Interfaces thermal desorption

E. S. Erancis, M. Wu, R B. Eamswoith and M. L. Lee, Supercritical fluid extraaion/gas cliromatography with thermal desorption modulator interface and niti O-specific detection for the analysis of explosives , 7. Microcolumn Sep. 7 23-28 (1995). 

Pankow JR, Isabelle LM, Kristensen TJ. 1982. Tenax-GC cartridge for interfacing capillary column gas chromatography with adsorption/thermal desorption for determination of trace organics. Anal Chem 54 1815-1819. 

By employing a laser for the photoionization (not to be confused with laser desorption/ ionization, where a laser is irradiating a surface, see Section 2.1.21) both sensitivity and selectivity are considerably enhanced. In 1970 the first mass spectrometric analysis of laser photoionized molecular species, namely H2, was performed [54]. Two years later selective two-step photoionization was used to ionize mbidium [55]. Multiphoton ionization mass spectrometry (MPI-MS) was demonstrated in the late 1970s [56—58]. The combination of tunable lasers and MS into a multidimensional analysis tool proved to be a very useful way to investigate excitation and dissociation processes, as well as to obtain mass spectrometric data [59-62]. Because of the pulsed nature of most MPI sources TOF analyzers are preferred, but in combination with continuous wave lasers quadrupole analyzers have been utilized [63]. MPI is performed on species already in the gas phase. The analyte delivery system depends on the application and can be, for example, a GC interface, thermal evaporation from a surface, secondary neutrals from a particle impact event (see Section 2.1.18), or molecular beams that are introduced through a spray interface. There is a multitude of different source geometries. 

Valuable information can be obtained from thermal desorption spectra (TDS) spectra, despite the fact that electrochemists are somewhat cautious about the relevance of ultrahigh vacuum data to the solution situation, and the solid/liquid interface in particular. Their objections arise from the fact that properties of the double layer depend on the interaction of the electrode with ions in the solution. Experiments in which the electrode, after having been in contact with the solution, is evacuated and further investigated under high vacuum conditions, can hardly reflect the real situation at the metal/solution interface. However, the TDS spectra can provide valuable information about the energy of water adsorption on metals and its dependence on the surface structure. At low temperatures of 100 to 200 K, frozen molecules of water are fixed at the metal. This case is quite different from the adsorption at the electrode/solution interface, which usually involves a dynamic equilibrium with molecules in the bulk. 

One of the key developments in the development of thermal desorption devices was the possibility for cryofocusing systems that have the advantage of injection-like samples. A short section of capillary tubing at liquid nitrogen temperatures (i.e., -160°C) traps the volatiles. When capillary columns replaced packed columns as the standard, complete flow from the desorption trap (5 ml/min minimum) to the capillary columns ( 1 ml/min) was possible through the use of cryofocusing. The split injection interface was another development that splits the flow so that only a part of the desorbed volatiles entered the column. While this allowed the need for cryofocusing to be circumvented, sensitivity was lost due to the split. 

A solid or liquid sample can be introduced to the analyzer by thermal desorption. The resultant vapors are swept through the inlet by the carrier gas and ionized by a radioactive 63Ni source. Discreet packets of ions are then pulsed down the flight tube under a controlled potential. The arrival of the ions at the detector is inversely proportional to the mass of the molecule. Thus, the smaller ions arrive at the detector first, and the larger ions arrive later. At that point the signal is amplified and read out via an appropriate computer interface. Both commercially available instruments are capable of generation and detection of both positive and negative ions. 

The effectiveness of using SEE coupled to GC with a thermal desorption modulator (TDM) interface and TEA detection to analyse explosives has been evaluated by Francis et al. [33] in soil samples and standards. 

Matz, G., Kibelka, G., Dahl, J., Leimeman, F. Experimental study on solvent-less sample preparation methods - membrane extraction with a sorbent interface, thermal membrane desorption application and purge-and-trap. J. Chromatogr. 830, 365-376 (1999)... 

