GC and MS

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 

By way of illustration, very simple spectra for four substances (A, B, C, D) are shown (a) separately and (b) mixed in unequal proportions. The mixture spectrum is virtually impossible to decode if A, B, C, D are not known beforehand to be present. 

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The vacuum residua or vacuum bottoms is the most complex fraction. Vacuum residua are used as asphalt and coker feed. In the bottoms, few molecules are free of heteroatoms molecular weights range from 400 to >2000, so high that characteri2ation of individual species is virtually impossible. Separations by group type become blurred by the sheer mass of substitution around a core stmcture and by the presence of multiple functionahties in a single molecules. Simultaneously, the traditional gc and ms techniques require the very volatiUty that this fraction lacks. 

Maximum benefit from Gas Chromatography and Mass Spectrometry will be obtained if the user is aware of the information contained in the book. That is, Part I should be read to gain a practical understanding of GC/MS technology. In Part II, the reader will discover the nature of the material contained in each chapter. GC conditions for separating specific compounds are found under the appropriate chapter headings. The compounds for each GC separation are listed in order of elution, but more important, conditions that are likely to separate similar compound types are shown. Part II also contains information on derivatization, as well as on mass spectral interpretation for derivatized and underivatized compounds. Part III, combined with information from a library search, provides a list of ion masses and neutral losses for interpreting unknown compounds. The appendices in Part IV contain a wealth of information of value to the practice of GC and MS. 

Masada. Y. Analysis of Essential Oils by GC and MS. New York John Wiley. 1976. 

The book is divided into four parts. Part I, The Fundamentals of GC/MS, includes practical discussions on GC/MS, interpretation of mass spectra, and quantitative GC/MS. Part II, GC Conditions, Derivatization, and Mass Spectral Interpretation of Specific Compound Types, contains chapters for a variety of compounds, such as acids, amines, and common contaminants. Also included are GC conditions, methods for derivatization, and discussions of mass spectral interpretation with examples. Part III, Ions for Determining Unknown Structures, is a correlation of observed masses and neutral losses with suggested structures as an aid to mass spectral interpretation. Part IV, Appendices, contains procedures for derivatization, tips on GC operation, troubleshooting for GC and MS, and other information which are useful to the GC/MS user. Parts I to III also contain references that either provide additional information on a subject or provide information about subjects not covered in this book. 

The full-scan mode is needed to achieve completely the full potential of fast GC/MS. Software programs, such as the automated mass deconvolution and identification system (AMDIS), have been developed to utilize the orthogonal nature of GC and MS separations to provide automatically chromatographic peaks with background-subtracted mass spectra despite an incomplete separation of a complex mixture. Such programs in combination with fast MS data acquisition rates have led to very fast GC/MS analyses. 

Thus, the question is whether such classes of molecules were present on the young Earth. The only witnesses capable of giving an answer to this question are meteorites (Deamer, 1988). The group of David Deamer studied Murchison material after extraction and hydropyrolysis (at 370-570 K, with reaction times of several hours or days). GC and MS analyses showed the presence of a series of organic compounds, including significant amounts of amphiphilic molecules such as octanoic (C ) and nonanoic acids (C9) as well as polar aromatic hydrocarbons. 

Compound 4 was epoxidized to give 15 (Scheme 3) and the MS fragmentation of 15 is given by the broken-line mlz (relative abundance). To ensure the location of the double bonds, the epoxidation and the MS of 15 was compared with the products of the ozonolysis identified by GC and MS (as marked by the broken lines)6. The use of the various derivatization products via oxidation, combined with other spectroscopic methods, is discussed in Section III. 

Figure 15.1. Gas chromatograph (right) attached to a mass spectrometer (left). Transfer line between GC and MS is hidden between the upper left of the GC and the upper right of the MS. 

In some cases, confirming identification of components obtained from soil, such as pesticides, is essential. Thus, the uncertainty in some analyses needs to be addressed. This can be accomplished by identifying the components using two entirely different methods such as IR spectroscopy and MS. Although GC-IR-MS methods can positively identify separated components, the IR component of the system is not nearly as sensitive as are the GC and MS components. This detracts from the usefulness of this method. However, in cases where the level of analyte is not limiting, which frequently occurs in soil extracts, this can be an excellent method to use. Also, with modern concentration techniques, it is neither difficult nor time-consuming to concentrate analytes to a level that is identifiable by IR spectroscopy [17,18],... 

The most precise procedure for detection of banned substances is a combination of GC and MS. Gas chromatography/mass spectrometry is a two-step process, where GC separates the sample... 

In the identification experiments, the GC and MS data of the analytes have to be compared with those of corresponding authentic samples. However, as mentioned already, odorants are often concealed in the gas chromatogram by major volatile compounds therefore, to avoid misidentification it is necessary to compare by GC-O the odour quality of the analyte with that of the authentic sample at approximately equal levels. The analyte, which has been perceived by GC-O in the volatile fraction, is only correctly identified if there is agreement in the sensorial properties, in addition to GC and MS data. 

