High-voltage techniques modification

A recent modification of mass spectrometric technique involves the use of a spark source. A spark is struck by means of a high voltage between two rods of the material under examination. Under this drastic treatment many substances decompose completely into their elements and give positive ions. The ion detector is usually a photographic plate which shows a line mass spectrum. A number of exposures are taken for each sample and a quantitative estimate of the presence of an element in the sample can be made from the exposure time. 

Another modification in which form thin sheet electrophoresis is employed is known as ionophoresis. In this method, devised by Sanger and his co-workers, high voltage electrophoresis is carried out on ion-exchange paper. It is a rapid method of great resolution and sensitivity and is used in Biochemistry for the separation of constituents of a highly complex mixture e.g. a mixture of oligoribonucleotides produced by partial enzymic hydrolysis of RNA. For separation of the constituents of this hydrolysate mixture, a two-dimensional technique is used in which the mixture is subjected to electrophoresis on cellulose acetate and then the partially separated mixture is transferred to a DEAE-cellulose... 

Conductimetric detection is a less sensible, but universal detection technique that has been applied as a detection mode in ME, either in the galvanic (a pair of electrodes is placed in the separation channel for liquid impedance measurement) [12, 13] or the contactless mode (no contact between the pair of electrodes and separation channel solution) [14-16], Both formats are illustrated in Figure 12.4. Contactless detection (CCD) is preferred for three reasons (i) the electronic circuit is decoupled from the high-voltage applied for separation (no direct coupling between the electronics and the liquid in the channel), (ii) the formation of glass bubbles at the metal electrodes is prevented, and (iii) electrochemical modification or degradation of the electrode surface is prevented. 

The results in these model systems confirm the introductory expectations that a defined local surface structuring and modification in the nanometer range can be realized by electrochemical means in combination with the inhomogeneous field distribution below the STM tip. However, the two-probe technique substrate-tip requires relatively high tunneling voltages. Attempts are being made to replace the two-probe technique by the conventional three-probe technique to drastically reduce C/t. 

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