Spectroscopy Nomenclature W e have compiled the following list of terms, their definitions, and abbreviations, which occur most frequently in papers on spectroscopy. The list indicates our preferred usages in an attempt to obtain some consistency in a field where much discrepancy exists. Sources used in this compilation were: ASTM Committee E-1 3 on Molecular Spectroscopy [I 966 revision of tentative definitions); H. K. Hughes et a / . [Anal. Chem., 24, 1349 (1 952)]; and Chemical Abstracfs. This list i s approved by the Nomenclature Committee of the ACS Division of Analytical Chemistry. Absorbance, A .
(Not optical density, absorbancy, or extinction.) Logarithm to the base 10 of the reciprocal of the transniittance il = loglo ( l / T ) .
a. (Not k ) . (Not absorbancy index, specific extinction, or extinction coefficient.) Absorbance divided by the product of the concentration of the substance and the sample path length,
Absorptivity,
a = -A
be
E. (Not molar absorbancy index, molar extinction coefficient, or molar absorption coefficient.) Product of the absorptivity, a , and the molecular weight of the substance.
Absorptivity, Molar,
A. Unit of length equal t o 1/6438.4696 of wavelength of red line of Cd. For practical purposes, it is considered equal t o 10-8 cm.
Angstrom,
Beer’s Law. (Representing Beer-Lambert law.) Absorptivity of a substance is a constant with respect to changes in concentration.
Quantity of the substance contained in a unit quantity of sample. (In absorption spectrometry it is usually expressed in grams per liter.)
Concentration, c.
Frequency.
Kuniber of cycles per unit time.
The region of the electromagnetic spectrum extending from approximately 0.78 to 300 micrometers.
Infrared.
Unit of length equal t o 10-6 meter. (Do not use micron.)
Micrometer, I?. Nanometer, nm.
Unit of length equal t o
meter.
(Do not use millimicron.) (Not 1 or d.) Internal cell or sample length, usually given in centimeters.
Sample Path Length, b.
Instrument with an entrance slit and dispersing device that uses photography to obtain a record of spectral range. The radiant power passing through the optical system is integrated over time, and the quantity recorded is a function of radiant energy. Spectrometer, Optical. Instrument with an entrance slit, a dispersing device, and with one or more exit slits, with which measurements are made a t selected wavelengths within the spectral range, or by scanning over the range. The quantity detected is a function of radiant power. Spectrometry. Branch of physical science treating the measurement of spectra.
Spectrograph.
Spectrometer with associated equipment, so that i t furnishes the ratio, or a function of the ratio, of the radiant power of tv7o beams as a function of spectral wavelength. These two beams may be separated in time, space, or both. Transmittance, T . (Not transmittancy or transmission.) The ratio of the radiant power transniitted by a sample to the radiant power incident on the sample. Ultraviolet. The region of the electromagnetic spectrum from approximately 10 t o 380 nni. The term T’i.ithout further qualification usually refers to the region from 200 t o 380 nm. Visible. Pertaining t o radiant energy in the electromagnetic epectral range visible t o the human eye (approximately 380 to 780 mi). Spectrophotometer.
(One word.) The distance, measured along the line of propagation, betweeno two points that are in phase on adjacent waves-units A., pm, and nm.
Wavelength.
(One word.) Kumber of waves per unit length. The usual unit of wavenumber is the reciprocal centimeter, cm.-l I n terms of this unit, the wavenumber is the reciprocal of the wavelength when the latter is in centimeters in vacuo.
Wavenumber.
Guide for Measures of Precision and Accuracy It is important to know what an author means when he writes of precision and accuracy. The following definitions are applicable when one operator provides a series of test results and no prior information on the method i s used. They are endorsed b y ANALYTICAL CHEMISTRY-after careful study b y members of its Advisory Board and on advice of qualified statisticiansand suggested when results reported are suitable for statistical treatment (based on 5 or more determinations). Series. A number of test results which possess common prop-
erties that identify them uniquely. Mean. The sum of a series of test results divided by the number in the series. Arithmetic mean is understood. Precision Data. Measurements which relate to the variation among the test results themselves-Le., the scatter or dispersion of a series of test results, without assumption of any prior information. The followingmeasures apply: VARIASCE.The sum of squares of deviations of the test results from the mean of the series after divisions by one less than the total number of test results. The square root of the variance. STANDARD DEVIATION. RELATIVE STANDARD DEVI.4TlON. The standard deviation of a series of test results as a percentage of t h e mean of
this series. This term is preferred over “coefficient of variation.” RdNGE. The difference in magnitude between the highest test result and the lowest test result in a series. Accuracy Data. Measurements which relate to the difference between the average test results and the true result when the latter is known or assumed. The following measures apply: MEANERROR. The average difference with regard to sign of the test results from the true results. Also equal to the difference between the average of a series of test results and the true result. RELaTIVE ERROR.The mean error of a series of test results as a percentage of the true result.
