Chapter 3: Specifications
SPD-M20A Service Manual
10
3.2 Operating principles
The instrument employs a D2 (deuterium) lamp and a W (tungsten) lamp as light sources.
Lighting of the lamps occurs automatically via the LC workstation (personal computer).
The light from the two lamps is mixed by a half mirror and is reflected by a concave mirror
(M1), whereupon it passes through the flow cell, is reflected by a mirror (M2) and is
spectrally resolved by the monochromator. A photodiode array (512 elements) measures
the intensity of the light at each wavelength. The signals undergo A/D conversion at each
element, and after signal processing, are sent to the LC workstation via a CPU through
the TCP/IP interface.
3.3 Description of the optical system
1) The light from the D2 lamp and W lamp is mixed with a half mirror (LM ASSY), and
after being reflected by mirror M (M1 ASSY), the light impinges upon the flow cell.
2) The flow cell has a lens on the incident side and a flat glass plate (cell window) on
the emission side.
3) The light exiting the cell is reflected by a mirror M2 (M2 ASSY), and enters the
monochromator through a variable slit.
4) The variable slit can switch between a 50 m and 360 m slit width, which
correspond to spectral band widths of 1.2 nm and 8 nm.
5) The light that is scattered by the grating in the spectrometer is projected as an
objective image of the spectrum on a photodiode array (PDA). The surface of the
photodiode array has 512 light receiving elements aligned in an array, where each
element is assigned to 1.2 nm of the spectrum extending over a range of 190-800
nm.
6) The photoelectric current from each of the light-receiving elements is integrated over
the "integration time" and is read at a constant interval. The signal from each
element is converted into a digital signal which is then converted to an absorbance
level.
7) The "integration time" changes depending on the lamp or adjustable conditions. The
time can be automatically set using the PDA Utility software.
8) The shutter in front of the flow cell is automatically inserted into the respective light
path when measuring the dark (dark current) signal or when the filter is undergoing
automatic wavelength correction or during an automatic wavelength check.
The figure on the following page is a view of the optical system from above and from the
side.
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