Instrument-free quantitative colorimetric analysis using adsorption-band length in a packed silica gel column

A simple and instrument-free colorimetric method for quantitative analysis is reported, in which analyte concentration is determined by measuring the length of a colored adsorption band formed in a packed silica gel column. The proposed method employs a miniature silica gel column that acts as a signal transducer after it adsorbs the colored compound from a solution during its flow in the column. The length of this band increases proportionally with analyte concentration, which enables quantitative detection via simple distance measurement. A theoretical model was developed to describe the relationship between solute concentration, adsorption behavior, and band propagation along the column. The principle was validated via the detection of both iron ions and enzyme-mediated glutamic acid. For Fe²⁺ analysis, the o-phenanthroline complexation method shows a level of sensitivity comparable to that of conventional spectrophotometry, which enables an almost quantitative recovery of trace iron in tap water. The limit of detection (LOD) and the limit of quantification (LOQ) were estimated to be 0.20 μM and 0.60 μM for the proposed method and 0.23 μM and 0.70 μM for spectrophotometry, respectively. The approach was further extended to glutamate detection using a cascade reaction involving glutamate oxidase and horseradish peroxidase with N-benzoyl leucomethylene blue as the chromogenic substrate. The LOD and the LOQ of the proposed method were 0.08 and 0.25 μM, and both values are superior to the 0.24 μM and 0.73 μM obtained using a microplate reader. By integrating preconcentration with a distance-based readout, this method provides a simple yet highly sensitive analytical platform and establishes distance as a quantitative signal for colorimetric detection.