ASTM E573-96
(Practice)Standard Practices for Internal Reflection Spectroscopy
Standard Practices for Internal Reflection Spectroscopy
SCOPE
1.1 These practices provide general recommendations covering the various techniques commonly used in obtaining internal reflection spectra. 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.
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Designation: E 573 – 96
Standard Practices for
Internal Reflection Spectroscopy
This standard is issued under the fixed designation E 573; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope reflecting surface (see Fig. 1). In internal reflection spectros-
copy, IRS, this phenomenon is applied to obtain absorption
1.1 These practices provide general recommendations cov-
spectra by measuring the interaction of the penetrating radia-
ering the various techniques commonly used in obtaining
,
2 3
tion with an external medium, which will be called the sample
internal reflection spectra. Discussion is limited to the
(2,3). Theoretical explanation for the interaction mechanisms
infrared region of the electromagnetic spectrum and includes a
for both absorbing and nonabsorbing samples is provided by
summary of fundamental theory, a description of parameters
Snell’s law, the Fresnel equations (4), and the Maxwell
that determine the results obtained, instrumentation most
relationships (5).
widely used, practical guidelines for sampling and obtaining
useful spectra, and interpretation features specific for internal
NOTE 1—To provide a basic understanding of internal reflection phe-
reflection.
nomena applied to spectroscopy, a brief description of the theory appears
in Appendix X2. For a detailed theoretical discussion of the subject, see
2. Referenced Documents
(4).
2.1 ASTM Standards:
6. Parameters of Reflectance Measurements
E 131 Terminology Relating to Molecular Spectroscopy
6.1 Practical application of IRS depends on many precisely
E 168 Practices for General Techniques of Infrared Quanti-
controlled variables. Since an understanding of these variables
tative Analysis
5 is necessary for proper utilization of the technique, descriptions
E 284 Terminology of Appearance
of essential parameters are presented.
3. Terminology 6.2 Angle of Incidence, u—When u is greater than the
critical angle, u , total internal reflection occurs at the interface
3.1 Definitions of Terms and Symbols—For definitions of c
between the sample and the internal reflection element, IRE.
terms and symbols, refer to Terminologies E 131 and E 284,
When u is appreciably greater than u , the reflection spectra
and to Appendix X1. c
most closely resemble transmission spectra. When u is less
4. Significance and Use
than u , radiation is both refracted and internally reflected,
c
generally leading to spectral distortions. u should be selected
4.1 These practices provide general guidelines for the good
far enough away from the average critical angle of the
practice of internal reflection infrared spectroscopy.
sample—IRE combination that the change of u through the
c
5. Theory
region of changing index (which is related to the presence of
the absorption band of the sample) has a minimal effect on the
5.1 In his studies of total reflection at the interface between
shape of the internal reflection band. Increasing u decreases the
two media of different refractive indices, Newton (1) discov-
number of reflections, and reduces penetration. In practice,
ered that light extends into the rarer medium beyond the
there is some angular spread in a focused beam. For instru-
ments that utilize f4.5 optics in the sample compartment, there
These practices are under the jurisdiction of ASTM Committee E-13 on
is a beam spread of 6 5°, but the beam spread in the IRE is
Molecular Spectroscopy and are the direct responsibility of Subcommittee E13.03
on Infrared Spectroscopy. smaller because of its refractive index. The value will increase
Current edition approved April 10, 1996. Published June 1996. Originally
as lower f-number optics are utilized. This beam spread
published as E 573 – 76. Last previous edition E 573 – 90.
produces a corresponding distribution of effective paths and
Internal Reflection Spectroscopy, IRS, is the accepted nomenclature for the
effective depth of penetrations.
technique described in these practices. Other terms are sometimes used which
include: Attenuated Total Reflection, ATR; Frustrated Total Reflection, FTR;
6.3 Number of Reflections, N—N is an important factor in
Multiple Internal Reflection, MIR; and other less commonly used terms. In older
determining the sensitivity of the IRE. Where multiple reflec-
literature, one may find references to Frustrated Total Internal Reflection, FTIR.
tions are employed, internal reflection occurs a number of
This should not be confused with Fourier Transform Infrared Spectroscopy FT-IR.
times along the length of the IRE depending on its length, l,
Other terms sometimes used for referring to the internal reflection element are:
ATR crystal, MIR plate, or sample plate.
thickness, t, and on the angle of incidence, u, of the radiant
Annual Book of ASTM Standards, Vol 03.06.
beam.
