while a blue reaction is
obtained from the Simon test. The latter test can be used to
differentiate this drug from amphetamine via the selective reaction
of the Simon’s reagents with the secondary amine within
methamphetamine. These tests are currently widely used as the
presumptive tests for methamphetamine in most forensic
laboratories because they are rapid, simple, and reliable. However
they are currently only used to produce qualitative results.
Quantitative analysis of methamphetamine is generally achieved
within a forensic science laboratory using gas chromatography
which may coupled with flame ionization detector (GC-FID) and/or
mass spectrometry (GC–MS) [3–9].
Our previous work has shown a great potential for digital
image-based analysis to be used to develop fast and direct
quantitative determinations [10–12] which extend the potential
value of colourimetric presumptive test methods. The method has
been used for rapid quantitative analysis by color test for
amphetamine and methamphetamine [10], opiates [12], and the
explosive trinitrotoluene [11]. It has also been reported in other
analytical applications [13–19]. These applications have been
based on the analysis of basic red green blue (RGB) color data
obtained from digital images [12,14] generated by a digital camera.
During image processing, the reflected light from the colored
products of a colourimetric test pass through three different filters,
red, green, and blue, and is detected and recorded by image sensors such as charge-coupled devices (CCD) [14,19] or complementary
metal oxide semiconductors (CMOS). The final color of the digital
image is composed from the additive data of the three RGB filters
after scaling and adjustment to compensate for variations in the
conditions of capture.