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cell, the respective light enters a silicon photodiodes (D1 and D2) where it is
converted to an electrical signal that is proportional to the light intensity.
The light entering the cells, or in other words, the wavelength being measured
during analysis, is selected by changing the angle of the grating. A stepping
motor is used to move the grating via a pulley and belt. When the instrument
power switch is turned on, mirror M1 moves to the angle position for the W lamp,
then again to the D2 lamp position. The lamp switching photomicrosensor detects
the position and the D2 lamp will turn on. When the mirror M1 position is
switched, the home position for the filter assembly is simultaneously detected.
Once the lamp goes on, the grating stepping motor first searches for the home
position of the wavelength setting, then verifies the wavelength using the D2
lamp and Hg lamp emission lines. This instrument measures the amount of light
passing through the cell as the wavelength is varied and automatically identifies
the wavelengths corresponding to the emission line spectrum peaks. If these
wavelengths are within one nanometer of the emission line wavelengths, then
“CHECK GOOD” will appear on the display. Then the grating rotates according
to the wavelength setting and the lamp position switches also. The sample cell
has a flow-through type structure. The reference cell is simply a hole that lets
light pass through. The Mercury lamp is used to check or calibrate wavelengths.
The characteristic emission line of Hg lamps at 253.7 nm is used as a reference
for checking accuracy in the ultraviolet region.
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