Specialty SEC Columns
We can assume that the lowest-particle-size packing within each designation in Table II will produce the most efficient columns.
In addition, the smallest- and largest-pore-size packings will be especially useful for small molecules and oligomers, and
for ultrahigh-molecular-weight polymers, respectively. For a given sample molecular weight range and chemical composition,
this table should prove useful for selecting appropriate single-pore columns or for putting together a useful column set.
Available column dimensions are not included in these tables, because most column sizes are readily obtainable or can be custom
packed. Column suppliers, in general, offer a broad range of column lengths and inner diameters for high-speed and high-throughput
SEC separations, including large-internal-diameter columns (>20 mm) for semipreparative and preparative separations.
Table IV: Characteristics of specialty high-performance SEC packings, continued*
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Many specialty SEC columns are now available from vendors, as listed in Tables III and IV. Columns capable of operating at
220 °C are now available from Agilent; silica packings also can be used at elevated temperatures because of their innate stability.
Several suppliers now have packings that are custom polymerized to prevent particle fines from either forming or sloughing
off the column, giving spurious spikes if light-scattering detection is used. Columns can also be conditioned for light-scattering
applications by extensive elution with the mobile phase of choice.
There are some interesting packings termed universal or multimode that can be used for SEC, as well as for enthalpic HPLC. In principle, one can first analyze a sample using SEC by eluting
with, for example, a nonpolar mobile phase under isocratic conditions. For the second stage of analysis, gradient elution
might be used with a different set of mobile phases to elute and separate adsorbed components left on the column from the
SEC run.
Polymer Standards Service has an interesting asymmetric column configuration that has dimensions of 5 cm × 2 cm i.d. for high
efficiency separations of only a few minutes (see Figure 6).
Figure 6: High-temperature, high-speed, high-flow rate SEC of polyethylene samples in under 3 min. A 50 mm ×20 mm polyolefin
mixed-bed column (10 µm) was used at a flow rate of 5 mL/min, with trichlorobenzene as the mobile phase at 145 °C with a Waters
150 chromatograph. Adapted from Polymer Standards Service GmbH, Germany.
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Lastly, as listed in Table IV, specialty SEC columns have been developed for determining 1. ultrahigh-molecular-weight synthetic
polymers under low-shear-rate conditions to avoid polymer shear degradation; 2. the MWD of cationic and anionic polyelectrolytes;
3. the MWD of ds-DNA and high-molecular-weight RNA and DNA fragments; and 4. the size separation of DNA-wrapped carbon nanotubes.
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