Raquel Sánchez

Effect of the silicon chemical form on the emission intensity in inductively coupled plasma atomic emission spectrometry for xylene matrices

Silicon is present in petrol products under very different chemical forms. The effect of the silicon chemical form on the sensitivity was studied by inductively coupled plasma atomic emission spectrometry (ICP-AES). Solutions containing sixteen different silicon organic compounds with the same silicon concentration were prepared in xylene. The studied compounds were: six siloxanes, nine silanes and a silicate. Their boiling points ranged from 101 °C (hexamethyldisiloxane) to 310 °C (2,3,4-(epoxycyclohexyl)ethyltrimetoxysilane). The results indicated that the ICP-AES signal depended strongly on the silicon compound. Thus, for a commonly used sample introduction system consisting of a cross-flow pneumatic nebulizer coupled to a Ryton double pass spray chamber, the signal varied by a factor of up to 17, depending on the compound. For most studied cases, some siloxanes yielded higher emission intensities than silanes. In addition, under some circumstances, hexylmethyldichlorosilane and dimethyldodecylchlorosilane also produced higher signals than the other studied silanes. In order to elucidate the possible mechanisms for these changes, a series of experiments was performed to verify whether the origin could be assigned either to the plasma or to the sample introduction system. Using the Mg II/Mg I ratio, no significant change in the plasma characteristics was actually observed. As regards the sample introduction system, (i) the primary aerosol, (ii) the transport and memory effects using continuous and discrete introduction modes, an alternative spray chamber, and a total consumption system (the TISIS system), and (iii) the tertiary aerosol, via an impactor, were studied. Additional experiments were carried out with an ultrasonic nebulizer. In addition, the Si behaviour was compared to other elements, such as Cu, Mg, Mn and Zn. The results indicated that the Si behaviour was not related to the pneumatic nebulizer and suggested that a fraction of silicon was transported towards the plasma as a vapour when it was present as hexamethyldisiloxane (i.e., the compound with the lowest boiling point among the species studied). In any case, the variability of the signal induced by the different silicon chemical forms was less significant for the TISIS than for cyclonic and ultrasonic nebulizer. The observed results were specific to silicon as the signal of the other elements did not significantly change with silicon chemical form. As a consequence, internal standardization was not appropriate for mitigating interferences on silicon when petroleum products were analyzed.

Metal and metalloid determination in biodiesel and bioethanol

Biofuel quality control involves the determination of metal and metalloid content. These species play a very important role because they may modify the efficiency of biofuel production as well as the stability of these products. Furthermore, some metals are toxic and generate environmental concerns whereas others are used as additives. Normally, products such as biodiesel and bioethanol are mixed with conventional fossil fuels (diesel and gasoline, respectively). Therefore, metals come from the raw product employed for biofuel production (seeds, sugars…) as well as from the production and storage process or even from the added fuels. The determination of the final metal and metalloid concentration in biofuels is a challenging subject because of several reasons. On the one hand, their content is usually low (i.e., from several μg L−1 to mg L−1) and, hence, sensitive techniques should be used. Besides all these, calibration with organic complex matrices becomes more difficult and degrades the accuracy of the determination. Several approaches have been evaluated to carry out this kind of analysis going from spectrochemical to electroanalytical techniques. Within the first group, Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) and Mass Spectrometry (ICP-MS) are often employed together with atomic absorption methods. The different procedures applied will be discussed in the present review emphasizing the most widely employed ones. On this subject, fundamental as well as applied studies related to the biofuel analysis through ICP-OES and ICP-MS will be shown to illustrate the current difficulties associated with these determinations. Comments regarding the possible solutions proposed to overcome the drawbacks encountered will be made.

Effect of solvent dilution on the ICP-AES based silicon sensitivity, the aerosol characteristics and the resulting organic solution properties in the analysis of petroleum products

The effects of the solvent dilution factor on the physical properties of the resulting organic solutions, the aerosol characteristics and the silicon sensitivity were studied in ICP-AES for four different petroleum products by using near total sample consumption systems. The four samples were two gasoline products having very different volatilities along with a kerosene and a diesel sample. Petroleum product samples were diluted with xylene using four sample dilutions; 1:2, 1:5, 1:10 and 1:50. The sample introduction systems were a single pass spray chamber associated with a micronebulizer and a demountable Direct Injection High Efficiency Nebulizer (d-DIHEN). A cyclonic spray chamber also associated with a micronebulizer was taken as the reference system. Silicon was used as the test element, because it has been previously demonstrated that the ICP-AES Si sensitivity was significantly modified according to its chemical form. Silicon was spiked in each diluted solution with the same concentration to test sensitivity. When considering the dilution factor as the key variable, it was found that for the two gasoline samples and the kerosene one, the higher this variable, the lower the sensitivity. This result was explained in terms of changes in the solution volatility and/or in the aerosol characteristics. It was also observed that the total sample consumption systems were less sensitive to changes in the properties of the resulting organic solutions than the system based on the cyclonic spray chamber. However, for the latter chamber, the properties of the resulting organic solution had a marked influence on the extent of the effect of the silicon chemical form on the sensitivity. This fact demonstrated the appearance of an undefined interaction between the analyte and the organic solution during the aerosol transport step. However, both the single pass spray chamber and the d-DIHEN mitigated this effect for all the samples.

