A review of impurities in methamphetamine seizures.

A review of the newly identified impurity profiles in methamphetamine seizures.Show more

Forensic Science International: Synergy two (2020) pages 194 to 205.

Isaac Onoka and pals.

Abstract.
Forensic intelligence of synthetic illicit drugs suffers a problem of continuous introduction of new synthetic methods, modification of the existing routes of manufacture, and adulterations practiced by criminal networks. Impurity profiling has been indispensable in methamphetamine intelligence based on precursors, synthetic routes, and chemical modifications during trafficking. Law enforcement authorities maintain the credibility and integrity of intelligence information through constant monitoring of the chemical signatures in the illicit drug market.
Changes in the synthetic pattern result in new impurity profiles that are important in keeping valuable intelligence information on clandestine laboratories, new synthetic routes, trafficking patterns, and geographical sources of illicit Methamphetamine.
This review presents a critical analysis of the methamphetamine impurity profiles and more specifically, profiling based on impurity profiles from Leuckart, Reductive amination, Moscow, Emde, Nagai, Birch, Moscow route; a recent nitrostyrene route and stable isotope signatures. It also highlights the discrimination of ephedrine from pseudoephedrine sources and the emerging methamphetamine profiling based on stable isotopes.

One. Introduction.
Methamphetamine is a schedule two controlled substance according to the Single Convention on Narcotic drugs and the United Nations Convention against Illicit Traffic in Narcotic Drugs and Psychotropic Substances, 1988. It is a highly addictive drug with a potent central nervous system (CNS) stimulant properties.
The United Nations Office on Drugs and Crime, UNODC, report MA as the most abused drugs worldwide. For the last two decades, the use of MA has been increasing in many countries worldwide. In Japan, about 15,000 drug arrests were from cases related to MA accounting for 90 percent of all reported violations. Previous studies have documented the prevalence of methamphetamine over other synthetic drugs elsewhere.
The impurity profiling of MA provides the linkage of illicit drug seizures based on the chemical signatures contained in the seized illicit drugs.
The method uses organic and inorganic impurities which are by-products of reactions in the final formulation of MA. It has successfully been used to establish intelligence information in France, Australia, Thailand, China, Philippines, Japan, USA, Spain, Korea and in many other places worldwide.
Recently, the emerging complementary profiling method based on stable isotopes has drawn the interest of many researchers; its details will be included in this review.
The chemical analysis of illegal drugs provides valuable information about the conspiracy links and trafficking routes, categorizing the seizures based on the signatures, thereby identifying their origins. As a complementary law enforcement investigative work, it provides a background intelligence information concerning the number of sources of drugs, whether those sources are within a country or are internationally based and also unveiling the points of distribution and distribution networks. Similarly, the impurity profiles identify the emergence of new clandestine laboratories and their associated synthetic methods, which, in turn provides background intelligence information.

Chemical profiling marks the principal purpose of the intelligence of illicit drugs by establishing a link between the clandestine laboratories, suppliers, and users. The chemical information obtained from a drug can indicate its synthetic method, adulterations during trafficking, and the operations of the criminal networks.
Generally, the fundamental role of a forensic chemist in drug profiling is to extract the chemical signatures that can be used to establish the degree of commonality of seizures with their origins or a specific group of other samples, as well as linking the signatures with the possible synthetic methods, conditions, and post-production modifications. A comprehensive examination and comparison of the chemical signatures has found growing applications strategically at the macro level and tactically at the micro-level. These advances have helped the police and criminal investigators at both national and global scales to establish the dynamics of illicit drug markets, locate the drug traffickers, establish conspiracy links between dealers and users.
In this decade, extensive number of research studies on the chemical profiling of MA and its derivatives are focusing on the determination of organic and inorganic impurities, determination of synthetic routes, and synthesis of the impurities, identification of the impurities concentration in body fluids, characterization and the extraction and separations of the impurities.
In this paper, we critically analyze the impurity profiles of MA synthesized from ephedrine, pseudoephedrine, and 1-phenyl-2-propanone “P2P” precursors and subsequently analyze their potential use for intelligence perspective.

