Direct exposure probes

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] 

The idea behind the rapid heating of a DEP is to achieve evaporation faster than thermal degradation of the sample. [50,51] This principle is realized in perfection with energy-sudden methods (Chaps. 9, 10). 

If the analyte is exposed to energetic electrons the method is called direct electron ionization (DEI) or desorption electron ionization (DEI), and accordingly it is termed direct or desorption chemical ionization (DCI) if the analyte is immersed into the reagent gas under conditions of chemical ionization (Chap. 7). 

There are different types of DEPs in that some of them rely on conductive heating from the ion source block and/or the heated tip of a modified DIP, [49] and others - now widespread in use - that are capable of rapid resistive heating of a little loop made of chemically inert metal wire (rhenium). Resistively heated probes allow rates of several hundred °C s and temperatures up to about 1500 °C (Fig. 5.16). As a consequence of rapid heating, fast scanning, e.g., 1 s per scan over the m/z range of interest, is required to follow the evaporation of the analyte. 

In either case, the use of a DEP allows to extend the temperature range for evaporation. In addition, it reduces thermal degradation as a result of heating the analyte faster than its thermal decomposition usually proceeds, and therefore expands the range of applications for El and Cl to some extent. Whatsoever, employing direct exposure probes is by far no replacement of real desorption ionization methods. [52,53] 

Example The DEI technique has been applied to obtain mass spectra of four underivatized amino acids [35]. The method allowed for the observation of molecular ions and some primary fragment ions in contrast to conventional El conditions which do not yield molecular ions. However, these additional signals are comparatively weak (Fig. 5.13). 

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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). 

Fig. 5.16. Tip of a direct exposure probe of a (JC-oaTOF mass spectrometer for El, Cl, FI). By courtesy of Waters Corporation, MS Technologies, Manchester, UK.

In Cl, the analyte is introduced into the ion source the same way as described for El, i.e., via direct insertion probe (DIP), direct exposure probe (DEP), gas chromatograph (GC), or reservoir inlet (Chap. 5.3). 

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... 

Fig. 1 Schematic representation of a mass spectrometer depicting its main components and the different modes used. Abbreviations DIP direct insertion probe DEP direct exposure probe GC gas chromatography LC liquid chromatography CE capillary chromatography TEC thin-layer chromatography FEE field-flow fractionation APCI atmospheric pressure ionization El electron impact Cl chemical ionization FAB fast-atom bombardment PD plasma desorption MALDI matrix-assisted laser desorption ionization ED laser desorption TSP thermospray ESI electron spray ionization HSI hypherthermal surface ionization Q quadropole QQQ triple quadropole TOE time-of-fiight FTMS Fourier transform mass spectrometer IT ion trap EM electrom multiplier PM photomultiplier ICR ion cyclotron resonance.

Flamini, R. and Dalla Vedova, A. (2004) Fast determination of the total free resver-atrol content in wine by direct-exposure-probe, positive-ion-chemical-ionization and collisional-induced-dissociation mass spectrometry (DEP/PICI-MS/MS), Rapid Commun. Mass Spectrom. 18(17), 1925-1931. 

Mass spectrometry was also employed to confirm the structure of one Series 1 and one Series 2 compound, utilizing a Finnegan 4000 direct exposure probe mass spectrometer. 

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). 

The field-desorption spectra of several aldoses and ketoses ionized by attachment of potassium ions led to the identification of the characteristic fragments formed by loss of small molecules, and related secondary ion analysis was carried out on various purine and pyrimidine nucleosides. Also in the nucleoside field a set of mathematical procedures has been applied to the spectra derived from 125 compounds and has led to pattern recognition and interpretation. Underivatized nucleosides have been studied by a method based on pulsed laser and fission fragment-induced desorption, and also by a chemical ionization technique dependent on a direct exposure probe. ... 

Planner, R.D. and Gardner, H.W. Mass spectrometry of isomeric fatty acid hydroperoxides by chemical ionization via direct exposure probe. Lipids 20,126-131 (1985). 

Cyclic hydroperoxides have been characterised by chemical ionisation MS with a direct exposure probe [262]. 

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. 

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