Instant Notes: Analytical Chemistry

Section F – Combined techniques

F2 SAMPLE IDENTIFICATION

USING MULTIPLE

SPECTROMETRIC TECHNIQUES

In order to study and identify any unknown analytical sample using spectro-

metric techniques, the analytical chemist must first obtain good quality spectra

and then use these to select the information from each technique that is of most

value. It is also important to recognize that other analytical observations should

be taken into account. For example, if the sample is a volatile liquid and the

spectral information suggests that it is an involatile solid, clearly, there is

conflicting evidence.Table 1summarizes the information of most value that may

be obtained from the common spectrometric techniques.

The use of computerized library databases can assist in the matching of spectra

to recorded examples (Topic H4). If difficulties are found in distinguishing between

two possibilities for the sample identity, then it may be necessary to consult refer-

ence texts or additional computer databases so that an exact match is found. Some

databases give information that helps when working with samples that are new

Information from
each spectrum

Key Notes

The information that may be obtained from ultraviolet-visible, infrared,

proton and carbon-13 nuclear magnetic resonance and mass spectra is

complementary, and it is much easier to identify the structure of a

compound if all the spectra are considered.

Each spectrometric technique provides characteristic data to assist in the

eventual identification of the sample. These have been considered in the

individual topics, but need to be combined to extract the maximum

information.

Examples of the use of the multidisciplinary approach to sample

identification exemplify a general scheme of interpretation.

Related topics Spectrometric techniques (Section E)

Information from

each spectrum

Spectrometric

identification

Applications

Table 1. Information from common spectrometric techniques

Information Techniques

IR MS NMR UV

Molecular formula h

Functional groups h m m m

Connectivity m m h m

Geometry/stereochemistry m h

h, high value; m, moderate value.