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Differential Refractive Index Detector RI 8120

Model 8120 - The differential refraction index detector

Designed for high sensitivity

For highest sensitivity and baseline stability the RI detector 8120 for HPLC has been designed as a differential refractometer detector. It measures continously the differential refractive index Dn between the pure eluent and the eluate from the HPLC column in its differential flow cell. Key properties are in the highly symmetric construction of the optical bench and a build-in heat exchanger which guarantee very small temperature effects on the baseline..

Many analytical applications

Since the volume of the flow cell is 8 µL only, the RI detector 8120 meets all the requirements for high resolution chromatographic separations. No additional peak broadening can be observed with standard 250 x 4.6 mm columns. For preparative applications the modified model 8120P is available.

Thermal stability

Due to the very low heat dissipation of the LED and due to the construction of the optical bench the RI detector 8120 has an outstanding temperature stability. Therefore, no external thermostatting is required.

Linearity and stability

The optics of the RI detector 8120 are designed to give the highest possible linearity. Therefore, a long wavelength almost light emitting diode (LED with l=950 nm) is used instead of a white tungsten lamp. The use of a LED also avoids non-linearity caused by dispersion effects. In addition to that, silicon photodiodes have been built into the instrument as sensors for the position of the light beam. This position is then converted into an electrical signal in a highly linear way.

Functionality and Simplicity of Operation

The reference (i.e. eluent) channel of the differential refractometer detector RI 8120 is flushed from the sample channel by a push-button actuated 2-way valve. Thus, a second (reference) flow system is avoided. Operation of the RI 8120 is further simplified by the build-in option to perform a complete optical and electrical auto-zeroing by pressing a single push-button.

Measuring Principle

The refractive index detector is constructed as a flow-through differential refractometer which continously mesures the difference Dn of the refractive index of the eluate and the pure eluent. Fig. 1 shows schematically the path of the light beam through the instrument. The light emitted from the LED 1 passes slit 2 before being deflected by the semi-transparent mirror 3 and traversing the two halves of the measuring cell 4, the semi-transparent mirror 3, and the plane parallel glass plate 6 (zero glas). Finally, the light beam is split into two partial beams by prism 7. The intensities of these beams at the photodiodes 8a and 8b serve as a measure for the position of the original light beam and, thus, of the difference Dn of the refractive indices in the cell halves. Dn and the light beam are connected by Snellius' law according to fig. 2. With the angle of incidence a, the angle of reflection b and the refractive indices n1 and n2 i.e. those of the pure eluent and the eluate, the following holds: For small deflection angles g and two passes of the light beam through the cell the equation is: For small deflection angles g this means that g is proportional to Dn between the two cell chambers.

Specifications

RI 8120 RI 8120 P Sensitivity range 0, 1, 2, 5, 20, 50, 100, 200, 500, 1000, 2000 DRI 0, 1, 2, 5, 20, 50, 100, 200, 500, 1000, 2000 DRI Noise (Cell filled with water) ±2 x 10-8Dn ±2 x 10-8Dn Baselinedrift at constant temperature 1 x 10-7Dn/h 1 x 10-7Dn/h Light source LED LED Wavelength of light source 950 nm ± 20 950 nm ± 20 Auto zero range ±1% from total sensitivity range ±1% from total sensitivity range Time constant 0.05 s / 0.1 s / 0.5 s / 2 s 0.05 s / 0.1 s / 0.5 s / 2 s Recorder output 0 - 100 mV 0 - 100 mV Integrator output 0 - 1 V 0 - 1 V Marker - internal - external +10 mV +5 mV +10 mV +5 mV Flow cell - volume - angle of deflection - pressure limit 8 µl 45° 0.02 MPa 22 µl 5° 0.02 MPa Temperature range 15 - 50°C 15 - 50°C Purge Filling of the reference cell 2-way valve 2-way valve Display alternatively a) RI-actual b) range c) time constant d) polarity 5 digit LED display 5 digit LED display External control - by contact closures - by current loop or via RS232 autozero marker all functions autozero marker all functions Power requirements 110 / 220 V ± 10% 50 / 60 Hz 110 / 220 V ± 10% 50 / 60 Hz Dimensions 381 x 255 x 130 mm 381 x 255 x 130 mm Weight 9.5 kg 9.5 kg

Ordering Information

Analytical RI Detector Model 8120 P/N 8120 0000 Flow cell 45° P/N 8110 0845 Preparative RI Detector Model 8120P P/N 8120 0000P Flow cell 5° P/N 8110 2203