One of the primary applications of optical emission spectroscopy based on inductively coupled plasmas (ICP OES) is to analyze elements in very low concentrations. If you consider metals such as mercury or metalloids like arsenic, then the reason for these analyses is not hard to understand. Soil sample investigations, foodstuff testing and drinking water analyses are all carried out for the health of people and the environment, but the environmental sector is just one of many important applications for ICP OES. Today, ICP OES plays a crucial role in other fields as well. Trace elements in fuels can cause premature wear on engines. The slightest impurities in metals can compromise the most important properties of those materials. High-frequency tests with low detection limits have therefore become the standard in many fields.
ICP OES takes on this role, superseding other methods of elemental analysis. Short analysis times, multi-element detection and exceptional device sensitivity are hallmarks of this method. The latest generation of ICP OES devices makes elemental analysis even more user-friendly, economical, and flexible.
Atomic spectroscopy basics
Atoms absorb light of the same wavelengths that they themselves emit when luminescing. Because atoms can only absorb energy in specific quanta, the light they emit consists of different frequencies. Consequently, by looking at the spectrum of the absorbed or emitted light, it is possible to distinguish atoms clearly – two points of departure for elemental analysis.
Absorption or emission for atomic spectroscopy?
Although these theoretical fundamentals were discovered as early as the 19th century, they have only been employed in analysis since the middle of last century. It wasn't until 1955 that the Australian Alan Walsh applied the theory of light absorption by atoms to chemical analysis. The idea: Atomic absorption spectroscopy (AAS), which entails measuring the concentration of elements in a sample by using light absorption. The first AAS devices hit the market about a decade after that. These atomize samples in a flame, known as the atomizer unit, in front of a light source of a specific wavelength. Measuring the intensity of the light before and after the atomization unit gives an absorption signal that provides information about the content of analytes in the sample.
Atomic emission spectroscopy (AES), more commonly known as optical emission spectroscopy (OES), takes the other route. It relies on the fact that excited atoms emit electromagnetic radiation. Based on the type of external energy input, OES with inductively coupled plasma (ICP OES) can be distinguished from other variants.
The principle of operation behind ICP OES
The essential characteristic is in the name itself: ICP OES uses an inductively coupled plasma to excite the atoms in the sample. A generator supplies argon gas with enough energy for it to jump to the next physical state. With a high particle density, the plasma reaches temperatures of 5,000 to 10,000 K. The measurement solution is then injected into the plasma. It takes a fraction of a second for the sample to dry out, melt and finally to vaporize. The gas molecules, which are now also being excited, are then atomized and ionized. They emit the electromagnetic radiation that is used for the actual analysis. Transfer optics direct the radiation to another optical component that can separate out the various wavelengths. After the waves are split, a detector registers the intensity of each wavelength, which is proportional to the concentration of the respective analyte. As the analysis depends on the correlation between the light intensity and the element concentration, calibration standards come into play. These calibration standards help to derive a mathematical function which relates radiation to concentration.
Structure of an ICP OES device
Generate the plasma, insert the sample to excite the atoms, then split and measure the resulting radiation: Those are the most important steps for ICP OES in a nutshell. The following overview translates these steps to the technical components of an ICP OES device.
- Plasma torch: This component is responsible for maintaining the plasma. The torch also has an injector which introduces the previously atomized sample into the plasma from the upstream sample insertion system.
- High-frequency generator: The plasma is coupled to a high-frequency generator responsible for feeding energy to the plasma. It operates at frequencies of 27 or 40 MHz, which are reserved internationally for this purpose.
- Sample insertion system: Many components come together here. A peristaltic pump delivers the measurement solution to the atomizer. There, a gas stream disintegrates the fluid into droplets. The downstream spray chamber serves to remove larger droplets from the resulting aerosol. The injector in the torch is connected to this component.
- Transfer optics: The transfer optics are not responsible for separating the wavelengths. They only serve to transmit the radiation to the dispersing optics.
- Monochromator/ polychromator: Separation into the component wavelengths can be accomplished in two ways: either sequentially (monochromator) or simultaneously (polychromator). With ICP OES it is critical that the optics can distinguish adjacent lines that are extremely close to each other.
- Detector: A CID or CCD sensor is used to detect the signals. The incident light on the sensor induces a change in charge which is processed as a signal.
