INTRODUCTION:

Any drug product or component is not 100 % pure. In small amount or acceptable amount of impurities are always present in any drug substance.

Impurities are defined as per various guidelines, As per ICH an impurity in new drug substance are addressed from two perspectives: Chemistry aspects include classification and identification of impurities, report generation, listing of impurities in specifications and a brief discussion of analytical procedures; and Safety aspects include specific guidance for qualifying those impurities that were not present or were present at substantially lower levels in batches of a new drug substance used in safety and clinical studies¹.

In general pharmaceutical impurities may defined as, “The unwanted chemical that remain with the active pharmaceutical ingredients (APIs) or develop during formulation or upon aging of both API and formulated APIs to medicine². Recently ranitidine issue is observed, FDA alerting health care professionals to All pharmaceutical companies voluntary recall of all ranitidine product from the market. The medications are being recalled because they may contain N- nitroso dimethylamine (NDMA).

FDA has recommended all pharmaceutical companies to recall their ranitidine if testing shows levels of NDMA above the acceptable daily intake limit (96 ng/day). FDA has set the acceptable daily intake limit for NDMA at 96 ng or 0.32 ppm for ranitidine. Many companies received warning letters from FDA. Pharmaceuticals components are generate impurities at various stages like during process of manufacturing, storage and transportation. These impurities should be limited as per acceptable limits otherwise they may produce toxicity if they present above the limit.

Metal impurities in pharmaceutical product may cause various toxicity; it may degrade the active pharmaceutical ingredients and hence shorten their shelf life and give undesirable effects in human body. Hence, the amount of metal impurities in raw material and final dosage forms should be under control. Regulatory agencies instructed that the pharmaceutical company must monitor and control toxic elements is controlled by their respective pharmacopeias³ .

Metal impurities like Pb, Cd and As are toxic at much lower levels. Pb is known to influence renal tumors, increase blood pressure and cardiovascular disease in adults. The human brain is affected by lead intake. Excessive intake of cadmium affects mostly the kidney and to a lower extent the reproductive system, while arsenic is known to cause cancer, impairment of the reproductive system and atherosclerosis³ .

The FDA in Uttar Pradesh has found high levels of lead and monosodium glutamate in two dozen packets of Maggie noodles tested and issued a notice to Nestle to recall 2 lakh packets of the product from the state³. US research estimates 64 million homes still contain lead paint. According to this report 5 – 15 million of these homes have been identified as ‘very hazardous’. Many children are currently affected by lead toxicity in the US and most of these are younger than 6 years. Lead can affected IQ levels and contributes to learning disabilities, hyperactivity and aggressive or disruptive behavior³

Classification of impurities:

It is possible to categorize impurities into the following categories:

· Organic impurities

· Inorganic impurities

· Residual solvents

Organic impurities will arise throughout the producing method and/or storage of the new drug substance. They will be known or unidentified, volatile or non – volatile and include:

· Starting materials eg. In the synthesis of amlodipine besylate traces of 4-(2- chlorophenyl)-3-

ethoxycarbonyl-5-methoxycarbonyl-6-methyl-2-[(2- phthalimidoethoxy) methyl]p-1-4-dihydroxy

pyridine is synthesis related impurity.

· By-products eg. In the case of paracetamol bulk production, diacetylated paracetamol may as a byproduct.

· Intermediates

· Degradation products eg. The rate of hydrolysis mannitol containing methyl prednisolone sodium is significantly higher than lactose containing methyl prednisolone.

· Reagents, legends and catalysts

Inorganic impurities may result from the producing method. They are commonly famous and known and include:

· Reagents, legends and catalysts

· Heavy metals or other residual metals

· Inorganic salts

· Other materials (e.g., filter aids, charcoal)

Solvents are inorganic or organic liquids used as vehicles for the preparation of solutions or suspensions in the synthesis of a new drug substance. Since these are generally of known toxicity, the selection of appropriate controls is easily accomplished¹.

