Reproducible quantitative analysis by reflectance measurements
requires consideration of several factors that can influence the uniformity
and intensity of the spot test reaction. Investigation of several
surfactants revealed that use of DTAB generated the best AR signal,
with no observable precipitation when in contact with the Fe(III)
salt. The order of reagent addition was studied, and tests were performed
to identify any need to acidify the solution or adjust the
ionic strength (no such adjustments were necessary). Optimum signal
and spot homogeneity were achieved when 20 μL of the chromogenic
reagent (containing FeCl3 and DTAB) were added prior to
addition of 20 μL of furosemide solution.