Difference between Assay and Purity

  Substances are hardly found in a pure state. If it is an element, it forms various combinations between them or with other elements in order to exist. Not only elements, molecules and compounds also tend to mix with a large number of other species in nature. Therefore, often all the substances exist as mixtures.

What is the Difference Between Assay and Purity?
  • The difference between assay and purity is that an assay is the determination of one of the main component in a sample whereas purity is the determination of impurities in a sample. Furthermore, an assay may provide either a quantitative measurement while purity can be indicated quantitatively or qualitatively.
  • Assay and purity are two types of measurements used to determine the components of a sample. Both of them are important aspects of chemistry and biochemistry.

What is an Assay?
  • An assay is the analysis of a component of a sample. The type of measurement can be either qualitative or quantitative. That means; an assay measures the presence, amount or the functional activity of a component in a sample. 
  • Assay methods normally incorporate accurate and precise analytical methods. There are various types of assays. Depending on the sample to be analyzed and the other requirements, you can choose the most appropriate assay type. Chemical 
  • assays are done using techniques like chromatography, titrations, etc.
  • Bioassays are another type of assays carried out to quantify the effect of samples in biological systems. It includes studies of drugs on micro-organisms, virulence studies on human, bioassay of hormones, etc.
  • On the contrary, assays are a type of widely used analyzing methods of samples in pharmacology, laboratory medicine, biochemistry, biotechnology, immunology, environmental biology, etc. Here, the analyte or the main target component can be a drug, biochemical substance, a cell of an organism or an organic sample.
  • Moreover, assays can be classified based on different aspects. Based on the type and number of measurements taken, there are two types of assays; they are endpoint assays, which measure signals after a fixed incubation time, and kinetic assays, which take measurements multiple times over a fixed time intervals. 
  • Also, based on the number of analytes detected, there are two types of assays. They are the single target assays, which measure a single component at a time, and multiplex assays, which measure several components simultaneously during the same assay.
It is calculated against standard. It may be of following types:
  1. On as is basis = (Area of sample / Area of standard) x (conc. of standard / conc. of sample) x potency or assay of standard
  2. On anhydrous basis = (Assay on as is basis / 100 – moisture) x 100
  3. On dried basis = (Assay on as is basis / 100 – LOD) x 100

Similarities Between Assay and Purity
Assay and purity are two types of analyzing methods used to determine components of a sample both in chemistry and biochemistry.
Both methods can quantify the amount of a particular component in a sample.

What is Purity?
  • Purity is a measurement of percentage composition of a chemical which serves as an impurity in a particular sample. It can be considered a type of quantitative assay. As an example, laboratory-grade ethanol comes as 95% ethanol. That means; the 95% of the sample contains ethanol, and water takes the rest of the 5% of it.
  • One of the main analytical method used to determine the purity of a sample is chromatography. Both gas chromatography (GC) and liquid chromatography (LC) are capable of determining the purity of s sample. However, analytical standards are not available to measure most types of impurities. 
  • Therefore, it can be difficult to determine or quantify the exact amount of those impurities in the sample. In such cases, the purity of the sample is given as the area% assay of the main component instead of a purity assay. Here, the sum of the areas of the peaks for impurity is used to give the purity. Thus, this method of determining the impurities based on the area given by impurities is known as the Area normalization method.
It is calculated by area normalization method, it can be directly find out by chromatograms obtained from HPLC.
% Purity = (Area of desired peak / Sum of area of all peaks) x 100

Difference Between Assay and Purity
An assay refers to an investigative (analytic) procedure in laboratory medicine, pharmacology, environmental biology and molecular biology for qualitatively assessing or quantitatively measuring the presence, amount, or functional activity of a target entity (the analyte) while purity refers to the degree to which a substance is undiluted or unmixed with extraneous material, typically expressed as a percentage (%). Thus, this is the main difference between assay and purity.

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