Differential Refractometry - Waters ACQUITY Refractive Index Detector Overview And Maintenance Manual

1 The ACQUITY Refractive Index Detector
Presence of sample changes the photodiode signal:
Dual-element
photodiode
Collimating lens
Sample side
of flow cell
Reference side
of flow cell
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 solute refracts a beam
of light more than a dilute solution does. Therefore, high concentrations of
sample yield large peaks.

Differential refractometry

The detector 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 Δn, expressed in refractive
index units (RIU).
The refractive index detector measures Δn values as small as 7 × 10
detecting the difference in the amount of light falling upon each of the
elements of the dual-element photodiode (see the previous figure).
20 October 13, 2014, 715003547 Rev. C
Incident light
Sample in sample
side of flow cell
Reference side
of flow cell
–9 RIU by