Selection of an analytical method

1. JSS MAHAVIDYAPEETHA
JSS SCIENCE AND TECHNOLOGY UNIVERSITY
SELECTING AN ANALYTICAL METHOD
Presented by :
RAMYA SAVITHRI K
UP222505
Department of Chemistry

2. Introduction
Accuracy
Precision
Sensitivity
Selectivity
Robustness
Ruggedness
Scale of operation
Equipment, time and cost
Making the final choice
References

3. Analytical chemistry is a branch of chemistry which deals with the study of the seperation,
identification, qualitative and quantitative determination of the compositions of different
substances.
An analytical method is the application of a technique to a specific analyte in a specific matrix.
The best method is chosen based on the requirements of the analysis. Some of the criterias which
are considered for selecting the method are,
1. Accuracy
2. Precision
3. Sensitivity
4. Selectivity
5. Robustness
6. Ruggedness
7. Scale of operation
8. Equipment,Time and cost

4. 1. ACCURACY
• Accuracy is how closely the result of an experiment agrees with the “true” or “expected”
value.
• It is defined as ‘A measure of agreement between the experimental value and
theoretical value’
• Errors are of two types,
a. Absolute error [ Ea]
Ea=Xi-Xt
b. Relative error [Er]
Er= Xi-Xt x 100
Xt
If the relative error is within 1%, the value is said to be highly accurate.
If the relative error is between 1-5%, the value is moderately accurate.
If the relative errror is more than 5%, the value is considered to be of low accuracy.

5. 2. PRECISION
• When a sample is analysed several times, the individual results are rarely the
same. Instead the results are randomly scattered
• It is defined as ‘A measure of the variabilities of results obtained in
different trials of the same experiment.’
• The closer the agreement between the individual analysis, the more precise the
results.
• Precision doesn’t imply accuracy.
In determining the concentration of K+ in serum,
The results in (a) are more precise than those in (b), but (b) may have more
accurate values.

6. 3. SENSITIVITY
• Sensitivity is the ability of a method to demonstrate that two samples have different
amount of analyte.
• It is defined as ‘The change in signal per unit change in the amount of analyte’.
• It is equivalent to the proportionality k
If nA is the number of moles of analyte, SA is the signal obtained by analyte,
and CA is the concentration of analyte, k is given by
𝐒𝐀
𝐧𝐀
= k or SA = knA , In total analysis method.
𝐒𝐀
𝐂𝐀
= k or SA =kCA , In concentration method.
• The method with greatest k value [Sensitivity] is best able to discriminate among smaller
amounts of analyte.

7. 4. SELECTIVITY
• An analytical method is selective, if its signal is a function of only the amount of
analyte present in the sample.
• It is defined as ‘The measure of methods of freedom from interferences’
• The signal of sample, containing interferent is given by,
SSample = SA+ SI
where, SA & SI are signals of analyte and interferent respectively
or
SSample = kAnA+ kInI (In total analysis method)
SSample = kACA+ kICI (In concentration method)
where,kA & kI are sensitivities of analyte and interferent respectively.
nA & nI are no: of moles and CA & CI are concentrations of analyte and
interferent respectively.

8. • The selectivity of a method is given by selectivity coefficient KA,I,
KA,I =
𝐤𝐈
𝐤𝐀
where kI and kA are sensitivities of interferent and analyte respectively.
• The selectivity coefficient greater than +1 or lesser than -1 indicates that the
method is more selective for the interferent than for the analyte.
or kI =KA,I*kA
Substituting this equation in the above equations,
SSample = kA (nA+ KA,I*nI) (In total analysis method) ............(1)
SSample = kA (CA+ KA,I*CI) (In concentration method) ............(2)
• For a method to be selective, KA,I*nI should be smaller than nA in (1) and KA,I*CI
should be smaller than CA in (2).

9. 5. ROBUSTNESS
• For a method to be useful it must provide reliable results. Unfortunately, methods are
subject to a variety of chemical & physical interferences that contribute uncertainity
to the analysis.
• When a method is relatively free from chemical interferences it can be applied to the
determination of analytes in a wide variety of sample matrices. Such methods are
considered robust.
6. RUGGEDNESS
• Random variations in experimental conditions also introduce uncertainity.
• If a method’s sensitivity is highly dependent on experimental conditions such as
temperature, acidity or relaxation time, then slight changes in those conditions may
lead to significantly different results.
• A rugged method is relatively insensitive to changes in experimental conditions

10. 7. SCALE OF OPERATION
• Another way of narrowing the choice of methods is to consider the scale on which the
analysis must be conducted.
• Three limitations of particular importance are
i. The amount of sample available for the analysis.
ii. The concentration of analyte in the sample.
iii. The absolute amount of analyte needed to obtain a measurable signal.
The first and second limitations define the scale of operation.
The last limitation positions a method within the scale of operation.

12. 8. EQUIPMENT, TIME AND COST
Analytical methods can be compared in terms of their need for equipment, the time
required to complete an analysis, and the cost per sample.
I. Equipment
Methods relying on insrumentation are equipment-intensive and may require significant
operator training.
II. Time
The time needed to complete an analysis for a single sample is often fairly similar from
method to method. This is however not completely true, because, much of this time is
spent preparing the solutions and equipments are in place, the number of samples that can
be analysed per hour differs substantially from method to method. This is a significant
factor for laboratories that handle a high volume of sample.

13. III. Cost
The cost of analysis is determined by many factors including the cost of
necessary equipments and reagents, the cost of hiring analysts and the number
of samples that can be processed per hour.
In general methods relying on insruments cost more per sample than other
methods.

14. MAKING THE FINAL CHOICE
• Selecting a specific method requires a careful balance among these design
criterias.
• The most important criteria among these that should be considered during the
selection of analytical method is accuracy.
For example,
In determining the concentration of lead in drinking water, Graphite Furnace
Atomic Absorption Spectroscopy is the commonly used method because, it meets
all those criterias.

15. ANALYTICAL CHEMISTRY by B.K. Sharma
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