Sample-insertion technique, direct

In the direct sample-insertion technique, the sample is physically placed in the atomizer. For solids, the sample may be ground into a powder, which is then placed on or in a probe that is inserted directly into the atomizer. With electric are and spark atomizers, metal samples arc frequently introduced as one or both electrodes that are used to form the arc or spark. 

In the direct insertion technique, the sample (liquid or powder) is inserted into the plasma in a graphite, tantalum, or tungsten probe. If the sample is a liquid, the probe is raised to a location just below the bottom of the plasma, until it is dry. Then the probe is moved upward into the plasma. Emission intensities must be measured with time resolution because the signal is transient and its time dependence is element dependent, due to selective volatilization of the sample. The intensity-time behavior depends on the sample, probe material, and the shape and location of the probe. The main limitations of this technique are a time-dependent background and sample heterogeneity-limited precision. Currently, no commercial instruments using direct sample insertion are available, although both manual and h ly automated systems have been described. ... 

In direct insertion techniques, reproducibility is the main obstacle in developing a reliable analytical technique. One of the many variables to take into account is sample shape. A compact sample with minimal surface area is ideal [64]. Direct mass-spectrometric characterisation in the direct insertion probe is not very quantitative, and, even under optimised conditions, mass discrimination in the analysis of polydisperse polymers and specific oligomer discrimination may occur. For nonvolatile additives that do not evaporate up to 350 °C, direct quantitative analysis by thermal desorption is not possible (e.g. Hostanox 03, MW 794). Good quantitation is also prevented by contamination of the ion source by pyrolysis products of the polymeric matrix. For polymer-based calibration standards, the homogeneity of the samples is of great importance. Hyphenated techniques such as LC-ESI-ToFMS and LC-MALDI-ToFMS have been developed for polymer analyses in which the reliable quantitative features of LC are combined with the identification power and structure analysis of MS. 

The qualifiers continuous and discrete as applied to pervaporation refer to different aspects of the process. In fact, analytical pervaporation is a continuous technique because, while the sample is in the separation module, mass transfer between the phases is continuous until equilibrium is reached. Continuous also refers to the way the sample is inserted into the dynamic manifold for transfer to the pervaporator. When the samples to be treated are liquids or slurries, the overall manifold to be used is one such as that of Fig. 4.18 (dashed lines included). The sample can be continuously aspirated and mixed with the reagent(s) if required (continuous sample insertion). Discrete sample insertion is done by injecting a liquid sample, either via an injection valve in the manifold (and followed by transfer to the pervaporator) or by using a syringe furnished with a hypodermic needle [directly into the lower (donor) chamber of the separation module when no dynamic manifold is connected to the lower chamber]. In any case, the sample reaches the lower chamber and the volatile analyte (or its reaction product) evaporates, diffuses across the membrane and is accepted in the upper chamber by a dynamic or static fluid that drives it continuously or intermittently, respectively, to the detector — except when separation and detection are integrated. 

Zaray Gy., Broekaert J. A. C. and Leis F. (1988) The use of direct sample insertion into a nitrogen-argon inductively coupled plasma for emission spectrometry I. Technique optimization and application to the analysis of aluminium oxide, Spectrochim Acta, Part B 43 241—253. 

Karanassios V and Wood TJ (2000) Direct sample insertion. In Beauchemin D, Gregoire DC, Gunther D, Karanassios V, Mermet J-M and Wood TJ, eds. Discrete sample introduction techniques for inductively coupled plasma mass spectrometry. In Barcelo D, ed. Wilson and Wilson s Comprehensive analytical chemistry, Vol 34, pp. 503-560. Elsevier, Amsterdam. 

It is possible to introduce a sample directly into the chemical ionization source on a tungsten or rhenium wire. A drop of sample in solution is applied to the wire, the solvent is allowed to evaporate, and the sample inserted into the Cl source. The sample molecules are desorbed by passing a current through the wire, causing it to heat. The analyte molecules then ionize by interaction with the reagent gas ions as has been described. This technique is called desorption chemical ionization and is used for nonvolatile compounds. 

In this method, the sample is injected directly onto the column. Here, the sample is not contained in the glass insert. Columns with a small inside diameter are unsuitable for this technique of sample itroduction. As on-column injection is a splitless method, only low-concentration samples can be injected. This method is suitable for polar and thermally unstable components. 

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). 

Yes, but we need to discuss relationships previously derived from statistical mechanics that relate to the population of atoms and ions among the various quantized electronic energy levels. Unless a solid sample is being directly introduced into the plasma by one of the direct insertion techniques, a metal ion dissolved in water, is most likely to be found. This... 

Field desorption refers to the technique in which the sample is deposited directly on the emitter before it is inserted into the ion source. This is an ambiguous term, because it implies that it is the electric field tiiat causes desorption and ionization of the analyte from the probe. It is well-known, however, that the field is only one factor in the process "field ionization" is only one of the ionization processes that may occur. Thus, most practihoners use the term "field desorption" to refer to the sample introduchon technique and not necessarily to the method of ionization. 

Particles were collected directly in the flame using a TEM sampling probe technique [14]. This technique consists of placing a Transmission Electron Microscope (TEM) grid directly onto the tip of a thin probe, which then is rapidly driven into and out of the burner. Particles are driven onto the grid by thermophoresis. The location of the tip of the probe is adjusted such that when it is inserted into the burner, the surface of the TEM grid lays exactly in the middle of the laser beam. This enables... 

The first mass spectra of TATP were recorded by introduction of the sample with a direct-insertion probe [48,49]. The El mass spectrum was very uninformative and contained its major ions in the low-mass region, with a very low-abundance molecular ion. It had some resemblance to the spectrum of acetone. The Cl mass spectra, using methane [49] and isobutane [48,49] contained a distinct [M + H] ion at m/z 223. A GC-MS analysis of TATP was later performed, using El [51-53] and Cl-methane [51]. The resulting mass spectra were similar to those obtained by the solid-probe technique. 

Direct sample insertion allows the direct analysis of used oils [222] and of microamounts of sediments [226] as well as the determination of volatile elements in refractory matrices [224]. Difficulties lie in the calibration and in the signal acquisition. The latter necessitates a simultaneous and time-resolved measurement of the transient signals for the line and background intensities to be made in trace analysis. This may become easier when applying CCD detection with the appropriate software. Also in the case of direct sample insertion the use of thermochemical reagents has been found to be usefiil [469]. An extremely sensitive technique lies... 

Desorption chemical ionization (DCI) places the sample onto a direct-insertion probe located within the chemical ionization plasma [98-100]. Cationic surfactants produce molecular ions and decomposition ions useful for quantitative analysis [91,101a]. The DCI technique is less informative for anionic or nonionic surfactants. 

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