Further Reading
7.2 Optical Emission Spectrometry
7.2.2 Optical Emission Spectrometry with Plasma Sources
7.2.2.2 Sample Introduction Systems
Modern ICP-OES instruments are very capable multi-element systems using power- ful optical, electronic and computer technology. However, the sample introduction into plasma is often referred as the weakest ring of the chain or the Achille’s heel for the system as suggested by Browner and Boom.6
Sampling Solutions: Mostly liquid samples are handled in ICP-OES and usually these are aqueous. The problem is the transportation of sufficient amount of analyte into the plasma without altering plasma characteristics to obtain a reproducible and accurate analytical signal. The sample transport system consists of a nebulizerto form small droplets, which is followed by a spray chamberto eliminate the large droplets and finally the sample flow of Ar to introduce these selected droplets with a size of less than 5 µm into the center of the plasma. Typical sample gas flow is 1.0 L min−1Ar and commonly the rate of liquid sample transport into the system is 1.0 mL min−1. The overall efficiency of the system for transportation of analyte species from the sample solution into the plasma is 1–10%; this figure can somewhat be improved by using better means of nebulization. An overall liquid sampling system with a nebulizer is shown in Figure 7.2.
Nebulizers are classified and used in the following groups:
Pneumatic nebulizer. The gas flow in a chamber creates a vacuum into which the sample solution can be sucked by the process known as the Venturi effect. The prin- ciple of pneumatic nebulization is the same as that is used for flame AAS. The important difference between these systems are typical flow rates which are about 5.0 and 1.0 mL min−1 for AA flames and Ar ICP, respectively. This difference is caused by the fact that a rather high total gas flow is used in AA flame, such as 10–20 L min−1, where Ar ICP sample flow is about only 1.0 L min−1. The most common kinds of pneumatic nebulization systems are concentric,cross-flow and V-groove nebulizers as shown schematically in the Figure 7.3.
Ultrasonic nebulizer. The sample is directed onto a piezoelectric crystal plate vibrating at a frequency of 0.2–10 MHz. The nebulization takes place on the vibrat- ing plate; the aerosol thus formed is directed through first a heated chamber, then a cooled one; desolvation is thus realized as the solvent volatilized in the heated cham- ber will be condensed in the following one. In addition to improved nebulization, the sample is directed into plasma without its solvent. Consequently, there is no cooling effect in plasma. Flow rate for the transport of fine, dry sample particles into plasma is lower as compared to pneumatic nebulizers. This results in longer analyte residence times in plasma up to 2–3 ms that is another cause of improvement in sensitivity. The main advantage of this nebulizer is about 10 times enhancement in sensitivity.
Difficulties and memory effects are experienced with samples of high solid content.
Accumulation of non-volatile components on the vibrating crystal causes alteration of the vibrating frequency, that will in turn reduce the nebulization efficiency.
Grid nebulizer. Hildebrand grid nebulizer7 uses two successive platinum grids through which the sample solution is forced to pass. The aerosol is produced after the first grid; the second grid that is about 2 mm away from the first one breaks up
144 Chapter 7
Figure 7.2 Schematic sample introduction systems for ICP-OES. (A) impact-bead spray chamber, (B) double-pass type spray chamber. N, nebuliser; SC, spray chamber;
P, plasma sample flow; Ar, Ar flow; IB, impact bead; S, sample flow; AS, aerosol sample (small droplets, d ⬍5 µm); and W, waste
the aerosol into smaller particles. The sample mist is directed to plasma through a spray chamber. Schematic representation of a grid nebulizer is given in Figure 7.4.
Extensive use of solutions containing aqua regia may harm the platinum grids.
In order to ensure a reproducible sample solution transport into a nebulizer, a peristaltic pump is almost always used. However, if there are significant differences in viscosities of sample and standard solutions, nebulization efficiencies will be dif- ferent and thus the net analyte transport efficiency to plasma will show variations;
internal standard technique can be used to correct the induced errors.
Vapour generation techniques.For sampling solutions, in addition to introducing solutions directly to nebulizers,hydride generation for As, Se, Sb, Sn, Bi, Te, Ge, In and Pb and cold vapour formationtechniques for Hg and Cd are also frequently used in ICP-OES. The principles are the same as described in Chapter 6. Automated sys- tems based on flow injection are often used for these vapour generation techniques.
Figure 7.3 Schematic representations of some pneumatic nebulizers. A, concentric nebulizer (low solid content); B, cross-flow nebulizer (high solid content); C, V-groove nebulizer (high solid content); S, sample flow (usually by a peristaltic pump); and Ar, Ar flow
Sampling Solids: Analysis of solids has also been possible although it is not as easy and convenient as in the case of liquid samples. Numerous ways of solid sam- pling have been proposed, the most popular techniques are electrothermal vaporiza- tion (ETV) and laser ablation(LA).
In principle, ETV is a graphite vaporizer–atomizer; its essential function is vapor- ization rather than atomization when used with ICP-OES. The working principles of graphite atomizer are given in Chapter 6. When coupled to ICP-OES, however, the transport of vaporized sample into plasma becomes important; the locations of vapor- ization and measurement have thus been separated. All the advantages of ETA, such as ashing with a convenient temperature programming and using chemical modifiers are also valid for ETV. Both tube and rod types of atomizers have been used.
In contrast to nebulization of solutions, LA technique can produce high transport efficiencies, approaching to 100% in some cases, but only for a brief period of few seconds. The signal obtained is transient. This technique is based on vaporization of solid samples by using a powerful laser beam such as a Nd-YAG laser. Since the laser beam can be focused on areas as small as micrometer dimensions, LA tech- nique can be used for localized analysis of heterogeneous materials. Depth profiling is also possible. A schematic representation for LA system is given in Figure 7.5.