27 2.6 DETECTION TECHNIQUES FOR POTENTIALLY TOXIC ELEMENTS IN WATER
28 Figure 2.1: A system block diagram of the F-AAS
2.6.2 Graphite Furnace-Atomic Absorption Spectrometry (GF-AAS)
The technique is similar to F-AAS except that the flame is replaced by an electrically heated closed graphite tube having transparent end windows. The graphite tube generates a cloud of atoms which is exposed to the light from the hollow cathode lamp.
A higher atom density and longer residence time in the furnace tube provide a lower LOD. The LODs are improved by a factor of up to 1000 times compared to F-AAS, thus detecting as low as ppb range (Johnson et al., 1997). The GF-AAS has limited dynamic ranges which can also detect concentrations in the ppt range of 100-1000.
Therefore, solutions must be held in a narrow range of concentrations.
2.6.3 Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES)
An ICP-AES is a type of technique used for determination of potentially toxic elements in different sample matrices. The plasma source discharge atomic emissions in a radial or axial configuration. The emitted atoms are collected with a lens and imaged onto an entrance slit of a wavelength selection device. A monochromator is used as a wavelength selection device. The use of ICP-AES is competitive with most other spectroscopic analysis of elements with regards to sample throughput, sensitivity and detection limits for analytes (Bernazzani and Paquin, 2001).
29 Figure 2.2 shows the simplified diagram of the process of introducing a sample till the results output for analysis in an ICP-AES. In this figure, the sample in a solution form is introduced into the nebuliser chamber via a peristaltic pump attached to an automatic sampler.
A peristaltic pump operates by sequentially compressing flexible tubing with evenly spaced and rotating rollers that pull the liquid through the system. The flow of the sample and argon gas through the small aperture of the nebuliser creates small droplets that form a mist of µm-sized particles in the spray chamber (Hou and Jones, 2000). The small droplets from the sample travel with the argon flow and enter the torch region while the larger droplets are carried to the drain system.
In the plasma region, evaporation, atomisation, and ionisation occur at maximum temperatures reaching 10 000K. Visible and UV radiation emitted from the sample constituents enter the monochromator through a small slit where the wavelengths are separated by a grating before being captured and measured by the detector. From the detector, intensity of the emitted lines is measured converting those readings to the actual sample concentration which can be processed from the computerised data output (Hou and Jones, 2000).
Since ICP-AES is used for multi-element quantification, two types of the instrument are commonly known for the elemental analysis.
The sequential spectrometer is one of the instruments selected whereby it uses a monochromator to scan different emission lines in sequence. The wavelength of choice during analysis is controlled and chosen by an analyst on the instrument software, manually.
The second common instrument is the direct reading spectrometer which uses a polychromator with as many as 64 detectors located at exit slits in the focal point (Hou and Jones, 2000).
30 Figure 2.2: Schematic diagram of a typical ICP-AES
2.6.4 Inductively Coupled Plasma-Mass Spectrometry (ICP-MS)
An ICP-MS is a hyphenated method that combines a high-temperature inductively coupled plasma (ICP) source with a mass spectrometer (MS). Figure 2.3 shows the schematic diagram of the ICP-MS from sample introduction till the processed results.
A liquid sample is introduced into the ICP plasma as an aerosol by aspirating the liquid into the nebuliser. The hot region of the ICP torch converts the elements in the aerosol to gaseous atoms where ionisation takes place in the plasma region. The ICP source converts the atoms of the elements in the sample to ions. These ions are then separated by mass spectrometer and detected (Jones, 1992).
Figure 2.3: Schematic diagram of ICP-MS
ICP-AES
31 The type of sample matrix can influence the detection capabilities, which can in turn affect the degree of ionisation that will occur in the plasma or allow the formation of species that may interfere with the analyte determination (Jarvis et al., 1992). The speed, accuracy and multi elements that can be determined in a single analysis make the ICP-MS a very cost-effective technique. The advantage using plasma compared to other ionisation methods, such as flame ionisation, are that complete ionisation is guaranteed and as such prevents oxide formation since it occurs in a chemically inert environment.
Additionally, the benefits associated with the use of ICP-MS include the ability to perform speciation studies when coupled with separation techniques such as HPLC.
The technique enables measurement of isotopic composition in nutritional studies and to identify sources of environmental exposure (Hill, 2007; Petridis et al., 2014).
Consequently, the ICP-MS limitations are based on the amount of total dissolved solids in the samples of interest. High total dissolved solids cause instrumental drifts, decreased sensitivity and detection capability due to the blockages that may occur in the apertures of the cones. To account for this, proper dilutions should be carried out prior sample analysis to minimise the high levels of total dissolved solids (McCurdy and Proper, 2014).
2.7. ANALYTICAL PERFORMANCE CHARACTERISTICS FOR DETERMINATION