Qualitative Analysis

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Qualitative Analysis

Qualitative analysis refers to analysis that examines aspects of a sample that do not have a numerical value. This type of analysis is more subjective.

Various techniques have been devised to permit the analysis of the structure and function of microorganisms. Some techniques are qualitative in their intent. That is, they provide a “yes or no” answer. Other techniques are quantitative, and so provide numerical information about a sample.

Assessing the growth of a bacterial sample provides examples of both types of analysis techniques. An example of a qualitative technique would be the growth of a bacterial sample on a solid growth medium, in order to solely assess whether the bacteria in the sample are living or dead. An example of a quantitative technique is the use of that solid growth media to calculate the actual number of living bacteria in a sample.

Microscopic observation of microorganisms can reveal a wealth of qualitative information. The observation of a suspension of bacteria on a microscope slide (the wet mount) reveals whether the bacteria are capable of self-propelled motion. Microorganisms, particularly bacteria, can be applied to a slide as a so-called smear, which is then allowed to dry on the slide. The dried bacteria can be stained to reveal, for example, whether they retain the primary stain in the Gram stain protocol (Gram positive) or whether that stain is washed out of the bacteria and a secondary stain retained (Gram negative). Examination of such smears will also reveal the shape, size, and arrangement (singly, in pairs, in chains, in clusters) of the bacteria. These qualitative attributes are important in categorizing bacteria.

Microscopy can be extended to provide qualitative information. The incorporation of antibodies to specific components of the sample can be used to calculate the proportion of the samples in a population that

KEY TERMS

Electromagnetic radiation— The energy of photons, having properties of both particles and waves. The major wavelength bands are, from short to long: cosmic, ultraviolet, visible or “light,” infrared, and radio.

Functional group— A particular arrangement of atoms in an organic compound that determines the dominate characteristics of that compound. An alcohol, for example, contains the grouping R-OH, where R represents a collection of carbon and hydrogen atoms and -OH identifies the compound as an alcohol.

Reagent— A chemical added to a suspect material to produce a known reaction response. If the reaction response is observed as expected the identity of the material is assumed to be known.

Spectroscopy— That branch of science in which a varying perturbing energy (electromagnetic, magnetic, etc.) is directed on a sample. The magnitude of the response as the energy varies is plotted and the resulting spectrum characteristic of the arrangement of atoms within the molecules of the sampleis noted.

possess the target of interest. Fluorescent-labeled antibodies, or antibodies combined with a dark appearing molecule such as ferritin, are useful in such studies. The scanning confocal microscope is proving to be tremendously useful in this regard. The optics of the microscope allows visual data to be obtained at various depths through a sample (typically the sample is an adherent population of microorganisms). These optical thin sections can be reconstructed via computer imaging to produce a three-dimensional image of the specimen. The use of fluorescent-tagged antibodies allows the location of protein within the living biofilm to be assessed.

Bacterial growth is another area that can yield qualitative or quantitative information. Water analysis for the bacterium Escherichia coli provides an example. A specialized growth medium allows the growth of only E. coli. Another constituent of the growth medium is utilized by the growing bacteria to produce a by-product that fluoresces when exposed to ultraviolet light. If the medium is dispensed in bottles, the presence of growing E. coli can be detected by the development of fluorescence. However, if the medium is dispensed in smaller volumes in a grid-like pattern, then the number of areas of the grid that are positive for growth can be related to a mathematical formula to produce a most probable number of living E. coli in the water sample. Viable bacterial counts can be determined for many other bacteria by several other means.

Chemical tests are widely used for qualitative analysis. If an unknown produces the same results when reacted with a certain chemical reagent as does a material of known composition, they may be identical. To be absolutely sure more then one confirmatory test is made, for although reagent A may, when added to both a known and an unknown substance, produce identical responses, reagent B when used for testing might react only with the known and not with the unknown. The analytical chemist who performs these tests must be knowledgeable both in selecting the proper test reagents and in knowing the expected results.

The tentative identification was correct if the melting point of the mixture is identical to the literature value melting point for the pure substance but incorrect if a substantially lower melting point is observed.