Some problems require inert HPLC systems that can withstand pressures of more than 350 bar. The current HPLC systems are made of steel, which is resistant to almost all mobile phases . Halogenated acids are an exception. The steel capillaries also have a similar chemical resistance and can withstand pressures above 1000 bar. Nevertheless, the eluents lead to a permanent abrasion of the materials so that metal ions can dissolve from the steel and possibly influence the substance, which can damage the columns or make detection more difficult. Even with the use of titanium considerable amounts of released metal ions are still detected.
The solution could be the use of PEEK. The polyetheretherketone is a high temperature resistant thermoplastic that is resistant to almost all organic and inorganic chemicals. Inconsistencies have been observed with UV irradiation, concentrated nitric acid and some hydrogen halides. Limitations for the use of PEEK sample loops are: no strongly alkaline eluents, no pressures above 250 bar and no temperatures above 300 ° C. By using plastic rather than steel in the devices and capillaries, the amount of metal ions can be significantly reduced, but, for example, metal-containing pressure transducers are irreplaceable.
Due to the weak points in the use of steel and the restrictions with the use of PEEK, a passivation of the HPLC devices is almost unavoidable. Passivation is the spontaneous formation or the targeted production of a corrosion-preventive protective layer. Spontaneous passivation is observed, for example, in the case of chromium. Due to a thin oxide layer, the material is protected from the atmosphere and further oxidation so that corrosion is very slow. For other metals, this property of chromium is utilized by treating it with chromic acid, thus forming a protective layer from the chromates.
In order to equip the HPLC devices for special applications, the passivation has proved to be a reliable and tested measure. It is necessary to repeat the operation 1-2 times a year that the effect decreases with time.
The following method has proved successful:
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