On-site semi-quantitative analysis for ammonium nitrate detection using digital image colourimetry

• Published in: Science & justice Vol. 55; no. 6; pp. 437 - 445
• Main Authors: Choodum, Aree, Boonsamran, Pichapat, NicDaeid, Niamh, Wongniramaikul, Worawit
• Format: Journal Article
• Language: English
• Published: England Elsevier Ireland Ltd 01-12-2015; Forensic Science Society
• Subjects: Ammonia; Ammonium nitrate; Chemical reactions; Color; Digital image colourimetry; Forensic sciences; Griess’s test; Pathology; RGB colour system; Sensitivity analysis; Webcam; Ammonium nitrate; Griess’s test; Digital image colourimetry; Webcam; RGB colour system
• ISSN: 1355-0306; 1876-4452
• DOI: 10.1016/j.scijus.2015.05.001

Abstract Digital image colourimetry was successfully applied in the semi-quantitative analysis of ammonium nitrate using Griess’s test with zinc reduction. A custom-built detection box was developed to enable reproducible lighting of samples, and was used with the built-in webcams of a netbook and an ultrabook for on-site detection. The webcams were used for colour imaging of chemical reaction products in the samples, while the netbook was used for on-site colour analysis. The analytical performance was compared to a commercial external webcam and a digital single-lens reflex (DSLR) camera. The relationship between Red-Green-Blue intensities and ammonium nitrate concentration was investigated. The green channel intensity ( IG ) was the most sensitive for the pink-violet products from ammonium nitrate that revealed a spectrometric absorption peak at 546 nm. A wide linear range (5 to 250mgL − 1 ) with a high sensitivity was obtained with the built-in webcam of the ultrabook. A considerably lower detection limit (1.34 ± 0.05mgL − 1 ) was also obtained using the ultrabook, in comparison with the netbook (2.6 ± 0.2mgL − 1 ), the external web cam (3.4 ± 0.1mgL − 1 ) and the DSLR (8.0 ± 0.5mgL − 1 ). The best inter-day precision (over 3 days) was obtained with the external webcam (0.40 to 1.34%RSD), while the netbook and the ultrabook had 0.52 to 3.62% and 1.25 to 4.99% RSDs, respectively. The relative errors were + 3.6, + 5.6 and − 7.1%, on analysing standard ammonium nitrate solutions of known concentration using IG , for the ultrabook, the external webcam, and the netbook, respectively, while the DSLR gave − 4.4% relative error. However, the IG of the pink-violet reaction product suffers from interference by soil, so that blank subtraction (| IG -IGblank | or | AG -AGblank |) is recommended for soil sample analysis. This method also gave very good accuracies of –0.11 to − 5.61% for spiked soil samples and the results presented for five seized samples showed good correlations between the various imaging devices and spectrophotometer used to determine ammonium nitrate concentrations. Five post-blast soil samples were also analysed and pink-violet product were observed using Griess’s test without zinc reduction indicating the absence of ammonium nitrate. This demonstrates significant potential for practical and accurate on-site semi-quantitative determinations of ammonium nitrate concentration.