Detectors, characteristics sensitivity

Principles and Characteristics The main reasons for hyphenating MS to CE are the almost universal nature of the detector, its sensitivity and the structural information obtainable, including assessment of peak purity and identity. As CE is a liquid-phase separation technique, coupling to the mass spectrometer can be achieved by means of (modified) LC-MS interfaces. Because of the low flow-rates applied in CE, i.e. typically below lOOnLmin-1, a special coupling device is required to couple CE and the LC-MS interface. Three such devices have been developed, namely a... 

Fluorescence detection can be up to four orders of magnitude more sensitive than UV absorbance, especially where laser induced excitation is used, mass detection limits being as low as 10-20—10 21 mole. Pre- and post-column derivatization methods are being developed to extend the applicability of fluorescence detection to non-fluorescent substances. Several types of electrochemical and mass spectrometric detector have also been designed. Detector characteristics are summarized in Table 4.21. 

Important performance characteristics of UV/Vis detectors are sensitivity, linearity, and band dispersion. These are controlled by design of the optics and the flow cell—more specifically by spectral bandpass, stray light characteristics, and the volume and path length of the flow cell. 

The detector used to sense and quantify the effluent provides the specificity and sensitivity for the analytical procedure. Table 1 summarizes significant detector characteristics. 

A great deal of evidence now exists to support the notion that the SIT detector approaches the characteristics of an ideal multichannel detector (20). Sensitivity on a per/channel basis is comparable to that of the the photomultipliers, so that almost... 

Another advantageous characteristic is the FPD s compatibility with temperature programming. The photomultiplier part of the detector is sensitive to heat, though from column temperatures between 180° and 200° up, a serious noise problem can arise. However, a few copper cooling coils soldered around the metal part of the detector can improve the situation considerably (68). 

The device functions in the same way as the conventional electron-capture detector with a radioactive source. The column eluent enters just below the third electrode, any electron-capturing substance present removes some of the free electrons, and the current collected by the fourth electrode falls. The sensitivity claimed for the detector is 0.2-1.0 ng, but this is not very informative as its significance depends on the characteristics of the column used and on the k of the solute peak on which the measurements were made. The sensitivity should be given as that solute concentration that produces a signal equivalent to twice the noise. Such data allow a rational comparison between detectors. The sensitivity or minimum detectable concentration of this detector is probably similar to the conventional pulsed ECD (viz. 1 X 10 g/mL). The linear dynamic range appears to be at least three orders of magnitude for a response index of r, where 0.97 [Pg.607]

UV/VIS detector characteristics. Compound-specific nondestructive concentration detector compound sensitivities differ over a wide range useable with isocratic or gradient mobile phases including buffer salts pre-, or post-column derivatization can be used to increase number of measurable compounds. 

Conductometric Detector Characteristics. Universal for aU anions or cations in solution (i.e., a bulk property of mobile phase detector) nondestructive concentration detector compound sensitivities differ over an order of magnitude range, useable... 

Detection systems used for LC analysis of pesticides are UV spectrometric detection (especially diode array detection - DAD - allowing peak confirmation by means of spectra comparison), fluorescence detection (FLD), electrochemical detection (ED), evaporative light scattering detection (ELSD), and MSD. Their characteristic sensitivities can be considered to vary in the following order ELSDdetection systems mentioned above, only ELS and MS are universal detectors. 

Hence, all compounds having a conductivity less than the carrier gas will be detected by the TCD, which is a universal concentration-sensitive detector. The TCD is nondestructive, and may be used for preparative separations. The detector has however low sensitivity, and the minimum detectable (M D) mass is about 10 ng even using He or H2 as carrier gas. Other detector characteristics can be found in Table 2.4. The TCD is commonly used for determination of light and permanent gases in packed or PLOT columns. The TCD is well suited for portable gas chromatographs because it is easily miniaturized and does not require extra gases. 

The FID can detect all organic compounds containing C and H, with the exception of formic acid and methane. It is a mass-sensitive detector. The minimum detectable mass is about 0.01-0.1 ng and the FID has a large dynamic range of 10. Other detector characteristics can be found in Table 2.4. 

The properties of the detection system employed for a chromatographic analysis can be as critical as the properties of the column itself and it is now well understood that column performance and detector performance are interactive. The properties of the detector controls the mass sensitivity and the concentration sensitivity of the whole of the chromatographic system. Detector characteristics also determine the minimum... 

Flame photometric detector, providing a mass flow dependent signal, the detector burns in a hydrogen-rich flame where analytes are reduced and excited. Upon decay of the excited species light is emitted of characteristic wavelengths. The visible-range atomic emission spectrum is filtered through an interference filter and detected with a photomultiplier tube. Different interference filters can be selected for sulfur, tin or phosphorus emission lines. The flame photometric detector is sensitive and selective. 

The near-ir spectmm of ethylene oxide shows two peaks between 1600—1700 nm, which are characteristic of an epoxide. Near-ir analyzers have been used for verification of ethylene oxide ia railcars. Photoionization detectors are used for the deterrnination of ethylene oxide ia air (229—232). These analyzers are extremely sensitive (lower limits of detection are - 0.1 ppm) and can compute 8-h time-weighted averages (TWAg). 

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