Attenuated total reflectance powder cell for infrared analysis of hygroscopic samples

Article abstract of DOI:10.1016/j.saa.2011.07.062

An attenuated total reflectance (ATR) sample cell has been designed, manufactured and subsequently used for the mid-infrared analysis of hygroscopic samples. This sample cell was installed as a simple drop-in replacement for the cell supplied with our commercially available Harrick Mvp-Pro FTIR-ATR accessory. Calcium chloride, a well-known desiccant that has a propensity to absorb water into its crystal lattice, was selected as non-infrared active substrate to accentuate the efficacy of the cell in preserving the anhydrous state of the sample by straightforward monitoring of the water bands. In contrast, mid-infrared spectra are presented that qualitatively demonstrate the rapid rate at which atmospheric moisture is incorporated into the anhydrous sample when analyzed using the conventional ATR cell assembly.

Full text of DOI:10.1016/j.saa.2011.07.062

Spectrochimica Acta Part A 82 (2011) 529–531
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Short communication
Attenuated total reflectance powder cell for infrared analysis
of hygroscopic samples
∗
M.D.S. Lekgoathi , J.P. le Roux
Department of Applied Chemistry, The South African Nuclear Energy Corporation Ltd., P.O. Box 582, Pretoria 0001, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
An attenuated total reflectance (ATR) sample cell has been designed, manufactured and subsequently
used for the mid-infrared analysis of hygroscopic samples. This sample cell was installed as a simple
drop-in replacement for the cell supplied with our commercially available Harrick Mvp-Pro FTIR–ATR
accessory. Calcium chloride, a well-known desiccant that has a propensity to absorb water into its crystal
lattice, was selected as non-infrared active substrate to accentuate the efficacy of the cell in preserving the
anhydrous state of the sample by straightforward monitoring of the water bands. In contrast, mid-infrared
spectra are presented that qualitatively demonstrate the rapid rate at which atmospheric moisture is
incorporated into the anhydrous sample when analyzed using the conventional ATR cell assembly.
Received 11 March 2011
Received in revised form 14 July 2011
Accepted 18 July 2011
Keywords:
Infrared
Hygroscopic
ATR
©
2011 Elsevier B.V. All rights reserved.
1
. Introduction
reflected beam’s evanescent wave serves to facilitate the collection
of reproducible spectra without signal saturation [2–5]. Further-
more, the introduction of ATR accessories having very small crystals
that rely only on a single reflection has further enhanced this tech-
nique’s versatility to accommodate non-deformable solid samples
that require high ATR crystal contact pressures for deriving quality
spectra [3]. In spite of all these improvements in the ATR tech-
nique, it has been our experience that often, current commercially
available fleet of ATR accessories do not allow for the convenient
handling of hygroscopic samples. Current ATR sampling crystal
plates do not prevent atmospheric moisture ingress into the sam-
ple. This shortcoming makes it difficult to attain ATR spectra of
anhydrous hygroscopic material. This paper reports on the design
and commissioning of a sample cell that can be used to analyze
hygroscopic samples. This cell was designed as a drop-in replace-
ment for the commercially available Harrick Mvp-Pro FTIR–ATR
accessory. Mention is also made of an extension of the design phi-
losophy that the cell can be used for in situ gas–solid interaction
studies.
In recent years, attenuated total reflection (ATR) has grown
steadily as the preferred sampling method for solid, liquid and
even thin film samples [1]. The reasons for this are that the tech-
nique addresses the most challenging aspects of infrared analyses,
namely sample preparation and reproducibility of spectra. Gener-
ally, sample preparation is easier for liquid transmission studies
than for solid transmission studies. The latter requires that the
sample be dispersed in a matrix, whether this is nujol mineral oil
or alternatively potassium bromide (KBr) powder. Our experience
in the laboratory is that the sample preparation process where a
solid is ground to a fine powder and subsequently mixed to a uni-
form dispersion is time consuming and in the case of the use of
nujol, an operation that is usually difficult to deal with. Special
care is also required when handling and storing KBr as a matrix
material since it is hygroscopic and the presence of moisture in
the matrix adversely affects the quality of the spectra. Due to the
complexity of the sample preparation process associated with the
respective matrix compounds, systematic error is difficult to avoid
and resultantly, spectral reproducibility is not easily achieved.
The advent of ATR as an internal reflection sampling technique
in infrared spectroscopy has witnessed the alleviation of the sam-
pling challenges associated with transmission studies as outlined
above. In most instances, samples can be analyzed using ATR in
a non-destructive manner without any sample preparation. The
reproducibility of the shallow penetration depth of the internally
2. The design
With reference to Fig. 1, the cell components are labelled with
the prefix C and the number associated with the prefix refers to
the specific component. A stainless steel body (C1) is flattened at
the bottom and fitted with a dowel pin (C2) to align it properly
during attachment onto a Harrick Mvp-Pro type ATR crystal plate,
which is a removable item from the Mvp-Pro ATR cell itself. The
sample compartment is fitted with a plunger (C4) used to drive
the applied force onto the sample and hence ensure better contact
with the crystal used. This sample pressurizing technique is similar
∗ Corresponding author. Tel.: +27 12 305 6839; fax: +27 12 305 6945.
E-mail address: mpho.lekgoathi@necsa.co.za (M.D.S. Lekgoathi).
1
386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.07.062

