Concept: Spectrophotometer Principle of Operation
This article explains the principle of operation behind the nova II UV-Vis/SW-NIR Spectrophotometer, the detector used in many of our analyzers.
The key difference between a spectrophotometer and conventional photometers is that photometers use 'non-dispersive' methods whereby measurement wavelengths are physically isolated using filters. For measuring multiple components, this will require the photometer to employ a moving part (filter wheel) or multiple line source lamps. By contrast, our spectrophotometer is solid state and has a single light source.
The Path of the Light Signal
The measurement cycle of the nova II is virtually instantaneous, but it is helpful to explain it in stages:
- The white light signal originates in the pulsed xenon light source.
- The signal travels via fiber optic cable to the entry point of the flow cell, where a collimator narrows the light beam. The signal travels directly across the flow cell path length, interacting with the continuously drawn process sample fluid.
- Now containing the distinct absorbance imprint of the current chemical composition in the sample, the signal exits the flow cell on the opposite end through a collimator and travels via fiber optic cable to the spectrophotometer inside the analyzer enclosure.
- The holographic grating physically separates (disperses) the signal into its constituent wavelengths, focusing each wavelength onto a corresponding photodiode within the 1024-element diode array.
- The light intensity spectrum measured by the diode array is processed by the analyzer CPU. The absorbance spectrum is calculated and visualized by plotting lost light intensity at each wavelength due to the process sample interactions.
From xenon lamp to diode array, the entire cycle occupies a few milleseconds and involves no moving parts.