Nebulization of sample

ICP-OES is an analytical system that can do simultaneous or sequential determination of up to 50 elements at all concentration levels with a high degree of accuracy and precision. Excellent vaporization-atomization-excitation-ionization is obtained with an argon-supported ICP operated at atmospheric pressure. The emitted spectra is observed with a polychromator or a scanning spectrometer may be used depending on whether simultaneous or sequential determinations are desired. This atomization-excitation process does not exhibit interelenent effects often seen in AAS, and ppb range detection is routine. Effective nebulization of samples needs to be improved on however, ICP and direct-current (DC) plasmas are extremely effective atomization sources that provide the most effective instrumental technique for simultaneous elemental analysis. 

Chapters 2 and 3 present an overview of single- and multi-collector ICP-MS instrumentation and their respective capabUities. Also appropriate ways to overcome spectral overlap are addressed. As the ICP is a robust ion source operated at atmospheric pressure, there are various means of sample introduction. Although pneumatic nebulization of sample solution is the standard approach, LA of solid material avoids the need for digestion in bulk analysis, and also allows spatially resolved information to be obtained. Recent advances in LA are discussed in Chapter 4, in which the fundamental technical challenges associated with the handling of transient signals are also considered. 

Suitable inlets commonly used for liquids or solutions can be separated into three major classes, two of which are discussed in Parts A and C (Chapters 15 and 17). The most common method of introducing the solutions uses the nebulizer/desolvation inlet discussed here. For greater detail on types and operation of nebulizers, refer to Chapter 19. Note that, for all samples that have been previously dissolved in a liquid (dissolution of sample in acid, alkali, or solvent), it is important that high-purity liquids be used if cross-contamination of sample is to be avoided. Once the liquid has been vaporized prior to introduction of residual sample into the plasma flame, any nonvolatile impurities in the liquid will have been mixed with the sample itself, and these impurities will appear in the results of analysis. The problem can be partially circumvented by use of blanks, viz., the separate examination of levels of residues left by solvents in the absence of any sample. 

Solutions can be examined by ICP/MS by (a) removing the solvent (direct and electrothermal methods) and then vaporizing residual sample solute or (b) nebulizing the sample solution into a spray of droplets that is swept into the plasma flame after passing through a desolvation chamber, where excess solvent is removed. The direct and electrothermal methods are not as convenient as the nebulization inlets for multiple samples, but the former are generally much more efficient in transferring samples into the flame for analysis. 

Under these conditions, the flow of sample liquid is predicted to be shown and this is similar to the flows observed with concentric nebulizers. 

Figure 19.7 shows a typical construction of a concentric-tube nebulizer. The sample (analyte) solution is placed in the innermost of two concentric capillary tubes and a flow of argon is forced down the annular space between the two tubes. As it emerges, the fast-flowing gas stream causes a partial vacuum at the end of the inner tube (Figure 19.4), and the sample solution lifts out (Figure 19.5). Where the emerging solution meets the fast-flowing gas, it is broken into an aerosol (Figure 19.7), which is swept along with the gas and eventually reaches the plasma flame. Uptake of sample solution is commonly a few milliliters per minute.

The dimensions of concentric-tube nebulizers have been reduced to give microconcentric nebulizers (MCN), which can also be made from acid-resistant material. Sample uptake with these microbore sprayers is only about 50 xl/min, yet they provide such good sample-transfer efficiencies that they have a performance comparable with other pneumatic nebulizers, which consume about 1 ml/min of sample. Careful alignment of the ends of the concentric capillary tubes (the nozzle)... 

In a concentric-tube nebulizer, the sample solution is drawn through the inner capillary by the vacuum created when the argon gas stream flows over the end (nozzle) at high linear velocity. As the solution is drawn out, the edges of the liquid forming a film over the end of the inner capillary are blown away as a spray of droplets and solvent vapor. This aerosol may pass through spray and desolvation chambers before reaching the plasma flame. 

The flows of gas and liquid need not be concentric for aerosol formation and, indeed, the two flows could meet at any angle. In the cross-flow nebulizers, the flows of gas and sample solution are approximately at right angles to each other. In the simplest arrangement (Figure 19.11), a vertical capillary tube carries the sample solution. A stream of gas from a second capillary is blown across this vertical tube and creates a partial vacuum, so some sample solution lifts above the top of the capillary. There, the fast-flowing gas stream breaks down the thin film of sample... 

In this cross-flow arrangement, a thin film of sample solution is obtained as it flows around the edge of a small opening, through which there is a fast linear flow of argon. The liquid film is rapidly nebulized along the rim of the orifice. 

To accommodate smaller liquid flows of about 10 pl/min, micro-ultrasonic nebulizers have been designed. Although basically similar in operation to standard ultrasonic nebulizers, in these micro varieties, the end of a very-small-diameter capillary, through which is pumped the sample solution, is in contact with the surface of the transducer. This arrangement produces a thin stream of solution that runs down and across the center of the face of the transducer. The stream of sample... 

