Reverse polarity machining

An active cathode (direct polarity machining) or active anode (reverse polarity machining) [63] can be used as a tool, of which the use of the former is most popular. As the discharge activity, and especially the discharge mechanism, differs for an active cathode and anode, the machining performances will also be different. 

In micro-ECM, the lEG is very narrow, which may cause deposition of metal ions as well as sludges on the microtool. Acidic electrolyte is suitable for micro-ECM due to its ability to dissolve metal hydroxide. However, it may cause deposition of metal ions and metal hydroxide on the microtool, which leads to poor machining. To overcome this, frequent replenishment of acidic electrolyte as well as frequent reversing of polarity may be considered as some of the effective approaches. 

Cyclic anodic polarization procedures based upon ASTM G5 (Reference Test Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements) have been used to evaluate the localized corrosion resistance of stainless steel alloys in paper machine white waters. The difference between the open circuit or naturally occurring corrosion potential and the pitting breakdown potential of various materials has been reported by many investigators [lO-Id]. Bowers [14] called the difference between the breakdown potential and the potential of the cathodic/anodic current reversal the margin of safety. He also noted the effect of the sulfate to chloride concentration ratio on localized corrosion of Types 304 and 316L stainless steels, These results permitted alternative materials of construction to be ranked and their limits of resistance to be defined. 

HPLC has been used in the extraction and purification of pollutants in environmental and biological samples. Its manifold applications are due to the availability of a number of stationary phases, particularly reversed phase columns of various dimensions. HPLC instruments used for this purpose are similar to the analytical machine but with the use of a different column, which depends on the nature of the pollutants to be extracted and purified. Many of the reports available in the literature deal with the use of HPLC as an extraction and purification technique for environmental and biological samples. For determinations of polar pesticides, HPLC appears to be the most appropriate technique for purification purposes [134, 135]. Smith etal. [136] used low pressure HPLC for the extraction of polychlorinated dibenzofurans and dioxins. Furthermore, Blanch era/. [137] used low pressure HPLC for the purification and extraction of PCBs from shark liver. Similarly, Ramos etal. [138] used the Smith etal. [136] method for the extraction of PCBs in dolphin liver. Bethan etal. [139, 140] used a LiChrosorb 100-Si HPLC column for the extraction of bromocyclen collected from river water and of ck-HCH in marine water. However, HPLC cannot achieve a reputation as the universal and highly applicable extraction technique, due to its low range with regard to preparative chromatography. Therefore, further advancement of the technique is required, especially with regard to the development of preparative columns. 

Besides the parameters discussed above, some other factors can also be used to optimize chiral resolution by CE. These parameters include the reversal of polarity, the volume of sample injected, the use of EOF modifiers and pre-derivatization of the chiral pollutants with a suitable reagent. In the normal CE machine, the anode (+) and cathode (—) are always at the inlet and outlet ends, respectively. In this modality, the EOF always tends to travel towards the cathode (detector). On the other hand, in the reverse mode, the direction of the EOF is away from the detector, and hence only negatively charged diastereomeric complexes with an electrophoretic mobility greater than the EOF will pass the detector. This format is typically used with capillaries that are coated with substances that reverse the net charge of the... 

---