ASTM E573-01(2021)
(Practice)Standard Practices for Internal Reflection Spectroscopy
Standard Practices for Internal Reflection Spectroscopy
SIGNIFICANCE AND USE
4.1 These practices provide general guidelines for the good practice of internal reflection infrared spectroscopy.
SCOPE
1.1 These practices provide general recommendations covering the various techniques commonly used in obtaining internal reflection spectra.2,3 Discussion is limited to the infrared region of the electromagnetic spectrum and includes a summary of fundamental theory, a description of parameters that determine the results obtained, instrumentation most widely used, practical guidelines for sampling and obtaining useful spectra, and interpretation features specific for internal reflection.
1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.3 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.
General Information
Relations
Standards Content (Sample)
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:E573 −01 (Reapproved 2021)
Standard Practices for
Internal Reflection Spectroscopy
This standard is issued under the fixed designation E573; 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 These practices provide general recommendations cov- 3.1 Definitions of Terms and Symbols—For definitions of
termsandsymbols,refertoTerminologiesE131andE284,and
ering the various techniques commonly used in obtaining
2,3
to Appendix X1.
internal reflection spectra. Discussion is limited to the
infrared region of the electromagnetic spectrum and includes a
4. Significance and Use
summary of fundamental theory, a description of parameters
that determine the results obtained, instrumentation most 4.1 These practices provide general guidelines for the good
widely used, practical guidelines for sampling and obtaining
practice of internal reflection infrared spectroscopy.
useful spectra, and interpretation features specific for internal
5. Theory
reflection.
5.1 In his studies of total reflection at the interface between
1.2 The values stated in SI units are to be regarded as
two media of different refractive indices, Newton (1) discov-
standard. No other units of measurement are included in this
ered that light extends into the rarer medium beyond the
standard.
reflecting surface (see Fig. 1). In internal reflection
1.3 This international standard was developed in accor-
spectroscopy, IRS, this phenomenon is applied to obtain
dance with internationally recognized principles on standard-
absorptionspectrabymeasuringtheinteractionofthepenetrat-
ization established in the Decision on Principles for the
ingradiationwithanexternalmedium,whichwillbecalledthe
Development of International Standards, Guides and Recom-
sample (2,3). Theoretical explanation for the interaction
mendations issued by the World Trade Organization Technical
mechanisms for both absorbing and nonabsorbing samples is
Barriers to Trade (TBT) Committee.
provided by Snell’s law, the Fresnel equations (4), and the
Maxwell relationships (5).
2. Referenced Documents
NOTE 1—To provide a basic understanding of internal reflection
2.1 ASTM Standards:
phenomena applied to spectroscopy, a brief description of the theory
E131Terminology Relating to Molecular Spectroscopy
appears in Appendix X2. For a detailed theoretical discussion of the
subject, see (4).
E168Practices for General Techniques of Infrared Quanti-
tative Analysis
6. Parameters of Reflectance Measurements
E284Terminology of Appearance
6.1 Practical application of IRS depends on many precisely
controlled variables. Since an understanding of these variables
isnecessaryforproperutilizationofthetechnique,descriptions
These practices are under the jurisdiction of ASTM Committee E13 on
of essential parameters are presented.
Molecular Spectroscopy and Separation Science and are the direct responsibility of
Subcommittee E13.03 on Infrared and Near Infrared Spectroscopy.
6.2 Angle of Incidence, θ—When θ is greater than the
Current edition approved April 1, 2021. Published April 2021. Originally
criticalangle, θ ,totalinternalreflectionoccursattheinterface
approved in 1976. Last previous edition approved in 2013 as E573–01 (2013). c
DOI: 10.1520/E0573-01R21. between the sample and the internal reflection element, IRE.
Internal Reflection Spectroscopy, IRS, is the accepted nomenclature for the
When θ is appreciably greater than θ , the reflection spectra
c
technique described in these practices. Other terms are sometimes used, which
most closely resemble transmission spectra. When θ is less
include: Attenuated Total Reflection, ATR; Frustrated Total Reflection, FTR;
than θ , radiation is both refracted and internally reflected,
Multiple Internal Reflection, MIR; and other less commonly used terms. In older
c
literature, one may find references to Frustrated Total Internal Reflection, FTIR.
generally leading to spectral distortions. θ should be selected
This should not be confused with Fourier Transform Infrared Spectroscopy FT-IR.
far enough away from the average critical angle of the
Other terms sometimes used for referring to the internal reflection element are:
sample—IRE combination that the change of θ through the
c
ATR crystal, MIR plate, or sample plate.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
the ASTM website. these practices.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E573−01 (2021)
6.4 Relative Refractive Index, n , of the Sample, n , and
21 2
IRE, n;(n =n /n )—Refractive index matching controls the
1 21 2 1
spectral contrast. If the indexes of the sample and the IRE
approacheachother,banddistortionscanoccur.Therefore,itis
necessary to select an IRE with a refractive index considerably
greater than the mean index of the sample.
