Direct sampling insertion devices

This type of device can also be used with ICP-MS [311], where it provides increased sample insertion efficiency and much simpler background spectra in relation to ETV. In addition, matrix interferences can be reduced by removing the matrix at lower temperatures before the sample probe is fully inserted into the plasma [312,313]. The very small peak widths obtained during vaporization of the sample into the plasma can cause serious problems when the ICP-MS software or electronics is too slow for acquiring a sufficiently large number of data points. 

A recent, improved contribution is the use of the Rh-Cu couple for in-torch vaporization in direct elemental ICP-MS analyses in solid microsamples (sample mass ca. 130 gg), slurries and liquid samples (sample volumes in the microlitre range) [314]. 

Jackson Ed., Electrothermal Atomization of Analytical Atomic Spectrometry, John Wiley, New York (1999). 

Sneddon Ed., Adyances in Atomic Spectroscopy, JAl Press, Stanford, CT. 1997, vols 3 4 (1997 1998, respectively). 

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Salin E. D. and Horlick G. (1979) Direct sample insertion device for ICP... 

Application of a wire loop direct sample-insertion device for ICP-MS, Anal Chem 58 975-976. 

In the first reported direct sample insertion device, the ICP torch was modified by replacement of the inner tube with a quartz tube that acted partially as a guide for the sample probe (Figure 112). A graphite sample probe is placed on the end of a solid quartz rod that can be inserted into the inner quartz tube. The bottom end of the quartz rod is attached to a sliding platform. The graphite electrode probe is moved into the plasma by moving the platform manually into the vertical position. Introduction of air into the plasma must be prevented. 

Figure 112 Schematic diagram of a direct sample insertion device for ICP-AES. (Adapted from E. D. Salin and G. Horlick, Anal. Chem., 1979, 51, 2284)...

The pneumatic nebulizer has for many years been the most universal sample insertion device for plasma-based spectrometry. The inherent lack of transport efficiency, coupled with the continuing need for increased sensitivity, has promoted research into the use of ultrasonic nebulizers to boost detection capabilities. Such research has focused on various aspects including fundamental aerosol properties [86-88], instrument development [89], nebulizer comparisons [90,91], desolvation effects [92,93], direct nebulization applications [94,95] and speciation [96]. 

These direct-insertion devices are often incorporated within an autosampling device that not only loads sample consecutively but also places the sample carefully into the flame. Usually, the sample on its electrode is first placed just below the load coil of the plasma torch, where it remains for a short time to allow conditions in the plasma to restabilize. The sample is then moved into the base of the flame. Either this last movement can be made quickly so sample evaporation occurs rapidly, or it can be made slowly to allow differential evaporation of components of a sample over a longer period of time. The positioning of the sample in the flame, its rate of introduction, and the length of time in the flame are all important criteria for obtaining reproducible results. 

The female mosquito s blood sampling ability has often been cited as an inspiration for development of microneedle-based systems. A few features of the mosquito anatomy and extraction ability are noteworthy. The mosquito s labium is about 3.5 mm long and narrows to an inner diameter of about 30 pm. The labium is applied to the skin with a hammer-like motion at the rate of 6-7 Hz for penetration. A muscle valve and mouth pump move in concert to create about 7 kPa of negative pressure that is sufficient to extract 1.9 pL of blood in 2 min.54 There is some indication that the mosquito can sense when a source of blood is reached and can change the direction of insertion while partway in the skin to achieve a greater rate of success. While the mosquito has served as inspiration for design and function parameters, man-made devices to date have relatively rudimentary functionality in comparison. 

Other Methods of Ionization. There are several other methods for ionization in addition to ESI and MALDI. However, most of them are not commonly used in proteomics. Some of these include chemical ionization, electron ionization, fast atom bombardment (FAB), and many others. Most of these lead to disintegration or fragmentation of analyte molecules and are not commonly used in proteomics. However, FAB has some application in the analysis of proteins and peptides, because this is a soft ionization procedure and does not cause the fragmentation of molecules under analysis. In the FAB method, a nonvolatile matrix such as m-nitrobenzyle alcohol is used to hold the analyte molecules. Analyte molecules are vaporized and ionized by bombardment with the high-energy beam of xenon or cesium from a probe inserted directly into the device containing the sample. Ionized molecules thus obtained are then subjected to separation by the mass... 

The acids are usually titrated with a methanol solution of tetramethylammonium hydroxide with phenolphthalein as the indicator. The solution of the salts formed is introduced directly into the device for sample insertion heated at 360-400°C. 

Another consideration is the beam cross section. Typical beam cross section at SSRL (SPEAR ) is on the order of several to tens of mm, while that on typical APS lines is in the tens of microns. For practical sample size, e.g., 2 cm, this means that the beam size cannot be larger than about 40 pm at a critical angle of 2 mrad. Any larger part of the beam will miss the sample (Fig. 6) and probably incur dramatic noise production. In practice this greatly reduces the utility of the GI experiment as so much incident beam is rejected by collimation (e.g., with SPEAR with a 2 mm vertical height beam, 98% of the beam is discarded). Thus the APS beams are ideal for samples of this size, and all of the output from an insertion device can in principle be directed on a GI sample of a few mm. 

Several techniques have been proposed during the last two decades for the direct introduction of solids into atomizers, thus avoiding the need to dissolve or decompose the sitmple. These techniques include (1) direct manual insertion of the solid into the atomization device, (2) electrothermal vaporization of the sample and transfer of the vapor into the atomization region, (3) arc, spark, or laser ablation of the solid to produce a vapor that is then swept into the atomizer, (4) slurry nebulizalion in which the finely divided solid sample is carried into the atomizer as an aerosol consisting of a suspension of the solid in a liquid medium, and (5) sputtering in a glow discharge device. None of these procedures yields results as satisfactory as those... 

Solid samples can be analyzed using a plasma torch by first ablating the solid to form an aerosol, which is swept into the plasma flame. The major ablation devices are lasers, arcs and sparks, electrothermal heating, and direct insertion into the flame. 

Amongst other devices used to produce the required atoms in the vapour state are the Delves cup which enables the determination of lead in blood samples the sample is placed in a small nickel cup which is inserted directly into an acetylene-air flame. The tantalum boat is a similar device to the Delves cup in this case the sample is placed into a small tantalum dish which is then inserted into an acetylene-air flame. The use of these devices, especially for small sample volumes, has now been largely superseded by the graphite furnace. 

Direct insertion probe pyrolysis mass spectrometry (DPMS) utilises a device for introducing a single sample of a solid or liquid, usually contained in a quartz or other non-reactive sample holder, into a mass spectrometer ion source. A direct insertion probe consists of a shaft having a sample holder at one end [70] the probe is inserted through a vacuum lock to place the sample holder near to the ion source of the mass spectrometer. The sample is vaporized by heat from the ion source or by heat from a separate heater that surrounds the sample holder. Sample molecules are evaporated into the ion source where they are then ionized as gas-phase molecules. In a recent study, Uyar et al. [74] used such a device for studying the thermal stability of coalesced polymers of polycarbonate, PMMA and polylvinyl acetate) (PVAc) [75] and their binary and ternary blends [74] obtained from their preparation as inclusion compounds in cyclodextrins. 

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