Shutter millisecond

In plasma chromatography, molecular ions of the heavy organic material to be analy2ed are produced in an ionizer and pass by means of a shutter electrode into a drift region. The velocity of drift through an inert gas at approximately 101 kPa (1 atm) under the influence of an appHed electric field depends on the molecular weight of the sample. The various sonic species are separated and collected every few milliseconds on an electrode. The technique has been employed for studying upper atmosphere ion molecule reactions and for chemical analysis (100). 

The collimated optics of the set-up demonstrated can be substituted by fibre cities. This type of equipment is shown in Fig. 4.11 using a bifurcated fibre and a diode array. Under these conditions white light has to be used to produce a reflection spectrum monitored by the diode array. However, white light causes undefined photochemical conditions. For this reason a third fibre arm is used, which guides monochromatic radiation to the sample. If the white radiation is blocked by a shutter, it may not disturb the monochromatic photoirradiation. Since modem diode arrays take spectra within a few milliseconds the time of measurement stays short compared to the total photochemical process time. This type of equipment can be used to monitor the photoprocess during irradiation of photoresists [101]. 

The setup gives a reasonable safety against detector damage. However, it must be noted that it does not give absolute safety. If a microscope lamp is switched on when the shutter is open and the detector is active there is a delay of some milliseconds until the shutter closes. To avoid risk completely, another shutter must be placed in front of the lamp and operated exclusively with the detector shutter. 

Figure 8.2 shows a mobility spectrum in which the ion current is plotted as a function of arrival time obtained from an IMS DTIMS used for explosive detec-tion.2 In this negative-polarity spectrum, the intensity of the current at the Faraday plate is plotted as a function of the time after the ion shutter is opened to introduce an ion swarm into the drift region of the IMS. When an ion swarm arrives at the electrode, an increase in current produces a peak representing the arrival time of the swarm. The arrival time axis is normally recorded in milliseconds, and the measured mobility is determined by the relation... 

A typical pseudo-2D experimental setup for optical measurements is shown below in Fig. 4.2. A glass plate makes up the front of the reactor, which is fiUed with inert particles. Fluidization gas, possibly treated with a small amount of steam to prevent static charging of the particles, is fed from below through a porous plate. To prevent blurring of the particles on the images, a fast shutter time is required (in the order of milliseconds), and therefore additional Hghting is usually necessary. 

A half-wave plate (HWP) and a polarizer (GLP) are positioned after the oscillator and are used to variably attenuate the laser output power to the desired input power required by specific experiments. Using a beam sampler (M ), a small portion of the laser beam is directed into a beam diagnostic unit (AC). In it, the laser pulse is characterized both in the time and frequency domains by employing an autocorrelator and a spectrometer. The laser beam is then expanded to match or overfill the back aperture of the objective lens. This is accomphshed using two positive lenses with the appropriate focal lengths. At the focal point of the first lens, a pinhole (SF) is carefully positioned to spatially filter the laser beam. An electro-mechanical shutter (S), used to control laser exposure times in the sample, is placed before this assembly. If exposure times shorter that a few milliseconds are required, faster response shutters such as acousto- or electro-optic modulators can be used. 

Phosphorescence is a delayed emission often much weaker than fluorescence from the same chromophore. In a standard spectrum, phosphorescence often appears as small ripples in the tail of the fluorescence spectrum. To remove the fluorescence from the spectrum, two approaches are taken. Traditionally one used a mechanical shutter or a rotating cam with slits that block the exciting light while passing the emitted light to allow only delayed emission to reach the photomultiplier tube. Mechanical choppers have millisecond resolution and permit decay... 

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