Pattaraporn Karamahito ^a, Jirayu Sitanurak ^b,c , Duangjai Nacapricha ^b,c , Prapin Wilairat ^b,c,Korbua Chaisiwamongkhol ^d,e , Apichai Phonchai ^a,^*a Division of Health and Applied Sciences, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand ^b Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Mahidol University, Bangkok 10400, Thailand ^c Flow Innovation-Research for Science and Technology Laboratories (FIRST Labs), Thailand ^d School of Science, Mae Fah Luang University, Chiang Rai 57100, Thailand ^e Center of Chemical Innovation for Sustainability (CIS), Mae Fah Luang University, Chiang Rai 57100, Thailand

Since 2010 many countries do not permit the use of sibutramine for obesity treatment owing to risks of heart attack or stroke. However, it is still found adulterated in some slimming products. A distance-based paper device was developed for quantitative measurement of sibutramine by visual inspection. The device is a filter paper printed with a thermometer-shaped pattern but having circular zones at both ends. A small volume of sample, loaded at one end, flows along the channel, reacting with the pre-deposited Dragendorff’s reagent to form orange–red precipitate of sibutraminetetraiodobismuthate complex. The length of the orange-red complex is directly proportional to amount of sibutramine in the sample. A ruler scale printed parallel to the channel allows the length of the orangered precipitate to be directly observed and recorded. The measurement takes approximately five minutes. The device has a linear range of 0.22–0.90 mmol L −1 sibutramine, with coefficient of determination (r2 ) > 0.9868. The limit of quantification of the device (10SD intercept/slope calibration) is 0.22 mmol L −1 sibutramine, with intra-day and inter-day precisions < 4.4 %RSD. The device was applied to analysis of various slimming products. Recovery of spiked samples ranged from 86 to 106 %, (5 %RSD). The limit of quantification was 20.5 mg sibutramine per gram of sample. The results are in agreement with gas chromatographic method at 95% confidence level. This is the first paper-based analytical device for measuring adulterated sibutramine in slimming products. It is simple, low-cost, easy-to-use and instrument-free for on-site analysis. It can be used by consumers to check for adulteration in over-the-counter slimming products.