Conductivity detection systems

There are reports that the use of robotic milking systems will significantly reduce mastitis, linked at least partially to the machines more efficient teat disinfection compared to human operators. However, the more frequent visit to be milked and more gentle application and removal of milking equipment from the teats may also contribute to reduced mastitis incidence. On the other hand, the absence of human operators in robotically milked herds may lead to late detection and treatment of mastitis. However, this may be overcome by the introduction of conductivity detection systems which allow early signs of mastitis to be detected (Hogeveen and Meijering, 2000). 

The electric-field-jump method with electric conductivity detecting system can be applied to reactions occuring on the order of milliseconds to microseconds. The rise time of the applied electric field is much faster than 0.1 ys. The strength of the electric field is 20 kV/cm. The details of the electric-field-jump apparatus can be found elsewhere (9). 

Organic modifiers have been frequently employed in CE to increase the solubility of hydrophobic solutes in the aqueous buffer system. Unfortunately, many organic modifiers are UV absorbent and cannot be used without considerable loss of sensitivity of detection. A contactless conductivity detection system has been developed which extends the application range of UV-absorbing solvents [ 119]. As both natural pigments and synthetic dyes absorb in the visible part of the spectra, the application of UV-absorbing organic modifiers in their CE analysis does not cause detection problems. 

Very recently, an interesting study of the liquid holdup in a packed column with cocurrent gas-liquid upflow was carried out by the Pittsburgh Energy Research Center.22 In this study, a series of radial conductivity measurements was performed on air-water flow through a 10.2-cm-i.d. clear acrylic column, with various packings. Three packing sizes (1.9-cm x 1.9-cm, 0.635-cm x 0.635-cm, and 0.32-cm x 0.32-cm cylinders) were examined. The conductivity detection system comprised two solid stainless-steel rods fitted with Teflon jackets, leaving only the... 

Rapid automated systems, employing high-temperature combustion for removal of the nitrogen from the sample and a thermal conductivity detection system for measurement of the released... 

Several experimental techniques may be used, such as acid/base titration, electrical conductivity measurement, temperature measurement, or measurement of optical properties such as refractive index, light absorption, and so on. In each case, it is necessary to specify the manner of tracer addition, the position and number of recording stations, the sample volume of the detection system, and the criteria used in locating the end-point. Each of these factors will influence the measured value of mixing time, and therefore care must be exercised in comparing results from different investigations. 

The electrical conductivity detector is probably the second most commonly used in LC. By its nature, it can only detect those substances that ionize and, consequently, is used frequently in the analysis of inorganic acids, bases and salts. It has also found particular use in the detection of those ionic materials that are frequently required in environmental studies and in biotechnology applications. The detection system is the simplest of all the detectors and consists only of two electrodes situated in a suitable detector cell. An example of an electrical conductivity detector sensing cell is shown in figure 13. 

This technique detects substances qualitatively and quantitatively. The chromatogram retention time is compound-specific, and peak-height indicates the concentration of pollutant in the air. Detection systems include flame ionization, thermal conductivity and electron capture. Traditionally gas chromatography is a laboratory analysis but portable versions are now available for field work. Table 9.4 lists conditions for one such portable device. 

Anions of weak acids can be problematic for detection in suppressed IEC because weak ionization results in low conductivity and poor sensitivity. Converting such acids back to the sodium salt form may overcome this limitation. Caliamanis et al. have described the use of a second micromembrane suppressor to do this, and have applied the approach to the boric acid/sodium borate system, using sodium salt solutions of EDTA.88 Varying the pH and EDTA concentration allowed optimal detection. Another approach for analysis of weak acids is indirect suppressed conductivity IEC, which chemically separates high- and low-conductance analytes. This technique has potential for detection of weak mono- and dianions as well as amino acids.89 As an alternative to conductivity detection, ultraviolet and fluorescence derivatization reagents have been explored 90 this approach offers a means of enhancing sensitivity (typically into the low femtomoles range) as well as selectivity. 

Nowak, M. and Seubert, A., Application of experimental design for the characterization of a novel elution system for high capacity anion chromatography with suppressed conductivity detection, /. Chromatogr. A, 855, 91,1999. 