Minimization of the effect of silicon chemical form in xylene matrices on ICP-AES performance

It has been shown previously that the influence of the silicon chemical form could be minimized in ICP-AES by using high efficiency sample introduction systems. In this work, the effects of two of the most different silicon chemical forms have been minimized in ICP-AES by using near total sample consumption systems. The two tested devices have been a single pass spray chamber equipped with wall heating (heated torch integrated sample introduction system, h-TISIS) and a demountable direct injection high efficiency nebulizer (d-DIHEN). A room temperature TISIS and a cyclonic spray chamber have been taken as the reference systems. Solutions containing silicon as six different compounds in xylene have been prepared and analyzed through ICP-AES. It has been observed that, at low liquid flow rates, the h-TISIS improves the sensitivity and the limits of detection as compared to the d-DIHEN. In contrast, shorter wash-out times are obtained for the d-DIHEN. As regards to the effect of the silicon chemical form, h-TISIS and d-DIHEN have minimized it with respect to a conventional cyclonic spray chamber.

Air-segmented, 5-μL flow injection associated with a 200 °C heated chamber to minimize plasma loading limitations and difference of behaviour between alkanes, aromatic compounds and petroleum products in inductively coupled plasma atomic emission spectrometry

A method based on the use of a high temperature single pass spray chamber and the injection of a sample plug into an air carrier gas stream was developed to mitigate non spectral interferences caused by organic samples and petroleum products and to reduce plasma loading in Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES). The studied solvents were eleven alkanes (hexane, heptane, octane, nonane, decane, undecane, dodecane, tridecane, tetradecane, pentadecane, hexadecane), xylene, kerosene and tetralin. As regards to the real samples two gasolines, a kerosene and a diesel sample were taken. The evaluated sample introduction systems were a 12 cm3 inner volume single pass spray chamber (also called Torch Integrated Sample Introduction System, TISIS) with and without heating and a 40 cm3 inner volume cyclonic spray chamber that was taken as a reference device. A characterization of the matrix effect in continuous aspiration mode at a 30 μl min−1 flow rate was initially performed. Drop size distributions were measured for the aerosols generated by the nebulizer (primary aerosols) and those leaving the spray chamber (tertiary aerosols). The results proved that the median of the aerosol volume drop size distribution (D50) for primary aerosols took values from 13.2 to 15.3 μm. Meanwhile, tertiary ones changed more significantly as a function of both the solvent nature and the chamber temperature. They went from 2 to 4 μm for the TISIS at room temperature, whereas at 100 °C D50 was included within the 0.7 to 3.1 μm range. The analyte mass transported towards the plasma was also measured and it was confirmed that this parameter was directly related to the solvent volatility. Thus, at room temperature, efficiencies went from 20 to 60% for hexadecane and octane, respectively. ICP-AES sensitivities changed significantly as a function of the solvent. For real samples, heating of the chamber walls mitigated the interferences, thus, while at room temperature, gasoline samples provided more than one order of magnitude higher signals than diesel samples, at 100 °C this signal improvement factor was only of five. All these problems were mostly overcome when the segmented injection of a 5 μl sample plug was performed. It was concluded that, for all the solutions at 200 °C heating temperature the injected sample volume (c.a., 5 μl) evaporated completely before its further introduction into the plasma. Therefore, differences in analyte mass transported as a function of the solution matrix were mitigated.

Improving the analytical performances of ICP-AES by using a high-temperature single-pass spray chamber and segmented-injections micro-sample introduction for the analysis of environmental samples

A new method based on the use of a high-temperature single-pass spray chamber and the injection of a sample plug into an air carrier gas stream was developed to mitigate non-spectral interferences caused by inorganic concomitants and to reduce plasma loading in inductively coupled plasma atomic emission spectrometry (ICP-AES). The evaluated sample introduction systems were a 10 cm3 inner-volume single-pass spray chamber (also called the Torch Integrated Sample Introduction System, TISIS) with and without heating and a cinnabar spray chamber, taken as a reference device. The temperature of the spray chamber was raised up to 350 °C. Sensitivity, memory effects, limits of detection and non-spectral interferences were evaluated. The results proved that the higher the chamber walls temperature, the higher the peak height and the lower the memory effects. The single-pass spray chamber heated at 350 °C provided lower limits of detection (0.3–2.3 μg l−1) compared with the reference spray chamber (1–33 μg l−1) and a significant reduction of matrix effects. This device was successfully applied to the analysis of environmental certified reference materials, such as marine sediments and animals. The calibration curve was obtained by modifying the mass of the analyte injected, thus requiring only one standard solution. The results were also compared with those obtained by external calibration (both at room temperature and at 350 °C) and single-point standard addition. The analytical bias was lower than 5%, showing that this sample introduction system is adequate to remove the matrix effects due to inorganic concomitants, allowing the accurate analysis of environmental samples.

