2. Materials and methods
2.1. Materials
Crystal methamphetamine standard (Ice; purity ∼98.5%) and Yaba (as samples seized by law enforcement agencies) were obtained from the Drug Control Division, Food and Drug Administration Thailand (license number: 1003.2/790). Methamphetamine standard solutions were prepared with ultrapure water (Barnstead EasyPure II, Thermo fisher scientific, OH). Acetaldehyde was purchased from Aldrich Chemical Co. Ltd. (Dorset, England), and anhydrous sodium carbonate was purchased from Fisher Chemicals, Fisher Scientific UK Limited (Loughborough, UK). Sodium nitroprusside dihydrate (>98.0%) was purchased from Fluka (Sigma–Aldrich Chemie, Steinheim, Germany).
2.2. Colourimetric presumptive test (Simon test).
Two reagents were required for the Simon test, 10% (v/v) acetaldehyde in aqueous sodium nitroprusside solution (1%, w/v) (reagent 1) and 2% (w/v) sodium carbonate in water (reagent 2) [1]. The reagents were prepared under optimized conditions as follows: 10 μL of reagent 1 was added to 20 μL of the methamphetamine solution in a micro-tube. This was followed by 80 μL of reagent 2. The solution was then mixed by shaking and left to stand for 2 min prior to detecting its color intensities.
Each experiment was repeated 3 times. The linear range was investigated from 0.05 to 10 mg mL−1. The average intensities of the red, green and blue colors from 3 replications were used to establish a calibration graph for each color. The limit of detection was calculated using standard methods (limit of detection = yB + 3SB where yB is the intercept of the calibration curve and SB is the standard deviation of the blank) [22]. Precision was expressed as the percentage relative standard deviation for each color from the 3 replicate analysis.