Samples introduction systems

The physical status of the samples determines the choice of the analytical approach. In particular, liquid samples can be processed by nebulization 

The liquid sample introduction system most commonly used on an ICP-MS is very similar to that used on a flame Atomic Absorption Spectrometer or an ICP-OES. Liquid samples can be directly injected using a pneumatic nebulizer and a spray chamber. 

The ICP-OES nebulizers can aspirate up to 1-2% dissolved solids (a dry wine has 2-4% total extract). Some nebulizers, such as the Babington and cone-spray nebulizers, were studied to handle as much as 15-20% dissolved solids, but are not ideally conceived for use in ICP-MS. Pneumatic nebulizers are commonly used to generate an aerosol of samples with dissolved components below 0.2%. Pneumatic devices are made from glass or different kinds of polymers and use argon as nebulizer gas. The characteristics of some of the most popular pneumatic nebulizers are detailed below. 

Crossflow design. This nebulizer suffers from relatively reduced analytical sensitivity and precision if compared with the concentric one. It is devised for routine use and is probably the best choice for samples that contain a high concentration of dissolved solids or heterogeneous samples with small contents of undissolved matter. The aerosol is produced on the nebulizer tip where the drained sample collides with a perpendicular jet of argon gas. 

Microflow design is based on the concentric principle. Typically it operates with higher gas pressure at less than 0.1 mL/min sample flow 

The sample introduction system, comprising the peristaltic pump, nebulizer, spray chamber, and drain system, takes the initial abuse from the sample matrix, and as a result, is an area of the ICP mass spectrometer that requires a great deal of attention. The principles of the sample introduction area have been described in great detail in Chapter 3, so let us now examine what kind of routine maintenance it requires. 

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Sample introduction system. A system used to introduce sample to a mass spectrometer ion source. Sample introduction system, introduction system, sample inlet system, inlet system, and inlet are synonymous terms. 

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. 

Detection limits for a particular sample depend on a number of parameters, including observation height in the plasma, applied power, gas flow rates, spectrometer resolution, integration time, the sample introduction system, and sample-induced background or spectral overlaps. ... 

There are several types of sample introduction systems available for GC analysis. These include gas sampling valves, split and splitless injectors, on-column injection systems, programmed-temperature injectors, and concentrating devices. The sample introduction device used depends on the application. 

Every mass spectrometer consists of four principal components (Fig 1) (1) the source, where a beam of gaseous ions are produced from the sample (2) the analyzer, where the ion beam is resolved into its characteristic mass species (3) the detector, where the ions are detected and their intensities measured (4) the sample introduction system to vaporize and admit the sample into the ion source. There is a wide variety in each of these components and only those types which are relevant to analytical and organic mass spectrometry will be emphasized in this survey. The instrumentation... 

Sample introduction system i 1 Sample Ion Unresolved accelerated Mass Resolved 1 Detectorj Recorder... 

Ion extraction. The aspirated or laser ablated sample is transported from the sample introduction system into the center of the torch by a 1 1/min flow of Ar carrier gas where it is immediately dissociated and ionized by energy transfer with the hot -6000 K temperature of the surrounding Ar plasma. Ionization efficiencies are >95% for U and Th (Jarvis et al., 1992). For laser ablation sampling, helium may be employed as the carrier... 

INTERFACE REQOIREHENTS FOR CHROMATOGRAPHIC SAMPLE INTRODUCTION SYSTEMS IN NASS SPECTRCHtETRY... 

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]. 

In Figure 8.12, the basic set-up of an ICP-MS instrument is presented as a block diagram, consisting of a sample introduction system, the inductively coupled argon plasma (ICP) and the mass-specific detector. By far the most commonly applied sample introduction technique is a pneumatic nebuliser, in which a stream of argon (typically 1 I.min ), expanding with high... 

Diversity in sample introduction systems (nebulisation, coupbng, sobd sampbng)... 

