Free sample aspiration

The physical properties of the liquid sample (a, tj and p) also influence d0, the efficiency and the analytical signals. Differences in the physical properties of the liquid samples to be analyzed thus lead to so-called nebulization effects, tj has a large influence. It influences d0 through the second term in Eq. (218). However, in the case of free sample aspiration it also influences do through Ql, which is given by the Poiseuille law as ... 

Nebulization effects As discussed earlier, differences in the physical properties of the different sample and calibration solutions lead to variations in the aerosol droplet size and thus also in the efficiency of the nebulizer and the sample introduction. This effect is strongest in the case of free sample aspiration and relatively low nebulizer gas flow and can be minimized (see Section 3.1). 

Precision For absorbances greater than 0.1-0.2, the relative standard deviation for atomic absorption is 0.3-1% for flame atomization, and 1-5% for electrothermal atomization. The principal limitation is the variation in the concentration of free-analyte atoms resulting from a nonuniform rate of aspiration, nebulization, and atomization in flame atomizers, and the consistency with which the sample is heated during electrothermal atomization. 

The total consumption type of burner consists of three concentric tubes as shown in Fig. 21.5. The sample solution is carried by a fine capillary tube A directly into the flame. The fuel gas and the oxidant gas are carried along separate tubes so that they only mix at the tip of the burner. Since all the liquid sample which is aspirated by the capillary tube reaches the flame, it would appear that this type of burner should be more efficient that the pre-mix type of burner. However, the total consumption burner gives a flame of relatively short path length, and hence such burners are predominantly used for flame emission studies. This type of burner has the advantages that (1) it is simple to manufacture, (2) it allows a totally representative sample to reach the flame, and (3) it is free from explosion hazards arising from unbumt gas mixtures. Its disadvantages are that (1) the aspiration rate varies with different solvents, and (2) there is a tendency for incrustations to form at the tip of the burner which can lead to variations in the signal recorded. 

The solution to be nebulized is usually pumped to the nebulizer using a peristaltic pump, unlike for FAAS, where the solution uptake is by free aspiration. The solution is pumped through polymeric tubing [usually poly(vinyl chloride)] and also connecting tubing (usually Teflon) to the nebulizer. Both of these materials can be manufactured to a high degree of purity, hence contamination is minimized. The solution is pumped at a rate of 1 -2 ml min, which is much slower than the 5-10 ml min uptake rate for FAAS. This tends to favour the formation of fewer but smaller droplets, which results in less noise but a lower overall sample transport efficiency. 

A flame, where the solution of the sample is aspirated. Typically, in FAAS the liquid sample is first converted into a fine spray or mist (this step is called nebulisation). Then, the spray reaches the atomiser (flame) where desolvation, volatilisation and dissociation take place to produce gaseous free atoms. Most common flames are composed of acetylene-air, with a temperature of 2100-2400 °C, and acetylene-nitrous oxide, with a temperature of 2600-2900 °C. 

Atomic Absorption Spectroscopy. One of the more sensitive instruments used to detect metal-containing toxicants is the AA spectrophotometer. Samples are vaporized either by aspiration into an acetylene flame or by carbon rod atomization in a graphite cup or tube (flameless AA). The atomic vapor formed contains free atoms of an element in their ground state, and when illuminated by a light source that radiates light of a... 

Nebulizers and Spray Chambers The nebulizer converts the sample liquid into an aerosol. Unlike FAAS/FAES, where solution uptake is by free aspiration, the solution to be nebulized in ICP is usually moved by a peristaltic pump. 

CCI3F (F-11) and CCI2F2 (F-12) shows the bucket samples to be free of contamination. The extraction flask is filled by attaching a stainless steel funnel (1 L) to the flask and filling the funnel with seawater from the bucket. About 650 mL of water is aspirated into the flask without allowing any air to enter. The flask is then pressurized with 15 psi of zero air. The results are equal volumes of air and water equilibrated at 25 °C and 1-atm pressure. Containing the sample under positive pressure reduces contamination... 

Sample dissolution is probably one of the most common operations in analytical chemistry and is carried out by dissolving in a suitable solvent to a suitable concentration that the analyte of interest can be reproducibly measured. If the composition of the non-aqueous solution is amenable to combustion in a flame or plasma, direct aspiration is possible. Unfortunately, ICP-AES instruments do not have the same solvent tolerance as AAS and require that the solvent selected be stable, non-quenching and non-interfering. Calibration standards are usually prepared in the same metal-free solvent, keeping in mind the effect of sample in the solvent. If the nebulisation efficiency of sample/solvent mixture is different to standards prepared in the same solvent only, then corrective actions must be taken so this anomaly can be taken into consideration. 

Most of the assays now carried out with isoluminol derivatives are solid-phase competitive assays (Fig. 2), in which the analyte to be determined competes with the labeled analyte for the available binding sites on antibodies that are immobilized on the solid phase. Thus, plastic microspheres have been used to immobilize antibodies for estriol (K12) or thyroxine (W9). Estradiol antibodies have been immobilized onto plastic tubes (K7) or beads (K16). In all of the above cases, the amount of immunoconjugated hormone label is inversely proportional to the amount of free hormone in the analytical sample. Unconjugated labels are removed by decantation or aspiration from the solid phase, and the specific, immunoconjugated labels are measured in a luminometer. This approach has been successfully applied to progesterone analyses in either serum (D6) or saliva (D5). A review of several separation-based assays with isoluminol analogs was presented a few years ago by Kohen et al. (K18). 

Sample preparation with flame methods can often be kept to a minimum. As long as chemical or spectral interferences are absent, essentially all that is required is to obtain the sample in tbe form of a diluted and filtered (for particulates) solution. It often makes no difference what the chemical form of the analyte is because it will be dissociated to the free elemental vapor in the flame. Thus, several elements can be determined in blood, urine, cerebral spinal fluid, and other biological fluids by direct aspiration of the sample. Usually, dilution with water will be required to prevent clogging of the burner. 

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