Differential Refractometry - Waters 2410 Operator's Manual

By keeping wavelength, temperature, and pressure constant, the changes in RI measured by the
refractometer are due only to changing sample concentration. A solution with a high concentration
of a solute refracts a beam of light more than a dilute solution. Therefore, high concentrations of
1
sample yield large peaks.

1.2.2 Differential Refractometry

The 2410 differential refractometer can measure extremely small changes in refractive index to
detect the presence of sample. The small difference in RI between a reference solution and a sample
solution is referred to as ∆n. ∆n is expressed in refractive index units (RIU).
The 2410 differential refractometer measures ∆n values as small as 5 × 10
–8
RIU by detecting the
difference in the amount of light falling upon each of the elements of the dual-element photodiode
(see Figure 1-4).
External Angle of Deflection
The amount of light falling upon the elements of the photodiode is determined by the external
angle of deflection ( φ ), as shown in Figure 1-5. The φ determines the magnitude of the shift (∆x) of
the image cast on the photodiode by the light beam.
Figure 1-5 illustrates the external angle of deflection ( φ ) and its dependence on the difference in RIs
between the reference and sample sides of the flow cell.
Theory of Operation 1-7