Detector pressure sensitivity

Detector Sensitivity, or Minimum Detectable Concentration Pressure Sensitivity Flow Sensitivity Temperature Sensitivity... 

The pressure sensitivity of a detector will be one of the factors that determines the long term noise and thus can be very important. It is usually measured as the change in detector output for unit change in sensor-cell pressure. Pressure sensitivity and flow sensitivity are to some extent interdependent, subject to the manner in which the detector functions. The UV detector, the fluorescence detector and the electrical... 

Here the points suspected of leaking at the pressurized test specimen (see Fig. 5.4, d) are carefully traced with a test gas probe which is connected with the leak detector by way of a hose. Either helium or hydrogen can be detected with the INFICON helium leak detectors. The sensitivity of the method and the accuracy of locating leaky points will depend on the nature of the sniffer used and the response time for the leak detector to which it is connected. In addition, it will depend on the speed at which the probe is passed by the leak points and the distance between the tip of the probe and the surface of the test specimen. The many parameters which play a part here make it more difficult to determine the leak rates quantitatively. Using sniffer processes it is possible, virtually independent of the type of gas, to detect leak rates of about 10 mbar l/s. The limitation of sensitivity in the detection of helium is due primarily to the helium in the atmosphere (see Chapter 9, Table VIII). In regard to quantitative measurements, the leak detector and sniffer unit will have to be calibrated together. Here the distance from the specimen and the tracing speed will have to be included in calibration, too. 

Refractive index detectors are useless in gradient elution because it is impossible to match exactly the sample and the reference while the solvent composition is changing. Refractive index detectors are sensitive to changes in pressure and temperature (—0.01 °C). Because of their low sensitivity, refractive index detectors are not useful for trace analysis. They also have a small linear range, spanning only a factor of 500 in solute concentration. The primary appeal of this detector is its nearly universal response to all solutes, including those that have little ultraviolet absorption. 

The pressure sensitivity of a detector is extremely important as it is one of the detector parameters that determines both the long term noise and the drift. As it influences long term noise, it will also have a direct impact on detector sensitivity or minimum detectable concentration together with those other characteristics that depend on detector sensitivity. Certain detectors are more sensitive to changes in pressure than others. The katherometer detector, which is used frequently for the detection of permanent gases in GC, can be very pressure sensitive as can the LC refractive index detector. Careful design can minimize the effect of pressure but all bulk property detectors will tend to be pressure sensitive. 

The pressure sensitivity (Dp) should be given as the output in millivolts for unit pressure change in the detector e.g. as mV/p.s.i or mV/kg/m. The pressure sensitivity can be used to calculate the pressure change (Np) that would provide a signal equivalent to the detector noise (N ),... 

Pressure Sensitivity - (Dp) - The pressure sensitivity of a detector is the output that results from unit change in pressure. It is usually specified in V/p.s.i. or V/kg/m. ... 

As the detector filament is in thermal equilibrium with its surroundings and the device actually responds to the heat lost from the filament, the katherometer detector is extremely flow and pressure sensitive. Consequently, all katherometer detectors must be carefully... 

Very often baseline problems are related to detector problems. Many detectors are available for HPLC systems. The most common are fixed and variable wavelength ultraviolet spectrophotometers, refractive index, and conductivity detectors. Electrochemical and fluorescence detectors are less frequently used, as they are more selective. Detector problems fall into two categories electrical and mechanical/optical. The instrument manufacturer should correct electrical problems. Mechanical or optical problems can usually be traced to the flow cell however, improvements in detector cell technology have made them more durable and easier to use. Detector-related problems include leaks, air bubbles, and cell contamination. These usually produce spikes or baseline noise on the chromatograms or decreased sensitivity. Some cells, especially those used in refractive index detectors, are sensitive to flow and pressure variations. Flow rates or backpressures that exceed the manufacturer s recommendation will break the cell window. Old or defective source lamps, as well as incorrect detector rise time, gain, or attenuation settings will reduce sensitivity and peak height. Faulty or reversed cable connections can also be the source of problems. 

This can be achieved by on-line analysis of the concentration profile. As depicted in Fig. 6.35, one or more detectors are placed in the recycle stream. They are positioned in front of the recycle pump, because some detectors are sensitive to high pressure. Due to the shifting of external streams the detector, /travels" through every process section during one cycle (see below). 

The problems that can be caused by highly pressure-sensitive detector cells e.g., RI detector cells) have been solved in different cornnisrcially available detectors by the construction of cells with greater volumes and larger outlet diameters. This is possible because the volume spreading of the peaks in the detection cell is not so critical as in analytical work. 

Once the chromatographic separation on the column has been conducted, the composition of the eluent at the column end must be determined using a detector. In all HPLC detectors, the eluent flows through a measuring cell where the change of a physical or chemical property with elution time is detected. The most important parameter of the detector is sensitivity, which is influenced by the noise and baseline drift, the absolute detection limit of the detector, the linearity, the detector volume (band broadening), and the effects of pressure, temperature and flow (pulsation, gas bubbles). 

Matsuda et al. [10] have proposed the use of a pressure-sensitive paint (PSP) technique, which is based on the interaction of atoms or molecules with photons, to measure the pressure inside micro/nanochannels. This technique is limited to gaseous flow, and has drawbacks for high-pressure and low-speed applications. Moreover, the surface where the pressure is to be measured must be visible to the detector. The temperature sensitivity of the PSP technique must also be considered during the calibration. In conclusion, although several techniques have been suggested by many researchers, the development of measurement techniques for use on the microscale is a challenging topic, and this topic seems likely to remain open-ended in the near future. 

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