Background-limited

In practice, the NEP of a room-temperature THz spectrometer is usually limited by fluctuations (shot-noise) in the ambient blackbody radiation. Usmg an optical bandwidth Av = 3 THz (limited by, for example, a polyethylene/diamond dust window), a field of view (at nomial incidence) 0 = 9 and a detecting diameter (using a so-called Winston cone, which condenses the incident radiation onto the detecting element) laboratory applications, the background-limited NEP of a bolometer is given by... 

When equation 12 is vaUd, the detector is said to be a background-limited infrared photodetector (BLIP). When this is the case, attempts often are made to improve D by cold shielding which reduces ( ). The ideal D is shown in Figure 3 as a function of wavelength with background photon flux as a parameter. The line of termination in the lower left corner represents TN values for a 180° (27T) detector field of view, 300 K ambient background... 

Back contact solar cell, 23 46 Back-end volatility, 12 398-399 Backflow, 11 323 Backflow bag filters, 26 709 Background-limited IR photodetector (BLIP), 19 135... 

Table V. Exponent of Dependence of SNR, MDC, and MAT on Three Instrumental Variables for Signal-Limited and Background-Limited Detection Cases in the Poisson Noise Limit...

The numerical entry represents the power to which the row variable, varied alone, must be raised to be proportional to the column variable to the first power. For example, the column variable SNR is proportional to the row variable Rs1/2 in the signal-limited case and to Rs1 in the background-limited case. 

In the interests of improved electrochemical background limits and reactant stability, it is important to employ solvents that are as free as possible of nucleophiles and proton sources. Special attention always goes to the removal of water. The most important media are carefully purified acetonitrile, dimethyl-formamide, benzonitrile, and tetrahydrofuran. Popular supporting electrolytes are tetra-n-butylammonium perchlorate (TBAP) and fluoroborate (TBABF4). Solutions are usually prepared by vacuum-line methods (Chap. 18) or in a dry box (Chap. 19) to exclude oxygen from the systems and to avoid contamination by water. 

Note that the quantum limit on coherent receivers is 0.5hv/k or 0.024 K/GHz (Wright, 1999). This corresponds to 0.5 photons per mode. The 0.3 K/GHz for cryogenic HEMTs is about 7 photons/mode. At 72 GHz the CMB has only n = 0.4 photons per mode, where n = (exp[hv/kT] — l)-1 is the mean number of photons per mode. Thus a background-limited incoherent detector could be much more sensitive than a coherent radiometer using HEMTs. 

Consider a bolometric radiometer with an 18 GHz at 72 GHz with a diffraction-limited throughput, Ail = A2. A background limited (BLIP) system would have a temperature sensitivity of 19 /jK in 1 second. This is already 6 times better than the 107 /iK in one second needed to reach the point of diminishing... 

It is important to recognize that it is not necessary to operate an electrochemical detector in the "plateau" region of the voltammogram. In fact, some substances do not reach a plateau prior to the background limit imposed by the mobile phase. 

From Figure 1.1.4, one can see that the open-circuit potential is not well defined in the system under discussion. One can say only that the open-circuit potential lies somewhere between the background limits. The value found experimentally will depend upon trace impurities in the solution (e.g., oxygen) and the previous history of the Pt electrode. 

With Hg, the anodic background limit occurs when Hg is oxidized to Hg2Br2 at a potential near 0.14 V NHE (0.07 V V5. Ag/AgBr), characteristic of the half-reaction... 

In general, the background limits depend upon both the electrode material and the solution employed in the electrochemical cell. 

Use a spreadsheet program to calculate and plot the current-potential curve for this system scanning from the anodic background limit to the cathodic background limit. Take the appropriate standard potentials from Table C.l and values for other parameters (mo, a,. .. ) from Problem 3.3. 

The considerations involved in understanding background currents in SWV are exactly those encountered in the treatment of DPV. If is greater than five cell time constants, there is no appreciable charging current contribution, either to the individual current samples or to the differences. Faradaic background processes do contribute and often control the detection limits of SWV. At solid electrodes or near background limits, the effects on the forward and reverse currents can be sizable, but they are often suppressed effectively in the difference currents. 

All reactions except 4 are reversible. That process will not show a wave at platinum before the cathodic background limit is reached. 

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