The adsorption isotherm and accurate measurements of of the amount of adsorption had occurred by 1910. It remained for Langmuir to introduce the concept of the "checkerboard" surface and the adsorption of an atom on the board, presumably by a chemical reaction. Taylor recognized that the surface may not be uniform and introduced the term "active site" for the special ones that could cause catalysis and contrasted slow and fast adsorption. In the 1930s, the work of Emmett and Brunauer, and then Brunauer, Emmett and Teller, utilized the adsorption of nitrogen at its boiling temperature to obtain an accurate measure of the total surface area. Emmett and Brunauer also used a combination of chemical and physical adsorption to identify the total surface area as well as the fraction of surface occupied by promoter materials. Eischens in the 1950s introduced the infrared technique to measure the nature of the adsorption on metallic and acidic catalytic sites and initiated the use of instrumental techniques for these measurements. The techniques used to effect the measurement of these values have advanced to where today automated instrumentation does the measurements but still utilizes these basic techniques to identify and quantify catalytic sites.