One point one. The synthetic schemes of methamphetamine and the specific route impurities.
The Forensic intelligence of illicit drugs is an exciting subject and very challenging. In totality, it embraces the determination, identification, and characterization of the individual components in the final formulation of illicit drugs regarded in this review as the impurities, intermediates, and the contaminants. Depending on the level of operation of a clandestine laboratory, an illegal drug is an assemblage of constituents carrying the information about the synthetic route, condition-specifics, reagents, adulterations during trafficking, synthetic batches and sometimes the chemical process level of the cooks (purity).
Like in any other reaction, each synthetic scheme has byproducts emanating from the conversion of the precursors to MA.
For instance, some constituents are by-products of the reaction conditions; others are formed from the conversion of precursors to intermediates and from intermediates to MA while others are intentionally added as cutting agents for potency or weight and as artifacts described by Broseus. Therefore, the final formulation of MA is characterized by a variation of the relative abundance of the major by-products, intermediates, and impurities that define a chemical signature.
Although different MA seizures produced from the same precursor, using similar route and the same reagents have related impurity profiles; some intra and inter batch variations may still occur due to varying reaction conditions. This variation is essential to distinguish chemists (cooks) by attaching a specific profile to reaction conditions practiced by a particular clandestine laboratory.
Based on a certain probability of a link corresponding to the calculation of a threshold, two or more exhibits will have the same chemical profiles supporting the fact that they originate from the same batch, with the strength of support increasing as profiles become more complex. Otherwise, a range not meeting the threshold can only distinguish the samples rather than discriminating against their cooks.
For quite a long time, the clandestine synthesis of MA employed three major precursors, namely ephedrine, pseudoephedrine, and the 1-phenyl-2-propanone “P2P”. With “P2P”, Figure one, the Leuckart route (Six) and reductive amination are the most commonly used routes for the synthesis of MA. In contrast, ephedrine, pseudoephedrine precursors, Figure two, convert to MA through the Nagai, one, Emde, two, Hypo, three, Moscow, Six, Rosenmund, Five, and Birch, Nazi, seven pathways.

As one of the main MA precursors, the “P2P” reaction scheme involves the reductive amination reactions Figure one, one to four, and the Leuckart (HCL, H2O). The reductive amination reaction of “P2P” to MA is achieved through Palladium Hydrogen, and methylamine, NaBH4, and methylamine, NaBH3CN, and methylamine, HCl, H2O, Platinum, H2, and methylamine, and Mercury, Aluminum, and methylamine, however, the aluminum, mercury amalgam in a slightly acidic media method is reported to be the most commonly used method in Europe and USA. Although the method has long history, 1-phenyl-2-propanol formed from the direct reduction of the precursor, “P2P”, remains the potential intelligence impurity profile. Described by Verweij in 1989, the Leuckart route, figure one, six, is achieved by the addition of N-methylformamide, methylamine or formic acid followed by H2SO4 or HCl to form MA. By means of N-methyl-formamide, the reaction results in a Leuckart route determinant; the N-formyl-methamphetamine disputed by Qi and others, and Barron; and non-synthetic route determinants namely:
Di-benzyl-ketone, R-benzyl-N-methylphenethylamine, and N-methyl-di-phenethylamine.
The synthetic-route character of N-formyl-methamphetamine argued by Barron and Qi was resolved by the identification of alpha, alpha prime, dimethyl-di-phenethylamine and N, alpha, alpha prime tri-methyl-di-phenethylamine by Barron.
The two impurities were later confirmed by Vanitha having identified them in Leuckart based MAs only.
The Nagai route figure two, one is associated with the formation of several chemicals. The impurities are formed from the nucleophilic substitution reaction of the OH group of ephedrine pseudoephedrine to form ido-ephedrine or iodo-pseudoephedrine.
The intermediary iodine is liable to internal nucleophilic attack from the adjacent nitrogen to form cis- and trans phenylaziridines which is reduced to MA or hydrolysed to “P2P”. In prolonged acidic conditions, the latter undergo condensation to form methylnaphthalenes, reported to be the specific synthetic route signatures.
The conversion of ephedrine, pseudoephedrine to MA via Emde route is the dominant synthetic route in South East Asia. In contrast to the Nagai route, the Emde reaction scheme is augmented by SN1 substitution, intramolecular nucleophilic displacement, or SN2 substitution, intermolecular displacement, of the OH in ephedrine, pseudoephedrine with chloride to form a racemic mixture ephedrines of variable impurity concentrations.
The plus-chloro-pseudoephedrine and minus-chloro-ephedrine can then undergo a cyclic ring closure to form cis and trans phenylaziridines, respectively.
Accordingly, plus-nor-pseudo-ephedrine and minus-norephedrine alternative precursors undergo similar reaction to form plus-chloro-methyl-pseudoephedrine and minus-chloro-methyl-ephedrine.
These intermediates may eliminate the HCl to form 1-propenyl-benzene and 2-propenylbenzene or can undergo a rearrangement to form 1-dimethylamino-1-phenyl-2-chloropropane.

