Introduction

Atomic spectroscopy is one of the most widely used techniques in analytical chemistry for quantitative elemental analysis. There are certain conditions when the analysis needs to be carried out to determine the elemental composition. These conditions are such as

• how much iron is in an ore sample?

• how much lead is in your drinking?

• What is the content of mineral water?

• Are the water samples containing toxic elements?

In the laboratory, elemental analysis can be performed using an atomic spectroscopy instrument. This experiment is designed to give you a little experience with AAS (Atomic Absorption Spectrophotometry) and AES (Atomic Emission Spectrophotometry) for the quantitative determination of a few elements (Figure 4.1). In this experiment, you will use flame atomic absorption spectrophotometry (AAS) to determine the concentrations of Ca²⁺, Mg²⁺ and iron (Fe) in mineral waters. Atomic emission spectroscopy (AES) is a method for the determination of alkali metals in water samples. These metals are excited in flames and can be determined by flame emission. In this experiment, you will use AES to determine concentrations of sodium (Na⁺) in the mineral waters. You are required to bring a few types of bottles of mineral water for testing purposes.

METHODOLOGY

Procedure for Determination of calcium, magnesium and iron in water using AAS

1. Request the standard solution of calcium, magnesium and iron from the lab assistants. Record the concentration of these standard solutions.

2. Prepare 5 standard solutions for each element using 50 mL volumetric flask. The concentrations of the standard solutions should be within the range as presented in Table 4.1.

3. Set up the flame atomic spectrophotometer (please consult the Lab Assistant). Measure the complete set of standards and unknown samples before switching to another element. Use AAS mode for the measurement of calcium, magnesium and iron. For the unknown samples, you can use mineral water and tap water samples. You may need to dilute the unknown samples before measurement if too concentrated. Record the dilution factor.

Procedure for Determination of sodium in water using AES

4. Use the AES mode for the measurement of sodium. Prepare the stock solution of sodium by accurately weighing out about 0.510 g (+- 0.001 g) of sodium chloride, quantitatively transfer into a 200 mL volumetric flask, dissolve in deionized water, dilute to the mark, and mix thoroughly. Using the stock solution, prepare 5 standard calibration solutions with the concentration ranging from 0.10 – 0.50 ppm.

5. To determine the sodium concentration, dilute any unknown sample(s) if the measured absorbance is too large – i.e., outside of the range of the standards. Record the dilution factor.

REPORT

I. Tabulate and plot the absorbance vs concentration for the calcium, magnesium, and iron measurements. Derive the calibration equations and calculate the concentration of the selected elements in unknown samples.

II. Tabulate and plot the emission intensity vs sodium concentration for the NaCl standards and derive the calibration equation. You should probably use a polynomial equation (e.g., a cubic equation) to fit the data if the calibration curves are nonlinear. Calculate the concentration of sodium in the unknown samples.

III. Use Microsoft Excel to tabulate and prepare the absorbance vs concentration calibration plot for the sodium, calcium, magnesium, and iron measurements. Define Limit of Detection (LOD) and Limit of Quantitation (LOQ). Calculate the LOD and LOQ based on the calibration plots by using the following equations.

IV. For sodium, if your calibration curve is fitted to polynomial or quadratic model, draw a tangent line near to the lowest concentration. Generate the linear equation of the tangent and estimate the LOD and LOQ as the stated in step III.

V. Discuss the obtained result by referring to Malaysia Drinking water standard.

Where Sy and S are the standard deviations of the response and slope of the calibration plot, respectively, you can use the LINEST function in Excel to calculate the Sy and S (Video 1) or Data Analysis function in Excel (Video 2).

Question

I. Why is flame emission a more sensitive technique for some cations, mainly the alkaline and earth alkali cations, while atomic absorption has greater sensitivity for other cations, such as the transition metal ions?

II. Explain why AAS is so selective, i.e. why do other elements not usually interfere in the analysis?

III. Why LOD and LOQ are important in chemical analysis.