LAAN-A-FT-E017
SHIMADZU APPLICATION NEWS
● SPECTROPHOTOMETRIC ANALYSIS
No.
A395
Points to Consider with Respect to ATR Spectra of
Inorganic Compounds
It is widely known that ATR spectra obtained using the
ATR measurement method have a different peak
intensity ratio than peaks collected using transmission
measurement. However, what is not so well known is
that a slight shift in peak position may also occur. This
shift is a phenomenon seen only with large peaks, and
in the case of organic substances, since the peak shift
is small, about 10 cm-1, it is not much of a problem.
However, in the case of inorganic compounds, very
large disparities may be seen. This Application News
introduces points to consider with respect to ATR
spectra of inorganic compounds.
■ Peak Shifts in ATR Spectra
In the ATR method, as the penetration depth is
proportional to the wavelength, it is a characteristic
that peak intensities on the high-wavenumber side
become smaller as compared with peaks in
transmission spectra. Moreover, the peak positions in
ATR spectra may be shifted slightly to the lowwavenumber side as compared with those in
transmission spectra. This shift is influenced by the
refractive index of the sample, and is especially seen
in large peaks. When measurement of organic
substances is conducted using a diamond or ZnSe
Abs
prism, large peaks may shift about 10 cm-1.
On the other hand, in the case of inorganic
compounds, very large peak shifts may occur. Figs.1
and 2 show spectra of silica gel (SiO2) and calcium
carbonate, respectively. As shown in Fig.1, the SiO2
peak at 1093 cm-1 in the transmission spectrum is
seen in the vicinity of 1062 cm-1 in the ATR spectrum.
And, in Fig.2, the peak at 1429 cm-1 in the
transmission spectrum is seen in the vicinity of
1390 cm-1 in the ATR spectrum.
1093.64
1429.25
Abs
950.91
804.32
877.61
711.73
1793.8
1390.68
1062.78
871.82
711.73
950.91
796.60
1793.80
1500
1400
1300
1200
1100
1000
900
800
700
1/cm
Fig.1 Spectra of Silica Gel (upper : transmission, lower : ATR)
2000
1800
1600
1400
1200
1000
Fig.2 Spectra of Calcium Carbonate
(upper : transmission, lower : ATR)
800
1/cm
No.A395
However, the amount of this peak shift is not constant.
Fig.3 shows an ATR spectrum of polystyrene
containing calcium carbonate. The peaks at 1417 cm-1
and 873 cm-1 are due to the calcium carbonate;
however, this shift is small when compared to the ATR
spectra of Fig.2.
Thus, in ATR spectra of inorganic substances, peak
shifts of 30 cm-1 or more may be seen in large, broad
peaks. However, it is important to note that this
amount of shift (peak position) will vary depending on
the state of the sample, such as whether it is a pure or
mixed substance.
1417.68
Abs
2000
873.75
1800
1600
1400
1200
1000
800
600
1/cm
Fig.3 ATR Spectra of Polystyrene with Calcium Carbonate
(upper) and Polystyrene (lower)
■ ATR Spectra of Glass
As mentioned above, in ATR spectra of inorganic
substances, a large shift in peaks may occur toward
the low wavenumbers as compared to peak positions
in transmission spectra, and the change in peak
position may vary depending on whether the sample is
pure or a mixture of substances. Here we introduce
some examples of peak shift caused by contact
status.
Fig.4 shows ATR spectra which were measured while
different levels of force were applied to the prism in the
measurement of a glass plate sample. As the applied
force increased, the peak intensity increased. For
comparison, Fig.5 shows the spectra of Fig.4 with
corrected peak intensity in the vicinity of 770 cm-1. The
difference in peak shape due to applied force is clearly
evident.
It is necessary to note that this difference may affect
the results when conducting glass identification, of
course, and when obtaining difference spectra in
analysis of adhering material on glass or in analysis of
thin films.
It is common for an inorganic substance to be included
in analysis of an unknown sample, such as in
contaminant analysis. When using the ATR method to
conduct analysis of an inorganic compound, or when
conducting analysis of an unknown sample that may
contain an inorganic component, please take note of
the points introduced here to make effective use of the
ATR method.
1
Abs
0.8
Abs
0.6
0.4
clamping force
0.2
0
1600
1500
1400
1300
1200
1100
1000
900
800
700
600
1/cm
1600
Fig.4 ATR Spectra of Glass
1500
1400
1300
1200
1100
1000
900
800
700
600
1/cm
Fig.5 ATR Spectra of Glass (Corrected at 770 cm-1)
NOTES:
✽This Application News has been produced and edited using information that was available when the
data was acquired for each article. This Application News is subject to revision without prior notice.
SHIMADZU CORPORATION. International Marketing Division
3. Kanda-Nishikicho 1-chome, Chiyoda-ku, Tokyo 101-8448, Japan Phone: 81(3)3219-5641 Fax. 81(3)3219-5710
Cable Add.:SHIMADZU TOKYO
Printed in Japan 3100-07703-10A-IK