Absolute spectrophotometric assays

In most studies of DPO activity, the main objective is usually a simple comparison of the potential of a particular tissue to undergo enzyme-catalyzed browning, for example, a comparison of the potential for enzymic browning of different apple or mushroom cultivars. Related to this are comparative studies of different inhibitors and processing regimes to control enzymic browning. In these circumstances, it is usually sufficient to provide comparative measurements rather than absolute values of enzyme activity, in which case results can be expressed in arbitrary units such as AmV/min for 02 electrode assays or AA/min for spectrophotometric assays. If more precise units are required, the 02 electrode results should be expressed as Anmol 02/min/(j.g protein. 

One cannot fail to note the vast expansion of the collection in the last few decades. Surely this was not fueled by additional biologic assays. Underlying the initial growth phase was the widespread utilization of spectrophotometry for identification and assay. Separation science was the second phase in pharmaceutical industry control laboratories. As a corollary, USP and NF method selection moved in the same direction. Spectrophotometric identity tests and assays are more reliable, especially for compliance testing, when performed in the relative mode, which uses a reference standard, rather than the absolute mode, which is the norm in titrimetry. There is some residual difference of opinion in other countries on this point, but that is rendered moot by the widespread adoption of separation science by the pharmaceutical industry and, thus, by the compendia. It is a characteristic of chromatographic methods that a reference standard be required, sometimes more than one for a procedure. The accumulation of modern tests and assays results in 5 to 10 uses for many reference standards. 

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