Detectors noise characteristics

Ge(Li) Detector Characteristics. Resolution measurements for the 18-cm.8 Ge(Li) detector were made with the anticoincidence shield in the inoperative mode, with a normal operating bias of 1700 volts, and with a preamplifier designed in our Laboratory (3, 4), and operated in conjunction with a Tennelec TC-200 linear amplifier. Resolution at 1.33 M.e.v. was 2.62 k.e.v., FWHM (Figure 4). The electronic pulser resolution for the amplifier system at a slightly lower energy was 1.86 k.e.v., the total capacitance of the detector was 28 pF, the noise slope was 0.035 k.e.v./pF, and the leakage current at 1700 volts was 0.5 X 10"9 amp. 

The most important detector characteristic is the signal it produces, of course, and that topic is treated throughout this chapter. In this section we will define two other important characteristics, noise and time constant. 

The level of noise restricts the minimum signal that can be detected and attributed to an analyte, so it is important to keep it to a minimum. A detector characteristic that is often more meaningful than the noise is the ratio of the signal-to-noise, SIN. In most chromatographic work it is... 

The two detector characteristics just discussed, noise and time constant, are important detector specifications too. However, this section will focus on the detector signal and its relationship to quantitative analysis. There are slight but significant differences between the specifications for concentration and mass flow rate types of detectors that will necessitate some duplication in the discussion. Recall that the concentration detector signal is proportional to concentration (e.g., g/mL) and the mass flow rate detector to mass flow (e.g., g/sec). 

Detectors. A detector capable of continuously monitoring effluent from the column is essential for efficient HPLC analyses. Considerations in connection with detector performance include absolute and relative sensitivity, drift characteristics, noise, linearity, specificity, and band spreading resulting from detector design. The selection of a proper detector is essential for successful analysis, both from the standpoint of sensitivity and elimination of effects of interfering compounds (specificity). 

Photoelectric gain should be maximized, but without compromising other detector characteristics like response time or noise level. 

A choice of instrumentation always involves trade-off considerations between signal-to-noise ratio, spectral and spatial resolution, telescope aperture, detector characteristics, frequency response, bit rate, and many other parameters. Clearly, the overall scientific objective should dictate the instrumental approach, but, in reality, the chosen solution must also fit within the framework of available resources such as volume, weight, power consumption, time required to complete the design, and cost. Within reason, these limitations are generally also subject to trade-off considerations. To limit ourselves in the presence of a multitude of instrumental approaches consider the Noise-Equivalent-Spectral-Radiance, NESR, of a radio-metric instrument, Eq. (5.8.14),... 

The unit cell type that best fits a specific application depends on many variables such as noise, real estate, dynamic range, and power. These were mentioned in Sections 7.3.1-7.3.4. Table 7.1 summarizes and compares those considerations. Figure 7.21 is a notional application space in terms of the magnitude of flux on the detector and the detector characteristics. 

For PMMA/additive dissolutions, it was not possible to identify any additive characteristic mass peaks, either by direct laser desorption or with matrix-assistance (dithranol, DHBA or sinapinic acid, 4-hydroxy-3,5-dimethoxy-cinnamic acid). This has again been ascribed to very strong interaction between PMMA and additives, which suppresses desorption of additive molecules. Also, partial depolymerisation of pho-tolytically labile PMMA by laser irradiation may play a role, which leads to saturation of the detector by PMMA fragment-ions and disappearance of additive mass peaks below noise level. Meyer-Dulheuer [55] has also reported MALDI-TOFMS analysis of a coating/2-ethylhexyldiphenylphosphate sample. Quantitative determination of the additives by means of MALDI-ToFMS proved impossible. Possibly the development of reproducible (automated) sample handling procedures or thin films might overcome this problem. 

Radiation detector output signals are usually weak and require amplification before they can be used. In radiation detection circuits, the nature of the input pulse and discriminator determines the characteristics that the preamplifier and amplifier must have. Two stages of amplification are used in most detection circuits to increase the signal-to-noise ratio. 

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