Injection valves volume-based sampling with

A well-defined volume of the gaseous sample is directed towards the flow manifold, where it interacts with a liquid medium, and the chem-iluminometric determination of nitrous acid in an indoor environment [18] is a good example. A calibrated vacuum pump aspirated the volume of air through a diffusion scrubber and the membrane-separated analyte was collected into a buffered stream that directed it towards an 8-port injection valve after passing a de-bubbler, the analyte zone was directed towards the detector. With this approach, a 120 pp tv detection limit was attained. The characteristics, potential and limitations of analytical procedures involving diffusion scrubbers and other devices for gas sampling aimed at in-line flow-based determinations have been discussed elsewhere [19]. 

On the one hand, in the determination without preconcentration, valve 2 is switched On while valve 3 is switched Off to inject an appropriate volume of sample (time-based injection) without passing through the SPE column, and at the same time valve 4 is actuated to inject thiocyanate. The red complex thus formed is measured with the spectrophotometer. On the other hand, in order to increase sensitivity the determination can be carried out with preconcentration. Valve 2 and valve 3 are switched On to inject an appropriate volume of sample (time-based injection) passing through the column. Then valve 1 and valve 2 are switched Off while valve 3 is kept in On position in order to... 

To ensure insertion of an accurate volume of sample into the carrier stream, injectors are used during FIA experiments. Several concepts of injector have been described in the literature. Two injection devices must be detached (a) the rotary valve [21], which two- to six-port models are the most versatile and (b) the proportional injector (manually operated) developed by researchers from CENA/USP-Brazil [22]. Their operation is based on two well-defined positions loading and unloading positions. With the valve in the loading position, a defined volume is filled with sample. In the unloading position, the volume contained in the loop is inserted into the carrier stream. 

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