Page 271 - Handbook of Instrumental Techniques for Analytical Chemistry

Infrared Spectroscopy 261

Pellets are used for solid samples that are difficult to melt or dissolve in any suitable IR-trans-
mitting solvents. The sample (0.5 to 1.0 mg) is finely ground and intimately mixed with approxi-
mately 100 mg of dry potassium bromide (or other alkali halide) powder. Grinding and mixing can
be done with an agate mortar and pestle, a vibrating ball mill (Wig-L-Bug from Crescent Dental Man-
ufacturing), or lyophilization. The mixture is then pressed into a transparent disk in an evacuable die
at sufficiently high pressure. Suitable KBr disks or pellets can often be made using a simpler device
such as a Mini-Press. To minimize band distortion due to scattering of radiation, the sample should
be ground to particles of 2 µm (the low end of the radiation wavelength) or less in size. The IR spectra

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produced by the pellet technique often exhibit bands at 3450 cm and 1640 cm due to absorbed
moisture.
Mulls are used as alternatives for pellets. The sample (1 to 5 mg) is ground with a mulling agent
(1 to 2 drops) to give a two-phase mixture that has a consistency similar to toothpaste. This mull is
pressed between two IR-transmitting plates to form a thin film. The common mulling agents include
mineral oil or Nujol (a high-boiling hydrocarbon oil), Fluorolube (a chlorofluorocarbon polymer), and
hexachlorobutadiene. To obtain a full IR spectrum that is free of mulling agent bands, the use of mul-
tiple mulls (such as Nujol and Fluorolube) is generally required. Thorough mixing and reduction of
sample particles of 2 µm or less in size are very important in obtaining a satisfactory spectrum.
Gas cells can be used to examine gases or low-boiling liquids. These cells consist of a glass or met-
al body, two IR-transparent end windows, and valves for filling gas from external sources. They pro-
vide vacuum-tight light paths from a few centimeters to 120 m. Longer path lengths are obtained by
reflecting the IR beam repeatedly through the sample using internal mirrors located at the ends of the
cell. Sample gas pressure required to produce reasonable spectra depends on the sample absorbance and
the cell’s path length. Typically, a good spectrum can be acquired at a partial pressure of 50 torr in a
10-cm cell. Analysis of multicomponent gas samples at parts-per-billion levels can be successfully per-
formed.
Microsampling accessories such as microcells, microcavity cells, and micropellet dies are used to
examine microquantities of liquids (down to 0.5 µL) and solids (down to 10 µg ). Beam-condensing
devices are often used to reduce the beam size at the sampling point. Extra practice is recommended
for performing this type of microanalysis.
Attenuated total reflectance (ATR) accessories are especially useful for obtaining IR spectra of dif-
ficult samples that cannot be readily examined by the normal transmission method. They are suitable
for studying thick or highly absorbing solid and liquid materials, including films, coatings, powders,
threads, adhesives, polymers, and aqueous samples. ATR requires little or no sample preparation for
most samples and is one of the most versatile sampling techniques.
ATR occurs when a beam of radiation enters from a more-dense (with a higher refractive index)
into a less-dense medium (with a lower refractive index). The fraction of the incident beam reflected
increases when the angle of incidence increases. All incident radiation is completely reflected at the in-
terface when the angle of incidence is greater than the critical angle (a function of refractive index). The
beam penetrates a very short distance beyond the interface and into the less-dense medium before the
complete reflection occurs. This penetration is called the evanescent wave and typically is at a depth of
a few micrometers (µm). Its intensity is reduced (attenuated) by the sample in regions of the IR spec-
trum where the sample absorbs. Figure 15.11 illustrates the basic ATR principles.
The sample is normally placed in close contact with a more-dense, high-refractive-index crystal
such as zinc selenide, thallium bromide–thallium iodide (KRS-5), or germanium. The IR beam is di-
rected onto the beveled edge of the ATR crystal and internally reflected through the crystal with a single
or multiple reflections. Both the number of reflections and the penetration depth decrease with increas-
ing angle of incidence. For a given angle, the higher length-to-thickness ratio of the ATR crystal gives
higher numbers of reflections. A variety of types of ATR accessories are available, such as 25 to 75°
vertical variable-angle ATR, horizontal ATR, and Spectra-Tech Cylindrical Internal Reflectance Cell