Atomic emission spectrophotometry (AES)

Introduction

Atomic emission spectroscopy plays an important role in the control of sodium, potassium and lithium in a number of raw materials and formulations.

Atoms contain various energy states, as illustrated in Figure 6.1 for the sodium atom. The normal unexcited state is the ground state. Sodium contains one electron in its outer (3p) orbital and, if energy is gained by the atom, this electron may be excited to a higher state and then subsequently lose its excess energy by falling back to a lower energy orbital. Thus, when a sodium salt is heated in a flame, the outer electrons in the volatilised atoms are excited and then return to the ground state with emission of energy, which appears, for example, as yellow light (wavelength 589.3 nm). The major line in the sodium emission spectrum is due to an electron falling from the 3p excited state to the 3s ground state; the atomic emission spectrum of sodium contains two other major lines, at 819.5 nm and 330.2 nm, due to the transitions shown in Figure 6.1. Atomic emission lines are very narrow (< 0.01 nm). Only a limited number of elements are sufficiently excited by thermal energy for AES measurements to be carried out. Common elements with emission lines suitable for utilisation in their quantitation are Ca, Ba, Na, Li and K.

Fig. 6.1 Electronic transitions within a sodium atom giving rise to the main bands in its emission spectrum.

Instrumentation

An atomic emission spectrophotometer (Fig. 6.2) is composed of the following components:

Fig. 6.2 Schematic diagram of an atomic emission spectrophotometer.

Examples of quantitation by AES

In order to measure a sample by AES, a calibration curve is constructed by aspirating solutions of known concentration into the flame.

Assay of sodium and potassium ions in an i.v. infusion

Standard solutions of sodium chloride (NaCl) and potassium chloride (KCl) in water were prepared and diluted appropriately to give a calibration curve across the working range across the range of the instrument (ca 0.05–1 mg/100 ml). The assay was then carried out by diluting the infusion until its concentration was close to that at the mid-point of the calibration series. Water is used as a blank. The following results were obtained:

(i) •Weight of NaCl used to prepare standard solution = 0.5092 g

• Weight of KCl used to prepare standard solution = 0.1691 g Both standards were transferred to the same 1000 ml volumetric flask and diluted to 1000 ml.

(ii) Dilutions were carried out on standards:

• Step 1: 20 ml of the standard solution was transferred to a 100 ml volumetric flask and was diluted to 100 ml (diluted standard solution)

• Step 2: A calibration series was prepared by transferring the following volumes of diluted standard solutions to 100 ml volumetric flasks 0, 5, 10, 15 and 25 ml.

(iii) The infusion solution was diluted as follows:

• Step 1: 5 ml to 250 ml

• Step 2: 10 ml to 100 ml.

(iv) The instrument was used with a sodium filter to establish the sodium calibration curve and then the sodium in the sample. The instrument was switched to a potassium filter in order to determine the potassium calibration curve and the potassium in the sample. Table 6.1 shows the readings obtained for sodium (Na) and potassium (K) in the calibration solutions as well as the concentrations of Na and K in the calibration solutions (calculated below). Calculate the concentrations of Na and K in the infusion solution in mmoles/l. Atomic weights: Na = 23; K = 39.1; Cl = 35.5.

Table 6.1 Data used in Calculation example 6.1

Calculation example 6.1

Dilutions of standards

Step 1: 20 to 100 ml (× 5).

Step 2: Point 1 = 5 to 100 (× 20).

Total dilution = 5 × 20 = 100.

Concentrations in solution used for point 1.

The rest of the points in the calibration series are simply × 2, × 3, × 4 and × 5. These values give the concentrations in Table 6.1.

The equations of the lines obtained for the above data were:

For Na, y = 99.0 × + 0.722.

For K, y = 222 × + 1.3.

Reading of diluted sample for Na = 70.2. Reading of diluted sample for K = 70.6.

Dilution of sample

Step 1: 5 to 250 ml (× 50).

Step 2: 10 ml to 100 ml (× 10), Total dilution 50 × 10 = 500.

Concentration of Na in infusion

Substituting into the equation of the line for Na.

Dilution factor = × 500.

Self-test 6.1

From the data given in Calculation example 6.1, calculate the concentration of K in the infusion.

Answer: 39.9 mmoles/l

Self-test 6.2

From the following data calculate the potassium content per tablet in effervescent KCl bicarbonate tablets.

• Weight of 20 tablets = 35.6751 g

• Weight of tablet powder taken for assay = 0.1338 g

The sample is dissolved in 500 ml of water and then 5 ml of the sample solution is taken and diluted to 100 ml.

• Weight of KCl used to prepare standard = 0.1912 g

The standard was dissolved in 100 ml of water and 5 ml of the standard solution was diluted to 250 ml.

The diluted standard solution was used to prepare a calibration series by transferring 0, 5, 10, 15, 20 and 25 ml to 100 ml volumetric flasks and making up to volume.

The following readings were obtained for the calibration series: 0, 20.3, 40.1, 60.3, 80.1 and 100.

• Reading obtained for potassium in the diluted sample solution = 73.9.

Answer: (From a computer-fitted calibration curve) K 496.2 mg per tablet

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