GC/MS system

As described above, the mobile phase carrying mixture components along a gas chromatographic column is a gas, usually nitrogen or helium. This gas flows at or near atmospheric pressure at a rate generally about 0,5 to 3.0 ml/min and evenmally flows out of the end of the capillary column into the ion source of the mass spectrometer. The ion sources in GC/MS systems normally operate at about 10 mbar for electron ionization to about 10 mbar for chemical ionization. This large pressure... 

Once a mass spectrum from an eluting component has been acquired, the next step is to try to identify the component either through the skill of the mass spectroscopist or by resorting to a library search. Most modem GC/MS systems with an attached data station include a large library of spectra from known compounds (e.g., the NIST library). There may be as many as 50,000 to 60,000 stored spectra covering most of the known simple volatile compounds likely to be met in analytical work. Using special search routines under the control of the computer, one can examine... 

The next step is to show that the response for the analysis of any target compound is linear. This step is known as the initial calibration and is achieved by the analysis of standards for a series of specified concentrations to produce a five-point calibration curve (Figure 41.2a, b). On subsequent days, a continuing calibration must be performed on calibration check compounds to evaluate the calibration precision of the GC/MS system. 

The combined GC/MS system provides more information than is obvious from the simple sum of the two separate instruments. 

Important to environmental analysis is the ability to automate the injection, as weU as the identification and quantitation of large numbers of samples. Gc/ms systems having automatic injectors and computerized controllers have this capabiUty, even producing a final report in an unattended manner. Confirmation and quantitation are accompHshed by extracting a specific ion for each of the target compounds. Further confirmation can be obtained by examining the full scan mass spectmm. 

To distinguish between azobenzene and benzophenone, assuming reference spectra are not available for these compounds, it is a good idea to examine the mass spectra of aromatic ketones, such as acetophenone, butyrophenone, diphenyldiketone, and so forth. Complete identification is assured by obtaining or synthesizing the suspected component and analyzing it on the GC/MS system under the same GC conditions. If the retention time and the mass spectrum agree, then the identification is confirmed. 

Cool the reaction mixture and inject 1-2 fA into the GC/MS system. TBDMS Procedure... 

To aid in determining the number of carboxyl groups, prepare a derivative using trideuterated Methyl-8 (Pierce cat. no. 49200), using the same procedure previously given. Inject 1-2 pd of the trideuterated methyl ester separately or mix equal portions of the nondeuterated methyl ester with the trideuterated methyl ester, and inject 2 pd immediately into the GC/MS system. From the mass difference, it is easy to determine the number of carboxyl groups present. 

Every column (including chemically bonded columns) will have some column bleed. The amount of column bleed will increase with increasing column temperature, film thickness, column diameter, and column length. The base line starts to rise approximately 25-50° below the upper temperature limit of the stationary phase. After a column is installed in a GC/MS system, a background spectrum should be obtained for future reference. 

Inject 1-2 fA chloroform or methylene chloride (bottom layer) into the GC/MS system. 

The GC-MS conditions are given in Table 11. Response factors are specific for the commercial standard, the particular clean-up procedure, and the GC-MS system used [24]. 

Flumetralin was extracted from tobacco using Soxhlet extraction. A 5-g amount of Florisil (5% deactivated) was transferred directly on to the filter disk of a Soxhlet extractor followed by another 5 g of Florisil mixed with 5 g of ground tobacco sample as an upper layer. A 60-mL volume of hexane and 3mL of a 4 agmL internal standard solution were placed in a 250-mL round-bottom flask prior to attaching the Soxhlet extractor. The unit was placed on a heating mantle and the hexane was refluxed through the extractor at the rate of about 250 mLh for 4.5 h. After cooling, 0.5 pL of the extract was injected directly into a GC/FCD or GC/MS system. 

Inject an aliquot of the gas chromatography (GC)-ready sample solution into the GC/MS system. 

Inject the cleaned up sample into the GC/MS system operated under the same conditions as employed for standardization. Compare the peak areas of the analytical samples with the calibration curve. Determine the concentrations of the At-methyl derivative of flutolanil and At,0-dimethyl derivative of the metabolite M-4 present in the sample. 

Inject a 2-qL aliquot of the cleaned-up sample into the GC/MS system operated under the same conditions as employed for standardization. 

Figure 7.14 Schematic set-up of a GC-MS system with various injectors (HS, headspace LI, liquid injector Py, pyrolyser)...

Table 7.90 shows the main characteristics of the polymer/additive analysis procedure, consisting of dissolution followed by p. SEC-GC. An advantage of this approach is the minimisation of the extraction losses inherent in conventional sample preparation techniques. The determination of higher-molecular-weight additives is restricted, due to the upper interface temperature attainable in the GC-MS system. The usefulness of the tool for quantitative analysis has been ascertained [948,976],... 

Yano, I. Analysis of bacterial metabolites and components by computerized GC/MS system—From shorter chain acids to very long-chain compounds up to C80. Rapid Meth. Autom. Microbiol. Immunol. (4th Int. Symp.) 1985,239-247. 

The spray paint can was inverted and a small amount of product was dispensed into a 20 mL glass headspace vial. The vial was immediately sealed and was incubated at 80°C for approximately 30 min. After this isothermal hold, a 0.5-mL portion of the headspace was injected into the GC/MS system. The GC-MS total ion chromatogram of the paint solvent mixture headspace is shown in Figure 15. Numerous solvent peaks were detected and identified via mass spectral library searching. The retention times, approximate percentages, and tentative identifications are shown in Table 8 for the solvent peaks. These peak identifications are considered tentative, as they are based solely on the library search. The mass spectral library search is often unable to differentiate with a high degree of confidence between positional isomers of branched aliphatic hydrocarbons or cycloaliphatic hydrocarbons. Therefore, the peak identifications in Table 8 may not be correct in all cases as to the exact isomer present (e.g., 1,2,3-cyclohexane versus 1,2,4-cyclohexane). However, the class of compound (cyclic versus branched versus linear aliphatic) and the total number of carbon atoms in the molecule should be correct for the majority of peaks. 

Liquid and gaseous products are analyzed by gas chromatography with respectively a Varian 3900 GC and a Varian 4900 micro GC. The identification of the liquid decomposition products is carried out by mass spectrometry on a GC/MS system (Shimadzu GC14A coupled with a Nermag/Quad service R10-10 C/U). 

Pyrolysis-Gas Chromatography-Mass Spectrometry. In the experiments, about 2 mg of sample was pyrolyzed at 900°C in flowing helium using a Chemical Data System (CDS) Platinum Coil Pyrolysis Probe controlled by a CDS Model 122 Pyroprobe in normal mode. Products were separated on a 12 meter fused capillary column with a cross-linked poly (dimethylsilicone) stationary phase. The GC column was temperature programmed from -50 to 300°C. Individual compounds were identified with a Hewlett Packard (HP) Model 5995C low resolution quadruple GC/MS System. Data acquisition and reduction were performed on the HP 100 E-series computer running revision E RTE-6/VM software. 

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