The dispersive system collects light emitted from the plasma and separates the various wavelengths so they can be measured for qualitative and quantitative analysis. Collection of light should be done with the highest efficiency to ensure sensitivity of measurements. The system should allow measurement in the 160-800 nm range, and down to 120 nm for some specific applications.
Fig. 25: Czemy-Turner dispersive system.
Several dispersive systems can be used in ICP-OES: Czerny- Turner, Paschen-Runge, and Echelle optics can be found.
Czerny-Turner In this optical mounting, the dispersive system is made of two collimating mirrors and one grating (or dual grating systems mounted on a turret). The light collected from the plasma through the lens of the entrance collimating system is reflected by the entrance mirror and wavelengths are separated using the grating (usually a high density grating is used, typically 2400 to 4320 gr/mm). The second mirror focuses the light on the detector. Optical components are reduced to the minimum to improve light transmission efficiency. Czerny-Turner optics provides constant resolution over the measured spectrum.
The rotation of the grating allows coverage of the whole spectral range and ensures full wavelength coverage, i.e. all wavelengths can be accessed and measured. The movement of the grating can be done using a sine bar or a direct drive system that leads to higher speed of movement and better repeatability for the positioning.
Fig. 26: Paschen-Runge dispersive system.
Paschen-Runge optics uses a concave grating to separate wavelengths. The grating is also used as the collimating system. High density gratings are used, typically 2400 to 4343 g/mm. Light is collected from the plasma using a lens, and is dispersed using the grating that is in a fixed position. Dispersed light is focused on a circle, the Rowland circle, where all detectors are placed. Many detectors should be used to measure signals. Paschen-Runge optics provides constant resolution over the measured spectrum but does not allow full wavelength coverage due to the relative positions of the entrance slit and of the diffracted light.
Echelle optics uses an Echelle grating that is a low density grating, typically in the 50-100 gr/mm range. A system is required before or after the grating to separate the different orders that are overlapping. This order dispersion can be done using a prism or more specific dispersers. Optical components may be fixed or moving according to the design of the instrument. Light is dispersed in a two-dimensional figure called Echellogram and detection should then be done on this 2-dimension figure.
Table 1: Theoretical resolution as a function of the grating groove density and the order of measurement.
The better the groove density is, the better the resolution is. The resolution is the ability of the dispersive system to separate two narrow peaks. It is usually expressed as the full width at half maximum of the peak. High resolution helps achieve high performance for matrices containing many elements or elements emitting many lines across the spectrum.
Table 2: Practical limitation of the wavelength coverage as a function of the grating groove density and the order of measurement.
The groove density also defines the wavelength range that can be accessed. The more the density is important, the less the wavelength range is large.
When a grating diffracts light, it follows a simple rule that is: sinα+sinβ=k.n.λ with α the incident angle, β the refracting angle, k the order, n the groove density and λ the wavelength.
For a given grating (n fixed), a given position of the grating (α and β fixed), several wavelengths λ can be observed: λ in the 1st order, λ/2 in the 2nd order, λ/3 in the 3rd order…
To avoid any issue, order filters are used in spectrometers. For a given wavelength (λ fixed), a given grating (n fixed) and a given incident angle (α), one wavelength can be observed at various diffracting angles β. The order has an effect on the resolution of the system and may help to improve resolution. Usually, 1st and 2nd orders are used only as light intensity decreases with the order.
Focal length of an instrument has an influence on resolution and on the amount of light reaching the detector. The more the focal length increases, the better the resolution is, but less light reaches the detector. Practically, ICP-OES spectrometers can use up to 1 meter focal length to improve resolution without sacrificing to the detection limits. HORIBA Scientific's ICP-OES spectrometers are equipped with 1 meter focal length optics ensuring for demanding applications and light throughput.
Stability of the instrument may be affected by optics movement if repeatability of the movement is not good. Recent improvements in mechanics lead to huge improvements in repeatability of position and thus huge improvements in stability. In addition, the use of a reference line that allows checking of the position before any acquisition helps to achieve excellent repeatability and then excellent stability of the optics. HORIBA Scientific's ICP-OES spectrometers use a direct drive high precision system for grating movements and a reference line to check for the practical position vs. theoretical one.
