Thermal desorption direct mass spectrometry

The main difference between field ionization (FI) and field desorption ionization (FD) lies in the manner in which the sample is examined. For FI, the substance under investigation is heated in a vacuum so as to volatilize it onto an ionization surface. In FD, the substance to be examined is placed directly onto the surface before ionization is implemented. FI is quite satisfactory for volatile, thermally stable compounds, but FD is needed for nonvolatile and/or thermally labile substances. Therefore, most FI sources are arranged to function also as FD sources, and the technique is known as FI/FD mass spectrometry. 

Desorption ionization (DI). General term to encompass the various procedures (e.g., secondary ion mass spectrometry, fast-atom bombardment, californium fission fragment desorption, thermal desorption) in which ions are generated directly from a solid or liquid sample by energy input. Experimental conditions must be clearly stated. 

For non-volatile sample molecules, other ionisation methods must be used, namely desorption/ionisation (DI) and nebulisation ionisation methods. In DI, the unifying aspect is the rapid addition of energy into a condensed-phase sample, with subsequent generation and release of ions into the mass analyser. In El and Cl, the processes of volatilisation and ionisation are distinct and separable in DI, they are intimately associated. In nebulisation ionisation, such as ESP or TSP, an aerosol spray is used at some stage to separate sample molecules and/or ions from the solvent liquid that carries them into the source of the mass spectrometer. Less volatile but thermally stable compounds can be thermally vaporised in the direct inlet probe (DIP) situated close to the ionising molecular beam. This DIP is standard equipment on most instruments an El spectrum results. Techniques that extend the utility of mass spectrometry to the least volatile and more labile organic molecules include FD, EHD, surface ionisation (SIMS, FAB) and matrix-assisted laser desorption (MALD) as the last... 

In a study on the identification of organic additives in rubber vulcanisates using mass spectrometry, Lattimer et al. [22] used direct thermal desorption with three different ionisation methods El, Cl and FI. Also, rubber extracts were examinated directly by four ionisation methods (El, Cl, FD and FAB). The authors did not report a clear advantage for direct analysis as compared to analysis after extraction. Direct analysis was a little faster, but the extraction methods were considered to be more versatile. 

T-MS). The main direct mass-spectral methods are thermal desorption and pyrolysis mass spectrometry. Several factors favour the efficiency at which volatiles can be removed from a polymeric matrix ... 

SFE-GC-MS is particularly useful for (semi)volatile analysis of thermo-labile compounds, which degrade at the higher temperatures used for HS-GC-MS. Vreuls et al. [303] have reported in-vial liquid-liquid extraction with subsequent large-volume on-column injection into GC-MS for the determination of organics in water samples. Automated in-vial LLE-GC-MS requires no sample preparation steps such as filtration or solvent evaporation. On-line SPE-GC-MS has been reported [304], Smart et al. [305] used thermal extraction-gas chromatography-ion trap mass spectrometry (TE-GC-MS) for direct analysis of TLC spots. Scraped-off material was gradually heated, and the analytes were thermally extracted. This thermal desorption method is milder than laser desorption, and allows analysis without extensive decomposition. 

Oezel, M.Z., Goegues, R, Lewis, A.C., (2006) Determination of Teucrium chamaedrys volatiles by using direct thermal desorption-comprehensive two-dimensional gas chromatography-time-of-flight mass spectrometry. J. Chromatogr All 14 164-169. 

Ozel, M.Z., Gogus, R, Hamilton, J.E, Lewis, A.C. (2004) The essential oil of Pistacia vera L. at various temperatures of direct thermal desorption using comprehensive gas chromatography with time-of-flight mass spectrometry. Chromatographia 60 79-83. 

Field-desorption mass spectrometry (FDMS), where no evaporation prior to ionization is required, has been successfully used in the analysis of in volatile phosphonium salts113, although a direct thermal process gave similar spectra114. In the case where the FD spectra are complex, a chemical ionization technique may give wider applicability115. The cation is the base peak for monophosphonium salts when the [2M + anion]+ cationic species is the one for bisphosphonium compounds. 

