Analytical Chemistry Chapter 3: Vocabulary Overview

An analysis provides chemical or physical information about a sample.
The component in the sample of interest is called the analyte, and the remainder of the sample is the matrix.
In an analysis, we determine the identity, concentration, or properties of an analyte by measuring one or more of its chemical or physical properties.
Example: Municipal water departments analyze samples of water to determine the concentration of fecal coliform bacteria by measuring the number of bacterial colonies that form during a carefully defined incubation period.

A technique is any chemical or physical principle that can be used to study an analyte.
A method is the application of a technique for a specific analyte in a specific matrix.
A procedure is a set of written directions that tell us how to apply a method to a particular sample, including information on how to collect the sample, handle interferents, and validate results.
A protocol is a set of stringent guidelines that specify a procedure that an analyst must follow if an agency is to accept the results.
Example: Methods for determining the concentration of lead in water, such as graphite furnace atomic absorption spectroscopy (GFAAS), may have different procedures for different matrices (e.g., water, soil, blood).

Analytical techniques can be classified into two general classes:
- Total analysis techniques: respond to the absolute amount of analyte in the sample (e.g., mass, volume).
- Concentration techniques: respond to the analyte's concentration in the sample (e.g., spectroscopy, electrochemistry).
Total analysis techniques are often called "classical" techniques and typically involve chemical reactions.
Concentration techniques are often called "instrumental" techniques and typically involve measuring an optical or electrical signal.

Accuracy is a measure of how close the result of an experiment is to the true or expected result.
Precision is a measure of the variability of individual results.
Accuracy and precision are not the same thing, and a method can be precise but not accurate.
Example: Two sets of results for the concentration of K+ in serum, with one set showing more precision than the other, but neither set necessarily being accurate.

Sensitivity is a measure of the method's ability to detect small amounts of analyte.
Selectivity is a measure of the method's freedom from interferences.
A method's sensitivity is equivalent to the proportionality constant, kA, in the equation relating the signal to the amount of analyte.
Selectivity is often confused with specificity, but specificity is a measure of whether the signal depends only on the analyte.
A method's selectivity coefficient, KA,I, is a measure of its selectivity for an interferent relative to the analyte.### Determining the Selectivity Coefficient
The value of kCa is determined using Equation 3.3.2, which is kCa = SCa / CCa = 1.
In the presence of Zn2+, the signal is given by Equation 3.4.2: Ssamp = kCa CCa + kZn CZn = 100.5.
Solving for kZn gives its value as 0.5.
The selectivity coefficient is KCa,Zn = kZn / kCa = 0.5.

Wang and colleagues developed a fluorescence method for analyzing Ag+ in water.
When analyzing a solution that contains 1.0 × 10^−9 M Ag+ and 1.1 × 10^−7 M Ni2+, the fluorescence intensity (signal) was +4.9% greater than that obtained for a sample of 1.0 × 10^−9 M Ag+.
The selectivity coefficient for this analytical method is KAg,Ni = 4.5 × 10^7 M^−1.

A selectivity coefficient provides a useful way to evaluate an interferent's potential effect on an analysis.
Solving Equation 3.4.3 for kI gives kI = KA,I × kA.
Substituting into Equation 3.4.1 and Equation 3.4.2, and simplifying gives Ssamp = kA {nA + KA,I × nI} and Ssamp = kA {CA + KA,I × CI}.
An interferent will not pose a problem as long as the term KA,I × nI in Equation 3.4.5 is significantly smaller than nA, or if KA,I × CI in Equation 3.4.6 is significantly smaller than CA.

Barnett and colleagues developed a method to determine the concentration of codeine in poppy plants.
The selectivity coefficient for the interferent, 6-methoxycodeine, relative to that for the analyte, codeine, is KA,I = 0.15.
To achieve an accuracy of better than ±0.50%, the concentration of 6-methoxycodeine must be less than 3.3% of codeine's concentration.

For a method to be useful, it must provide reliable results.
Methods are subject to a variety of chemical and physical interferences that contribute uncertainty to the analysis.
A robust method is relatively free from chemical interferences and can be used to analyze an analyte in a wide variety of sample matrices.
A rugged method is relatively insensitive to changes in experimental conditions.

The scale of operation defines the analytical method's limitations, including the amount of sample available, the expected concentration of analyte, and the minimum amount of analyte that will produce a measurable signal.
Samples can be divided into macro, meso, micro, and ultramicro categories based on their size and analyte concentration.