The sequence of sample and reagent zones can be assembled in two ways:
The single interface sequencing is simplest , self optimizing and should be always considered first. The sandwich sequencing is used when several reagents have to be used, and if conditions (such as pH) for consecutive reactions are different.
It is important to realize that the mechanism of dispersion and ensuing product formation is similar for both configurations, because the reaction product (yellow) is formed at the interface between the sample and reagent zone. Therefore it is essential to maximize zone overlap by increasing the amplitude of the forward flow. As the sample zone (A) is pushed into the holding coil (HC), axial dispersion is promoted, since the center of the stream travels at twice the mean flow velocity. The resulting parabolic profile telescopes the trailing zone toward the leading edge of the forward zone, while the inner core of the leading zone is filled by the protruding edge of the second zone.
Sequencing & Zone Overlap
2.2.8.
As this happens, the radial dispersion promotes mixing of adjacent parallel layers of sample (red) and reagent (blue). Upon flow reversal (B), the flow velocity profile is suddenly inverted. First radial mixing is promoted by local turbulence, while the axial dispersion is partially reduced, as the stacked zones travel downstream toward the flow cell (C). It follows that the amplitude of the forward flow is the most important parameter, since it controls zone penetration.
C