Classification Of Metals:

Class 1:

The elements arsenic (As), cadmium (Cd), mercury (Hg), and lead (Pb) are human toxicants that have limited or no use in the manufacture of pharmaceuticals. Their presence in drug products typically comes from commonly used materials (e.g., mined excipients). Because of their unique nature, these four elements should be evaluated during the risk assessment, across all potential sources of elemental impurities and routes of administration. The outcome of the risk assessment will determine those components that may require additional controls, which may in some cases, include testing for Class 1 elements. It is not expected that all components will require testing for Class 1 elemental impurities; testing should only be applied when the risk assessment identifies it as the appropriate control to ensure that the PDE will be met.

Class 2:

Elements in this class are generally considered as route-dependent human toxicants. Class 2 elements are further divided in sub-classes 2A and 2B, based on their relative likelihood of occurrence in the drug product.

Class 2A elements have relatively high probability of occurrence in the drug product, thus should be evaluated in the risk assessment across all potential sources of elemental impurities and routes of administration (as indicated). The class 2A elements are cobalt (Co), nickel (Ni), and vanadium (V).

Class 2B elements have a reduced probability of occurrence in the drug product related to their low abundance and low potential to be co-isolated with other materials. As a result, they can be excluded from the risk assessment unless they are intentionally added during the manufacture of drug substances, excipients or other components of the drug Contains Nonbinding Recommendations 8 product. The elemental impurities in class 2B include: silver (Ag), gold (Au), iridium (Ir), osmium (Os), palladium (Pd), platinum (Pt), rhodium (Rh), ruthenium (Ru), selenium (Se), and thallium (Tl).

Class 3:

The elements in this class have relatively low toxicities by the oral route of administration (high PDEs, generally > 500µg/day) but could warrant consideration in the risk assessment for inhalation and parenteral routes. For oral routes of administration, unless these elements are intentionally added, they do not need to be considered during the risk assessment. For parenteral and inhalation products, the potential for inclusion of these elemental impurities should be evaluated during the risk assessment, unless the route-specific PDE is above 500 µg/day. The elements in this class include barium (Ba), chromium (Cr), copper (Cu), lithium (Li), molybdenum (Mo), antimony (Sb), and tin(Sn)¹⁵.

Classification of Metal Impurities As Per Different Guidelines

Table 1. USP Classification of metal impurities³

CLASS	ELEMENTS
Class 1	As, Cd, Hg, Pb
Class 2A	Co, V, Ni
Class 2B	Tl, Au, Pd, Ir, Os, Rh, Ru, Se, Ag, Pt
Class 3	Li, Sb, Ba, Mo, Cu, Sn, Cr

Table 2. ICH Classification of metal impurities³

CLASS	ELEMENTS
Class 1	As, Cd, Hg, Pb
Class 2A	Co, V, Ni
Class 2B	Tl, Au, Pd, Ir, Os, Rh, Ru, Se, Ag, Pt
Class 3	Li, Sb, Ba, Mo, Cu, Sn, Cr

Table 3. EMA Classification of metal impurities³

CLASS	Elements
Class 1	-
Class 1A	Pt, Pd
Class 1B	Ir, Rh, Ru, Os
Class 1C	Cr, Ni, V, Mo
Class 2	Cu, Mn,
Class 3	Fe, Zn

Sources of Metal Impurities:

Toxic metals like As, Pb, Cd, Hg, Se, Cr, Al, Ni and Cu come in to the human body through the food chain including medicines, atmospheric air and drinking water leading to health problems.

Many other elements, which were not frequently used in the past, e.g., the rare earth elements (REE) and platinum group of elements (PGE) are progressively being used in new modern industries for the production of many new materials, finished products including drugs and pharmaceuticals, and for several technological applications. These elements which are also discover their way into different environmental pathways especially those related to the ground and surface waters; likely have their own contribution to the environmental pollution and human health.

Technological revolution in pharmaceutical sciences and healthcare in addition to modern living conditions have increase intake of remarkable quantities of these toxic elements in to the human body leading to health problems. Impurities such as As, Cd, Cu, Sn, Sb, Pb, Bi, Ag, Hg, Mo, In, Os, Pd, Pt, Rh, Ru, Cr, Ni and V in pharmaceutical-drug substances/ products may arise from different sources like metal catalysts and metal reagents used during the synthesis of an active pharmaceutical substance (e.g., from naturally derived plant or mineral sources) and the excipients (e.g., fillers, binders, stabilizers, flavors, colors and coatings), impurities from manufacturing equipment (e.g., leaching from pipes), water and the container closure system.

