Other Types of Detection

Other types of detection include refractive index, fluorescence, or mass selective detectors. The use of other types of detectors, such as those based on fluorescence, may be used for assaying compounds that can be specifically detected at low concentrations in the present of nonfluorescent species. However, as few compounds are naturally fluorescent, they require chemical modification, assuming they have a suitable reactive group, to give a fluorescent derivative. 

Other Types of Detection Systems Scintillation-type transducers consist of a crystalline phosphor dispersed on a thin aluminum sheet that is mounted on the window of a photomultiplier tube. When ions (or electrons produced when the ions strike a cathode) impinge on the phosphor, they produce flashes of light that are detected by the photomultiplier. A specialized version of this type of device is the Daly detector, which consists of an aluminized cathode in the shape of a knoh (the Daly knob) held at a very large negatKc voltage opposite a. scintillation transducer,. Analyte ions collide with the cathode producing secondary electrons that are then attracted to the sur-... 

Since its introduction in 1975 ion chromatography (IC) has been used in most areas of analytical and environmental chemistry. Although the conductivity detector is still the most popular, other types of detection can be applied for different analytes. These include the following methods electrochemical (e.g., amperometric, potentiometric), photometric (UV/Vis, chemiluminescence), and spectrometric detectors used mainly in hyphenated techniques. 

Optical detectors are by far the most commonly used of the analytical methods available for continuous monitoring. In fact, in around two-thirds of all FIA applications reported the use of this type of detector. However, several other types of detection methods are available for the determination of ions acetate and acetic acid involving, for instance, potentiometric, enzymatic, spectrophotometric, fluorometric, and chemiluminescence techniques. However, molecular absorption spectrophotometers continue to be the preferred choice in FIA, and in analytical chemistry in general, on account of their high versatility (Calatayud, 1996). Studies employing FIA and other methods for the determination of acetic acid and acetate ions are described in the following sections. 

Instead of MS, other types of detection can be used in GC. The most widely used are thermal conductivity detection and flame ionization detection. In addition, various more specific types are available, e.g., electron-capture detection, thermionic detection, and flame photometric detection. A comparison of some characteristics is given in Table 3. On-line combinations with spectrometric detection, other than MS, are available, e.g., Fourier-transform infrared (FT-IR) and atomic emission detection (AED). 

The application of fundamentally new ECT (Russia patent Jf 2063025) has made it possible to provide high-efficiency defect control accompanied by detecting both small surface defects and more rough under-surface defects under non-magnetic metal layer of 7 mm thick, or surface defects under protection coatings, dye, corrosion, hermetic and other type of layer of 10 mm thick. 

The second application was checked in a feasibility study. The preliminary results show that segregations in Inconnel IN 718 can be detected within the volume. It can be assumed that segregations also in other type of material can be detected by this method. Several details, however, have to be checked before a routine use of this susceptibility based method. Among these details are to determine the susceptibility contrast of v ious segregations in various matrix materials as well as sizing and localisation procedures of the segrations. 

In the other types of mass spectrometer discussed in this chapter, ions are detected by having them hit a detector such as an electron multiplier. In early ICR instruments, the same approach was taken, but FT-ICR uses a very different teclmique. If an RF potential is applied to the excitation plates of the trapping cell (figure B 1.7.18(b)) equal to the cyclotron frequency of a particular ion m/z ratio, resonant excitation of the ion trajectories takes place (without changing the cyclotron frequency). The result is ion trajectories of higher... 

Other types of mass spectrometer can use point, array, or both types of ion detection. Ion trap mass spectrometers can detect ions sequentially or simultaneously and in some cases, as with ion cyclotron resonance (ICR), may not use a formal electron multiplier type of ion collector at all the ions can be detected by their different electric field frequencies in flight. 

Further, peak overlap results in nonlinear detector response vs concentration. Therefore, some other detection method must be used in conjunction with either of these types of detection. Nevertheless, as can be seen from Figure Ilf, chiroptical detection can be advantageous if there is considerable overlap of the two peaks. In this case, chiroptical detection may reveal that the lea ding and tailing edges of the peak are enantiomerically enriched which may not be apparent from the chromatogram obtained with nonchiroptical detection (Fig. He). 

Such effects principally cannot be observed in multi band detectors such as a UV diode array detector or a Fourier transform infrared (FTIR) detector because all wavelengths are measured under the same geometry. For all other types of detectors, in principle, it is not possible to totally remove these effects of the laminar flow. Experiments and theoretical calculations show (8) that these disturbances can only be diminished by lowering the concentration gradient per volume unit in the effluent, which means that larger column diameters are essential for multiple detection or that narrow-bore columns are unsuitable for detector combinations. Disregarding these limitations can lead to serious misinterpretations of GPC results of multiple detector measurements. Such effects are a justification for thick columns of 8-10 mm diameter. 

Detectors. Although electrical conductance has been widely used for detecting ions in ion chromatography, the scope of the technique has been considerably extended by the use of other types of detector. It is convenient broadly to classify detectors into two series. 

The third pathway for DFN-induction involves detection of microbial nucleic acids by other types of... 

Note that the interfacing of LC techniques with MS puts significant constraints on the solvents that can be used i.e., they must be volatile, with a low salt concentration, for MS compatibility. Narrow-bore columns, which use much smaller amounts of salt and organic modifier, appear to have potential for facilitating IEC-MS applications.40 Despite the excellent sensitivity of MS detection for most elements, however, there are cases where matrix effects can interfere. In this situation, combination of IEC with atomic emission spectrometry (AES) or atomic absorption spectrometry (AAS) may be preferable, and can also provide better precision.21 32 4142 Other types of... 

Three types of detection methods for TLC may be distinguished, namely physical, microchemical and biological-physiological. The more common detection methods for polymer additives in TLC are given in Figure 4.8. Detection is an off-line process, thus several detection techniques may be used one after the other. 

The FTMS instrument operates in a very different fashion from most other types of mass spectrometers. With FTMS, the principal functions of ionisation, mass analysis and ion detection occur in the same space... 

It is certainly clear that a coulometric titration, like any other type of titration, needs an end-point detection system in principle any detection method that chemically fits in can be used, be it electrometric, colorimetric, photoabsorptionmetric, etc. for instance, in a few cases the colour change of the reagent generated (e.g., I2) may be observed visually, or after the addition of a redox, metal or pH indicator the titration end-point can be detected photoabsorptiometrically by means of a light source and photocell combination. Concerning the aforementioned coulometric titration of Fe(II), it is... 

---