Spectrometry
rnenclatur
ANALYTICAL CHEMISTRY has compiled the following list of terms frequently used in papers on spectrometry. The list includes CHEMISTRY and endorsed by its advisory board. definitions and abbreviations, and represents usage preferred by ANALYTICAL Sources used in the compilation are: Chemical Absfracfs, H. K. Hughes, et a/. [ANAL.CHEM.24, 1 3 4 9 ( 1 952)], and the ASPM Committee E-1 3 on Absorption Spectroscopy (E 131-66 T). Absorbance, A . (Not optical density, absorbancy, or extinction.) Logarithm t o the base 10 of the reciprocal of the transmittance A = log,, (1/7’). Absorptivity, a. (Not k ) . (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, C. (Not molar absorbancy index, molar extinction coefficient, or molar absorption coefficient.) Product of the absorptivity, a, and the molecular Reight of the substance. Angstrom, A. Unit of length equal t o 1/6438.4696 of wavelength of red line of Cd. For practical purposes, cm. it is considered equal t o Beer’s Law. (Representing Beer-Lambert law.) Absorptivity of a substance is a constant with respect t o changes in concentration. Concentration, c . Quantity of the substance contained in a unit quantity of sample. (In absorption speatrometry it is usually expressed in grams or moles per liter.) Frequency.
Number of cycles per unit time.
Infrared. The region of the electromagnetic spectrum extending from approximately 0.78 to 300 microns. Micron,
p.
Unit of length equal to 10-6 meter.
Millimicron, mp. Unit of length equal to one thousandth of a micron. Almost b u t not exactly equal to 10 angstroms. Sample Path Length, b. (Not 1 or d). Internal cell or sample length, usually given in centimeters.
uide for Measures
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 treating the measurement of spectra. Spectrophotometer. Spectrometer with associated equipment, so t h a t it 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. Transmittance, T . (Yot transmittancy or transmission.) The ratio of the radiant power transmitted by a sample t o the radiant power incident on the sample. Ultraviolet. The region of the electromagnetic spectrum from approximately 10 to 380 mp. The term without further qualification usually refers to the region from 200 t o 380 mp. Visible. Pertaining t o radiant energy in the electromagnetic spectral range visible to the human eye (approximately 380 to 780 mp). Wavelength. (One word.) The distance, measured along the line of propagation, between two points t h a t are in phase on adjacent waves-units k.,mp, and p . Wavenumber. (One word.) Number of waves per unit length. The usual unit of wavenumber is the reciprocal centimeter, om.-’ I n terms of this unit, the wavenumber is the reciprocal of the wavelength when the latter is in centimeters in vacuo.
cision
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 by ANALYTICAL CHEMISTRY-after careful study by 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-i.e., 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. STASDARD DEVIATION.The square root of the variance. RELATIVESTANDARD DEVIATION.The standard deviation of B series of test results as a percentage of the mean of
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 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 re sults as a percentage of the true result.
VBL. 40, NO. 14, DECEMBER 1968
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