146 www.restek.com
800-356-1688 or 814-353-1300
ume increases, lower efficiency is experienced as band broadening increases. Typically,
column IDs less than 3.0 mm, considered narrow bore columns, require systems with
minimized extra column volume. Table III defines the classification of columns accord-
ing to internal diameter or bore. Another contributor to overall system volume and
column ID choice is the system delay volume. Delay volume is the volume contained
between the pumps and the column, often including the mixing chamber and injection
valve. Delay volume is especially significant during gradient analysis. Narrow bore
columns often require lower flow rates, and these lower flow rates will not sweep the
delay volume in high volume systems quickly. This extends analysis time and creates an
increased gradient lag time. For fast gradient analysis and LC/MS, narrow bore columns
and systems with low extra column volume are recommended.
Physical Characteristics
Silica Type
The physical characteristics of the support material can be selected to control retention
and peak shape. The base silica, commonly porous spherical particles, used in the man-
ufacturing of the column can first be selected by type, namely Type A, Type B, or Base
Deactivated. Type B silica is typically higher in purity and provides limited silanol activ-
ity. When analyzing basic compounds, especially without the use of mobile phase mod-
ifiers, Type B silica is recommended for more symmetric peak shape. Type A and Base
Deactivated silica are recommended for acidic, neutral, and slightly basic compounds.
Another criterion for choosing a column line is the porosity of the silica. The pore size,
or pore diameter, which is commonly expressed in Å, is the average diameter of the
silica pores. This relates inversely to available surface area. Smaller pore volumes create
a larger surface area in a given particle and, therefore, can be used to control the amount
of stationary phase bonded to the particle.
The carbon load, or % carbon in the packing material, is the measure of the amount,
or load, of stationary phase. Carbon load directly affects retention. Higher carbon loads
typically result in higher retention characteristics. Figure 1 illustrates the relative reten-
tion capacities of commercially available columns for hydrophobic compounds.
Allure® columns were designed for maximum retention of small molecules by utiliz-
ing high carbon load, surface area, and ligand density. In contrast, Viva columns,
considered wide pore, have a large pore diameter and are used for the analysis of
larger molecules as commonly seen in biological separations. Table IV summarizes the
physical characteristics and recommended uses for Restek column lines.
Silica columns commonly have a temperature limit of 80 °C. Increased temperature can
be used to decrease mobile phase viscosity and, therefore, lower the back pressure of a
Table IV
Physical characteristics and recommended uses for Restek columns,
based on silica lines.
Table III
Common classifications
for LC columns by internal diameter.
Classification
Internal Diameter
Capillary <1.0 mm ID
Micro bore
1.0 mm ID
Narrow bore
2.1-3.0 mm ID
Standard bore 3.2-4.6 mm ID
Semi-prep
10 to 21.2 mm ID
Prep
30 to 50 mm ID
Column
Pore Surface Carbon
Line
Size Area Load Range*
(Å)
(m
2
/g)
(%) Usage
Allure
60
450 12–27 Very high retention (highest retention available)
High purity
5 µm particle size only
Ultra II
100 300 11–19 High retention
High purity
Full range particle size - 1.9, 2.2, 3 and 5 µm for UHPLC and HPLC
Ultra
100
300
2–20 High retention
High purity
3 and 5 µm particle size only
Pinnacle II 110
180
2–13 Moderate retention
Acidic Type A (not for RP analyses of bases)
3 and 5 µm particle size only
Pinnacle DB 140 150
4–11 Moderate retention
Base deactivated silica
1.9, 3 and 5 µm particle sizes
Viva
300
100
3.5–9 Low retention
Wide-pore silica for biological separations
Table II
Optimal flow rates for
various particle diameters and
column internal diameters.
Column ID
Optimal flow rate
(mm)
(mL/min.)
1.9 µm dp 2.2 µm dp 3µm dp 5 µm dp
4.6
1.50
1.00
3.2
0.73
0.50
3.0
1.12
1.00
0.65
0.40
2.1
0.55
0.47
0.31
0.20
1.0
0.07
0.05
*Ranges are based on phases available for each silica line. See column product listings for more specific
information.
Website : www.chromtech.net.au E-mail : info@chromtech.net.au TelNo : 03 9762 2034 . . . in AUSTRALIA Mar