Waters 2414, Overview And Maintenance Manual

Page: 19 / 130

June 11, 2015, 715004754 Rev. A
Page 19
The refractometer detects the position of the refracted light beam, creating a signal
that differs from the baseline signal.
The following figure shows how refraction by the sample in the flow cell changes the
proportion of light on each element of the photodiode.
Figure 1–3: Presence of sample changes the photodiode signal
By keeping wavelength, temperature, and pressure constant, the changes in RI
measured by the refractometer are caused only by the 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 sample yield large
peaks.
1.2.2 Differential refractometry
The 2414 RI 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
Δ
n .
Δ
n is expressed in refractive index
units (RIU).
The 2414 RI detector measures
Δ
n values as small as 7
×
10
–9
RIU by detecting the
difference in the amount of light falling upon each of the elements of the dual-element
photodiode (see the figure, above).
1.2.2.1 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 the figure, below. The
φ
determines the
magnitude of the shift (
Δ
x ) of the image cast on the photodiode by the light beam.
Dual-element
photodiode
Collimating lens
Sample side
of flow cell
Reference side
of flow cell
Incident light
Reference side
of flow cell
Sample in sample
side of flow cell