Chemistry

Calibration and standardization is of utmost importance in every analytical method. These procedures, which usually involve the use of chemical standards, determine the relationship between the analyte response and analyte concentration (Skoog et al., 2004).

The experiment aimed to demonstrate two methods of calibration and standardization, method of external standards and standard addition through the analysis of copper concentration using Atomic Absorption Spectrometry.

The method of external standards involved the preparation of a series of external standards of known concentration, prepared separately from the unknown sample. The analyte response (i.e. absorbance) of the standard solutions as well as the unknown was measured and recorded (Table 1) and plot of the absorbance against the analyte concentration, known as the calibration curve (Figure 1), was made. The curve was linearized and the equation of the best fit line was obtained. Based on this equation, the concentration of copper in the unknown was calculated to be 3.29 ppm.

Table 1. Absorbance data of the standard and unknown copper solutions for the method of external standards.
SampleConcentration (ppm)Absorbance110.0873220.1681330.2438440.3547550.4166Unknown3.28570.2760

The second method used was standard addition, more specifically the multiple additions method, which involved the addition of standard solutions of the analyte to the sample. In this method, known increasing concentrations of the internal standards are prepared and a constant amount of the unknown sample is added to each of the standard solutions and a calibration curve is obtained (Table 2, Figure 2). The concentration of the analyte in the unknown sample was calculated from the obtained equation of the best fit line of the curve by getting the x-intercept (setting y  0 and solving for x) and multiplying it by the dilution factor (DF) to account for the dilution that occurred during sample preparation. Using this method, the calculated concentration of the copper in the unknown was 0.75 ppm.

Table 2. Absorbance of the standard copper solutions for the standard addition method.
SampleConcentration (ppm)Concn of added std in 25cm3 sple soln (ppm)Absorbance1100.144221.60.296333.20.413444.80.547556.40.669

The concentration of the analyte in the unknown sample was also calculated by the spectrophotometer used in the experiment. The results from the manual and instruments calculations are summarized and compared in

Table 3.
Table 3. Summary of the concentration of the analyte in the unknown sample using the two methods.
MethodExternal StandardsStandard Additionx DFManual calculation3.251.880.75Instrument calculation3.291.730.69

As seen in the table, there are differences between the calculated values. The uncertainty of the unknown in each method can be determined by calculating the sample standard deviation and sample variance, which are a measures of precision (Skoog et al., 2007) often used in analytical chemistry.

Based on the results of the experiment, it can be deduced that both methods gave relatively good approximations of the analyte concentration in the unknown (as compared to the instrument results) however, statistical calculations still need to be done to compare the precision of the methods. The method of external standards is often used when there are no interference effects from the matrix components of the analyte solution (Skoog et al., 2007). The advantages of this method include the relative ease in the preparation of the solutions and simple calculations. However, it has limited application to due to the errors that may be caused by matrix interferences. On the other hand, the method of standard additions, is more advantageous because it a potential compensation for unexpected matrix and complex interferences. It is quite a powerful method when employed properly, that is, when a good blank reading has been obtained and the calibration curve of the analyte is linear in the sample matrix (Skoog et al., 2007). These were both achieved in the experiment especially the second requirement since the calibration curve had an R2  0.998, indicating excellent linearity. However, a notable disadvantage of this method is the more tedious work involved in the preparation of the solutions.

In summary, two methods in determining the analyte concentration of an unknown sample through spectrometry were employed in the experiment, the method of external standards and the standard addition method. The first method is a less tedious method whoch requires the determination of the response of the standard solutions containing a series of known concentrations of the analyte as well as the unknown sample and directly calculating the analyte concentration in the unknown from the equation of the best fit line of the calibration curve of the standard solutions. However, this method is disadvantageous due to possible errors which may be brought about by matrix interferences. The second method which is the standard addition method compensates this kind of error since the standard solutions of different concentrations are incorporated in unknown sample containing the analyte. Similarly, a calibration curve is plotted and the equation of the best fit line is obtained from which the concentration of the analyte in the unknown sample can also be calculated. Statistical analysis such as calculation of the sample standard deviation and variance can be conducted to evaluate the precision of the results.