Photomultiplier detection

Emission spectrometer incorporating sample and counter electrodes, means of excitation, prism or grating monochromator, photomultiplier detection system, microprocessors or computers for data processing, interference correction and data display. 

Emission spectrometer incorporating a sample nebulizer, grating monochromator, photomultiplier detection system and microprocessor controller. Excitation by dc-arc plasma jet, or inductively coupled plasma. Laser excitation sometimes used. 

A reference channel (quantum counter or photodiode) has two advantages (i) it compensates for the time fluctuations of the lamp via a ratiometric measurement (ratio of the output signals of the photomultiplier detecting the fluorescence of the sample to the output signal of the reference detector) (ii) it permits correction of excitation spectra (see below). 

An experimental set-up suitable for measuring dye sensitized injection currents and dye sensitized delayed fluorescence of the crystal is shown in Fig. 23. To shield the photomultiplier (detection of the fluorescence) from the incident light beam the two light choppers in Fig. 23 are rotating with a corresponding phase shift. 

The photomultiplier detects both the thermal emission from the determinant and also any other atomic or molecular emission from either concomitant elements present in the sample or from the flame itself. Figure 8, for example, shows a typical section of a flame emission spectrum. While it is possible for some determinations by FES to work at a single fixed wavelength, as in flame AAS, it is advisable, at least initially, to scan the emission spectrum in the vicinity of the wavelength of interest to confirm the absence of spectral interferences. In any event, regular re-zeroing and aspiration of an appropriate standard to check for signal drift is essential. 

Figure 3-6. A refrigerated scintillation spectrometer. The large bottom cabinet is a deepfreeze unit that contains the sample holders and photomultiplier detection units. The typewriter is used for data print out. (Courtesy of Packard Instrument Company, Inc.)...

Conventional scintillation counters such as the Microbeta (Wallac/Perkin Elmer, Turku, Finland) or the TopCount (Packard, Meriden, USA) use photomultiplier detection systems that count 8 or 12 wells at a time, resulting in a readout time of 40 minutes per 384-well microplate. Bialkali photocathodes (Sb-Rb-Cs or Sb-K-Cs) used in standard photomultipliers have a maximum spectral response at about 420 nm, with a quantum efficiency for detection of up to 30%. Thus, the aforementioned instruments are ideally suited for filtration assays and SPA assays with the blue-emitting YSi and PVT beads. 

Figure 15. Relationship between the plasma background emission intensity and its associated noise measured with a photomultiplier detection system.

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. 

For multi-element analysis, the significantly more energetic plasma sources are therefore by far superior to flames in most regards. Today, flames are only used for the determination of alkali metals, as these can be excited at low temperatures and give simple spectra free of interferences. The determination of these metals is especially important for the analysis of biological fluids, so that emission in acetylene-air flames is still routinely used in highly automated, and simplified systems based on single or multiple interference filters and photomultiplier detection (see Fig. 12.23). The systems often also include automatic addition of an internal standard and dilution [39]. 

Fig. 12.28 Schematic diagram of a polychromator based on a Paschen—Runge mount and photomultiplier detection, (a) General optical set-up (b) side view and arrangement of the photomulitpliers.

In Munich, we have developed our spectrometer around a Jarrell-Ash double monochromator equipped with holographic gratings, using photomultiplier detection [48-51]. An extracavity multiple-reflection cell [49] as well as intracavity excitation with transfer of the Raman-scattered light with optical fibres [50,51] were applied for signal enhancement. The multiple reflection cell has been transferred to Florence and installed at a Jobin-Yvon UlOOO spectrometer equipped with a charge-coupled device (CCD) camera [52]. 

Laser flash photolysis experiments were carried out with a home made apparatus that is schematised in Fig. 3.2. In our set-up a Surelite Nd YAG laser (pulse width <10 ns) was used as excitation source and a bight-pressure Xenon arc-lamp was used as analysing light. The signal after the sample was captured by a mono-chromator/photomultiplier detection system, recorded by a Tektronix TDS640A digitizer oscilloscope and transferred to a PC computer. 

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