A sensitive and affordable approach is described for thein-situmeasurement of ambient formaldehyde. Air is sampled around a 100 microliter aqueous drop containing 3-methyl-2-benzothiazoline hydrazone. After a desired period of sampling (typ. 5 min) and a waiting period of 10 min for the reaction to be completed, a second reagent (FeCl3) is added to the drop by means of a conjoined conduit. A blue product is formed and is read after an additional 10 min of reaction by a fiber-optic/light emitting diode based photodetector. A fresh drop is then formed and the process begins anew. As demonstrated here, the limit of detection is #x223C;6.25 fig m#x2212;3HCHO but can be significantly improved by using longer sampling times and a sampling rate higher than 100 ml min#x2212;1used in most of this work. This is the first example of a chromogenic drop sensor that utilizes sequential reagent addition.
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