ASTM E131-10(2023)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: E131 − 10 (Reapproved 2023)
Standard Terminology Relating to
1, 2
Molecular Spectroscopy
This standard is issued under the fixed designation E131; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 3. Terminology
1.1 The values stated in SI units are to be regarded as absorbance, A—the logarithm to the base 10 of the reciprocal
standard. No other units of measurement are included in this
of the transmittance, (T).
standard.
A 5 log ~1/T!52log T (1)
10 10
DISCUSSION—In practice the observed transmittance must be substi-
1.2 This international standard was developed in accor-
tuted for T.Absorbance expresses the excess absorption over that of a
dance with internationally recognized principles on standard-
specified reference or standard. It is implied that compensation has
ization established in the Decision on Principles for the
been effected for reflectance losses, solvent absorption losses, and
Development of International Standards, Guides and Recom-
refractive effects, if present, and that attenuation by scattering is small
mendations issued by the World Trade Organization Technical
comparedwithattenuationbyabsorption.Apparentdeviationsfromthe
Barriers to Trade (TBT) Committee. absorption laws (see absorptivity) are due to inability to measure
exactly the true transmittance or to know the exact concentration of an
absorbing substance.
2. Referenced Documents
absorption band—a region of the absorption spectrum in
2.1 ASTM Standards:
which the absorbance passes through a maximum.
E135Terminology Relating to Analytical Chemistry for
Metals, Ores, and Related Materials
absorption coefficient, α—a measure of absorption of radiant
E168Practices for General Techniques of Infrared Quanti-
energy from an incident beam as it traverses an absorbing
−αb
tative Analysis
medium according to Bouguer’s law, P/P = e .
o
E204Practices for Identification of Material by Infrared
DISCUSSION—In IRS, α is a measure of the rate of absorption of
Absorption Spectroscopy, Using the ASTM Coded Band
energy from the evanescent wave.
and Chemical Classification Index (Withdrawn 2014)
absorption parameter, a—the relative reflection loss per
E284Terminology of Appearance
reflection that results from the absorption of radiant energy
E386Practice for Data Presentation Relating to High-
at a reflecting surface: a=1− R, and R=the reflected
Resolution Nuclear Magnetic Resonance (NMR) Spec-
fraction of incident radiant power.
troscopy (Withdrawn 2015)
E456Terminology Relating to Quality and Statistics
absorption spectrum—a plot, or other representation, of
absorbance, or any function of absorbance, against
2.2 Other Document:
wavelength, or any function of wavelength.
ISOGuide30–1981(E)Terms and definitions used in con-
nections with reference materials
absorptivity, a—the absorbance divided by the product of the
concentration of the substance and the sample pathlength,
a = A ⁄bc. The units of b and c shall be specified.
This terminology is under the jurisdiction of ASTM Committee E13 on DISCUSSION—1—The recommended unit for b is the centimetre. The
Molecular Spectroscopy and Separation Science and is the direct responsibility of
recommended unit for c is kilogram per cubic metre. Equivalent units
3 3
Subcommittee E13.94 on Terminology.
are g/dm , g/L, or mg/cm .
Current edition approved Jan. 1, 2023. Published January 2023. Originally
approved in 1957. Last previous edition approved in 2015 as E131– 10 (2015).
DISCUSSION—2—The equivalent IUPAC term is “specific absorption
DOI: 10.1520/E0131-10R23.
coefficient.”
Forotherdefinitionsrelatingtonuclearmagneticresonance,seePracticeE386.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
absorptivity, molar, ε—the product of the absorptivity, a, and
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
the molecular weight of the substance.
Standards volume information, refer to the standard’s Document Summary page on
DISCUSSION—The equivalent IUPAC term is “molar absorption coef-
the ASTM website.
4 ficient.”
