Vicente Hernandis

Liquid-sample introduction in plasma spectrometry

Plasma-spectrometry techniques, namely inductively coupled plasma atomic emission spectrometry (ICP-AES) and plasma-based mass spectrometry (MS), are the most commonly used in analytical laboratories for elemental analysis in a wide variety of samples. In these techniques, the quality of the analysis strongly depends on appropriate selection of the sample-introduction system. For liquid samples, it basically comprises a nebulizer, which transforms the bulk solution into an aerosol, and a spray chamber, which modifies the characteristics of this aerosol and transports it to the plasma base through an injector tube. Sometimes, a desolvation system is incorporated to reduce the solvent load into the plasma. This article describes the different components of the sample-introduction system, emphasizing their main advantages and drawbacks. A review of the processes that affect the aerosol between generation and reaching the plasma is also included.

Comparison of characteristics and limits of detection of pneumatic micronebulizers and a conventional nebulizer operating at low uptake rates in ICP-AES

Three micronebulizers, the high-efficiency nebulizer (HEN), the microconcentric nebulizer (MCN) and the micromist (MM), were compared with a conventional pneumatic concentric nebulizer working at low liquid flow rates in ICP-AES. The gas back-pressure, the free liquid aspiration rate, the drop size distribution of primary and tertiary aerosols, the solvent and analyte transport rates, the emission intensity and the limits of detection were measured. The solvent evaporation inside the spray chamber proved to be a very important transport phenomenon when working at very low liquid flow rates. The micronebulizers produced finer primary aerosols, higher solution transport rates through the spray chamber and higher sensitivities than the conventional pneumatic concentric nebulizer. The HEN used in this work provided slightly lower ICP-AES limits of detection than the other two micronebulizers, but at the expense of a higher back-pressure.

Effect of analyte and solvent transport on signal intensities in inductively coupled plasma atomic emission spectrometry

Abstract Aerosols generated with several different nebulizers have been characterized by their major aerosol characteristics, namely the Sauter mean drop diameter, D3,2, the analyte mass transport, Wtot, the solvent mass transport, Stot, the analyte transport efficiency, ϵn, and the solvent transport efficiency, ϵs. These characteristic aerosol parameters have been compared with the magnitudes of analytical signals, Inet, generated in an inductively coupled plasma, in order to examine the possible existence of interrelationships between them. For three of the nebulizers tested, a reasonably linear correlation between Wtot and Inet was observed with water as solvent. For the fourth, a crossflow-type nebulizer, which produced a tertiary aerosol with a significantly larger mean drop size than the other nebulizers, the relationship between Wtot and Inet was not linear. For the organic solvents tested, methanol and n-butanol, solvent/plasma interactions were sufficiently strong as to make the relationship between Wtot and Inet quite complex.

Fundamental studies on pneumatic generation and aerosol transport in atomic spectrometry: effect of mineral acids on emission intensity in inductively coupled plasma atomic emission spectrometry☆

Abstract The mechanism of the mineral acid interference has been studied in ICP-AES. For this study five mineral acids have been evaluated (HCl, HNO3, HClO4, H2SO4 and H3PO4) in four concentrations (0, 0.5, 5 and 30%). In order to investigate this interference emission signal, sample uptake rate, primary and tertiary drop size distributions, total analyte transport rate and excitation temperature have been measured. From the results obtained, it seems that this interference is contributed by a reduction of the analyte transport rate and, also, by a decrease in the plasma temperature. The degree of the contribution to the interference of each one of these causes depends on the type of acid and sample uptake mode. The physical properties of the acid solutions are in the origin of the interference. These physical properties modify the sample uptake rate and/or the primary drop size distribution of the aerosols. The acids evaluated can be classified in two groups. The first group would consist of HCl, HNO3 and HClO4, and the second one of H2SO4 and H3PO4. In natural uptake mode the interference is mainly due to changes in sample uptake rate, and in controlled uptake mode to changes in primary drop size distribution of the aerosols. In both sample uptake modes a density-effect may appear on increasing acid concentration. All these factors tend to decrease the analyte transport rate and, hence, the emission signal. Finally, a cooling effect of the plasma due to a higher load of acids is superimposed to these causes. We think that from this study the mineral acid interference in ICP-AES, with pneumatic nebulization, should be better understood.

