14
www.restek.comGC COLUMNS
Selecting a GC Column
Checking for leaks, using a Restek® electronic
leak detector, is an easy way to protect your
instrument and column from damage.
All Restek columns have published minimum and maximum operating
temperatures that establish the working range for the stationary phase. Note
that these ranges vary with the thickness of the coating.
What Do the Temperature Limits Mean?
Film thickness also directly affects phase ratio (β), which must be accounted for when
changing to a column with a different inner diameter. When inner diameter increases,
film thickness (d
f
) must also increase in order to provide comparable resolution and
retention. Table III shows β values for common column dimensions; similar values
indicate similar separations on different ID columns.
Efficiency (N)
Column efficiency (N) is the column length divided by the height equivalent of a theo-
retical plate (HETP). The effective number of theoretical plates is affected by how well
the phase has been coated onto the column walls, and it is measured by how narrow
the peaks are when they elute out of the column. Higher column efficiency (N) results
in greater resolution between peaks. Inner diameter also influences efficiency; a simple
rule of thumb is the smaller the column ID, the more efficient the column.
Capillary columns are made in various lengths, typically 10, 15, 30, 60, and 105 meters.
Longer columns provide more resolving power, but will also increase analysis time and
cost more. When column length is doubled, analysis time will increase by as much as
a factor of two. However, doubling the column length increases resolution by only ap-
proximately 40% since the column length term is under the square root function in the
resolution equation. When selecting column length, the increase in resolution obtained
in a longer column must be weighed against the increase in cost and analysis time.
Conclusion
A basic understanding of the resolution equation allows analysts to make more effec-
tive column choices. Phase choice should be influenced primarily by separation fac-
tor, which can be approximated by considering the structures of both the phase and
the analyte, as well as by referencing retention indices or existing applications. Reten-
tion factor and efficiency also affect peak separations and should be considered when
choosing column inner diameter, film thickness, and length. By better understanding
these factors, analysts can simplify the column selection process, optimize separations,
and increase lab productivity.
Table III:
Phase Ratio (β) Values for Common Column Dimensions*
Film Thickness (d
f
) /
β
Value
Column ID 0.10 µm 0.25 µm 0.50 µm 1.0 µm 1.5 µm 3.0 µm 5.0 µm
0.18 mm
450
180
90
45
30
15
9
0.25 mm
625
250
125
63
42
21
13
0.32 mm
800
320
160
80
53
27
16
0.53 mm
1325
530
265
128
88
43
27
*
β
= r/2d
f
(r=internal radius of tubing; d
f
= phase film thickness)
Rxi®-5Sil MS Columns
(fused silica)
ID d
f
(µm)
temp. limits
0.25 mm 0.25 -60 to 320/350 °C
0.32 mm 0.50 -60 to 320/350 °C
0.53 mm 1.50 -60 to 320/330 °C
The second temperature is the
maximum
temperature-programmed operating tempera-
ture
, the temperature to which the column can
be heated for short periods of time (i.e., during a
temperature-programmed analysis). If only one
temperature is listed, it is both the isothermal and
the maximum temperature.
Many phases list two maximum operating tempera
tures. The first temperature is the
maximum isother
mal operating temperature
. This is the temperature
to which the columns are guaranteed to meet the
minimum bleed specification (i.e., lowest bleed level).
The
minimum operating temperature
de-
fines the lowest usable temperature before
the stationary phase solidifies. Operating the
column below the minimum temperature will
not harm the phase, but poor peak shape and
other chromatography problems may occur.