UV-Visible and Infrared Spectroscopic Methods in Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 217

Pure liquids : Sandwiched between two halide crystal disks made of
NaCl or KBr. A clean crystal window can be used as a reference
background.
Diluted solutions : Put in cells (NaCl, KBr) or IR cards; Carbon tetra-
chloride (CCl 4 ) and carbon disulfide (CS 2 ) are the common solvents used
for dilution. A longer path length than pure liquid is needed.
Solid samples : Grinding sample with a mineral oil (Nujol) to form liquid
slurry (mull). Otherwise, make a KBr pellet by grinding solid sample
with solid KBr and apply high pressure to the dry mixture to form a thin
transparent film.
Recommended pathlength for near-IR: 5-10 cm for 800-1100 nm or
0.1-2 cm for 1100-3000 nm (0.1-1 cm for UV-VIS). In low concentration
air pollution monitoring, the cell length can be very long.
Sodium chloride crystal cells must be placed in desiccators to protect from
atmospheric moisture. They require periodic polishing to remove ''fogging''
due to moisture contamination. For the same reason, maintaining IR
instrument at moisture-free condition is essential. Silver chloride windows
are often used for wet samples or aqueous solutions. They are soft and will
gradually darken due to exposure to visible light.
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