6.4.1 The refractive index of a material undergoes abrupt
changes in the region of an absorption band. Fig. 3 (6) shows
thechangeinrefractiveindexofasampleacrossanabsorption
band as a function of wavelength. When an IRE of index n is
A
NOTE 1—The ray penetrates a fraction of a wavelength (d ) beyond the
p selected,theremaybeapointatwhichtheindexofthesample
reflecting surface into the rarer medium of refractive index n (the
is greater than that of the IRE.At this wavelength, there is no
sample), and there is a certain displacement (D) upon reflection. θ is the
θatwhichtotalinternalreflectioncantakeplace,andnearlyall
angleofincidenceoftherayinthedensermedium,ofrefractiveindex, n ,
of the energy passes into the sample. The absorption band
at the interface between the two media.
FIG. 1Schematic Representation of Path of a Ray of Light for resulting in this case will be broadened toward longer
Total Internal Reflection
wavelengths, and hence appear distorted. When an IRE of
index n is selected, there is no point at which the index of the
B
sample exceeds it. On the long wavelength side, however, the
region of changing index (which is related to the presence of
refractive indexes approach each other. This results in an
the absorption band of the sample) has a minimal effect on the
absorptionbandthatislessdistorted,butthatisstillbroadened
shapeoftheinternalreflectionband.Increasingθdecreasesthe
on the long wavelength side. With an IRE of index n,a
number of reflections, and reduces penetration. In practice,
C
considerably higher refractive index than that of the sample,
there is some angular spread in a focused beam. For instru-
the index variation of the sample causes no obvious distortion
ments that utilize f4.5 optics in the sample compartment, there
of the absorption band.
is a beam spread of 6 5°, but the beam spread in the IRE is
smaller because of its refractive index.The value will increase
6.5 Depth of Penetration, d —The distance into the rarer
p
as lower f-number optics are utilized. This beam spread
medium at which the amplitude of the penetrating radiation
−1
produces a corresponding distribution of effective paths and
falls to e of its value at the surface is a function of the
effective depth of penetrations.
wavelength of the radiation, the refractive indexes of both the
IREandthesample,andtheangleofincidenceoftheradiation
6.3 Number of Reflections, N—N is an important factor in
at the interface.
determining the sensitivity of the IRE. Where multiple reflec-
6.5.1 The depth of penetration, d , can be calculated as
tions are employed, internal reflection occurs a number of
p
follows:
times along the length of the IRE depending on its length, l,
thickness, t, and on the angle of incidence, θ, of the radiant
λ
d 5 (3)
p 2 2 ½
beam.
2 π sin θ 2 n
~ !
NOTE 2—The length of an IRE is defined as the distance between the
λ
where: λ 5 5wavelengthofradiationintheIRE.
centers of the entrance and exit apertures.
n
6.3.1 Absorption occurs with each reflection (see Fig. 2),
giving rise to an absorption spectrum, the intensity of which
depends on N. For single-pass IREs, N can be calculated using
the following relationship:
l
N 5 cotθ (1)
S D
t
For double-pass IREs:
l
N 5 2 cotθ (2)
S D
t
Many single-pass IREs employ approximately 25 reflec-
tions.
NOTE3—NmustbeanoddintegerforIREsintheshapeofatrapezoid,
and an even integer for IREs in the shape of a parallelogram.
Solid Line—Refractive index of sample.
Dotted Line—Absorption band of sample.
Dashed Lines—Refractive indices of reflector plates.
FIG. 2Multiple Internal Reflection Effect FIG. 3Refractive Index Versus Wavelength
E573−01 (2021)
The depth of penetration increases as the angle of incidence
decreases, and becomes infinitely large as θ approaches the
critical angle (see Figs. 4 and 5) (7).
6.6 Effective Path Length, d —The effective pathlength, or
e
relative effective thickness, d , for the beam for each reflection
e
is defined by Harrick (4) in detail, and is different for
'-polarized than for i-polarized radiation. For bulk materials,
when θ=45°, d = ⁄2 d , and the average effective thickness
e' ei
isaboutequaltothepenetrationdepth, d .Forlargerangles, d
p e
is smaller than d and for smaller angles, d is larger than d . FIG. 5Variation of Penetration Depth with Wavelength of
p e p
Radiation in Sample (7)
The total effective pathlength is equal to N times the effective
pathlength, d .An example of the effect of θ on N· d is shown
e e
in Fig. 6.
