Instant Notes: Analytical Chemistry

The second stage is to conduct a computer search of an appropriate database

or library, for example of drugs, to test for an exact match or to compile an

inverted list (Topic H4).

The use of IR and NIR spectrometry for qualitative measurements is extensive

and wide ranging, and for this purpose transmission spectra are conventionally

recorded as a function of wavenumber. In order to make quantitative measure-

ments, it is necessary to convert the transmittance readings to absorbance, A,

the relation between the two being:

A = log (100/T%)

This also allows any absorbance by solvents or other components of the sample

to be subtracted from the analyte peak.

A(total) =A(sample) +A(background)

This also allows the propersubtraction of solvents or other components.

For example, if the spectrum of a machine oil without additives is measured

in a 0.1 mm NaCl cell, and then the same procedure is followed for a sample

with small amounts of additives, subtraction of the absorbance spectra will give

the spectrum of the additives in absorbance form.

Gas analysis by IR spectrometry using long path length cells has been used to

measure concentrations of anaesthetic gases. For example, nitrous oxide, N 2 O,

shows a strong absorbance at 2200 cm-^1 at which the wavenumber of neither

water vapour nor carbon dioxide interfere. Measurement of the concentrations

between 2 and 50 ppm is possible with a 15 m path length gas cell. Trichloro-

methane (chloroform) gives a strong, sharp peak at 770 cm-^1 and may be

measured down to 0.1 ppm.

In addition, IR spectrometry has been used to measure the mineral contents

Quantitative
measurements

246 Section E – Spectrometric techniques

Aromatic bands at 1600, 1590, 1500?

pa pa

Present (p)

Strong band at 830?

Absent (a)

Sharp band at 1420?

Modified

epoxides

Sharp band

at 1420

Broad band

at 690

papa

Broad, strong

band at 1050

Polyvinyl

chloride

copolymers

pa

Polyvinyl

alcohol

Cellulose

esters

Acrylics;

aliphatic

polyesters

Plasticized

polyvinyl chloride

pa

Strong band

at 1333–1212

Alkyd resins Cellulose ethers

Fig. 8. Decision tree for the identification of a polymer when a carbonyl absorption is

present.