Analytical Forensic Pharmacology/Polarimetry

Polarimetry is very useful in the analysis of drugs of abuse. Illicit drugs such as methamphetamine can be prepared in a variety of ways (As will be discussed later in Appendix I). Different steps in the synthesis of drugs often lead to structural differences known as enantiomers. In enantiomers, structures are said to be chiral. These structures may have the same molecular mass and an identical number of the same particles but they are put together in mirror images – much like a person’s left and right hand. The key to chirality is that structures that are enantiomers cannot be superimposed. (note - that in the example still using your hand, it is easy to think that hands can be superimposed by placing each hand palm to palm. However if you are looking at each hand from the same perspective with both palms and fingers oriented in the same direction, the thumbs will point in different directions.  The same is true for organic molecules).

Polarimetry is one way to designate between these mirror images. Each chiral structure rotates light in one way or the other. To identify whether a compound rotates light one way or another it can be placed in a diffraction grid matrix assembly (Figure 4) which can be set up to measure optical rotation. Polarimeters can be purchased commercially, and these analyses can be conducted quite simply. However the assembly of instrumentation that measures optical rotation is also quite simple and the section that follows will make it easier to understand the processes involved in polarimetry based on the construction of a polarimeter.

The two main parts of a polarimeter are a light source with a single wavelength and a diffraction grating. If using regular light as your source, a light polarizer is also needed. For this simple experiment however a helium-neon laser was utilized as the monochromatic light source with a wavelength of 632.8 nm.

When light would pass through the diffraction grating, a pattern like that seen in Figure 4 is observed.

In a diffraction grid matrix like the one seen on the left, the points of light are marked on a piece of graph paper on which the laser is projecting for calibration.

By knowing these properties of light, the angles of the original set up, and the original position of light it can be calculated how much any object placed in the path of the light would rotate the pattern shown in Figure 4 by using the following formula.

Huygens’ principle. mλ=dsinө where m=any integer λ=the wavelength d=Distance and ө=angle

Once again, it is important to note the control for λ. If white light was used for a polarimeter, the results would not be able to be measured since the different frequencies of light would produce different angles at the gradient as is seen in (FIGURE -5).

Similarly, if a green laser were put through a gradient at the same time as a red laser at the same distance and angle, the positioning of the grid would be tighter with green due to the shorter wavelength.