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Espectroscopia de Emissão Atômica com Plasma Indutivamente Acoplado: Instrumentação
O plasma indutivamente acoplado (ICP) é a fonte de plasma comumente utilizada na espectroscopia de emissão atômica (AES), uma técnica que detecta e an…
A espectroscopia de emissão de plasma usa três tipos básicos de instrumentos.
Os espectrômetros sequenciais de emissão de plasma medem uma linha de emissão por vez, enquanto os instrumentos multicanal e de transformada de Fourier medem simultaneamente várias linhas de emissão.
Os espectrômetros sequenciais normalmente usam um monocromador de grade, onde a grade é girada para focar diferentes comprimentos de onda na fenda de saída.
Por exemplo, em espectrômetros de varredura de giro, o monocromador gira rapidamente para um comprimento de onda próximo a uma linha de emissão e, em seguida, gira em pequenos passos através dessa linha.
Em contraste, os espectrômetros echelle de varredura têm um tubo fotomultiplicador móvel atrás de uma placa de abertura, com a grade echelle espalhando os comprimentos de onda em uma ampla matriz que pode ser detectada uma fenda de cada vez.
Alguns instrumentos multicanais usam um policromador de grade para medir várias linhas de emissão simultaneamente com tubos fotomultiplicadores posicionados atrás de fendas de saída fixas.
Outros instrumentos multicanais usam espectrógrafos para medições simultâneas, empregando dispositivos bidimensionais de injeção de carga ou dispositivos de carga acoplada como transdutores.
Embora os espectrômetros com transformada de Fourier ofereçam ampla cobertura de comprimento de onda, eles não são amplamente utilizados no AES devido às suas várias limitações.
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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.