The route specific potential of the intermediary aziridines was contradicted by Ko and Salouros having identified 1-methylamino-1-phenyl-2-chloropropane as a vapour-phase nucleophilic product of the aziridine and another amine. The latter was recogonized in “Moscow” and Nagai related methods and could not qualify as a route specific impurity for Emde route. Ko instead identified and proposed chloro-propane, chloro-ephedrine, chloro-pseudoephedrine as route specific impurity for the Emde method.
Other non-route specific impurities are included in the text.
The “Moscow” method Figure two, four, is achieved by a reaction between ephedrine, pseudoephedrine with red phosphorus and iodine in water. Its mechanism is treasured in the regenerative role of red phosphorus. Skinner as supported by NicDaeid proposed a scheme based on the oxidation of P by I2 to di-phosphorus tetra-iodide, P2I4, followed by the decomposition of P2I4 in water to form phosphoric acid and phosphonium iodide. The mixture them converts to hydroiodic acid (HI) and phosphine (PH3) upon heating.

The former protonates the OH of ephedrine, pseudoephedrine to form aziridine intermediates which potentially reduce to MA as in the case of the Nagai route. Birch, Nazi route, figure two six, is a reduction reaction of ephedrine, pseudo-ephedrine using excess alkali metal for example lithium, sodium in liquid ammonia to form (CMP) , notated as a propanamine. The impurity is the most commonly encountered MA impurity prepared by the Birch route. Its reaction scheme is based on the role of alkali metals preferably lithium as a proton source for the OH of ephedrine, pseudoephedrine. As lithium protonates the precursor, NH3 facilitates the reduction of the aromatic rings to form a methyl-amino-propane. This primary impurity associated with the lithium-ammonia method normally results in high CMP to MA ratio limiting the isolation of the impurity.
Martinez proposed potassium permanganate and aqueous base for effective CMP isolation.

One point two. A paradigm shift in methamphetamine precursor production.

As a result of the crackdown measures taken against the production, trafficking and the availability of the “P2P” and ephedrine, pseudo-ephedrine, access to the precursors has shifted to the illicit manufacture of the precursors through readily available starting materials with new routes leading to the emergence of new impurity profiles.
For quite a long time, the synthesis of phenyl-2-propanone was through a vast number of starting materials such as alpha-phenyl-aceto-acetonitrile, alpha-phenyl-beta-methyl-eneglycol, aphenylisopropyl alcohol phenyl-acylmalonic ester, phenylacetyl chloride, alpha-methylstyrene with thallium nitrate, and benzene via o,o-diprotonated nitro olefin, beta-methyl-beta-Nitrostyrene, and phenylacetic acid “PAA”.
Although several “P2P” synthetic schemes were available in the 1980s, the illicit production of “P2P” was mainly through Phenylacetic acid (PAA) via acetic anhydride and lead two acetate; and beta-methyl-beta-Nitrostyrene via iron, H plus.
A recent twist in the production of “P2P” has recently involved the nitrostyrene method (NTS). This emerging synthetic scheme results in nitrostyrene recently identified in MA samples seized in Mexico and the USA. The NTS method uses benzaldehyde and nitroethane in Knoevenagel reaction to form a nitrostyrene, yellow solid, which converts to “P2P” in the presence of iron powder and hydrochloric acid.
The evolution of the “P2P” clandestine chemistry is further confirmed by the re-emergence of a new impurity profile in place of alpha-benzyl-N-methylphenethylamine and trans-N-methyl-4-methyl-5-phenyl-4-penten-2-amine from the usual foul-smelting of a crystalline PAA. The synthesis of “P2P” from PAA, figure three “A”, utilizes the then easily available ethyl phenyl-acetate “ETPA”. However, a recent decline of “ETPA” and its associated esters and amides resulted in a shift in the “P2P” precursors, figure three b, resulting in the emergence of new characteristic impurities recently reported in Australia and observed in the USA.
The “P2P” produced from the PAA method and nitrostyrene (NTS) convert to MA with route-specific markers intelligently used to trace the sources of “P2P”.
The dynamics of the operations of criminal MA networks is one of the exciting profiling topics that appeals to close monitoring by intelligence agencies. A recent Drug Enforcement Administration (DEA) MA Profiling Program (MPP) done in the USA recorded trade-off impurity profiles assigned to pseudoephedrine route to those assigned to “P2P” precursors. According to the MA impurity profiles documented by this program, the impurity profiles derived from pseudoephedrine decreased significantly since 2007 with increasing impurity profiles derived from “P2P”.
This shift was associated with a spike in unknown synthetic route assignments and a sharp decrease, 84 percent, in samples assigned to a “P2P”-based recipe in the first quarter of 2015.

