Systems for ICP-MS

FIGURE 9.18 FI separation system for ICP-MS based on the PrepLab instrument. 

The Japanese product Muromac A-1, which has the same imminodiacetic functional group as Chelex-100, was reported to be free from the troublesome swelling properties of the latter, and has been used successfully in a number of AA and ICP spectrometric applications with column preconcentrations [13,14]. A prototype column packed with another imminodiacetic resin was used in a preconcentration system for ICP-MS [66]. 

KreschoUek,T., Holcombe, J.A. (2007) Dry analyte introduction system for ICP-MS optimization utilizing a dry plasma. Journal of Analytical Atomic Spectrometry, 22, 171-174. 

Figure 5.1 Conventional liquid sample introduction system for ICP-MS and aerosol transport processes taking place inside the spray chamber. 

More recently, Montaser and co-workers have developed a new low cost DIN called the direct injection high etficiency nebuliser (DIHEN). The DIHEN is entirely made of glass and is similar in construction to a HEN, but it is longer. The main advantages provided by the DIHEN with respect to other conventional liquid sample introduction systems for ICP-MS are higher sensitivities, better signal stability and lower limits of detection. The dead volume of the DIHEN can be made lower than 10 nL, which significantly reduces the wash-out times for elements such as iodine, mercury and boron. As a result, this nebuliser has also proven to be suitable as an interface between separation techniques and ICP-MS. " Note that in these later studies, with a modified low dead volume DIHEN, the liquid flow rate can be lowered down to 0.5 pL/min. 

As a support system for ICP-MS-Unked immunoassays, the Maxisorp plates are limited by capacity and are thus not suited to the detection of minor proteins in complex mixtures. However, these plates have the advantage of being able to bind all proteins, making the detection and quantitation of multiple analytes in a single sample possible. In the following section, another type of direct ELISA plate (maleic anhydride) is investigated for its potential use in the simultaneous quantitation of two proteins using ICP-MS-linked immunoassays. 

In ICP-AES and ICP-MS, sample mineralisation is the Achilles heel. Sample introduction systems for ICP-AES are numerous gas-phase introduction, pneumatic nebulisation (PN), direct-injection nebulisation (DIN), thermal spray, ultrasonic nebulisation (USN), electrothermal vaporisation (ETV) (furnace, cup, filament), hydride generation, electroerosion, laser ablation and direct sample insertion. Atomisation is an essential process in many fields where a dispersion of liquid particles in a gas is required. Pneumatic nebulisation is most commonly used in conjunction with a spray chamber that serves as a droplet separator, allowing droplets with average diameters of typically <10 xm to pass and enter the ICP. Spray chambers, which reduce solvent load and deal with coarse aerosols, should be as small as possible (micro-nebulisation [177]). Direct injection in the plasma torch is feasible [178]. Ultrasonic atomisers are designed to specifically operate from a vibrational energy source [179]. 

This paper presents an overview of the current research issues and commercialization efforts related to laser ablation for chemical analysis, discusses several fundamental studies of laser ablation using time-resolved shadowgraph and spectroscopic imaging, and describes recent data using nanosecond laser pulsed ablation sampling for ICP-MS and LIBS. Efforts towards commercialization of field based LIBS systems also will be described. 

A schematic diagram of an ICP-MS instrument is shown in Fig. 5.1. The TCP part bears an almost exact resemblance to the ICP used for atomic emission spectrometry, with the obvious exception that it is turned on one side. Indeed, sample introduction systems, radiofrequency generators and the nature of ICP itself are often the same for ICP-MS and ICP-AES systems, with the usual variations between individual manufacturers. 

Q. How do sample introduction systems used for ICP-MS compare with those used for ICP-AES ... 

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... 

Hydride generation for analytical use was introduced at the end of the 1960s using arsine formation (Marshal Reaction) in flame atomic absorption spectrometry (FAAS). A simple experimental setup for a hydride generator is shown in Figure 5.18. Today, hydride generation,91,92 which is the most widely utilized gas phase sample introduction system in ICP-MS, has been developed into... 

However, nowadays some other different mass spectrometers are used for ICP-MS time-of-flight (TOP) systems for multielemental analysis of transient signals, ion trap analysers for ion storage, multicollector instruments for precise isotope ratio measurements and double-focusing sector field mass spectrometers for high mass resolution, but still the majority of instruments are equipped with quadrupole filters, which are simpler and cheaper. 

