Inductively coupled plasma optical emission spectroscopy (ICP-OES) is an elemental analysis technique that derives its analytical data from the emission spectra of elements excited within a high-temperature plasma.
The sample (typically a liquid) is introduced into the plasma and the optical system (spectrometer) is used to separate element-specific wavelengths of light and to focus the resolved light onto the detector as efficiently as possible. The spectrometer is comprised of two sections, the fore-optics, and either a mono- or polychromator. When the light exits the mono- or polychromator, it is focused onto the detector and the derived signals are processed to quantify the elemental composition.
An ICP-OES instrument consists of four basic components: the sample introduction system, excitation source (plasma), spectrometer (for wavelength selection), and detector (Figure 1).
Before emission can occur, the solvent in which the sample is dissolved must be evaporated. Also, the remainders of the sample must be vaporized and existing molecules split into atoms. These are all tasks of the plasma. Exactly what is a plasma?
An ICP-OES plasma is a gas, which has been significantly ionized inside an oscillating radio frequency (RF) field. The RF field causes the gaseous ions to oscillate with the field and this results in extreme heat. The temperature developed inside a plasma can be as high as 10,000°C. One example of a plasma-like state in nature is lightning. In space, stars like the sun are largely composed of plasma.
Generation of the plasma is performed in the plasma "torch" that consists of three quartz tubes: the outer torch tube, the auxiliary tube, and the injector tube (Figure 2). Between the outer tube and the auxiliary tube, a tangential cool gas flow is introduced. This gas contains the plasma and keeps it away from the outer torch tube, protecting it from melting down. The auxiliary gas flow is used to elevate the bottom of the plasma from the injector tube. The sample aerosol is introduced into the plasma via a thin injector tube that typically has an aperture of 1–2 mm; through this opening, a thin jet of sample aerosol is emitted and "punches" a hole into the center of the plasma.
An efficient and powerful radio frequency (RF) generator helps develop and maintain the plasma, which is capable of completely dissociating almost any sample matrix, thereby reducing oxide formation and other chemical interferences to a minimum.
Challenges during the development of an RF generator may include size, robustness, efficiency, reliability, and ease-of-service.
The stability of an RF generator depends largely on its ability to adjust for changing conditions in the plasma due to different samples or sample matrices; this is facilitated by switching power conditions to suit the variations, a phenomenon also called ‘matching’.
Historically, there have been two main approaches to the control and matching of RF generators: crystal controlled, and free-running. Crystal-controlled generators match and lock the frequency of the RF generator to the oscillation of a reference crystal. Free-running generators match the power generated to the power required by the plasma and allow the frequency to vary slightly.
In ICP-OES, the plasma can usually be observed in two ways:
The radial plasma view offers less sensitivity than the axial view; however, it is preferable when analyzing difficult samples such as organics or very high amounts of dissolved solid matrices. The axially viewed plasma offers greater sensitivity than radial view. However, because the plasma is viewed along its entire length, the quantity of light observed from both analyte and background emissions is increased. Therefore, this view has higher susceptibility to spectral interferences.
There are typically two torch configurations available in ICP-OES systems, each realized through an individual instrument - A dedicated radial view instrument and a dual view instrument with both axial and radial observation options. In dual-view systems, the torch is characteristically oriented horizontally. In this design, heat and fumes from the plasma are extracted to the top and the mirrors in the fore-optics stay clean and unbiased by heat. This configuration is used for relatively clean aqueous samples.
A dedicated radial view instrument has a vertical torch and is often used for complicated matrices and organic samples because its torch configuration is more robust.
When it comes to UV wavelength analysis by ICP-OES, it is imperative that optical pathways leading into the spectrometer be purged. This is performed by purging the cones used for light collection with the gas flow originating from the spectrometer (Figure 3).
Most commonly, echelle-based optical designs are applied to produce the typical emission spectrum of an ICP-OES. These consist of an echelle grating and a prism, and multiple focusing mirrors.
Instead of a monochromator, where only one wavelength is observed at a time, a polychromator can be used to simultaneously determine multiple elements in a sample. This simultaneous detection improves stability of the analysis and reduces sample consumption and total analysis time considerably.
As the light from the plasma enters the polychromator, it is selectively focused through an entrance slit. Once the light has entered the system, it is focused onto the prism. The prism separates the light by wavelength in a single dimension and at low resolution. An echelle grating orders the separated light from the prism in a second dimension. This creates a high-resolution, two-dimensional spectrum, known as an echellogram. After passing through these optical elements, a mirror collects and focuses the now completely dispersed spectrum onto the detector.
Historically, light intensity was measured with photomultiplier technology. Currently, solid-state charge transfer devices (CTDs) are the detectors of choice for ICP-OES and have almost completely replaced photomultiplier tubes. Depending on the structure of the detector and the way in which signals are processed, two types of CTDs are used: charge injection devices (CIDs), and charge coupled devices (CCDs).
CIDs allow for individual, random access, and pixel-by-pixel integration due to the readout electrodes being located at every pixel site. This process can be carried out non-destructively and the CID has excellent anti-blooming capability. Such capability allows for an optimum signal-to-noise ratio at each integration, enabling both intense and weak light emissions to be recorded simultaneously. CIDs are composed of a light-sensitive surface that is subdivided into several thousand pixels, each of which is individually addressed by column and row electrodes (Figure 6) and permits thorough collection and readout of signals.
CIDs usually have full wavelength coverage of 160 nm to 900 nm. Fullframe imaging captures all of the data from the CID, regardless of the method elements specified. This Fullframe is then stored and can be used for retrospective analysis, batch analysis, or contamination identification of samples. These features allow the analyst to subtract Fullframes from each other, which is particularly useful for matrix stripping and contamination identification, producing more accurate results.
Conversely, a CCD sequentially transfers the charge from each pixel site to a buffer and then to an output register. Pixels may be processed either by rows or segments. During the process of reading a CCD, the charge in the pixel is destroyed.
Not only the right instrumentation for accurate and precise measurement is of great importance but also how the instrument control and data is handled. This is taken over by specific software for the ICP-OES instrumentation. The Software for ICP-OES needs to manage the instrument and accessory control, is the tool for data acquisition, evaluation and post-analysis data handling.
ICP-OES software typically comprises a range of features. First of all the instrument has to be started for which various routines like performance checks can be applied. Then for accurate results, a background correction technique has to be applied. This could be synchronous background correction next to the peak or fitted one. Drift control is very important when it comes to complex and varying sample matrices. Correction for drift is typically done by referencing every analyte to an internal standard element. A blank, containing the matrix of the samples is the reference point. Whenever direct spectral interferences occur, an interference correction factor has to be calculated and applied to all samples.
Another, sample preparation related, feature is the special blank correction. Typically a preparation blank is prepared when for example doing sample digestions and this is automatically subtracted from the sample result.
Quality control samples are run to observe the performance of the method and can be used to automate sample re-analysis due to certain parameters like for example the internal standard being out of range.
And finally an ICP-OES software is capable of creating exports and reports to the user’s need for example to feed the results into an existing LIMS system.
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