Chromatography Columns 2013+ - page 81

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GCCOLUMNS | CAPILLARYCOLUMNS
Selecting a GC Column
Table I Retention indices forRestekphases
Phase Benzene Butanol Pentanone Nitropropane
Rtx-1 651 651 667 705
Rtx-5/Rtx-5MS 667 667 689 743
Rtx-20 711 704 740 820
Rtx-1301/Rtx-624 689 729 739 816
Rtx-35 746 733 773 867
Rtx-200 738 758 884 980
Rtx-50 778 769 813 921
Rtx-1701 721 778 784 881
Rtx-65TG 794 779 825 938
Rtx-225 847 937 958 958
Stabilwax 963 1158 998 1230
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Table II Typical characteristics for columnswith the samephase ratio, such
as0.10mm IDx0.10μmand0.18mm IDx0.18μm, etc.
Retention,k
Thecapacityof thecolumn relates tohowmuchmaterial canbe injectedontoacolumn
without adversely affecting peak shape. If the amount of a compound (mass) exceeds
the capacity of a column (WCOT), the peakwill front,which sometimes can look like
a“shark fin”.Thegoal is to select acolumnwith sufficient capacity such thatpeak shape
will not suffer. Peak symmetry is typically used to calculate the degree of sample over-
load. There are twoprimary column-relateddimensions that affect capacity, assuming
the proper column phase was selected: column internal diameter (ID) and phase film
thickness (µ).
When selecting column ID, consideration should include the type of injection, the
detector beingused, and the concentrationof sample (amount on-column).The injec-
tion technique is an important consideration because the ID of the columnmay need
to be selected based on whether a split, splitless, cool on-column injection, or other
sample transfer to the column is being used. The second consideration is the detector
andhowmuch flow it canoptimallyworkunder. For example, someMSdetectors can
onlyhandle column flow rates of up to 1.5mL/min.; therefore, a 0.53mm ID column,
which requireshigher flows forproper chromatography, isnot anoption for thisdetec-
tor. The third consideration is sample capacity of the column. If the concentration of
the sample exceeds the column capacity, loss of resolution, poor reproducibility, and
peak distortion will result. Table II shows several typical column characteristics for
various column IDs.
Column ID
Characteristic 0.10mm 0.15mm 0.18mm 0.25mm 0.32mm 0.53mm
Helium Flow
(@ 20cm/sec.) 0.16mL/min. 0.3mL/min. 0.3mL/min. 0.7mL/min. 1.2mL/min. 2.6mL/min.
Hydrogen Flow
(@ 40cm/sec.) 0.32mL/min. 0.6mL/min. 0.6mL/min. 1.4mL/min. 2.4mL/min. 5.2mL/min.
Sample Capacity
(max load per component) <10ng <40ng <50ng 50–100ng 400–500ng 1000–2000ng
Theoretical Plates/Meter 8000 4000 3500 3200 2500 1800
Table I lists the Kovats retention indices for the more common stationary phases.
Assigning a retention index to eachprobe listedprovides a basis for comparing several
stationary phases and their relative retention to one another for a set of molecular
probes. For example,whenKovats indices are identical on two columnphases, then the
resulting separations will be identical. If, however, a Kovats value of one probe varies
significantly from the value on another phase for the same probe, then the resulting
compound elution order will differ. Thus, the Kovats indices are useful for comparing
column selectivity for different types of compounds amongdifferent phases.
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