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"The service provided by Viscotek representatives is unmatched by any other in the industry... A wonderful addition to the polymer community."
- E.F, Ph.D., Academic Institution
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Nanoparticle and Protein Sizing
Viscotek is proud to introduce its range of Dynamic Light Scattering Detectors featuring rapid, accurate and sensitive sizing for proteins, biomolecules, nanoparticles & polymers and a pair of outstanding technologies unique to Viscotek.
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"The GPCmax and Triple Detector System is state-of-the-art while remaining affordable and easy to use."
- M. G., Ph.D., Academic Institution
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Application Notes
Our database of GPC/SEC application notes explain the use of concentration, viscometer and light scattering detectors to obtain a distribution of absolute molecular weight, size and intrinsic viscosity, as well as information on conformation, aggregation, branching and copolymer composition. |
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GPC Theory: Triple Detection |
Triple detection sets the standard for current GPC technology and has become the preferred method for characterizing natural and synthetic polymers and proteins. It employs a concentration detector,
viscometer and light scattering detector acting in concert, with each detector providing complementary but different information:
- The light scattering detector provides a direct measurement of molecular weight and eliminates the need for column calibration.
- The viscometer detector provides a direct measurement of intrinsic viscosity or molecular density, and allows the determination of molecular size, conformation and structure.
- Concentration is measured with an RI or UV, and is necessary for the determination of both molecular weight and intrinsic viscosity.
Triple detection is able to accomplish all that Universal
Calibration does - and more – without the need for lengthy column calibration. Simply running a single narrow standard will enable calibration of all three detectors as well as performing corrections for inter-detector shift and inter-detector peak broadening effects.
Furthermore, triple detection has a unique advantage in that it provides absolute molecular weight, molecular size (to less than 1 nm) and intrinsic viscosity, as well as information on conformation, branching
and aggregation in a single, simple GPC experiment. It is the only detection technique capable of giving this tremendously
rich collection of data on-line in a continuous flow mode, and perhaps can best
be visualized graphically.
 Molecular Weight data can be combined with information
such as Branching and Hydrodynamic Radius to give a clear indication of the structure
of the sample across its Molecular Weight distribution.
As previously mentioned, in Triple Detection molecular weight is determined directly
by the light scattering detector.
In order to determine Molecular Weight, all commercial
light scattering detectors utilize the Rayleigh Equation, which simply states that the intensity of the scattered light is equal to an optical constant times the concentration times the molecular weight.
According to the Rayleigh Equation, the intensity of the scattered light must be
measured at zero angle. Since such a measurement would require you to look directly
into the incident beam, which is not possible, the intensity must be estimated at zero angle and there are essentially three different ways to do this:
- Measure the scattering at a very low angle (LALS). At angles of 10 degrees or less, the error is minimized and can be ignored.
- Measure the scattering at multiple angles and extrapolate to zero (MALS). This is accomplished by plotting the light scattering signal as a function of angle.
- Measure the scattering at 90 degrees and correct to zero using viscosity data (RALS).
All three methods will provide accurate molecular weight information for molecules
under 10 nm where there is no angular dissymmetry, which refers to the tendency
of molecules to scatter light with different intensity at different angles with
increasing molecular size. However, for larger molecules which are over 10 nm in
size, LALS is the only method for determination of molecular weight without extrapolation
or correction. Because it requires no extrapolation or correction.
LALS is the simples and most theoretically pure method of determining molecular
weight. In the LALS approach, the intensity of the scattered light is measured at
the lowest possible angle (7°), eliminating any major errors.
Historically, LALS detectors have suffered from poor signal to noise because of
"spikes" due to particulates, making it difficult to measure low molecular weight
macromolecules. However, increasing purity in solvents, the ubiquitous use of filters,
and advances in GPC/SEC column technology and LC pumps have effectively eliminated
the presence of these particulates in most samples. In addition, current GPC/SEC
software programs can employ de-spiking algorithms to reduce the data impact of
any particulates that might be present.
Perhaps most importantly, there have been recent innovations in the detector technology
itself. Current patent-pending LALS designs from Viscotek are now available which
effectively eliminate the noise historically associated with low angle measurements,
creating a light scattering detector which can be employed in all applications to
determine an absolute molecular weight without extrapolation or correction.
All three techniques also suffer from the inherent limitations of light scattering detectors:
- Detectability (signal to noise) problems with low molecular weight polymers, polymer/solvent systems with poor dn/dc and broad distributions with low molecular weight tails.
- Interference problems due to copolymers with varying dn/dc, chiral polymers that depolarize the incident beam, overlapping aggregates that are unresolved and absorbing or fluorescent polymers.
 In the cases like the above where light scattering cannot be employed, using the viscometer found in the Viscotek
triple detector system, along with the Universal Calibration
technique, is the only alternative for determining molecular weight.
Universal Calibration is a column calibration method of determining molecular weight
distribution that does not require the standards and samples to have identical structures.
The viscometer detector is also used in triple detection for measuring molecular
size and intrinsic viscosity and generating information on conformation, aggregation
and branching.
 A common addition to the Triple detector system is the UV/VIS detector which acts
as a supplemental concentration detector. This Tetra Detector System
yields another
level of detail; since we have two concentration detectors, it is possible to determine
Molecular Weights for co-polymers, in addition to being able to quantify the amounts
of the two co-monomers present across the distribution. Furthermore, a UV detector
enables us to calculate Molecular Weight when neither the concentration nor the
dn/dc is known. This is often the case when dealing with proteins. With knowledge
of the protein's extinction co-efficient it is possible to determine it's concentration
via the UV detector. Now that the concentration is known, it's possible to determine
the dn/dc from the RI detector, and hence all their parameters are available to
determine Molecular Weight.
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