A fully loaded OMNISEC GPC/SEC system includes four different detectors: Refractive index (RI), UV-Vis photodiode array (PDA), right angle + low angle light scattering, and viscometer, plus the potential for an additional light scattering detector in the form of a SEC-MALS 20. As discussed in a previous post and illustrated below in the multi-detector pyramid, these detectors respond to different characteristics of your sample. Moreover, they can be combined to calculate a variety of data, depending on which detectors are present. In this post, I’ll go through each grouping and discuss what data is available with various GPC/SEC detector combinations.
Single detector systems (RI or UV)
Data available: relative molecular weight moments
These systems are generally RI-only or UV-only systems because those detectors respond directly to a sample’s concentration. These detectors (often called “concentration detectors”) offer the relative concentration of each data slice within a sample’s distribution. A conventional calibration curve generated from a series of standards assigns each data slice a molecular weight. From that, you can obtain the relative molecular weight moments and dispersity of your sample.
It is important to remember that the molecular weight values are relative to the standards used to generate the calibration curve. Additionally, no structural information (e.g. intrinsic viscosity) is available.
When you add a viscometer to a concentration detector to create a universal calibration method you increase the amount and accuracy of available data. In addition to molecular weight values (no longer relative), you also gain structural information. This is especially useful if your samples do not produce sufficient light scattering data. You can use a UV detector instead of an RI detector, but your standards and samples need to be UV active, and you must know the dA/dc value of your narrow standard. Sample concentration, and thus percent recovery, is also available.
RI (or UV) + light scattering
Data available: absolute molecular weight moments, radius of gyration (Rg), concentration
The advantage of using a light scattering detector (MALS or RALS+LALS), along with a concentration detector, is that you obtain absolute molecular weight values. This data is more accurate, independent of a calibration curve, and more convenient to calculate. You can use a UV detector, but you still need the dn/dc value of your standard and sample. The dn/dc value is a component of the equation used to calculated molecular weight from the light scattering data.
For large samples, molecular size in the form of radius of gyration (Rg) can be determined from light scattering data.
This configuration represents one of the two typical triple detector systems. The image above shows some of the calculated data in the Derived Data view. Branching data calculated using a viscometer and a light scattering data is more reliable than the branching data available through a universal calibration. This is because a light scattering detector measures a sample’s molecular weight independently of viscosity, which is not the case with universal calibration. Like the previous detector combination, a UV detector can be used instead of a refractive index detector, but knowledge of the dn/dc value of the standard and sample is required.
RI + UV + light scattering
Data available: absolute molecular weight moments, radius of gyration (Rg), composition, concentration
Often used for protein analyses, this is the other common triple detector arrangement. The two concentration detectors, RI and UV, allow access to compositional analysis. In addition to absolute molecular weight, the light scattering response can help detect aggregates and other high molecular weight species. In terms of molecular size, Rg is possible from the light scattering detector, but without a viscometer Rh is not.
All four detectors, as found in a fully loaded OMNISEC system, offer all of the data described in the multi-detector pyramid above. In addition, you can pair a SEC-MALS 20 with a tetra-detector OMNISEC to make sure you have everything covered!
In conclusion, I hope this helps you understand the capabilities of your GPC/SEC system. Even better, maybe it will help you decide what detectors to target in a new system! Furthermore, if you have any questions please don’t hesitate to contact us or email me directly at firstname.lastname@example.org.