14.13 : 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 elements in a sample. This method is often called inductively coupled plasma atomic emission spectroscopy (ICP-AES).

There are three main types of inductively coupled plasma atomic emission spectroscopy (ICP-AES) instruments: sequential, simultaneous multichannel, and Fourier transform instruments, with the latter being less commonly used. Sequential ICP-AES analyzes each element individually, where the instrument is programmed to move from one element's line to another, pausing a few seconds at each to measure line intensities satisfactorily. On the other hand, multichannel instruments are designed to measure the intensities of emission lines for multiple elements simultaneously or nearly so. While sequential instruments are more straightforward, they require more time and sample consumption, making them costlier in the long run.

Sequential and multichannel emission spectrometers can utilize either a classical grating spectrometer or an echelle spectrometer. Grating monochromators, often featured in sequential instruments, use a holographic grating with 2400 or 3600 grooves per millimeter. The grating is rotated with a digitally controlled stepper motor to focus different wavelengths sequentially and precisely on the exit slit. Slew-scan spectrometers are sequential instruments that scan very rapidly to a wavelength near a line of interest before slowing down to scan across the line in small steps. This method minimizes time spent in non-useful wavelength regions.

On the other hand, an echelle spectrometer can operate as either a scanning instrument or a simultaneous multichannel spectrometer. Simultaneous multichannel instruments incorporate either a polychromator or a spectrograph. Polychromators contain a series of photomultiplier tubes for detection, but spectrographs use two-dimensional charge-injection devices (CIDs) or charge-coupled devices (CCDs) as transducers. In some multichannel emission spectrometers, photomultipliers are located behind fixed slits along the focal curve of a grating polychromator.

Though not widely used in AES, Fourier transform spectrometers offer benefits like wide wavelength coverage, speed, high resolution, highly accurate wavelength measurements, large dynamic range, compact size, and large optical throughput. Overall, AES instruments provide different capabilities, from sequential scanning to simultaneous multichannel detection and Fourier transform analysis, enabling researchers and analysts to choose the most suitable approach for their specific analytical needs.