Detection theoretical limits

Direct time-dependent detection is limited by the response time of detectors, which depends on the frequency range, and the electronics used for data acquisition. In the most favourable cases, modem detector/oscilloscope combinations achieve a time resolution of up to 100 ps, but 1 ns is more typical. Again, this reaction has been of fiindamental theoretical interest for a long time [59, 60]. 

Fig. 9. Spectral sensitivity of detectors where the detector temperatures in K are in parentheses, and the dashed line represents the theoretical limit at 300 K for a 180° field of view, (a) Detectors from near uv to short wavelength infrared (b) lead salt family of detectors and platinum siUcide (c) detectors used for detection in the mid- and long wavelength infrared. The Hg CdTe, InSb, and PbSnTe operate intrinsically, the doped siUcon is photoconductive, and the GaAs/AlGaAs is a stmctured supedattice and (d) extrinsic germanium detectors showing the six most popular dopants.

The beating of a faint source with a high power coherent source is a well known process to detect its phase and amplitude. The same detection equipment allows the evaluation of the power of the source with theoretical limits similar to a noiseless photon counter. Such detection apparatus are limited by the bandwidth of the electronic component as this bandwidth is rapidly increasing, this may be a competitive solution for quantum limited detection in the far infra red. The phase of a thermal source is an useless information ... 

The resolution of an FPD is theoretically limited by the number of channels (512-2048). In practice, it is even less because the image suffers some broadening as it passes from the first MCP to the photodiode array (Fig. 4.63). Therefore, instruments with FPD can normally be switched from FPD to SEM detection, e.g., by vertical electrostatic deflection of the ion beam (Finnigan MAT900). Furthermore, quadrupole lenses or an inhomogeneous ESA behind the magnet are employed to achieve variable dispersion, i.e., to zoom the m/z range of simultaneous... 

In indirect detection, the theoretical limit of detection (LOD) is given by ... 

Fig. 3. The theoretical increase in receptivity (abundance X sensitivity) obtainable by isotope enrichment and inverse ]H detection of13C, 15N and 195Pt. In practice, inverse 1H- 195Pt detection is limited by the broad linewidths of the 195Pt satellites.

The detection limit for a silver chloride electrode is shown in Fig. 13.8, which is far above the theoretical limit predicted from the solubility product. It has been shown in solutions of buffered silver ion activity that the limit can be reached. Note that the sensitivity is linked to resolution of the potential measurement and not with the detection limit. 

The transmission for ns UV pulses is clearly dependent on the fluence in a range of a few mj cm-2 to several J cm"2. With 248-nm irradiation, only a slight increase of the transmission ratio, TH/TL, could be detected, whereas with the XeCl excimer laser irradiation much higher transmission ratio values are reached which are close to the theoretical limit. 

In anodic stripping voltammetry of amalgams and metal deposits, there is no theoretical limit of detection of metal ions. If the accumulation potential is on the plateau of the pseudopolarogram and the solution is stirred, a steady state is established and the concentration of metal ions is linearly proportional to the duration of the accumulation ... 

The theoretical detection limit is plotted for three band-widths, 1 kHz, 1 MHz, and 1 GHz. Also shown are the corresponding pulsewidths, 0.5 ms, 0.5 /zs, and 0.5 ns. This ultimate performance is essentially only obtainable with the vacuum photomultiplier and then only within a factor of 4 (25% quantum efficiency) at wavelengths of 0.5 /xm and shorter. Avalanche photodiodes can also approach the theoretical limit at high bandwidths, as discussed in Section V.B.3. Although the avalanche diode in the Geiger mode can count individual photoevents, it is not operable in a continuous or analog signal mode. 

For a majority of the existing thermal detectors there is a trade-off between flie specific detectivity and the maximum response speed— a detector may be smaller and faster, or larger and more sensitive. There is a frequency-dependent thermodynamical limit of the detectivity of thermal detectors. Instead of that theoretical limit, a more realistic Havens limit is used in practical situations [5]. Generally speaking, thermal detectors tend to be at the slower side of the response. 

In the past, specifications for metal ion content and particles have been arbitrarily reduced for each new generation (smaller geometry) of devices. Recently, research has been conducted on the correlation of device performance with various impurities. While impurity specifications will continually drop, only troublesome impurities will be reduced. This change in strategy of purity management will reduce the burden on chemical suppliers, especially as specifications move toward 1 part per trillion (ppt) for metals and to the theoretical limits of particle size detection of optical counters. Semiconductor Equipment and Materials International (SEMI) has recently initiated an effort to reduce the number of specified metals down to 17 (Table 15.4) from over 30. 

The detection limit metric, as used here, is different from the sensitivity metric. Unlike sensitivity, which deals only with absolute signal, detection limits consider signal-to-noise ratio (S/N). Detection limits also consider the entire analytical method including sample extraction efficiencies and injection volumes in addition to the response characteristics and noise levels of the mass spectrometer signal. Detection limits are therefore analytically more relevant than sensitivity. Sensitivity measurements allow one to estimate the theoretical limits for the quantity of a compound that can be detected, whereas detection limits define what can actually be achieved in a practical setting to provide a meaningful result toward the solution of some application of mass spectrometry. 

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