Ion Clouds in the Inductively Coupled Plasma Torch: A Closer Look through Computations.

We have computationally investigated the introduction of copper elemental particles in an inductively coupled plasma torch connected to a sampling cone, including for the first time the ionization of the sample. The sample is inserted as liquid particles, which are followed inside the entire torch, i.e., from the injector inlet up to the ionization and reaching the sampler. The spatial position of the ion clouds inside the torch as well as detailed information on the copper species fluxes at the position of the sampler orifice and the exhausts of the torch are provided. The effect of on- and off-axis injection is studied. We clearly show that the ion clouds of on-axis injected material are located closer to the sampler with less radial diffusion. This guarantees a higher transport efficiency through the sampler cone. Moreover, our model reveals the optimum ranges of applied power and flow rates, which ensure the proper position of ion clouds inside the torch, i.e., close enough to the sampler to increase the fraction that can enter the mass spectrometer and with minimum loss of material toward the exhausts as well as a sufficiently high plasma temperature for efficient ionization.

Simulation and experimental studies on plasma temperature, flow velocity, and injector diameter effects for an inductively coupled plasma.

An inductively coupled plasma (ICP) is analyzed by means of experiments and numerical simulation. Important plasma properties are analyzed, namely, the effective temperature inside the central channel and the mean flow velocity inside the plasma. Furthermore, the effect of torches with different injector diameters is studied by the model. The temperature inside the central channel is determined from the end-on collected line-to-background ratio in dependence of the injector gas flow rates. Within the limits of 3% deviation, the results of the simulation and the experiments are in good agreement in the range of flow rates relevant for the analysis of relatively large droplets, i.e., ∼50 µm. The deviation increases for higher gas flow rates but stays below 6% for all flow rates studied. The velocity of the gas inside the coil region was determined by side-on analyte emission measurements with single monodisperse droplet introduction and by the analysis of the injector gas path lines in the simulation. In the downstream region significantly higher velocities were found than in the upstream region in both the simulation and the experiment. The quantitative values show good agreement in the downstream region. In the upstream region, deviations were found in the absolute values which can be attributed to the flow conditions in that region and because the methods used for velocity determination are not fully consistent. Eddy structures are found in the simulated flow lines. These affect strongly the way taken by the path lines of the injector gas and they can explain the very long analytical signals found in the experiments at low flow rates. Simulations were performed for different injector diameters in order to find conditions where good analyte transport and optimum signals can be expected. The results clearly show the existence of a transition flow rate which marks the lower limit for effective analyte transport conditions through the plasma. A rule-of-thumb equation was extracted from the results from which the transition flow rate can be estimated for different injector diameters and different injector gas compositions.

Optimized transport setup for high repetition rate pulse-separated analysis in laser ablation-inductively coupled plasma mass spectrometry.

An optimized laser ablation setup, proposed for high repetition rate inductively coupled plasma mass spectrometry (ICPMS) analyses such as 2D imaging or depth profiling, is presented. For such applications, the particle washout time needs to be as short as possible to allow high laser pulse frequencies for reduced analysis time. Therefore, it is desirable to have an ablation setup that operates as a laminar flow reactor (LFR). A top-down strategy was applied that resulted in the present design. In the first step, a previously applied ablation setup was analyzed on the basis of computational fluid dynamics (CFD) results presented by D. Autrique et al. (Spectrochim. Acta, B 2008, 63, 257-270). By means of CFD simulations, the design was modified in such a way that it operated in the LFR regime. Experimental results demonstrate that the current design can indeed be regarded as an LFR. Furthermore, the operation under LFR conditions allowed some insight into the initial radial concentration distribution if the experimental ICPMS signal and analytical expressions are taken into account. Recommendations for a modified setup for more resilient spatial distributions are given. With the present setup, a washout time of 140 ms has been achieved for a 3% signal area criterion. Therefore, 7 Hz repetition rates can be applied with the present setup. Using elementary formulas of the analytical model, an upper bound for the washout times for similar setups can be predicted. The authors believe that the presented setup geometry comes close to the achievable limit for reliable short washout times.

Production of ultrafine particles by nanosecond laser sampling using orthogonal prepulse laser breakdown.

The particle size distribution and composition of nanosecond laser-generated aerosols from brass samples in atmospheric argon has been measured by low-pressure impaction and subsequent quantitative analysis of the aerosols by total reflection X-ray fluorescence. Ablation was performed applying a Nd:YAG laser at 1.06 microm both without and with a prepulse plasma breakdown generated by a second Nd:YAG laser at 2-60 micros prior to the ablation pulse. The beam of the prepulse laser had orthogonal direction to the ablation laser beam, and the breakdown was produced 2.5 mm above the ablation spot. Ultrafine aerosol particles (<50 nm) were generated in the double-pulse experiment representing practically the total mass impacted, while in single-pulse ablation the proportion of large particles (>0.1 microm) was dominating. The predominance of ultrafine aerosols in the prepulse experiment indicates that particle formation from vapor-phase condensation is the major mechanism, while the appearance of large particles in single-pulse ablation points at fragmentary evaporation in the laser-produced plasma. It was also shown that the total mass impacted in double-pulse ablation increases almost linearly with the power of the prepulse laser. The better atomization and the larger sample mass ablated can be assumed to be the main reasons for the increase of the line intensities in double-pulse laser-induced breakdown spectrometry with orthogonal prepulses reported by several research groups.

Dependence of lipoprotein-lipase-catalyzed triacylglycerol hydrolysis on droplet size of synthetic monodisperse emulsions measured with static light scattering.

Well-defined triolein emulsions of low polydispersity were prepared by shearing a crude emulsion in a modified Couette cell, resulting in radii in the range of 300 to 900 nm. These emulsions were used as synthetic substrates for lipoprotein lipase, a key enzyme for the hydrolysis of serum triacylglycerols. The change in radius with time was studied with on-line static light scattering at 37 degrees C. An optimum radius of about 750 nm was found for this reaction.

Dynamic and static properties of the concentrated micellar solution and gel phase of a triblock copolymer in water.

The dynamic and static behavior of the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer "P94" is studied in aqueous solution. This polymer forms transparent micellar phase structures through the whole phase diagram. The characteristics of these structures can be tuned by variation of temperature and concentration. The transparent gel at high concentrations and at ambient temperatures is the main interest in this study. The transition from the ergodic solution to this nonergodic gel or glass state can be reached in two ways: at constant temperature with an increase of concentration or at constant concentration with increasing temperature. We characterize the system in both ways with dynamic and static light scattering and with small-angle x-ray scattering experiments. At 40 degrees C, no concentration dependent change in the micellar structure of the system can be detected from the small-angle x-ray scattering results. Even in the gel we find globular particles. This is important for the evaluation and interpretation of the dynamic light scattering measurements. In the ergodic phase region at constant temperature, we find that a ratio of 2.17 gives a perfect fit to the experimental data when we allow the effective volume fraction to be proportional to the mass fraction. In the gel region we find two diffusional decays in the intensity correlation functions and an arrested part. The diffusion coefficients from the faster decay stay constant inside the gel and the nonergodicity parameter increases linearly with concentration. Both, the fast and slow diffusional modes show no angular dependence of their scattering intensities, indicating their origin from small objects, while the arrested mode comes, as expected, from structures larger than the resolution of the experiments.