Several authors [386,387] have discussed the spectroscopic and nonspectroscopic (matrix) interferences in ICP-MS. ICP-MS is more susceptible to nonspectroscopic matrix interferences than ICP-AES [388-390]. Matrix interferences are perceptible by suppression and (sometimes) enhancement of the analyte signal. This enhanced susceptibility has to be related to the use of the mass spectrometer as a detection system. In fact, since both techniques use the same (or comparable) sample introduction systems (nebulisers, spray chambers, etc.) and argon plasmas (torches, generators, etc.), it is reasonable to assume that, as far as these parts are concerned, interferences are comparable. The most severe limitation of ICP-MS consists of polyatomic... 

The extension of inductively coupled plasma (ICP) atomic emission spectrometry to seawater analysis has been slow for two major reasons. The first is that the concentrations of almost all trace metals of interest are 1 xg/l or less, below detection limits attainable with conventional pneumatic nebulisation. The second is that the seawater matrix, with some 3.5% dissolved solids, is not compatible with most of the sample introduction systems used with ICP. Thus direct multielemental trace analysis of seawater by ICP-AES is impractical, at least with pneumatic nebulisation. In view of this, a number of alternative strategies can be considered ... 

The MC-ICP-MS consists of four main parts 1) a sample introduction system that inlets the sample into the instrument as either a liquid (most common), gas, or solid (e.g., laser ablation), 2) an inductively coupled Ar plasma in which the sample is evaporated, vaporized, atomized, and ionized, 3) an ion transfer mechanism (the mass spectrometer interface) that separates the atmospheric pressure of the plasma from the vacuum of the analyzer, and 4) a mass analyzer that deals with the ion kinetic energy spread and produces a mass spectrum with flat topped peaks suitable for isotope ratio measurements. 

When an analyte is transferred into the ion source by means of any sample introduction system it is in thermal equilibrium with this inlet device. As a result, the energy of the incoming molecules is represented by their thermal energy. Then, ionization changes the situation dramatically as comparatively large amounts of energy need to be handled by the freshly formed ion. 

The physicochemical aspects of the ionization process in general, ion internal energy, and the principles determining the reaction pathways of excited ions have already been addressed (Chap. 2). After a brief repetition of some of these issues we will go more deeply into detail from the analytical point of view. Next, we will discuss technical and practical aspects concerning the construction of El ion sources and sample introduction systems. Finally, this chapter directly leads over to the interpretation of El mass spectra (Chap. 6). 

Note Sample introduction systems such as reservoir inlets, chromatographs, and various types of direct probes (Chap. 5.3) are of equal importance to other ionization methods. The same holds valid for the concepts of sensitivity, detection limit, and signal-to-noise ratio (Chap. 5.2.4) and finally to all sorts of ion chromatograms (Chap. 5.4). 

Note The detection of signals from previously measured samples in the mass spectrum of the actual analyte is usually termed memory or memory effect. It is caused by contaminations of ion source or sample introduction system. The best way to reduce memory effects is to use the lowest amount of sample necessary to produce good spectra, to keep ion source and ion source housing at elevated temperature, and to allow some minutes for pumping between subsequent measurements. 

For the purpose of sample introduction, any sample introduction system (also sample inlet system or inlet) suitable for the respective compound can be employed. Hence, direct probes, reservoir inlets, gas chromatographs and even liquid chromatographs can be attached to an El ion source. Which of these inlet systems is to be preferred depends on the type of sample going to be analyzed. Whatever type the inlet system may be, it has to manage the same basic task, i.e., the transfer of the analyte from atmospheric conditions into the high vacuum of the El ion source Table 5.1 provides an overview. 

Pattillo, A.D. Young, H.A. Liquid Sample Introduction System for a Mass Spectrometer. Anal. Chem. 1963, 35, 1768. 

Fig. S.6 Gas sample introduction system for mercury determination by atomic fluorescence.

Gunn et al. [44] described the apphcation of a graphite-filament electrothermal vaporization apparatus as a sample introduction system for optical emission spectroscopy with an inductively coupled argon plasma source. Good detection levels were reported for the elements, and details of the interfacing requirements between the ICP and the graphite filament were explored. 

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 ... 

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