Effect of sodium during the aerosol transport and filtering in inductively coupled plasma atomic emission spectrometry

Robust conditions, i.e. high R.F. power and low carrier gas flow rate were used to minimize the role of the plasma and to emphasize the contribution of the sample introduction system in the effect of Na in ICP-AES. Two combinations of pneumatic nebulizers and spray chambers were selected to obtain different Na effects. A conespray nebulizer was used with a cyclone spray chamber, and a cross-flow nebulizer was coupled to a double-pass spray chamber. A decrease in the ICP-AES analyte signal was observed with Na solutions under so-called robust conditions, irrespective of the sample introduction system employed. The cross-flow nebulizer associated with the double-pass spray chamber was more sensitive to Na presence than the other combination. These observations can be partially explained by a decrease in the solvent transport rate observed in presence of Na. It has been found that, in every case, the presence of Na did not modify the characteristics of the primary aerosol, while the tertiary aerosol is significantly modified. Finer droplets were obtained at the exit of the spray chamber when Na was present. Also, recirculation of the aerosol led to a significant element enrichment of the largest droplets for the Na solutions. It can be concluded that the effect of Na occured during the aerosol transport and filtering through the spray chamber.     

Dispersion effects of laminar flow and spray chamber volume in capillary electrophoresis-inductively coupled plasma-mass spectrometry: a numerical and experimental approach

Band broadening related to laminar flow and spray chamber dead volume is a potential problem in flow injection (FI)-inductively coupled plasma-mass spectrometry (ICP-MS). We studied these two dispersion effects with a sheath flow capillary electrophoresis (CE)-ICP-MS interface. A numerical model was used to simulate advection diffusion processes in the CE-capillary and dispersion in the spray chamber. Experimental results of FI with this CE-ICP-MS interface agree well with numerical modeling results. Dispersion due to laminar flow depends strongly on capillary diameter and analyte diffusion coefficient and to a lesser extent on laminar velocity and capillary length and typically amounts to one order of magnitude peak width increase. Three spray chambers of 5, 20 and 150 ml dead volume showed an increase in band broadening and peak tailing with increasing dead volume. The use of standard Scott-type spray chambers (>90 ml volume) increases peak widths by 5-10 s regardless of injection time. The use of a low dead volume spray chamber is recommended for experiments where resolution is critical. The modeling approach can be extended to the coupling of other flow injection techniques, like micro-LC and nano-LC with ICP-MS.     

Comparison of several spray chambers operating at very low liquid flow rates in inductively coupled plasma atomic emission spectrometry

Four different spray chambers were evaluated in ICP-AES at very low liquid flow rates: a double-pass (Scott type), a conventional cyclonic, and two low-volume cyclonic-type spray chambers (i.e., Cinnabar and Genie). A glass concentric pneumatic micro nebulizer (Atom Mist) was used in conjunction with all four chambers. The liquid flow rate was varied from 10 to 160 microL min(-1). The conventional cyclonic spray chamber gave rise to coarser tertiary aerosols, higher analyte and solvent transport rates, higher sensitivity and lower limits of detection than the remaining ones. The low-volume spray chambers afforded analytical figures of merit similar to the double-pass one, despite their very different designs. However, these spray chambers exhibited shorter wash-out times. The matrix effects were significant only for the double-pass. This fact allowed the analysis of reference samples by employing aqueous standards at a minimum level of sample consumption. The recoveries obtained for the cyclonic spray chambers and several certified samples were close to 100%, being always lower in the case of the double-pass spray chamber.     

Optimization of HPLC-ICP-AES for the determination of arsenic species

High performance liquid chromatography coupled to ICP-AES detection provides a rapid, reliable and sensitive method for arsenic speciation. The separation of As(III), As(V), DMA and MMA was achieved with ion exchange chromatography coupled to an axially-viewed sequential ICP-AES. After optimization of the chromatographic parameters (pH and concentration of the mobile phase), a careful study of the interface was conducted. Five nebulizers associated to three spray chambers were tested. Response of the ICP to each arsenic species was strongly affected by the selection of the nebulizer and spray chamber, however similar responses were obtained for each arsenic species. Best signal-to-noise ratios were obtained by using a microconcentric nebulizer and a cyclone spray chamber and did not affect the chromatographic resolution. Detection limits better than 10 microg L(-1) were obtained for As(III), DMA, MMA and 20 microg L(-1) for As(V), which is a significant improvement over previously published results.     

