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Inductively Coupled Plasma Atomic Emission Spectroscopy: Instrumentation
Inductively coupled plasma (ICP) is the common plasma source used in atomic emission spectroscopy (AES), a technique that detects and analyzes various…
Plasma emission spectroscopy uses three basic instrument types.
Sequential plasma emission spectrometers measure one emission line at a time, whereas multichannel and Fourier-transform instruments simultaneously measure multiple emission lines.
Sequential spectrometers typically use a grating monochromator, where the grating is rotated to focus different wavelengths on the exit slit.
For example, in slew scan spectrometers, the monochromator rapidly rotates to a wavelength close to one emission line and then rotates in small steps across that line.
In contrast, scanning echelle spectrometers have a moving photomultiplier tube behind an aperture plate, with the echelle grating scattering the wavelengths in a broad array that can be detected one slit at a time.
Some multichannel instruments use a grating polychromator to measure multiple emission lines simultaneously with photomultiplier tubes positioned behind fixed exit slits.
Other multichannel instruments use spectrographs for simultaneous measurements, employing two-dimensional charge-injection devices or charge-coupled devices as transducers.
Although Fourier transform spectrometers offer wide wavelength coverage, they are not widely used in AES due to their various 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.