Section one point three. The emerging methamphetamine impurity profiles.
In response to the crackdown measures imposed on the production and trafficking of MA and its precursor chemicals, clandestine laboratories circumvent the law enforcement authorities by deriving the precursors from uncontrolled substances such as phenyl acetic Acid (PAA), nitrostyren, and legal medicine resulting into the emergence of new impurity profiles.
The emergence of impurity profiles such as dimethyl-amphetamine and p-methoxy-amphetamine was recently documented by Stojanovska and supported by a literature collection of impurity profiles and synthetic route of manufacture of methyl-amphetamine, 3,4-methylenedioxymethylamphetamine, amphetamine, di-methyl-amphetamine, and p-methoxy-amphetamine as well as the recently identified less potent l-methamphetamine in place of d-methamphetamine in the United States.
Since their identification in seized MA, several MA impurity profiling reports reveal profiles that are potentially important for strategic, tactical and operational intelligence of MA in the USA, France, Australia, Korea, Iran, China, Philippines, Japan and Thailand.

One point three point one. Impurities from metal catalytic hydrogenation.

Metal catalytic hydrogenation of ephedrine, pseudoephedrine and “P2P” is one of the oldest MA synthetic methods.
Using ephedrine, pseudoephedrine, the clandestine laboratories often use palladium via the Rosenmund route, lithium and NH3 via Birch route, and nickel via Emde route. The reaction involves the reduction of the C-X, X-halo, phosphate, and sulfate, figure four, rather than the benzylic OH group to form methamphetamine.
Using “P2P”, an imine intermediate MA base is formed from a reaction of between “P2P” with methylamine. The MA freebase is then distilled and directly converted to hydrochloride salt. Figure five represents a reductive amination for the conversion of “P2P” to MA hydrochloride.
Reaction (c) occurs through heterogeneous reactions with internal or external sources of hydrogen in the presence of Palladium, Palladium Carbon, Palladium BaSO4, Platinum, Platinum, Carbon, Copper Oxide, CaSO4, BaSO4, Raney Nickel.
Tracking the traces of metals in the final formulation of MA has been used to determine the synthetic routes. MA manufactured from ephedrine, pseudoephedrine through the Emde and Nagai methods was found to contain two specific chemicals as route-specific impurities. Since their identification, they have been used as Emde route-specific signatures.
Furthermore, a chloro-propane derivitive has recently been used as an additional route specific marker impurity synthesized from ephedrine via chloro-ephedrine by the Emde route, figure six. The metal catalysis reaction of (1R, 2S)-plus-ephedrine or (1S, 2S)-plus-pseudoephedrine results in the formation of chloro-ephedrine, chloro-pseudoephedrine which is hydrogenated to (S)-plus-Methamphetamine.
Recent profiling of reductive amination of “P2P” made from PAA, lead two based MA, elucidated trans-N-methyl-amine and alpha-benzyl-N-methylphenethylamine.
Trans-N-methyl-4-methyl-5-phenyl-4-penten-2-amine used as a route-specific marker was presumed as an acetone and P2P condensation product, however, this assumption could not explain why the little amount of the impurity and its associated intermediates produced even if the “P2P” was refluxed in acetone for a long time.

