Spectroscopic Methods for On-Site Drug Testing: A Comprehensive Review

Review, Pages 10 (2327 words)

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Abstract

In this review various spectroscopic methods that could be employed in the analysis of illegal substances are summarised, and the suitability of each method for drug testing at events as a harm reduction/health promotion strategy is considered. The discussed techniques are; infrared (IR) spectroscopy, ion mobility spectroscopy (IMS), ultraviolet (UV) spectroscopy, Raman spectroscopy and near infrared spectroscopy (NIRS). All the methods are assessed on their ability to provide quick, accurate results at a relatively low cost. The training required to use the instrument was also considered, however the most important factor was the suitability of the technique for on-site testing at nightlife events.

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Introduction

In recent years, deaths due to substance abuse in the UK have reached an all-time high. In 2017 there were 3,756 deaths related to drug poisoning in England and Wales, two thirds of these deaths were related to drug misuse and the majority of deaths were due to accidental poisoning.

1 In 2017, cocaine related deaths had risen for the 6th straight year to 432 people and it was reported that average purity of cocaine on the streets had risen for the 5th straight year, meanwhile ecstasy deaths had increased by a factor of 8 since 2010.

Meanwhile across Europe there have been wildly varying purity levels of a number of drugs tested over the years such as ecstasy, MDMA and amphetamines.3 Varying purity levels and the possibility of the drug being mixed with another more sinister substance is of concern to users who now can’t be certain what they are consuming; if it is stronger than expected or perhaps not what it is purported to be then this can have huge consequences for the individual and their health.

Because of the ever-increasing number of deaths and the concerns surrounding purity levels, it has started to become more common for harm reduction agencies such as ‘The Loop’ in the UK to explore ways to help identify and quantify illicit drugs for the public.2 This can be done at places such as music festivals, or nightclubs, where according to one study, drugs such as MDMA, cocaine, MDA, amphetamines and ketamine were found to be common in urine samples tested.

Testing before use allows the individual to know exactly what they are taking so that they are able to make safe, informed decisions. The use of such services is proven to affect the individuals use of the drugs and also allows the agency to notify public health authorities if there is an increase in the presence of lethal substances circulating.3,4,11 It is still stressed though that the government and the agencies involved in testing do not condone consuming said substance. Currently ‘The Loop’ uses Fourier-transform infrared (FTIR) spectroscopy and UV spectroscopy as two of its main techniques, however this review will look at a wider range of spectroscopic techniques that could also be used for drug analysis.2

Spectroscopic methods are extremely useful in determining what a material is or confirming the presence of a substance, and involve the analysis of the different spectra produced by the material when it interacts with or emits electro-magnetic radiation. Several of the methods in this review are already validated methods by The Scientific Working Group for the Analysis of Seized Drugs (SWGDRUG) however not all of the discussed methods are useful in terms of on-site use at events due to the size of the machines and cost or expertise involved in operating them.6,8

There are several issues that currently face this type of illicit drug testing; there are legal and moral issues, of course, however the methods and the problems with these methods is what will be discussed in this review. Some of the most accurate equipment used to analyse materials, such as mass spectrometers or NMR spectrometers, often need a laboratory setting and are very large with a great cost associated to them, making them unsuitable for the type of testing needed for harm reduction services.

One of the most common ways to test rapidly for drugs of abuse is the analysis of urine samples (or hair and nail samples) using gas chromatograph mass spectrometry (GC-MS), this is also an unsuitable technique for harm-reduction services to use as it clearly defeats the point of the service.10 Some methods can offer false positives and are not reliable or discriminatory enough for professionals to be confident in offering advice to the user, so are also not suitable in an environment where the individual’s health is potentially at risk.7 This review will discuss the range of spectroscopic techniques available; how they work, the equipment needed and the advantages and disadvantages of the technique when it comes to checking drugs at point of care/on site. Figure 1 shows a brief summary of the methods discussed.

Discussion

IR Spectroscopy

IR spectroscopy is the measurement of the amount of IR radiation that is absorbed or emitted by a sample as a function of the wavelength of the radiation.16 The absorption spectra obtained are unique to the molecules making up the sample, this is due to the absorption bands in the IR region being characteristic of the molecule, and allow the functional groups of the molecule in question to be determined. The spectrum produced can also easily be compared to a database of spectrums using a computer algorithm that finds the closest match within a very small discrepancy, this is particularly useful in the harm-reduction environment where workers need fast results, however is only possible where reference spectra are available.19 Advancements in IR spectroscopy include FTIR spectroscopy which is a fast, non-destructive technique that can provide information on solid, liquid and gaseous samples.

FTIR spectroscopy involves finding the ratio of the spectra produced with the sample’s absorption compared to without the sample’s absorption, which is done by transforming the interferograms produced by the spectrometer with and without the sample in the machine.16 In recent years handheld and portable FTIR spectrometers have begun to become commercially available.17

As already mentioned, IR spectroscopy is an extremely fast, reliable and discriminatory method that requires only a small amount of the sample for testing. It is able to distinguish between different isomers and diastereoisomers (however not between enantiomers), and can analyse solids, liquids and gases. It is also able to test quantitatively and qualitatively (as most compounds have IR active vibrational modes), provided that it is an already known compound and has a spectrum on the database available for comparison.8 In terms of harm reduction settings, FTIR spectroscopy is already actively being used due to its ‘low cost per test and excellent detection ability’, the fact that it is a non-destructive technique is also a considerable benefit when it comes to working with drugs that people plan to later use.2,8 The advancement in handheld and portable FTIR spectrometers make the method extremely attractive for the type of service being considered as it makes on-site testing at events much more accessible, especially considering that some devices have been made specifically to cater to individuals with only an intermediate knowledge level.