Few of these metals can also be considered as active drug ingredients rather than as contaminants to increase the useful effects on human health. In fact, some active pharmaceutical ingredients (API’s) include metals and metalloids by design. These include antimicrobials containing iron, silver and gold; imaging agents using barium, gadolinium, iron, manganese and sodium, lithium drugs for psychotic illness, and platinum-based chemotherapy agents. For example, the platinum compounds; cisplatin, carboplatin and oxaliplatin are extensively used in cancer therapy and the potential of ruthenium compounds is also being presently investigated.

Aluminum is used in antacids, zinc is a part of insulin suspensions and iron is used for treatment of anemia. Metals for instance Pt and Pd are excellent catalysts and hence generally used in drug synthesis and as such can be potentially present in the final product as catalyst residues. In spite of exercising maximum care, pharmaceutical raw materials may be contaminated by factors like environmental conditions, selective use, or as a result of natural processes.

Furthermore, any product or raw material can come into contact with a broad range of materials during manufacture and processing. Sometimes, storage conditions can influence leaching (heat, UV radiation and storage time). In addition, metal ions also can affect the stability and shelf life of the formulation, catalyze the degradation of the API’s leading to the formation of unqualified degradates, or create a toxicity threat on their own.

These impurities have to be monitored in pharmaceuticals for primarily two reasons. Few metals are known to be toxic and have to be controlled during the entire manufacturing process starting from the testing of source material to the final products. Some metal impurities are toxic even at low levels and hence should be closely monitored to ensure safety of human health. Thus metals and metalloid impurities are acquiring an increasing focus for pharmaceutical regulators anticipating high standards of QC/QA for pharmaceuticals with regard to efficacy and patient safety.

The recent changes in the European Pharmacopoeia (EP), the United States Pharmacopoeia (USP) and the International Conference on Harmonization of Technical Requirements for Registration of Pharmaceuticals for Human Use (ICH) regulations for metal impurities and new strategies require companies to adopt new strategies for heavy metal analyses.

Compared to the earlier existing heavy metal analytical procedures, the new strategies involve elimination of the specificity issue, expand the range of elements detected almost simultaneously and decrease detection levels. With the new standards, drug substance/product, manufacturers will have to make sure that all their products adhere strictly to the new requirements^4.

Different Agency Which Provide Information Regarding Metal Impurities:

· Agency for Toxic Substances and Disease Registry (ATSDR) of the US Department of Health and Human Services.

· Integrated Risk Information System (IRIS) of the US Environmental Protection Agency & World Health Organization (WHO) International Program on Chemical Safety (IPCS).

· Joint Expert Committee on Food Additives (JECFA) of the WHO and the Food and Agriculture Organization.

· State of California Office of Environmental Health Hazard Assessment (OEHHA) reproductive/developmental toxicity and carcinogenicity information for articles marketed in California.

· Chemical-specific assessments that address the most current issues are also published by federal and state agencies.

Different Toxicity Observed Due to Metal Impurities:

The types of toxicity caused by metals are chronic, sub chronic and acute. The major concerns are related to neurotoxicity, nephrotoxicity, hepatictoxicity, cardiovascular effects, reproductive/developmental toxicity, neuro developmental toxicity, immune toxicity and carcinogenicity. Many agencies give information about metal toxicities^3:

Lead Toxicity:

Chronic lead exposure, even at very low levels, has been related with decreased IQ (intelligence quotient) in children. Lead is the most important toxic heavy element in the atmosphere. Lead is a highly toxic metal affecting almost every organ in the body. The toxicity in children is however of a greater effect than in adults. Long-time exposure to lead has been reported to cause anemia, along with an increase in blood pressure, and that mostly in old and middle aged population. Serious damage to the brain and kidneys, both in adults and children, were found to be linked to exposure to heavy lead levels resulting in death. In pregnant women, high level exposure to lead may cause miscarriage. Chronic lead exposure was found to reduce fertility in males. Blood disorders and damage to the nervous system have a high occurrence in lead toxicity^5.

Vanadium Toxicity:

Vanadium occurs in soil, water and air. Inhaling air with vanadium pentoxide can result in coughing^7. Vanadium exposure may cause respiratory dysfunction, hematologic and biochemical alterations, renaltoxicity, reproductive and developmentaltoxicity, immuneotoxicity, mutagenicity and neurotoxicity may also occurs when readily exposed^8.

