimensions of columns. Similar values indicate similar elution for different IDs.
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Mar 2011