The last approved version of this historical standard is referenced on
www.astm.org.
acceptance angle, n—for an optical fiber,themaximumangle,
Available fromAmerican National Standards Institute (ANSI), 25 W. 43rd St.,
4th Floor, New York, NY 10036, http://www.ansi.org. measuredfromthelongitudinalaxisorcenterlineofthefiber
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E131 − 10 (2023)
to an incident ray, within which the ray will be accepted for aperture of an IRE, A'—that portion of the IRE surface that
transmission along the fiber by total internal reflection. can be utilized to conduct light into the IRE at the desired
DISCUSSION—If the incidence angle exceeds the acceptance angle,
angle of incidence.
optical power in the incident ray will be coupled into leaky modes or
rays, or lost by scattering, diffusion, or absorption in the cladding. For apodization—modification of the ILS function by multiplying
a cladded step-index fiber in the air, the sine of the acceptance angle is
the interferogram by a weighting function the magnitude of
givenbythesquarerootofthedifferenceofthesquaresoftherefractive
which varies with retardation.
indexes of the fiber core and the cladding, that is, by the relation as
DISCUSSION—This term should strictly be used with reference to a
follows:
weighting function whose magnitude is greatest at the centerburst and
decreases with retardation.
2 2
sin A 5 = n 2 n (2)
1 2
where A is the acceptance angle and n and n are the refractive in-
1 2
attenuated total reflection (ATR)—reflection that occurs
dexes of the core and cladding, respectively. If the refractive index is a
when an absorbing coupling mechanism acts in the process
function of distance from the center of the core, as in the case of
of total internal reflection to make the reflectance less than
graded index fibers, then the acceptance angle depends on the distance
unity.
from the core center. The acceptance angle is maximum at the center,
and zero at the core-cladding boundary. At any radius, r, the sine of the DISCUSSION—In this process, if an absorbing sample is placed in
acceptance angle of a graded index fiber is defined in compliance with contact with the reflecting surface, the reflectance for total internal
that of a step-index fiber as follows: reflection will be attenuated to some value between zero and unity (O
2 2
sin A 5 =n 2 n (3)
r 1 2 power can take place.
where A is the acceptance angle at a point on the entrance face at a
r
distance, r, from the center, n is the refractive index of the core at a attenuation index, κ—a measure of the absorption of radiant
r
radius, r, and n is the refractive index of the cladding. In air, sin A
energy by an absorbing material. κ is related to the absorp-
and sin A are the numerical apertures. Unless otherwise stated, accep-
r
tion coefficient by: nκ= αc /4πν, where c =the speed of
o o
tance angles and numerical apertures for fiber optics are those for the
light in vacuo, ν=the frequency of radiant energy, and
center of the endface of the fiber, that is, where the refractive index,
n=the refractive index of the absorbing medium.
and hence the numerical aperture, is the highest.
background—apparent absorption caused by anything other
accuracy—the closeness of agreement between an observed
than the substance for which the analysis is being made.
value and an accepted reference value (see Terminology
E456).
baseline—any line drawn on an absorption spectrum to estab-
DISCUSSION—The term accuracy, when applied to a set of observed
lish a reference point representing a function of the radiant
values, will be a combination of a random component and a common
power incident on a sample at a given wavelength.
systematic error or bias component. Since in routine use, random
components and bias components cannot be completely separated, the
basic NMR frequency, ν —the frequency, measured in hertz
reported “accuracy” must be interpreted as a combination of these two
(Hz), of the oscillating magnetic field applied to induce
components.
transitions between nuclear magnetic energy levels.
active fiber optic chemical sensor, n—a fiber optic chemical
sensor in which a transduction mechanism other than the
bathochromic shift, n—change of a spectral band to longer
intrinsic spectroscopic properties of the analyte is used to wavelength (lower frequency) because of structural modifi-
modulate the optical signal.
cations or environmental influence; also known as “red
DISCUSSION—Examples include a pH sensor composed of a chemical
shift.”
indicator substance whose color changes with pH, and an oxygen
sensor coupled to an optical fiber bearing a chemical indicator whose beamsplitter—a semireflecting device used to create, and
fluorescence intensity depends on oxygen concentration.
often to recombine, spatially separate beams.
