Solvents nebulisation efficiencies

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. 

Figure 3.7 shows the results of a study of nebulisation efficiencies of solvents compatible with the ICP-AES plasma source under normal conditions (as listed in Table 3.4). An accurate volume of 50.0 ml of each solvent was nebulised through the plasma under normal conditions. The resultant waste volume was collected into a measuring cylinder in triplicate to give the values listed in Table 3.4. The efficiency was calculated from the average. The behaviour of each solvent was also noted under Remarks [15]. 

The sample transport system, nebuliser and spray chamber are designed to ensure the maximum amount of sample reaches the atomisation source without quenching it. Only a few solvents can be used that are compatible with direct injection to ICP-OES (see Table 3.5) and these solvents have been studied as part of nebulisation efficiency. 

Conclusion to Study of Non-Destructive Methods of Metal Analysis of Oil Products. The results in Table 5.9 show accurate results for analysis of metal spiked low viscosity (Conostan 20 blend) oil when analysed against a standard calibration curve in solvents kerosene, decalin and tetralin, respectively. The scatter of results for the six measurements of each sample is acceptable. The results for higher viscosity (Conostan 75 blend) oil gave consistently lower values, which illustrates the effect of viscosity on the nebulisation efficiency. 

This manifold architecture has been more widely used with flame atomic absorption spectrometry and other detection techniques using a nebuliser [176] which are less affected by the Schlieren effect. The reported sensitivity and /or selectivity enhancements are due to the beneficial influence of solvent addition on the nebuliser efficiency. The nebuliser also acts as a phase-separating device, predominantly directing one phase towards detection and the other towards waste surprisingly, this latter aspect has not been fully investigated. 

ES ionisation can be pneumatically assisted by a nebulising gas a variant called ionspray (IS) [129]. ESI is conducted at near ambient temperature too high a temperature will cause the solvent to start evaporating before it reaches the tip of the capillary, causing decomposition of the analyte during ionisation and too low a temperature will allow excess solvent to accumulate in the sources. Table 6.20 indicates the electrospray ionisation efficiency for various solvents. 

As the vast majority of LC separations are carried out by means of gradient-elution RPLC, solvent-elimination RPLC-FUR interfaces suitable for the elimination of aqueous eluent contents are of considerable use. RPLC-FTTR systems based on TSP, PB and ultrasonic nebulisa-tion can handle relatively high flows of aqueous eluents (0.3-1 ml.min 1) and allow the use of conventional-size LC. However, due to diffuse spray characteristics and poor efficiency of analyte transfer to the substrate, their applicability is limited, with moderate (100 ng) to unfavourable (l-10pg) identification limits (mass injected). Better results (0.5-5 ng injected) are obtained with pneumatic and electrospray nebulisers, especially in combination with ZnSe substrates. Pneumatic LC-FI1R interfaces combine rapid solvent elimination with a relatively narrow spray. This allows deposition of analytes in narrow spots, so that FUR transmission microscopy achieves mass sensitivities in the low- or even sub-ng range. The flow-rates that can be handled directly by these systems are 2-50 pLmin-1, which means that micro- or narrow-bore LC (i.d. 0.2-1 mm) has to be applied. 

Physical (transport) interferences. This source of interference is particularly important in all nebulisation-based methods because the liquid sample must be aspirated and transported reproducibly. Changes in the solvent, viscosity, density and surface tension of the aspirated solutions will affect the final efficiency of the nebulisation and transport processes and will modify the final density of analyte atoms in the atomiser. 

The most effective solvents for use in atomic absorption are medium weight, low volatile aliphatics, alcohols and ketones. Frequently used solvents are methyl isobutyl ketone (MIBK) and ethyl propionate. These solvents have viscosities and surface tensions such that the efficiency of nebulisation is increased. 

The advantage of using APDC is the metal salts are readily soluble in most organic solvents and will separate them from high concentrations of other solutes that could cause difficulties in nebulisation and atomisation. Large bulk of aqueous sample may be extracted efficiently into a smaller volume of an organic solvent and this can be further... 

ETV, as a sample introduction method for ICP-MS elemental analysis, offers several advantages over conventional nebulisation systems. These advantages are related to higher analyte transmission efficiency, use of reduced sample volumes, achievement of very low detection limits and the ability to remove solvent and matrix components, which help in avoiding spectral and non-spectral interferences (see Chapter 3 for a detailed discussion of ETV-ICP-MS). 

When an aerosol is generated at liquid flow rates of the order of several microlitres to tens of microlitres per minute, the relative significance of the aerosol transport phenomena is different with respect to that at hquid flow rates in the mL/min range. Solvent evaporation becomes much more pronounced and droplet coalescence is much less significant under the former nebulisation conditions. Both effects combine to increase the analyte transport efficiency. 

In ICP-MS, the desolvation of an aerosol can be beneficial in several cases (i) when using efficient nebulisers such as ultrasonic nebulisers or thermospray devices, (ii) when analysing samples containing organic solvents, (hi) when an increase in the sensitivity is required and (iv) when trying to remove polyatomic interferences. 

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