**3. Fine-tuning of the analytical technique**

The analytical technique has to be fine-tuned prior to analysis:

## **3.1 Determine the stability of the cadmium hollow cathode lamp**

To determine the stability of the lamp, the absorbance value of at least three element standards of different concentrations 0.2, 0.6 and 1.0 μg Cd/L must be monitored for at least 30 minutes throughout three hours, as shown in **Figure 5**.

Subsequently, the normality and homogeneity of the data must be analyzed by applying the Shapiro–Wilk test with N-1 degrees of freedom and Levene test with degrees of freedom 1 and 2, determined as K-1 and (k-N)-K (N = absorbance

**Figure 5.** *Cadmium hollow cathode lamp stability [70].*

#### *Crystallization and Applications*

readings and K = number of times). One-way ANOVA statistical treatment is applied next to determine whether there are any significant differences between the absorbances obtained for each standard during the three hours of analysis.

In the Shapiro–Wilk test, the Ho was accepted, since the calculated W was lower than the tabulated W (0.999) and the homogeneity of variances with calculated F was lower than the tabulated F (5.100), indicating that the means obtained are representative values of the absorbances of each one of the standards, with good variation coefficients (<3.5%) [70].

The one-way ANOVA used to measure the absorbance of each of the standards for the metal, showed a constant value for the sum of squares, as for the root mean square (0.000). We can also see that the calculated F is lower than the tabulated F at a 95% confidence level, indicating that there were no significant differences between the values of the absorbances at different times when the analysis was performed [70].

## **3.2 Optimizing the calcination and atomization temperature to determine cadmium by graphite furnace**

In order to obtain a higher sensitivity for cadmium determination, the calcination and atomization temperature in the graphite furnace must be optimized. To do so, a standard of 1 μg/L of cadmium must be prepared and the atomization temperature set by varying the calcination temperature from 500 to 800 °C in 50 °C intervals. Once this is done, the highest absorbance is observed (which in our study was between 500 and 550 °C) and another variation of the calcination temperature in this range is made, but this time by modifying it in 10 °C intervals. Once the calcination temperature has been optimized, a standard of 0.8 μg/L of Cd is prepared and read on the atomic absorption equipment, setting the calcination temperature at 800 °C and varying the atomization temperature from 900 to 1800 °C. This variation is made at intervals of 100 °C. **Table 2** shows the optimum temperatures at 530 and 1750 °C [54, 70].

#### **3.3 Quantification method**

Cd quantitative analysis was performed using the calibration curve for which standard solutions of the metal were prepared as follows:

The calibration curve was constructed based on the stock solution of 1000 μg Cd/L by preparing 5 mL of six cadmium standards in a range of 0.1 to 1.0 μg Cd/L in 0.2% HNO3 solution. For each Cd standard, four absorbance readings are taken in the AAS-HG.

The Pearson's Correlation Coefficient showed that the calibration curve for cadmium has a high r value, above 0.9950, demonstrating that there is a positive correlation between the absorbance of the metal and its concentration [54, 70].


**Table 2.**

*Optimal graphite furnace temperature programming for Cd determination.*