Comparison of several spray chambers operating at very low liquid flow rates in inductively coupled plasma atomic emission spectrometry

Four different spray chambers were evaluated in ICP-AES at very low liquid flow rates: a double-pass (Scott type), a conventional cyclonic, and two low-volume cyclonic-type spray chambers (i.e., Cinnabar and Genie). A glass concentric pneumatic micro nebulizer (Atom Mist) was used in conjunction with all four chambers. The liquid flow rate was varied from 10 to 160 μL min^–1. The conventional cyclonic spray chamber gave rise to coarser tertiary aerosols, higher analyte and solvent transport rates, higher sensitivity and lower limits of detection than the remaining ones. The low-volume spray chambers afforded analytical figures of merit similar to the double-pass one, despite their very different designs. However, these spray chambers exhibited shorter wash-out times. The matrix effects were significant only for the double-pass. This fact allowed the analysis of reference samples by employing aqueous standards at a minimum level of sample consumption. The recoveries obtained for the cyclonic spray chambers and several certified samples were close to 100%, being always lower in the case of the double-pass spray chamber.     

Influence of nebulizer design and aerosol impact bead on analytical sensitivities of inductively coupled plasma mass spectrometry

The characteristics of the aerosols generated by pneumatic concentric nebulizers as well as sensitivities were studied in ICP-MS with a total of 26 nebulizers: 17 of the A-type, 6 of the C-type, and 3 of the K-type. Although of the same design, discrepancies in sensitivity were observed among the A1-30 type nebulizers. Free delivery rate, relation between the median of the volume drop size distribution, D₅₀, and the capillary i.d., effect of the nebulizer gas exit cross sectional area on D₅₀, energy transfer efficiency from the gas to the liquid stream and spatial homogeneity within the aerosol cones were studied. For a given nebulizer design, the gas exit cross-sectional area has shown to critically influence the aerosol characteristics. The aerosol generation mechanism has been explored and it has been concluded that, for A-type nebulizers the nebulization is more efficient than for C and K-type ones. The spray chamber design has also a marked effect on the results according to the particular nebulizer used. It has been observed that sample capillary was not perfectly centered with regard to the gas exit bore for several nebulizers. In order to minimize the problems associated with this bad alignment (i.e., different sensitivity depending on the nebulizer), spray chambers equipped with impact beads may be used, but to the detriment of the sensitivity.     

Studies about the origin of the non-spectroscopic interferences caused by sodium and calcium in inductively coupled plasma atomic emission spectrometry. Influence of the spray chamber design

In the present work a systematic study about the characterization of the performance of three spray chambers in terms of inductively coupled plasma atomic emission spectrometry (ICP-AES) analytical figures of merit and matrix effects caused by sodium and calcium at high concentrations was carried out. In addition, experiments were conducted in order to understand the origin of the non-spectroscopic interferences caused by sodium and calcium in ICP-AES. The chambers used were a double pass (DP) a cyclonic (CC) and a home made single pass (SP). In all the cases a high efficiency nebulizer was operated at liquid flow rates ranging from 20 to 200 μl min⁻¹. The results revealed that the ICP-AES sensitivities were higher for the SP than for the two remaining spray chambers. The data concerning the matrix effects caused by concomitants (i.e. sodium and calcium) indicated that the extent of these effects was higher for the DP than for the SP and CC. In the presence of these elements in excess, finer tertiary aerosols were generated than for water. Nonetheless, similar primary aerosols were generated irrespective of the matrix tested. Several experiments were conducted in order to elucidate the mechanism leading to the matrix effects caused by sodium and calcium in terms of aerosol transport towards the plasma. It was concluded that a combination of droplet charge effects and a reduction in the extent of solvent evaporation could be responsible for these effects.     

