Sample direct exposure probe

Direct-exposure probe. Provides for insertion of a sample on an exposed surface, such as a flat surface or a wire, into (rather than up to the entrance of) the ion source of a mass spectrometer. 

Direct introduction of a sample, either in solid or liquid state, in the ion source of a mass spectrometer may be achieved through two procedures the first one is based on the use of a direct insertion probe (DIP) the second one necessitates a direct exposure probe (DEP). Direct introduction followed by heating of the sample in the ion source of the mass spectrometer is also known as direct temperature resolved mass spectrometry (DTMS). 

Employing a direct exposure probe (DEP) may be helpful in case of analytes that cannot be evaporated from a sample vial without complete decomposition. [44] Here, the analyte is applied from solution or suspension to the outside of a thin wire loop or pin which is then directly exposed to the ionizing electron beam. This method has also been termed in-beam electron ionization. Early work describing the direct exposure of a sample to the electron beam came from Ohashi, [45,46] Constantin, [47] and Traldi. [48,49]... 

Cl in conjunction with a direct exposure probe is known as desorption chemical ionization (DCI). [30,89,90] In DCI, the analyte is applied from solution or suspension to the outside of a thin resistively heated wire loop or coil. Then, the analyte is directly exposed to the reagent gas plasma while being rapidly heated at rates of several hundred °C s and to temperatures up to about 1500 °C (Chap. 5.3.2 and Fig. 5.16). The actual shape of the wire, the method how exactly the sample is applied to it, and the heating rate are of importance for the analytical result. [91,92] The rapid heating of the sample plays an important role in promoting molecular species rather than pyrolysis products. [93] A laser can be used to effect extremely fast evaporation from the probe prior to CL [94] In case of nonavailability of a dedicated DCI probe, a field emitter on a field desorption probe (Chap. 8) might serve as a replacement. [30,95] Different from desorption electron ionization (DEI), DCI plays an important role. [92] DCI can be employed to detect arsenic compounds present in the marine and terrestrial environment [96], to determine the sequence distribution of P-hydroxyalkanoate units in bacterial copolyesters [97], to identify additives in polymer extracts [98] and more. [99] Provided appropriate experimental setup, high resolution and accurate mass measurements can also be achieved in DCI mode. [100]... 

Open-access LC/MS formats have spawned new dimensions in access and data management. The use of a direct exposure probe (DEP) for automated sample introduction has been developed for quick (ca. 3 min) molecular weight determination of new lead compounds and quantitative analysis. Fig. 7 illustrates an automated direct probe system for molecular weight determination. Versatile software packages for data manipulation and processing has been a popular... 

A direct exposure probe usually has a rounded glass tip. The sample is dissolved in solvent, a drop of the solution is placed on the end of the probe and the solvent is allowed to evaporate. A thin film of sample is left on the glass tip. The tip is inserted into the ion source and heated in the same manner as the direct insertion probe. Much less sample is introduced into the ion source and the spectrometer is less likely to be contaminated as a result. 

As mentioned before, MS is a useful technique to analyze unknown organic molecules by studying their fragmentation pattern. However, since there are several requirements for samples to be analyzed, when an organic molecule is unsuitable to be studied by gas chromatography, another suitable analytical technique can be chosen, for example, liquid chromatography (EC), direct injection probe (DIP), and direct exposure probe (DEP). 

Direct exposure probe, DEP sample particles or film of analyte on resistively heated metal filament solids of extremely low volatility, especially if thermally labile... 

In principle, any type of magnetic or quadrupole mass spectrometer can be utilized for the analytical pyrolysis of organic materials, if a direct introduction system capable of producing a desired tempera-ture/time profile is available. For example, direct insertion probes (DIPs) and direct exposure probes (DEPs) are Avidely used for sample introduction and such probes are supplied with control units that allow heating and temperature programming of the sample up to 500-800°C. Therefore, such modules should be considered as the most readily available probes for Py-MS studies. 

Direct-inlet probe. A shaft or tube having a sample holder at one end that is inserted into the vacuum system of a mass spectrometer through a vacuum lock to place the sample near to, at the entrance of, or within the ion source. The sample is vaporized by heat from the ion source, by heat applied from an external source, or by exposure to ion or atom bombardment. Direct-inlet probe, direct-introduction probe, and direct-insertion probe are synonymous terms. The use of DIP as an abbreviation for these terms is not recommended. 

The pressurized sample can be introduced into the probe without direct exposure to the tube. The baseplate and cap of the safety shield are removed, and the remaining assembly is made to rest on top of the magnet above the probe stack. The tube is restrained by means of a removable loop of string or lasso around the stem drive (part 3 in Figure 5) while the plastic bolts supporting the spinner are removed. The tube can then be lowered to the top of the probe stack where it can be supported by the eject air system and lowered into the probe. 

A new ionization method called desorption electrospray ionization (DESI) was described by Cooks and his co-workers in 2004 [86]. This direct probe exposure method based on ESI can be used on samples under ambient conditions with no preparation. The principle is illustrated in Figure 1.36. An ionized stream of solvent that is produced by an ESI source is sprayed on the surface of the analysed sample. The exact mechanism is not yet established, but it seems that the charged droplets and ions of solvent desorb and extract some sample material and bounce to the inlet capillary of an atmospheric pressure interface of a mass spectrometer. The fact is that samples of peptides or proteins produce multiply charged ions, strongly suggesting dissolution of the analyte in the charged droplet. Furthermore, the solution that is sprayed can be selected to optimize the signal or selectively to ionize particular compounds. 

In view of the ionization of samples of low volatility, desorption chemical ionization (DCI) is one of the most promising new techniques. The sample is placed on a heatable probe tip or on a field desorption emitter probe and introduced directly into the plasma of a chemical ion source. It was shown that by such an exposure of the sample to the ion plasma, mass spectra can be obtained at much lower temperatures than usually required . This extraordinarily simple sample handling and the apparently more intense and better reproducible ion beam than in field desorption ionization have obviously induced a rapidly growing field of applications of DCI. This development is also supported by a growing number of DCI probe devices available from different manufacturers at relatively reasonable prices. Although many applications, for instance to underivatized peptides , to cyclic adenosine monophosphate, guanosine, and thiophosphoric acid pesticides , as well to creatinine and arginine have been reported, the analytical potential of DCI is not yet exhausted and will obviously result in many new possibilities in analytical research (for recent publications see ). 

Modulated radiation from a standard spectrometer is directed onto a sample and detected by a microprobe (see Fig. 20) [137, 138]. The IR imager s thermal probe is the same as used in micro-thermcd techniques. It detects photo-thermal response of a region on the specimen heated by exposure to the beam from a FTIR... 

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