Spectrometry
Nomencla tu re
We have compiled the following list of terms, their definitions, and abbreviations, which occur most frequently in papers
on spectrometry. 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 Absorption Spectroscopy ( 1 959 revision of tentative definitions); H. K. Hughes et a/. [ANAL.CHEM.24, 1 3 4 9 ( 1 9 5 2 ) and Chemical Absfracts. Our advisory board members considered the terms and the following list includes their suggestions.
1;
Absorbance, A . (Not optical density, absorbancy, or extinction.) Logarithm to the base 10 of the reciprocal of the transmittance A = loglo ( 1 j T ) . Absorptivity, a. (ru’otk.) (Not absorbancy index, specific extinction, or extinction coefficient.) Absorbance divided by the product of the concentration of the substance (in g./l.) and the sample path length (in cm.), a = -A
bc
Absorptivity, Molar, E . (Not molar absorbancy index, molar extinction coefficient, or molar absorption coefficient.) Product of the absorptivity, a, and the molecular weight of the substance.
Spectrograph. 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 treatin measurement of spectra.
the
Angstrom, A. (Not b.) Unit of length equal to 1/6438.4696 of wavelength of red line of Cd. For practical purposes, i t is considered equal to lo-* cm.
Spectrophotometer. Spectrometer with associated equipment, so that i t furnishes the ratio, or a function of the ratio, of the radiant power of two beams as a function of spectral wavelength. These two beams may be separated in time, space, or both.
Beer’s Law. (Representing Beer-Lambert law.) Absorptivity of a substance is a constant with respect to changes in concentration.
Transmittance, T. (Not transmittancy or transmission.) The ratio of the radiant power transmitted by a sample to the radiant power incident on the sample.
Concentration, c. Quantity of the substance contained in a unit quantity of sample. (In absorption spectrometry i t is usually expressed in grams or moles per liter.)
Ultraviolet. The region of the electromagnetic spectrum from approximately 10 to 380 m p . The term without further qualification usually refers to the region from 200 to 380 mp.
Number of cycles per unit time.
Visible. Pertaining to radiant energy in the electromagnetic spectral range visible to the human eye (approximately 380 to 780 mp).
Frequency.
Infrared. The region of the electromagnetic spectrum extending from approximately 0.78 to 300 microns. Micron, p . Unit of length equal to lo4 meter. Millimicron, mp. Unit of length equal to one thousandth of a micron. Almost but not exactly equal to 10 angstroms. Sample Path Length, 6. (Not I or d.) Internal cell or sample length, usually given in centimeters.
Wavelength. (One word.) The distance, measured along the line of propagation, between two points that are in phase on adjacent waves-units A., mp, and+ Wavenumber. (One word.) Number of waves per unit length. The usual unit of wavenumber is the reciprocal centimeter, cm.-’ I n terms of this unit, the wavenumber is the reciprocal of the wavelength when the latter is in centimeters in vacuo.
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 is 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.
Series. A number of test results which possess common properties 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 following measures apply: VARIANCE.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. STANDARD DEVIATION.The square root of the variance. RELATIVE STANDARD DEVIATION.The standard deviation of a series of test results as a percentage of the mean of
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this series. This term is preferred over “coefficient of variation.’, RANGE. The difference in magnitude between the highest test result and the lowest test result in aseries. 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. RELATIVEERROR. The mean error of a series of test results as a percentage of the true result.