Fig. 34 are presented as ij-U-y curves. Without sulfide treatment a 1.05 eV band gap is resolved, whereas the gap shrinks completely after passivation. Though results seem in line with a passivation of electronic surface states by the sulfide treatment, since no band gap is resolved on clean GaAs(llO) in vacuum [53] , one may wonder whether the results of Fig. 34 a are not related to the overlayer instead of the substrate. Recent studies have indeed shown that the density of states is not reduced after sulfur coating [87], in contrast to initial assumptions [86]. Moreover, thermal desorption of the sulfide layer opens a band gap [164], as in Fig. 34 b, which is consistent with the existence of the monolayer of oxygen at the interface between GaAs and the layer [161]. In vacuum a wide band gap is also found locally at places where oxygen is adsorbed on clean GaAs(llO) [53]. 

As an alternative to thermal desorption, analytes can be desorbed from the fibre by using organic solvents at a special interface and subsequently analysed by HPLC [222]. 

When the extracted analytes are to be retained directly on the chromatographic column or at the retention interface, their insertion can be accomplished in various ways, namely (a) by injection into the column, whether directly (SFC, GC) or with the aid of a cooling system (GC, HPLC) (b) by split-splitless injection (SFC, GC) (c) by using a programmed temperature vaporizer (GC) or (d) by injection into a cold trap and subsequent thermal desorption (GC) or elution (HPLC). 

The PBI was originally developed as a monodisperse aerosol generating interface for chromatography (MAGIC) by the research group of Browner [80-81]. The design objective of MAGIC was the development of an LC-MS interface with El capabilities, minimum peak distortion, and without a thermal desorption step, as is required in the MBI. 

Insertion/introduction of the needle into the GC port, depression of the plunger, and thermal desorption of the analytes. Alternatively, the analytes are washed out of the fiber by the HPLC mobile phase via a modified HPLC six-port injection valve and a desorption chamber that replaces the injection loop in the HPLC system. The SPME fiber is introduced into the desorption chamber, under ambient pressure, when the injection valve is in the load position. The SPME-HPLC interface enables mobile phase to contact the SPME fiber, remove the adsorbed analytes, and deliver them to the separation column. Analytes can be removed via a stream of mobile phase (dynamic desorption) or, when the analytes are more strongly adsorbed to the fiber, the fiber can be soaked in mobile phase or another stronger solvent for a specific period of time (e.g., 1 min) before the material is injected onto the column (static desorption) (Fig. 6). 

The region of the cyclic voltammogram, corresponding to anodic removal of Hathermal desorption spectra of platinum catalysts. However, unlikely the thermal desorption spectra, the cyclic-voltammetric profiles for H chemisorbed on Pt are usually free of kinetic effects. In addition, the electrochemical techniques offer the possibility of cleaning eventual impurities from the platinum surface through a combined anodic oxidation-cathodic reduction pretreatment. Comparative gas-phase and electrochemical measurements, performed for dispersed platinum catalysts, have previously demonstrated similar hydrogen and carbon monoxide chemisorption stoichiometries at both the liquid and gas-phase interfaces (14). 

The amount of solute transferred to the column can be diminished by formation of a frozen CO2 plug in its head. Any water present in the sample may also plug the column restrictor with ice deposits. Both shortcomings can be circumvented using a hot injector as the interface or a cooled thermal desorption injector. 

A study of methanol adsorption on platinum under UHV conditions or at a gas/solid interface is also of interest. Not many papers dealing with methanol adsorption in a UHV chamber [135,136] are available. The adsorption takes place without a reaction on Pt( 111) at lo w temperatures (100 K), and based on thermal desorption experiments it was concluded that a monolayer of methanol adsorbate desorbs at 180 K. The heat of adsorption of molecular methanol was estimated to be 46 kJ mol-1 on unreconstructed Pt(l 11) [137]. Infrared spectroscopy has been applied for the study of methanol adsorption on Pt(l 11) [138], and it was shown that a 0.36 monolayer of methanol corresponds to the saturation of the desorption peak found at 180 K. The methanol multilayer coverages were also found, but had different infrared frequencies that were associated with the methyl and C-0 stretching modes (Scheme 11.1). 

In the following sections, we will first consider metal—silicon and metal/III—V compound systems, covering electronic and chemical interface effects. We will then present a general treatment of the adsorption-desorption models which have been proposed, including some discussion of the interpretation of thermal desorption spectra, and finally we will discuss to what extent the two sets of data can be reconciled. We will not be concerned with metal films in device fabrication, nor the formation of additional phases by heat treatment. We shall only deal with interactions and interface chemistry which is directly relevant to adsorption-desorption behaviour. 

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