Page 270 - Handbook of Instrumental Techniques for Analytical Chemistry

260 Handbook of Instrumental Techniques for Analytical Chemistry

What It Does

It is possible to obtain an IR spectrum from samples in many different forms, such as liquid, solid, and
gas. However, many materials are opaque to IR radiation and must be dissolved or diluted in a trans-
parent matrix in order to obtain spectra. Alternatively, it is possible to obtain reflectance or emission
spectra directly from opaque samples. Some popular sampling techniques and accessories are discussed
here.
Liquid cells are used for dilute solutions of solid and liquid samples that are dissolved in relatively
IR-transparent solvents. Sampling in solution results in enhanced reproducibility and is often the pre-
ferred choice. Unfortunately, no single solvent is transparent through the entire mid IR region. The an-
alyst usually chooses solvents that have transparent windows in the region of interest. The conventional
–1
popular solvents are carbon tetrachloride for the region between 4000 and 1330 cm and carbon disul-
–1
fide for the region between 1330 and 625 cm . Both solvents are quite toxic, and thus must be handled
carefully. One may replace carbon tetrachloride with the less-toxic tetrachloroethylene or methylene
chloride and substitute carbon disulfide with n-hexane or n-heptane. Polar solvents such as water and
alcohols are seldom used because they absorb strongly in the mid IR range and react with alkali-metal
halides, such as NaCl, commonly used for cell windows.
Acquiring acceptable IR spectra of aqueous samples requires use of special types of liquid cells
such as thin cells of BaF 2 , AgCl, or KRS-5(a mixed thallium bromide–thallium iodide). Aqueous solu-
tion measurements can also be accomplished with attenuated total reflectance (ATR) accessories,
which are discussed later in this chapter.
Typically, solutions of 0.05 to 10% in concentration are handled in IR cells of 0.1 to 1 mm in thick-
ness. Concentration of 10% and cell path length of 0.1 mm represent one practical combination. In a
double-beam spectrometer, a compensating cell is filled with pure solvent and placed in the reference
beam. In the single-beam FT instrument, the solvent bands are mostly removed by obtaining the differ-
ence spectra through subtraction of solvent spectra from sample spectra. Both fixed-thickness and vari-
able-thickness liquid cells are available commercially. They normally consist of metal frame plates, IR-
transmitting windows, and gaskets that determine the path length of the cells.
Salt plates of IR-transmitting materials can be used for semivolatile and nonvolatile liquid sam-
ples. Sodium chloride disks are the most popular and economical choice for nonaqueous liquids. Sil-
ver chloride or barium fluoride plates may be used for samples that dissolve or react with NaCl plates.
A drop of the neat sample is squeezed between two salt plates to form a film of approximately 0.01
mm in thickness. The plates can be held together by capillary attraction, or they may be clamped in a
screw-tightened holder or pressed to form a good contact in a press fit O-ring supported holder. It is
also possible to place a film of samples on salt plates by melting a relatively low-melting solid and
squeezing it between two plates. Sodium chloride salt plates can usually be cleaned with dry methyl-
ene chloride or acetone. This smear technique is one of the simplest ways to obtain IR spectra.
Thin films of nonvolatile liquids or solids can be deposited on an IR-transmitting salt plate by sol-
vent evaporation. The sample is first dissolved in a reasonably volatile solvent. A few drops of the re-
sulting solution are placed on the plate. After evaporating off the solvent, a thin film of sample is
obtained for subsequent spectra acquisition.
Disposable IR cards have been developed recently by 3M to accommodate samples that are liquids,
are soluble in reasonably volatile solvents, or can be smeared on flat surfaces. The cards are made up
of a cardboard holder containing a circular IR-transmitting window made of a microporous substrate
–1 –
(polytetrafluoroethylene substrate for 4000 to 1300 cm or polyethylene substrate for 1600 to 400 cm
1
). Samples are generally applied to the cards by the techniques used for salt plates. The substrate bands
can be subtracted from the sample spectra. Besides the convenience, the disposable IR cards are non-
hygroscopic, and thus can handle water-containing samples.