José Luis Todolí

Development of novel and sensitive methods for the determination of sulfide in aqueous samples by hydrogen sulfide generation-inductively coupled plasma-atomic emission spectroscopy

Two new, simple and accurate methods for the determination of sulfide (S(2-)) at low levels (microgL(-1)) in aqueous samples were developed. The generation of hydrogen sulfide (H(2)S) took place in a coil where sulfide reacted with hydrochloric acid. The resulting H(2)S was then introduced as a vapor into an inductively coupled plasma-atomic emission spectrometer (ICP-AES) and sulfur emission intensity was measured at 180.669nm. In comparison to when aqueous sulfide was introduced, the introduction of sulfur as H(2)S enhanced the sulfur signal emission. By setting a gas separator at the end of the reaction coil, reduced sulfur species in the form of H(2)S were removed from the water matrix, thus, interferences could be avoided. Alternatively, the gas separator was replaced by a nebulizer/spray chamber combination to introduce the sample matrix and reagents into the plasma. This methodology allowed the determination of both sulfide and sulfate in aqueous samples. For both methods the linear response was found to range from 5microgL(-1) to 25mgL(-1) of sulfide. Detection limits of 5microgL(-1) and 6microgL(-1) were obtained with and without the gas separator, respectively. These new methods were evaluated by comparison to the standard potentiometric method and were successfully applied to the analysis of reduced sulfur species in environmental waters.

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 the effect of acetic acid with axially and radially viewed inductively coupled plasma atomic emission spectrometry: influence of the operating conditions

Acetic acid was selected as a highly perturbing matrix for the comparison of matrix effects between axial and radial viewing modes in ICP-AES. The effect of acetic acid (5 and 10% v/v) on the intensities of various ionic lines in ICP-AES was studied under extreme operating conditions,i.e., robust and non-robust conditions. A cross-flow nebulizer associated with a double-pass spray chamber was chosen as a sample introduction system. The influence of the operating conditions on the Mg II 280 nm/Mg I 285 nm line intensity ratio was also studied. This work confirmed that, under so-called robust conditions, i.e., high rf power and low carrier gas flow rate, the acetic acid caused an enhancement of the signal intensity, regardless of the energy line and the viewing mode, which could be assigned to a change in the aerosol formation and transport. An increase in the solvent transport rate and a significant modification of the primary, and especially the tertiary, aerosol drop size distributions were observed in the presence of acetic acid. Hence, finer aerosols were obtained when acetic acid was present.

Heated-spray chamber-based low sample consumption system for inductively coupled plasma spectrometry

A systematic study about the effect of the heating of a modified version of the so-called Torch Integrated Sample Introduction System (TISIS) has been performed. The results showed that when working at liquid flow rates of 20–40 µl min−1, it was possible to raise the chamber wall temperature up to 100 °C without degradation of plasma thermal characteristics. Furthermore, a sheathing gas stream was added to the spray chamber so as to improve the aerosol transport towards the plasma. Under these conditions, sensitivity enhancements ranging from 8 to 15 with regard to those found under standard conditions (i.e., with neither chamber heating nor sheathing gas inlet) were reached for the different ionic and atomic emission lines studied. As a result, limits of detection at 20 µl min−1 were improved by a factor between 2.5 and 13. These LODs were only 1.7 to 2 times lower than for a cyclonic spray chamber at an uptake rate of 1 ml min−1. Two direct injection nebulizers were used for comparison: the conventional direct injection high efficiency nebulizer (DIHEN) and its demountable version (d-DIHEN). When TISIS was used under standard conditions quite similar results were found as those provided by d-DIHEN, although at low nebulizer gas flow rates (i.e., 0.1 l min−1), the DIHEN gave rise to higher sensitivities. However, by heating the TISIS chamber at 100 °C, sensitivities were enhanced by a factor up to one order of magnitude with respect to both the DIHEN and d-DIHEN. The result was ascribed both to the production of a finer tertiary aerosol and to the improved plasma excitation conditions. Furthermore, lower limits of detection were encountered for the hot chamber as compared to direct injection nebulizers. Hence, the developed system could be considered to be very promising for the analysis of very low liquid sample volumes through ICP techniques.

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.

Rapid and sensitive determination of carbohydrates in foods using high temperature liquid chromatography with evaporative light scattering detection.

