Atomic transport efficiency

Backstrom and co-workers [1] have demonstrated significant improvements in nebuHzer efficiencies by increasing analyte transport efficiency, at a solvent load acceptable for the atom reservoir in question, and therefore improving detection Hmits. Conventional nebuHzer systems do not allow this because an increased analyte transport efficiency will give a too high a solvent load in the atom reservoir. 

An analyte transport efficiency of nearly 100% has been obtained with an interface for flame atomic absorption spectrometry (FAAS) [3]. It has been used for the determination of lead in blood [5] and for coupHng with a high-performance Hquid chromatograph (HPLC) [6]. 

In addition to conventional aspiration, using a nebulizer and spray chamber, samples may be introduced in to atomic spectrometers in a number of different ways. This may be because a knowledge of speciation (i.e. the organometallic form or oxidation state of an element) is required, to introduce the sample while minimizing interferences, to increase sample transport efficiency to the atom cell or when there is a limited amount of sample available. 

The introduction of hydrides into plasma-based instmmentation has also been achieved. The sensitivity increases markedly when compared with conventional nebulization because of the improved transport efficiency of the analyte to the atom cell (close to 100%). Often, a membrane gas-liquid separator is usee ensure that aerosol droplets of liquid do not reach the plasma. 

Over the years many analytical spectroscopists have attempted to improve upon this situation, but the only reliable way to improve transport efficiency with pneumatic nebulizers is apparently to restrict the aspiration rate.17,18 Reduced aspiration rate means that the nebulizer energy is distributed to less aerosol per unit time, resulting in a finer droplet size distribution finer droplets (e.g. < 2 pm in diameter) are more likely to be transported through the spray chamber. Alternatively, the determinant may be introduced to the flame in gaseous form, or in a small cup. Such approaches are discussed in Chapter 6. However often the approach taken is to use electrothermal atomization rather than a flame,6,19 but this is outwith the scope of the present small volume. 

From the late 1960s onwards, a number of research groups around the world began to investigate alternatives to pneumatic nebulization for sample introduction, in an attempt to overcome transport efficiency limitations. The most successful approaches were those which involved heating small, discrete liquid samples, and sometimes even solid samples, directly on a metal filament, boat, or cup which could be positioned reproducibly into a flame. However, since the temperature of the metal would be lower than that of the flame itself, the techniques were confined to the determination of relatively easily atomized elements such as arsenic, bismuth, cadmium, copper, mercury, lead, selenium, silver, tellurium, thallium, and zinc. 

The use of radiotracers is very helpful for the understanding as well as for the optimization of the analyte volatilization in furnace AAS, and with this element losses and their causes at all levels of the atomization processes can be quantitatively followed. This has been studied in detail for a number of elements such as As, Pb, Sb and Sb in furnace atomization by Krivan et al. (see e.g. Ref. [280]). The results, however, may differ considerably from those when the furnace is used as an evaporation device only and the vapor produced is transported into a second system for signal generation, as has been studied extensively by Kantor et al. (see e.g. Ref. [281]). Here the transport efficiencies were calculated for the case of transport of the vapors released through the sampling hole, and similar considerations could be made when releasing the vapors end-on. 

The energy efficiency of plasma-chemical Krp2 synthesis with surface stabilization of products is here determined mostly by the energy cost of F2 dissociation and by the effectiveness of F-atom transportation to the surface of the krypton film. 

Inductively coupled plasma atomic emission spectrometry (ICP-AES) involves a plasma, usually argon, at temperatures between 6000 and 8000 K as excitation source. The analyte enters the plasma as an aerosol. The droplets are dried, desol-vated, and the matrix is decomposed in the plasma. In the high-temperature region of the plasma, molecular, atomic, and ionic species in various energy states are formed. The emission lines can then be exploited for analytical purposes. Typical detection limits achievable for arsenic with this technique are 30 J,g As/L (23). Due to the rather high detection limit, ICP-AES is not frequently used for the determination of arsenic in biological samples. The use of special nebulizers, such as ultrasonic nebulization, increases the sample transport efficiency from 1-2% (conventional pneumatic nebulizer) to 10-20% and, therefore, improves the detection limits for most elements 10-fold. In addition to the fact that the ultrasonic nebulizer is rather expensive, it was reported to be matrix sensitive (24). Inductively coupled plasma atomic emission spectrometry is known to suffer from interferences due to the rather complex emission spectrum consisting of atomic as... 