Annual Book of ASTM Standards, Vol 06.01.
The boldface numbers in parentheses refer to the list of references at the end of
NOTE 2—The length of an IRE is defined as the distance between the
these practices.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
E 573
NOTE 1—The ray penetrates a fraction of a wavelength (d ) beyond the
p
reflecting surface into the rarer medium of refractive index n (the
sample), and there is a certain displacement (D) upon reflection. u is the
angle of incidence of the ray in the denser medium, of refractive index, n ,
at the interface between the two media.
FIG. 1 Schematic Representation of Path of a Ray of Light for
Total Internal Reflection
Solid Line—Refractive index of sample.
Dotted Line—Absorption band of sample.
Dashed Lines—Refractive indices of reflector plates.
centers of the entrance and exit apertures.
FIG. 3 Refractive Index Versus Wavelength
6.3.1 Absorption occurs with each reflection (see Fig. 2),
tive indexes approach each other. This results in an absorption
giving rise to an absorption spectrum, the intensity of which
band that is less distorted, but that is still broadened on the long
depends on N. For single-pass IREs, N can be calculated using
wavelength side. With an IRE of index n , a considerably
the following relationship: C
higher refractive index than that of the sample, the index
l
variation of the sample causes no obvious distortion of the
N 5 cot u (1)
S D
t
absorption band.
For double-pass IREs:
6.5 Depth of Penetration, d —The distance into the rarer
p
l medium at which the amplitude of the penetrating radiation
N 5 2 cot u (2)
−1
S D
t falls to e of its value at the surface is a function of the
wavelength of the radiation, the refractive indexes of both the
Many single-pass IREs employ approximately 25 reflec-
IRE and the sample, and the angle of incidence of the radiation
tions.
at the interface.
NOTE 3—N must be an odd integer for IREs in the shape of a trapezoid,
6.5.1 The depth of penetration, d , can be calculated as
p
and an even integer for IREs in the shape of a parallelogram.
follows:
6.4 Relative Refractive Index, n , of the Sample, n , and
21 2
l
IRE, n ;(n 5 n /n )—Refractive index matching controls d 5 (3)
1 21 2 1 p 2 2 ½
2 p ~sin u2 n !
the spectral contrast. If the indexes of the sample and the IRE
l
approach each other, band distortions can occur. Therefore, it is
where: l 5 5 wavelength of radiation in the IRE.
n
necessary to select an IRE with a refractive index considerably
The depth of penetration increases as the angle of incidence
greater than the mean index of the sample.
decreases, and becomes infinitely large as u approaches the
6.4.1 The refractive index of a material undergoes abrupt
critical angle (see Figs. 4 and 5) (7).
changes in the region of an absorption band. Fig. 3 (6) shows
6.6 Effective Path Length, d —The effective pathlength, or
e
the change in refractive index of a sample across an absorption
relative effective thickness, d , for the beam for each reflection
e
band as a function of wavelength. When an IRE of index n is
A
is defined by Harrick (4) in detail, and is different for
selected, there may be a point at which the index of the sample
’-polarized than for{-polarized radiation. For bulk materi-
is greater than that of the IRE. At this wavelength, there is no
als, when u5 45°, d 5 ⁄2 d , and the average effective
e’ e{
u at which total internal reflection can take place, and nearly all
thickness is about equal to the penetration depth, d . For larger
p
of the energy passes into the sample. The absorption band
angles, d is smaller than d and for smaller angles, d is larger
e p e
resulting in this case will be broadened toward longer wave-
than d . The total effective pathlength is equal to N times the
p
lengths, and hence appear distorted. When an IRE of index n
B
effective pathlength, d . An example of the effect of u on N· d
e e
is selected, there is no point at which the index of the sample
is shown in Fig. 6.
exceeds it. On the long wavelength side, however, the refrac-
6.7 Absorption Coeffıcient, a—As in transmission spectros-
copy, the absorptivity of a material affects the fraction of the
incident radiation that is absorbed, and hence the spectral
contrast. The internal reflectance of bulk materials and thin
films, for small abosrptivities, is as follows:
R 5 12a d (4)
e
FIG. 2 Multiple Internal Reflection Effect The reflectance for N reflections is:
E 573
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 u.
