Reproducibility of assays performed in the mini pFI mode varied between rsd 1.1 % (1.2.34.) for nitrite assay, to rsd. 3.3% (1.2.38.) for nitrate assay, the latter being based on complex microfluidic manipulations. This level of precision is in agreement with precision of spectrophotometry (Christian 2014) used here as detection technique. Also application of mini pFI to nutrient assays resulted in equal or better sensitivity and LOD compared to cFI or miniSI. Since these three techniques provide similar results, it is useful to review their merits and features relevant for their application in laboratory, field or research.
The key difference between cFI, mini pFI and miniSI is, that In order to accommodate different assays, cFI manifold must be physically reconfigured, while mini pFI or miniSI instrument does not need to be reconfigured, because it will accommodate diverse assays, by means of flow programming, implemented by software. This flexibility makes optimization of reagent based assays easier, and allows the same apparatus to be used for a wide variety of assays, based on reactions with different reaction rates.
Briefly, in the traditional continuous Flow Injection:
Volume of the injected sample is adjusted changing volume of the injection loop.
Concentration of reagent in sample/reagent mixture is adjusted by changing diameter of peristaltic tubing that feeds reagent into the confluence point..
Incubation time is adjusted by changing the length of reaction coil(s) between injector and detector.
In the upgraded forward flow pFI (A) incubation time is adjusted by flow programming.
In mini pFI (B) or miniSI format :
Volume of injected sample is adjusted by selecting sample volume aspirated into HC
Concentration of reagent in sample/reagent mixture is adjusted by flowrates between HC1 and HC2, controlled by software script.
Incubation time is adjusted by selecting the length of stop flow period.
Additional benefits of pFI and minSI are:
Sample and reagent consumption is minimized and waste generation is substantially reduced.
Solutions are pumped only during sample processing cycle
By using DI water is used as a carrier, the flow path is thoroughly washed after each measuring cycle
In summary, the advantage of the traditional cFI is that has been widely used, it is well established and simple to implement. The mini pFI, introduced here for the first time, is in early stage of development, and its applications for real life assays have yet to be worked out. The miniSI is more robust than mini pFI by virtue of simpler apparatus, as it uses only a single pump. It is therefore preferable for single and two reagent assays. Multireagent assays and large sample volumes can be automated by mini pFI . Since both methods use the same apparatus, switch between the two is easy to make.
For the research applications, flow programming offers variety of microfluidic manipulations (1.2.27.) that present unprecedented research opportunities for enhancement of flow based instrumental analysis
G. Christian, “Analytical Chemistry”, 7h Ed. p.12.to 13., Willey NY 2