Band broadening evaluation by back-and-forth capillary electrophoresis

The purpose of this study is to develop a method for precise evaluation of band broadening in capillary electrophoresis. A capillary electrophoresis system with on-column twin detectors is constructed. Back-and-forth electrophoresis permits zones to make a round trip between the twin detectors placed at distant positions on the capillary. The system is capable of eliminating extra-column effects and discriminating between reversible and irreversible band-broadening mechanisms.     

Peak broadening from an electrothermal vaporization sample introduction source into an inductively coupled plasma

Signal broadening using electrothermal vaporization with inductively coupled mass spectrometry (ETV-ICPMS) occurs at a rate much faster than would be predicted by simple longitudinal diffusion. A Monte Carlo simulation that focused on particle motion within the transport tubing was created to elucidate the causes of this dispersion within ETV-ICPMS. Several parameters, including the diffusion coefficient, tube diameter, transport tube length, and flow rate were varied to discern their role in signal broadening. Using typical instrumental parameters, the parabolic flow profile generated by laminar flow of the carrier gas was shown to be the primary cause of dispersion. Manipulating the aforementioned variables to lessen the effects of laminar flow led to a decrease in dispersion. Conversely, increasing the role of laminar flow promoted broadening. The broadening processes should be applicable to any transient introduction system where material must be transported to a detection system. Due to the difference in the rate of broadening expected for particles of different sizes, the simulation was used to calculate the average size of particles generated in the ETV using different mass amounts of sample. No change in particle size (∼1 nm) was seen for mass amounts ranging from 10–10 000 pg, which suggests that the particle number is increased with increasing sample mass rather than the average particle size. Using this method of determining particle size, it might be possible to further evaluate the mechanisms of physical ‘carrier’ action.     

Characterization of a double-pass spray chamber for ICP spectrometry by computer simulation (CFD)

Computational Fluid Dynamics (CFD) is applied to fundamental investigations on the aerosol flow and its modification in spray chambers typically used in ICP–OES spectrometry. Detailed information is gained on the flow field (argon flow), the droplet motion and on the droplet deposition at different places on the walls inside the chamber, which is not accessible experimentally. The temporal course of the calculated mass flow rate at the outlet of a double-pass Scott type spray chamber is compared with an analytical signal received from the ICP spectrometer. The shape of the calculated time-resolved mass flow corresponds well with the shape of the analytical signal. The information on the detailed functions of various types of spray chambers gained by CFD can be used for optimization of their performance.     

High efficiency nebulization for helium inductively coupled plasma mass spectrometry

A pneumatically-driven, high efficiency nebulizer is explored for helium inductively coupled plasma mass spectrometry. The aerosol characteristics and analyte transport efficiencies of the high efficiency nebulizer for nebulization with helium are measured and compared to the results obtained with argon. Analytical performance indices of the helium inductively coupled plasma mass spectrometry are evaluated in terms of detection limits and precision. The helium inductively coupled plasma mass spectrometry detection limits obtained with the high efficiency nebulizer at 200 μL/min are higher than those achieved with the ultrasonic nebulizer consuming 2 mL/min solution, however, precision is generally better with high efficiency nebulizer (1–4% vs. 3–8% with ultrasonic nebulizer). Detection limits with the high efficiency nebulizer at 200 μL/min solution uptake rate approach those using ultrasonic nebulizer upon efficient desolvation with a heated spray chamber followed by a Peltier-cooled multipass condenser.     

Signal instability in inductively coupled plasma emission due to electrostatic effects in the nebulizer spray chamber

Electrostatic fields created by the charge on the spray in the pneumatic nebulizer are found to be an intermittent source of several types of ICP emission instability. The electrostatic effects act to reduce the sample aerosol density leaving the spray chamber. Simple diagnostics allow unambiguous confirmation of the presence of electrostatic signal depression and suggest techniques for its control.     

Evaluation of spray chambers for use in inductively coupled plasma-atomic emission spectrometry

A laboratory-built spray chamber featuring aerosol collection at the centre of the chamber by means of a funnel is described and compared with a commercially available, dual tube chamber. The influence of some chamber design parameters on the emission signal intensity and stability, the nebulizer efficiency and chamber clean-out time is studied.     

