ASSAY MEANS PURITY, RIGHT?
02 NOV 2021
Appropriate Examination of purity (assay) of a chemical is one of the major checkpoint in buyer-supplier relationship. In this blog, Dr. Suhas Ketkar reveals what is to 'assay' and how it is carried out for Organosilane and organometallic chemicals, also, the challenges in examining the purity.
The verb assay means to “examine a chemical to determine its purity.” In our context, assay provides a metric that refers to the chemicals purity, usually expressed as a percent assay. On occasion, assay refers to the purity of the chemical, e.g., 99% or 99.9% pure. There is no test to determine the purity of a chemical, so in practice, the amount of impurities is quantitatively analyzed and the assay is determined by subtracting the sum of impurities from 100, to infer the assay of the chemical. This leads to a caveat emptor or “Let the Buyer Beware” situation since the supplier can choose to measure only certain impurities and if the customer does not know how to perform the analysis, they will have no clue about the actual purity of the material. For Organosilane (OS) materials, a Gas Chromatography (GC) analysis is typically performed to analyze for impurities. Analytical standards usually are not available for most of the impurities and in some cases, the impurities have not been identified. Because of this, it is often difficult to quantify the exact amount of impurities present in the chemical. In these cases, the assay is given as an area percent assay instead of the purity assay. The sum of areas of the impurity peaks is used to determine the assay. It is implicitly assumed that the response of the GC detector is uniform for all the impurities. This is a reasonable assumption if the detector is a Thermal Conductivity Detector (TCD). However, if it is critical to control the amount of one of the impurities, the assay information will not give the complete picture relevant to the end user. In these cases, additional analysis would have to be performed to quantify the impurity of interest, in addition to the assay determination. Organometallic (OM) materials are typically not amenable to be analyzed by GC. They might have to dissolve in a solvent before GC analysis. Impurities present in the OM materials might be adsorbed on the column and might not be detected. For this reason, Nuclear Magnetic Resonance (NMR) analysis is often used to determine the assay of the OM materials. Unfortunately, if the chemical of interest is paramagnetic, it presents additional challenges for assay by NMR. For paramagnetic materials, the NMR shifts are large and peaks are broadened. If the GC and the NMR analysis is not possible for the chemical of interest, sometimes the assay (or purity) is specified in terms of metallic impurities. Metals analysis using inductively coupled plasma mass spectrometry (ICP-MS) is used to determine the number of elemental impurities in the chemical. Assay based on metals analysis provides information regarding the metallic portion of the OM chemical but fails to provide any information on the organic ligands in the chemical. It is possible that the actual assay, based on the impurities in the chemical, is much lower than the assay information provided on the certificate of analysis. The following table illustrates the need for the supplier to specify what the assay means accurately. Depending upon how the assay is defined, a product can be characterized as having a higher assay than it warrants.
We have developed methods to determine relevant impurities, based on the knowledge of the synthesis route used to produce the chemical. In addition, we provide information on the method used to determine the assay so that the end user has all the relevant information critical to the success of their process.