Platinum Toxicity:

The toxicological effects of platinum in humans are repeated sneezing, rhinorrhoea, chest tightness, wheezing, shortness of breath and cyanosis, while a proportion of these also developed scaly erythematous dermatitis with urticaria. Symptomatology of toxicity including watering of the eyes, cough, dyspnoea and severe asthma, itching^8.

Lithium Toxicity:

Symptoms of intoxication of lithium include coarse tremor, hyperreflexia, nystagmus, and ataxia. Patients often show varying consciousness levels, ranging from mild confusion to delirium. Renal toxicity is observed due to lithium. Toxicity includes impaired urinary concentrating ability, nephrogenic diabetes insipidus, sodium-losing nephritis, nephritic syndrome.

Control of Metal Impurities As Per New Norms Of USFDA:

Control of elemental impurities is one part of the overall control strategy for a drug product that assures that elemental impurities do not exceed the PDEs. When the level of an elemental impurity may exceed the control threshold, additional measures should be implemented to assure that the level does not exceed the PDE. Approaches that an applicant can pursue include but are not limited to:

· Modification of the steps in the manufacturing process that result in the reduction of elemental impurities below the control threshold through specific or non-specific purification steps

· Implementation of in-process or upstream controls, designed to limit the concentration of the elemental impurity below the control threshold in the drug product

· Establishment of specification limits for excipients or materials (e.g., synthetic intermediates)

· Establishment of specification limits for the drug substance

· Establishment of specification limits for the drug product

· Selection of appropriate container closure systems Periodic testing may be applied to elemental impurities according to the principles described in ICHQ₆A^15.

Limits Of Metal Impurities In Different Dosage Forms As Per Different Guidelines:

Quality control in pharmaceutical industry includes evaluation of quality throughout the entire process of manufacturing starting from raw materials and API’s to the finished drug products including the package materials. Analysis of impurities can be very challenging as they need to be controlled at μg/g and ng/g levels.

Concentration limits are set by various regulatory agencies which are as following:

Table 4. Limits of metal impurities as per USP ⁴

Metal	Oral Exposure		Parenteral Exposure		Inhalation Exposure
Metal	PDE (µg/day)	Conc. (ppm)	PDE (µg/day)	Conc. (ppm)	PDE (µg/day)	Conc. (ppm)
Cd	25	2.5	2.5	0.25	1.5	0.15
Pb	5	0.5	5	0.5	5	0.5
As	1.5	0.15	1.5	0.15	1.5	0.15
Hg	15	1.5	1.5	0.15	1.5	0.15
Ir	100	10	10	1	1.5	0.15
Os	100	10	10	1	1.5	0.15
Pd	100	10	10	1	1.5	0.15
Pt	100	10	10	1	1.5	0.15
Rh	100	10	10	1	1.5	0.15
Ru	100	10	10	1	1.5	2.5
Cr	-	-	-	-	25	1
Mo	100	10	10	5	10	0.15
Ni	500	50	50	1	1.5	3
V	100	10	10	10	30	10
Cu	1000	100	100	10	100	10

Table 5. Limits of metal impurities as per ICH ⁴

Metal	Class	Oral Exposure		Parenteral Exposure		Inhalation Exposure
Metal	Class	PDE(µg/day)	Conc. ppm	PDE(µg/day)	CONC. ppm	PDE(µg/day)	CONC. Ppm
Cd	1	5	0.5	2	0.2	2	0.2
Pb	1	5	0.5	5	0.5	5	0.5
As	1	15	1.5	15	1.5	2	0.2
Hg	1	30	3.0	3	0.3	1	0.1
Co	2A	50	5.0	5	0.5	3	0.3
V	2A	100	10	10	1	1	0.1
Ni	2A	200	20	20	2	5	0.5
Tl	2B	8	0.8	8	0.8	8	0.8
Au	2B	100	10	100	10	1	0.1
Pd	2B	100	10	10	1	1	0.1
Ir	2B	100	10	10	1	1	0.1
Os	2B	100	10	10	1		0.1
Rh	2B	100	10	10	1	1	0.1
Ru	2B	100	10	10	1	1	0.1
Se	2B	150	15	80	8	130	13
Ag	2B	150	15	10	1	7	0.7
Pt	2B	100	10	10	1	1	0.1
Li	3	550	55	250	25	25	2.5
Sb	3	1200	120	90	9	20	2
Ba	3	1400	140	700	70	300	30
Mo	3	3000	300	1500	150	10	1
Cu	3	3000	300	300	30	30	3
Sn	3	6000	600	600	60	60	6
Cr	3	11000	11000	1100	110	3	0.3

