Instant Notes: Analytical Chemistry

focused on it, and hydride generation (Topic E7) are also used. Electrothermal

vaporization may also be employed for solids. As detailed in Topic E4, the

sample undergoes a sequence of processes to generate excited atoms.

The optics are aligned with the base of the plume where atomic relaxation is

most prevalent. The emitted radiation from the ICP torch is focused into the

monochromator and detected by a photomultiplier tube (PMT) or ‘polychro-

mator’ detector. The output is then processed and displayed under computer

control as the inductively coupled plasma-atomic emission spectrum (ICP-AES).

ICP-AES can detect a greater number of elements at low concentrations

than other atomic emission or atomic absorption techniques. For example,

at 1-10 ppb ICP-AES can measure over 30 elements, while AES and AAS are

restricted to around ten.

By extracting the atoms from the cooling plasma, the high sensitivity and selec-

tivity of the mass spectrometer (see Topic E14) may be exploited. Figure 2shows

a schematic of an ICP-MS system.

A horizontal ICP torch is placed next to a water-cooled aperture placed in the

sampling cone. The sample, initially at atmospheric pressure, is skimmed down

through water-cooled nickel cones through small orifices into progressively

lower pressure regions until the sample ions enter the mass spectrometer (see

Topic E14).

Usually a quadrupole mass spectrometer is used, but double focusing instru-

ments are also possible. Two modes of operation are employed. Either the mass

spectrometer may be set to select a single m/zratio and monitor a single ion, or

the mass spectrum may be scanned to provide a complete overview of all m/z

ratios and ions.

Since the ICP torch can produce ions as well as atoms from the sample, it

provides a ready source for the mass spectrometer. Problems may arise due to

interferences.

● Isobaric interference occurs where different elements produce ions of the

same m/zratio, for example at m/z =40, Ca and Ar both produce abundant

ions, as does^40 K. At m/z=58,^58 Ni and^58 Fe mutually interfere.

● Polyatomic interference occurs when molecular species, or doubly charged

Inductively
coupled plasma-
mass
spectrometry
(ICP-MS)

E5 – Inductively coupled plasma spectrometry 211

Slide valve

Skimmer cone

Pressure

~ 0.0001 mbar ~ 2.5 mbar

ICP

Sampling cone

Fig. 2. Schematic of an ICP-MS system. Reproduced from J. Baker, Mass Spectroscopy,

1999, 2nd edn, with permission from Her Majesty’s Stationery Office.