Multi-element AAS

Multi-element AAS has been reviewed [112], as well as ETAAS [104] and instrumental aspects of GFAAS [113]. Various monographs on analytical atomic absorption spectrometry are available [52,96,114,115], and on GFAAS [116] and ETAAS [117] more in particular. 

Vinas et al. (1993a), determined copper in biscuits and bread using a fast-program slurry electrothermal atomic absorption procedure and Miller-Ihli (1988) also used EAAS after slurry preparation for simultaneous multi-element AAS, as did Littlejohn et al. (1985) who introduced slurried food samples into the graphite furnace for analysis. Haswell and Barclay (1992), carried out on-line microwave digestion of slurry samples with direct flame atomic absorption spectrometric elemental detection. 

Although the ICP is somewhat more sensitive in terms of reported detection limits than DCP, the former cannot tolerate as high a dissolved solids content as the latter. Therefore, on the original silicate materials, the detection limits are similar. Another advantage of PES compared to AA is that commercial PES spectrometers can be configured for simultaneous multi-elemental analysis, while the current commercial multi-elemental AAs are sequential. The base price of PES equipment is higher than AA but if the sample load is high, the increased productivity of multi-elemental PES may result in a lower cost per analysis. 

Background correction by wavelength-modulation is not widely used, but seems to have considerable potential for simultaneous multi-element AAS with continuum sources. Alternatively, high-resolution echelle gratings may be used that can detect both the elemental line and the background at the same time. 

In 1996 Harnly [50] was still optimistic when he concluded that multi-element AAS will ultimately be successful because it will offer figures of merit which are comparable or superior to those available today for single element AAS , and a CS AAS instrument. .. would provide analytical capabilities comparable to ICP-MS with considerably less complexity . In 1999, however, he became more pessimistic [54] when he provokingly concluded The cost of development and construction of the ideal detector would be expensive. In the current economic climate, where development of technology for future products is sacrificed for quarterly profits, the cost of the detector may be sufficient to block the development of multi-element CS AAS, regardless of the enhanced analytical capabilities . 

In industrial research laboratories, AAS (in particular FAAS) is no longer being used to the same extent as in the past, despite the aforementioned important improvements in AAS technology. More rapid, multi-element methods have gradually taken over, such as ICP-AES, ICP-MS and NAA. However, the determination of one element is faster with AAS than with an ICP technique. Also, ICP-AES does not supersede GFAAS in terms of sensitivity and selectivity. 

Plasma sources were developed for emission spectrometric analysis in the late-1960s. Commercial inductively coupled and d.c. plasma spectrometers were introduced in the mid-1970s. By comparison with AAS, atomic plasma emission spectroscopy (APES) can achieve simultaneous multi-element measurement, while maintaining a wide dynamic measurement range and high sensitivities and selectivities over background elements. As a result of the wide variety of radiation sources, optical atomic emission spectrometry is very suitable for multi-element trace determinations. With several techniques, absolute detection limits are below the ng level. 

All metals at trace concentration, or in trace quantities, can be analyzed by atomic absorption (AA) spectrophotometry in flame or graphite furnace (electrothermal reduction) mode. A rapid, multi-element analysis may use... 

On the other hand, multi-element ET-AAS shows some weak aspects, namely ... 

The multielement function of the plasma-based techniques has been a source of challenges in the AAS field. This has resulted in the fast sequential technique, which is a simple way to mimic the multi-element function. However, it only works for FAAS applications. Moreover, simultaneous multielement ET-AAS systems for analysis have also been placed on the market, although there are some spectral limitations. It also has the drawback of using the same time and temperature programme for all elements. In the future, further developments in the multielement technique can be envisaged which will resort to continuum sources as well as CCD and other multiwavelength detectors. 

Some refractory elements cannot be determined by ET-AAS at the levels usually present in waters. That is the case with M. El Himri et al. [28] developed a fast and accurate procedure, without any prior treatment, to analyze tap and mineral waters from Spain and Morocco for this highly toxic element. ICP-MS was employed. The analytical isotope selected was 238U, with Rh as internal standard. An LoD of 2ngl 1 was obtained. The estimated repeatability was 3 percent at the concentration level of 73 ng l-1. The method was validated by comparison with a radiochemical procedure devised for natural samples and by analysis of a Certified Reference Material (CRM). Multi-element capabilities of ICP-AES have also been employed for surveys of trace elements. Al-Saleh and Al-Doush [29] reported the concentrations of dissolved Be, Cd, Cr, Cu, Fe, Mg, Mn, Hg, Ni, Se, Sr, V, and Zn in 21 samples of retail bottled waters from Riyadh, Saudi Arabia. It was found that Cd, Fe, Hg, Ni, and Zn were present at concentrations higher than the limits recommended by the EU and World Health Organization (WHO) guidelines. 

The association of a spectrometer with a liquid chromatograph is usually to aid in structure elucidation or the confirmation of substance identity. The association of an atomic absorption spectrometer with the liquid chromatograph, however, is usually to detect specific metal and semi-metallic compounds at high sensitivity. The AAS is highly element-specific, more so than the electrochemical detector however, a flame atomic absorption spectrometer is not as sensitive. If an atomic emission spectrometer or an atomic fluorescence spectrometer is employed, then multi-element detection is possible as already discussed. Such devices, used as a LC detector, are normally very expensive. It follows that most LC/AAS combinations involve the use of a flame atomic absorption spectrometer or an atomic spectrometer fitted with a graphite furnace. In addition in most applications, the spectrometer is set to monitor one element only, throughout the total chromatographic separation. 

Analytical determinations of heavy metals in the atmospheric particulate matter are performed using INAA and ETA-AAS. INAA is a multi-elemental and nondestructive technique. Using such a technique, it is possible to analyze the sample without any kind of pretreatment, which avoids the possibility of additional contamination and, at the same time, allows for a high degree of sensitivity. However, there are some heavy metals of a great interest, such as Cd, Ni and Pb, that cannot be determined with sufficient precision due to spectral interference or are insensitive to the neutron irradiation (e.g., Pb). These are analyzed by means of the ETA-AAS technique. 

ICP-AES offers rapid, multi-element determinations. Its sensitivity is lower than that of either ICP-MS or AA-GTA, but it can handle higher levels of total dissolved solids than ICP-MS and is much faster than AA-GTA. The results obtained are in comparison with all the equipment used by about 100 laboratories that are participating in the Environmental Laboratory Approval Program. The quality of the performance relating to equipment as well as the analyst in such international collaborative programs implies the use of certified reference materials for calibration and control samples to ensure a certain level of traceability of the measurements and finally the degree of assurance of reliability. 

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