Understanding ICP-OES
Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) is an invaluable scientific testing technique used to determine the elemental composition of a sample. ICP-OES is highly sensitive, accurate, and precise, making it a popular method in a wide range of fields, including chemistry, geology, environmental science, and materials science.
ICP-OES works on the principle that atoms and ions can absorb energy, which excites their electrons, causing them to move from the ground state to an excited state. In ICP-OES, the energy source is a plasma generated by an argon gas, operating at a temperature of 10,000 Kelvin.
As the excited electrons return to their ground state, they release energy in the form of light at very specific wavelengths. The wavelength of the emitted light is determined by the type of atom or ion and the energy levels the electron is moving between. The amount of light emitted at each wavelength is proportional to the number of atoms or ions making the transition. The Beer-Lambert law describes the relationship between the concentration of the element and the intensity of the emitted light.
To put the ICP-OES principle into practice, a sample is first introduced into the plasma, which causes the atoms and ions in the sample to become excited. The excited atoms and ions then release energy in the form of light at specific wavelengths. A detector measures the intensity of the emitted light at each wavelength. The measured intensity is compared to a calibration curve, which relates the intensity of the emitted light to the concentration of the element in a known solution. Using this calibration curve, the concentration of each element in the sample can be determined.
The versatility and precision of ICP-OES make it a crucial tool for many applications. ICP-OES is capable of detecting and quantifying a broad range of elements, from the major components to trace elements, in a variety of sample types, including solids, liquids, and gases. ICP-OES is also highly sensitive, able to detect elements at concentrations as low as parts per billion.
In conclusion, ICP-OES is a powerful analytical technique that relies on the principle that excited atoms and ions emit light at specific wavelengths. By measuring the intensity of the emitted light, ICP-OES can determine the concentration of elements in a sample, providing valuable information for research, quality control, and regulatory compliance in a variety of industries.
How does ICP-OES work?
ICP-OES instrumentation
An ICP-OES instrument operates by utilizing the ICP-OES principle to excite atoms or ions with an argon plasma. The intensity of the light emitted when the electrons in the atoms or ions return to the ground state, or a lower energy state, is then measured. Based on a calibration graph, the concentration of specific elements in a solution can be calculated.
Sample introduction
The sample introduction system starts with pumping the liquid sample into the nebulizer. The nebulizer then utilizes a stream of argon gas to turn the liquid into a fine aerosol. This aerosol then enters the spray chamber where larger droplets are removed, and the remaining aerosol continues into the plasma torch.
Plasma torch: The energy source for ICP-OES analysis
The plasma torch is the heart of the ICP-OES instrument, providing the energy source for the analysis. It is composed of three concentric glass tubes, with argon gas flowing between the two outermost tubes. An electric current is passed through the metal coil around the glass torch, creating a magnetic field that sparks a stream of argon gas to produce the plasma.
The plasma torch resembles a flame on a glass candle, with a metal coil around the candle. A spark is discharged into a stream of argon gas, creating the plasma. Energy is transferred from the metal coil around the torch to the argon gas, sustaining the plasma.
The aerosol of the sample is carried up the middle of the plasma torch by another stream of argon gas. The heat of the plasma evaporates the solvent in the sample and breaks down the sample molecules into atoms and ions. The energy from the plasma excites the electrons in the atoms and ions, moving them up to higher energy levels.
Radio-frequency generator
Radio-frequency (RF) generator is an essential component in the ICP-OES instrument, which generates the radio frequency energy that passes through the metal coils surrounding the plasma torch. The power of the RF can be controlled by the instrument operator, allowing for adjustment of the energy level of the plasma. For samples with low concentrations of elements, the RF power might need to be increased while decreasing the flow of the sample into the torch. These changes result in a slower movement of the sample aerosol through a higher energy plasma, increasing the chances of all atoms and ions emitting light and being measured. Therefore, the RF generator plays a crucial role in achieving accurate and precise results in ICP-OES analysis.
