Page 144 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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5.6 Performance

Performance characteristics for quadrupole and magnetic sector instruments are shown in Table 5.2.
The main advantages that ICP-MS has over other techniques are its low detection limits, in the 1-10 pg
-1
ml range for quadrupole instruments (Fig. 5.9), large linear dynamic range and rapid multi-element
capability. However, ICP-MS also suffers from a number of interferences.
Spectroscopic interferences arise when an interfering species has the same nominal m/z as the analyte
of interest. The interfering species can be either an isotope of another element (which are well
documented and hence easily accounted for), or a molecular ion formed between elements in the
sample matrix, plasma gas, water and entrained atmospheric gases. The molecular ions are less easy to
correct for since they will vary depending on the nature of the sample matrix. Some common molecular
ion interferences are shown in Table 5.3. Many of these interferences can be overcome by choosing an
alternative isotope of the analyte which is free from interference, although a sacrifice in sensitivity may
result. If a 'clean' isotope is not available, then one recourse is to separate the analyte from the matrix
before the analysis using chemical extraction and chromatography, or to save up and buy a magnetic
sector instrument which is capable of resolving the interfering species from the analyte. Many of these
interferences are thought to be formed in the interface due to a secondary discharge. This discharge
can be eliminated by operating the plasma at low power, typically 600 W, and modifying the torch or
RF coil. Under these so-called 'cool plasma' conditions interferences due to ArO , for example, can be
+
eliminated, although the sensitivity for refractory elements and those with high ionization potentials
may be reduced.

Non-spectroscopic interferences are caused by the sample matrix, and are manifest as an apparent
enhancement or suppression of the analyte signal in the presence of a concentrated sample matrix.
Such effects are thought to be caused primarily by space charge in the ion beam, whereby
Table 5.2 Performance characteristics for ICP-MS.
Parameter Quadrupole Magnetic sector

-1
Detection limit (pg ml ) 1-10 0.01-0.1 a
1-10 b
Linear dynamic range 10 6 10 6

10 dual mode 10 dual mode
10
9
Precision (% RSD) 1-2 1-2
Mass resolution 400 400-10,000

a Resolution 400.
b Resolution 3500.