Sample modulation measurements

Ideally, time-resolved and imaging measurements require that the OPD be held precisely constant at each sampling point. On the other hand, for photoacoustic and most sample modulation measurements, the position of one of the interferometer mirrors is varied sinusoidally with an amplitude of one, two, or more wavelengths of the HeNe laser by a piezoelectric transducer while the average position is held constant at each sampling point. This mode of operation is called phase modulation. 

OTHER SAMPLE MODULATION MEASUREMENTS WITH STEP-SCAN INTERFEROMETERS... 

Prior to describing the possible applications of laser-diode fluorometry, it is important to understand the two methods now used to measure fluorescence lifetimes these being the time-domain (Tl)/4 5 24 and frequency-domain (FD) or phase-modulation methods.(25) In TD fluorometry, the sample is excited by a pulse of light followed by measurement of the time-dependent intensity. In FD fluorometry, the sample is excited with amplitude-modulated light. The lifetime can be found from the phase angle delay and demodulation of the emission relative to the modulated incident light. We do not wish to fuel the debate of TD versus FD methods, but it is clear that phase and modulation measurements can be performed with simple and low cost instrumentation, and can provide excellent accuracy with short data acquisition times. 

Figure 13.6 Principle of phase-modulation measurement and elements of a phase fluorometer (a) sample measurement (b) cross-correlation phase fluorometer,...

The volume-sampling module (Module 4) consists of three submodules. The purpose of submodule 4.1 is the gas suction or pump (not needed in pressurised gasification). In the. submodule 4.2 a gas meter measures the volume of gas flow. Additionally a flow meter, a pressure indicator and a temperature indicator are needed. 

The pulse cavitation peak phenomenon, where under an appropriate choice of the pulse width the cavitation yield reached an apparently maximum value, was first studied in a reverberating field [84,85], Frequencies of823-kHz and 1,7-MHz ultrasonic waves modulated by rectangular wave with an on off ratio of 1 1 and pulse width from 100 ps to 270 s were used to irradiate both an aqueous terephtha-late solution and deionized water, respectively. After sonication, the fluorescence and electroconductivity changes of the samples were measured. All the experimental data showed that a maximum value of cavitation yield occurred in the pulse width range between 10 and 100 ms. The theoretical result predicted by the authors based on the sonicated system resonance with the modulation envelope appeared to be in agreement with experimental data. In further investigations a similar phenomenon was observed for the 1.7-MHz ultrasound modulated by sine wave [86,87],... 

Figure 3.16. Electroclinic effect in ferroelectric LCEs (a) Chemical structure of sample, (b) Measurement geometry the beam in the interferometer passes twice through the film to measure the electrically induced thickness modulation, (c) The viewing angle is turned by 90° around the layer normal compared with that in (b). Source Lehmann et al., 2001.

The fluctuations in the fluorescence signal from a sample are used in FCS to probe the processes that cause them. In terms of a diffusing sample, the effective number (concentration) fluctuations of a fluorescent species in an open sample volume defined by a fluorescence microscope are monitored (see Figure 2.1). The number fluctuations of the fluorescent species can be due to random diffusion of molecules into and out of the sample volume, chemical reactions, or structural changes— indeed any effect that generates, extinguishes, enhances or modulates a fluorescence signal from a species in the sample volume. Measurements of the... 

The type of recording electronics used depends on whether pulsed excitation (either pulse sampling or timesingle photon covmting) or phase/ modulation measurement are being made. Therefore, we discuss the electronics in the context of the method. 

A d.c. voltage was applied to the polyacetylene gap, which acts as a pulser upon excitation and biases the second gap. The quantity measured is the total charge at the output of the Si gap as a function of the relative delay between the two light pulses. The signal was amplified with a broadband preamplifier and integrated with a fast-sampling module of a boxcar system. 

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