Photomultiplier detection photodiode array

The reaction cell has a White cell optical system (see Chapter ll.A.lc) with a pulsed xenon lamp light source. Once the radicals are formed, they are detected by their absorptions in the UV using the Xe lamp and a monochromator-photomultiplier or photodiode array detector. Thus the absorption spectra of the free radicals generated in the system can be measured and the absorption at a particular wavelength used to follow their reaction kinetics. 

Figure 21. Experimental arrangement for monitoring optical emission from an r.f plasma. The photomultiplier tube (PMT) and picoammeter detection electronics are frequently replaced with photodiode arrays and photographic film in many spectroscopic studies.

There is also a standard test method for determination of major and minor elements in coal ash by inductively coupled plasma (ICP)-atomic emission spectrometry (ASTM D-6349). In the test method, the sample to be analyzed is ashed under standard conditions and ignited to constant weight. The ash is fused with a fluxing agent followed by dissolution of the melt in dilute acid solution. Alternatively, the ash is digested in a mixture of hydrofluoric, nitric, and hydrochloric acids. The solution is analyzed by (ICP)-atomic emission spectrometry for the elements. The basis of the method is the measurement of atomic emissions. Aqueous solutions of the samples are nebulized, and a portion of the aerosol that is produced is transported to the plasma torch, where excitation and emission occurs. Characteristic line emission spectra are produced by a radio-frequency inductively coupled plasma. A grating monochromator system is used to separate the emission lines, and the intensities of the lines are monitored by photomultiplier tube or photodiode array detection. The photocurrents from the detector... 

While the SIT vidicon detection system provides unique and exciting spectroscopic detection capabilities, inherent limitations of vidicon detection prevent it from replacing other detectors such as photographic emulsions, photomultiplier tubes, or linear photodiode arrays. Rather, the SIT vidicon is complementary to the more traditional spectroscopic detectors. 

Photodiode Array Versus Photomultiplier Detection. The advantages of photodiode array detection, PDA, as compared to photomultiplier tube, pmt, detection for emission spectroscopy are well known (19). These advantages are especially important for the specific examples we discuss here, namely, upconverting emission spectroscopy. This is dramatically demonstrated in Figure 7 where we compare single channel pmt versus multichannel PDA detection of a small portion of the upconverted fluorescence spectrum of coumarin 520 in ethanol solvent at room temperature. 

Figure 7. A comparison of photodiode array multichannel detection to photomultiplier tube single channel detection for the upconverted emission spectrum of coumarin 520.

The steps involved in spectrophotometric measurement in the ultraviolet and visible range include irradiation of a sample at a specific wavelength, detection (commonly using a photomultiplier tube or a photodiode array), and transduction into an electronic signal. Specifications and sample analysis procedures are listed in the USP General Chapter <851 >. 

The detectors used in UV-vis spectroscopy are typically photomultiplier tubes, photodiodes, or charge-coupled devices (CCDs). Photomultipliers are very sensitive to UV and visible radiation and have fast response times, but with amplification factors of 10 electrons per incident photon, they are limited to measuring low light intensities. Photodiode or CCD arrays allow parallel detection of many wavelengths of radiation simultaneously if the array is positioned at the output focal plane of the monochromator. With many hundreds or thousands of chaimels, UV-vis spectra can be can be acquired very rapidly. 

The first measurement we make when starting a fluorescence study is not usually a fluorescence measurement at all but the determination of the sample s absorption spectrum. Dual-beam differential spectrophotometers which can record up to 3 absorbance units with a spectral range of 200-1100 nm are now readily available at low cost in comparison to fluorimeters. The wide spectral response of silicon photodiode detectors has made them preeminent over photomultipliers in this area with scan speeds of a few tens of seconds over the whole spectral range being achieved, even without the use of diode array detection. 

In routine spectrophotometers, photomultiplier tubes are replaced by photodiodes (Fig. 11.11), which have excellent sensitivity, linearity and dynamic range. The photoelectric threshold, in the order of 1 eV, allows detection up to wavelengths of 1.1 pm. In diode array systems, each rectangular rectangular diode (15 pm x 2.5 mm) is associated with a capacitor. The electronic circuit sequentially samples the charge of each capacitor. While a photomultiplier tube measures the instant intensity in watts, a diode measures the emitted energy in joules over a time interval. 

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