DISCUSSION—Beamsplitters are often made by depositing a film of a
aliasing—the appearance of features at wavenumbers other
high refractive index material onto a flat transmitting substrate with an
than their true value caused by using a sampling frequency
identical compensator plate being held on the other side of the film.
less than twice the highest modulation frequency in the
interferogram; also known as “folding.”
beamsplitter efficiency—the product 4RT, where R is the
reflectance and T is the transmittance of the beamsplitter.
analytical curve—the graphical representation of a relation
between some function of radiant power and the concentra-
Beer’s law—the absorbance of a homogeneous sample con-
tion or mass of the substance emitting or absorbing it.
taininganabsorbingsubstanceisdirectlyproportionaltothe
concentration of the absorbing substance (see also absorp-
analytical wavelength—any wavelength at which an absor-
tivity)
bance measurement is made for the purpose of the determi-
nation of a constituent of a sample.
bias—a systematic error that contributes to the difference
angle of incidence, θ—the angle between an incident radiant
between a population mean of the measurements or test
beam and a perpendicular to the interface between two
results and an accepted or reference value (see Terminology
media.
E456).
DISCUSSION—Bias is determined by the following equation:
anti-Stokes line (band)—a Raman line (band) that has a
frequency higher than that of the incident monochromatic
n
bias 5 e¯ 5 e (4)
i
(i51
beam. n
E131 − 10 (2023)
where: with a core of higher refractive index material used to
achieve total internal reflection.
n = the number of observations for which the accuracy is
DISCUSSION—The cladding confines electromagnetic waves to the
determined,
core, provides some protection to the core, and also transmits evanes-
e = the difference between a measured value of a property
i
cent waves that usually are bound to waves in the core.
and its accepted reference value, and
e¯ = the mean value of all the e.
concentration, c—the quantity of the substance contained in a
I
unit quantity of sample.
Bouguer’s law—the absorbance of a homogeneous sample is
DISCUSSION—For solution work, the recommended unit of concen-
directly proportional to the thickness of the sample in the
tration is grams of solute per litre of solution.
optical path.
core, n—of an optical fiber, the center region of an optical
DISCUSSION—Bouguer’s law is sometimes also known as Lambert’s
waveguide through which radiant energy is transmitted.
law.
DISCUSSION—In a dielectric waveguide such as an optical fiber, the
boxcar truncation—identical effective weighting of all points
refractive index of the core must be higher than that of the cladding.
inthemeasuredinterferogrampriortotheFouriertransform; Most of the radiant energy is confined to the core.
allpointsoutsideoftherangeofthemeasuredinterferogram
correlation coefficient (r)—a measure of the strength of the
take a value of zero.
linear relationship between X and Y, calculated by the
equation:
buffer—in fiber optics, see fiber optic buffer.
n
X Y
~ !
(i51 i i
bulk reflection—reflection in which radiant energy is returned
r 5 (7)
xy n 1/2 n 1/2
2 2
exclusively from within the specimen. X Y
~ ! ~ !
(i51 i (i51 i
DISCUSSION—Bulk reflection may be diffuse or specular.
where:
centerburst—the region of greatest amplitude in an interfero-
n = the number of observations in X and Y.
gram.
DISCUSSION—X and Y areanytwomeancorrectedvariables.Forthe
i i
DISCUSSION—For unchirped or only slightly chirped interferograms,
simple linear regression only,
this region includes the “zero path difference point” and the “zero
2 1/2
retardation point.”
r 5 R 5 signof b R (8)
~ !~ !
xy 1
certified reference material, n—a reference material, the
where:
composition or properties of which are certified by a
R = the coefficient of multiple determination.
recognized standardizing agency or group.
critical angle, θ —theanglewhosesineisequaltotherelative
DISCUSSION—Acertified reference material produced by the National c
refractive index for light striking an interface from the
Institute of Standards and Technology (NIST) is designated a Standard
−1
Reference Material (SRM).
greater to the lesser refractive medium: θ =sin n , where
c 21
n =the ratio of the refractive indices of the two media.
chemical shift (NMR), δ—the defining equation for δ is the
DISCUSSION—Total reflection occurs when light is reflected in the
following:
more refractive of two media from the interface between them at any
angle of incidence exceeding the critical angle.
