1 Introduction to Chemical Analysis

There are many situations in which it is useful to know what elements or molecules are present in a sample, or if one or more specific elements or molecules of interest (i.e. analytes) are present. Such situations occur frequently in manufacturing, natural resource sectors, and other industries; in agriculture, food and water quality and safety; in environmental assessment; in forensics and security; in health care, medicine, and pharmaceuticals; and, of course, in many fields of scientific research and development. The obvious challenge is that elements and molecules cannot be directly seen, classified, and counted by eye—special methods of analysis are needed.

Classical Methods of Analysis

Classical methods of chemical analysis are usually volume-based or mass-based, leveraging reactivity and other chemical properties of analytes to yield macroscopic observables (e.g. colour changes, precipitates) that can be linked to analyte quantity. Titrations are perhaps the most famous type of classical volumetric analysis. Although classical methods work well in some situations—potentially providing good accuracy and precision at low cost—serious limitations are encountered as samples get more complex and smaller in quantity. Classical methods are also poorly suited to discovery and screening analyses, where there is not a predetermined analyte of interest and where there is limited foreknowledge about the composition of a sample.

Instrumental Methods of Analysis

Instrumental methods of chemical analysis are technology-based, leveraging the interactions of atoms or molecules with light, other types of radiation, electric and magnetic fields, and other forms of energy. In general, an instrument applies a stimulus (e.g. light/radiation, electric potential, kinetic energy) to a sample and the response (e.g. residual stimulus, emitted light/radiation, electric current, velocity or trajectory) is transduced into an electrical quantity (e.g. current, voltage). The analog electrical signal is converted to a digital signal to interface with a computer or other data acquisition and recording system.

It should be noted that transduction does not produce a current or voltage that is easily converted to the quantity of analyte. In most cases, a direct conversion using fundamental scientific theory is impossible or impractical. Instead, calibration is required. Lab-made standard samples with known quantities of analyte(s) are prepared and measured using an instrument. The resulting plots of instrument signal versus known quantity of analyte enables determination of the unknown quantity in a sample of interest.

Broad and strong knowledge of instrumental methods makes a scientist highly employable.

Figures of Merit

Figures of merit are used to discuss and compare methods of analysis. The limit of detection is the smallest amount of analyte that can be reliably detected. The dynamic range is the range over which the instrument signal scales predictably and significantly with analyte quantity. The sensitivity is the change in signal per unit quantity of analyte.

As instruments and analysis methods are never perfect, other important quantities are the signal-to-noise (S/N) ratio, the signal-to-background (S/B) ratio, and the selectivity. “Signal” refers to the instrument response to analyte; “background” refers to what is measured by the instrument for a sample without analyte; and “noise” refers to random variation in the instrument response. Although a larger signal per unit quantity of analyte is often favourable, it can be negated by high levels of noise. Conversely, a smaller signal for analyte may be useful if the noise is very small or negligible. The goal is thus to maximize the S/N ratio. When noise scales in proportion to the background, it is also useful to maximize the S/B ratio. “Selectivity” is the degree to which a method is sensitive to analyte versus non-analyte interferences in a sample.

Connections

Subsequent chapters discuss common instrumental methods of analysis and their underlying physicochemical basis.

• Light is the stimulus and/or the measured analyte response in Ch. 6, Ch. 8, Ch. 10, Ch. 12, Ch 14.
• Electric potential is the stimulus in Ch. 16 and electric current is the measured analyte response.
• In Ch. 27, kinetic energy is part of the stimulus and velocity or trajectory is the measured analyte response.
• Physical and chemical behaviours are used to separate mixtures of analytes prior to detection in Ch. 18, Ch. 20, Ch. 21, Ch. 23, and Ch. 24.

Post-Reading Questions

1. Convert the text description of an instrumental method into a flow chart or similar diagram, including labels such as response, sample, stimulus, and transduction. Include at least one example of a stimulus and at least one example of a response. Define transduction.
2. Explain why calibration is required for most instrumental methods.
3. Define the six figures of merit discussed in this chapter.

Topic Learning Objectives

The chapter is a primer for the following learning objectives, which will be covered in lecture and/or with additional assigned reading:

• Distinguish between classical and instrumental methods of chemical analysis.
• Identify the stimulus, response, and transduction when learning about a specific type of instrumental analysis.
• Discuss how figures of merit are useful for characterizing and comparing between methods of analysis.
• Link the qualitative definitions of figures of merit with quantitative ones.
• Match figures of merit with features on a calibration plot (i.e. signal vs. concentration) or graphical data from an instrument (e.g. plot of a signal peak).