information paper

Spectroscopic Nomenclature*

Leopold May

Department of Chemistry, The Catholic University of America, Washington, D.C. 20017 Basic uni t s of measu remen t s involving spectroscopic te rms are presented. Uni t s used in spec- t roscopy are redefined. Terms commonly in use and the uni t s approved by the In te rna t iona l Union of Pure and Applied Chemis t ry ( IUPAC) are included.

INDEX HEADING: Spectroscopic nomencla ture .

With the advent of the International System of Units or SI (from the French, Le Systgme International d'Unites), changes have been made in the basic units of measurements, some of which involve spectroscopic terms commonly used. The basic units of the system are given in Table I.

A set of prefixes (Table II) has been designated for decimal multiples of SI units. An exception is the use of kilogram for mass. The gram (g) is defined as 10 -3

kg and is a submultiple of the SI unit, kg. These new definitions necessitate changes in the

units used in spectroscopy. Some of the terms are re- defined below. Also included are terms currently in use and the units approved by the International Union of Pure and Applied Chemistry (IUPAC). Absorbance, A Logarithm to the base 10 of the

Absorptivity, a

Absorptivity, molar, e

reciprocal of the transmittance (T)

A = log10 ( l / T )

(not used: absorbancy, extinc- tion, or optical density). Absorbance divided by the prod- uct of the sample path length (b) and the concentration of the absorbing substance (c).

a = A/bc

IUPAC-Specific absorption coef- ficient (not used: absorbancy index, extinction coefficient, or specific extinction). Product of the absorptivity (a) and the molecular weight of the absorbing substance. IUPAC-Molar absorption coeffi- cient (not used: molar absorb- ancy index or molar extinction coefficient).

* Edi tors Co m m en t s : The SI uni t s discussed in this paper will be the guideline for fu ture papers submi t t ed to Applied Spectroscopy. Authors should adhere to these uni t s in sub- m i t t i n g papers af ter 1 J a n u a r y 1974.

Angstrom,

Beer's law

Unit of length = 10 -1° m and 0.1 nm. Not a SI unit, should be replaced by nm. Absorptivity of a substance is a constant with respect to changes in concentration. Also known as Beer-Lambert law.

TABLE I. SI base units .

Quantity Name Symbol

L e n g t h mete r m Mass k i logram kg Time second s Electr ic cur ren t ampere A T h e r m o d y n a m i c t empera tu re kelvin K Luminous in t ens i ty candela cd A m o u n t of subs tance mole mol

TABLE II . SI Prefixes.

Factor Prefix Symbol Factor Prefix Symbol

1012 tera T 109 giga G 106 mega M 103 kilo k 10 2 hecto h 101 deca da 10 -1 deci d

10 -2 centi c 10 -3 milli m 10 -6 micro 10 -9 nano n 10 -52 pico p 10 -15 femo f 10 -18 atto a

Concentration, c

Frequency, u Infrared region Micrometer, ~m

Nanometer, nm

Sample path length, b

Transmittance, T

Quant i ty of the substance con- gained in a unit quant i ty of sample. SI units: kg /m ~, mol/mL Equivalent units: g/1 or mg/cm ~. cycles/s, SI units is hertz, Hz. 0.78 to 300 ~m. Unit of length = 10 -G m (not used: microns). Unit of length = 10 -9 m (not used: millimicron). Internal cell or sample length, cm (not used: 1 or d). Ratio of the radiant power trans- mitted by a sample to the radi-

Volume 27, N u m b e r 5, 1973 APPLIED SPECTROSCOPY 419

Ultraviolet region

Visible region

Wavelength , X

an t power incident on the sample (not used: t r ansmi t t aney or transmission). 10 to 380 nm. Usual ly refers to region f rom 200 to 380 nm. Spectral range visible to h u m a n eye. 380 to 780 nm. Dis tance measured along line of propagat ion, between two points t h a t are in phase on adjacent waw~s: um and nm.

Wavenumber , ~ N u m b e r of waves per uni t length = 1/X. Usual ly em -1.

1. C. H. Page and P. Vigoureux, Eds., The International Sys- tem of Units (SI), Nat. Bur. Stand. Spec. Publ. 330, 1971.

2. H. K. Hughes et al., Anal. Chem. 24, 1349 (1952). 3. ASTM Committee E-13 on Molecular Spectroscopy, Tenta-

tive Definitions, 1966. 4. M. L. McGlashan, Manual of Symbols and Terminology for

Physicochemical Quantities and Units, Pure Appl. Chem. 21, 1, (1970).

420 Volume 27, Number 6, 1973