Figure six. Metal catalytic reduction of ephedrine and pseudo-ephedrine.

Furthermore, an attempt to produce “P2P” using a Dakin-West and lead two acetate conditions were futile and could not yield the expected:
Trans-N-methyl-4-methyl-5-phenyl-4-penten-2-amine as an impurity.

The best reasoning so far centers the argument on the role of a low-level 4-carbon acetate unit as an intermediate. Since acetic acid undergoes decarboxylation in aqueous solution over a range of temperatures, a route-specific marker impurity amine, results from an intramolecular reaction of lead acetate with P2P via chelation controlled transition states followed by decarboxylation. Figure seven shows the proposed mechanism for the formation of this route specific marker.
The reductive amination of P2P is also associated with the formation of:
Alpha-benzyl-N-methylphenethylamine, as a synthetic route characteristic impurity. The MPP identified the contaminant at the DEA’s Special Testing and Research Laboratory.
The emergence of new impurity profiles in MA analysis suggests, possibly, a change in the synthetic route parameters or the synthesis of precursor chemicals. The foul-smelting of a crystalline PAA results in new impurity profiles monitored in seized MA samples. The impurities associated with the modified “P2P” synthetic pathway are:
Alpha-benzyl-N-methylphenethylamine, and:
Trans-N-methyl-4-methyl-5-phenyl-4-penten-2-amine.
They have been used to track MA synthesized from PAA.
The emergence of N-butyl-amphetamine and N-cyclo-hexyl-amphetamine in seized MA has recently triggered interest in the nitroalkene chemistry. The two impurities result from a Knoevenagel reaction of benzaldehyde and nitroethane to form a nitrostyrene.
Toske referred to this method as a nitrostyrene method (NTS) or a nitropropene method. The catalytic activities of butylamine, cyclohexylamine influence the conversion of the “P2P” precursors. The catalysts react with benzaldehyde to form imine, which then reacts with the nitroalkane to form a nitrostyrene as an intermediate. The reaction mixture at this step contains nitrostyrene and extractable cyclohexylamine, butylamine with a significant reaction potential. Based on a reaction proposed by Hass, nitrostyrene converts to “P2P” in the presence of iron powder and hydrochloric acid. The extractable cyclohexylamine, butylamine can then react with “P2P” to form the stable N-butyl-amphetamine, and N-cyclo-hexyl-amphetamine elucidated in MA seizures. Figure eight “A” and “B” represents the formation of N-butyl-amphetamine and N-cyclo-hexyl-amphetamine.
Since 2015, the two impurities were detected in MA seizures collected in the USA. The identification of the two impurities has been fundamental in tracking the “P2P” based MA synthesized by the nitrostyrene chemistry.

Section one point three point two. Emerging impurity profiles from pharmaceutical compounds.
In response to the crackdown measures taken against controlled substances, ephedrine, and pseudoephedrine, other adaptation strategies used by clandestine laboratories are co-ingredients of legal medicines, direct extraction from ephedra plants, as well as direct synthesis from easily available starting materials. Although Lee reports less common MA crystals containing pharmaceutical impurities, Barker and Antia had a different opinion on the most common sources of ephedrine and pseudoephedrine used to synthesize crystal MA.
The latter as supported by Liu who also considered medicinal drugs as the most common sources of pseudoephedrine and ephedrine.
Synthesis from legal medicines is the most common coping strategy practiced by clandestine laboratories to avoid strict measures from the law enforcement authorities. The legal medicines approach result in MA whose final formulation contains pharmaceutical signatures that used, to reflect the trends in precursor chemicals, manufacturing sources, and the trafficking patterns organized by the criminal networks. Unlike by-products, there is limited literature linking pharmaceutical impurities to the synthetic route of MA.
More recently, the MA profiling based on synthetic pharmaceutical signatures has been done in Korea, Iran, China, Japan and Thailand. Several studies have been done in this field, more research is required to unveil the potential of pharmaceutical impurities beyond their existence as sole impurities into the final MA formulations. Tracking the by-products down to their origin and their point of entry may provide the potential of establishing synthetic routes using pharmaceutical contaminants.
In a profiling program conducted in Korea between 2006 and 2011, Acetaminophen, Caffeine, Phenacetin, Ambroxol, Chlorpheniramine, Desloratadine, Barbital, Ketamine, Procaine, and Dimethylsulfone were elucidated as characteristic pharmaceutical impurities. The profiling program linked these impurities with cold medicines, cold relievers, ingredients of analgesic drugs, expectorant, and dietary supplements extracted together with ephedrine, pseudo-ephedrine. In contrast, others added as adulterants during trafficking.