Quantification of nickel, vanadium and manganese in petroleum products and biofuels through inductively coupled plasma mass spectrometry equipped with a high temperature single pass spray chamber

A heated Torch Integrated Sample Introduction System (hTISIS) has been applied to the analysis of petroleum products and biofuels through Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Three elements have been determined because of their importance in the petroleum industry: V, Ni and Mn. Sample injection has been accomplished by means of the introduction of a low sample volume (2.5 μl) into an air carrier stream. A peak has been thus obtained. Two sets of samples have been selected: five solvents (xylene, kerosene, nonane, undecane and hexadecane) and five real samples (biodiesel, diesel, kerosene, superethanol and gasoline). The chamber temperature has been varied when introducing either solvents or real samples. In both cases it has been found that sensitivity peaked at 110 °C heating temperature. However, non-spectral interferences caused by differences in the matrix composition became less severe as this variable was increased and they were virtually eliminated at temperatures of 150 °C (alkanes) and 200 °C (real samples). When comparing with a default spray chamber (i.e., conical chamber with an impact bead) 3 to 6 times lower LODs were obtained. At 150 °C, this parameter has taken values of approximately 80 ng l−1 for V and Ni to 140 ng l−1 for Mn. At 200 °C heating temperature it has been possible to carry out accurate ICP-MS determinations by applying external calibration. Additional advantages of the present approach were that no oxygen was required to avoid soot deposition at the sampler cone and that nickel, instead of platinum cones, was used.

Fully Automatic In-Syringe Magnetic Stirring-Assisted Dispersive Liquid–Liquid Microextraction Hyphenated to High-Temperature Torch Integrated Sample Introduction System-Inductively Coupled Plasma Spectrometer with Direct Injection of the Organic Phase

A proof of concept study involving the online coupling of automatic dispersive liquid-liquid microextraction (DLLME) to inductively coupled plasma optical emission spectrometry (ICP OES) with direct introduction and analysis of the organic extract is herein reported for the first time. The flow-based analyzer features a lab-in-syringe (LIS) setup with an integrated stirring system, a Meinhard nebulizer in combination with a heated single-pass spray chamber, and a rotary injection valve, used as an online interface between the microextraction system and the detection instrument. Air-segmented flow was used for delivery of a fraction of the nonwater miscible extraction phase, 12 μL of xylene, to the nebulizer. All sample preparative steps including magnetic stirring assisted DLLME were carried out inside the syringe void volume as a size-adaptable yet sealed mixing and extraction chamber. Determination of trace level concentrations of cadmium, copper, lead, and silver as model analytes has been demonstrated by microextraction as diethyldithiophosphate (DDTP) complexes. The automatic LIS-DLLME method features quantitative metal extraction, even in troublesome sample matrixes, such as seawater, salt, and fruit juices, with relative recoveries within the range of 94-103%, 93-100%, and 92-99%, respectively. Furthermore, no statistically significant differences at the 0.05 significance level were found between concentration values experimentally obtained and the certified values of two serum standard reference materials.

Aerosol-Phase Extraction Method for Determination of Ca, K, Mg, and Na in Biodiesel through Inductively Coupled Plasma Optical Emission Spectrometry

A novel extraction method was developed, optimized, and validated for the elemental analysis of organic samples. The method, called aerosol-phase extraction (APE), is based on nebulization of the extracting aqueous solution (0.1 mol·L-1 nitric acid) on the sample. Extraction was performed at the interface of generated extractant droplets as they entered into contact with the samples. Afterward, the phases were allowed to separate and Ca, K, Na, and Mg were determined in aqueous phase by means of inductively coupled plasma optical emission spectroscopy (ICP-OES). Measurement of aerosol characteristics demonstrated that a water-in-oil emulsion was generated. Therefore, once the aqueous solution was dispersed into the sample, the phases spontaneously separated. Furthermore, the interfacial specific surface area exhibited values on the order of 1 m2·mL-1, hence enhancing the extraction kinetics over conventional extraction methods. Key variables affecting the extraction yield were the nebulization gas flow rate, liquid flow rate, extraction time, acid concentration, nebulizer tip to sample surface gap, and morg/maq ratio. Once the optimal conditions were selected, the method was applied and validated for the determination of Ca, K, Na, and Mg by ICP-OES in 0.5 mL biodiesel samples with an expanded uncertainty lower than 2%. With the APE method, the extraction time was around 1 min, whereas conventional methods employed to perform this kind of extraction required from 4 to 50 min. Additionally, the APE involved preconcentration of analytes, thus lowering the limit of detection (LOD) to the nanograms per milliliter level (i.e., LODs based on the 3sb criterion were 32, 20, 19, and 24 ng·mL-1 for Ca, K, Na, and Mg, respectively). Furthermore, accuracy of quantification of Ca, K, Na, and Mg concentration by APE was not significantly different as compared to that afforded by conventional liquid-liquid extraction. Finally, Ca, K, Na, and Mg contents were determined in four real samples in the 0.5-13 mg·kg-1 range. The obtained results were not statistically different from those encountered with a microwave-based digestion method.