14.13
Induktiv gekoppelte Plasma-Atomemissionsspektroskopie: Instrumentierung
Induktiv gekoppeltes Plasma (ICP) ist die meistverwendete Plasmaquelle für die Atomemissionsspektroskopie (AES), eine Technik, die verschiedene Elemen…
Bei der Plasmaemissionsspektroskopie werden drei grundlegende Gerätetypen verwendet.
Sequentielle Plasmaemissionsspektrometer messen jeweils eine Emissionslinie, während Mehrkanal- und Fourier-Transformationsinstrumente gleichzeitig mehrere Emissionslinien messen.
Sequentielle Spektrometer verwenden in der Regel einen Gittermonochromator, bei dem das Gitter gedreht wird, um verschiedene Wellenlängen auf den Austrittsspalt zu fokussieren.
Bei Slew-Scan-Spektrometern dreht sich der Monochromator beispielsweise schnell auf eine Wellenlänge nahe einer Emissionslinie und dreht sich dann in kleinen Schritten über diese Linie.
Im Gegensatz dazu haben Rasterechelle-Spektrometer ein bewegliches Photomultiplierrohr hinter einer Aperturplatte, wobei das Echelle-Gitter die Wellenlängen in einem breiten Array streut, das schlitzweise detektiert werden kann.
Einige Mehrkanalgeräte verwenden einen Gitter-Polychromator, um mehrere Emissionslinien gleichzeitig mit Photomultiplier-Röhren zu messen, die hinter festen Ausgangsschlitzen positioniert sind.
Andere Mehrkanalgeräte verwenden Spektrographen für gleichzeitige Messungen und verwenden zweidimensionale Ladungsinjektionsgeräte oder ladungsgekoppelte Geräte als Wandler.
Obwohl Fourier-Transformationsspektrometer eine breite Wellenlängenabdeckung bieten, werden sie aufgrund ihrer verschiedenen Einschränkungen in der AES nicht häufig eingesetzt.
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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.