Elemental Analysis and Heavy Metals

Elemental Analysis and Heavy Metals

Heavy metal residues in pharmaceutical products

In accordance with the monographs of the European and US Pharmacopoeias (Ph. Eur.; USP), for reasons of patient safety, the heavy metals in pharmaceutical finished products and raw materials must not exceed the specified limit values and must be tested for that. Heavy metals can be introduced into the product by catalysts, synthesis reagents and even by the manufacturing process itself.

As a result of the replacement of the USP heavy metal limit tests by the new, element-specific chapters <232> and <233>, these testing methods have been defined for the US market. As an FDA-approved and GMP certified contract laboratory in Switzerland, we will carry out analyses in accordance with all common Pharmacopoeias and also in accordance with the new USP regulations.

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The definition of heavy metals 

The concept of a “heavy metal” is not clearly scientifically defined. Depending on the classification criteria (density, periodic number etc.), differing elements are sometimes classified here. In technical terms, every metal with a density greater than 5 g/cm3 is considered to be a heavy metal. In common parlance, “heavy metal” is typically understood to mean an element that is toxic. This is however only partially correct, as elements that are essential for humans in small quantities also come into this category.

The origin of heavy metals in nature 

Heavy metals are largely found in nature as minerals and ores. They get into the environment as a result of being extracted, from erosion or from volcanic activity. Heavy metals are used in a number of technical applications and processes and can get into the environment or into products unintentionally.

Sample digestion: examples of microwave digestion (middle) and ashing in a muffle furnace (right)
Sample digestion: examples of microwave digestion (middle) and ashing in a muffle furnace (right)

Heavy metals in medical plants 

Intake of heavy metals occurs in the growth process of plants by absorption from water, from the ground and by aerosols from the ambient air. Contamination can also occur from the spillage of pesticides or sewage sludge containing heavy metal.

Heavy metals in finished products and raw materials 

In the manufacturing of pharmaceutical products, catalysts containing heavy metals are often involved in the synthesis. Heavy metals can also transfer into the process by abrasion or by leaching (e.g. Fe, Ti, Cu, Cr and so on.) If they are not removed efficiently, then the tainted products could get into the market.

Legal regulations

For many years, it has been known that certain heavy metals exhibit toxic effects even at low concentrations. As a result, limit values for the protection of the patients have been defined in the legislation and in the various pharmacopoeias (e.g. Ph. Eur., USP, JP, BP).

Detection methods 

If testing is not performed for a specific heavy metal, the most common source of evidence nowadays come from a limit test being carried out. 
After treatment, the heavy metal is complexed with thioacetamide or precipitated as a sulphide. Then, one compares the resulting colouring of the sample solution against that from a reference lead solution.

These limit tests still form the majority of testing for heavy metals in the current national and international Pharmacopoeias (e.g. Ph. Eur. 2.4.8). Thereby, it is possible however to make only a semi-quantitative statement about the total contents of heavy metals in the sample, and in addition - and additionally, only for those heavy metals that actually form dark coloured complexes or sulphides.

Spectroscopic tests are to be found only in individual monographs and methods (e.g. nickel in polyols and oils, lead in sugar) to date.

There is no sign of any changes in the near future with regard to the methods in the Ph. Eur. and JP. This is in contrast to the US Pharmacopoeia.

Sample preparation for limit testing for heavy metals
Sample preparation for limit testing for heavy metals

New testing standards in the US Pharmacopoeia (USP)

From May 2014, chapters <232> <<Elemental Impurities – Limits>> and <233> <<Elemental Impurities – Procedures>> completely replace the old heavy metals test and are valid for all monographed drugs. Chapter <232> serves the purpose of defining the allowable limit values for 15 individual heavy metals in all pharmaceutical products. For excipients and active ingredients, these limit values are only valid when they are referred to explicitly in a monograph.

USP: The allowed daily dosage for each element is determined in the finished product 

Depending on the method of administration (oral, inhalation or parenteral), the permissible daily exposure (PDE) is defined for each element depending on its toxicity. In consideration of the maximum daily dose DD of a pharmaceutical product, it can be calculated what the maximum concentration cmaxof an element is then allowed:

Formula: PDE ≥ c × DD → cmax = PDE/DD


USP: Measuring heavy metals in raw materials 

If the end product is not tested and only the raw materials are, the allowed daily dosage must not be exceeded by the total of the impurities. The total of the impurities is calculated from the sum of all mass contents for all heavy metal components, with regard to the maximum daily dose.

Formula: PDE ≥ (∑M(cM × mM)) × DD

USP: Must the end product always be tested?

Testing of the end product is not necessary according to these guidelines if the manufacturer can prove, using validated processes and continual monitoring of the supply chain, that impurities do not appear in their products. The manufacturer must however prove that there is no possibility of additional contaminations in the manufacturing process.