Table 6. Limits of metal impurities as per EMA ⁴

Metal	Class	Oral Exposure		Parenteral Exposure
Metal	Class	PDE(µg/day)	CONC. (ppm)	PDE(µg/day)	CONC. (ppm)
Pt, Pd	1A	100	10	10	1
Ir, Rh, Ru, Os	1B	-	-	-	-
Mo, Ni, Cr, V	1C	300	30	30	3
Cu, Mn	2	2500	250	250	25
Fe, Zn	3	13000	1300	1300	130

Figure 3. Portable XRF

The method uses continuous wavelet transform to filter the signal and noise components of the spectrum. The use of a portable total reflection XRF is another viable option. Raman and NIR could also be used to identify or verify materials. Materials failing the rapid screening should be sent to Quality Control Labs for further testing.

Instruments For Regular Testing:

Until recently heavy metals (Ag, As, Bi, Cd, Cu, Hg, Mo, Pb, Sb and Sn) were being tested by a 110-year old traditional colorimetric qualitative (at best semi- quantitative) test that indicated the content of metallic impurities by a colored sulfide precipitate.

This test involves sample ignition and ashing at 600°C (800°C EP) and addition of H2S and thioacetamide after pH adjustment. The metal sulfide color thus developed is visually (subjective) compared against that of 0.001% (10μg/ml) Pb standard prepared in a similar way.

Although still widely accepted and used in pharmaceutical industry, these methods based on the intensity of the color of sulfide precipitation are non-specific, insensitive, time-consuming, labor intensive, and more often yield low recoveries or no recoveries at all.

Spectrophotometry:

UV/Visible spectrophotometry has become popular because it is relatively cheap, rapid and simple. Spectrophotometry uses light in the visible, ultraviolet, and near infrared ranges. According to BeerLambert law, absorbance of a solution is directly proportional to the concentration of the absorbing species in solution and the path length. Thus, for a fixed path length, UV/VIS spectrophotometry can be used to determine the concentration of several metals although some desired constituents are self-colored, UV/Visible spectrophotometry also sometimes involves the use of ligands which selectively bind to metals such as iron (II) and copper (II) to produce colored complexes with a higher molar absorptivity to enable sensitive determination of these metals in different pharmaceutical samples.

Developed a solvent extraction and spectrophotometric method for the determination of Mo (VI) in pharmaceutical samples by using acetophenone 2′, 4′- dihydroxysemicarbazone as an analytical reagent. Methods have been developed not only for the determination of metals but also specific organometallic compounds in pharmaceutical samples.

Figure 4. UV Visible Spectrophotometer

Developed a novel spectrophotometric method by complexation of the drug with ortho-phenylenediamine and monitoring the absorbance of green colored complex at 706nm for the accurate determination of cisplatin hydrochloride which is an important chemotherapeutic drug for cancer treatment.

The method has been validated and successfully applied for the assay and dissolution studies of cisplatin hydrochloride tablets during quality control in pharmaceutical industries.

Atomic Absorption Spectrometry (AAS):

F-AAS is a widely accepted analytical technique for the determination of metals at μg/ml level and below in different types of materials including pharmaceuticals and drugs. This technique is based on the principle that the amount of light absorbed is a measure of the concentration of a particular analyte at a particular wavelength.

By comparing absorption of a known standard with that of an unknown, the concentration can be computed in an unknown sample. GF-AAS is a technique which offers better sensitivity and involves injection of a small amount of solution to be analyzed into a small graphite tube and thus is suitable for the analysis of metals at ultra-trace levels (ng/ml). Mercury by ‘cold vapor’ method and some ‘volatile’ elements like As and Sb can be measured as their hydrides.

