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Espectroscopia de emisión atómica con plasma acoplado inductivamente: instrumentación
El plasma acoplado inductivamente (ICP) es la fuente de plasma común que se utiliza en la espectroscopia de emisión atómica (AES), una técnica que det…
La espectroscopia de emisión de plasma utiliza tres tipos básicos de instrumentos.
Los espectrómetros de emisión de plasma secuencial miden una línea de emisión a la vez, mientras que los instrumentos multicanal y de transformada de Fourier miden simultáneamente múltiples líneas de emisión.
Los espectrómetros secuenciales suelen utilizar un monocromador de rejilla, en el que la rejilla se gira para enfocar diferentes longitudes de onda en la rendija de salida.
Por ejemplo, en los espectrómetros de barrido giratorio, el monocromador gira rápidamente a una longitud de onda cercana a una línea de emisión y luego gira en pequeños pasos a través de esa línea.
Por el contrario, los espectrómetros echelle de barrido tienen un tubo fotomultiplicador móvil detrás de una placa de apertura, con la rejilla echelle dispersando las longitudes de onda en una amplia gama que se puede detectar una rendija a la vez.
Algunos instrumentos multicanal utilizan un policromador de rejilla para medir múltiples líneas de emisión simultáneamente con tubos fotomultiplicadores colocados detrás de ranuras de salida fijas.
Otros instrumentos multicanal utilizan espectrógrafos para mediciones simultáneas, empleando dispositivos bidimensionales de inyección de carga o dispositivos de carga acoplada como transductores.
Aunque los espectrómetros de transformada de Fourier ofrecen una amplia cobertura de longitud de onda, no se utilizan ampliamente en AES debido a sus diversas limitaciones.
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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.