Passivation surface analysis requirements

Since the initiation of pitting is the localized penetration of the passive film, understanding of this step requires information on the structure of passive films and the mechanisms whereby they can be destroyed locally. Understanding of either of these is complicated by the thinness of the films and the question of the passive film structure when formed by and existing in the aqueous environment as compared with its structure when removed from this environment. The latter is necessary for the use of most of the surface analysis techniques applicable to structure evaluation. As a consequence, specific conclusions as to the structure are frequently inferred rather than more directly established. 

Passive samplers place an adsorptive surface (frequently charcoal-based) a fixed distance from a windshield or semipermeable membrane in an enclosure in which the windshield or membrane is exposed to the air being sampled. These devices generally require that a minimum air movement across the windshield or membrane occurs in order to ensure that the device is sampHng properly. The concentration of the contaminants of interest will determine the minimum exposure time of the passive sampler. The major advantages of this type of sampler are that the collection devices are small, require no electrical power and are very easy to use. After exposure, which can, for some devices, extend to periods as long as 6 months or more, the adsorbent is removed and extracted. The extract may have to be reduced in volume prior to analysis. As with other non-specific adsorbents, the passive sampler adsorptive surface will trap a wide variety of compounds. Oxidation of the adsorbed compounds may be a problem if the passive sampler is left for a long period in a gas that contains oxidants. 

Several biocompatible HPLC and UHPLC systems are also available, engineered with a bio-inert flow path made of non-stainless steel materials (such as titanium or PEEK) for protein analysis. Stainless steel equipment and tubing should be avoided for protein analysis by aqueous/salt-based methods because protein chelation with metals can negatively affect separation (19). These interactions can either occur with metal contaminating the column or with corroded surfaces within the instrument. In addition to affecting separation, corrosion can result in physical damage to the system, such as pump seal failure and compromised performance of the detector cells. Stainless steel systems may require periodic passivation for reliable usage (20). 

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