∆ν
δ 5 310 (5)
ν
R
depth of penetration, d —in internal reflection spectroscopy,
p
the distance into the less refractive medium at which the
where ν is the frequency with which the reference sub-
R
−1
amplitude of the evanescent wave is e (that is, 36.8%) of
stance is in resonance at the magnetic field used in the ex-
its value at the surface:
periment and ∆ν is the frequency difference between the ref-
erence substance and the substance whose chemical shift is
λ
d 5 (9)
p 2 2 1/2
being determined, at constant field. The sign of ∆ν is to be 2π sin θ 2 η
~ !
chosen such that shifts to the high frequency side of the ref-
where: n =n /n = refractive index of sample divided by
21 2 1
erence shall be positive.
that of the IRE; λ = λ⁄n =wavelength of radiant energy in
1 1
DISCUSSION—If the experiment is done at constant frequency (field
the sample; and θ=angle of incidence.
sweep) the defining equation becomes
derivative absorption spectrum—a plot of rate of change of
∆ν ∆ν
δ 5 3 1 2 310 (6)
S D
absorbance or of any function of absorbance with respect to
ν ν
R R
wavelength or any function of wavelength, against wave-
chirping—the process of dispersing the zero phase difference
length or any function of wavelength.
points for different wavelengths across the interferogram, so
difference absorption spectrum—a plot of the difference
thatthemagnitudeofthesignalisreducedintheshortregion
between two absorbances or between any function of two
of the interferogram where all wavelengths would otherwise
absorbances, against wavelength or any function of wave-
constructively interfere.
length.
clad—see cladding.
diffuse reflection—reflection in which the flux is scattered in
cladding, n—of an optical fiber, a layer of a optically trans- many directions by diffusion at or below the surface (see
parent lower refractive index material in intimate contact Terminology E284).
E131 − 10 (2023)
digitization—the conversion of an analog signal to digital fiber optic sensor, n—a device that responds to an external
values using an analog-to-digital converter “sampling” or stimulus and transmits through an optical fiber a modulated
“digital sampling.” optical signal, indicating one or more characteristics of the
stimulus.
digitization noise—the noise generated in an interferogram
DISCUSSION—Examples include sensors which provide a suitable
throughtheuseofananalog-to-digitalconverterwhoseleast
signal or impulse to a meter. Such sensors might be found as the active
significant bit represents a value comparable to, or greater
elements in pH meters, strain gages, or pressure gages.
than, the peak-to-peak noise level in the analog data.
fiber optics, n—the branch of science and technology devoted
dilution factor—the ratio of the volume of a diluted solution
to the transmission of radiant energy through fibers made of
to the volume of original solution containing the same
transparent materials.
quantity of solute as the diluted solution.
DISCUSSION—Transparent materials include glass, fused silica, and
plastic. Optical fibers in fiber optic cables may be used for data
double modulation, n—a technique in which a modulated
transmission, and for sensing, illumination, endoscopic, control, and
signal is further varied by a second means.
display purposes, depending on their use in various geometric
DISCUSSION—As an example, a spectrometer could generate a modu- configurations,modesofexcitation,andenvironmentalconditions.The
lated signal while at the same time that signal is further varied by an fibers may be wound and bound in various shapes and distributions
external higher frequency modulator; on detection, the conventional singly or in bundles. Bundles may be aligned or unaligned. Aligned
bundles are often used to transmit and display images.
spectrometric signal is filtered out so that only the high frequency
signal is recorded.
filter—a substance that attenuates the radiant power reaching
double-pass internal reflection element—an internal reflec-
the detector in a definite manner with respect to spectral
tion element in which the radiant power transverses the
distribution.
length of the optical element twice, entering and leaving via
filter, neutral—a filter that attenuates the radiant power
the same end.
reaching the detector by the same factor at all wavelengths
effective pathlength (or effective thickness), d —in internal
within a prescribed wavelength region.
e
reflection spectroscopy,theanalogofthesamplethicknessin
fixed-angle internal reflection element—an internal reflec-
transmission spectroscopy that represents the distance of
tion element which is designed to be operated at a fixed
propagation of the evanescent wave within an absorbing
angle of incidence.
sample in IRS. It is defined from the relationship: R=1 −
αd , and is related to the absorption parameter by: a= α d .
e e
fluorescence—the emission of radiant energy from an atom,
molecule, or ion resulting from absorption of a photon and a
evanescent wave—the standing wave that exists in the less
subsequenttransitiontothegroundstatewithoutachangein
refractive medium, normal to the reflecting surface of the
total spin quantum number.