The HREELS, Auger electron spectroscopy (AES) and thermal desorption spectrometry (TDS) experiments were carried out in a UHV chamber described previously.6 Briefly, the chamber was equipped with a HREELS spectrometer for vibrational analysis, a single-pass cylindrical mirror analyzer for AES measurements and a quadrupole mass spectrometer for TDS measurements. The HREELS spectra were collected in the specular direction with an incident energy of 3.5 eV and with a spectroscopic resolution of 50-80 cm-1. The TDS data were obtained by simultaneously monitoring up to 16 masses, with a typical heating rate of about 1.5 K s-1. 

Ionization Methods/Processes. The recent development of several new ionization methods in mass spectrometry has significantly improved the capability for the analysis of nonvolatile and thermally labile molecules [18-23]. Several of these methods (e.g., field desorption (FD), Californiun-252 plasma desorption (PD), fast heavy ion induced desorption (FHIID), laser-desorption (LD), SIMS, and fast atom bombardment (FAB) or liquid SIMS) desorb and ionize molecules directly from the solid state, thereby reducing the chance of thermal degradation. Although these methods employ fundamentally different excitation sources, similarities in their mass spectra, such as, the appearance of protonated, deprotonated, and/or cationized molecular ions, suggest a related ionization process. 

Aroma compounds from vanilla beans have been extracted using several extraction procedures, using alcohols and organic solvents (Galletto and Hoffman, 1978 Dignum et al., 2002), direct thermal desorption (Hartman et al., 1992 Adedeji et al., 1993) and solid-phase microextraction (SPME) (Sostaric etal., 2000), followed by identification of the compounds by gas chromatography-mass spectrometry (GC-MS). 

In order to reveal molecular ions, we moved to desorption/chemical ionization mass spectrometry. This direct chemical ionization (DCI), initially described by Me Lafferty in 1973 68 , was widely used and improved recently with the aim of decreasing sharply thermal degradation of samples under vaporization and to extend mass spectrometry capability both to poorly volatile and to very fragile compounds having high molecular masses 69-74). 

Perez-CoeUo, M. S., Sanz, J., Cabezudo, M. D. (1997). Analysis of volatile components of oak wood by solvent extraction and direct thermal desorption-gas chromatography-mass spectrometry. J. Chromatogr. A, 778, 427 34. 

Schnelle-Kreis, J., Orasche, J., Abbaszade, G., Schafer, K., Flarlos, D.P., Hansen, A.D.A., Zimmermann, R. Application of direct thermal desorption gas chromatography time-of-flight mass spectrometry for determination of non-polar organics in low-volume samples from ambient particulate matter and personal samplers. Anal. Bioanal. Chem. 401, 3083-3094 (2011)... 

After transport to a laboratory, gases are introduced into an analytical instrument for quantitative determination of the constituents of interest. Soil air in a container is introduced directly to the instmment, whilst adsorbed gas is released by thermal of chemical desorption. The instrumental methods most widely used for gas analyses include gas chromatography, mass spectrometry and atomic absorption spectrophotometry. For quantifying the radiation scars on film, image analysis methods are employed. 

Crystalline products with the structure of AIPO4-II have been produced using four different amines as structure-directing species. A method for quantifying the amount of amine in the precursors has been developed. This involves the use of thermal desorption/mass spectrometry in combination with thermo-gravimetry. This method also provides information as to the distribution of the water within the structure. 

Hartman T.G., Lech J, Karmas K., Salinas J., Rosen R.T. and Ho C.T. (1993) Flavor characterization using adsorbent trapping-thermal desorption or direct thermal desorption-gas chromatography and gas chromatography-mass spectrometry. 16 th I FT Basic Symp. Ser., New-Orleans, June 19-20, 1992. Ho Manley, Eds Flavor Measurement. Marcel Dekker, New York, pp. 37-69. 

Electrochemists often use mass spectrometry as a tool for the identification of electrolysis products ex situ, but the approach is conventional and requires no amplification here. Mass spectrometry (MS) can also be used to sample volatile species produced at a porous electrode connected directly to a mass spectrometer. Alternatively, the solution in the electrochemical cell can be introduced into the mass spectrometer inlet by a thermospray or electrospray approach. Moreover, electrodes can be removed from the cell and introduced into a UHV chamber and their surface examined by MS using conventional desorption techniques, such as laser or thermal desorption, or bombardment of the surface with an ion beam (secondary-ion mass spectrometry or SIMS) to produce the ions that are mass-analyzed. 

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