Ultrasonic leaching

This section discusses various applications of discrete ultrasonic leaching that are classified according to the nature of the leached species (inorganic, organometallic or organic) without regard to whether a bath or a probe was used. 

Polychlorinated biphenyls (PCBs) are also frequently included in pollutant monitoring programmes owing to their substantial persistence and accumulation in the environment. This has raised the need to develop methods for their routine analysis. USAL as a sample preparation method is an effective choice for the fast, efficient and straightforward removal of these compounds, as shown by the method for the determination of 146 PCBs in heron eggs [119], where ultrasonic leaching resulted in improved precision, efficiency and reliability under the operational conditions proposed. USAL was used prior to the... 

D.-H. Chen, F. Lazaro, M.D. Luque de Castro, M. Valcarcel, Direct spectrophotometric determination of total boron in soils with ultrasonic leaching in automatic flow systems, Anal. Chim. Acta 226 (1989) 221. 

Plant Azomethine H Spectrophotometry 0.1-6.0mgr ultrasonic leaching Dry-ashing... 

Salim OM, Mahir I, Mahmut Bayramo glu (2005) Leaching of silver from solid waste using ultrasound assisted thiourea method. Ultrason Sonochem 12(3) 237-242... 

Swamy KM, Sukla LB, Narayana KL, Kar KN, Panchanadikar VV (1995) Use of ultrasound in microbial leaching of nickel from laterites. Ultrason Sonochem 2(1) S5-S9... 

Ultrasonic agitation during leaching has recently been reported to increase the catalytic activity of skeletal nickel for the hydrogenation of benzene to cyclohexane, with the enhanced activity related to changes in the catalyst structure and surface species [47],... 

Calibration and quantification procedures are easier in LA-ICP-MS compared to other solid-state mass spectrometric techniques because the laser ablation and the ICP ion source operate at normal pressure and the laser ablation of solid samples and ionization of analytes are separated in space and time. Therefore the advantage of solution calibration in ICP-MS can be applied in this solid-state analytical technique. The introduction of solution based calibration, which is only possible in LA-ICP-MS, was an innovative step in the development of this sensitive mass spectrometric technique. A number of different calibration approaches using aqueous standard solutions in the dual gas flow technique have been discussed by various authors.74 75 In the dual gas flow injection technique , the nebulized standard solution and the laser ablated sample material are mixed in the -piece and the two gas flows from the nebulizer (e.g. ultrasonic nebulizer) and laser ablation chamber are added. Using solution based calibration with the addition of a standard solution, Leach et alP determined minor elements in steel reference materials with a relative accuracy of a few %. In comparison to the so-called dual gas flow technique proposed in the literature, where the argon flow rates through the nebulizer and ablation cell add up to 11 min-1 (e.g. 0.451 min-1 and... 

P. Bermejo-Barrera, O. Muniz-Naveiro, A. Moreda-Pineiro and A. Bermejo-Barrera, The multivariate optimisation of ultrasonic bath-induced acid leaching for the determination of trace elements in seafood products by atomic absorption spectrometry. Anal. Chim. Acta, 439(2), 2001, 211-227. 

Upon receipt of the samples in the laboratory, they were cleaned with distilled water in an ultrasonic cleaner aided by a small brush, air dried, and photographed for later reference. It was assumed that the surface leaching of these chunks was small compared to the surface area available for leaching after the samples were ground. A representative sample from each core was... 

Root samples must be washed to reduce contamination, but even so complete clean up is impossible for many soil types. Sometimes a brief period of ultrasonic treatment is used in a water bath. Reliable studies of leaching losses from plant roots are scarce because they are so difficult to conduct, and the results are rarely unequivocal. 

Derivatization and Measurement by Different GC-ICP-MS Techniques A simple and rapid sample pretreatment procedure is based on acid leaching (5 min) of the biological material, followed by simultaneous in situ derivatization and extraction (40 min) in the presence of NaBH4 and nonane, buffered at pH 7.0, in an ultrasonic bath. The separation is done with GC, while two independent on-line detecting techniques (MIP-AES and ICP-MS) are used. This approach affords LoDs better by a factor of 2 than those offered by ICP-MS [64]. 

Another example of ultrasound use is leaching of organic impurities from different kinds of samples. The main analytes of interest are PAHs, which are widespread in soil, sediment, dust, and particulate samples [55]. USE is recommended as a fast, efficient, and direct environmental sample preparation method for determination of PCBs, nitrophenols, pesticides, or polymer additives. Organometallic and biologically active compounds (such as vitamins A, D, and E) present in samples in trace quantities, can be extracted from animal and plant tissues with the aid of ultrasonic wave energy [59]. Table 6.6 presents some typical applications of USE in trace analysis of biological and environmental samples [60]. 

Some contradictory results are obtained in the comparison of results in the determination of arsenic after ultrasonic slurry sampling — the use of sampling is not most appropriate in this case as the sampling step is before the slurry formation — ultrasound-assisted extraction — leaching has been the correct word in this case — and microwave-assisted digestion [28], which will be treated in detail in Chapter 5. 

Ultrasonic energy is frequently used to accelerate the dissolution of solid samples under soft conditions of temperature, pressure and chemical reagents. Similar to direct dissolution by agitation, US-assisted soft digestion is not used to the same extent as other operations of the analytical process such as leaching, derivatization or detection. The simplicity of this operation with some types of samples and the operator s lack of awareness of its error contribution are responsible for the absence of optimization studies for this process. Inappropriately conducted soft digestion can result in major errors and affect the quality of the results. 

USE OF ULTRASONIC BATHS VERSUS ULTRASONIC PROBES FOR LEACHING... 

Except in some special cases where the users themselves have designed and produced their own ultrasonic devices, US equipment for leaching consists of commercial ultrasonic baths or probes. 

The US intensity received by the target sample in ultrasonic baths is strongly influenced by variables such as the level of transmitting liquid, US intensity, shape of the leaching vessel and position of the vessel inside the bath. In addition, not all cleaning baths operate at the same frequenoy and not always is such a frequency stated this frequently hinders accurate reproduction of reported leaching methods. 

Comparative studies have also revealed that probes allowed more expeditious leaching than did ultrasonic baths [5]. In any case, ultrasonic baths have been massively used for leaching on account of their aooessibility. 

Position of the sampie vessei. Although the position of the sample container with respect to the transducer is not a characteristic variable of US application, it determines the amount of energy that is received by the sample. This variable, exclusive of ultrasonic baths, should be optimized in both DUSAL and CUSAL methods. When only one sample is leached, the precision is not affected provided the sample vessel is always in the same position — cavitational effects can be maximal or not in this situation, however. If several samples are simultaneously treated, then the precision is probably affected because the irradiation profile is not uniform throughout the bath. One example is the DUSAL of cadmium and lead from foods, where the iodine method was used to locate the best position for cavitational effects [5]. This requires the use of mapping techniques. 

Once all variables have been optimized, a kinetic study of the leaching efficiency as a function of the ultrasonication time can be used to examine the course of the leaching process and determine the time required for complete removal of the target species. 

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