Sample introduction systems requirements

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]. 

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. 

Although ICP-MS has been used for analysis of nuclear materials, often the entire instrument must be in an enclosed hot enclosure [350]. Sample preparation equipment, inlets to sample introduction systems, vacuum pump exhaust, and instrument ventilation must be properly isolated. Many of the materials used in the nuclear industry must be of very high purity, so the low detection limits provided by ICP-MS are essential. The fission products and actinide elements have been measured by using isotope dilution ICP-MS [351]. Because isotope ratios are not predictable, isobaric and molecular oxide ion spectral overlaps cannot be corrected mathematically, so chemical separation is required. 

Micellar Pg Surfactants may clog the sample introduction system. Smaller concentrations of organic solvents are required. 48, 52... 

For some elements, fit-for-purpose sample preparation is required in order to obtain reliable analytical results. Iodine is a well-known case, its determination being complicated by loss of iodide (as HI) from HNO3 solutions, memory effects due to volatilization in the sample introduction system and by matrix effects... 

Mass spectra can only be obtained from compounds which are in the vapor-phase. The vapor pressure required to obtain a spectrum depends on the kind of sample introduction system if the sample is first evaporated in the gas container of the spectrometer and from there introduced into the ion source, a vapor pressure of about 10-2 mm Hg is necessary, while for direct introduction of the substance into the ion source a vapor pressure of only 10-6 mm is needed,2 usually sufficient to obtain spectra of very polar and nearly nonvolatile compounds, e.g., amino acids. Therefore, direct introduction systems2-9 (see also Biemann,10 p. 33) available since the pioneering work of Reed2 in all commercial instruments should be used in spite of experimental difficulties, if thermal or catalytic decomposition of the sample is to be expected. If the vapor pressure is so low that the sample cannot be vaporized sufficiently in the ion source, protecting of polar OH and NH groups by methylation or acetylation may produce a derivative of volatility enough to obtain a spectrum. 

The amount of sample required to obtain a spectrum is of the order of 0.01 mg (direct sample introduction system) to 1 mg (indirect sample introduction system). Purity of the sample is desirable, but in many cases not essential. Usually up to 10% of impurity (much more even of low molecular weight compounds) does not seriously interfere with the interpretation. In some instances, information about the structures of the different compounds may be obtained even from mixtures. 

With the ever-increasing need to improve quality and productivity in the analytical pharmaceutical laboratory, automation has become a key component. Automation for vibrational spectroscopy has been fairly limited. Although most software packages for vibrational spectrometers allow for the construction of macro routines for the grouping of repetitive software tasks, there is only a small number of automation routines in which sample introduction and subsequent spectral acquisition/data interpretation are available. For the routine analysis of alkali halide pellets, a number of commercially available sample wheels are used in which the wheel contains a selected number of pellets in specific locations. The wheel is then indexed to a sample disk, the IR spectrum obtained and archived, and then the wheel indexed to the next sample. This system requires that the pellets be manually pressed and placed into the wheel before automated spectral acquisition. A similar system is also available for automated liquid analysis in which samples in individual vials are pumped onto an ATR crystal and subsequently analyzed. Between samples, a cleaning solution is passed over the ATR crystal to reduce cross-contamination. Automated diffuse reflectance has also been introduced in which a tray of DR sample cups is indexed into the IR sample beam and subsequently scanned. In each of these cases, manual preparation of the sample is necessary (23). In the field of Raman spectroscopy, automation is being developed in conjunction with fiber-optic probes and accompanying... 

Due to the very high sensitivity of the ICP-MS technique, memory (carry over) effects may occur when analytes from a previous sample are measured in the current sample. In cases where analysis of highly polluted soil digests is carried out, memory effects can occur, they may be indications of problems in the sample introduction system. Severe memory interferences may require disassembly and the cleaning of the entire sample introduction system, including the plasma torch and the sampler and skimmer cones. Due to these memory... 

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... 

The ICP spectrometer has much in common with the flame AA spectrometer in its pneumatic sample introduction system. However, usually less compromise is required in the flow-rate during sample introduction, because optimum uptake rates of ICP spectrometers are much lower than flame AA systems. Nevertheless, various attempts were made to further improve the sensitivity by other FI techniques, including direct injection into the torch [20], and thermospray interfacing [21]. 

All mass spectrometers comprise a series of components to effect this (Fig. 5.1). The separation of ions in the gas phase requires the analyser to be held under high vacuum (better than 10 Torr) to reduce ion/gas molecule collisions. Thus a sample introduction system is required which maintains this. This introduction system can maintain the sample in an electrically neutral state, but often ionisation is effected during the sampling process. If the sample is neutral then ionisation takes place in an ionisation source. The mixture of ions formed is electrically accelerated into the mass analyser. This is the heart of the spectrometer and uses a variety of electrical and/or magnetic fields to separate ions and present them to a detector capable of amphfying the weak electrical signals generated by the ions to allow the response to be collected by a further analytical tool, usually a conputerised data system. The computer also controls the operation of the mass analyser and often will control the sample introduction system as well. 

The sample introduction system used will depend on the type of sample and whether a chromatographic separation is required. The usual technique for separated and relatively pure samples is the direct insertion probe which carries a small amount of solid sample through an air lock into the high vacuum of the mass spectrometer. The sample would then be vapourised by applying gentle heat, and the vapours ionised in an ionisation source. Samples already in the gas phase, such as vapours or the eluent from gas chromatography, can be introduced directly into the ionisation source at low flows ( 1 mLmin ). Usually two ionisation techniques are used for these samples, electron ionisation (El) or chemical ionisation (Cl). 

Probably the most common separation systems used in the laboratory today require the sample to be in solution (e.g. HPLC, CE). The solvent may be aqueous or solvent based. However, onemL of such solution yields far too much vapour (1-2L) to be accommodated by a mass spectrometer s vacuum system. Thus the aim of a sample introduction system for such solutions would require the sample to be ionised and the solvent to be separated from these sanple ions. In addition the interface must maintain the integrity of the chromatography. The chromatographic separation must be maintained as well as allowing sufficient analyte through to generate a mass spectmm. A number of methods have been developed to do this, but the two main techniques used today are electrospray and atmospheric pressure chemical ionisation (APCI for short). These are described below under ionisation techniques. 

Mass spectrometers with simple sample introduction systems are not effective when analyzing complex mixtures, although multianalyzer systems can address this problem to some extent (Section 3.3.2). Samples of biological or environmental origin nsnally require chromatographic separation with subsequent, sequential introduction of the constituent species into the ion source. Gas chromatography (GC), liquid chromatography (variously abbreviated as LC, HPLC, and UPLC), and capillary electrophoresis (CE) can be interfaced directly with mass spectrometers (Section 2.1.2). 

The general requirements for sample introduction in fast-GC include the accurate delivery of a small injection size, typically using a fast injection device. The effectiveness of the sample introduction system will be measured by its ability to deliver a sufficiently narrow injection band. In 1966, Sternberg described how column performance is degraded by the design of the injection port, dead volume, and surface activity in the flow lines, detector cell design, and electronic time constants. These effects are summarized in eqn [9], where the subscript o denotes all contributions other than the injector and the column ... 

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