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Spectroscopie d'émission atomique à plasma inductif : instrumentation
Le plasma inductif (ICP) est la source de plasma couramment utilisée dans la spectroscopie d'émission atomique (AES), une technique qui détecte et ana…
La spectroscopie d’émission de plasma utilise trois types d’instruments de base.
Les spectromètres d’émission plasma séquentiels mesurent une raie d’émission à la fois, tandis que les instruments multicanaux et à transformée de Fourier mesurent simultanément plusieurs raies d’émission.
Les spectromètres séquentiels utilisent généralement un monochromateur à réseau, où le réseau est tourné pour focaliser différentes longueurs d’onde sur la fente de sortie.
Par exemple, dans les spectromètres à balayage orienté, le monochromateur tourne rapidement à une longueur d’onde proche d’une raie d’émission, puis tourne par petits pas sur cette ligne.
En revanche, les spectromètres à échelle à balayage ont un tube photomultiplicateur mobile derrière une plaque d’ouverture, le réseau à échelle dispersant les longueurs d’onde dans un large réseau qui peut être détecté une fente à la fois.
Certains instruments multicanaux utilisent un polychromateur à réseau pour mesurer plusieurs raies d’émission simultanément avec des tubes photomultiplicateurs positionnés derrière des fentes de sortie fixes.
D’autres instruments multicanaux utilisent des spectrographes pour des mesures simultanées, en utilisant des dispositifs d’injection de charge bidimensionnels ou des dispositifs à couplage de charge comme transducteurs.
Bien que les spectromètres à transformée de Fourier offrent une large couverture de longueur d’onde, ils ne sont pas largement utilisés dans l’AES en raison de leurs diverses limitations.
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Q1: What are the three main types of ICP-AES instruments?
ICP-AES instruments fall into three categories: sequential, multichannel, and Fourier transform types. Sequential instruments measure one emission line at a time by rotating a monochromator to focus different wavelengths on an exit slit. Multichannel instruments simultaneously measure multiple emission lines using either a polychromator with photomultiplier tubes or a spectrograph with charge-coupled devices. Fourier transform spectrometers offer wide wavelength coverage but are rarely used in atomic emission spectroscopy due to various limitations.
Q2: How does a grating monochromator work in sequential spectrometers?
A grating monochromator uses a holographic grating with 2400 or 3600 grooves per millimeter. A digitally controlled stepper motor rotates the grating to focus different wavelengths sequentially and precisely on the exit slit. This allows sequential spectrometers to measure one emission line at a time by positioning the grating so that each desired wavelength aligns with the exit slit for intensity measurement.
Q3: What is the difference between slew-scan and scanning echelle spectrometers?
Slew-scan spectrometers rapidly rotate the monochromator to a wavelength near an emission line, then scan across that line in small steps to minimize time spent in non-useful wavelength regions. Scanning echelle spectrometers use a different approach: they employ an echelle grating that scatters wavelengths in a broad array, with a moving photomultiplier tube behind an aperture plate detecting wavelengths one slit at a time.
Q4: How do multichannel instruments measure multiple elements simultaneously?
Multichannel instruments use either a polychromator or spectrograph for simultaneous detection. Polychromators contain multiple photomultiplier tubes positioned behind fixed exit slits along the focal curve of a grating, each detecting a different emission line. Spectrographs employ two-dimensional charge-injection devices or charge-coupled devices as transducers to capture multiple wavelengths at once, enabling rapid analysis of multiple elements.
Q5: What are the advantages and disadvantages of sequential versus multichannel ICP-AES?
Sequential instruments are more straightforward and cost-effective initially but require more time and sample consumption since they measure each element individually. Multichannel instruments measure multiple elements simultaneously or nearly so, reducing analysis time and sample usage, making them more efficient for routine analyses. The choice depends on analytical throughput needs and budget constraints for specific applications.
Q6: Why are Fourier transform spectrometers rarely used in atomic emission spectroscopy?
Although Fourier transform spectrometers offer significant advantages including wide wavelength coverage, high speed, high resolution, accurate wavelength measurements, large dynamic range, compact size, and large optical throughput, they are not widely used in atomic emission spectroscopy due to various limitations. These limitations outweigh their benefits for most AES applications, making traditional grating and echelle spectrometers more practical choices.
Q7: What detection devices are used in multichannel spectrographs?
Multichannel spectrographs employ two-dimensional transducers for simultaneous wavelength detection. The primary devices used are charge-injection devices and charge-coupled devices, which capture multiple emission lines across a broad wavelength range at once. These two-dimensional detectors enable rapid, simultaneous measurement of multiple elements, making spectrographs ideal for high-throughput analytical applications.