Amplifiers and Recorders

Beyond the preamplifier, the amplifier requirements are essentially independent of the detector. Because the preamplifier requirements for thermal detectors are unique and more stringent than for photodetectors, only the case of thermal detectors will be considered here. 

The unique requirements of the preamplifier with respect to thermal detectors have their origin in the generally low-impedance and low-signal levels connected with these. These rms signal levels, of about 10 volt, require the utmost care in handling to avoid being swamped 

Transistors generally have too high an equivalent noise input for direct coupling to a thermal detector and they have too low an input impedance for use of the type of transformer described above. However, 

The subsequent use of the rectified signal depends on the photometric system employed. In single-beam systems it is fed directly to a potentiometric recorder. However, in a double-beam optical null system the rectified signal is remodulated, this time at the line frequency (e.g., 60 Hz). The object of this is to obtain an ac signal which can be amplified to sufficient power to drive a servomotor, which then positions the optical attenuator to establish a null signal. Line-frequency remod- 

In order to record a spectrum the pen of the instrument recorder must indicate the position of the wedge while the abscissa is scanned at a rate dependent on the scanning of the spectrophotometer. The following will briefly describe the most common methods of accomplishing these functions. 

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Chopper is a rotating wheel whose function is to break the steady stream of light from the hollow-cathode-lamp into an intermittent light that gives rise to a Pulsating current into the photocell this current is duly amplified and recorded. 

Light from the source(s) is focused into the cell, that consists of sample and reference sample and the two chambers are separated by a diagonal sheet of glass. After passing through the cell, the light is diverted by a beam-splitter (B) to two photocells (Pj and P2 respectively. A change in the observed refractive index (RI) of the sample stream causes a difference in their relative output, which is adequately amplified and recorded duly. 

The contribution of slow detector response can be ne ected when the base peak width is at least 40 times larger than r [cf. Eq. 3)]. In practice it is difficult to correct for such distortion because the time constant concept is only an approximation. It is not very reproducible and is sensitive to changes in the characteristics of the various elemehts of the electronics. Furthermore, detectors, amplifiers, and record are not first-order systems mid their response is only iippruxinntted by an exponential function (44). The response time is therefore defined by the time neces-... 

In the conventional NMR experiment, a radio-frequency field is applied continuously to a sample in a magnetic field. The radio-frequency power must be kept low to avoid saturation. An NMR spectrum is obtained by sweeping the rf field through the range of Larmor frequencies of the observed nucleus. The nuclear induction current (Section 1.8.1) is amplified and recorded as a function of frequency. This method, which yields the frequency domain spectrum f(ai), is known as the steady-state absorption or continuous wave (CW) NMR spectroscopy [1-3]. 

Raleigh horn. The collected scattered light is transmitted by an optical fiber to a photomultiplier. The photomultiplier current is amplified and recorded. 

In this type of calorimeter, the heat flows through a thermocouple, and then the voltage potential, produced by the thermocouple and which is proportional to the thermal power, is amplified and recorded in an x-y plotter (see Figure 6.3) [3,31,34,49], The concrete thermal effect produced is the integral heat of adsorption, which is measured with the help of the heat-flow calorimeter using the equation [50]... 

All fluorescence techniques involve the measurement of intensity of emission. They may be carried out by eye or more quantitatively by the use of phototubes especially of the multiplier type. By these means fluorescence emission from systems containing as low as 10-10 M in fluorescent compounds can be detected. Such trace amounts of material do not affect the system. Continuous non-destructive measurements can be carried out and the photocurrent can be amplified and recorded in the usual fashion. 

The IR pulse is split into a weak probe beam, which passes down a computer-controlled variable delay line with up to 12 ns of delay and a strong pump beam. The pump and probe pulses are counterpropagating and focused into the center of the SCF cell. Typical spot sizes (1/e radius of E-field) were oj0 120 pm for the pump beam and oj0 60 pm for the probe beam. A few percent of the transmitted probe beam is split off and directed into an InSb detector. A reference beam is sent through a different portion of the sample. The reference beam is used to perform shot-to-shot normalization. The pump beam is chopped at half the laser repetition rate (900 Hz). The shot-to-shot normalized signal is measured with a lock-in amplifier and recorded by computer. 

Differential staircase voltammetry — In this variation of staircase - voltammetry the current is sampled twice on each tread of the staircase potential-time waveform. The difference between the two currents sampled on the same step is amplified and recorded as a function of the... 

Potentiometric systems, which are less commonly used, measure changes in electric potential caused by interaction of 02 with the cathode. Appropriate electronic equipment transduces the amperometric output into a voltage, which is amplified and recorded on a conventional strip chart recorder. 

The differential signal resulting from the presence of a solute in the column eluent sensor cell is amplified and recorded. The ionization detector, as originally described by Boer and taken directly from his original publication, is shown in figure 4. 

IR spectroscopy appears to be a very promising tool for oxidation studies. The rate of formation of carbon monoxide, carbon dioxide and formic acid and the disappearance of ozone was measured this way with the O3/O2 + CH4 system . The system used by Burt and Minkoff for the combustion studies is shown in Fig. 61. Light from a Nernst filament is split in two and passed alternately through two heated cells and F2 containing either fuel + N2 or fuel + Oj. The beams are rejoined and fed into a Wadsworth monochromator containing a CaF2 prism and finally focussed onto a thermopile, from which a particular signal may be amplified and recorded. 

Photoionization detector A chromatographic detector that uses intense ultraviolet radiation to ionize analyte species the resulting currents, which are amplified and recorded, are proportional to analyte concentration. 

The basic components of a DTA apparatus are a temperature-controlled furnace containing sample and reference cells and a pair of matched temperature sensors connected to recording apparatus, as indicated in Figure 1.1. The temperature sensors (usually thermocouples) are in contact with the sample and reference or their containers, and the output is amplified and recorded. DTA data may be plotted as a function of sample temperature, reference temperature (as is usually the case), or time. In both DTA and DSC, the measurement relies on the occurrence of a temperature difference between a sample and reference (AT) as a result of the thermal event in question. 

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