Use of Atomic Spectrometry in Pharmaceutical Analysis

• Jie Pan

Introduction

Pharmaceutical analysis is a critical issue for human beings, not only to verify the effectiveness of drugs, but also for the safety issues. The toxicity of various elements has been well studied and documented for many years. There are many ways to prove effective elements or compounds in drugs. For more than 100 years, the main focus on standard for testing pharmaceuticals elements has been the Heavy Elements.^[1] However, there is no clear definition of heavy elements has showed authority. Most of the concern regarding elements in pharmaceuticals was associated with following elements such as: antimony(Sb), arsenic(As), Cadmium(Cd), copper(Cu), iron(Fe), lead(Pb) and zinc(Zn). The purpose in detecting pharmaceutical materials is not just for drug products, the active pharmaceutical ingredients(API’s), raw material and intermediates for metals and metalloids also very important for analysis of drugs. For these reasons, atomic spectroscopy has been developed as one of the most powerful tool for detecting metals and metalloids in drugs.^[2]

Atomic Spectroscopy techniques

Atomic spectroscopy involving the measurements of the optical properties of free atoms. An atomization source is a system for generating atomic vapor from a sample. Classification of atomic method is depends on the source: flame atomic absorption spectroscopy(FAAS), graphite furnace atomic absorption spectroscopy(GFAAS), inductively coupled plasma-atomic emission spectroscopy(ICP-AES), direct-current argon plasma, electric arc and electric spark and inductively coupled plasma-mass spectroscopy(ICP-MS).

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Most common techniques of atomic spectroscopy are FAAS and GFAAS, which based on the Beer-Lambert Law. FAAS is considered to be less sensitive technique than GFAAS, with FAAS the sensitivities in the range of low parts per million(ppm, w/w), requiring milliliter quantities, while GFAAS is capable of low parts per billion(ppb. w/w) with requiring microliter quantities of sample. FAAS is considered the one with less-expensive and also easier skill level for an analyst as well as less time consuming when compared with GFAAS. Regardless of the technique, both FAAS and GFAAS require the use of a hollow cathode (HCL) or electrodeless discharge lamp (EDC) for each radiation source.^[3]

ICP-AES and ICP-MS have seen greater use within the pharmaceutical industry in more recent years. Both of the techniques can be applied for variety of sample types and capable of rapid, multi-element analyses. ICP-MS offering much greater sensitivity, which down to parts per trillion(ppt) than ICP-AES-pm to ppb, however, has more potential spectral interferences.

Atomic absorption spectroscopy (AAS)

Atomic absorption spectroscopy (AAS) is based on absorption of radiation by atoms. Main components included for AAS are: light source, sample holder, wavelength selector and detector. HCL (hollow cathode lamp) is the most common source, which applied for atomic absorption measurement.^[4] Daryoush Afzali et. al. applied this technique on the detecting of palladium.

Palladium is attracting a lot of attention in various fields including industry, technology and medicine. However, the long-term exposure to palladium may affect the human health. Therefore, it is our task to trace the residue of palladium, which is quite meaningful. A method for preconcentration of palladium on modified multi-walled carbon nanotubes columns has been developed. Palladium analysis requires analytical methods of high sensitivity, selectivity, and the control of interference effects. Previous research used GFAAS and ICP-MS to detect palladium, however, the mainly drawback is solvent extraction methods such as emulsion formation, different extracting efficiencies, and low sensitivity. Daryoush Afzali et. al. focused on repairing conventional solvent extraction methods for isolating environmental pollutants with solid phase extraction (SPE) techniques, which applied prior to spectrometric determination. Compared with liquid–liquid extraction, SPE utilizing solid sorbents are simpler and faster, reduce organic solvent consumption and yield higher enrichment factors. Different solid phase extractors such as Amberlite XAD resins, polyurethane foam, activated carbon, and silica gel with chelating groups have been the most widely used collectors. Carbon nanotubes (CNTs) have been chosen as solid phase extractor because of its unique thermal, mechanical, electronic, and chemical properties. The extremely large surface area and the unique tubular structure make CNTs a promising adsorbent material. The modified method using multi-walled carbon nanotubes (MWCNTs), were oxidized with concentrated HNO₃ and then modified with 5-(40-dimethylamino benzyliden)-rhodanine. Following with using as a solid sorbent for preconcentration of Pd(II) ions. The effects of experimental parameters, including pH of the sample solution, sample flow rate, eluent flow rate, and eluent concentration were investigated. The procedure offers a useful, rapid, and reliable enrichment technique for preconcentration of Pd(II) in various samples. Under the optimum condition, palladium in aqueous samples was concentrated to about 200-fold.^[5]

