What is a percent purity assay

What is the difference between assay and purity?

The Main difference between assay and purity that is An assay is the determination of one of the main components in a sample, while purity is the determination of impurities in a sample. In addition, an assay can provide either a quantitative or a 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 are important aspects of chemistry and biochemistry.

Important areas

1. What is an assay?
- Definitions, facts, types
2. What is purity?
- Definition, facts, investigation methods
3. What are the Similarities Between Assay and Purity?
- Overview of common functions
4. What is the difference between assay and purity?
- Comparison of the main differences

key terms

Analysis methods, range 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 functional activity of a component in a sample. In chemistry, an assay is a test procedure for a metal or ore in order to determine its constituents and quality. On the contrary, assays are a kind of widely used analysis method for samples in pharmacology, laboratory medicine, biochemistry, biotechnology, immunology, environmental biology, etc. Here the analyte or the main target component can be a drug, a biochemical substance, a cell of an organism or an organic sample.

Figure 1: Samples for a spectrophotometer assay

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

  1. Processing and processing of samples
  2. Target-specific discrimination
  3. Signal amplification
  4. Signal detection
  5. Signal enhancement and noise filtering

In addition, assays can be classified according to different aspects. There are two types of tests, depending on the type and number of measurements taken. These are endpoint assays, in which signals are measured after a fixed incubation time, and kinetic assays, in which several measurements are made over fixed time intervals. There are two types of assays based on the number of analytes detected. These are single-target assays, in which only one component is measured at a time, and multiplex assays, in which several components are measured simultaneously during the same assay.

What is purity

Purity is a measure of the percentage of a chemical that is used as an impurity in a given sample. It can be thought of as a kind of quantitative test. For example, 95% ethanol is used as laboratory grade ethanol. That means; The 95% of the sample contains ethanol and the remainder of the 5% of the sample is taken up by water.

Figure 2: Range normalization method in chromatography

One of the most important analytical methods for determining the purity of a sample is chromatography. Both gas chromatography (GC) and liquid chromatography (LC) can determine the purity of the sample. However, analytical standards are not available to measure most types of contaminants. Therefore, it can be difficult to determine or quantify the exact amount of these impurities in the sample. In such cases, the purity of the sample is reported as the area percentage test of the major component instead of the purity test. Here the sum of the areas of the peaks for contamination is used to obtain the purity. Therefore, this method of determining 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 analytical methods that are used to determine components of a sample in both chemistry and biochemistry.
  • Both methods can quantify the amount of a particular component in a sample.

Difference Between Assay and Purity


An assay refers to an examination (analysis) method in laboratory medicine, pharmacology, environmental biology and molecular biology for the qualitative assessment or quantitative measurement of the presence, amount or functional activity of a target unit (the analyte), while the purity refers to the degree where a substance is undiluted or not mixed with foreign material, typically expressed as a percentage (%). So this is the main difference between assay and purity.


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

Type of component analyzed

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

Type of measurements

In addition, an assay can provide either a quantitative, semi-quantitative, qualitative, or functional measurement, while purity is a quantitative measurement. This is another difference between assay and purity.


An assay is a quantitative or qualitative measurement of a specific component in a sample. The main component of the sample is analyzed here. In contrast, purity is the analysis of impurities in the sample. It is mainly a quantitative measurement. Therefore, the main difference between test and purity is in the nature of the components and measurements.


1. "Assay". Wikipedia, Wikimedia Foundation, December 10, 2018,