Figure 5. Atomic Absorption Spectroscopy

Figure 6. Schematic diagram of Atomic Absorption Spectroscopy

Cold vapor-AAS is especially valuable for measuring low amounts of mercury and this technique was used for the detection of mercury by absorption at 254nm (9 ng/ml – limit of detection) in pharmaceuticals. A major advantage of cold vapor AAS is the inherent separation of mercury from the matrix.

Both F-AAS and GF-AAS allow reliable determination of metallic impurities in pharmaceutical quality control operations. Investigated different plant materials for metal contents and detected measurable amounts of Ca, Cu, K, Li, Mg, Mn, Na, Ni, and Zn in phyto pharmaceutical derivatives of Hypericum perforatum by AAS techniques, in order to establish their normal concentration range and consider their role in plants useful for human medicinal treatment.

Metal monitoring as a pattern recognition method is a promising tool in characterization and/or standardization of phyto medicines. These techniques also allow reliable determination of mineral content in pharmaceutical quality control of medicinal plants. For example, Cu in wenglitong capsules was determined by F-AAS for evaluating quality of the capsules. AAS techniques are still popular in some laboratories although they are often restricted by poorer sensitivity (in the case of F- AAS) and not being multi-element analytical techniques.

X – Ray Florescencespectrometry:

In recent times XRF analysis has become increasingly attractive when compared to other techniques, especially due to the ease of sample preparation. XRF spectrometry involves irradiation of the sample with high energy excitation X-rays and measurement of element-specific fluorescence X-rays at a particular wavelength or energy from the sample.

Samples can be in solid, powder or liquid form. Since it is a non-contact analysis, problems such as memory effects commonly experienced in solution analysis, are not encountered. As it is a non-destructive technique, it is possible to reuse the sample after measurements. Both forms, namely, wavelength dispersive- XRF (WD-XRF) and energy dispersive-XRF (EDXRF) techniques have been successfully applied for the determination of Zn, Fe and Ni in API’s.

Have utilized WDXRF for monitoring heavy metal impurities (Fe, Zn, Cr and Ni) in drug substances and validated them according to the specification of the European Agency for the Evaluation of Medicinal Products (EMEA) and ICH. Earlier Linder et al. used dibenzyldithiocarbaminate as a co-precipitant during routine determination of 12 heavy metals in pharmaceuticals by XRF. In recent times, total- reflection XRF (TXRF) has been found to be useful for the determination of trace elements in drugs. Investigated samples of lecithin, insulin, procaine and tryptophan are form different origins. Element concentrations provide fingerprints which offer the possibility of discriminating between different batches of the analyzed substances originating from different production or purification processes.

Figure 7. XRF Analysis

TXRF facilitates characterization of such samples without extensive pre- treatment, and provides fast multi-element determination of elements having atomic numbers 14 < Z < 92 based on matrix-independent quantification by means of an internal standard. Though there are several studies on the application of XRF techniques in pharmaceutical industry, because of the higher detection limits, they are not very popular for quantitative determinations of metal impurities in pharmaceutical samples.

Instrumental Neutron Activation Analysis (INAA):

INAA is a relatively straightforward analytical technique for determining elemental abundance in a wide range of materials. This technique relies on the measurement of characteristic radiation from radio nuclides formed directly or indirectly by neutron irradiation of the material of interest.

The energy of the emitted gamma rays is used to identify the nuclide and the intensity of the radiation can be used to determine its abundance. In the last fifty years, this analytical technique has been found to be extremely useful in the determination of trace and minor elements in many types of environmental, geological, plant, food and pharmaceutical materials.

The advantages include,

(I) The method is non-destructive, hence the same sample can be used for other measurements

(II) Sample size can be very small, often as little as a milligram

(III) Detection limits for many elements are in the ng/grange;

(IV) No sample preparation is required

(V) Over 40 elements can be measured simultaneously.

Because of these advantages, INAA used to be a very popular analytical technique compared to other analytical methods until ICPMS came in to use.

As NAA does not require sample dissolution, it has a great advantage over solution techniques such as AAS, ICPAES and ICP-MS. Determination of fourteen trace elements (K, Sc, Cr, Fe, Co, Zn, Br, Rb, Cs, La, Sm, Eu, Au and Th) in natural drug samples using INAA.

