Question

An analytical method which can be applied in a wide variety of sample matrices is A. robust B. sensitive C. accurate D. selective

Answer 1

A. Robust:

For a method to be useful it must provide reliable results. Unfortunately, methods are subject to a variety of chemical and physical interferences that contribute uncertainty to the analysis. When a method is relatively free from chemical interferences, we can use it on many analytes in a wide variety of sample matrices. Such methods are considered robust.

B. Sensitive:

The ability to demonstrate that two samples have different amounts of analyte is an essential part of many analyses. A method’s sensitivity is a measure of its ability to establish that such differences are significant. Sensitivity is often confused with a method’s detection limit, which is the smallest amount of analyte that we can determine with confidence. Sensitivity is equivalent to the proportionality constant, k_A. If DSA is the smallest difference that we can measure between two signals, then the smallest detectable difference in the absolute amount or relative amount of analyte is

∆n_A = ∆S_A/k_A or     ∆C_A = ∆S_A/k_A

Suppose, for example, that our analytical signal is a measurement of mass using a balance whose smallest detectable increment is ±0.0001 g. If our method’s sensitivity is 0.200, then our method can conceivably detect a difference in mass of as little as

∆n_A = (±0.0001 g / 0.200) = ±0.0005 g

For two methods with the same ∆S_A, the method with the greater sensitivity—the larger k_A—is better able to discriminate between smaller amounts of analyte.

C. Accurate:

Accuracy is how closely the result of an experiment agrees with the “true” or expected result. We can express accuracy as an absolute error, e

e = obtained result - expected result

or as a percentage relative error, %e_r

%e_r = ((obtained result − expected result) / expected result) × 100

A method’s accuracy depends on many things, including the signal’s source, the value of k_A in equation 3.1 or equation 3.2, and the ease of handling samples without loss or contamination. In general, methods relying on total analysis techniques, such as gravimetry and titrimetry, produce results of higher accuracy because we can measure mass and volume with high accuracy, and because the value of k_A is known exactly through stoichiometry.

D. Selective:

Selectivity is a measure of a method’s freedom from interferences. The selectivity of a method for the interferent relative to the analyte is defined by a selectivity coefficient, K_A,I

KA,I = kI / kA

which may be positive or negative depending on the sign of k_I and k_A. The selectivity coefficient is greater than +1 or less than –1 when the method is more selective for the interferent than for the analyte.