As the precision of the ICP-MS isotope ratio is poor compared with the precision using TIMS, the range of applications for ICP-MS have traditionally been limited to measuring induced changes in the isotopic composition of a target element (for example, to calibrate by means of isotope dilution). However, the introduction of multicollector ICP-MS systems to enhance precision and accuracy in isotopic analysis opened up novel applications. 

Use of inductively coupled plasma-mass spectrometry (1CP-MS) coupled to a laser-ablation sample introduction system (LA-ICP-MS) as a minimally destructive method for chemical characterization of archaeological materials has gained favor during the past few years. Although still a relatively new analytical technique in archaeology, LA-ICP-MS has been demonstrated to be a productive avenue of research for chemical characterization of obsidian, chert, pottery, painted and glazed surfaces, and human bone and teeth. Archaeological applications of LA-ICP-MS and comparisons with other analytical methods are described. 

By the late 1990s and into the 2000s, a number of additional groups became involved in automated fluidic separations for radiochemical analysis, especially as a front end for ICP-MS. Published journal articles on fluidic separations for radio-metric or mass spectrometric detection are summarized in Tables 9.1 through 9.5. The majority of such studies have used extraction chromatographic separations, and these will be the main focus of the remainder of this chapter. Section 9.4 describes methods that combine separation and detection. Section 9.5 describes a fully automated system that combines sample preparation, separation, and detection. 

FIGURE 9.15 Design of a SI separation system for on-line separation of actinides for ICP-MS detection, where the SI system handles clean solutions and reagents outside the radioactivity containment glove box, while the injection valve and separation column downstream handle radioactive solutions. 

Compared with the ICP, other atomic spectrometric detectors are not widely coupled to HPLC. Several interfaces have been described for AAS detector. Methods include a rotating platinum spiral collection system (Ebdon et al., 1987) and a flow injection thermospray sample introduction system (Robinson and Choi, 1987). Post-column hydride generation is also popular with AAS detection as will be described later. Pedersen and Larsen (1997) used an anion-exchange column to separate selenomethionine, selenocysteine, selenite and selenate with both FAAS and ICP-MS. The detection limits for the FAAS system were lmg H1 compared with 1 fig l-1 for ICP-MS. HPLC-MIP systems have been described to an even lesser extent. These either use elaborate interfaces to overcome the problems of quenching the low-power plasma (Zhang and Carnahan, 1989) or use a modified argon/oxygen mixed gas plasma (Kollotzek et al., 1984). 

Desolvation systems can provide three potential advantages for ICP-MS higher analyte transport efficiencies, reduced molecular oxide ion signals, and reduced solvent loading of the plasma. Two different approaches have been used for desolvation in ICP-MS. The heated spray chamber/condenser combination has been discussed it is the most commonly used system. The extent of evaporation of the solvent from the aerosol and cooling to reduce vapor loading varies from system to system. The second approach is the use of a membrane separator to remove solvent vapor before it enters the ICP. 

Liquid chromatography (LC) is the most commonly used technique for trace element speciation with ICP-MS detection. The mobile phase flow rates used with most LC techniques (0.5-2.0 mL min-1) are compatible for ICP-MS introduction using conventional sample introduction systems (pneumatic nebulization with cross flow and concentric nebulizers and double-pass spray chambers). An interface, known as a transfer line, must be constructed to allow connection between the outlet of the LC column and the nebulizer of the ICP-MS. Inert plastic tubing is commonly used for this purpose with the inner diameter and length kept to 20-50 cm in order to minimize peak broadening. 

Iodine and Se speciation in breast milk provides an example of the use of CE in hyphenated systems with ICP-MS detection. By employing CE, Michalke and colleagues determined selenoaminocids and identified two chemical forms of iodine, I- and thyroxine, which were present in comparable amounts in milk [115-117]. Other authors used SEC and IEC for I speciation in various types of milk and infant formulae (see Table 8.3) and found I- as the main species, with the exception of breast milk and formulae. The latter were found to contain less I than commercial and human milk, and mostly as an unidentified macromolecular compound. 

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