Why is Closer Better for LALS? – Viscotek GPC System

 

Introduction 


  Currently, GPC/SEC (Gel Permeation Chromatography/Size Exclusion Chromatography) experiments widely use laser light scattering detectors to measure molecular weight.  

  The advantage of directly measuring molecular weight from LS (Light Scattering) detector signals without the need to use calibration with polymers or standard proteins, makes it preferred by many users. Additionally, molecular weight measured with LS detector is irrespective of polymer type and structure. 

 

  Therefore, the molecular weight measured by GPC-LS is referred to as absolute molecular weight. In reality, all GPC-LS detectors require calibration and the majority of LS detectors in use today calculate molecular weight through data extrapolation or data correction, which creates a significant gap for it to be called absolute. 

 


  This application note explains the theory related to this issue and explains whether LALS (Low Angle Light Scattering) is the only LS technology that measures scattering angle proportional to molecular weight.

Theory of Light Scattering 


First, the correlation between measured capacity (intensity of the scattered light) and desired results (weight average molecular weight) needs to be examined. These two factors are connected through the well-known Releigh equation. 

 

R=o indicates the scattering intensity at o degrees, Mw refers to the average molecular weight, c denotes the concentration of the solution, K is the optical constant representing dn/dc, and A2 stands for the second virial coefficient. 

 


In low concentration solutions (e.g., typical GPC conditions), the concentration is so low that this effect can be ignored. This simplifies the equation as shown below. 

 

With this simplified formula, Mw can be directly obtained from the scattering light values. However, it is important to understand that this equation concerns scattering light at 0 degrees (e.g., 0°). 

Of course, measuring scattered light at 0° is impossible due to the incident laser beam, so scattering light must be measured at several angles (Figure 1). However, this makes the process complex due to the dependency of scattering light on scattering angles and molecular sizes. 

If the molecular size is very small, this angle dependency effect can be ignored. Problems arise in solutions for molecular sizes greater than 12nm. (1) 

Conclusion 

With the advent of advanced LALS detectors, GPC/SEC laboratories can accurately measure the molecular weight of samples. 

Using LALS can prevent issues related to data extrapolation or data fitting. Newly launched LALS products are small, easy to handle, and suitable for integrated multi-detector systems with viscometers, allowing simultaneous determination of molecular weight and structure. (6, 7, 8) 

This overthrows the conventional belief that more angles are better. In fact, when measuring scattered light, closer is better.