Development of microfluidic paper based analytical devices for determination of nitrate and nitrite in gunshot residues

Abstract

Microfluidic paper-based analytical devices (µPADs) have been fabricated to determine nitrate and nitrite ions in samples of gunshot residues (GSR). The µPADs were created by screen printing with beeswax and the analysis of nitrite ions was based on the color reaction using a Griess reagent.  For nitrate analysis, the nitrate ions were converted into nitrite ions by using Zn metal as a reducing agent. The devices were used to quantify the nitrate and nitrite contents from the color intensity of the µPADs images read by an ImageJ software in RGB color space.  It was found that the limits of quantification for both ions were 0.02 nmol. The storage of µPADs in the dark at ambient temperature and freeze condition for 7 days provides stability for nitrite determination while the stability of µPADs for nitrate measurement is 2 days in that storage condition. The GSR samples for analysis were collected from the cartridge cases and the hands of the shooters.  The nitrite and nitrate contents in GSR determined by the µPADs were compared to those measured by the ion chromatography (IC) technique.  The good correlations between the two data sets were observed for both nitrite and nitrate contents as suggested by the correlation coefficients of 0.983 and 0.968for the two ions respectively.  However, the amounts of nitrite and nitrate ions in GSR measured by µPADs were slightly higher than those determined by the IC technique. The slopes of correlation graphs were 1.295 and 1.445 for nitrite and nitrate ions respectively.  The µPADs developed in this study provides a low-cost and convenient method for GSR analysis and may be used in the field-work detection of GSR