IRE during internal reflection.
DISCUSSION—The initial and final states of the transition are usually
extrinsic fiber optic chemical sensor, n—a fiber optic chemi- both singlet states. The average time interval between absorption and
−6
fluorescence is usually less than 10 s.
cal sensor in which modulation of the optical signal is not
effected through a change in the properties of the fiber itself.
folding—see aliasing.
DISCUSSION—Examples include a pH sensor composed of a chemical
Fourier transform (FT)—the mathematical process used to
indicatorimmobilizedattheendoftheopticalfiber,andasensorbased
convert an amplitude-time spectrum to an amplitude-
on Raman, fluorescence, infrared, visible, or other spectral information
gathered in the acceptance cone of the fiber. frequency spectrum, or vice versa.
DISCUSSION—In FT-IR spectrometry, retardation is directly propor-
far-infrared—pertaining to the infrared region of the electro-
tional to time; therefore the FT is commonly used to convert an
magnetic spectrum with wavelength range from approxi-
amplitude-retardation spectrum to an amplitude-wavenumber
-1
mately 25µm to 1000 µm (wavenumber range 400cm to
spectrum, and vice versa.
-1
10 cm ).
Fourier transform infrared (FT-IR) spectrometry—a form
fast Fourier transform (FFT)—a method for speeding up the of infrared spectrometry in which an interferogram is ob-
computation of a discrete FT by factoring the data into tained; this interferogram is then subjected to a Fourier
sparse matrices containing mostly zeroes. transform to obtain an amplitude-wavenumber (or wave-
length) spectrum.
fiber optic buffer, n—material placed on or around a cladded
DISCUSSION—1—The abbreviation FTIR is not recommended.
optical fiber to protect it from mechanical damage.
DISCUSSION—2—When FT-IR spectrometers are interfaced with
DISCUSSION—Mechanical damage can be caused by such things as
other instruments, a slash should be used to denote the interface; for
microbends and macrobends formed during manufacture, spooling,
example, GC/FT-IR; HPLC/FT-IR, and the use of FT-IR should be
subsequent handling, and pressure applied during use. Buffers may be
explicit; that is, FT-IR not IR.
bonded to the cladding and may also serve the purpose of preventing
ambient energy from entering the core.
frequency, ν—the number of cycles per unit time.
fiber optic chemical sensor, n—a fiber optic sensor that
DISCUSSION—The recommended unit is the hertz (Hz) (one cycle per
responds to a chemical stimulus. second).
E131 − 10 (2023)
frustrated total reflection (FTR)—the reflection which oc- intercorrelation coefficient, (r ) —a measure of the linear
XX
curs when a nonabsorbing coupling mechanism acts in the association between values of the same type of variable
process of total internal reflection to make the reflectance expressed as a correlation coefficient, (r).
DISCUSSION—The variables X and Y are replaced by X and X in the
less than unity.
j k
equation for the correlation coefficient, r.
DISCUSSION—In the process the reflectance can vary continuously
between zero and unity if: (1) An optically transparent medium is
interferogram, I (δ)—record of the modulated component of
within a fraction of a wavelength of the reflecting surface and its
the interference signal measured as a function of retardation
distancefromthereflectingsurfaceischanged,or(2)Boththeangleof
by the detector.
incidence and the refractive index of one of the media vary in an
appropriate manner. DISCUSSION—1—An alternate symbol is I(x).
In these cases part of the radiant power may be transmitted through
DISCUSSION—2—The recommended symbol for the spectrum com-
the interface into the second medium without loss at the reflecting
puted from I(δ)is B(ν). An alternate symbol is B(σ).
surface such that transmittance plus reflectance equals unity. It is
possible, therefore to have this process taking place in some spectral
interferogram, double-sided—interferogram measured with
regions even when a sample having absorption bands is placed in
approximately equal retardation on either side of the center-
contact with the reflecting surface.
burst.
high-resolution NMR spectrometer—anNMRapparatusthat
interferogram, laser reference—sinusoidal interferogram of
is capable of producing, for a given isotope, line widths that
a laser source measured at the same time as the signal
arelessthanthemajorityofthechemicalshiftsandcoupling
interferogram.
constants for that isotope.
