Mass spectrometry silicone extractables

The use of separation techniques, such as gel permeation and high pressure Hquid chromatography interfaced with sensitive, silicon-specific aas or ICP detectors, has been particularly advantageous for the analysis of siUcones in environmental extracts (469,483—486). Supercritical fluid chromatography coupled with various detection devices is effective for the separation of siUcone oligomers that have molecular weights less than 3000 Da. Time-of-flight secondary ion mass spectrometry (TOF-sims) is appHcable up to 10,000 Da (487). 

DGE a AC AMS APCI API AP-MALDI APPI ASAP BIRD c CAD CE CF CF-FAB Cl CID cw CZE Da DAPCI DART DC DE DESI DIOS DTIMS EC ECD El ELDI EM ESI ETD eV f FAB FAIMS FD FI FT FTICR two-dimensional gel electrophoresis atto, 10 18 alternating current accelerator mass spectrometry atmospheric pressure chemical ionization atmospheric pressure ionization atmospheric pressure matrix-assisted laser desorption/ionization atmospheric pressure photoionization atmospheric-pressure solids analysis probe blackbody infrared radiative dissociation centi, 10-2 collision-activated dissociation capillary electrophoresis continuous flow continuous flow fast atom bombardment chemical ionization collision-induced dissociation continuous wave capillary zone electrophoresis dalton desorption atmospheric pressure chemical ionization direct analysis in real time direct current delayed extraction desorption electrospray ionization desorption/ionization on silicon drift tube ion mobility spectrometry electrochromatography electron capture dissociation electron ionization electrospray-assisted laser desorption/ionization electron multiplier electrospray ionization electron transfer dissociation electron volt femto, 1CT15 fast atom bombardment field asymmetric waveform ion mobility spectrometry field desorption field ionization Fourier transform Fourier transform ion cyclotron resonance... 

In an attempt to overcome the significant difficulties that the presence of water vapor poses to the analysis of very volatile compounds, purge-and-membrane extraction techniques have been developed that largely prevent the introduction of water into the analytical system. Typical implementations of this form of sample introduction have been called by its developers membrane extraction with a sorbent interface (MESI),97 or membrane introduction mass spectrometry (MIMS).98 " They are based on a silicone hollow-fiber membrane that is inserted into the sample to be monitored, and the passing of a certain volume of inert gas through the membrane. Volatile compounds permeate the membrane and are swept to the adsorbent trap from which they are desorbed into the GC. This method of sample introduction is particularly suited for field and process monitoring and for dirty samples, since it prevents any nonvolatile compounds from entering the analytical system.100... 

Both fractions obtained from crackers extracted less than one week after baking had strong cracker-like aromas. The Freon 113 fraction had a cracker, roasted grain, cooked rice aroma while the ethyl acetate fractions had a cracker, sweet baked good, burnt butter aroma. A preliminary analysis of the two fractions by gas chromatography-mass spectrometry on a 25 m by. 22 mm methyl silicone column between 700 and 1800 retention indices showed the compounds listed in Table 2. 

Singh and Drewes described an improved method for the analysis of acetylcholine and choline in canine brain and blood samples by capillary gas chromatography-mass spectrometry [203]. Frozen samples were mixed with butyryl choline (internal standard) and extracted. The GC-MS system was equipped with an electron-impact ionizer and a 15 m capillary column coated with a bonded methyl-silicone phase (HP-SE 54). The column was operated at 60° C for 3 min, and then temperature programmed (at 25°C/min) to 200°C. The ion at mjz 58 was selected for monitoring, and the separation of the three compounds was achieved in less than 5 min. 

Second method ca. 0.5 g sediment was extracted with methanol/tropolone after addition of HCI. Tripropyltin was added as internal standard. Derivatiza-tion was performed by addition of pentylmagnesium bromide (2 mol L ) followed by clean-up with silica gel. Separation was by CGC (column of 25 m length, 0.2 mm internal diameter, methylphenyl silicon as stationary phase, 0.11 pm film thickness He as carrier gas at 130 mL min injector temperature at 260 °C column temperature ranging from 80 to 280 °C detector (transfer line) temperature at 280 °C). Detection was by mass spectrometry. Calibration was by calibration graph, using butyltin chloride compounds as calibrants. 

NMR and GC/MS Analyses. Reaction products were analyzed by GC-mass spectrometry (MS) and NMR. For GC/MS, silylation of methyl-esterified extract was achieved with a mixture of TMSI + pyridine (1 4, vol/vol) for 30 min at room temperature. GC/MS analysis was performed with a Hewlett-Packard Model 5890 gas chromatograph interfaced with a Model 5971 mass selective detector operating at 70 eV. The capillary column used was a Hewlett-Packard HP-5-MS cross-linked 5% phenylmethyl silicone, 30 m x 0.25 mm i.d., film thickness 0.25 xm. The carrier gas was helium and its flow rate was 0.65 mL/min. The GC column was programmed from 65 to 260°C at a rate of 20°C/min and then kept at 260°C for 20 min. NMR spectra were obtained with a Bruker model ARX-400 spectrometer (Billerica, MA) equipped with a 5-mm dual probe ( C NMR, 100... 

Figure 3.11 summarizes such key experimental points. As a first point, we have to choose the appropriate ionization method for the detection of small metabolites, we have alternative choices other than MALDI, such as secondary ion mass spectrometry (SIMS) [15], nanostructure-initiator mass spectrometry (NIMS) [20,21], desorption/ionization on silicon (DIOS) [22], nanoparticle-assisted laser desorptiopn/ ionization (nano-PALDI) [23], and even laser desorption/ionization (LDI) [24,25]. We consider that MALDI is stiU the most versatile method, particularly due to the soft ionization capability of intact analyte. However, other methods each have unique advantages for example, SIMS and nano-PALDI have achieved higher spatial resolution than conventional MALDI-IMS, and above aU, these mentioned alternative methods are all matrix-free methods, and thus can exclude the interruption of the matrix cluster ion. Next, if MALDI is chosen, experimenters should choose a suitable matrix compound, solvent composition, and further matrix application method for their target analyte. All these factors are critical to obtain sufficient sensitivity because they affect efficiency of analyte extraction, condition of cocrystallization, and, above all, analyte-ionization efficiency. In addition, based on the charge state of the analyte molecule, suitable MS polarity (i.e., positive/ negative ion detection mode) should be used in MS measurement. Below, we shall describe the key experimental points for MALDI-IMS applications of representative metabolites. 

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