ICP-AES and ICP-MS

Inductively coupled plasma utilizes an argon plasma to excite and ionize elemental species, which is the most important feature of argon ICP plasma is the temperature can be achieved around 6000-10000 K. The ions formed by the ICP discharge are typically positive ions, M⁺ or M²⁺, therefore, elements that prefer to form negative ions such as Cl, I, F, etc., are very difficult to determine via ICP-AES. Samples are aspirated into the plasma by means of a nebulizer, which generates small droplets that pass through a spray chamber and then through the center tube of a concentric torch. Desolvation, vaporization, atomization and ionization of the sample occur in the high temperatures of the plasma, and the collisions of the ions and electrons of the argon plasma ionize and excite the analyte atoms. As the ions generated within the plasma pass into the mass spectrometer, the ions are separated in the magnetic field according to their mass-to-charge ratio (m/z). Due to the heat of the plasma, the ionic components is reduced, the mass range for ICP-MS typically covers from 6 to 240 atomic mass units (amu). Therefore, offers an advantage over ICP-AES。 The nature of interferences in ICP-MS is typically due to the formation of multiply charged ions, oxides and polyatomic isobaric interferences formed in the plasma.^[2]

Multi-elements detection in pharmaceutical analysis helps improve efficiency on analyzing and also reduce the sample amount. Its not only save our time, but also increase accuracy and explore more mechanism details in pharmacy. In a series of papers, by using ICP-AES to study the effects of aging. Tohno et al. measured multi-elements Ca, Mg, P and S in the four human cardiac valves showed that Ca and P accumulated most in the aortic valve, about 12–19 fold higher than in the tricuspid valve, which showed the least accumulation.^[6-7] Lin and Jiang et. al. used slurry sampling of hypertensive drug tablets to introduce a dry aerosol for ETV-ICP-MS measurement of Cd, Cr, Mo, Pb, Pd and Pt. Electrothermal vaporization (ETV) is one of alternative technique to solution nebulization, which is coupled with ICP-MS. This new combination allows the possibility to perform direct analysis of solids. Most of the traditional techniques require sample pretreatment, such as acid digestion and dry ashing. These pretreatment procedures bring issues such as time consuming, with the consequent risk of sample contamination and analyte loss. In this situation, Lin and Jiang et. al. proposed USS-ETV-ICP-MS as an alternative technique for the direct determination of Cr, Mo, Pd, Cd, Pt and Pb in three antihypertensive tablets (Cozaar, Norvasc and Bisoprolol). The innovative method by coupling electrothermal vaporization with ICP-MS provides a simple way to determine Cr, Mo, Pd, Cd, Pt and Pb in drug tablets without complicated sample pretreatment. By these means, the method precision was increased to 5% RSD (25% for Pt). Moreover, It has been demonstrated to be effective in alleviating various spectral interferences in ICP-MS analysis.^[8]

Conclusion

For the patient safety perspectives, the need for the analysis of elements in pharmaceuticals to qualify product is becoming more important. Nowadays, since the variety of instrumental techniques, such as flame and graphite furnace AA, which is mature and traditional to newer technologies, ICP-AES and ICP-MS, make it possible to monitor multi-elements at same time with the concentration from sub-ppb’s to percent’s.^[9-10] As we mentioned above, the selection of sample pretreatment is the key to the success of an analysis. The elimination of pretreatment of sample can be achieved now, which avoid risk of sample contamination and analyte loss. In the future, there is no doubt that detection limit will be reduced and the sensitivity of instrument will be increase. More sensitive technique such as ICP-MS and also modified ICP-MS will play an important role. Easier skill and low expenses of this technique should be developed. Only various techniques which can deal with difficult sample matrices and low limits of detection can help us to meet the challenges to address both product safety and product quality issues.^{[2, 11]}

References

[1] A. Taylor, S. Branch, D. Halls, M. Patriarca, M. White, Journal of analytical atomic spectrometry 2002, 17, 414-455.

[2] N. Lewen, Journal of pharmaceutical and biomedical analysis 2011, 55, 653-661.

[3] M. Ahmed, M. A. Qadir, M. Q. Hussain, American Journal of Analytical Chemistry 2014, 05, 674-680.

[4] J. S, V. K, A. S, Int. J. Res. Pharm. Chem. 2012, 2, 146-163.

[5] D. Afzali, R. Jamshidi, S. Ghaseminezhad, Z. Afzali, Arabian Journal of Chemistry 2012, 5, 461-466.

[6] Y. Tohno, S. Tohno, T. Minami, Y. Moriwake, F. Nishiwaki, K. Hashimoto, H. Yamamoto, Biological Trace Element Research 2000, 77, 107.

[7] Y. Tohno, Y. Takano, S. Tohno, Y. Moriwake, T. Minami, F. Nishiwaki, M. Yamada, M. Utsumi, K. Yuri, Biol. Trace Elem. Res. 2000, 77, 119.

[8] M.-L. Lin, S.-J. Jiang, Journal of analytical atomic spectrometry 2011, 26, 1813.

[9] A. Taylor, M. P. Day, S. Hill, J. Marshall, M. Patriarca, M. White, Journal of analytical atomic spectrometry 2013, 28, 425.

[10] A. Taylor, M. P. Day, J. Marshall, M. Patriarca, M. White, Journal of analytical atomic spectrometry 2012, 27, 537.

[11] N. H. Bings, J. O. Orlandini von Niessen, J. N. Schaper, Spectrochimica Acta Part B: Atomic Spectroscopy 2014, 100, 14-37.