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).     

The influence of the sample introduction system on signals of different tin compounds in inductively coupled plasma-based techniques

The influence of the sample introduction system on the signals obtained with different tin compounds in inductively coupled plasma (ICP) based techniques, i.e., ICP atomic emission spectrometry (ICP–AES) and ICP mass spectrometry (ICP–MS) has been studied. Signals for test solutions prepared from four different tin compounds (i.e., tin tetrachloride, monobutyltin, dibutyltin and di-tert-butyltin) in different solvents (methanol 0.8% (w/w), i-propanol 0.8% (w/w) and various acid matrices) have been measured by ICP–AES and ICP–MS. The results demonstrate a noticeable influence of the volatility of the tin compounds on their signals measured with both techniques. Thus, in agreement with the compound volatility, the highest signals are obtained for tin tetrachloride followed by di-tert-butyltin/monobutyltin and dibutyltin.The sample introduction system exerts an important effect on the amount of solution loading the plasma and, hence, on the relative signals afforded by the tin compounds in ICP–based techniques. Thus, when working with a pneumatic concentric nebulizer, the use of spray chambers affording high solvent transport efficiency to the plasma (such as cyclonic and single pass) or high spray chamber temperatures is recommended to minimize the influence of the tin chemical compound. Nevertheless, even when using the conventional pneumatic nebulizer coupled to the best spray chamber design (i.e., a single pass spray chamber), signals obtained for di-tert-butyltin/monobutyltin and dibutyltin are still around 10% and 30% lower than the corresponding signal for tin tetrachloride, respectively. When operating with a pneumatic microconcentric nebulizer coupled to a 50 °C-thermostated cinnabar spray chamber, all studied organotin compounds provided similar emission signals although about 60% lower than those obtained for tin tetrachloride. The use of an ultrasonic nebulizer coupled to a desolvation device provides the largest differences in the emission signals, among all tested systems.     

Two‐dimensional chromatography as a tool for studying band broadening in size‐exclusion chromatography

Comprehensive 2‐D size‐exclusion chromatography (SEC×SEC) has been realized. SEC×SEC is not a useful technique for characterizing complex polymers. However, it is potentially an elegant tool to study band‐broadening phenomena. If narrow fractions can be collected from the first dimension, the band broadening in the second dimension is only due to chromatographic dispersion. This would allow a clear distinction to be made between chromatographic band broadening (column and extra‐column) and SEC selectivity (band broadening due to sample polydispersity). In comparison with MALDI‐MS, SEC×SEC allows the study of polymers across a much broader molar‐mass range.     

Band-broadening in capillary zone electrophoresis with axial temperature gradients

It is widely accepted that Joule heating effects yield radial temperature gradients in capillary zone electrophoresis (CZE). The resultant parabolic profile of electrophoretic velocity of analyte molecules is believed to increase the band-broadening via Taylor-Aris dispersion. This typically insignificant contribution, however, cannot explain the decrease in separation efficiency at high electric fields. We show that the additional band-broadening due to axial temperature gradients may provide the answer. These axial temperature variations result from the change of heat transfer condition along the capillary, which is often present in CZE with thermostating. In this case, the electric field becomes nonuniform due to the temperature dependence of fluid conductivity, and hence the induced pressure gradient is brought about to meet the mass continuity. This modification of the electroosmotic flow pattern can cause significant band-broadening. An analytical model is developed to predict the band-broadening in CZE with axial temperature gradients in terms of the theoretical plate height. We find that the resultant thermal plate height can be very high and even comparable to that due to molecular diffusion. This thermal plate height is much higher than that due to radial temperature gradients alone. The analytical model explains successfully the phenomena observed in previous experiments.     

