For example, heavy metals might be deliberately used in the process (e.g. catalysts), or occur naturally within the plant or mineral sources that are used to produce active ingredients or excipients. They may also occur as undetected contaminants from starting materials or reagents, or come from the process e.g. leaching from pipes and other equipment.
Heavy Metal Contamination
Regardless of how metal ions might get into a product, pharmaceutical producers need to carry out their own tests of supplies to demonstrate the absence of impurities before using these materials, and test their own production to prevent release of contaminated product into the market.
Long established requirement
Heavy metal testing is not a recent phenomenon. The national pharmacopoeias have included heavy metal tests for many years, and many of these have found their way into the relevant European Pharmacopoeia 2.4.8. Similarly, the USP has included a general test for heavy metals since volume VIII of 1905. This was a sulphide precipitation method used to detect antimony, arsenic, cadmium, copper, iron, lead and zinc. As it happens, the test was more concerned with prevention of mislabelling than prevention of heavy metal contamination, since heavy metal salts were often used in therapy and so one had to know which salts were present in a treatment.
The detection of residual contamination came in 1942 with the introduction of volume XII in which a lead-containing standard was included in the test. The purpose was to detect potentially poisonous heavy metal residuals, such as lead and copper, since at the time, these were widely used in production equipment. As it happens, metals like iron, chromium and nickel were not revealed by the test.
The USP recently decided to revise the 'wet chemistry' methods described in USP Chapter <231> because of their perceived limitations.
One such limitations is that the original methods involve subjective visual examination and comparison of the sample solution with a lead standard, and it is this subjectivity which can create problems. Like the method of 1905, the compendial methods use a reaction to form the sulphide of any metal ions present and the total metal content is reported against the lead standard response as a limit test.
The validity of this comparison relies on several assumptions, all of which can be challenged. For example, the compendial method assumes that each of the heavy metals in the sample matrix will react in the same way as lead to form a sulphide species. This is despite the fact that many sulphides are known to be insoluble and despite some elements being known to have a far more intensely coloured sulphide than lead.
Similarly, the compendial method assumes that the reaction kinetics for lead sulphide are close to those for the other metal sulphides and that reaction kinetics are not greatly affected by the sample matrix. A final major and unsafe assumption is that the heating and/or ashing step of the method has no impact on volatile metals.
These are by no means the only reasons to challenge the validity, applicability and reliability of the compendial methods, and it is no surprise that additional chapters for the control of specific metals and other inorganic impurities have been added to the pharmacopoeia over the years. Significant amongst these in the USP is Chapter <730> Plasma Spectrochemistry, which was introduced approximately two years ago and gives laboratories the opportunity to use techniques such as inductively coupled plasma with either mass spectrometry or atomic emission spectroscopy (ICP-MS, ICP-AES).
Graphite Furnace Atomic Absorption
RSSL Pharma is able to offer ICP-MS as well as graphite furnace atomic absorption (for volatile elements such as mercury and arsenic) and standard atomic absorption spetroscopy. The advantage of ICP methods is that they can provide specific detection and quantification for each of the elements expected to give rise to a positive response in the compendial methods. Most importantly, any subjectivity is eliminated with ICP. The ICP technique is also quicker in some cases, requires a smaller sample size and gives a higher recovery of all the elements of interest. The sample preparation method for ICP, for example, is less likely to lead to the loss of the volatile elements.
Any revision to Chapter <231> is sure to include modern analytical technology. In its stimuli article for the proposed revision, the USP Ad Hoc Advisory Panel on Inorganic Impurities and Heavy Metals and the Expert Committee proposed that "the selection of an instrumental technique and a procedure for the evaluation of the inorganic impurities specified … requires the evaluation of a large number of variables including, among others, sensitivity, precision, accuracy, compatibility, time, and cost. The method selected may include plasma spectrochemistry, atomic absorption spectroscopy, or any other method that displays requisite accuracy (trueness and uncertainty) and established sensitivity and specificity."
Hence the proposed revision opens the door for heavy metal limit testing to be carried out using ICP-MS and other plasma spectrochemistry.
Room for improvement
Even with the use of more sophisticated detection technology it is worth remembering that any standard method always runs the risk of either being too general, or unable to deal with particular circumstances.
According to the heavy metals that need to be detected, some variation on the standard method may need to be applied, and RSSL's skill in method development and validation will undoubtedly be needed for certain clients and certain drugs.
For example, in a water soluble sample, solution viscosity and salt strength must always be taken into account, because both of these properties will have severe effects on the performance and life of the nebuliser in the ICP instrumentation.
In the case of water insoluble samples, decisions must also be made about which solvents it is appropriate to use because the choice of solvent will have major impacts on the cost of the analysis and lifetime of equipment.
These observations merely demonstrate some of the areas of complexity in performing heavy metal analysis and even with the most sophisticated analytical equipment available, accurate analysis still relies on the experience and expertise of the chemists performing the work.