The dispersing optics, i.e. the monochromator or polychromator, have a major impact on the quality of results obtained from ICP OES. Measurement sensitivity and accuracy under real conditions depend heavily on the optics. Polychromators have virtually no moving parts, which can result in faster analyses and more stable operation. Sequential devices, on the other hand, have a better spectral resolution. There are also spectrometers which combine the benefits of both, known as scanning array spectrometers. Here (in devices like the PlasmaQuant 9100 Series), the wavelengths are focused sequentially as in a traditional sequential spectrometer. However, the system also measures a spectral range surrounding the analysis line. The duplex monochromator is a cornerstone in the superior performance of these instruments, particularly benefitting those users with samples that have line-heavy matrices.
Detection requirements for trace elements are continually increasing. Low detection limits and short analysis times, and all at low operational expenses: Many laboratories face the challenge of reconciling these divergent requirements. Its wide range of applications demonstrates that ICP OES is employed in practically every sector:
- Oil and gas: Political regulations greatly impact fuels and their use. For example, gasoline is subject to ever-stricter limits on harmful elements like lead and advanced engine technologies are also pushing up requirements on the quality of fuel. Elements like vanadium, iron and nickel can damage components through corrosion processes. Volatile organic compounds like gasoline are therefore one of the toughest sample matrices for elemental analysis. Soot formation, commonly observed here, is one of the core challenges for ICP OES. However, state-of-the-art ICP OES devices with a high plasma power can handle the analyses of these matrices with high sensitivity and excellent long-term stability.
- Food and agriculture: Cooking oils and fats are important in the food sector, and also play an important role in the cosmetics industry. Care is required in the manufacturing process: Secondary processing steps come after the seed oils are pressed. These are used to adjust desired properties in terms of odor, taste or shelf life, for example. Chemical changes in the product occur during the process, which makes it important for analytical testing to follow the product through the manufacturing process. It ensures that the content of undesired trace elements is within acceptance criteria. For example, iron and copper stemming from the manufacturing process can reduce the shelf life of oil. The same goes for beverage manufacturing: In breweries, one use for ICP OES is to monitor copper impurities from the brewing kettle. The resulting metallic aftertaste is not exactly a customer favorite.
- Power plants and energy: In Europe, there is no getting around the shift to renewable energy. It is steadily becoming a larger proportion of the energy mix. Cogeneration plants enjoy flexibility in their choice of fuel, but when using alternative renewable fuels, the limits on the concentrations of potassium or sodium impurities have to be observed. Otherwise, these alkali metals can cause severe corrosion on the blades of the gas turbines. Scientists use ICP OES to help exploit renewable energy sources in sustainably sourced energy generation.
- Geology, mining and metallurgy: The elements in the group of rare earth metals are indispensable materials in technology products in every field. Batteries and electric motors are among the most prominent applications. But increasing demand is threatening a stable supply, making it necessary to extract even difficult-to-reach deposits. The challenge begins before extraction even starts, however, as detecting the desired elements in the first place is very difficult. Matrix-rich rock samples have to be analyzed reliably. Two characteristics of modern ICP OES instruments are especially in demand here: Stable plasma and a high-resolution optical system. Stringent requirements also apply to application in metal production, where even the slightest amount of trace elements can significantly alter the performance of high-tech materials. High-resolution ICP OES devices are a must-have here as well, guaranteeing reliable measurement of impurities.
- Chemistry and materials science: Urea is a breakdown product from metabolism; a synthetic form of it is used in cosmetics. Urea is also used to scrub exhaust gases from combustion. Using selective catalytic reduction, it is possible to target the reduction of nitrogen oxides. For the treatment of exhaust gases from diesel motors using urea, standards define upper limits for the content of iron, copper or zinc to the order of one-tenth of a milligram per kilogram. Other applications go even further in their purity requirements. Their quality control requirements are putting more pressure on contract laboratories to detect these trace amounts continuously and precisely. For ICP OES devices, this means that they must combine high sensitivity with high long-term stability.
Simple and effective are the traits which qualify AAS for use in even the most unfavorable conditions. AAS instruments will always have a role in routine analysis thanks to their robustness and user-friendliness. Complex laboratory analyses with higher accuracy requirements, on the other hand, are largely reliant on ICP OES.
At least that is the current state of play. Modern ICP OES devices are increasingly finding their way into routine analysis tasks. Offering innovative solutions and better user-friendliness, they are suitable for this area of application as well. Unlike AAS, ICP OES devices with lower detection limits have no need for flammable gases. As a result, these devices can run with minimal supervision even in shift operation. A high level of potential automation is another driver of this trend. Finally, their wide working range with minimal sample dilution and high matrix tolerance contribute to the attractiveness of ICP OES devices.
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