5
30
M.D.S. Lekgoathi, J.P. le Roux / Spectrochimica Acta Part A 82 (2011) 529–531
Fig. 2. FTIR–ATR spectra of anhydrous calcium chloride collected 0.5 min (top) and
0 min (bottom) using a dry sample cell. A straight line is observed because the
1
sample is infrared inactive in the mid infrared region.
Fig. 1. An exploded view drawing for the dry sample holder used for ATR-infrared
analysis of moisture sensitive samples. The number next to the prefix C refers to the
particular component.
has a built-in pressure reader. The powder sample was loaded into
the cell then the plunger and a screw cap were fitted into place
(see Section 2). The vent valve (C7) was tightened before the whole
cell assembly was transferred to a Bruker T27 FTIR spectrometer
fitted with a Harrick MvPro ATR accessory. An appropriate force
of 70 N was applied onto the plunger of the sample cell to ensure
better contact between the sample and the diamond crystal. The
pressure was applied by means of a pressure applicator which is an
intricate part of the ATR accessory and is outfitted with a built-in
sensor and pressure readout. A spectrum of an anhydrous sample
was collected using this new cell. In a separate experiment, another
sample of calcium chloride was exposed to air atmosphere from
several seconds to minutes, and the infrared spectra were collected
using a commercially available sample cell.
to the one used by Balik and Simendinger [6], where pressurized
gas was used to maintain contact between the polymer sample and
the ATR crystal. A screw cap (C3) secures the fitted plunger into
place. The vent valve (C7) is used to relieve the gas pressure when
the plunger is pushed into place after loading the sample within
an inert gas atmosphere. An inlet with a valve (C9) may be used as
an alternative to the side screw to draw vacuum or allow flow of
various gases into the analyte, hence allowing the device to be used
as a reaction cell during an infrared measurement. The O-rings (C5,
C6, and C8) ensure that the cell assembly is leak tight.
To attach the new sample cell to the existing ATR crystal plate,
the existing powder cell is first detached from the crystal plate and
then replaced with a new sample cell shown in Fig. 1. This is conve-
niently executed by simply undoing and subsequently retightening
the single screw that normally fixes the sample cell onto the crystal
plate. For the purpose of deriving FTIR–ATR spectra of a hygroscopic
sample, the complete sample cell and crystal plate assembly is
firstly detached from the ATR accessory. The assembly is thereafter
transferred into a dry nitrogen filled glove box and then charged
with the sample. The sample is finally sealed in the cell assembly
by inserting the plunger (C4), then securing this in place by means
of its screw cap (C3) and then finally tightening the vent valve (C7).
4. Results and discussion
Calcium chloride does not absorb mid-infrared radiation, and
therefore unity infrared beam reflectance is expected. Indeed anhy-
drous samples of calcium chloride analyzed using the designed
sample cell have demonstrated unity reflectance even after leaving
them in the sample cell for a period of 10 min (Fig. 2).
When calcium chloride samples are exposed to air atmosphere
during ATR infrared analysis using a conventional ATR sample cell,
−1
moisture bands are observed. The 1630 and 3400 cm bands are
associated with crystalline water’s bending and stretching modes
respectively (Fig. 3).
3
. Experimental
The water content of hydrated calcium chloride was monitored
over a period of time from 0.5 to 10 min and there was an indication
of increasing moisture band intensity (Fig. 3), suggesting that the
sample was still adsorbing water.
3
.1. Materials
Calcium chloride dihydrate from Merck (99.5% purity) was
◦
dehydrated in an oven at 150 C and then handled inside a glove
box with a circulating dry nitrogen atmosphere. All sampling was
done inside a dry nitrogen glove box.
The rapid rate at which calcium chloride hydrolyzes was stud-
ied using FTIR–ATR as shown in Fig. 3. The two prominent bands
−
1
are the 1629 cm due to the bending mode of water, normally, in
−
1
crystalline form, and the 3451 cm due to the –O–H stretch. The
−
1
noise in the 2000–2200 cm spectral region is due to interference
by the diamond crystal used for the ATR-infrared measurements.
Other hydrolyzing samples such as fluorinated inorganic com-
pounds are known to produce toxic gases when exposed to
moisture, for example, lithium hexafluorophosphate decomposes
rapidly in the presence of moisture (300 ppm) to give off HF gas
3.2. Apparatus and procedure
The anhydrous ATR cell which was designed, manufactured and
described in this work (Fig. 1) was used inside a dry nitrogen glove
box to ensure that the influence of atmospheric water vapour on
the sample was eliminated. This sample holder was attached to a
Harrick MvPro ATR plate fitted with a diamond crystal. The top of
this plate attaches the sample cell and the bottom can be attached to
or detached from the main Mvp-Pro ATR accessory. This accessory
[
7], a poisonous and corrosive substance. As a result, the use of a
closed system such as the sample cell described here is important
to maintain sample integrity and eliminate health hazards.

M.D.S. Lekgoathi, J.P. le Roux / Spectrochimica Acta Part A 82 (2011) 529–531
531
hygroscopic sample can now be obtained in an inert environment.
This is particularly important for the infrared analysis fluorides
and chlorides which are known to be very reactive to atmospheric
moisture and rapidly decompose to release toxic gases.
An inlet with a valve may be used to replace the side screw on the
new cell and in this configuration, the cell provides special infrared
analysis conditions such as vacuum or specific gas environment to
study solid–gas interactions.
Acknowledgement
The authors would like to thank The South African Nuclear
Energy Corporation for funding.
References
Fig. 3. FTIR–ATR spectra of hydrolyzed calcium chloride samples. The spectra from
top to bottom indicate samples exposed to air for 0.5, 3, 5, 7 and 10 min respectively.
The amount of moisture in each sample depends on the time the sample was exposed
to air atmosphere.
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. Conclusion
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The analysis of calcium chloride as a representative sample of
a hygroscopic species has shown that the designed sample cell
does avoid atmospheric moisture from interfering with the ana-
lyte. Anhydrous CaCl2 quickly hydrates when a conventional ATR
sample cell is used during ATR measurements, and the extend
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