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Many different methods and techniques can be used to determine the elemental composition of a molecule in a laboratory. In some cases, even the specific isotopes of the elements present in the analysis sample can be deduced. In this article, we’re going to look at the analytical instruments used in the laboratory to perform elemental analysis on an unknown substance.
Atomic Absorption Spectroscopy (AAS)
Atomic absorption spectroscopy (AAS) is a technique that provides a quantitative determination of a sample’s elemental make-up, by examining how the atoms absorb wavelengths of light. It is a technique widely used for identifying metal and semi-metal ions present in sunscreens, pharmaceuticals, food, and soil samples; as well as for clinical analyses of biological matter.
To see what is in a sample, it is first atomized into a gaseous state. This is either done with a flame or an electrothermal atomizer. Once the metals in the sample are ionized, light is shone at them, and a detector analyzes the ions; deducing which wavelengths have been absorbed.
As each element only absorbs specific wavelengths of light, this process allows the metallic elements to be worked out. To determine the quantity of each element, a calibration curve needs to generated, which can then measure the degree of absorption of a specific wavelength as a function of concentration. Once there is a calibration curve in place, the frequency of atoms in the sample can be deduced by comparing it against the reference.
Inductively Coupled Plasma (ICP)
Inductively coupled plasma (ICP) is a class of techniques, where the ICP part of an instrument is used to dissociate a sample into free atoms. To do this, an inert gas (usually argon or nitrogen) is passed through a chamber with the sample, where a radio frequency generator, electrical discharge, and a thermal torch are used to convert the sample into a plasma.
The plasma is coupled within the chamber through the generation of a magnetic field that arises from a high-frequency electrical current passing through a cooled induction coil. This creates a magnetic field with rapid oscillations.
The coil ionizes the inert gas and sample ions in the chamber, and the resulting ions then interact with the oscillating magnetic field. The inert gas ions are collided to create more ionized particles, and the electrons are accelerated to the point where an eddy current is formed. This further increases the degree of ionization in the gaseous ions and increases the internal temperature.
This process ultimately leads to the generation of a plasma at the top of the torch. Once a plasma has been created, it then travels through to an analytical analyzer. For elemental analysis, this is either mass spectrometry (MS) or optical emission spectroscopy (OES).
Inductively Coupled Plasma-Mass Spectrometry (ICP-MS)
ICP-MS is a widely used elemental analysis technique in the food and beverage, sunscreen, cosmetics, pharmaceutical, forensic and toxicology industries; as well as a trace analysis method in museums.
After forming a plasma, the ions are focused by a series of lenses in an intermediate region before entering the mass spectrometer (often a quadrupole mass spectrometer), where they are separated by their mass-to-charge ratio to determine which elements are present.
Once they have been separated, they are detected, and the detector counts the number of each element. This enables the deduction of which types of elements are present, and how many there are, in the sample.
Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES)
ICP-OES is the second type of ICP that is used for elemental analysis, and it is used throughout the pharmaceutical industry. It is also used in the wine industry for detecting the presence of heavy metals, the automotive industry, and the food industry.
This method relies on the highly ionized atoms returning to their ground state and releasing wavelengths of light. All excited atoms produce a wavelength as their energy is lowered to the ground state, and these specific wavelengths are picked up by a photodetector. The result is a photo-emission spectrum, where each type of element is determined by the wavelength emitted (and its subsequent position of the spectrum), and the number of atoms is determined by the intensity of the wavelength—i.e., a higher intensity means more atoms.
X-ray Fluorescence (XRF)
There are many types of X-ray fluorescence (XRF) that can be used to perform an elemental analysis, and these include wavelength-dispersive X-ray fluorescence (WD-XRF), energy-dispersive X-ray fluorescence (EDXRF) and total-reflection X-ray fluorescence (TXRF).
While many variations have slightly different operating and analysis principles, we’re going to look at the standard XRF instrument. XRF instruments are commonly used throughout the art and archaeological, pharmaceutical, food and beverage, nanotechnology, and medical industries.
In XRF, a sample is irradiated with high-energy X-rays, and when an atom in the sample is struck with enough energy, one of the electrons from the inner sub-shells is ejected from the nucleus. To make the atomic nucleus stable, a different electron from an outer sub-shell migrates towards the inner sub-shell that has an electron vacancy.
This means that the electron moves from a higher energy state to a lower energy state, and to compensate for this, the excess energy is released as a fluorescent X-ray. The energy of the emitted X-ray is equal to the difference between the lower and higher energy states of the electrons, and this enables the elemental composition of the atom to be deduced. The intensity of the emitted X-ray is used to determine how many atoms of each element are present in the sample.
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