Herbal medicines were analyzed for 31 elements by INAA using 5-minute and 6-hour thermal neutron irradiation followed by high-resolution gamma-ray spectrometry. Determination of mercury in some traditional Indian drugs by using both ICP – AES and INAA techniques. some used INAA for the accurate and precise determination of the concentration levels of several minor and trace components (Na, Mg, Al, Cl, K, Sc, Ti, V,Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Br, Rb, Sr, Zr, Mo, Ag, Cd, Sb, I, Cs,Ba, La, Ce, Nd, Sm, Eu, Tb, Dy, Yb, Lu, Hf, Ta, Hg, Th and U) in three radiopharmaceuticals (HMPAO, DMSA and DTPA).

Quantification of heavy toxic elements is required for the registration of radiopharmaceuticals. Despite the above advantages, INAA is certainly not a popular analytical technique as it is time-consuming, not independent, requires a reactor nearby and involves longer cooling times for certain elements.

Inductively Coupled Plasma Atomic Emission Spectrometry (ICP –AES):

ICP-AES technique involves measurement of light emitted by the elements in a sample when introduced into an ICP source. The measured emission intensities are then compared to the intensities of standards of known concentrations to obtain the respective elemental concentrations in an unknown sample.

The technique can simultaneously measure up to 60 elements with high sensitivity and an extraordinarily wide linear dynamic range which is perhaps the most outstanding feature of the ICP-AES. The general chapter USP includes two analytical procedures involving ICP-AES and ICP-MS for determination of elemental impurities in pharmaceuticals and includes a comprehensive validation procedure to ensure acceptability of results. Compared with F-AAS, ICP-AES provides lower detection limits, has multi element capability and a wider linear dynamic range.

Applied ICP-AES method for quantitative determination of As, Cd, Cu, Cr, Fe, Hg, Ir, Mn, Mo, Ni, Os, Pb, Pd, Pt, Rh, Ru, V and Zn in medicinal tablets. Sample preparation was performed by microwave-assisted acid digestion using a mixture of 65% HNO3 and 37% HCl (3:1, v/v). Also reported memory effects of Hg and Os, and potential spectral interference for Ir, Os, Pb, Pt and Rh. Limits of detection were at least a factor of ten below the USP limit concentrations confirming that the ICP-AES technique is well suited for quantitative determination of elemental impurities in pharmaceutical samples.

Inductively Coupled Plasma Mass Spectrometry (ICP -MS):

ICP-MS combines a high-temperature ICP source with a mass spectrometer. The ICP source converts atoms of the elements in the sample to positively charged ions. These ions separated on the basis of mass-to-charge ratio in a mass spectrometer, are directed to a detector. Since the first linking of an ICP ion source with a quadrupole mass spectrometer in 1980, development of commercial instruments in 1983, further advances such as the advent of HR- ICP-MS and ICP- TOF-MS, during the last three decades have brought this technique to a point where this technique can deliver detection limits of one part in 1015 for a majority of elements in the periodic table.

These techniques have contributed to a better understanding of the role of even elements such as REE and PGE in different compartments of the environment. For example, recent studies have indicated that Pt, Pd and Rh are getting accumulated in to the biosphere through vehicular emissions. Diet also is an important source of PGE intake in addition to vehicular emissions which, if ignored, can result in serious consequences. ICP-MS was successfully used to determine Pt, Pd and Rh impurities in pharmaceuticals with detection limits of 15, 2.8, 2.5ng/g for Pd in enalapril maleate, Pt in calcium folinate and Rh in levodopa, respectively.

Identified limitations of the traditional heavy metal colorimetric procedure compared to ICP-MS for analysis of elemental traces in pharmaceutical materials and recommended a multi-element ICP-MS survey method as an alternative to the antiquated ‘heavy metals limit test’ prescribed by USP, EP, and BP, for drug substances, intermediates and raw materials. The survey method is simple, fast, sensitive, semi-quantitative to quantitative, and includes all the elements which can be analyzed by atomic spectroscopy.

ICP-MS and TXRF methods were applied for screening of metal impurities in pharmaceuticals. Determination of heavy metal contents in dicyclomineHCl, ethambutol, pyrazinamide and furazolidone drugs by using ICP-MS. The drugs were analyzed for Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd, Hg, Pb by selecting suitable isotopes. Cr, Fe, Ti and Cu were observed to be highest in dicyclomineHCl, ethambutol, pyrazinamide and fura-zolidone respectively. Ni and Hg were absent in all the four drugs, while traces of Cd were present in ethambutol and pyrazinamide.

