Transducers multichannel

Use of bilayer lipid membranes as a generic electrochemical transducer is an exciting future for food biosensors. A taste sensor with multichanneled lipid membrane electrode was recently developed (93). The electric patterns generated from the sensor are similar to human response. The sensor can distinguish different brands of beer. More details on the taste sensor can be found in Chapter 16 of this book. 

Although the above lipid membranes had the ability to sense the taste by responding to many taste substances, information was insufficient to recognize quality of the taste. This weakness was overcome by means of a multichannel sensor, where transducers were composed of lipid membranes immobilized with a polymer [16-23]. We investigated responses of the sensors to various taste solutions. The electrode showed five different response patterns to five primary tastes with small experimental deviations. The patterns looked alike when the applied substance elicited the same taste in humans. 

Acetylcholineesterase Miniaturized multichannel transduc-tor with planar Au electrode which was first covered with a choline-selective liquid membrane made from 66% PVC-polyvinyl acetate (PVA), 33% 2-nitrophenyl octyl ether plasticizer and 1% ion-pair choline phosphotungstate. A second layer of 2% AChE in the PVA-polyethylene dispersion was spread on the top. The electrode was used as working electrode versus Ag/AgCl for potentiometric measurement of Ch and ACh in 0.1 M Tris buffer at 7.4. Optimum pH range for the sensor was 7-9. The calibration graph was linear from 0.02-10 mm ACh and detection limit was 5 pM. Response time was 3-5 min. Sensor was suitable for determination of ACh in biological fluids. [86]... 

The Mossbauer transmission spectra were recorded in the constant acceleration mode with an Elscint Mossbauer drive unit and a model MFG 3A Elscint function generator, an MVT-3 linear velocity transducer and an MD-3 transducer driving unit, y-ray detection was done with a Reuter-Stokes Kv-CH4 proportional counter driven by an Ortec 401A/456 high voltage power supply. Voltage pulses were introduced into an Ortec 142 PC preamplifier and an Ortec 571 spectroscopy amplifier. Data were collected on a Tracor-Northern NS-701A multichannel analyzer. The data were later analyzed on an IBM 360/370 computer. 

X-ray powder diffraction experiments were performed on a Phillips x-ray diffractometer with a Diano interface as described earlier (15). Mossbauer spectroscopy studies were all done at 77K with a homebuilt spectrometer consisting of an Elscint transducer, a Canberra amplifier and multichannel analyzer, and a Nal Harshaw... 

Output signals from the pressure and ultrasonic transducers and from the thermocouples were registered using multichannel recorders (HOUSTON INSTRUMENT, OMNISCRIBE B-5000). 

Multichannel Photoelectric Spectrometers After the appearance of pholoeleclric transducers in the IWOs, multichannel pholoeleclric spectrometers became commercially available. 

Fach transducer in a multichannel instrument has its own amplifier, pulse-height selector, scaler, and counter or integrator. These instruments are equipped with a computer for instrument control, data processing, and display of analytical results. A determination of twenty or more elements can be completed in a few seconds to a few minutes. 

Most energy analyzers are of the type illustrated in Figure 21-4, in which the electron beam is dellected by the electrostatic field of a hemispherical capacitor. The electrons thus travel in a curved path from the lens to the multichannel transducer, I he radius of curvature depends on the kinetic energy of the electrons and the magnitude of the electrostatic field. An entire spec-... 

The normal experimental technique is to scan rapidly through the velocity range and repeat this scan many times imtil data of the required accuracy has been accumulated. The Doppler motion is provided by an electromechanical drive system controlled by a servo -amplifier. Usually, the source is attached to the drive shaft and driven either in a saw-tooth or a triangular constant acceleration wave form. The transducer is coupled to a multichannel analyser operating in the multiscaler mode, and the servo-amplifier is controlled by the channel advance frequency. The dwell time in each channel, corresponding to a specific velocity increment, is 100 ps, and while the channel gate is open it accepts pulses from the detector. 

Edell, D., A peripheral nerve information transducer for amputees long-term multichannel recordings from rabbit peripheral nerves. IEEE Trans. Biomed. Eng., 1986. 33 203-214. 

A transducer, such as a photomultiplier tube (or a multichannel charge-coupled device, CCD) that will convert the light intensity into electric current magnitude. 

A Raman spectrophotometer analyzes the radiation scattered by molecules when they are illuminated with monochromatic exciting radiation. Currently, most Raman spectrometers are either FT instruments equipped with cooled germanium transducers or multichannel instruments based upon CCDs. These transducers, in contrast to photomultiplier tubes, are sensitive to radiation at 785 nm produced by diode lasers, which provide Raman excitation of many compounds without significant fluorescence. CCDs are not sensitive to the 1064-nm radiation from an Nd/YAG laser. 

The Mossbauer measurement requires the generation of a precise, controllable relative motion between the source and the absorber. A large variety of drive systems has been developed. The majority of drives work on electromechanical, mechanical, hydrauHc, and piezoelectric principle. The spectrometers can be classified into constant-velodty spectrometers and velocity-sweep spectrometers. The mechanical drives, hke a lead screw or a cam, move with constant velocity. They have advantages for the thermal scan method and because their absolute velocity calibration is straightforward. The velocity-sweep spectrometers are usually of electromechanical nature (like loudspeaker-type transducers) and normally used in conjunction with a multichannel analyzer. The most commonly used M(t) functions are rectangular (constant velocity), triangular (constant acceleration), trapezoidal, and sinusoidal. A typical Mossbauer spectrometer is shown schematically in O Fig. 25.24. 

The block scheme of a Mossbauer spectrometer. A, absorber CR, cryostat with temperature controller TC (optional, for low-temperature measurements) S, source moved by velocity transducer VT of driving unit DR FG, function generator VC, velocity calibrator (optional) LI, laser interferometer (optional) DET, detector HV, high-voltage power supply PA, preamplifier AM, amplifier SCA, single channel analyzer MCA, multichannel analyzer and PC, computer, OP, output... 

A basic Mossbauer spectrometer set-up (Fig. 4.8) consists of a Co radioactive source mounted on a velocity transducer (drive unit), where it is repeatedly moved between zero and maximum velocity (typically a triangular wave of frequency 10-20 Hz), and a proportional counter, giving an absorption spectrum in the multichannel analyzer (MCA). 

The essential components of a modem Mdss-bauer spectrometer as illustrated in the block-diagram of Figure 5 are the velocity transducer, the wave form generator and synchronizer, the multichannel analyzer, y-ray detection system, a cryostat or oven for low and temperature dependent measurements, a velocity calibration device, the source and the absorber, and a read-out unit. 

Modern multichannel transducers consist of an array of small photosensitive elements arranged either linearly or in a two-dimensional pattern on a single semiconductor chip. The chip, which is usually silicon and typically has dimensions of a few millimeters on a side, also contains electronic circuitry to provide an output signal from each of the elements either sequentially or simultaneously. For spectroscopic studies, a multichannel transducer is generally placed in the focal plane of a spectrometer so that various elements of the dispersed spectrum can be transduced and measured simultaneously. 

A spectrometer is an instrument that provides information about the intensity of radiation as a function of wavelength or frequency. The dispersing modules in some spectrometers are multichannel so that two or more frequencies can be viewed simultaneously. Such instruments are sometimes called polychromators. A spectrophotometer is a spectrometer equipped with one or more exit slits and photoelectric transducers that permit the determination of the ratio of the radiant power of two beams as a function of wavelength as in absorption spectroscopy. A spectrophotometer for fluorescence analysis is sometimes called a speciroftttorometer. 

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