In our experience, second source can have a variety of definitions which depend on the customer’s industry and accreditation. The term "source" can refer to the supplier of the starting material, the chemical form of starting material, the lot number of the starting material, the supplier of a prepared standard, the lot number of a prepared standard, etc. In order to provide our customers with standards that meet their specific second source definitions, we must know how the customer defines second source.
In Methods 200.7 and 200.8, the US EPA requires second source materials for QCS (quality control sample) testing. These samples are used periodically to verify instrument performance by confirming the calibration standards. According to the methods, QCS samples are to be prepared from a different source than the calibration standards. Here, the term “source” refers to the standard vendor. It does not guarantee that the starting material is of a different chemical form or that the starting material lot comes from different supplier and/or has a different lot number.
The definition provided by The NELAC Institute (TNI) gives the user more clarity. In defining second source, TNI emphasizes independent preparation of calibration and verification standards. There are a number of ways this can be achieved:
- Calibration and verifications standards with different lot numbers prepared from same starting material
- Calibration and verifications standards with different lot numbers prepared from different starting materials
- Calibration and verifications standards prepared by different vendors
The policy at Inorganic Ventures is to use a different lot of a certified concentrate when preparing second source blends, if available. Our approach highlights cases 1 and 2 of TNI’s definition. The COA for second source blends explains the policy to the user:
Second Source: Whenever possible, this solution was manufactured from a second set of concentrates in our manufacturing facility
Typically, this is the same starting material since most of our raw starting materials are fixed. The chemistries we choose are primarily based upon starting material considerations (purity, safety, availability, ease of use, etc.) as well as ISO Guide 34 compliance. In order to produce an ISO Guide 34 compliant standard, we must assess and incorporate uncertainties due to stability and homogeneity into the certified uncertainty. Any change in starting materials warrants new stability and homogeneity studies to ensure that the new chemistry is not unstable/inhomogeneous. In short, having multiple raw starting materials for individual products is not cost-effective, and is not a requirement based upon commonly accepted definitions.