Optical Spectrometer
The optical spectrometer is responsible for analyzing the light emitted by the excited electrons as they return to a lower energy level, emitting light at specific wavelengths depending on the element and energy level. When a sample contains multiple atoms, many wavelengths of light are emitted. These wavelengths are called analytical lines or emission lines, which are determined by the energy levels of the electrons. Although any analytical line for a specific element can be used for analysis, the most intense one is typically chosen.
To direct the emitted light, mirrors and other optical components are used to guide it into the spectrometer. Inside the spectrometer, the light is separated into precise wavelengths, allowing for accurate intensity measurement. A detector then measures the separated light at each wavelength.
Detector
In recent years, there have been significant changes in the detectors employed by ICP-OES instruments. Currently, charge-coupled devices (CCD) that resemble computer chips are predominantly used. The chip's surface is divided into pixels that gauge the photons of light of varying wavelengths.
Computer Controllers
Computerized instrument control involves collecting information from the instrument and feeding it to a controlling computer, typically a standard PC running on MS Windows. Specialized control software then calculates the concentration of each element in the sample, using the pre-analyzed calibration. This software also handles statistical analysis of results, saving of instrument settings as analytical methods, and generates reports on the analysis.
Modern ICP instruments are equipped with a network of sensors and utilize advanced algorithms to monitor their own health. They can notify the user when maintenance is required or when something needs to be fixed to ensure the accuracy of the results. These ICP instruments, provide a deeper understanding of the samples being analyzed and can suggest the optimal wavelengths to choose for obtaining the correct answer every time.
ICP-OES procedure
Element Selection: Select the elements to be measured in the sample, such as sulfur (S), lead (Pb), and phosphorus (P).
Sample Preparation: Prepare solutions of the samples using conventional techniques of quantitative chemical analysis.
Calibration Solution Preparation: Prepare a set of calibrating solutions, each containing accurately known concentrations of the selected elements within the range of expected concentrations in the sample solutions.
Plasma Analysis: Deliver the calibrating solutions and sample solutions into the plasma and measure the intensities of light at appropriate analytical lines, such as 181.972 nm for S, 220.253 nm for Pb, and 213.618 nm for P.
Calibration Curve Preparation: Prepare calibration graphs for each element from the emission intensities of the calibrating solutions.
Concentration Determination: Determine the concentrations of the selected elements in each sample solution from the calibration graphs. Calculate the concentrations in the original sample from the measured concentrations of the elements in the sample solution and the known dilution factor.
Results: Present the results of the ICP-OES analysis that list the concentration of the selected elements in µg/L or mg/L.
How can metals by analyzed by ICP-OES?
The procedure for measuring metals using ICP-OES is similar to that of nonmetals such as Cl, Br, I, S, and P. The steps involved are:
Select the elemental metals to be measured in the sample, such as iron (Fe), lead (Pb), and copper (Cu).
Prepare solutions of the samples using conventional techniques of quantitative chemical analysis.
Prepare a set of calibrating solutions, each with accurately known concentrations of the analyte elements (Fe, Pb, and Cu), with the concentration range for each element in the set chosen to include the expected concentration of that element in the sample solutions (if known).
Introduce the calibration and sample solutions into the plasma and measure the intensities of light at appropriate analytical lines. For example, the emission wavelength for Fe might be 238.204 nm, for Pb 220.253 nm, and for Cu 324.754 nm.
Prepare calibration graphs for each element from the emission intensities of the calibration solutions.
Determine the concentrations of the elements in each sample solution from the calibration graphs.
Calculate the concentrations in the original sample from the measured concentrations of the elements in the sample solution and the known dilution factor.
ICP metal analysis delivers results that list the concentration of the selected elements, typically in µg/L or mg/L.
What’s the difference between ICP-OES and ICP-AES?
Inductively coupled plasma optical emission spectroscopy (ICP-OES) and inductively coupled plasma atomic emission spectroscopy (ICP-AES) refer to the same analytical technique.