Background-limited performance

Much higher sensitivity and much lower noise can be achieved by means of a highly temperature dependent resistive device operated at low temperature the liquid Helium cooled Ge bolometer. In most instances present day bolometers have an intrinsic noise characteristic which is well below the intensity fluctuation limit imposed by the statistical fluctuation in the photon flux incident on the detector. This is the so-called "background limited" performance limit. 

To briefly indicate how this background limited performance limit is determined the number of photons N striking a detector per second can be calculated from the total power on the detector divided by the energy of the average photon. The uncertainty in this number N is given by ViJ. The power fluctuation then becomes /n times the energy of the photon. 

This noise can be reduced only by reducing the number of photons per second striking the detector. Current bolometers incorporate liquid Helium cooled field of view masks and liquid Helium cooled optical bandpass filters so that only useful radiation is permitted to strike the detector element. In this manner reduced background limited performance is achieved. 

The photon noise is simply due to the uncertainty, a, in the number of photons per second, i.e., the detected signal level. In the photon noise limit the signal-to-noise ratio is proportional to the square root of the photon arrival rate. When the measured noise is the photon noise in the background radiation, the detection is said to have reached background limited performance (BLIP). 

Unlike most other atomic spectroscopic techniques, which normally require repetitive measurements on a blank sample, an estimate of XRF detection limit performance can be made from a knowledge of the appropriate background count rate and elemental sensitivity. Using the theoretical basis outlined above, it can be shown that the lower limit of detection (LLD) (i.e., the 3cr, detection limit) can be calculated from ... 

FIGURE 10 Graph of detector performance indicating mode of operation, bandwidth, and pulse response. SL, signal or photon noise limited. BL, background limited. AL, amplifier noise limited. Various semiconductors are centered at their approximate wavelength cutoff. 

BLIP (background-limited infrared jiiotodetecta-) sometimes means the best performance one can obtain - and that impUes QE-FF = 1.00. Here, we choose to express the best possible performance with a detector of any specified QE-FF product. 

It is useful to envision a perfect array as a benchmark for how far we have progressed. My personal definition is an array with roughly 10 pixds, each, of which has detective quantum efficiency (DQE) > 50% and works at the background limit in any foreseeable application. Evm more ambitious definitions cotdd be formulated and more ambitious arrays would produce advances in many types of measurement. Howeva, from watchii my colleagues in optical astronomy, I have noted that many considerations besides performance become important once my perfect array becomes available. 

In modern times, most analyses are performed on an analytical instrument for, e.g., gas chromatography (GC), high-performance liquid chromatography (HPLC), ultra-violet/visible (UV) or infrared (IR) spectrophotometry, atomic absorption spectrometry, inductively coupled plasma mass spectrometry (ICP-MS), mass spectrometry. Each of these instruments has a limitation on the amount of an analyte that they can detect. This limitation can be expressed as the IDL, which may be defined as the smallest amount of an analyte that can be reliably detected or differentiated from the background on an instrument. 

Third, the bulk of the items in Table 1 address method performance. These requirements must be satisfied on a substrate-by-substrate basis to address substrate-specific interferences. As discussed above, interferences are best dealt with by application of conventional sample preparation techniques use of blank substrate to account for background interferences is not permitted. The analyst must establish a limit of detection (LOD), the lowest standard concentration that yields a signal that can be differentiated from background, and an LOQ (the reader is referred to Brady for a discussion of different techniques used to determine the LOD for immunoassays). For example, analysis of a variety of corn fractions requires the generation of LOD and LOQ data for each fraction. Procedural recoveries must accompany each analytical set and be based on fresh fortification of substrate prior to extraction. Recovery samples serve to confirm that the extraction and cleanup procedures were conducted correctly for all samples in each set of analyses. Carrying control substrate through the analytical procedure is good practice if practicable. 

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