The detection of the migrating sample boundary in CE can be accomplished by UV, fluorescent, electrochemical, radiochemical, conductivity, and mass spectrometry (MS) means. The use of high-sensitivity detection systems is always a key issue in CE applications. The sensitivity of HPCE detectors may be at least 2 to 3 orders of magnitude better than that of HPLC detectors. Since the detection cell volume is very small, the concentration sensitivity... 

Many IC techniques are now available using single column or dual-column systems with various detection modes. Detection methods in IC are subdivided as follows [838] (i) electrochemical (conductometry, amper-ometry or potentiometry) (ii) spectroscopic (tJV/VIS, RI, AAS, AES, ICP) (iii) mass spectrometric and (iv) postcolumn reaction detection (AFS, CL). The mainstay of routine IC is still the nonspecific conductometric detector. A significant disadvantage of suppressed conductivity detection is the fact that weak to very weak acid anions (e.g. silicate, cyanide) yield poor sensitivity. IC combined with potentiometric detection techniques using ISEs allows quantification of selected analytes even in complex matrices. The main drawback... 

Some commercially available detectors have a number of detection modes built into a single unit. Fig. 2.4o is a diagram of the detector used in the Perkin Elmer 3D system, which combines uv absorption, fluorescence and conductivity detection. The uv function is a fixed wavelength (254 nm) detector, and the fluorescence function can monitor emission above 280 nm, based on excitation at 254 nm. The metal inlet and outlet tubes act as the electrodes in the conductance cell. The detection modes can be operated independently or simultaneously, using a multichannel recorder. In the conductivity mode, using NaCl, a linear range of 103 and a noise equivalent concentration of 5 x 10 8 g cm-3 have been obtained. 

Since the magnetic moments are smaller, now we have a smaller susceptibility and therefore much smaller signal, requiring more sensitive detection systems. These are resonance or SQUID (see Section 14.5) techniques. Thermal response time are shorter, since pure metals can be used with good thermal conductivity and fast spin-lattice relaxation. The parameter to be measured is the nuclear susceptibility ... 

Several kinds of detection systems have been applied to CE [1,2,43]. Based on their specificity, they can be divided into bulk property and specific property detectors [43]. Bulk-property detectors measure the difference in a physical property of a solute relative to the background. Examples of such detectors are conductivity, refractive index, indirect methods, etc. The specific-property detectors measure a physico-chemical property, which is inherent to the solutes, e.g. UV absorption, fluorescence emission, mass spectrum, electrochemical, etc. These detectors usually minimize background signals, have wider linear ranges and are more sensitive. In Table 17.3, a general overview is given of the detection methods that are employed in CE with their detection limits (absolute and relative). 

Other detection systems, such as conductivity detector or refractive index detection are generally applicable for the determination of common anionic surfactants [1]. However, they are less sensitive than other techniques and are used more often for the characterisation of pure surfactants, than for their determination at low concentrations. 

For the cationic surfactants, the available HPLC detection methods involve direct UV (for cationics with chromophores, such as benzylalkyl-dimethyl ammonium salts) or for compounds that lack UV absorbance, indirect photometry in conjunction with a post-column addition of bromophenol blue or other anionic dye [49], refractive index [50,51], conductivity detection [47,52] and fluorescence combined with postcolumn addition of the ion-pair [53] were used. These modes of detection, limited to isocratic elution, are not totally satisfactory for the separation of quaternary compounds with a wide range of molecular weights. Thus, to overcome the limitation of other detection systems, the ELS detector has been introduced as a universal detector compatible with gradient elution [45]. 

A pressure perturbation results in the shifting of the equilibrium the return of the system to the original equilibrium state (i.e., the relaxation) is related to the rates of all elementary reaction steps. The relaxation time constant associated with the relaxation can be used to evaluate the mechanism of the reaction. During the shift in equilibrium (due to pressure-jump and relaxation) the composition of the solution changes and this change can be monitored, for example by conductivity. A description of the pressure-jump apparatus with conductivity detection and the method of data evaluation is given by Hayes and Leckie (1986). 

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