2.4 Transmission Methods
2.4.1 Liquids and Solutions
There are several different types of transmission solution cells available. Fixed-pathlength sealed cells are useful for volatile liquids, but cannot be taken apart for cleaning. Semi-permanent cells are demountable so that the windows can be cleaned. A semi-permanent cell is illustrated in Figure 2.9. The spacer is usually made of polytetrafluoroethylene (PTFE, known as ‘Teflon’) and is available in a variety of thicknesses, hence allowing one cell to be used for various pathlengths.
Variable pathlength cells incorporate a mechanism for continuously adjusting the pathlength, while a vernier scale allows accurate adjustment. All of these cell
Back plate Gasket Gasket Back
window Front window
Front plate
Syringe port Spacer
Infrared radiation
Figure 2.9 Schematic of a typical semi-permanent liquid cell. From Stuart, B., Biological Applications of Infrared Spectroscopy, ACOL Series, Wiley, Chichester, UK, 1997. University of Greenwich, and reproduced by permission of the University of Greenwich.
types are filled by using a syringe and the syringe ports are sealed with PTFE plugs before sampling.
DQ 2.1
Which type of solution cell would you consider to be the easiest to maintain?
Answer
The demountable is by far the easiest to maintain as it can be readily dismantled and cleaned. The windows can be repolished, a new spacer supplied and the cell reassembled. The permanent cells are difficult to clean and can become damaged by water. The pathlengths need to cali-brated regularly if quantitative work is to be undertaken. Variable path-length cells suffer from similar disadvantages and they are difficult to take apart. The calibration therefore suffers and the cells have to be calibrated regularly.
An important consideration in the choice of infrared cells is the type of window material. The latter must be transparent to the incident infrared radiation and alkali halides are normally used in transmission methods. The cheapest material is sodium chloride (NaCl), but other commonly used materials are listed in Table 2.1.
Certain difficulties arise when using water as a solvent in infrared spec-troscopy. The infrared modes of water are very intense and may overlap with the sample modes of interest. This problem may be overcome by substituting water with deuterium oxide (D2O). The infrared modes of D2O occur at different wavenumbers to those observed for water because of the mass dependence of
Experimental Methods 27 Table 2.1 Summary of some optical materials used in transmission infrared spectroscopy.
From Stuart, B., Modern Infrared Spectroscopy, ACOL Series, Wiley, Chichester, UK, 1996. University of Greenwich, and reproduced by permission of the University of Greenwich
Window material
Useful range (cm−1)
Refractive index
Properties
NaCl 40 000 – 600 1.5 Soluble in water; slightly soluble in alcohol; low cost; fair resistance to mechanical and thermal shock;
easily polished
KBr 43 500 – 400 1.5 Soluble in water and alcohol; slightly soluble in ether; hygroscopic; good resistance to mechanical and thermal shock
CaF2 77 000 – 900 1.4 Insoluble in water; resists most acids and bases; does not fog; useful for high-pressure work
BaF2 66 666 – 800 1.5 Insoluble in water; soluble in acids and NH4Cl; does not fog; sensitive to thermal and mechanical shock KCl 33 000 – 400 1.5 Similar properties to NaCl but less
soluble; hygroscopic CsBr 42 000 – 250 1.7 Soluble in water and acids;
hygroscopic
CsI 42 000 – 200 1.7 Soluble in water and alcohol;
hygroscopic
the vibrational wavenumber. Table 2.2 lists the characteristic bands observed for both H2O and D2O. Where water is used as a solvent, NaCl cannot be employed as a infrared window material as it is soluble in water. Small path-lengths (∼ 0.010 mm) are available in liquid cells and help reduce the intensities of the very strong infrared modes produced in the water spectrum. The small path-length also produces a small sample cavity, hence allowing samples in milligram quantities to be examined.
SAQ 2.2
What would be an appropriate material for liquid cell windows if an aqueous solution at pH 7 is to be examined?
Liquid films provide a quick method of examining liquid samples. A drop of liquid may be sandwiched between two infrared plates, which are then mounted in a cell holder.
Table 2.2 The major infrared bands of water and deuterium oxide. From Stuart, B., Bio-logical Applications of Infrared Spectroscopy, ACOL Series, Wiley, Chichester, UK, 1997.
University of Greenwich, and reproduced by permission of the University of Greenwich Wavenumber
(cm−1)
Assignment
3920 O–H stretching
3490 O–H stretching
3280 O–H stretching
1645 H–O–H bending
2900 O–D stretching
2540 O–D stretching
2450 O–D stretching
1215 D–O–D bending
DQ 2.2
The method of liquid films is normally not used for volatile (with a boiling point less than 100◦C) liquids. Why would this be necessary?
Answer
A common problem encountered in obtaining good quality spectra from liquid films is sample volatility. When the spectrum of a volatile sam-ple is recorded, it progressively becomes weaker because evaporation takes place during the recording period. Liquids with boiling points below 100◦C should be recorded in solution or in a short-pathlength sealed cell.
Before producing an infrared sample in solution, a suitable solvent must be chosen. In selecting a solvent for a sample, the following factors need to be considered: it has to dissolve the compound, it should be as non-polar as pos-sible to minimize solute–solvent interactions, and it should not strongly absorb infrared radiation.
If quantitative analysis of a sample is required, it is necessary to use a cell of known pathlength. A guide to pathlength selection for different solution concen-trations is shown in Table 2.3.