Shimadzu ICPE-9000, Инструкция по эксплуатации

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4 Basic Principles
4-7
The prism divides the light so that it radiates in a direction perpendicular to the direction in which the
light is dispersed by the diffracted grating. The light is separated according to order and all the
wavelengths are dispersed onto a two-dimensional plane. "Fig. 4-3" shows the echelle pattern.
Short Long
Wavelength for
each order
Orde
r
Low order
(long wavelength)
High order
(short wavelength)
Fig. 4-3 Echelle Pattern
4.4 Detector
Light can be converted to electrical signals using a photomultiplier tube or a semiconductor detector.
A photomultiplier tube uses the fact that electrons are generated when light strikes a photoelectric
surface. It accelerates the electrons in a vacuum and amplifies the signal and detects them as current from
an anode. A semiconductor detector uses the fact that electrons are generated in a solid-state element (Si)
by incident photons. It accumulates electrons and performs detection by measuring the amount of electric
charge.
In order to measure two-dimensional spectra, the echelle spectrometer uses a semiconductor detector.
Approximately one million (1,024 × 1,024) independent elements (pixels) with vertical and horizontal
dimensions of a few tens of microns are arranged within an area of one square inch. After accumulation
time elapses, the amount of electric charge is read (i.e., measured) for each pixel. The spectral intensity
of the target element is obtained as the amount of electric charge that accumulated in the pixel a certain
number along and a certain number up.
Semiconductor detectors come in CCD (charge coupled device) and CID (charge injection device) types.
Repeated reading is possible with a CID but all the pixels are read in one operation with a CCD. The
ICPE-9000 uses a CCD.
The sensitivity of a photomultiplier tube is adjusted by changing the applied voltage. With a
semiconductor detector, however, the sensitivity adjustment is not possible for individual pixels. Also,
there is a limit to the amount of charge that can accumulate in a pixel. If a pixel is irradiated with light of
an intensity that exceeds this amount, an overflow occurs. In this case, the accumulation time is
decreased to prevent an overflow and the number of repetitions is increased. On the other hand, if the
light intensity is relatively low, the accumulation time is increased and the number of repetitions is
decreased.