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Assay means purity, right?

The verb assay means to “perform an examination on a chemical to determine how pure it is”. In our context, assay provides a metric that refers to the chemicals purity, usually expressed as a percent assay. On occasion it is also referred to as 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 im-purities 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 the analysis is being performed, 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 are normally 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 % assay instead of the purity assay. The sum of areas of the impurity peaks are 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, added 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 be dissolved in a solvent prior to 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 amount 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 accurately specify what the assay means. Depending upon how the assay is defined, a product can be characterized as having a higher assay than it warrants.

Tris(ethylcyclopentadienyl)yttrium99.399.9877
Tris(ethylcyclopentadienyl)yttrium99.599.9995
Tetrakis(dimethylamino)hafnium99.899.9996
Titanium isopropoxide99.998399.9

At Versum Materials, methods are developed 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.

Author

Suhas Ketkar has been with the company for 26 years and is the director of advanced analytical technology. Suhas graduated from the University of Texas at Austin with a Ph.D. in Physics and from Wharton/University of Pennsylvania with a master’s degree in management of technology.

Suhas.Ketkar@nullversummaterials.com