NOTE 1—Fractional penetration depth of electromagnetic field in rarer
Points represent experimental measurements and solid curves are theo-
bulk medium for total internal reflection versus angle of incidence for a
retical calculations (4).
number of interfaces. The penetration depth is infinitely large at the
critical angle and is about one tenth the wavelength at grazing incidence
FIG. 6 Total Effective Pathlength Versus Angle of Incidence
for relatively high index media. l 5l/ n is the wavelength in the denser
1 1
medium.
FIG. 4 Relative Penetration Depth Versus Angle of Incidence
depending on the IRS system in which it is used. A small
region or the entire area of the sampling faces can be sensitive,
as seen for the dispersive systems shown in Fig. 7. It must be
emphasized that, in general, there is no relationship between
the size of the sensitive sampling area and the optical efficiency
of the IRS system, provided that the slit height of the dispersive
spectrophotometer is filled. In fact, it is preferred that an IRE
have insensitive edges so that gasket materials or sample
holders do not cause spectral interference. It is important that
samples be positioned so that they lie completely across the
width of the sensitive area. For accessories utilizing single-
reflection prisms and hemicylinders, the entire sample face
FIG. 5 Variation of Penetration Depth with Wavelength of
Radiation in Sample (7)
should be covered. If this area is not completely covered by the
sample, radiation bypasses the sample and the effect will be
N N
similar to a transmission cell with an air bubble in it. Knowing
R 5 ~12ad ! (5)
e
N the sensitive sampling area on an IRE is important when the
6.7.1 If ad << 1, R ’ 1− N · a· d , that is, the reflection
e e
sample is limited and it is desirable to place the sample on the
loss is increased by a factor of N. The relationships between the
IRE in the most efficient manner (8). The sensitive region of an
absorption coefficient, a, and the absorptivity, a, are given by
IRE sampling face may differ quite radically when used in an
Eq X2.13 and Eq X2.14.
6.8 Sampling Area—When multiple reflections are used, the
sampling area is somewhat analogous to the pathlength in
transmission spectroscopy. The amount of absorption by a
sample in contact with a multiple-reflection IRE is proportional
to the area of contact within the sensitive region. Sampling area
is proportional to 1/cos u and increases with increasing u.
6.8.1 The sensitive region of an IRE sampling face varies, FIG. 7 Sensitive Sampling Areas of IRE Plates
E 573
interferometer. The focused image is nearly circular and may spectrometer, a reference spectrum (or background) is usually
not fill the vertical dimension of the crystal but often will recorded using the same IRE with no sample in contact with
overfill the width of the IRE face. This results in vignetting and the crystal. Very careful cleaning and sampling procedures
introduces small wavenumber errors in Fourier Transform (more than usual) are required here. Spectral verification of
spectroscopy. The problem of overfilling the entrance aperture IRE cleanliness is essential. Internal reflection equipment
can be minimized by utilizing beam condensing optics, but this includes the following:
will increase the angular spread of the incident rays.
7.1.1 The IRAs designed to be placed into the sampling
compartment of a spectrophotometer. These are of the follow-
NOTE 4—It is recommended that an IRE with a vertical dimension only
ing types: (a) variable-angle single internal reflection; ( b)
slightly larger than the focused beam diameter be used. This ensures that
fixed-angle multiple internal reflection (u usually set at 45°),
the sensitive area encompasses the full crystal face.
and (c) variable-angle multiple internal reflection (u is either
6.9 Effıciency of Contact—In order to obtain an internal
continuously variable, usually between 30 and 60°, or a choice
reflection spectrum, it is necessary to bring the sample to a
of angles is preset by the manufacturer, usually at 30, 45, and
distance within the penetration depth, d . Physical contact of
p
60°. In order to have the u that is specified on the attachment,
the sample with the IRE may be sufficient to obtain a
an IRE for that same u must be used.) ( d) platforms for
qualitative spectrum. However, if the exact contact conditions
supporting fixed-angle plates in a horizontal position, and (e)
are not reproduced, a source of error may result, particularly
IRAs for supporting prism IREs of various geometry.
when interpretation requires a direct comparison with similarly
7.1.2 Goniometers—Goniometers are essential for absolute
obtained spectra, or when quantitative measurements are made.
intensity measurements.
6.10 Electric Field Strength—Spectral contrast is affected
7.1.3 Horizontal ATR Attachments—This family of acces-
by the strength of the electric field, that is, the amplitude of
...
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