Desorption conditions, optimization

The overall strategy consists in finding conditions where ODN adsorption occurs without avoiding grafting. Once chemical grafting is reached, desorption of non-covalently grafted ODN is performed in an appropriate conditions optimized as reported in the desorption part (basic pH, high salt amount in presence of excess non-ionic surfactant). 

Ref. [41] describes a procedure developed for the determination of eight organophosphoms insecticides in natural waters using SBSE combined with thermal desorption-GC-atomic emission detection (AED). Optimization of the extraction and thermal desorption conditions showed that an extraction time of 50 min and a desorption time of 6 min were sufficient. Addition of salt and adjustment of the pH were not necessary. Recoveries of seven of the compounds studied between 62 and 88%. For fenamiphos, which is highly water-soluble, recovery was only 15%. The very low detection limits, between 0.8 ng/1 (ethion) and 15.4 ng/1 (fenamiphos), indicate that the SBSE-GC-AED procedure is suitable for sensitive detection of OPPs in natural waters. 

Analyze using thermal desorption system after optimization of conditions. 

Determine conditions of optimal desorption of traps flow rate, temperature of traps, and transfer lines. 

The C02 absorption is hindered by a slow chemical reaction by which the dissolved carbon dioxide molecules are converted into the more reactive ionic species. Therefore, when gases containing H2S, NH3, and C02 contact water, the H2S and ammonia are absorbed much more rapidly than C02, and this selectivity can be accentuated by optimizing the operating conditions (23). Nevertheless, all chemical reactions are coupled by hydronium ions, and additional C02 absorption leads to the desorption of hydrogen sulfide and decreases the scrubber efficiency. 

For semi volatile compounds, inlet optimization is very simple. Classical splitless inlet conditions, followed by an initial column temperature cool enough to refocus the analyte peaks following the desorption, work well. Thus, a typical condition would be a temperature of about 250° C, a head pressure sufficient to maintain optimum GC column flow and an initial column temperature at least 100°C below the normal boiling point of the analyte. For semivolatile analytes, a classical splitless inlet liner can be used, as the cool column will refocus these peaks. The desorption time in the inlet must be determined by experimentation, but typically, runs between 1 and 5 minutes. 

The first step of this study is to optimize the desorption experiments particularly the mass balance of the desorption process expecially the separation between the solutes and C02. The first step of the process is the adsorption at atmospheric conditions of VOC (butyl acetate and/or xylenes). Two adsorbants were chosen to realize a comparison activated carbon and zeolithe. We studied the desorption characteristics as a function of pressure, temperature and C02 flow rate. 

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. 

---