Type: Analytical Essays
Sample donated: Julie Ellis
Last updated: February 28, 2019
In this experiment, bothmethods produced results within a +/-10% margin of error from the expectedacetaminophen concentration, which is in line with current pharmacologicalguidelines.
However, the standard addition method (SA) produced a result foracetaminophen concentration that was 2.7% lower than the result produced by thecalibration curve (CC) method. As both results fall within the +/-10% errorallowed by pharmacological guidelines, determining which method gave the moreaccurate result would require further research with a greater (n > 3) numberof repeat readings and, particularly for the standard addition method, agreater number of non-sample values. However, from the R2 valuealone, the CC method appears to have given data which conforms more accuratelyto the linearity expected with an R2 value of 0.965022738 compared to the value of 0.962757712for the SA method, a small but relevant difference.
Althoughthe results by this experiment appear to suggest that the CC method was ofhigher reliability and, if the tablet was indeed of 500 mg acetaminophen asstated on the packaging, greater accuracy, the SA method is in fact consideredin analytical terms to be more reliable and more accurate. This is dueprimarily to the presence of matrix effects, also known as multiplicativeinterferences, which give rise to biased results as a consequence of an alteredcalibration function.Matrixeffects may impact results via two primary routes: a translational effect,which alters the intercept of a calibration function and may manifest in allcalibration methods, and a rotational effect, which alters the slope but doesnot manifest in SA methods.
The rotational effect arises from interference producedby non-analyte substances present in the test solution but not in the standard whichmay attenuate or amplify the analyte signal. In the case of this experiment, althoughthe mechanically macerated tablet was added to a solution and the undissolvedbindings agents removed via filtration, other non-acetaminophenimpurities may have remained which could have impacted the amplitude of the DPVsignal recorded for the sample in a way which would not have occurred in thestandard solutions. Unlike CC methods which do not include the test solution incomparative measurements, SA methods overcome this rotational effect by includingthe same volume of test solution in each standard addition and hence overridingany impact non-analyte substances may have on the amplitude of the analytesignal.
Both CC and SA methods, however, are vulnerable to translational matrixeffects as stated above due to translational effects being independent ofanalyte concentration. SAmethods do have their limitations, particularly regarding the use ofextrapolation which is widely assumed to degrade the precision of the resultwhen compared to the use of interpolation in CC methods, although thisassumption has been challenged in recent years.iStandard addition also assumes true linearity of the analytical calibration overthe whole relevant concentration range, and that a small volume of standard solutiondoes not alter the matrix composition of the test solution, both of which wouldrequire experimental and statistical verification in a research environment. Theuse of a calibration curve also assumes linearity but has the additionalbenefit of multiple standard comparisons, thereby reducing random errors inpreparing and reading the standard solutions as they are averaged over severalstandards. This benefit could obviously be implemented for SA proceduresby increasing the number of additions tested.