⚛ Atomic Absorption Spectroscopy (AAS)

AAS

☢ Spectroscopy Basics

Spectroscopy refers to the interaction between radiation and matter.

Electrons in an atom can absorb a specific amount of energy to transition to a higher energy orbit, also known as an excited state. The energy absorbed equals the energy difference between the original and the new orbit.

Different elements have different energy levels in their orbits, which means that the amount of energy absorbed by electrons varies significantly between elements. When excited electrons return to their original orbit, or ground state, they emit the same amount of energy previously absorbed in the form of electromagnetic radiation (EMR).

🔬 Atomic Absorption Spectroscopy (AAS) Defined

Atomic Absorption Spectroscopy (AAS) is a quantitative technique used to analyze the concentration of metal ions in a sample using spectroscopy.

AAS is highly sensitive and can detect and measure trace amounts of metal species by measuring the amount of EMR absorbed.

📏 Beer-Lambert Law

The relationship between the amount of metal ion or atom in a sample and the extent of EMR absorption is described by the Beer-Lambert Law.

In other words, a lower concentration results in less absorbance, while a higher concentration results in more absorbance. Measuring the amount of EMR absorbed allows for the calculation of the exact concentration of a particular metal.

🎯 Specificity of AAS

AAS can analyze the concentration of a specific metal even when multiple metals are present in a sample. This is because the amount of EMR absorbed by metals is determined by the energy difference between orbits in the metal's atomic structure.

Since each metal atom has a unique structure (e.g., atomic radius), the amount of energy of EMR absorbed varies. For example, one metal might absorb a smaller amount of radiation, while another absorbs a greater amount. Therefore, AAS offers highly specific analysis for samples containing multiple metal species.

💡 Wavelength and Energy

The energy of radiation is directly proportional to its frequency, meaning higher frequency radiation has more energy. Consequently, shorter wavelength radiation also has more energy.

Metals absorb EMR of different energies, so EMR of different wavelengths is used to analyze each metal. This is because the energy levels of orbits are intrinsically different among different metals.

Before analyzing the concentration of a particular metal using AAS, it is important to determine the wavelength of EMR that the metal can absorb.

⚙ AAS Components and Functions

The key components of an AAS setup include:

• Hollow Cathode Lamp

• Flame Atomizer

• Nebulizer

• Monochromator

• Detector

Hollow Cathode Lamp

The hollow cathode lamp produces the EMR needed to excite electrons in the metal atom. The cathode is made from the same metal element being analyzed in the sample.

This EMR from the lamp matches the wavelength absorbed by the metal being analyzed. For example, a lead hollow cathode lamp produces EMR with the same wavelength as that absorbed by lead atoms in a water sample.

Flame Atomizer

A nebulizer sprays the liquid sample containing the metal into the flame. In AAS, the flame converts metal species in the sample into metal atoms in the ground state. This is why the flame is called the atomizer.

The EMR from the hollow cathode lamp passes through the hottest part of the flame, where metal atoms in the ground state are present. The metal atoms in the flame absorb this EMR to reach an excited state.

The amount of EMR before the flame (I0I0​) is always higher than what comes out of the flame (II). The Beer-Lambert Law states that the extent of EMR absorption depends on the concentration of the metal in the sample. More metal atoms in the flame result in more EMR absorption, causing less EMR to exit the flame.

Beer-Lambert Law Formula

A=log(I0I)=ϵ∗c∗lA=log(II0​​)=ϵ∗c∗l

Where:

• AA is the absorbance

• I0I0​ is the original intensity of EMR

• II is the final intensity of EMR after the flame

• ϵϵ is the extinction coefficient (absorptive property of the metal)

• cc is the concentration of the metal atoms

• ll is the path length of the flame

Monochromator

The monochromator receives the EMR with the characteristic wavelength of the metal being analyzed while eliminating other sources of EMR, such as background light. This allows the detector to analyze the intensity of the characteristic wavelength that belongs to the metal atom.

Detector

The detector measures the intensity of the radiation after it has passed through the monochromator.

🧪 Example: Analyzing Lead Concentration

Suppose we want to analyze the lead concentration in a water sample.

1. Determine the Wavelength: Lead atoms absorb a characteristic wavelength of 217 nm (as shown in the table earlier).

2. Use a Lead Hollow Cathode Lamp: This lamp produces EMR with a wavelength of 217 nm.

3. Pass EMR Through Flame: As the EMR passes through the flame, which atomizes the water sample, part of the radiation is absorbed by the lead atoms present.

4. As the EMR exits a flame, it...

Atomic Absorption Spectroscopy (AAS) 🧪

Measuring Absorbance and Calculating Concentration 🧮

The water sample is passed through a monochromator, which only receives radiation of 217 nanometers. The detector measures the intensity of this radiation to calculate the absorbance.

For example, the absorbance of a water sample is 0.58. To calculate the concentration, we must first create various standard solutions containing the same metal ion being analyzed (in this case, lead).

The absorbance of these solutions are measured in AAS and plotted on a graph against the concentration of the standard solution. A line of best fit is constructed using these data points.

This graph is referred to as the calibration curve of AAS.

Calibration Curve Example 📈

Here's an example of the absorbance values of various standard solutions of lead ions:

Six standard solutions with various concentrations of lead ions were created, and by analyzing these solutions in AAS, six different absorbance values for the solutions were obtained.

These are then plotted on a graph with the absorbance on the y-axis against the concentration of the standard solutions on the x-axis to allow for the construction of a line of best fit.

Now, this line of best fit can be used to determine the concentration of lead ions in the water sample. An absorbance value of 0.58 (obtained previously) gives a concentration of 3.5 parts per million (ppm) by using the calibration curve.

Parts Per Million (ppm) 📏

The unit ppm (parts per million) is a very common unit in the setting of AAS, as AAS is a very sensitive technique. We are able to use it to quantify the concentration of metals in very, very small traces or low concentrations of any sample.