While the pharmaceutical medicines demonstrate growing intelligence phenomena, tracking the specific legal drugs used in the MA production and the post-production modifications is an area of utmost interest.
Interestingly, in 2010, chlorpheniramine was identified in both Korea and Iran, indicating a cross-border operation of the criminal networks. The emergence of these impurities was a direct indication of the use of legal medicines and their associated analgesic and co-ingredients containing ephedrine or pseudoephedrine.
Furthermore, Lee identified a pharmaceutical recipe based on dimethyl sulfone from seized MA in Korea. The impurity was associated with the recipe used in medicinal drugs containing ephedrine, pseudoephedrine as well as an adulterant used in cutting MA. The impurity was previously identified in Korea, 1996 to 2003, and USA, 1996 to 2003, emerged in Australia, 1998 to 2002, re-surfaced in USA 2007, Korea 2006 to 2011 and Japan 2006 to 2007. The observed trend in the occurrence and re-emergence of dimethyl sulfone in the seized MA is potentially important in linking the operation of criminals in these countries. Although the determination of homogeneity might be very challenging, linking the dimethyl sulfone to its common source is essential for integrated intelligence.
Unlike other countries, a new profiling program based on pharmaceutical impurities conducted in China recorded a new trend of impurity profiles of MA synthesized from ephedrine, pseudoephedrine. Liu reported tablets with Theophylline-Ephedrine, Ephedrine-Diphenhydramine, Pseudoephedrine, Dextromethorphan, and Chlorpheniramine as a new set of legal medicines commonly used as a source of ephedrine, pseudoephedrine.
These drugs contain alkaline substances such as chlortrimeton, diphenhydramine, dextromethorphan, and tri-prolidine with the potential to form characteristic impurities. Their profiles information is not only used for monitoring the routine trends in precursor chemicals but also for the identification of the seized materials, smuggling patterns, and the determination of the synthetic routes.

Liu systematically determined a specific diamine, which was assigned a characteristic impurity derived from pharmaceutical products containing ephedrine, pseudoephedrine and diphenhydramine.
Unlike other pharmaceutical contaminants, the impurity is a product of a reaction between MA and traces of diphenhydramine derivative, which is co-extracted with ephedrine, pseudoephedrine.
As a pharmaceutical co-extract of ephedrine, ephedrine derived diphenhydramine, the traces of the “P2P” precursor undergo band dissociation with HI in an I-P route to form 2-iodo-N, N-di-methyl-ethan-amine as an intermediate and traces of diphenyl methanol. The 2-iodo-N, N-di-methyl-ethanamine then reacts with
MA to form a specific diamine as shown in Figure nine.
In this reaction, the diphenhydramine is present as co-ingredients of legal medicine used for the synthesis of ephedrine, pseudoephedrine. The control of such drugs is essential for monitoring and identification of illicit production of ephedrine, pseudoephedrine from legal medicines.

Section one point three point three.
Impurities discriminating ephedrine and pseudoephedrine synthetic routes.
Ephedrine and pseudoephedrine are the basic precursors commonly used to synthesize MA beside the Phenyl-2-propanone. A synthetic method using each of the precursor chemicals is associated with specific impurities that can intelligently discriminate against the MA synthetic method. From a forensic chemist’s viewpoint, tracking the impurities down to the level of discrimination ephedrine and pseudoephedrine is an ultimate goal. Many MA profiling methods based on ephedrine, pseudoephedrine end up with non-discriminatory results, deriving their conclusions from an unresolved analytical process.
Precursor discrimination based on identified impurities is another interesting intelligence work. In a recent study by Dujourdy, 43 target impurities in MA were successfully characterized and discriminated using chemometric methods.