Empirical model for estimating drop size distributions of aerosols generated by inductively coupled plasma nebulizers

Abstract Drop size distributions have been determined for two inductively coupled plasma concentric nebulizers using laser diffraction measurements. The influence on drop size distributions of (1) gas and liquid flows (2) physical properties of several solvents and gases, and (3) principal dimensions of the nebulizers are reported. An empirical equation is developed which allows the prediction of two key parameters of primary aerosols (e.g. Sauter mean diameter and span). The equation has been checked by estimating Sauter mean diameters and spans for two additional concentric nebulizers, and good agreement has been obtained between experimental and estimated values. By comparison, estimates of Sauter mean diameters obtained from the Nukiyama and Tanasawa equation have been shown to differ markedly from experimental values. From the present study it may be concluded that gas velocity is the most critical variable in nebulizer operation, and that in order to obtain aerosols with small mean drop sizes and narrow distributions, a nebulizer with small gas cross-sectional area should be used, and operated with high gas flows. Organic solvents possessing low surface tensions are shown to lead to smaller mean drop sizes than aqueous solvents under all comparable operating conditions.

Evolution of drop size distributions for pneumatically generated aerosols in inductively coupled plasma-atomic emission spectrometry

Abstract Drop size distributions of pneumatically generated aerosols used for inductively coupled plasma-atomic emission spectrometry have been measured using a laser Fraunhofer diffraction system at several locations along their trajectories in a spray chamber. The influence of: (1) nebulizing gas and liquid flows; (2) design and dimensions of nebulizers; and (3) physical properties of solvents on drop size distributions are reported. Droplet coalescence and evaporation appear to be the main factors influencing the aerosol size distributions in the stages immediately following primary aerosol formation. Differences in particle and gas velocities can also influence evaporation rates and coagulation processes. In the middle of the spray chamber, impaction loss processes predominate. The magnitude of such losses increases as the droplet mean size increases. At the bottom wall of the spray chamber, the aerosol distribution is modified mainly through inertial losses. For a given chamber, the coarser the aerosol distribution just before this point is, the greater the efficiency of the inertial loss process will be. For a fixed geometry spray chamber any factors which tend to increase the mean drop size will also tend to increase the efficiency of impaction, gravitational and inertial loss processes. Factors which tend to increase the mean drop size are low nebulizing gas flow and high liquid flow, high surface tension, low volatility, large cross-sectional area for the gas outlet, and the use of a cross-flow design nebulizer. Volatilization and, to a lesser extent, coalescence processes are increased by the factors that tend to decrease mean drop size. The higher ratio of surface area to mass enhances evaporation and the greater particle density increases the likelihood of particle collisions.

Experimental evaluation of the Nukiyama-Tanasawa equation for pneumatic nebulisers used in plasma atomic emission spectrometry

Drop size distributions for aerosols generated pneumatically were measured by means of a laser Fraunhofer diffraction system. The Sauter mean diameters of these aerosols were compared with those predicted by the Nukiyama-Tanasawa equation. In this study the following variables were considered: dimensions and design of the nebuliser nozzle, solvent nature and gas and liquid flow-rates. These parameters include all the variables contained in the Nukiyama-Tanasawa equation. Three main conclusions can be drawn from the results obtained: (i) in all situations predictions of the mean drop size made using the Nukiyama-Tanasawa equation are greatly in excess of the experimental results; (ii) under a given set of experimental conditions the Nukiyama-Tanasawa equation predicts that aerosols will be produced with larger mean diameters for some organic solvents than for water, whereas it is found experimentally that mean diameters for water aerosols are always larger than for organic solvents, and correlate well with their surface tension values; (iii) the nebuliser performance (i.e., its ability to generate as fine an aerosol as possible) increases as the nebulising gas cross-sectional area becomes, smaller, and also, though to a lesser extent, as the liquid cross-sectional area becomes smaller.

