Direct solid sampling, atomic spectroscopy

Metal Content. Calcium, sodium, and boron contents of paper samples were determined by using a direct solid-sampling technique in conjunction with flameless atomic absorption spectroscopy (20,24). A Varian Techtron AA-6 spectrophotometer was employed with a model 90 carbon rod atomizer. In addition to the direct solid-sampling technique, calcium content was also determined by digesting samples with concentrated hydrochloric acid for 30 min. 

Although most samples are commonly presented as liquids for atomic emission spectroscopy, direct solid sample analysis has the advantage that no major pretreatment or dissolution steps are required [44]. This minimises dilution errors or contamination from reagents and reduces the reagent and manpower cost per sample. In addition, improved detection Hmits may be obtained if microsamples or microanalysis are possible without any further dilution. However, the analyst has to ensure that the solid material sampled is representative of the bulk material. ICP-AES has generally a remarkable tolerance for total dissolved sohds compared to ICP-MS or flame AAS so that, depending on the overall matrix, between 2 and 25 % suspended sohds can be coped with. Therefore, most of the sohd sample introduction devices described below are dedicated for ICP-AES. 

Different analytical techniques are used for detection of the elemental composition of the solid samples. The simplest is direct detection of emission from the plasma of the ablated material formed above a sample surface. This technique is generally referred to as LIBS or LIPS (laser induced breakdown/plasma spectroscopy). Strong continuous background radiation from the hot plasma plume does not enable detection of atomic and ionic lines of specific elements during the first few hundred nanoseconds of plasma evolution. One can achieve a reasonable signal-to-noise ra-... 

Except for a few attempts to vaporize directly a solid sample by a rapid heating technique, the sample for atomic fluorescence is a liquid solution that can be aspirated into a flame. The same type of sample preparation therefore is required as is necessary for flame emission and atomic absorption spectroscopy. The sample must be in solution, free of solid particles, and of low viscosity. The comparison standards should have the same physical characteristics as the sample solution. If organic solvents are used for the sample solution, they also must be used for the comparison standards. 

Figure 7.7 (a) Diagram of a direct current arc source. The solid sample is packed into the cupped end of the lower graphite electrode. The graphite counterelectrode is also shown. (From Hareland, W., Atomic emission spectroscopy, in Ewing, G.W. ed.. Analytical Instrumentation Handbook, 2nd edn., Marcel Dekker, Inc., New York, 1997. With permission.) (b) A schematic of the lower electrode used to hold the sample. 

The sensitivity, accuracy, and precision of solid sample analysis were greatly improved by coupling of LA with ICP-OES/MS. The ablated species are transported with a carrier gas (usually argon) into the plasma torch. Additional atomization. excitation and ionization of the ablated species in a stationary hot plasma provide a dramatic increase in the sensitivity of emission detection (LA-ICP-OES) or detection of ions (LA-ICP-MS). The efficiency of the transport of ablated species into an ICP strongly depends on the size of the particles. The optimal conditions for ablation in the ca.se of LA-ICP differ significantly from the optimal conditions for LIBS because the efficient transport of the ablated matter to an ICP requires a fine aerosol (with solid particle diameters less than a few micrometers), whereas direct optical emission spectroscopy of the laser plume needs excited atoms and ions. 

Photoelectron spectroscopy provides a direct measure of the filled density of states of a solid. The kinetic energy distribution of the electrons that are emitted via the photoelectric effect when a sample is exposed to a monocluomatic ultraviolet (UV) or x-ray beam yields a photoelectron spectrum. Photoelectron spectroscopy not only provides the atomic composition, but also infonnation conceming the chemical enviromnent of the atoms in the near-surface region. Thus, it is probably the most popular and usefiil surface analysis teclmique. There are a number of fonus of photoelectron spectroscopy in conuuon use. 

In Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES), a gaseous, solid (as fine particles), or liquid (as an aerosol) sample is directed into the center of a gaseous plasma. The sample is vaporized, atomized, and partially ionized in the plasma. Atoms and ions are excited and emit light at characteristic wavelengths in the ultraviolet or visible region of the spectrum. The emission line intensities are proportional to the concentration of each element in the sample. A grating spectrometer is used for either simultaneous or sequential multielement analysis. The concentration of each element is determined from measured intensities via calibration with standards. 

Tin is readily measured in multielement analyses of air, water, and solid waste samples by inductively coupled plasma atomic emission spectroscopy. For individual analyses of tin, direct aspiration atomic absorption spectroscopy is usually used. Organotin can be extracted from environmental samples and determined by atomic spectrometric methods or gas chromatography, usually after derivatization. 

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