Through clustering, the impurities identified from ephedrine, pseudoephedrine, and Benzyl-methylketone.
In their work, 1-benzyl-3-methyl-naphthalene, and 1, 3-dimethyl-2-phenyl-naphthalene were used to signify a route associated with ephedrine precursor.
Previously, N-formyl-methamphetamine was considered a route-specific impurity for Leuckart route based MA; however, the impurity has recently been identified in a reductive amination based route for MA.
A realization of this challenge was reported by Khajeamiri in their work involving the reduction of ephedrine, pseudoephedrine with HI-red P. In their viewpoint, both ephedrine and pseudoephedrine react with HI, red P to form iodoephedrine, which undergoes a ring-opening to form commonly used route-specific impurities; the cis and trans-1, 2-dimethyl-3-phenylaziridine.
Khajeamiri articulated that 1, 2-dimethyl-3-phenylaziridine is derivatized into several other chemicals mentioned in the text.
Reporting N-benzyl-2-methylaziridine as an emerging impurity, Khajeamiri associated its formation with the conversion of 1, 2-dimethyl-3-phenylaziridine into N-benzyl-2-methylaziridine during the formation of MA from ephedrine and pseudoephedrine.
Additionally, Khajeamiri reported for the first time in 2012 the presence of Chlorpheniramine as a pharmaceutical-based impurity in MA. The impurity was later reported in Korea, 2013, and Iran.
The reports associated the impurity with pharmaceutical tablets used to synthesize pseudoephedrine precursors.
Because chlorpheniramine is a co-ingredient of pseudoephedrine tables only, it discriminates ephedrine and pseudoephedrine based MA.

Section one point three point four.
Emerging signatures from stable isotopes.
Stable isotope composition in a MA sample has recently been used to profile MA seized in the USA and Japan. The technique employs natural abundance stable isotope compositions in samples to establish chemical signatures for evaluating the links between MA seizures and their production batches. The isotopes have specific natural ratios; however, compartmental isotope ratios vary as per the geographical origin of the source which is the basis for MA profiling based on stable isotopes.
The stable isotope ratio is presented in delta values, delta in per mill, where a “mil” equals a thousand. The calculation of delta value as proposed by Barrie is shown in the equation:
Delta equals on thousand times R sample, minus R standard, all over R standard.

Where R Sample represents the ratio of the heavy to the light isotope measured for the sample while R Standard is the equivalent ratio for the standard.
Although the conventional analytical techniques through the existing GC, HPLC, GC-MS, and LC-MS-MS, ICP-MS, NMR have been very effective in determining the type of precursor chemicals synthetic route, and adulteration of illicit MA; it has not been able to discriminate the precursors produced by different methods in the sense of identifying their origins. Furthermore, reductive amination routes usually have few impurity profiles that may not grant a successful impurity profiling. The conventional techniques are also ineffective in traceability beyond sources of the starting materials.

In such circumstances, stable isotope analysis is a complementary technique that can individualize illicit MA samples based on the sources of their starting materials. Elsewhere, the method has been successfully used in tracing studies in food, identification of illegal migrants, past human activities, and reconstruction of human diet.
In forensic intelligence of illicit drugs, stable isotopes technique unveil the hidden intrinsic precursor signatures that break the limits of the conventional impurity profiling by linking the seized MA and their batches to their synthetic origin. Stable isotopes add value to the intelligence of illicit drugs based on impurity profiling.

