The Electronic Detection System

Standard PMTs have a lateral photocathode (side-on window), spectral response from 200 to 800 nm and a few connected dynodes (typically five out of twelve) for a faster response. The operating voltage V (typically V — —950 — 

As mentioned above a first condition for fast response of the detection system is the rise time of the photodetector. A second condition is relevant to the load resistance R. The fall time tf of the anode current depends on the values of the resistance / and the capacitance C, the latter due essentially to stray capacitances and the input capacitance of the oscilloscope, according to Eq. 8.9 

Thus for / /, = 50 Q, the value generally used for nanosecond detection, C must be below 40 pF to have tf 2 ns. Other crucial parameters are the bandwidth (BW) of the oscilloscope amplifier, which for nanosecond resolution must be 350 MHz 

If this condition is not fulfilled, especially for a single event signal, the reconstructed waveform does not correspond to the original one, because the information relevant to the high frequency components has been lost in the digitalization process. For example, for 2 ns resolution the sampling frequencyof the digital oscilloscope must be 5 x 10 sample/s. 

Nanosecond kinetic measurements need wide amplifier bandwidths which limit electronic filtering of high frequency noise. Thus noise is high. Further the statistical fluctuations in the number of photoelectrons, the so called shot noise of the PMT photocathode, is large because the number of incident photons is low in the short time intervals explored. In these conditions the S/N ratio is  

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For a simplified case, one can obtain the rate of CL emission, =ft GI /e, where /is a function containing correction parameters of the CL detection system and that takes into account the fact that not all photons generated in the material are emitted due to optical absorption and internal reflection losses q is the radiative recombination efficiency (or internal quantum efficiency) /(, is the electron-beam current and is the electronic charge. This equation indicates that the rate of CL emission is proportional to q, and from the definition of the latter we conclude that in the observed CL intensity one cannot distii pish between radiative and nonradiative processes in a quantitative manner. One should also note that q depends on various factors, such as temperature, the presence of defects, and the... 

The ion detection system consists of a high-gain electron multiplier and the signal digitizing system, along with a computer for data acquisition and manipulation. 

Electronic detection systems may range from simple intmder-detection devices monitored by basic control units to a variety of complex systems monitored by sophisticated computer-operated controls linked to 24-hour manned stations. Intruder-detection devices can be arranged into the following groups ... 

Colorimeter Also called color comparator or photoelectric color comparator. An instrument for matching colors with results about the same as those of visual inspection, but more consistent. Basically the sample is illuminated by light from the three primary color filters and scanned by an electronic detecting system. It is sometimes used in conjunction with a spectrophotometer, which is used for close control of color in production. 

This technique detects substances qualitatively and quantitatively. The chromatogram retention time is compound-specific, and peak-height indicates the concentration of pollutant in the air. Detection systems include flame ionization, thermal conductivity and electron capture. Traditionally gas chromatography is a laboratory analysis but portable versions are now available for field work. Table 9.4 lists conditions for one such portable device. 

The complexity of die flow injection manifold required by the three approaches was very similar. All of them necessitated electronic interfaces to control the propulsion and injection systems through the microcomputer in approaches I and II, and the injection and switching valves in manifold III. A passive electronic interface was also required in all three manifolds in order to acquire data fi om the biosensor/detection system. 

Figures 12-14 show the steady-state current dependence of the acetylcholine sensors on substrate concentration these sensors contained polymers C, F, and I, respectively, as the electron relay systems. For an applied potential of +250 mV vs. SCE, the time required to reach 95% of the steady-state current was typically 10-15 sec after addition of the acetylcholine sample. At lower potentials, the response time was slightly slower. For these systems, a detection limit (as defined by a signal-to-noise ratio of approximately 2) of approximately 0.5 to 1.0 /xM was achieved under N2-saturated conditions. The response of the sensors to choline was nearly identical to the acetylcholine response, which demonstrates the efficient conversion of acetylcholine to choline by acetylcholinesterase.

Giachetti et al. [60] compared the performance of mass selective detector (MSD), electron capture detector (ECD) and nitrogen-phosphorus detector (NPD) of gas chromatography systems in the assay of six nonsteroidal antiinflammatory drugs in the plasma samples. As a practical test, six NSAIDs (mefenamic, flufenamic, meclofenamic and niflumic acids, diclofenac and clonixin) added to plasma samples were detected and quantified. The analyses were carried out after solvent extraction from an acidic medium and subsequent methylation. The linearity of response was tested for all the detection systems in the range of 1-25 ng/mL. Precision and accuracy were detected at 1, 5 and 10 ng/mL. The minimum quantifiable level for the six drugs was about 1 ng/mL with each of the three detection systems. 

The UV detection system like any man-made product, is subject to failure. These failures could be detector tube failures, electronic or wiring failures, or optical system failure or contamination. 

Polyak, C. S., Elbert, Y, Pavlin, J. A., Kelley, P. W. (2002, March). The Electronic Surveillance System for the Early Notification of Community-based Epidemics (ESSENCE) GIS modeling in an early detection surveillance system for bioterrorist and natural disease threats. In Program and Abstracts of the International Conference on Emerging Infectious Diseases, 2002 (p. 31), Abstract retrieved March 11, 2007 from http // www.cdc.gov/iceid/program.htm... 

The sample ions are then detected by an ion detection system. Figure 19-51, that produces a signal proportional to the number of ions detected. The ion current signal from the ion detection system is received and amplified by the system electronics and is then passed on to the data system for further processing, storage, and display. 

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