In the present work, an evaporative light scattering detector was used as a high-temperature liquid chromatography detector for the determination of carbohydrates. The compounds studied were glucose, fructose, galactose, sucrose, maltose, and lactose. The effect of column temperature on the retention times and detectability of these compounds was investigated. Column heating temperatures ranged from 25 to 175°C. The optimum temperature in terms of peak resolution and detectability with pure water as mobile phase and a liquid flow rate of 1 mL/min was 150°C as it allowed the separation of glucose and the three disaccharides here considered in less than 3 min. These conditions were employed for lactose determination in milk samples. Limits of quantification were between 2 and 4.7 mg/L. On the other hand, a temperature gradient was developed for the simultaneous determination of glucose, fructose, and sucrose in orange juices, due to coelution of monosaccharides at temperatures higher than 70°C, being limits of quantifications between 8.5 and 12 mg/L. The proposed hyphenation was successfully applied to different types of milk and different varieties of oranges and mandarins. Recoveries for spiked samples were close to 100% for all the studied analytes.

Behaviour of a single-bore high-pressure pneumatic nebulizer operating with alcohols in inductively coupled plasma atomic emission spectrometry

The behaviour of a single-bore high-pressure pneumatic nebulizer (SBHPPN) with alcohols in (ICP-AES) was investigated. A standard Meinhard nebulizer was used for comparison. To this end, the drop size distribution of the primary aerosols, the analyte and solvent transport rates, Wtot and Stot, the fraction of solvent transported in liquid and vapour forms, Sliq and Svap, the excitation temperature of the plasma, Texc, and the molecular emission intensity of the C2 band were determined for solvents of different physical properties. The effect of the physical properties of the solvent on the nebulization process was also studied.The results show that the SBHPPN gives rise, for all the solvents studied, to primary aerosols that have smaller mean diameters than those produced by the Meinhard nebulizer for the same gas and liquid flows. The relative decrease in volume median diameter (Dv,50) when switching from the Meinhard nebulizer to the SBHPPN is more noticeable for alcohols than for water. Wtot is significantly higher for the SBHPPN than for the Meinhard nebulizer, particularly at high liquid flows. However, the differences between their Stot values are less pronounced. Under similar conditions, the SBHPPN gives rise, for all the solvents studied, to higher emission intensities than the Meinhard nebulizer. However, the relative signal enhancements achieved by changing from the SBHPPN to the Meinhard nebulizer are lower than the corresponding analyte transport enhancements. The relative signal enhancements achieved by switching from water to alcohols are lower for the SBHPPN than for the Meinhard nebulizer. This behaviour can be explained in terms of the higher Stot values associated with the SBHPPN, which lower Texc in comparison with the Meinhard nebulizer.

High-Temperature Liquid Chromatography Inductively Coupled Plasma Atomic Emission Spectrometry hyphenation for the combined organic and inorganic analysis of foodstuffs

The coupling of a High-Temperature Liquid Chromatography system (HTLC) with an Inductively Coupled Plasma Atomic Emission Spectrometer (ICP-AES) is reported for the first time. This hyphenation combines the separation efficiency of HTLC with the detection power of a simultaneous ICP-AES system and allows the combined determination of organic compound and metals. The effluents of the column were introduced into the spectrometer and the chromatograms for organic compounds were obtained by plotting the carbon emission signal at a characteristic wavelength versus time. As regards metals, they were determined by injecting a small sample volume between the exit of the column and the spectrometer and taking the emission intensity for each one of the elements simultaneously. Provided that in HTLC the effluents emerged at high temperatures, an aerosol was easily generated at the exit of the column. Therefore, the use of a pneumatic nebulizer as a component of a liquid sample introduction system in the ICP-AES could be avoided, thus reducing the peak dispersion and limits of detection by a factor of two. The fact that a hot liquid stream was nebulized made it necessary to use a thermostated spray chamber so as to avoid the plasma cooling as a cause of the excessive mass of solvent delivered to it. Due to the similarity in sample introduction, an Evaporative Light Scattering Detector (ELSD) was taken as a reference. Comparatively speaking, limits of detection were of the same order for both HTLC-ICP-AES and HTLC-ELSD, although the latter provided better results for some compounds (from 10 to 20 mg L(-1) and 5-10 mg L(-1), respectively). In contrast, the dynamic range for the new hyphenation was about two orders of magnitude wider. More importantly, HTLC-ICP-AES provided information about the content of both organic (glucose, sucrose, maltose and lactose at concentrations from roughly 10 to 400 mg L(-1)) as well as inorganic (magnesium, calcium, sodium, zinc, potassium and boron at levels included within the 6-3000 mg L(-1)) species. The new development was applied to the analysis of several food samples such as milk, cream, candy, isotonic beverage and beer. Good correlation was found between the data obtained for the two detectors used (i.e., ICP-AES and ELSD).