Methods pulse

An alternative approach to isotherm determination is offered by the so called Pulse methods There are two types of pulse methods, the elution of a pulse 

The method of elution on a plateau was first suggested by Helfferich in Science more than forty years ago [126], In the PP method, the chromatographic column is equilibrated with a constant stream of molecules in the mobile phase and a concentration plateau is established. A perturbation is then accomplished by injecting a sample containing an excess or a deficiency of the molecules as compared to the concentration at the plateau. [118-120, 127], The response at the column outlet will be small peaks, known as perturbation peaks, and their retention times are used to determine the adsorption isotherm parameters. The retention time of the perturbation peak is related to the isotherm through the equation  

Here t0 is the column hold-up time, F is the phase ratio and dq/dc is the derivative (slope) of the isotherm function that is evaluated at the plateau concentration Ct. The retention time of a small injection of a compound in a column equilibrated with pure mobile phase without the compound (such as for analytical chromatography) gives the retention time under linear conditions, through the equation  

---

In order to prevent this occurring a pulsed method of pumping is used with a repetition rate low enough to allow time for Tj — Sq relaxation. For CW operation either Tj must be sufflciently short or another dye has to be used for which T2 — Ti absorption does not overlap with the fluorescence. 

The desugarization by-product is normally sold as a low value molasses. Pulse method systems also produce a relatively high value betaine-rich (at least 50% on soHds) fraction. The concentrated betaine-rich by-product is used as a custom animal feed, whose European markets are well estabUshed and may provide a future opportunity in the U.S. feed industry. Beet sugar molasses contains from 3 to 6% betaine, by weight, about three-quarters of which may be recoverable as a potential by-product ( 40 50% purity). 

Test Method for Thermal Diffusivity of Carbon and Graphite by a Thermal Pulse Method... 

To study the lowest pressure region from about 10 MPa, the acceleration loading pulse method previously used by Setchell [88S01] has been employed. In this case the slowly rising stress pulse from high quality fused quartz is in the form of a ramp in pressure. Hence, a continuous response can be determined to stresses up to about 3 GPa. 

New techniques (multiple pulse methods) in structure elucidation [14] Acquisition of newly accessible heteronuclei (such as N and O) [75]... 

Each of these two procedures can be varied by proceeding from a low to a high current density (or potential) or from a high to a low current density (or potential) the former is referred to as forward polarisation and the latter as reverse polarisation. Furthermore, there are a number of variations of the potentiostatic technique, and in the potentiokinetic method the pwtential of the electrode is made to vary continuously at a predetermined rate, the current being monitored on a recorder in the pulse method the electrode is given a pulse of potential and the current transient is determined by means of an oscilloscope. 

The electrical double-layer structure of a Pt/DMSO interface has been investigated using the potentiostatic pulse method.805 The value of C at E = const, as well as the potential of the diffuse layer minimum, have been found to depend on time, and this has been explained by the chemisorption of DMSO dipoles on the Pt surface, whose strength depends on time. Eg=Q has been found11 at E = -0.64 V (SCE in H2O). 

Obviously, experiments designed to measure cross-sections as a function of energy are needed. At present, tandem experiments are not capable of high precision at low energies because one must assume details of collision mechanics and because it is difficult to estimate collection efficiencies in forward scattering geometry (15). The extension of all known techniques to lower energy (64, 65) and the further development of pulse methods (58) offer the possibility for advances in this area. 

Met. = Method. EC = electrocapillary method, PC = electrode capacitance measured with a pulse method, T = ten-sammetry. See also list of symbols. 

CS = coulostatic method, CV = cyclic voltamogram, FD = Faradaic distortion method, FR = Faradaic rectification, GD = galvanostatic double pulse method, IP = impedance method, PS = potential step method. See also list of... 

PC eleetrode eapaeitanee measured with a pulse method... 

The fluorescent lifetime of chlorophyll in vivo was first measured in 1957, independently by Brody and Rabinowitch (62) using pulse methods, and by Dmitrievskyand co-workers (63) using phase modulation methods. Because the measured quantum yield was lower than that predicted from the measured lifetime, it was concluded that much of the chlorophyll molecule was non-fluorescent, suggesting that energy transfer mechanisms were the means of moving absorbed energy to reactive parts of the molecule. 

Pulsed method. Using a pulsed or modulated excitation light source instead of constant illumination allows investigation of the time dependence of emission polarization. In the case of pulsed excitation, the measured quantity is the time decay of fluorescent emission polarized parallel and perpendicular to the excitation plane of polarization. Emitted light polarized parallel to the excitation plane decays faster than the excited state lifetime because the molecule is rotating its emission dipole away from the polarization plane of measurement. Emitted light polarized perpendicular to the excitation plane decays more slowly because the emission dipole moment is rotating towards the plane of measurement. 

Metal dispersions were obtained by the dynamic pulse method using either H2, CO or O2 chemisorption at 298 K (.J) ... 

A word should be said regarding the use of O2 chemisorption to measure Ru-Ir metal dispersions. The stoichlonetry of the CO adsorption on Ir (CO/Irjgj) was taken from the literature to be 0.5 (9-10). The measured CO/O2 chemisorption ratio on Ir was determined using the dynamic pulse method and found to be 1.55. These results give... 

Fig. 3.51 shows CO chemisorption data using the pulse method for a reduced Pt/A Os sample. It is clear that the peak height gradually increases as more pulses are given. From the... 

Fig. 5.18 Potentiostatic methods (A) single-pulse method, (B), (C) double-pulse methods (B for an electrocrystallization study and C for the study of products of electrolysis during the first pulse), (D) potential-sweep voltammetry, (E) triangular pulse voltammetry, (F) a series of pulses for electrode preparation, (G) cyclic voltammetry (the last pulse is recorded), (H) d.c. polarography (the electrode potential during the drop-time is considered constant this fact is expressed by the step function of time—actually the potential increases continuously), (I) a.c. polarography and (J) pulse polarography...

Part E. Pulse Method of Measuring Thermophysical Parameters... 

This chapter concludes with a brief description of one advanced technique, Electron Nuclear Double Resonance (ENDOR), the capabilities for which, unlike pulsed methods, may be added as a relatively minor modification to commercial CW ESR spectrometers. 

---