Solution introduction systems, in ICP

From the sample solution to be analyzed, small droplets are formed by the nebulization of the solution using an appropriate concentric or cross-flow pneumatic nebulizer/spray chamber system. Quite different solution introduction systems have been created for the appropriate generation of an aerosol from a liquid sample and for separation of large size droplets. Such an arrangement provides an efficiency of the analyte introduction in the plasma of 1-3 % only.6 The rest (97 % to 99%) goes down in the drain.7 Beside the conventional Meinhard nebulizer, together with cooled or non-cooled Scott spray chamber or conical spray chamber, several types of micronebulizers together with cyclonic spray chambers are employed for routine measurements in ICP-MS laboratories. The solvent evaporated from each droplet forms a particle which is vaporized into atoms and molecules... 

The APEX system (Element Scientific Inc., Omaha) as an improved Aridus nebulizer was introduced for ICP-MS in 2004 for more effective solution introduction at flow rates from 20-400 p,lmin-1.88 In this solution introduction system (see Figure 5.15), a microflow PFA nebulizer is combined with a heated cyclonic spray chamber followed by cooling of the nebulized aerosol in a condenser loop and using a multipass condenser cooled by a Peltier element. The APEX solution introduction system results in a significant increase of sensitivity (by a factor of ten in comparison to a standard nebulizer spray chamber arrangement) and a decreasing polyatomic formation rate.89... 

An ICP-OES instrument consists of a sample introduction system, a plasma torch, a plasma power supply and impedance matcher, and an optical measurement system (Figure 1). The sample must be introduced into the plasma in a form that can be effectively vaporized and atomized (small droplets of solution, small particles of solid or vapor). The plasma torch confines the plasma to a diameter of about 18 mm. Atoms and ions produced in the plasma are excited and emit light. The intensity of light emitted at wavelengths characteristic of the particular elements of interest is measured and related to the concentration of each element via calibration curves. 

Advances in TIMS-techniques and the introduction of multiple collector-ICP-MS (MC-ICP-MS) techniques have enabled the research on natural variations of a wide range of transition and heavy metal systems for the first time, which so far could not have been measured with the necessary precision. The advent of MC-ICP-MS has improved the precision on isotope measurements to about 40 ppm on elements such as Zn, Cu, Fe, Cr, Mo, and Tl. The technique combines the strength of the ICP technique (high ionization efficiency for nearly all elements) with the high precision of thermal ion source mass spectrometry equipped with an array of Faraday collectors. The uptake of elements from solution and ionization in a plasma allows correction for instrument-dependent mass fractionations by addition of external spikes or the comparison of standards with samples under identical operating conditions. All MC-ICP-MS instruments need Ar as the plasma support gas, in a similar manner to that commonly used in conventional ICP-MS. Mass interferences are thus an inherent feature of this technique, which may be circumvented by using desolvating nebulisers. 

The role of the sample introduction system is to convert a sample into a form that can be effectively vaporized into free atoms and ions in the ICP. A peristaltic pump is typically used to deliver a constant flow or sample solution (independent of variations in solution viscosity) to the nebulizer. Several different kinds of nebulizers are available to generate the sample aerosol, and several different spray chamber designs have been used to modify the aerosol before it enters the ICP Gases can be directly introduced into the plasma, for example, after hydride generation. Solids can be introduced by using electrothermal vaporization or laser ablation. 

When using a pneumatic nebulizer, an unheated spray chamber, and a quadrupole mass spectrometer, ICP-MS detection limits are 1 part per trillion or less for 40 to 60 elements (Table 3.4) in clean solutions. Detection limits in the parts per quadrillion range can be obtained for many elements with higher-efficiency sample introduction systems and/or a magnetic sector mass spectrometer used in low-resolution mode. Blank levels, spectral overlaps, and control of sample contamination during preparation, storage, and analysis often prohibit attainment of the ultimate detection limits. 

Changes in sensitivity (signal/concentration) can occur in ICP-MS, depending on the identity and concentration of elements in the sample solution and the solvent. Chemical matrix effects can be due to changes in the analyte transport efficiency from the nebulizer into the plasma or modification of ion generation in the plasma. The severity of this matrix effect depends on the concentration of matrix ions generated in the ICP, not the matrix-to-analyte ratio. Whenever the matrix ion current becomes significant compared to other ion currents, matrix effects are observed [166]. Therefore, sample introduction systems that increase the sample transport rate into the ICP suffer from chemical matrix effects at lower dissolved solid concentrations in the sample. 

Because these digestion schemes may result in widely varying acid concentrations in the final solution, the ICP-AES conditions require careful optimization for this work. Researchers have seen that increasing acid concentration often causes a depression in signal intensity for some lines when using pneumatic sample introduction systems [6]. The effect may be especially prominent under non-robust plasma conditions. The ICP-AES conditions were optimized using... 

Sensitivity is assessed by comparing signal level and stability of the background with analyte response. In early ICP/MS instruments, response was on the order of 106 ion counts/s per part-per-million (ug/mL) of analyte in a solution introduced at a flow rate of 1 mL/min. This is an analyte introduction rate of 1 ug/min, or 1014 atoms/s for the analyte 100Mo. The overall efficiency is thus 10 8 given 106 counts/s vs. 1014 atoms/s from the sample. Research and development has led to efficiencies that have steadily increased over the years. State-of-the-art instruments and high efficiency sample introduction systems can now achieve 10 3 (0.1 percent) overall efficiency in terms of counts detected per atom in the sample. Champion efficiencies as large as 0.5 percent have been reported (Rehkamper et al., 2001). For comparison, TIMS champion efficiencies are 1 and 5 percent, for uranium and plutonium, respectively. 

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