The usefulness of handheld devices has already proved beneficial in applications such as homeland security and terror defense.17 Drawbacks of IR spectroscopy seem to be the occasional high cost or expert knowledge required in operating the equipment, although this is not always the case and models are available for less advanced users. It is also possible for interference to occur which can make it harder to analyse a compound’s spectrum. Overall, it is clear why IR and FTIR spectroscopy is already being implemented by outreach workers.

Raman Spectroscopy

Analysis by Raman spectroscopy concerns the detection of scattered radiation after it interacts with matter. Small amounts of the scattered light are shifted in energy from the original frequency used due to the interaction of the laser light with the molecules vibrating in the molecule.12 The intensity of the shifted light is plotted against the frequency of the light resulting in a Raman spectrum.8 When a molecule is excited from its ground state (S0) to its excited energy state (S1 or S2) by an incident photon the molecule will often decay back to its ground state. If this decay is to a vibrationally excited ground state then an inelastically scattered photon will be emitted in the process.12 This is called Raman/inelastic scattering and is an optical effect. Information gathered by Raman scattering includes the molecular vibrational structure which can be used to identify a specific molecular fingerprint which will be characteristic of the molecule tested, thus the molecules making up the sample can be determined.12 To obtain Ramanspectrums, FT Raman spectrometers and dispersiveRaman spectrometers are commonly used. In the same way as FTIR spectrometers, handheld ‘point and shoot’ Raman spectrometers have become available that have a level of accuracy comparable to traditional laboratory spectrometers and can compare the results to a database.17

The most common illicit drug tested using Raman spectroscopy is cocaine. This is because it is able to distinguish between different types of cocaine as well as between cocaine and the common ‘cutting agents’ found in street cocaine.

This feature of Raman spectroscopy, to be able to differentiate between contaminants, is especially important when you consider that illegal drugs are often not pure by the time they reach the user. Another advantage of Raman spectroscopy is that measurements can be taken from outside of the drug’s packaging which makes the process slightly easier to carry out.12 The reason why Raman spectroscopy is such a powerful drug analysis technique is because; no sample preparation is needed, it provides fast and highly discriminatory qualitative results and is a non-destructive analysis method, allowing the sample to be re-tested if necessary.18

One of the main downsides of Raman spectroscopy is that the noise generated by fluorescent substances, such as heroin or some cutting agents, will interfere with the measurements being taken. FT Raman spectroscopy can be used to avoid this kind of interference as it is based on a wavelength of light (1064nm) that isn’t absorbed well by fluorescent molecules.12 Another disadvantage is the increase in the cost and expertise required to carry out a higher-level quantitative analysis. Costs of the equipment and expertise needed to operate that equipment vary depending on the quality of the machine and if it is specified to this type of testing or a wider range of uses, this is as would be expected though.

UV Spectroscopy

UV spectroscopy involves the use of UV light (light with a wavelength less than 400 nm) to cause the molecules of the sample being tested to be excited from their ground state energy levels to excited state energy levels. In order for this to happen the light directed through the sample must be of a particular, characteristic, wavelength and therefore a particular, characteristic, energy. The electronic structure of the molecule will dictate what wavelengths are absorbed or pass through the sample, so by using a particular wavelength a UV absorption spectrum characteristic of the molecule can be obtained.8 UV spectroscopy is commonly used by harm reduction services due to its relatively low cost and ability to be quantify the amount MDMA in ecstasy pills, this is because light with a wavelength of 286nm which isn’t absorbed by many other compounds will be absorbed by MDMA. However, UV spectroscopy has also been shown to have the capability to identify ketamine and cocaine at very low concentrations.20 UV-Vis spectrometers use light of wavelengths in the UV and visible range (< 800 nm), the intensity of the light passed through the sample is compared to the intensity of light absorbed by the sample, and by plotting the absorbance against the wavelength an absorption spectrum can be created.

An issue with using UV spectroscopy is its fairly low selectivity compared to the methods previously discussed in this review. If the absorbance for the impurity and the absorbance for the major component are the same for a particular wavelength of detection then the quantity of the impurity can be estimated, however if the impurity is more or less UV-active than the major component this can lead to the quantity of the impurity being over or underestimated. This type of inaccuracy could have dangerous implications if the impurity is harmful and is not picked up on by the individuals testing the illicit drug sample. What makes UV spectroscopy useful for the service being discussed is its low cost and relatively simple and fast technique.

NIRS

NIRS is a fast and non-destructive method of analysis that uses wavelengths of light from the near infrared region, i.e. wavelengths between roughly 780 nm and 2500 nm in the electromagnetic spectrum, absorptions in this region are complex and are as a result of overtone and combination vibrations in the molecule. NIRS can be used to give information on either specific sample areas or an average of a wider area. FT instruments can be used to carry out the measurements.

Whilst NIRS has proven useful in the testing of illicit drugs such as heroine, ketamine, methamphetamines and cocaine, this was analysing seized drugs and would most likely have therefore been carried out in a lab environment.22 Highly discriminatory hand-held versions of NIRS spectrometers are available which would make the method slightly more suitable for use by outreach workers.17 However, as a whole the method is still fairly complex requiring data collected to be processed using chemometric or least squares analysis. This would likely need a high amount of experience to be carried out and increased time frame for results making the method largely unsuitable for use by harm reduction workers. If the technology behind portable devices was to become more advanced so that results that have already been processed by a chosen method were to be output and compared to other available spectra, as is possible in hand-held IR spectrometers, this would be a much more suitable method.