OE Typical Elements

OE typically is organized though a set of element processes similar to a safety management system. The elements usually include some aspects of the following ... 

Approximately 70 different elements are routinely determined using ICP-OES. Detection limits are typically in the sub-part-per-billion (sub-ppb) to 0.1 part-per-million (ppm) range. ICP-OES is most commonly used for bulk analysis of liquid samples or solids dissolved in liquids. Special sample introduction techniques, such as spark discharge or laser ablation, allow the analysis of surfaces or thin films. Each element emits a characteristic spectrum in the ultraviolet and visible region. The light intensity at one of the characteristic wavelengths is proportional to the concentration of that element in the sample. 

Table 1 Typical detection limits (ppb) for iCP-OES (using a pneumatic nabuiizer for sample introduction) of the most sensitive amission line betwean 175 nm and 850 nm for each element.

Simultaneous ICP-OES. The simultaneous ICP-OES measures all elements at the same time. A large number of samples can be analysed in a short period of time making it useful for rapid analysis. They are very expensive and are used where routine multiple sample analysis is required on a regular basis, usually on the same elements and samples. Most simultaneous instruments are custom designed for a selection of elements at specific wavelengths and some instruments can have between 10 and 100 slits and are factory fixed for selected elements. Typical applications would be in the water industries where analysis for metal content would be important for health reasons, in the water supply to power stations where analysis is usually carried out before and after treatment prior to use so as to avoid contamination of turbine blades, in the food industries, in mineral exploration or any other routine analysis where metal analysis requirements do... 

ICP/OES can be conducted either simultaneously or sequentially. Simultaneous instruments rely on a polychromator or direct-reading spectrometer to read up to 60 elements from the same sample excitation. Sequential analyses use a computer-controlled, scanning monochromator system. The light emitted by the sample in the plasma source is focused on the entrance slit of the monochromator and the spectrum is scanned through the region of interest. Typically, it is possible to determine several elements per minute in the sample in a sequential spectrometer. 

Flame AAS is a well established technique which suffers from relatively few interferences and these are well characterised. The instrument is relatively inexpensive, easy to operate and, of the three techniques, requires the least operator experience. For most elements the LODs achieved with FAAS or with ICP-OES are comparable - within a factor of two or three of each other. The ICP approach is advantageous (i) for elements that form refractory oxides and (ii) for elements which are characterised by low ionisation energies. Included in the former are B, V, Ti and W which are only partially dissociated in the flame. Typical of the latter group are elements Mg, Ca, Be, Zr and the rare earth elements whose most sensitive emission lines are from ions rather than atoms. By contrast, the more volatile heavy elements such as Cd and Zn have slightly lower limits of detection by FAAS. 

Figure 11-17 compares detection limits for iCPMS with those for ICP optical emission spectroscopy (ICP-OES) and those for electrothermal atomic absorption spectroscopy (ETA AS) for selected elements. These data arc typical for most other elements in the periodic table. Generally, detection limits with mass spectromeiric detection range from 0.02 to 0.7 ppb wilh the majority of elements in the range of 0.02 to 0.1 ppb. 

Radio-frequency (RF) ICP source. In ICP-OES, the sample is usually introduced to the instrument as a stream of liquid. The sample solution is nebulized and the aerosol transported to the plasma. In the plasma, the sample undergoes the same process outlined in Table 7.2. The argon plasma serves to atomize, ionize, and excite the elements in the sample. The emitted radiation is sorted by wavelength in a spectrometer and the intensity is measured at each wavelength. A schematic of a typical ICP-OES system is presented in Figure 7.21. 

A novel technique of atomisation, known as vapour generation via generation of the metal hydride, has been evolved, which has increased the sensitivity and specificity enormously for these elements [5-7]. In these methods the hydride generator is linked to an AAS (flame graphite furnace) or inductively coupled plasma-optical emission spectrometer (ICP-OES) or an inductively coupled plasma mass spectrometer (IPC-MS). Typical detection limits achievable by these techniques range from 3 pg/1 (arsenic) to 0.09 pg/1 (selenium). 

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