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noise-filtering devices (or at best the lowest attenuation with the fastest response time) and then corrected to an attenuation of unity.   Detector Sensitivity or Minimum Detectable Concentration Detector sensitivity or minimum detectable concentration (MDC) is defined as the minimum concentration of solute passing through the detector that can be unambiguously discriminated from noise. The size of the signal that will make it distinguishable from the noise (the signal�to�noise ratio) is an arbitrary choice. It is generally accepted that with electronic measuring instruments discrimination is possible when the signal to noise ratio is two and this criteria has been adopted for chromatography detectors.   Thus for a concentration sensitive detector, the detector sensitivity (XD) is given by   (Rc) and (ND) being determined in the manner previously described

and a well�designed cell involves a careful compromise between cell radius and length to provide the maximum sensitivity. Most modern UV spectrometer sensor's have path lengths that range between 1 and 10 mm and internal radii that range from about 0.5 to 2 mm. From equation (2),   Where (A) is termed the absorbance?   Now (DA) is commonly employed to define the sensor sensitivity where the value of (DA) is the change in absorbance that provides a signal-to-noise ratio of two.   Thus   Where, (Dc) is the sensor concentration sensitivity or minimum detectable concentration.   Thus   The UV spectrometer used for monitoring the eluent from a GC column employs a source that provides light over a wide range of wavelengths and consequently, with the aid of an appropriate optical scanning system, absorption spectra of any substance eluted from a gas

nbsp; �It must be emphasized that detectors can not� be compared on the basis of their noise or response. They can only be compared on the basis of their sensitivity or signal-to-noise ratio at a specific solute concentration.   6. Detector Sensitivity � Minimum Detectable Concentration (MDC)� (XD)-Detector sensitivity can also be defined as that concentration that will produce a signal equivalent to twice the noise or, as that change in the physical property being measured that will provide a signal equivalent to twice the noise. The sensitivity defined in concentration units is, in general, more useful to the analyst.   7.Detector Dispersion

nbsp;                 Now,                                      where (A) is termed the absorbence Now (DA) is sometimes employed to define the detector sensitivity where the value of (DA) is the change in absorbence that provides a signal-to-noise ratio of two.                Thus                  where (Dc) is the detector concentration sensitivity or minimum detectable concentration

of the pipe as it passes through it. The light leaving the pipe is focused by means of two curved mirrors onto a cooled IR sensor. A cold baffle is situated between the pipe exit and the sensor to ensures that none of the IR emitted by the hot oven that thermostats the tube can fall on the sensor.   Courtesy of the Perkin Elmer Corporation   Figure 34. The Optical Arrangement of Light Pipe Interface   This simple device reduces the noise generated by the oven and increases the signal-to-noise ratio and thus the sensitivity. Details of the light pipe are shown in figure 34. This interface can be used with all types of GC columns including open tubular columns. The capillary column passes into the interface through a heated tube right up to the light pipe. Concentric to the column, and through the same heated tube is fed a stream of scavenging gas that carries the solute through the IR light pipe. This maintains the integrity of the separation at the expense of some

manner and consequently, the circuit shown in figure 50 may be only schematic. The column was connected directly to the sensor and the eluent passed through the annular channel between the central electrode and the sensor wall. The response of the radio frequency discharge detector was reported as 106mV for a concentration change of 10-3 g/ml of methyl laureate. The noise level was reported to be 0.05 mV, which would give the minimum detectable concentration for a signal�to�noise ratio of 2 as about 6 x 10-10 g/ml. This detector had the advantage of operating at atmospheric pressure and so no vacuum system was required. The effect of temperature on the detector performance was not reported, nor was its linearity over a significant concentration range. This detector appears not to have been made commercially.   The Ultrasound Whistle Detector The velocity of the propagation of sound through a gas depends on its density and, thus, the presence of a solute vapor in