The introduction of a gas phase sample into an atomizer has significant advantages over the introduction of solids or solutions. The transport efficiency may be close to 100%, compared to the 5-15% efficiency of a solution nebulizer. In addition, the gas phase sample is homogeneous, unlike many solids. There are two commercial analysis systems with unique atomizers that introduce gas phase sample into the atomizer. They are the cold vapor technique for mercury and the hydride generation technique. Both are used extensively in environmental and clinical chemistry laboratories. 

Spray Chambers and Desolvation Systems. A nebulizer must produce droplets less than 10 /im in diameter in order to achieve a high aerosol transport efficiency (the percentage of the mass of nebulized solution that reaches the plasma), and rapid desolvation, volatilization, and atomization of the aerosol droplets. Pneumatic nebulizers, especially, produce highly poly dispersive aerosols with droplets up to 100 jwm in diameter and these large droplets must be removed by a spray chamber. 

The nebulization rates achieved with the spray and nebulizer systems used in ICP spectrometry are much slower than those used in flame atomic absorption. The transport efficiency of the sample introduction systems is less than 3% in ICP spectrometry, whereas that in flame atomic absorption is about 15% or less. 

The requirements for ETV-ICP systems (both ICP-AES and ICP-MS) differ significantly from those for ETA-AAS. When using ETV-ICP, it is necessary to introduce a volatile sample into the gas stream. The atomization stage is then performed in the ICP unit. This is the reason why the technique is referred to as ETV in the case of ICP-AES and ICP-MS, but ETA (electrothermal atomization) in the case of AAS. The chemical matrix effects observed in AAS are negligible using ETV-ICP, whereas the transport efficiency of the sample from the ETV unit to the ICP unit is critical to the analytical performance in ETV-ICP-MS. 

With conventional nebuhsers, the aerosol size increases at a low nebuhser gas flow, reducing the transport efficiency and decreasing the emission from all lines. However, lower flow rates also increase the residence time and the excitation temperature enhancing the emission of ionic lines. For atomic lines, the excitation is improved either by the increased residence time. On the other hand also the ionisation rate is increased, resulting in a net decrease of atomic hne emission. 

Hydride generation for minor components such as arsenic, selenium, and tin continues to have advantages that include separation of the analyte from the matrix, increased transport efficiency to the point of atomization, and the capability for organometallic speciation. 

In 2007 (O Fig. 20.43c), the gas-flow rate was increased to 1,500 ml/min in order to increase the transport efficiency. The temperature gradient was between +32°C and — 164°C. As a result, the Hg deposition region broadened considerably to 14 detectors. Only about 30% of the Rn deposited on the last four detectors. The faster carrier gas transported three observed atoms of Cn further down to detectors at lower temperatures. Chains 3 and 5 were detected in detectors 11 (—21°C) and 14(—39°C). Chain 4 was detected in detector 26 (—124°C). From dew point measurements in the carrier gas, it has to be concluded that the detector surfaces held below — 95° C were covered by a thin ice layer (O Fig. 20.43a-c, vertical lines). Thus, it was concluded that four events (chains 1-3, and 5) are attributable to atoms of element 112 deposited on the gold surface, while chain 4 represents an atom of element 112 deposited on ice. 

Hydride generation is a common method for the detection of metalloids such as As, Bi, Ge, Pb, Sb, Se, Sn and Te, although other vapours, e.g. Hg or alkylated Cd, may also be determined. This technique improves the sensitivity of the analysis substantially. Since the sample is in the gas phase, the sample transport efficiency is close to 100%. The hydrides atomize readily in the flame, although this approach is usually used in conjunction with a quartz T-piece in the atom cell. Methods have been developed that trap the hydrides on the surface of a graphite tube for use with ETAAS. This leads to preconcentration and further improvements in detection limit. 

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