NDUCTIVELY COUPLED PLASMA SPECTROMETRY

E5 I NDUCTIVELY COUPLED PLASMA SPECTROMETRY

Key Notes

Principles

A gas plasma provides a very high temperature excitation source for atomic spectrometry. Quantitative analysis for a large number of elements may be achieved rapidly. By combination with a mass spectrometer, individual isotopes may be identified and quantified.

Inductively coupled

A high-voltage discharge into an argon flow creates a plasma, which is

plasmas

sustained by induction heating due to the field of a radiofrequency coil. The sample solution is nebulized into the plasma. The emitted radiation is analyzed using a monochromator and photomultiplier detector.

Inductively coupled

If part of the sample stream from the plasma is directed into a mass

plasma-mass

spectrometer, the resulting mass spectrum is used to analyze for elements

spectrometry

and to determine isotopic ratios.

Applications

Over 70 elements may be determined using these techniques, many down to ultra-trace levels.

Related topics

Flame atomic emission

Mass spectrometry (E14)

spectrometry (E4) Atomic absorption and atomic fluorescence spectrometry (E7)

Principles When heated to temperatures above 6000 K, gases such as argon form a plasma - that is a gas containing a high proportion of electrons and ions. The plasma may be produced by a DC arc discharge or by inductive heating in an induc- tively coupled plasma (ICP) torch.

Discharge of a high voltage from a Tesla coil through flowing argon will provide free electrons which will ‘ignite’ the gas to a plasma. If the conducting plasma is enclosed in a high frequency electromagnetic field, then it will accel- erate the ions and electrons and cause collisions with the support gas, argon, and the analyte. The temperature rises to around 10 000 K. At such tempera- tures, energy transfer is efficient and the plasma becomes self-sustaining. It is held in place by the magnetic field in the form of a fireball. The sample aerosol enters the fireball at high speed and is pushed through it, becoming heated and emerging as a plume, which contains the sample elements as atoms or ions, free of molecular association. As they cool to around 6000-7000 K, they relax to their ground state and emit their characteristic spectral lines. This technique is known as ICP-atomic emission spectrometry (ICP-AES) or sometimes as ICP-optical emission spectrometry (ICP-OES).

If part of the plume is diverted into a mass spectrometer, the isotopic masses

Section E – Spectrometric techniques

of individual elements present may be identified. This is the technique of ICP- mass spectrometry (ICP-MS).

Quantitative measurements are possible with both ICP-AES and ICP-MS. Inductively

Argon gas is supplied at 10-15 l min -1 through the three concentric quartz tubes coupled plasmas

of the torch, shown in Figure 1(a). The tangential flow of gas in the outer tube contains the plasma, while the central tube carries the nebulized sample droplets suspended in argon.

The plasma is established by high-voltage ignition and sustained by the magnetic field of the radiofrequency generator providing 2 kW of power at about 27 MHz. The sample is pumped into the nebulizer and the finest droplets carried forward by the gas, while other, larger drops flow to waste from the spray chamber. Viscous solvent systems should be avoided. High-solids nebu- lizers, where particulate matter and slurries are introduced into the ICP, have been developed. Laser ablation, where the sample is vaporized by a laser

(a) Plume

Tunnel

Fireball

RF coil

Sapphire jet

Concentric quartz tubes

Auxiliary argon Leakage argon

Sample aerosol in argon

Radio frequency

Spectrometer

generator

ICP PMT torch

To waste

Fig. 1. (a) The ICP torch. (b) Schematic of an ICP-AES spectrometer.

E5 – Inductively coupled plasma spectrometry

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.

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

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

a schematic of an ICP-MS system. spectrometry

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

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/z ratio 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/z ratio, 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

Slide valve

Sampling cone

Skimmer cone

ICP

Pressure ~ 0.0001 mbar

~ 2.5 mbar

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.

Section E – Spectrometric techniques

ions occur at the same m/z ratio as the analyte ion. For example, 32 S 16 O + and

31 P 16 O 1 H + both interfere with 48 Ti , and 40 Ar 16 O + interferes with 56 Fe + . ● Matrix effects may occur due to excess salts or involatile solids. Some of these interferences may be removed by the use of reaction cell tech-

nology where a gas such as helium is added so that, by ion-molecule reactions , interfering ions may be converted into noninterfering species or removed by a multipole filter device.

The sensitivity is generally very high for a large number of elements, typically ten times more sensitive than ICP-AES. Since ICP-MS can scan over a wide mass range, every element is detected simultaneously. Additionally, the isotopes are separated so that changes in isotope ratios produced from radio- active or other sources, or required for geological dating, may be measured accurately. If interferences occur, an alternative isotope may be available for quantitative analysis.

Applications With ICP-AES there is little interference from ionization, since there is an excess of electrons present. The high temperature ensures that there is less interference from molecular species or from the matrix. Since a large number of elemental emission lines are excited, line overlap, though rare, may occur. Figure 3 shows the simultaneous emission of a number of elemental lines from a sample. Up to

70 elements, both metals and nonmetals can be determined. Table 1 gives details of the comparative detection limits of the various atomic spectrometric techniques. The ICP-AES technique provides a wide linear range of detection. For example, for lead, the linear range extends from below

0.01 ppm to 10 ppm. Mercury in waste water may be determined by ICP-MS, using the most abundant mercury isotope, 202 Hg. Since lead from different sources may have different isotopic compositions, ICP-MS can be used to identify sources of envi- ronmental contamination. Tracer studies and measurements of isotopes after chromatographic separation of species have also proved the value of ICP-MS.

Cd 226,502

Ni 227,021

Ni 226,446 Ta 226,230

Wavelenght Fig. 3. Simultaneous determination of 7 elements by ICP-AES.

E5 – Inductively coupled plasma spectrometry

Table 1. Detection limits for atomic emission spectrometry/ppb ( mg l -1 ) Element

ICP-MS Al

Flame ICP-AES

10 1 0.08 Flame, flame atomic emission (see Topic E4); ICP-AES, inductively coupled plasma-atomic emission

spectrometry; ICP-MS, inductively coupled plasma-mass spectrometry.

Section E – Spectrometric techniques