Comparative Study of Several Nebulizers in Inductively CoupledPlasma Atomic Emission Spectrometry: Low-pressureversusHigh-pressure Nebulization

Five nebulizers for use in ICP-AES were compared. Two of them work at low pressure, a Meinhard and a V-groove nebulizer (VGN), and three at high pressure, a single-bore high-pressure pneumatic nebulizer (SBHPPN), a hydraulic high-pressure nebulizer and a thermospray (TN). The comparison was made using three solvents, water, ethanol and butan-1-ol, using the sample uptake rate ( Q l ) as a variable and studying its influence on drop size distribution, analyte transport rate and analytical behaviour, i.e. , emission intensity and limits of detection (LODs). The sample introduction system includes a desolvation unit. The Sauter mean diameters of the primary aerosols generated by the high-pressure nebulizers (HPNs) are between 1.5 and 5.8 times lower than those generated by the low-pressure nebulizers (LPNs), this reduction being more noticeable at high liquid flow rates. In addition, at high liquid flow rates, HPNs achieve higher analyte transport rates (between 2.4 and 19 times higher), higher emission signals (up to 1.8 times for methanol and up to 4.5 times for water, using the Mn II 257.610 nm line) and lower LODs for nine elements than the LPNs. Among HPNs, the SBHPPN gives rise to the best results at low Q l ( i.e. , 0.6 ml min -1 ), whereas at high Q l ( i.e. , 1.2 ml min -1 ) the results are similar for all three HPNs when using methanol and butan-1-ol. With water, at high Q l , the TN gives the best results. For all the nebulizers tested, organic solvents (methanol and butan-1-ol) provide better results than water, the relative improvement being more important for LPNs ( e.g. , with VGN at 1.2 ml min -1 , the improvement with methanol over water for Mn II is around sixfold) than for HPNs ( e.g. , when SBHPPN is used at 1.2 ml min -1 for Mn II this improvement is 4.5-fold).

A microwave-powered thermospray nebulizer for liquid sample introduction in inductively coupled plasma atomic emission spectrometry.

A new thermospray nebulizer based on the absorption of microwave radiation (MWTN) by aqueous solutions of strong acids is presented for the first time. To this end, a given length of the sample capillary is placed inside the cavity of a focused microwave system. A small piece of a narrower capillary tubing is connected at the tip of the sample capillary, outside the microwave cavity, to build up pressure. Drop size distributions of primary aerosols are exhaustively measured in order to evaluate the influence of several experimental variables (microwave power, liquid flow, irradiation length, inner diameter of the outlet capillary, nature and concentration of the acid) on the characteristics of the primary aerosol that are related to the emission signal. These experiments have been performed mainly to increase our understanding of the microscopic process of this new type of aerosol generation. A standard Meinhard nebulizer was employed for comparison. Under the best conditions the entire aerosol volume is contained in droplets smaller than 20 μm compared with 45% of the volume of the aerosol generated by the Meinhard. Hence, higher analyte and aerosol transport rates are to be expected for the MWTN compared with the Meinhard nebulizer. As any highly efficient nebulizer, MWTN requires a desolvation unit. For solutions 0.75 M in strong acid, the new nebulizer improves sensitivity (1.0-2.8 times), limits of detection (1.2-3.0 times), and background equivalent concentration (0.9-2.0 times) as compared to the standard Meinhard nebulizer, features many of the advantages of the conventional thermospray nebulizer, and overcomes some of its drawbacks (MWTN does not show corrosion problems and works at lower pressure, the aerosol characteristics are not modified when the PTFE capillary is replaced).

Determination of metals in lubricating oils by flame atomic absorption spectrometry using a single-bore high-pressure pneumatic nebulizer

The behaviour of a single-bore high-pressure pneumatic nebulizer (SBHPPN) as a tool for the analysis of lubricating oils by flame atomic absorption spectrometry (FAAS) was investigated. The effects of the sample oil content [from 10% to 100% (w/w) oil in 4-methylpentan-2-one, IBMK] and the carrier nature (IBMK and methanol) on the characteristics of the aerosols generated, on the analyte transport efficiency and on the analytical figures of merit in FAAS were studied. A pneumatic concentric nebulizer (PCN) was used for comparison. Increasing the oil content increases the viscosity of the sample. With the PCN this gives rise to coarser aerosols, making it impossible to nebulize samples with an oil content higher than 70% (w/w). Using the SBHPPN, the viscosity of the sample scarcely affects the characteristics of the primary aerosols. Hence, the SBHPPN is able, by using the appropriate carrier, to nebulize pure lubricating oils. Among the carriers tested, IBMK is the most advisable because it is fully miscible with all the oil samples. The SBHPPN provides higher sensitivities and lower limits of detection than the PCN. Compared with a method based on organic dilution, the use of the SBHPPN for the direct analysis of lubricating oils by FAAS makes it possible, in addition to increasing the analysis throughput, to detect elements at lower concentrations. Moreover, the SBHPPN provides similar results to those obtained using a previous acid digestion step.