6.7 Absorption Coeffıcient, α—As in transmission
spectroscopy, the absorptivity of a material affects the fraction
oftheincidentradiationthatisabsorbed,andhencethespectral
contrast. The internal reflectance of bulk materials and thin
films, for small abosrptivities, is as follows:
R 5 1 2 α d (4)
e
The reflectance for N reflections is:
N N
R 5 1 2 αd (5)
~ !
e
NOTE 1—Total effective pathlength versus angle of incidence for
polystyrene stain on silicon surface. The sharp drop with angle of
incidence is largely, although not entirely, due to decrease of N with θ.
Points represent experimental measurements and solid curves are theo-
retical calculations (4).
FIG. 6Total Effective Pathlength Versus Angle of Incidence
N
6.7.1 If αd << 1, R ≈1− N · α· d , that is, the reflection
e e
lossisincreasedbyafactorofN.Therelationshipsbetweenthe
absorption coefficient, α, and the absorptivity, a, are given by
Eq X2.13 and Eq X2.14.
6.8 Sampling Area—Whenmultiplereflectionsareused,the
sampling area is somewhat analogous to the pathlength in
transmission spectroscopy. The amount of absorption by a
sampleincontactwithamultiple-reflectionIREisproportional
NOTE 1—Fractional penetration depth of electromagnetic field in rarer
totheareaofcontactwithinthesensitiveregion.Samplingarea
bulk medium for total internal reflection versus angle of incidence for a
is proportional to 1/cos θ and increases with increasing θ.
number of interfaces. The penetration depth is infinitely large at the
6.8.1 The sensitive region of an IRE sampling face varies,
critical angle and is about one tenth the wavelength at grazing incidence
depending on the IRS system in which it is used. A small
for relatively high index media. λ = λ⁄n is the wavelength in the denser
1 1
regionortheentireareaofthesamplingfacescanbesensitive,
medium.
FIG. 4Relative Penetration Depth Versus Angle of Incidence as seen for the dispersive systems shown in Fig. 7. It must be
E573−01 (2021)
reflection attachment for obtaining internal reflection spectra
(Note 4). The optical efficiency of internal reflection infrared
systems can be nearly equal to theoretical. However, in some
IRAs only half of the spectrophotometer energy may be
available. Schematic diagrams of two types of fixed-angle
FIG. 7Sensitive Sampling Areas of IRE Plates IRAs are presented in Fig. 8. For double-beam operation, it is
preferred that an IRAidentical to that used in the sample beam
be used in the reference beam in order to compensate for
emphasized that, in general, there is no relationship between surface scatter, atmospheric absorptions, or absorptions in the
thesizeofthesensitivesamplingareaandtheopticalefficiency
IRE. When using an IRAin a FT-IR spectrometer, a reference
oftheIRSsystem,providedthattheslitheightofthedispersive spectrum (or background) is usually recorded using the same
spectrophotometer is filled. In fact, it is preferred that an IRE
IRE with no sample in contact with the crystal. Very careful
have insensitive edges so that gasket materials or sample cleaning and sampling procedures (more than usual) are
holders do not cause spectral interference. It is important that
required here. Spectral verification of IRE cleanliness is
samples be positioned so that they lie completely across the essential. Internal reflection equipment includes the following:
width of the sensitive area. For accessories utilizing single- 7.1.1 The IRAs designed to be placed into the sampling
reflection prisms and hemicylinders, the entire sample face compartment of a spectrophotometer. These are of the follow-
shouldbecovered.Ifthisareaisnotcompletelycoveredbythe ing types: (a) variable-angle single internal reflection; (b)
sample, radiation bypasses the sample and the effect will be fixed-angle multiple internal reflection (θ usually set at 45°),
similar to a transmission cell with an air bubble in it. Knowing and (c) variable-angle multiple internal reflection (θ is either
the sensitive sampling area on an IRE is important when the continuously variable, usually between 30° and 60°, or a
sample is limited and it is desirable to place the sample on the choice of angles is preset by the manufacturer, usually at 30°,
IREinthemostefficientmanner (8).Thesensitiveregionofan 45°, and 60°. In order to have the θ that is specified on the
IRE sampling face may differ quite radically when used in an attachment,anIREforthatsameθmustbeused.)(d)platforms
interferometer. The focused image is nearly circular and may for supporting fixed-angle plates in a horizontal position, and
not fill the vertical dimension of the crystal but often will (e) IRAs for supporting prism IREs of various geometry.
overfillthewidthoftheIREface.Thisresultsinvignettingand
7.1.2 Goni
...
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