Solution Introduction Systems in ICP-MS

Pneumatic nebuhzers are utihzed for the generation of an aerosol with droplet size distribution in the low p,m range and consequently for an effective solution introduction into the ICP ion source 

The use of a desolvating system improves the sensitivity and can extend the matrix effect with respect to conventional sample introduction apparams. The current trend is towards total consumption systems. Fundamental studies concerning the analysis of microsamples with common micronebulizer based systems have been reviewed by TodoU and Mermet.  

Characteristics such as the solution uptake rate, the sensitivity and the limit of detection for uranium determination using several nebulizers (Meinhard, MicroMist nebulizer, Q-DIHEN, 

Aridus, USN operated in the continuous mode versus the DS-5 nebulizer used in the flow injection mode) for solution introduction in ICP-SFMS (Element, Thermo Fisher Scientific, Bremen) for uranium determination in aqueous solution are summarized in Table 5.3. 

Bremen) for uranium determination in aqueous solution using different solution introduction systems. 

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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... 

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. 

To examine a sample by inductively coupled plasma mass spectrometry (ICP/MS) or inductively coupled plasma atomic-emission spectroscopy (ICP/AES) the sample must be transported into the flame of a plasma torch. Once in the flame, sample molecules are literally ripped apart to form ions of their constituent elements. These fragmentation and ionization processes are described in Chapters 6 and 14. To introduce samples into the center of the (plasma) flame, they must be transported there as gases, as finely dispersed droplets of a solution, or as fine particulate matter. The various methods of sample introduction are described here in three parts — A, B, and C Chapters 15, 16, and 17 — to cover gases, solutions (liquids), and solids. Some types of sample inlets are multipurpose and can be used with gases and liquids or with liquids and solids, but others have been designed specifically for only one kind of analysis. However, the principles governing the operation of inlet systems fall into a small number of categories. This chapter discusses specifically substances that are normally liquids at ambient temperatures. This sort of inlet is the commonest in analytical work. 

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. 

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. 

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