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By Dunja BozicSep 9 2019.jpg)
Image Credit: Rabbitmindphoto/Shutterstock.com
The elements critical for the quality and safety of a material are often present in very low concentrations. This requires highly sensitive instrumentation for the determination of elemental composition.
The term elemental analysis is typically defined as the determination of the amount of an element in a given sample, usually a weight percent.
As there are different elements in many different samples, there is a number of techniques more or less suitable for elemental analysis of a sample of interest.
Some of the most common techniques used in the laboratories today are X-ray fluorescence (XRF), absorption atomic spectroscopy (AAS), and inductively coupled plasma (ICP) techniques: ICP-optical emission spectroscopy (ICP-OES) and ICP-mass spectrometry (ICP-MS).
X-ray Fluorescence (XRF)
XRF is a non-destructive technique for the determination of elemental composition. A primary X-ray source radiates the sample causing excitation. The sample consequentially emits fluorescent (or secondary) X-ray which is measured by the instrument.
Each element produces a specific set of fluorescent X-rays. This set is unique for a given element and is thus called “a fingerprint”.
If a sample has many elements present, which is often the case, a wavelength dispersive spectrometer can be used, which allows the separation of complex emitted X-ray spectrum into wavelengths characteristic for a specific element.
Depending on the wavelength of an emitted beam, different types of detectors are suitable. Gas flow counters are commonly used for measuring long wavelengths, higher than 0.15 nm, typical of K spectra from elements lighter than Zn. The scintillation detector is suitable for shorter wavelengths. Both types can be used in tandem for the measurement of intermediate wavelength X-rays.
The intensity of the measured energy is proportional to the concentration of the element in the sample. XRF spectroscopy is thus suitable for both qualitative and quantitative analysis.
The limitations of this technique are mostly instrumental, as the majority of available instruments don’t have the ability for accurate and precise measurement of a wide range of elements, especially the ones with Z<11.
It is also “blind” for isotopes or ions of the same element in different valence states.
Nevertheless, its many advantages such as easy sample preparation, nondestructive analysis, rapid multi-element screening and a wide range of suitable matrices, from rocks through slurries to liquids, are the reason this technique is a valid analytical asset in the laboratories in many different research fields and industries.
Atomic Absorption Spectroscopy (AAS)
Atomic absorption spectroscopy (AAS) is based upon the detection of wavelengths of light absorbed by an element (usually 190 nm to 900 nm).
The AA spectrometer consists of a light source, a sample cell to atomize the sample and a detector.
As a source of light, several lamps are typically used for different elements.
There are two basic sample cells for atomization used in AAS: the flame burner and the electrothermal heating.
The amount of the absorbed light is dependent on element concentration in the sample.
If there is a sufficient amount of the element of interest in the sample, the flame cell (FAAS) can be used. This is a rapid technique and very simple to use. Its sensitivity is typically in the parts per million (ppm) range.
For trace analysis, electrothermal heating (ETAAS), also known as graphite furnace (GFAAS), can be used instead of a flame burner to increase the sensitivity. ETAAS requires more skill and is less rapid, but has lower detection limits and is more suitable for low concentrations of the element in the sample.
Inductively Coupled Plasma - Optical Emission Spectroscopy (ICP-OES)
Inductively coupled plasma spectrometry is a technique used for elemental analysis and trace analysis. The sample is injected into argon gas plasma in a liquid form. Solid samples require a preparation step prior to injection, such as extraction or acid digestion, but liquid and gas samples can be injected directly.
The sample solution gets converted to an aerosol which is quickly vaporized by ICP at a temperature of approximately 10 000 K. Elements are liberated as free atoms and possibly converted to ions. Both atoms and ions are promoted to the excited state. The photons emitted from these species are measured by optical spectrometry.
ICP-OES instruments mostly detect only a single wavelength at a time with a monochromator. If an element emits at several wavelengths, sequential scanning is a suitable solution. A polychromator designed to capture multiple wavelengths at a time can also be used.
Low detection limits ranging from parts per million (ppm) to parts per billion (ppb) are the main advantage of the ICP-OES technique.
The high temperature of the ICP allows excellent atomization and excitation of various species. This feature allows simultaneous analysis of more than 60 elements, which makes this way superior to the techniques such as FAAS.
Inductively Coupled Plasma - Mass Spectrometry (ICP-MS)
An ICP-MS combines a high-temperature inductively coupled plasma (ICP) source with a mass spectrometer (MS). Samples are introduced into an argon plasma in the form of aerosol drops. The aerosol is dried, the molecules dissociated and an electron removed from the components. The resulting singly-charged ions are filtered in the mass spectrometer. At a given time only one mass-to-charge ratio passes through the MS to the detector.
The intensity of a resulting pulse in the detector is proportional to the concentration of the element.
One of the great advantages of the ICP-MS technique is the ability to measure the individual isotopes of each element. The other is extremely low detections limits of one part per trillion (ppt). This technique is also relatively free from interferences which, if do exist, can be easily removed.
Source and Further Reading
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