What are spectroanalytical methods?

Spectroanalytical methods measure the wavelength and intensity of radiations that are either emitted or absorbed by a molecular or atomic species. These methods are useful for evaluating the electronic or atomic arrangement of the molecules of a given analyte.

What are the different types of spectroanalytical methods?

Spectroanalytical methods are used for identifying the elemental composition of a given sample. There are two important categories of such methods—atomic absorption spectrometry (AAS) and atomic emission spectrometry (AES). Both these methods provide the quantitative evaluation of chemical elements present in a given sample. These methods are further classified into several types based on the sources they use.

Atomic absorption spectrometry

Atomic absorption spectrometry or (AAS) is an important category of spectroanalytical methods. In this method, the analysis is based on light absorption by the free metallic ions. This method is used for determining an element’s concentration in an analyte sample. Using AAS, it is possible to identify more than 70 elements in a given sample solution.

The important AAS methods are Flame Atomic absorption spectrometry (FAAS) and graphite furnace atomic absorption spectroscopy (GFAAS).

For the determination of an unknown analyte, a known standard analyte content is required. It gives the relation between the measured absorbance and analyte concentration. Some other requirements for this analysis technique are:

• Atomizer: It is used to atomize the material present in the sample and analyse its atomic constituents. Commonly, flame is used as an atomizer (FAAS).
• Radiation source: There are two main classes of radiations sources used in this analysis. One is the line source and the other one is the continuum source. As the name suggests, line sources produce the analyte spectrum as lines. Hollow cathode lamps are the commonly used line sources in the analysis. Continuum sources are used for background correction, and their common example is the deuterium lamp.

• Spectrometer: A spectrometer comprises a monochromator and a detector. Micromotors are used for separating spectral lines according to wavelength. Spectrometers are used according to the radiation sources used. For line sources, medium-resolution spectrometers are used, and for continuum sources, high-resolution spectrometers are used.

All these parts and requirements are illustrated in the following schematic diagram.

Flame atomic absorption spectrometry

Flame atomic absorption spectrometry (FAAS) is an important type of atomic absorption spectroscopy. In this method, the analysis uses flame for atomizing the content in the sample. The commonly used flames are air-acetylene flame and nitrous oxide-acetylene flame.

The following diagram shows the schematic representation of a typical FAAS system.

Atomic emission spectrometry

Atomic emission spectrometry (AES) is another important category of spectroanalytical methods. This method utilizes the intensity of light emitted from various sources at a definite wavelength. These sources can be flame, plasma, spark or arc. AES method is used for the quantification of a particular element present in a sample.

In AES, with the help of electrical discharges from arcs, flame, spark etc., electrons in the atoms of the analyte get excited to a higher energy level from the ground level. While returning to lower energy levels, these electrons emit radiations of characteristic frequencies that are detected using a spectrometer. Using this data, the elemental composition of an unknown analyte is identified.

An emission spectrum can be produced through AES, which has several atomic spectral lines. The wavelength of these lines identifies the element, and the intensity gives the number of atoms present in that element. Samples can be excited using the sources mentioned below.

• Flame: The analyte material is brought into contact with the flame in the form of a solution, a gas, or it can be directly exposed to the flame. The flame’s heat evaporates the content, break the bonds between the atoms, and produces free atoms. Thus, the thermal energy from the flame excites the atoms and emits radiations of characteristic wavelength that can be detected using a spectrometer.

• Inductively coupled plasma: Inductively coupled plasma atomic emission spectroscopy (ICP AES) is a type of analysis method. It utilizes an inductively coupled plasma to excite the atoms and emit the light of suitable wavelength for detecting an element in the given material. It is also called inductively coupled plasma optical emission spectroscopy (ICP OES).

• Spark and arc: This type of analysis method is used for solid samples containing metallic elements. In this method, an electric spark is passed through the sample, and further heating excites the atoms present in it. These excited atoms produce radiations of characteristic wavelengths. A monochromator then disperses this radiation or light, and the detection process is carried out. Earlier methods of spark atomic emission spectroscopy were only qualitative. Now, both qualitative and quantitative spark atomic emission spectroscopic methods are available.

Laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) is an important atomic emission spectroscopic technique. In this analytical method, a highly energetic laser pulse is used for exciting the sample. It forms a plasma and then atomizes and excites the analyte sample.

LIBS can be used to analyze samples in any physical state (solid, liquid, or gas). In this method, the laser produces plasma that atomizes the atoms and excites the sample. For analysing the relative abundance of known constituent elements of sample material, LIBS is used. It can also be used for analysing the impurities present in the constituents.

The instrumentation of a common LIBS system comprises an Nd-YAG solid-state laser (produces energy in near-infrared (IR) region of 1064 nm wavelength), a spectrometer with a highly sensitive detector, and a monochromator or a polychromator. The following diagram illustrates a typical LIBS system.

SPECTRO analytical instruments

One of the most popular analytical instrument suppliers is SPECTRO. They make elemental analyzers using the principles of optical emission spectrometry and X-ray fluorescence spectrometry.

Common Mistakes

Students may get confused between atomic emission spectrometry and atomic absorption spectrometry. They may think that both are identical in their principle, which is incorrect. Both are different as AES is based on emission and AAS is based on absorption.

• Atomic absorption spectrometry is based on light absorption by the free metallic ions, while atomic emission spectrometry is based on the intensity of light emitted from various sources at a definite wavelength.
• Both are used for the detections of elements in a particular analyte sample.
• Compared to atomic emission spectrometry, atomic absorption spectrometry is more versatile as it can detect more than 70 elements.

Context and Applications

This topic is helpful in the professional exams of undergraduate and graduate courses, especially:

Bachelors in Chemistry

Bachelors in Analytical Chemistry

Bachelors in Industrial Chemistry

Masters in Industrial Chemistry

Masters in Pure and Applied Chemistry

Masters in Analytical Chemistry

Related Concepts

Florescence

Spectro chemistry

Spectrophotometry

Optical emission spectrometry

Practice Problems

Q1: Complete the name of the given spectroanalytical method:

ICP AES: ________Atomic Emission Spectroscopy

(a) Induced Current Plasma Atomic Emission Spectroscopy

(b) Inductively Coupled phosphorescence Atomic Emission Spectroscopy

(c) Induced Coupled Plasma Atomic Emission Spectroscopy

(d) Inductively Coupled Plasma Atomic Emission Spectroscopy

Correct option: (d)

Q2: Using atomic absorption spectrometry, a minimum of how many elements can be determined from a single sample solution?

(a) 2

(b) 10

(c) 70

(d) 100

Correct option: (c)

Q3: Which of the following methods does not use atomic absorption spectrometry?

(a) GFAAS

(b) FAAS

(c) ICP OES

(d) Both (a) and (b)

Correct option: (c)

Q4: LIBS is used to analyse the relative abundance of the constituent elements in a given analyte sample. It is possible only if:

(a) If one of the constituent atoms of the sample is known

(b) If the wavelength of the sample is known

(c) If all the constituents of the sample are known

(d) If the concentration of the sample is known

Correct option: (c)

Q5: The commonly used laser in LIBS is Nd-YAG solid-state laser. What is its wavelength?

(a) 1064 nm

(b) 750 nm

(c) 2190 nm

(d) 1090 nm

Correct option: (a)