Limitations of Analytical Methods

Limitations of Analytical Methods • The function of the analyst is to obtain a result as near to the true value as possible by the correct application of the analytical procedure employed.

Limitations of Analytical Methods • The level of confidence in the results will be very small unless there is a knowledge of the accuracy and precision of the method used as well as being aware of the sources of error in the measurement.

Data Handling • Accuracy and Precision • Statistics • Errors • Calibration Curves

Data Handling • Accuracy • The accuracy of a determination may be defined as the concordance between it and the true or most probable value.

Data Handling • Accuracy: Two possible ways of determining the accuracy. • Absolute Method: Using a synthetic sample containing known amounts of the constituents to be determined. • Comparative Method: Using a standard sample of the material in question.

Data Handling • Precision • Precision may be defined as the concordance or reproducibility of a series of measurements of the same quantity.

Data Handling • Precision • This definition can be further refined to take account the timing of the experiment. • Thus there is a distinction between a series of measurements made by one analyst on one day; REPEATABILTY, and measurements made by a number of analysts over several days; REPRODUCIBILTY.

Data Handling • Precision • Precision always accompanies accuracy, but a high degree of precision does not imply accuracy.

Data Handling • Inaccurate and Imprecise

Data Handling • Accurate but Imprecise

Data Handling • Accurate and Precise

Data Handling • Inaccurate but Precise

Data Handling • Statistics • The true or absolute value of a quantity cannot be established experimentally, so that the observed value must be compared with the most probable value. • Statistics provide a means of quantifying the precision of a set of measurements.

 x n x = Data Handling • Mean • It is found that the results of a series of determinations will vary slightly. • The average value is accepted as the most probable.

Data Handling • Estimates of Precision • Standard Deviation • Variance • Relative Standard Deviation • Coefficient of Variation

Data Handling • Standard Deviation • Defined as the square root of the sum of the squares of the deviation from the mean.

 ( x - x)2 n - 1 s = Data Handling • Standard Deviation

 ( x - x)2 n s = Data Handling • Standard Deviation

 ( x - x)2 n - 1 s2 = Data Handling • Variance • Is the square of the standard deviation.

R.S.D. = s / x Data Handling • Relative Standard Deviation • A further measure of precision is known as the Relative Standard Deviation (R.S.D.).

R.S.D. = 100s / x Data Handling • Coefficient of Variation • This measure is often expressed as a percentage as the coefficient of variation (C.V.)

Limitations of Analytical Methods