Sample Desorption

Irrespective of the desorption method used (solvent or thermal) it is essential to ascertain the recovery efficiency of the VOCs of interest by spiking sorbent tubes and canisters. 

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Prior to each use, a SPME fiber should be cleaned by exposing it for 2 to 10 min to a temperature that is within the range recommended by the manufacturer for desorption. Since the fiber will have been cleaned during sample desorption, subsequent sampling within a few hours does not require cleaning, provided the fiber is kept retracted and away from high levels of volatiles. 

Figure 1.4 Simple selection scheme for a suitable sorbent under different conditions Boiling point range, humidity of the air sample, desorption type. Please see text for other considerations (artifact formation, surface activity).

Generally, SPE consists of four steps (Figure 2.42) column preparation, or prewash, sample loading (retention or sorption), column postwash, and sample desorption (elution or desorption), although some of the recent advances in sorbent technology reduce or eliminate column preparation procedures. The prewash step is used to condition the stationary phase if necessary, and the optional column postwash is used to remove undesirable contaminants. Usually, the compounds of interest are retained on the sorbent while interferences are washed away. Analytes are recovered via an elution solvent. 

Prior to each TPD-TGA experiment, the samples were exposed to between 10 and 15 Torr of 2-propanol or 2-propanamine for approximately 5 min at 295K. This exposure was sufficient to fill a substantial fraction of the zeolite pore volume of each sample. Desorption measurements were performed following evacuation of the samples for 1 to 20 hrs to remove some of the weakly adsorbed species. While the evacuation time did affect the amount of weakly adsorbed species observed in TPD at lower temperatures, it had no affect on the well-defined, stoichiometric complexes observed in this study. Following an adsorption-desorption experiment, the sample... 

Mass spectrometry is one physical technique that does not (at least directly) involve electromagnetic radiation. However, some sample desorption and ionization processes do use high intensity pulses of laser light in techniques such as MALDI (Matrix-Assisted Laser Desorption Ionization) that have proved very useful in mass analysis of proteins and other biologic macromolecules. High resolution mass spectrometry derives from atomic/molecular beam studies in which the trajectories of ionized particles in a vacuum can be manipulated by static... 

The speed of extraction is controlled by the mass transport of the analytes from the sample matrix to the coating. This process involves convective transport in an air or liquid sample, desorption of the analytes from the solid surface when particulate matter is present, and diffusion of the analytes in the coating [205], In direct SPME sampling, the mass transfer rate is determined by the diffusion of analytes in the coating provided the sample matrix is thoroughly agitated. 

These data are selected from sorption values measured by Williams et al (2.) to show the large difference in Kvalues obtained by the conventional batch equilibration method and by desorption of residual DBCP in field samples. Desorption Kd values are clearly several times larger than those obtained by batch equilibration. Although it might be expected that desorption equilibration times much longer that those used in these tests (3 hours) might have resulted in smaller differences, recent measurements using 24-hour sorption and desorption equilibrations also indicated a several-fold difference in Kd between sorption of DBCP from recently added DBCP solution and desorption of DBCP added to dry soil a few weeks earlier (30). 

The moving slotted heater interface. Figure 3.18, mechanically sweeps a slotted heater over a short modulator capillary connected at either end to the separation columns [207,213,221-223]. The modulator capillary is coated with a thick film of stationary phase terminated in an uncoated zone from which the trapped components are rapidly desorbed when the slotted heater passes over them. The heater is paused for about 0.5 s at the end of each revolution to facilitate sample desorption and transfer. 

Analytes Sampling Desorption/ extraction Extract treatment Determination LOD References... 

Analytes Sampling Desorption/extraction Extract treatment... 

A type H2 hysteresis loop has a triangular shape and a very steep desorption branch. Such behavior was observed for many porous inorganic oxides and was attributed to pore coimectivity effects [80], which were often defined as the presence of pores with narrow mouths (inkbottle pores), but the latter identification may be grossly oversimplified [50]. Indeed, triangular hysteresis loops were observed even for highly ordered MCM-41 materials with pore sizes of about 4-5 nm [39,55,57]. For such samples, desorption (capillary evaporation) of nitrogen from primary mesopores takes place at relative pressures of 0.4-0.5, i.e., in the region where... 

Drying the adsorbent (extremely important when the solvent used for remaining matrix elimination is not fully miscible with the mobile phase) requires the switching of valve A to the LOAD position. Sample desorption is achieved by switching the valve B to the INJECT position. 

Though SFC interfacing in an online fashion with an matrix assisted laser desorption ionization (MALDI) source has not yet been reported, the analysis of fractions from an SFC separation has been facilitated by the recent design of an online sample desorption device for MALDI.In this device, the end of the column is connected to an integral restrictor positioned over a spot of C18 sorbent saturated with an appropriate sample matrix such as a-cyano-4-hydroxycinnamic acid. The technique was applied to the separation of a silicone oil. MALDI-MS elucidated the polymer distributions corresponding to fractions deposited over a range of SFC retention times. 

Li, J., Dewald, H.D., Chen, H. (2009) Online Coupling of Electrochemical Reactions with Liquid Sample Desorption Electrospray Ionization-Mass Spectrometry. Anal. Chem. 81 9716-9722. 

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