Lead nitrate in Heavy metals testing

I will appreciate being illustrated on the reasons (historical, practical, chemical…) that resulted in the use of lead nitrate as the source of lead for the classical USP heavy metal test and why heavy metals are expressed as lead (and not, for example, as mercury). Thanks

The Heavy Metals test described in USP General Chapter <231> was a classical, wet chemistry testing method that was developed more than 100 years ago. There are several reasons why the USP has chosen to eliminate the Heavy Metals test, but the primary reasons are as follows:

  • It lacked “specificity,” i.e., the ability to distinguish between and quantitate specific heavy metals of concern.
  • It lacked the “sensitivity” to quantitate the low levels of heavy metals that are expected by today’s industry standards.
  • It employed the use of reagents that are highly toxic. USP had previously announced its intent to move in the direction of “green” chemistry wherever possible.
  • It lacked “specificity,” i.e., the ability to distinguish between and quantitate specific heavy metals of concern.
    Historically, the testing for heavy metals was conducted exclusively on raw materials that were used in food, drug, cosmetic/personal care, and dietary supplement products. Unfortunately, heavy metal contamination from sources other than raw materials was rarely, if ever, assessed. This resulted in continued heavy metal contamination of such products, placing users at risk.

Since heavy metals contamination can occur from any number of sources, including but not limited to raw materials, USP testing will now focus on finished products. They will be tested in their marketed containers with labeling in place.

Also, the terminology has been changed from “heavy metals” to “elemental impurities”. This “new” focus ensures that metal contaminants, from all sources, are captured, including:

  • Raw materials.
  • Product manufacturing, filling and packaging processes.
  • Primary container/closure systems and labeling.
    The USP <231> test for Heavy Metals has now been eliminated (without replacement) from every USP Monograph in which it once appeared. There is no longer a general requirement to test raw materials for heavy metals.
    USP General Chapter <232> entitled ‘Elemental Impurities – Limits’ now establishes a limit for each elemental impurity of concern. USP <233> entitled ‘Elemental Impurities – Procedures,’ provides a choice of methodologies to conduct USP testing. The old Heavy Metals method described in USP <231> has now been eliminated and replaced with more modern methods using Inductively Coupled Plasma (ICP) and related technologies.
    The focus of testing for elemental impurities has shifted from raw materials to the packaged finished product, and such testing is now required in accordance with USP General Chapters <232> entitled Elemental Impurities – Limits, and <233> entitled Elemental Impurities – Procedures.
    You are no longer required to test your raw materials for Heavy Metals per USP <231>. That being said, you are now required to assess your products for the twenty-four (24) elemental impurities listed in USP <232> Elemental Impurities – Limits.

This assessment may be achieved through a science-based risk assessment or by performing actual testing. Testing may include raw materials, container/closure systems, in-process samples of product batches, and finished product batches which have resided in their marketed packaging and labeling through self-life.

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