USP: Which elemental impurities are to be analysed?

The USP lists limit values for 15 heavy metals. It should be observed that it is not always necessary to investigate all of these. After a risk-based estimation, determination is restricted to those elements which could unintentionally end up in the end product either through natural methods, or by being added (e.g. catalysts). Excluded from this are the so-called “bad 4”, arsenic, lead, cadmium and mercury which must always be integrated into the risk evaluation.

Standard limits for all formulations have been defined for excipients and active ingredients in pharmaceutical products whose daily dose exceeds 10g per day.

Important: To comply with GMP, all methods must be validated in a product-specific manner.

Testing of elemental impurities has also been a subject at the ICH (International Conference on Harmonization). The Q3D Elemental Impurities Working Group has already compiled a guideline on the subject.

High-purity water is a basic prerequisite for high-quality analyses
High-purity water is a basic prerequisite for high-quality analyses

Sample digestion

To be measured, solid matter samples must be present in a dissolved form (most commonly in aqueous solution). Digestion nearly always involves additional dilution of the sample and this makes even lower detection limits necessary. Matrix-destroying methods reduce interference caused by the matrix during measurement.

The following digestions are commonly used, depending on the matrix and the sample.

  • Dissolving in aqueous solution (mostly acidified)
  • Dissolving in an organic solvent
  • Extraction
  • Open digestions (ashing, fuming, ...)
  • Microwave digestions - for high throughput and high temperature
Sample digestion: Preparing the microwave containers for closed digestion 1
Sample digestion: Preparing the microwave containers for closed digestion 2
Sample digestion: Preparing the microwave containers for closed digestion 3
Sample digestion: Preparing the microwave containers for closed digestion 4

Spectrometric measurement techniques

Atomic absorption spectroscopy (AAS)

In AAS, the weakening (absorption) of an element-specific radiation through interaction with free atoms is used to quantitatively and qualitatively determine a number of elements. Free atoms can be generated by a number of different methods. Commonly used are flame (F), graphite tube (GR), cold vapour (CV) and hydride generation (HG).

AAS is, as a result of the radiated and element-specific radiation, an extremely selective method, particularly when used in combination with CV and HG. AAS is more sensitive when used in combination with GR, CV and HG, but is also more time-consuming. F-AAS in general demonstrates the lowest level of sensitivity but is however tolerant in relation to a high proportion of the sample matrix and to up to 100% organic solvents.

The biggest disadvantage to AAS is that classical AAS is an individual method, even when the latest developments in broadband emitters could turn AAS into a multi-method. As a result, determination of a number of elements in the same sample is time-consuming.

[Translate to english:] Graphitrohr-AAS: Autosampler
[Translate to english:] Graphitrohr-AAS: Autosampler

Optical emission spectrometry with inductively coupled plasma (ICP-OES) 

In ICP-OES, atoms, ions and molecules are excited in high-energy plasma. The emissions that the particles emit again in relaxation are then separated in the spectrometer component and detected.

As a multi-method with medium sensitivity, depending on the element and the matrix, ICP-OES provides stable and quantitative analysis for many elements to approximately 10 ppb. For checking the selectivity, a number of emission lines can be used in parallel. Because the number of emission lines can however lead to interference, this can in some cases prove to be disadvantageous. In general, the sensitivity for many elements is not as low as is the case with GR-AAS or ICP-MS. As a robust method, ICP-OES is however highly suitable for quantitative element determination or for qualitative overview analysis up to the middle levels of trace amounts.

Mass spectrometry with inductively coupled plasma (ICP-MS) 

In ICP-MS, the ions generated in the inductively coupled plasma will be separated by an interface in the high vacuum section of the spectrometer according to their mass-to-charge ratio. 
Despite the mostly sequential detection, it is a multi-method that is recommended due to its low limits of determination, particularly for the monitoring of many trace elements.

This method is particularly suitable for the new heavy metal tests in accordance with USP and EMEA. For measurement however, one must always accept a higher level of dilution as this highly-sensitive method is less matrix tolerant than, for example, ICP-OES.

Elemental analysis: Measurement of heavy metals in ICP-MS
Elemental analysis: Measurement of heavy metals in ICP-MS

Our services in detail:

  • Consultation on elemental analysis
  • Content and purity testing in accordance with pharmacopoeia methods
  • Method development
  • Product-specific method validations
  • Screening analysis
  • Carrying out classic visual heavy metal tests 
  • (e.g. Ph. Eur, JP, BP)
  • Additional services are according to the directory of services or upon request
ICP-MS: Optimised sample introduction with the FAST system
ICP-MS: Optimised sample introduction with the FAST system

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