Thermospray nebulizers are somewhat expensive but can be used on-line to a liquid chromatographic column. About 10% of sample solution is transferred to the plasma flame. The overall performance of the thermospray device compares well with pneumatic and ultrasonic sprays. When used with microbore liquid chromatographic columns, which produce only about 100 pl/min of eluant, the need for spray and desolvation chambers is reduced, and detection sensitivities similar to those of the ultrasonic devices can be attained both are some 20 times better than the sensitivities routinely found in pneumatic nebulizers. 

Introduction of sample solution via a nebulizer may need both spray and a desolvation chamber, but a well-designed, efficient nebulizer needs neither. 

Flow injection techniques can be used to inject sample volumes as small as 10 jiL into a flowing stream of water with little degradation of detection limits. Frit nebulizers have efficiencies as high as 94% and can be operated with as litde as 2 jiL of sample solution. 

The major advance in the way in which column eluate is deposited on the belt was the introduction of spray deposition devices to replace the original method which was simply to drop liquid onto the belt via a capillary tube connected directly to the outlet of the HPLC column. These devices, based on the gas-assisted nebulizer [5], have high deposition efficiencies, transfer of sample can approach 100% with mobile phases containing up to 90% water, and give constant sample deposition with little band broadening. 

Bjorn E, Freeh W, Hoffmann E, Liidke C (1998) Investigation and quantification of spectroscopic interferences from polyatomic species in inductively coupled plasma mass spectrometry using electrothermal vaporization or pneumatic nebulization for sample introduction. Spectrochim Acta 53B 1766... 

More recent determinations of serum iron have been reported by Schmidt 57), who simply diluted with lanthanum chloride solution, and by Tavenier and Hellen-doorn58), who deproteinized samples in the latter study, iron in the protein precipitate is analyzed to correct the serum iron level. Uny etal. 59) determined serum iron, using ultrasonic nebulization of the sample to increase the sensitivity. Olson and Hamlin 6°) have determined serum iron and total iron-binding capacity. Proteins are precipitated and iron (III) is released by heating with trichloroacetic acid. 

The ability to handle very small samples such as clinical specimens. A nebulizer, spray chamber, burner arrangement consumes several cm3 of sample per minute, most of which runs to waste. 

The preparation of aqueous solutions from solids is usually performed after the sample has been ground to a powder of uniform size. Sometimes, samples can be only sparingly soluble in water and therefore organic solvents may be used to dissolve the sample. Organic solvents can increase the sensitivities of atomic spectrometric analyses as a result of increases in the efficiencies of the nebulization of the analyte solutions. When organic solvents are used to dissolve samples non-selective ligands should be added to complex ionic species that would otherwise be insoluble in the organic solvent. 

The digestion of solid samples to produce a solution is discussed in Section 13.2. For solution-based ICP MS analysis, the liquid is taken up through a thin tube via a peristaltic pump. This feeds directly into the instrument nebulizer, where argon gas is introduced into the liquid and a fine mist of droplets is expelled from the tip of the nebulizer. This sample aerosol is sprayed into the condenser to reduce the size of the droplets, ensuring an even sample loading and preventing cooling of the plasma. About 1% of the sample solution uptake is transported to the plasma torch, and any unused solution is drained away and may be recycled. 

Fig. 3.38.The IUPAC names of Sudan azo dyes are as follows Sudan 1 = 1— [(2,4-dimethylphenyl)azo]-2-naphtalenol Sudan II = l-(phenylazo)-2-naphtol Sudan III = l-(4-phenylazophenylazo)-2-naphtol Sudan IV = o-tolyazo-o-tolyazo-beta-naphtol and Disperse Orange 13 = 4-[4-(phenylazo)-l-naphtylazo]-phenol. Azo dyes were separated in an ODS column (250 x 2.1 mm i.d. particle size 5 /xm) at 35°C. The isocratic mobile phase consisted of 0.1 per cent formic acid in methanol-0.1 per cent formic acid in water (97 3, v/v). The flow rate was 200 /xl/min. MS conditions were nebulizing and desolvation gas were nitrogen at the flow rates of 50 and 5551/h, respectively electrospray voltage, 3.0 kV cone voltage 25 V source temperature, 110°C desolvation temperature, 110°C. Azo dyes were extracted from the samples by homogenizing 1 g of sample with 10 ml of acetone, then the suspension was centrifuged and an aliquot of 3 ml of supernatant was mixed with 1 ml of deionized water, filtered and used for analysis. LC-ESI-MS/Ms SRM traces of standards and spiked samples are listed in Fig. 3.39. It was found that the detection and quantitation limits depended on both the chemical structure of the dye and the character of the accompanying matrix. LOD and LOQ values in chilli tomato sauce...

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