A new nebulizer for atomic spectrometry

The rotating disc nebulizer has been redesigned with respect to the principle parameters that determines its analytical performance. The flow pattern of the aerosol attained has been optimized by altering the shape of the inner chamber of the nebulizer to obtain optimum aerodynamic characteristics. The optimum angle of impact and "free flight" distance has been established using particle size distribution and mass transport efficiency as criteria. Analytical characteristics have been determined by monitoring the emission signal from aqueous standards. The accuracy has been assessed by using reference steel samples. Using standard solutions of different viscosity the performance of the nebulizer with respect to viscosity changes of the sample has been compared with that of a commercial Meinhardt nebulizer. The rotating disc nebulizer has been less affected by changes in viscosity making it possible to use this nebulizer with slurry and oil samples.     

Evaluation of several pneumatic micronebulizers with different designs for use in ICP–AES and ICP–MS. Future directions for further improvement

This paper reports characterization of the behavior of five pneumatic micronebulizers based on slightly different designs in inductively coupled plasma atomic-emission spectrometry and mass spectrometry (ICP–AES and ICP–MS). Two nebulizers were used as reference nebulizers, a high-efficiency nebulizer (HEN) and a micromist (MM). They were compared with a commercially available PFA (tetrafluoroethylene–perfluoroalkyl vinyl ether copolymer) nebulizer and with two new prototypes called the polymeric pneumatic concentric nebulizer (PMN) and the high-solids micronebulizer (HSM). The dimensions of the nebulizers, the gas back-pressure, and the free liquid uptake rates were measured. The study also included tertiary aerosol drop-size distributions, analyte transport rate, and analytical figures of merit, i.e. sensitivities and limits of detection, both in ICP–AES and ICP–MS. Recoveries for two food solid reference materials were also determined. Overall, the results indicated that the PFA and the HEN nebulizers provided the best results. These two nebulizers delivered a higher mass of analyte to the plasma and showed better sensitivies giving lower limits of detection than the PMN, HSM and MM. The results revealed that the liquid prefilming effect occurring before aerosol production in the PFA nebulizer promoted more efficient interaction of liquid and gas, thus affording good results even though gas back-pressure values could be maintained below 3 bar. In contrast, the HEN had to be operated at about 7 bar under the same conditions. Nebulizer design did not have a relevant effect on the recovery, which confirmed that the spray chamber plays an important role in terms of non-spectroscopic interferences.     

Alcohol and metal determination in alcoholic beverages through high-temperature liquid-chromatography coupled to an inductively coupled plasma atomic emission spectrometer

In the present work, an inductively coupled plasma atomic emission spectrometry (ICP-AES) system was used as a high temperature liquid chromatography (HTLC) detector for the determination of alcohols and metals in beverages. For the sake of comparison, a refractive index (RI) detector was also employed for the first time to detect alcohols with HTLC. The organic compounds studied were methanol, ethanol, propan-1-ol and butan-1-ol (in the 10-125 mg/L concentration range) and the elements tested were magnesium, aluminum, copper, manganese and barium at concentrations included between roughly 0.01 and 80 mg/L. Column heating temperatures ranged from 80 to 175 °C and the optimum ones in terms of peak resolution, sensitivity and column lifetime were 125 and 100 °C for the HTLC-RI and HTLC-ICP-AES couplings, respectively. The HTLC-ICP-AES interface design (i.e., spray chamber design and nebulizer type used) was studied and it was found that a single pass spray chamber provided about 2 times higher sensitivities than a cyclonic conventional design. Comparatively speaking, limits of detection for alcohols were of the same order for the two evaluated detection systems (from 5 to 25 mg/L). In contrast, unlike RI, ICP-AES provided information about the content of both organic and inorganic species. Furthermore, temperature programming was applied to shorten the analysis time and it was verified that ICP-AES was less sensitive to temperature changes and modifications in the analyte chemical nature than the RI detector. Both detectors were successfully applied to the determination of short chain alcohols in several beverages such as muscatel, pacharan, punch, vermouth and two different brands of whiskeys (from 10 to 40 g of ethanol/100 g of sample). The results of the inorganic elements studied by HTLC-ICP-AES were compared with those obtained using inductively coupled plasma mass spectrometry (ICP-MS) obtaining good agreement between them. Recoveries found for spiked samples were close to 100% for both, inorganic elements (with both HLTC-ICP-AES and ICP-MS) and alcohols (with both HTLC-ICP-AES and HTLC-RI hyphenations).