Mass spectral libraries determination

A thorough search of the reconstructed ion chromatogram (RIC) was made to determine the mass spectra of all detectable components. The spectra were compared with those of reference standards when available or with the spectra from the mass spectral library. Molecular weights of more abundant constituents are shown in Figure b, many additional minor components were also characterized. A complete list of compounds characterized in this sample by GC-MS is given in Table I. 

The components of the liquid products from various biomasses were determined by GC-MS analysis. The mass to charge ratios of the components were checked against the mass spectral library published by NIST, The peak areas shown on the GC spectra were calculated and are given in Table 4. The percentage values indicate the proportions of individual compounds in the liquid and do not represent the actual concentration of these compounds. 

The application of MS/MS or in-source-CID for identification of pollutants brought problems, since no product ion libraries were available. Product ion spectra either had to be interpreted for compound identification or standard comparison had to be performed provided that standards were available. To overcome this disadvantage, analysts prepared their own libraries suitable for the instrument generated on or for instruments of the same type. Attempts at generating mass spectral libraries for polar compounds determined by API methods were reported and the results obtained with their application in real environmental samples were discussed [291, 292]. The generation of IT-MS product ion spectra and their use for identification provided the most promising results so far. The preparation of a universally applicable product ion library for the identification of polar compounds, i.e. of a library which could work with various equipments for identification, still remains wishful thinking. 

Electron impact Controlled and reproducible fragmentation pattern Mass spectral libraries for identification of unknowns Confirmation of analyte identity (consistent ion abundance ratios) Determination of molecular structure Difficult determination of the molecular weight (molecular ion not always present in the spectrum)... 

Hewlett-Packard Mass Selective Detector (MSD)(HP 5971A). The ion source was run in the El mode at 170 °C using an ionisation energy of 70 eV. The scan rate was 0.9 scans/sec. Data from the MSD was stored and processed using a Hewlett-Packard Vectra QS20 computer installed with Mustang software and the Wiley Mass Spectral Library. Kovats indices were calculated against extemd hydrocarbon standards. Concentrations were determined from the internal standard, butyl hexanoate, and are not corrected for detector response. 

When your sample has been run, you will have an opportunity to search the NIST mass spectral library to determine the structures of the product(s) of the nitration. Determine the structures of the product(s) and the percentages of each component. There will likely be starting material left in the reaction mixture. It would be of interest to see how your product ratios compare to the values obtained from the literature (see References). 

The composition is determined based on relative response in the chromatograms. The identification of each component is based on direct comparison of the retention time and mass spectral fragmentation with the data published by Adams [12] and others [13-16]. The mass spectral library fit (Wiley 139 Library) for all identified components was more than 90%. It should be mentioned that the GC column used in this study (Rtx -5, 30m x 0.25 mm, 0.25pm film thickness) was similar to that reported by Adams [12]. 

Focus identification on the odor-active regions as determined by GC-sniffing and RI values as published in the Flavomet (http //www.nysaes.cornell.edu/flavornet). The fragmentation pattern obtained for the compounds can be compared with those in data banks like the Wiley/NBS Registry of Mass Spectral data (McLafferty and Stauffer, 2000) or the NIST 98 library (National Institute of Science and Technology). 

In the modern analytical laboratory, gas chromatography-mass spectrometry (GC-MS) is a vital tool in the characterization and identification of unknowns. The advantages of GC-MS are accuracy in quantitation, low detection limits, tentative identification of unknowns by spectral library search, and a high degree of reliability and versatility ( 1). Of all the chemicals known, however, only a fraction (=20 ) are amenable to analysis by GO. The remaining compounds, because of their high molecular weight, thermal instability, or ionic and/or polar character, are not suited to direct GC determination. 

It is often difficult to determine the degree to which the chemistry proceeded on the entire library population and whether peaks in a mass spectrum are due to the product, side reactions, reagents, solvents, or impurities. Diversity Sciences developed mass-spectral methods to distinguish all components that are cleaved from a solid support and implemented the method into the analytical construct. While early studies demonstrated promising results for fragmentation methods with tandem mass spectrometry (MS/MS), stable isotopes were routinely implemented as signature peaks for the identification of compounds that are produced from solid-phase reactions [27]. 

High mass resolution of ToF-SIMS instruments (10 to 10 amu) enables exact mass measurements, calculation of empirical formulae and more reliable unknown peak identification [130]. Accurate mass determination by ToF-SIMS is very simple because no special calibration procedures or calibration standards are required. The accuracy is about 10 ppm for atomic species and for molecules in the low mass range. For organic molecules in the high mass range an accuracy better than 5 ppm was obtained. In cases where the mass resolution is not sufficient, mass spectral fragmentation patterns may be compared to library spectra for identification. 

The fragmentation pattern of a compound is reproducible, and many libraries of EI-MS data are available. This allows one to compare the mass spectrum of a sample compound against thousands of data sets in a spectral library in a few seconds using a PC, thus simplifying the process of determining or confirming a compound s identity. 

Identification of substances by comparison with spectral libraries is no longer possible. The relative intensities (ion ratios) of the selected ions serve as quality criteria (qualifiers) (1 ion no criterion, 2 ions l criterion and 3 ions=>3 criteria ). This process for detecting compounds can be affected by errors through shifts in retention times and compromised peak area determination caused by the matrix. In residue analysis, it is known that with SIM analysis false positive findings occur in about 10% of the samples. Positive SIM data are confirmed in the same way as positive results from classical GC detectors by running a complete mass spectrum of the analytes suspected. Confirmation of positive results, and statistically of negative results as well, is required by international directives either by full scan, MS/MS, ion ratios or HRMS. 

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