Microwave Plasma Atomic Emission Spectrometry (MP– AES):

In 2011, a new commercial instrument representing yet another analytical technique called the MP-AES has been introduced, which appears to provide an attractive alternative to flame-AAS and ICPAES for the analysis of pharmaceutical samples. The MP-AES system utilizes a relatively new design of plasma torch, utilizing nitrogen gas as previously reported by Hammer. In contrast to previously reported systems, the newly commercialized MP-AES system couples energy from the microwave magnetic field rather than the microwave electric field, using a hollow rectangular metal section called a waveguide.

The utility of MP-AES has been demonstrated using several different geological and environmental matrices, including industrial effluents, water, sediments, soils, rocks and ores, as well as ethanol and gasoline. MP-AES is a potential analytical technique for the analysis of drugs and pharmaceutical samples for their inorganic contents, because it has a number of obvious advantages over techniques such as F-AAS and ICP-AES. Use of nitrogen microwave plasma represents a considerable cost saving compared with argon ICP, particularly as nitrogen generator with air compressor can be used to generate nitrogen plasma, effectively eliminating ongoing gas costs. Compared to AAS, microwave plasma does not require the use of flammable gases such as acetylene and associated gas cylinders and regulators, providing a safer laboratory environment.

Given the similarity of sample introduction systems between the MP-AES and conventional ICPAES, it is anticipated that sample introduction and sample pre- concentration strategies already validated for ICP-AES will be equally robust for MP- AES, which would make MP-AES an even more attractive alternative technique if governmental regulatory bodies approve. The measurement on concentrations of different elements in tap water by MP-AES indicates the potential this technique has for the analysis of elemental impurities in drugs and pharmaceutical samples.

Laser Ablation – ICP – MS (LA – ICP – MS):

LA-ICP-MS is a powerful and rapid analytical technique for multi elemental analysis in different types of materials. This technique does not require much sample preparation and solid/liquid samples can be analyzed directly by generating vapor from the surface of the sample, which is then swept into an ICP-MS and analyzed. A major disadvantage in using LA-ICP-MS method for the analysis of pharmaceutical and drug samples is the lack of matrix reference materials for validation and calibration purposes, as the instrument needs to be calibrated using reference materials similar to the samples.

Figure 8. Schematic diagram LA – ICP – MS

However, there are several studies related to the quantification of elemental impurities in active pharmaceutical ingredients by LA-ICP-MS. A recent study has highlighted the active role of LA-ICP-MS in the control of inorganic contaminants in pharmaceutical ingredients and has thus significance in the quality control of drug products. These studies also focus on the handling strategy of laboratory-made matrix calibration standards for the quantification of elemental impurities in an active pharmaceutical ingredient by LA-ICP-MS. Such techniques found to be useful even for probing tablet uniformity, were successfully used for the direct analysis of pharmaceutical tablets. Two-dimensional elemental images of thin tissue sections by LA-ICP-MS can have great promise as a tool for improved diagnosis and monitoring of certain diseases. LIBS is also beginning to see wider applications in the analysis of pharmaceutical tablets for inorganic contents. LIBS technique was also used to study tablet coating variability in pharmaceutical tablets, involving quantification of the amount of coating on a tablet by assigning an average coating thickness score.

CONCLUSION:

From this review, it can be understand that identification of metal impurities is very important step during manufacturing of any pharmaceutical formulation. Because it can provide most important information about metal toxicity, detection limitsofanytoxicmetal. There is requirement of technique which candetect the metal in between the process when metals exceed the permissible limits, which can cease the metal impurities in final product. It is obviously that identification and control of metal impurities in pharmaceuticals are tough or challenging task but at safety aspects it is necessary to permissible amount should be present in final product. Some modern analytical techniques are there which can detect the impurities as per requirements but in process detection techniques or instrumentation should be implemented.

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Received on 03.03.2021 Modified on 27.04.2021

Asian J. Pharm. Ana. 2021; 11(3):212-222.

DOI: 10.52711/2231-5675.2021.00038