D. PCBS. Vos et al. in 1970 were able to identify PCDFs (tetra- and penta-CDFs) in two samples of European PCBs but not in a sample of Aroclor 1260 (k2). The toxic effects of the PCBs were found to parallel the levels of PCDFs present. Bowes et al. examined a series of Aroclors as well as the samples of Aroclor 12 0, Phenoclor DP-6 and Clophen A-60, that had previously been analyzed ( 2). They used packed column GC and mass spectrometry, and found that the most abundant PCDFs had the same retention time as 2,3,7 8-tetra- and 2,3 , 7 8-penta-CDF their results are collected in Table IV 0 3). Using high-resolution GC and MS, Rappe and Buser (unpublished) have analyzed a number of commercial PCBs, and the results are also collected in Table IV. In general the PCBs contained quite a complex mixture of PCDFs, up to UQ different isomers. The highest level of PCDFs was found in a Japanese PCB used for two years in a heat exchange system, which was found to have about 10 yg/g. The dominating isomer was identified as P j tetra-CDF at a level of 1.25 yg/g (UU ]. 

Bringing the model mouth to temperature requires -20 min. Food preparation, i.e., cutting and measuring, takes -5 min. Initiating the model mouth takes -1 min, running the model mouth and sampling the effluent usually takes 1 min. Cleaning the model mouth requires 10 min, and may be done concurrently with GC analysis. The time needed for GC and MS analysis is as described for the RAS. 

Chloroligno sulphonic acids Combination of pyrolysis GC and MS with single ion monitoring... 

The rationale used in the interpretation of the mass spectra of methylalkanes has been presented in several reports 2- vs. 4-methylalkanes (Baker et al., 1978 Scammells and Hickmott, 1976 McDaniel, 1990 Bonavita-Cougourdan et al., 1991) 2,X- and 3,X-dimethylalkanes (Nelson et al., 1980 Thompson et al., 1981) and internally branched mono-, di- and trimethylalkanes (Blomquist et al., 1987 Pomonis et al., 1980). In the majority of reports, identification is based on GC and MS data, but the conclusions are not confirmed with standards or synthesis of the proposed structures. However, there are reports of chemical ionization (Howard et al., 1980) and electron impact of synthetic methyl-branched hydrocarbons (Carlson et al., 1978, 1984 Pomonis et al., 1978, 1980) and these have been very useful in confirming mass spectral fragmentation patterns with chemical structures. 

The performance of GC and MS is tested and retention indices are calibrated by injection of the OPCW test mixture. This mixture is a solution of nine n-alkanes with even carbon numbers (C8 to C24) and seven test compounds (nonscheduled chemicals) in dichloromethane (for details see Chapter 4). 

The performance of the system is tested by injecting 2 xl of OPCW check mixture running the Performance Check Injection method. The composition of the mixture is given in Annex 2. The retention times of the series of nine hydrocarbons in the check mixture are used by AMDIS for calibration of the RI the other seven compounds are used for assessing the performance of the GC and MS part of the system. Two components of the check mixture, chloromethylaniline and dibenzothiophene, are used for evaluation of isotopic ratios for chlorine and sulfur measured by the mass spectrometer. The example chromatogram of the check mixture is presented in Figure 5. 

We analysed ethanolic extracts of preputial glands of muskrats collected during a full annual cycle by one trapper, in Zeeuwsch-Vlaanderen, near the Belgian border. The majority of the glands were taken from adult males, but some glands of adult females and young females and males were also collected and analysed separately (GC and MS SE-30 capillary column). 

Collection of the GC effluent and subsequent MS analysis allowed assignment of a possible molecular formula as Ci9H280. Since boar taint can be eliminated by castration of male pigs, attention was focused on the testosterone and androsterone family of compounds as possible candidates. When the crude volatiles were treated with 2,4-dinitrophenylhydrazine, the boar taint odor was completely removed, implicating a ketone functionality for the lone oxygen atom. Anecdotal information implicated several androstene derivatives, including 47, which was described as having an intense, urine-like odor. 143 An authentic sample of 47 was prepared, and comparison of the GC and MS properties allowed the definitive structural identification of the boar taint compound. 

Again, GC and MS analyses led to the identification of the compound responsible for the odor, alcohol 48, the likely precursor to the 3-keto compound 47, which accumulates in the fat and should be less easily excreted compared to 48. 

Reference Standards. Reference standards for TLC, GC, and MS were obtained as follows. Pure abietic acid (mp 172-173 °C) was used as received. Dehydroabietic acid was prepared by oxidation of abietic acid with selenium dioxide to hydroxy abietic acid and subsequent dehydration with glacial acetic acid (11). 7-Ketodehydroabietic acid was prepared by oxidation of dehydroabietic acid with potassium permanganate and isolation of the product by way of the Girard reagent T (12). Pyroabietic acid (a mixture of dehydro- and dihydroabietic acids) was prepared by heating abietic acid with 10% palladium on charcoal to 250 °C for 1 h (13). 

For forensic purposes, at least two uncorrelated methods of identification are required (e.g. TLC and GC, or GC and MS). Immunoassay methods provide good exclusion evidence, but poor confirmation of identity, although they are vital for the detection of insulin, lysergide, and the cannabinoids. 

Rather low yields of ethylation products were obtained by reaction of 2,2,4-trimethylpentane with ethylene. It seems probable that the abstraction of hydrogen even from the tertiary carbon atom was difficult because it was a neohexyl carbon atom. It was suggested by gc and ms analysis that the major reaction did occur at the tertiary carbon atom, yielding... 

FIGURE 54.1. Large volume (200 00 pi) thermal desorption from TENAX (300 mg, TA 60-80 mesh) followed by 2D-GC and MS detection in El mode. (1) Pre-column. (2) Analytical column. Small part of the chromatogram of the first column is reinjected on the second anal 4ical column. From Trap and Van der Schans (2007), Figures 3 and 4. 

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