Light multichannel

No account of UV detectors would be complete without mention of the diode array (multichannel) detector, in which polychromatic light is passed through the flow cell. The emerging radiation is diffracted by a grating and then falls on to an array of photodiodes, each photodiode receiving a different narrow-wavelength band. A microprocessor scans the array of diodes many times a... 

Optical emision spectra nowadays are simply measured using a fiber optic cable that directs the plasma light to a monochromator, which is coupled to a photodetector. By rotating the prism in the monochromator a wavelength scan of the emitted light can be obtained. Alternatively, an optical multichannel analyzer can be used to record (parts of) an emission spectrum simultaneously, allowing for much faster acquisition. A spectrometer resolution of about 0.1 nm is needed to identify species. 

The basic components include a Nd YAG pulsed laser system which is coaxial with a He Ne pilot laser and visible light optical system. The latter system enables the analytical area of interest to be located. The TOF-MS has a flight path of 2m in length, with an ion detection system that includes an electron multiplier detector, a multichannel transient recorder, together with a computer acquisition and data processing system. 

In a multichannel YI, monochromatic laser light with a vacuum wavelength X is coupled into an input channel waveguide and split to N output parallel channels, as shown in Fig. 10.5 (in this figure a network of three Y-junctions is used to split the light to four output channels). 

At this point it is important to mention that the experimental setup used for luminescence decay-time measurements is similar to that of Figure 1.8, although the light source must be pulsed (alternatively, a pulsed laser can be used) and the detector must be connected to a time-sensitive system, such as an oscilloscope, a multichannel analyzer, or a boxcar integrator (see Chapter 2). 

To eliminate the need for recalibration during a measurement and to obtain additional information, the Royco instrument was supplemented with a Nuclear Data ND-60 Multichannel Analyser (MCA). The amplified signal of the Royco 225 (which is proportional to the cimount of light scattered from each particle) was connected to the input of the MCA which cem count and classify pulsed input signals into as many as 2048 channels and display the results on a cathode ray tube (CRT). This number of cells is of course much more than required to determine the PSD. The data were therefore grouped into eleven cells whose limits were consistent with those used earlier (5), emd the counts in these cells were then printed on a Texas Instruments 743 KSR Data Terminal interfaced with the MCA. 

In a third method for lifetime measurements, the levels to be investigated are excited by a short pulse and subsequently the number of fluorescence quanta is counted as a function of the delay time (e.g. with a multichannel analyzer and a time-to-pulse height converter). The experimental procedure is described by Bennet 2 and has been perfected by several authors using light pulses 22) or electron pulses 23) for excitation. 

The second-generation FOCS is shown in Figure 1. It consists of a He-Cd laser excitation source (Omnichrome model 139), a polychronator (Instruments SA model HR-320), an optical multichannel analyzer (either PAR-0MA2 or PAR-0MA3), and a coupler interface of the type described by Hirshfeld et al. (8) which couples the excitation light (4ill. 6 nm) into the optical fiber (Quartz Products QSF 1000) and... 

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