MA produced by the same hidden laboratories, following the same method with the same kind of a precursor but different sources, can be distinguished by examining the Carbon-13 and Nitrogen-15 values of their precursors. In this respect, the variation of stable isotopes in seized MA can also trace the diversion of medicinal ephedrine for the illicit manufacture of MA. Illicit ephedrine based MA was initially produced through a biosynthetic approach from the ephedra plant Figure ten “A”; however, a growing trend of total chemical synthesis, Figure ten “B” and semi-synthesis, figure ten “C” have dominated the market.
The ephedrine produced through methods “A”, “B” and “C”, above will have different stable isotopic compositions used to track MA seizures. In principle, isotopic variation is due to different enrichment factors during the biochemical synthesis of raw materials for the precursor chemicals as well as the isotopic fractionation during the synthetic processes. Although chemical synthesis is a reliable source of ephedrine, the extraction from natural sources is another potential source for clandestine laboratories. Therefore, an integrated approach with intelligence information collected beyond the starting materials is key to linking the operation of the criminal networks.
A complementary study on the use of stable isotope techniques in evaluating the links between different MA seizures was reported by Iwata, Benson, and Billault. In a study by Billault, a variation of Carbon-13 and Nitrogen-15 was used to cluster seized MA and successfully established a link between different MA cases. The study successfully discriminated against semi-synthetic ephedrine from bio-synthetic or synthetic ephedrine by using Carbon-13. However, based on the fact the Carbon-13 values of acetaldehyde from sugar are more optimistic compared to C3- photosynthetic plants or products derived from petroleum, it could not differentiate biosynthetic ephedrine from synthetic ephedrine.
Recently, a stable isotope technique was used to investigate the unique profiles of stable nitrogen isotopic composition in seized MA samples. In this work, the stable isotopes variation can be due to the isotopic variations in the starting materials, isotopic fractionation during the synthetic processes, and due to analytical errors. It is therefore essential to draw a conclusion based on the magnitude of the variation to eliminate the influence of isotopic fractionation and analytical errors. A variation of zero point nine mils Nitrogen-15 evidenced a difference in batches of production and subsequently, different ephedrine sources used as starting materials for the production of MA. Iwata further used the stable isotope technique to classify the MA seizures based on their synthetic batches. Based on the criteria proposed by Iwata, a change of zero point four mills represents a significant variation in batches.
Interestingly, stable isotope technology is making an in-road towards the discrimination of illicit synthetic, semi-synthetic illegal drugs based on their synthetic routes and their associated reactions conditions beyond its conventional use in discriminating the sources for MA. As a growing profiling method, it complements the impurity profiling by linking the synthetic routes to isotopes ratios.

Billault reported startling scientific research that seems to confirm the use of stable isotopes in linking MA batches to their respective synthetic routes. These results were the first to be reported in linking the Carbon-13 values of the precursors in distinguishing synthetic routes of seized MA and their derivatives. Having investigated the relationship between Carbon-13 and Nitrogen-15 of the precursors and those of 45 samples of MDMA, the authors demonstrated how the number of synthesis steps influenced the value of Carbon-13 in the seized MA samples and consequently discriminated the synthetic routes possessing more than one step.
Similarly, the discrimination of synthetic routes using stable isotopes is established by comparing the Nitrogen-15 values of the origin precursors and the seized MA. The values of Nitrogen-15 in MA are dependent on the source of nitrogen used, the route by which the MDMA is synthesized, and the experimental conditions employed.
Previous work by Billault discriminated MA based on their synthetic origins, synthetic routes, as well a close variation of the stable isotopes based reaction conditions of MA tested.

Conclusion.

In this review, we have discussed the impurities and stable isotopes signatures found in illicit MA. The signatures are critical in the intelligence of illegal drugs, linking the illegal drugs with their sources, synthetic methods, synthetic batches, and their geographical origin. Although stable isotopes have been influential in discriminating seizures based on their origin, it is evident from this review that its potential in profiling MA has not been fully explored.
The review highlights further how the integration of impurity profiling with stable isotopes signatures coupled with chemometric techniques complements the existing intelligence gaps. The review illustrates how an assortment of legally available chemicals and medicines used to mask the controlled substances. It has also been shown in this review how precursor chemicals come with their corresponding stable isotopes and tracking them down to their stable isotopes has been essential in discriminating the seizures based on their original starting materials resulting in comprehensive profiling.
Studies linking stable isotope technique in profiling MA are undoubtedly limited, perhaps because of the advanced instrumentations.
In furtherance of monitoring the dynamics of the MA, drug markets, and the advances of the criminal networks, future research is indispensable. Future research should focus on the diversity of both licit and illicit starting materials, complementing impurity profiling with stable isotopes, building knowledge with regards to the newly identified MA profiles, and finally; coupling the methods with chemometric techniques.