Balanced detector

Instruments for exact measurements usually have a sinusoidal current source and an electronic balance detector. The circuit is made as symmetrical as possible to avoid stray coupling. 

Fig. 5.6. A block diagram of an optical coherence tomography/Raman spectroscopy system C, circulator RSOD, rapid scanning optical delay BP, 785 bandpass BSO, beam shaping optics DM1, dichroic mirror at 990 nm DM2, dichroic mirror at 800-950 nm LP, long pass at 808 nm GP, galvanometer pair BD, balanced detector BPF, electronic band-pass filter AI-AO DAQ, analog input-output data acquisition (reprinted with permission from [34]. Copyright 2008 Optical Society of America)...

The following data provide useful guidance in the operation and optimization of procedures with the gas density balance detector in gas chromatography.1 The property values were calculated with... 

Fig. 55. Schematic view of the gas density balance detector. See J. H. Purnell p. 271.

An initial product identification can be carried out by comparing the retention times, under given conditions, of the unknown compound with that of suspected known compounds. To counteract minor variations in flow rate and temperature, coincidence of peaks can be checked with consecutive doping or marking of the mixture with known compounds. Coincidence should also be checked by using one or more different columns. In this context the gas density balance detector provides particular confidence in identification. 

A gas chromatographic method using a gas density balance detector has been described407 for the determination of chlorinated phenylchlorosilanes using a column packed with Celite 545 supporting 10% of Lukopren G 1000 (a silicone elastomer), with nitrogen as carrier gas. 

The MC-2 mass chromatograph from the Spex Industries determines molecular weight of GC eluent through differential gas density measurements. A sample is split into two equal fractions they are carried by two different gases, C02 and Freon 115, through two matched GC columns into density-balance detectors. The molecular weight of the unknown is obtained from... 

Figure 5.25 Gas density balance detector, mounted as shown.

Figure 17. A time diagram of the various pulses to the spectrograph.9 The lower pulse, which is on for a period of time t, is applied to the solenoid that controls the nozzle opening to introduce the sample into the Fabry-Perot cavity (see Figure 16). After an appropriate delay, the upper pulse, which is on for a period of time t3, is applied to PIN 1 which opens the microwave pulse to the Fabry-Perot cavity. The middle pulse, which is on for a time t4, is applied to PIN 2 in order to protect the balanced detector mixer (M3) from the high power microwave pulse.

If the amplitude of the amplifier output is normalized to unity, then this signal represents the sample absorption in per cent. If, furthermore, the amplifier has a logarithmic response (so-called balanced detectors are such devices), then the signal output is ln(/ //o), or log(/i//o), and thus is directly proportional to the absorbance A (see Equations (6.1) and (6.2)). Thus, an unknown absorber particle density N can easily be derived from the signal, provided the absorption cross-section a is known. Alternatively, one can calculate the absorption cross-section if the absorber particle density is predetermined. 

Figure 6.11 TypicaUaboratory set-up of an absorption spectrometer with multipass cell, balanced detector and wavelength monitoring. HR High reflector EC Fibre collimator...

Figure 14.5 Absorption signalofthe RbOj-line, using a tuneable diode Laser. Top linearabsorption signal, revealing th e diode s noise and amplitude variations with wavelength. Bottom same absorption, but normalized forthe diode laser characteristics using a balanced detector minor signal features (here diode laser side bands) are revealed due to the largely improved S/Nratio...

FIGURE 21 Functional block diagram of a detector using a pair of balanced photodiodes, a transimpedance amplifier (TIA), and operational amplifiers (op-amp s). This circuit is implemented in commercial balanced detectors such as PDB450C from Thorlabs. 

The sensitivity of this modulation spectroscopy is two to three orders of magnitude higher than in conventional spectroscopy and reaches a minimum detectable absorption of aL < 10 . Using multiple-path absorption cells (Herriot cells) and balanced detectors for detection of reference and transmitted beams, absorption coefficients as low as a < 10" ° cm" can still be measured. 

A thermobalance is built around a highly sensitive balance module, a furnace and a controlled atmosphere cabinet. Different types of balance detectors are available but the most commonly used is based on the principle of the null position balance. An example is given in Fig. 2.29. 

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