Primer análisis

Heavy metal contamination (HMC) is prevalent in many industries including: wastewater treatment,1 environmental monitoring,2 pharmaceutical manufacturing and food production.3 HMC raises significant health concerns due to the inherent toxicity of heavy metals even when present at trace concentrations.4 As a result, government bodies such as the World Health Organisation5 _{and the European Medicines Agency}

(EMA)6 have stringent guidelines on quantification of heavy metal impurities to ensure that they do not exceed safe concentration limits. Currently, the analytical techniques recommended for quantitative heavy metal detection are laboratory based inductively coupled plasma optical emission spectroscopy (ICP-OES) and ICP-mass spectrometry (ICP-MS).7 _{For ICP (MS or OES) prior to analysis complex matrices such as soil,}

pharmaceutical products and foodstuffs must first be broken down into solution form, typically by microwave digestion in concentrated acid.8 _{The solution is then diluted}

prior to ICP ionisation.

In the electrochemical field, anodic stripping voltammetry (ASV) has long been used as a means to detect heavy metals in aqueous environments9 due to its rapid analysis time, ease of use, inexpensive instrumentation, the potential for on-line (on-site) analysis, unlike ICP-MS (-OES), as well as the ability to reach ppb detection limits.10 As discussed in Chapter 1.7.1.2, in ASV, dissolved and labile heavy metals are electrochemically reduced onto an electrode and then oxidatively stripped from the surface, with the stripping peak(s) analysed. With liquid mercury functioning as the detection electrode, the position of the stripping peak can be used to inform on metal chemical identity, with peak area or height providing quantitative information on metal ion concentration.11 However, as mercury can no longer be employed, due to its own

67 toxicity concerns,12 wide cathodic window solid electrodes such as boron doped diamond (BDD)13 _{and iridium oxide}14 _{are required as an alternative. Interpretation of}

stripping peaks from metal deposition/dissolution on solid electrodes is complex as the deposit morphology, peak suppression and the appearance of intermetallic peaks can all affect the number, position and magnitude of peaks observed.15 This makes chemical identification and quantification of metal concentrations in solution challenging.

Furthermore, this approach ideally requires that no other redox active species (interferents) are present which can negatively impact on the metal deposition and stripping process. This is to avoid fouling of the surface with electrochemical intermediates/products of the redox process and interferent electrochemistry masking the analytical stripping signal. This is likely to be especially problematic when investigating solutions which contain high parts-per-million (ppm) concentrations of electroactive species, for example, in dissolved foodstuffs, e.g. L-ascorbic acid (vitamin C),16 riboflavin (vitamin B2)17 and in dissolved pharmaceutical tablets where the majority of active pharmaceutical ingredients (APIs) i.e. the drug molecule themselves,18 show a redox electrochemical signature.

Stand-alone XRF provides a simple, non-destructive alternative for heavy metal detection, requiring little, if any sample preparation.19 Typically, the sample of interest, usually in solid form, is excited with an X-ray beam of a chosen energy resulting in the emission of a unique fluorescent signature, allowing unequivocal elemental identification from Na11 to U92. In conventional energy dispersive-XRF, ppm detection limits are found.20 Unfortunately these are not appropriate for trace level metal

68 detection studies, which typically require detection sensitivities at the ppb level.21 To improve detection limits, pre-concentration procedures can be employed, such as precipitation, liquid-solid extraction and evaporation, however they still do not enable many of the required detection sensitivities to be achieved using XRF alone.22

EC-XRF capitalises on the advantages of both electrochemistry and XRF, overcoming the aforementioned disadvantages associated with each independent technique.23

“Preconcentration” of metal ions on the surface of the electrode is achieved using electrodeposition. However, no oxidative stripping step is employed for analysis, unlike ASV. Unequivocal chemical identification and subsequent quantification is instead made using the XRF component of the technique. Boron doped diamond (BDD) is utilised as the electrode substrate, to take advantage of its excellent electrochemical properties suitable for both electrodeposition and XRF including: a wide cathodic solvent window; low background currents and high resistance to fouling.24 Furthermore, the thin BDD substrate (250 µm) is freestanding i.e. is unsupported, and constitutes only B and C atoms, which are noninterfering elements in the XRF spectra.25 _{Previous EC-XRF ex-situ studies, which focused on determining}

detection sensitivities, employed model solutions containing only inert background electrolyte (0.1 M KNO3, pH 6) and the labile metal ions Cu2+ and Pb2+.23

In this work the ability of EC-XRF to provide the required detection sensitivities for more challenging solutions appropriate to both the pharmaceutical and food industry is investigated. Initial focus is placed on the ability of EC-XRF to monitor heavy metal contamination in a pharmaceutical API, acetaminophen (ACM). Note the API is always present at significantly higher concentrations than the contaminant, and the

69 vast majority of APIs show oxidative signatures.26 Contamination of drug intermediates and drug products with heavy metals can arise from many sources27

including raw materials, equipment, solvents, reagents and catalysts,28,29 with the latter being a key cause for concern. Palladium (Pd) contamination in the pharmaceutical industry is very common30 as Pd-derived catalysts are routinely used in API synthesis.31,32 Pd is of particular concern not only due to its toxicity, but also its ability to catalyse drug decomposition. For these reasons levels of Pd must not be greater than 10 ppm with respect to the API.6 The widespread applicability of the EC-XRF technique is further demonstrated by investigating the detection of our target metal (Pd) in the presence of high concentrations of other complex electroactive species appropriate to the food industry, including L-ascorbic acid, caffeine and riboflavin. Finally, the applicability of the technique to the detection of other metals contaminants present in solution is demonstrated.