Deflection detection methods

Deflection detection methods 317—323 detection limit 322 mechanical resonance 318 optical beam deflection 321 optical interferometry 319 vacuum tunneling 317 Density of states 23... 

RI detectors measure this deflection, and are sensitive to all analytes that have a different R1 than the mobile phase. There are two major limitations First, Rl detectors are very sensitive to changes in the temperature, pressure, and flow rate of the mobile phase, and so these measurement conditions must be kept stable in order to obtain low background levels. Second, Rl detectors are incompatible with chromatographic separations using gradient elution. Furthermore, because Rl detectors are nonselective, they must be used in conjunction with other detection methods if specificity is required. Nevertheless, they have found wide application in isocratic chromatographic analysis for analytes that do not have absorptive, fluorescent, or ionic properties, such as polymers and carbohydrates. 

There exist a number of readout techniques based on optical beam deflection, variation in capacitance, piezoresistance, and piezoelectricity. Piezoelectricity is more suited for a detection method based on resonance frequency than the method based on cantilever bending. The capacitive method is not suitable for liquid-based applications. The piezoresistive readout has many advantages, and it is ideally suited for handheld devices. 

Resistance variation can be detected by either null-balance or deflection-balance bridge circuits. In a null-balance bridge the sensor resistance change is balanced (zero output) by a variable resistance in a bridge adjacent arm. The calibrated null adjustment is an indication of the change in sensor resistance. The deflection-balance method, on the other hand, makes use of the amount of bridge unbalance in order to determine the change in sensor resistance. 

The deflections of a microcantilever can be measured with sub-Angstrom resolution using current techniques perfected for die AFM, such as optical reflection, piezoresistive, capacitance, and piezoelectric detection methods. One great advantage of flie cantilever technique is diat five resonance response parameters (resonance fiequency, phase, anqilitude, Q-flictor, and deflection) can be simultaneously detected. 

The particle size dependence of thermophoresis has been extensively studied for dilute polystyrene beads dispersed in aqueous solutions with very thin electric double layers under pH conditions of 7 8. The results of these investigations however are controversial and inconclusive. As shown in Fig. 6, Duhr and Braun [2] reported that the thermal diffusion coefficient Dj is proportional to particle diameter at room temperature by using the fluorescence detection method. Although Braibanti et al. [10] observed a similar dependence of Dj on particle size at a temperature of 45 °C through the beam deflection method, they found independence of with particle size at room temperature and at 5 °C, as shown in Fig. 6. In addition, for concentrated polystyrene beads at a temperature of 50 °C, Dj is not found to be dependent on particle size. 

Instrumentation. Commercial atomic force microscopes Explorer from Topometrix Inc., Nanoscope Illa Digital Instruments Co. Ltd.) which are based on the laser beam deflection detection scheme were used in conjunction with digital oscilloscopes of very stable low frequency (1-20 Hz) trigger system for lateral force (friction) measurements, and dual-phase lock-in amplifiers and function generators for force modulation measurements. Various triangular silicon nitride cantilevers were used. The lateral spring constants were determined with the "blind torsional calibration method discussed in more detail in the Appendix. 

In FM-AFM, a cantilever is mechanically oscillated at its resonance frequency using a piezo actuator placed adjacent to the cantilever. The induced cantilever vibration is detected typically with the optical beam deflection (OBD) method, as shown in Fig. 18.1. 

Detection of cantilever displacement is another important issue in force microscope design. The first AFM instrument used an STM to monitor the movement of the cantilever—an extremely sensitive method. STM detection suffers from the disadvantage, however, that tip or cantilever contamination can affect the instrument s sensitivity, and that the topography of the cantilever may be incorporated into the data. The most coimnon methods in use today are optical, and are based either on the deflection of a laser beam [80], which has been bounced off the rear of the cantilever onto a position-sensitive detector (figme B 1.19.18), or on an interferometric principle [81]. 

For SFM, maintaining a constant separation between the tip and the sample means that the deflection of the cantilever must be measured accurately. The first SFM used an STM tip to tunnel to the back of the cantilever to measure its vertical deflection. However, this technique was sensitive to contaminants on the cantilever." Optical methods proved more reliable. The most common method for monitoring the defection is with an optical-lever or beam-bounce detection system. In this scheme, light from a laser diode is reflected from the back of the cantilever into a position-sensitive photodiode. A given cantilever deflection will then correspond to a specific position of the laser beam on the position-sensitive photodiode. Because the position-sensitive photodiode is very sensitive (about 0.1 A), the vertical resolution of SFM is sub-A. 

The metals were coprecipitated with lead-ammonium pyrrolidine dithio-carbamate and detected by X-ray spectrometry following neutron activation. Magnetic fields deflect the p rays while the X rays reach the silicon (lithium) detector undeviated. The detectors have low sensitivity to y rays. The concentration of cobalt found by this method was 1.3 xg/l, about one-fifth of that measured previously, while that of copper, 2.0 xg/l, agreed with results obtained by some previous workers. The concentration of mercury was 1.2 xg/l. 

This temperature rise can be detected directly (laser calorimetry and optical calorimetry), or indirectly by measuring the change in either the refractive index (thermal lensing, beam deflection or refraction and thermal grating) or the volume (photo- or optoacoustic methods). This review will focus primarily on photoacoustic methods because they have been the most widely used to obtain thermodynamic and kinetic information about reactive intermediates. Other calorimetric methods are discussed in more detail in a recent review.7... 

Atomic force microscopy (AFM) or, as it is also called, scanning force microscopy (SFM) is the most generally applicable member of the scanning probe family. It is based on the minute but detectable forces - order of magnitude nano-Newtons -between a sharp tip and atoms in the surface [39]. The tip is mounted on a flexible arm called a cantilever, and is positioned at a subnanometer distance from the surface. If the sample is scanned under the tip in the x-y plane, it feels the attractive or repulsive force from the surface atoms and hence is deflected in the z direction. Various methods exist to measure the deflection, as described by Sarid [40]. Before we describe equipment and applications to catalysts, we will briefly look at the theory behind AFM. 

The technique employed is IR-FT photothermal beam deflection spectroscopy (PBDS). It is an off-shoot of photoacoustic spectroscopy (PAS) [1] and is based on the "mirage detection of the photothermal effect invented by Boccara et al. [2] and shown to result in a spectroscopic technique of remarkable versatility and utility. Some applications of "mirage spectroscopy," mainly in the visible, and theoretical treatments, have been described [3 6]. The method has now been developed for use in the IR. The spectrometer and techniques are described in detail elsewhere [7], but it will be useful to give a brief outline of the principles. 

Many different methods have been developed for detecting the minute deflection of the cantilever (Sarid, 1991). In this. section, we present several important ones, including vacuum tunneling (Binnig, Quate, and Gerber, 1986), mechanical resonance (Diirig, Gimzewski, and Pohl 1986), optical interferometry (Martin et al., 1988 Erlandson et al., 1988), and optical beam deflection (Meyer and Amer, 1988). 

The tunneling current between two metal electrodes separated by a vacuum gap varies about one order of magnitude per A. Therefore, vacuum tunneling provides an extremely sensitive method for detecting minute displacements. The first AFM, demonstrated by Binnig, Quate, and Gerber (1986), utilized vacuum tunneling to detect the cantilever deflection. 

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