Page 120 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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CCD detector consists of 224 linear photodetector arrays on a silicon chip with a surface area of 13 x
18 mm (Fig. 4.16). The array segments detect three or four analytical lines of high analytical sensitivity
and large dynamic range and which are free from spectral interferences. Each subarray is comprised of
pixels. The pixels are photosensitive areas of silicon and are positioned on the detector at x -y locations
that correspond to the locations of the desired emission lines generated by an echelle spectrometer. The
emission lines are detected by means of their location on the chip and more than one line may be
measured simultaneously. The detector can then be electronically wiped clean and the next sample
analysed. The advantages of such detectors are that they make available as many as ten lines per
element, so lines which suffer from interferences can be identified and eliminated from the analysis.
Compared with many PMTs, a CCD detector offers an improvement in quantum efficiency and a lower
dark current.

Q. How is amplification achieved in a photomultiplier tube?

Q. What type of photomultiplier would you choose to determine arsenic at the 197 nm line?

Q. What is dark current?

Q. Why are CCD detectors used widely in multi-element analysis?

4.4.7 Data Handling

Effective signal handling has become an increasingly important feature of ICP-AES instruments owing
to the large amount of data produced. The signal output from the detector is usually amplified,
converted into a digital signal with an analogue-to-digital converter (ADC) and input into a
microcomputer. This facilitates rapid and efficient handling of the data, and performs such tasks as
logging of samples, analytical wavelengths and plasma operating conditions, control of an autosampler
for unattended operation, calibration and calculation of the results and report writing. Long gone are the
days of chart recorders and analogue readout meters, although these still have their place in the research
laboratory—a fact often ignored by the manufacturers of modern instruments.

4.4.8 Performance Characteristics

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ICP-AES is characterized by low detection limits of the order of 1-100 ng ml , as shown in Fig. 4.17,
because of its high excitation temperature compared with the flame. It has a large linear dynamic
range