DISCUSSION—The zero crossings of this interferogram are used to
DISCUSSION—By this definition, a given spectrometer may be classed
control sampling of the signal interferogram. It may also be noted that
as a high-resolution instrument for isotopes with large chemical shifts,
other effectively monochromatic sources can be used in place of the
butmaynotbeclassedasahigh-resolutioninstrumentforisotopeswith
laser.
smaller chemical shifts.
interferogram, signal—interferogram of the beam of radiant
hole-burning, n—in luminescence, the photo-induced disap-
energy whose spectrum is desired.
pearanceofanarrowsegmentwithinabroaderabsorptionor
emission band.
interferogram, single-sided—interferogram in which sam-
DISCUSSION—Holes are produced by the disappearance of resonantly
pling is initiated close to the centerburst and continues
excited molecules because of photochemical or photophysical pro-
through that point to the maximum retardation desired.
cesses.
interferogram, white light—reference interferogram of a
infrared—pertaining to the region of the electromagnetic
broadband light source measured at the same time as the
spectrum with wavelength range from approximately
-1 signal interferogram and used to initiate data acquisition of
0.78µm to 1000 µm (wavenumber range 12800cm to 10
consecutive scans for signal-averaging.
-1
cm ).
interferometer—device used to divide a beam of radiant
infrared spectroscopy—pertaining to spectroscopy in the
energy into two or more paths, generate an optical path
infrared region of the electromagnetic spectrum.
difference between the beams, and recombine them in order
DISCUSSION—1—Spectroscopy and other related terms are defined in
toproducerepetitiveinterferencemaximaandminimaasthe
Terminology E135.
optical retardation is varied.
DISCUSSION—2—Common applications of infrared spectroscopy are
the identification of materials and the quantitative analysis of materials interferometer, Genzel—interferometer in which the beam is
(see, for example, Practices E204 and Practices E168).
focused in the plane of the beamsplitter and collimated
before the moving mirror(s).
instrument line shape (ILS) function—theFTofthefunction
by which an interferogram is weighted.
interferometer, lamellar grating—interferometer in which
DISCUSSION—This weighting may be performed optically, due to the
the beam is reflected from two interleaved mirrors, one of
finite optical throughput, or digitally, through multiplication by an
which is stationary while the other is movable.
apodization function, or both. The ILS function is the profile of the
DISCUSSION—This type of interferometer is generally used only for
spectrum of a monochromatic source producing a beam with the same
far infrared spectrometry.
throughput as the beam in the actual measurement being performed.
interferometer, Michelson—interferometer in which an ap-
instrument response time—the time required for an indicat-
proximately collimated beam of radiant energy is divided
ing or detecting device to undergo a defined displacement
into two paths by a beamsplitter; one beam is reflected from
following an abrupt change in the quantity being measured.
amovablemirrorandtheotherfromastationarymirror,and
integration period, π—the time, in seconds, required for the they are then recombined at the beamsplitter.
penorotherindicatortomove98.6%ofitsmaximumtravel
interferometer, rapid-scanning—interferometer in which the
in response to a step function.
retardation is varied rapidly enough that the modulation
DISCUSSION—For instruments with a first-order response, the integra-
frequenciesintheinterferogramaresufficientlyhighthatthe
tion period will be approximately equal to four times the exponential
interferogram signal can be amplified directly without addi-
time constant. It is equal to the period, classically defined, for a second
order, critically damped response system. tional modulation by an external chopper.
E131 − 10 (2023)
interferometer, refractively scanned—interferometer in isoabsorptive point—awavelengthatwhichtheabsorptivities
whichtheretardationbetweentwobeamsisgeneratedbythe of two or more substances are equal.
movement of a wedged optical element.
isosbestic point—thewavelengthatwhichtheabsorptivitiesof
two substances, one of which can be converted into the
interferometer, slow-scanning—interferometer in w
...