Optical Path In The Ri Detector 2300/2400 - Knauer Smartline 2300 Benutzerhandbuch

6 The Basic Principle of the RI Detector 2300/2400

Optical Path in the RI Detector 2300/2400

Light source (LED)
Slit
Concave mirror
Deviation mirror
Detector diodes
Measurement
cell
Zeroplate
I
1
I
2
Fig. 3 Optical Path in the RI Detector 2300/2400 (schematically)
A light beam emitted from a LED crosses the sample and the reference
cell of the RI Detector twice (see ). When both cells contain pure solvent,
the system is calibrated to zero by means of a parallel zeroplate that
positions the beam on the two detector diodes 1 and 2 in such a manner
that the measured light intensities (I and I ) of the two diodes are virtually
1 2
identical. When the sample cell contains a solution with a different
refractive index, the light beam is geometrically proportionally deflected
depending on the relative change of the refractive index (according to
Snell´s law).
This results in a change of the light intensities I and I , proportional to
1 2
concentration and refractive index of the sample solution. From these
intensity changes the signal value is calculated (as shown in the formulas
in section "Calculation of the Signal Value" on page 7) and indicated in
the display.
The built in measurement cell of the RI Detector 2300 has a measuring
angle of 45°, the cell of the preparative RI Detector 2400 an angle of 15°.
The 45° and 15° measurement cells are interchangeable with the effect
that the sensitivity increases approximately by a factor of 3 using the 45°
cell. The maximum flow rate will not be influenced by the measuring cell.
It depends only on the inner diameter of the capillaries in the device (0.3
mm or 0.7 mm for the Smartline 2300 and 1.0 mm for the Smartline
2400).
Measurements are performed at a wavelength of λ= 950 ± 30 nm. Signal
detection and signal processing methods allow the refractive index to be
given out in the "online" mode without absorption components. The
display of the main menu also allows a permanent control of the detected
light quantity. You will find additional information in section "Calculation of
the Signal Value". The autozero-range includes the complete
measurement range.
Refractive index detectors are universal detectors due to their ability to
detect nearly every substance dissolved in the eluent.