What is the Difference Between Assay and Purity

The main 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 or qualitative measurement while purity is typically a quantitative measurement.

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.

Key Areas Covered

1. What is an Assay
     – Definition, Facts, Types
2. What is Purity
     – Definition, Facts, Methods of Determination
3. What are the Similarities Between Assay and Purity
     – Outline of Common Features
4. What is the Difference Between Assay and Purity
     – Comparison of Key Differences

Key Terms

Analyzing Methods, Area Normalization Method, Assay, Chromatography, Purity

What is an Assay

An assay is the analysis of a component of a sample. Here, 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. In chemistry, an assay is a testing method of a metal or ore to determine its ingredients and quality. 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.

Figure 1: Samples for a Spectrophotometer Assay

Generally, a fixed amount of exogenous reactants or reagents are added in an assay. Therefore, the target component becomes the only variable factor during the analysis. Furthermore, the general steps involved in an assay can be:

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.

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.

Figure 2: Area Normalization Method in Chromatography

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.

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.

Difference Between Assay and Purity

Definition

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.

Significance

While an assay provides an experimentally determined value for the content or the potency of an analyte in the sample, purity provides an accurate estimate of all impurities in a sample. Hence, this is another difference between assay and purity.

Type of Component Analyzed

Also, the type of component analyzed is another difference between assay and purity. In an assay, the main component of the sample is analyzed while in purity, the impurities in the sample are analyzed.

Type of Measurements

Moreover, an assay may provide either a quantitative, semi-quantitate, qualitative or functional measurement while purity is a quantitative measurement. This is one other difference between assay and purity.

Conclusion

An assay is a quantitative or qualitative measurement of a particular component in a sample. Here, it analyzes the main component of the sample. In contrast, purity is the analysis of impurities in the sample. It is mainly a quantitative measurement. Therefore, the main difference between assay and purity is the type of components and measurements.

References:

1. “Assay.” Wikipedia, Wikimedia Foundation, 10 Dec. 2018, Available Here
2. Ketkar, Suhas. “Assay Means Purity, Right? – Versum Materials.” Versum Materials, Available Here

Image Courtesy:

1. “Assay Set Up” By Vivien Rolfe (CC BY-SA 2.0) via Flickr

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