Mass spectra computer comparison

Assuming that the mass spectrometer has sufficient mass resolution, the computer can prepare accurate ma.ss data on the m/z values from an unknown substance. To prepare that data, the system must acquire the mass spectrum of a known reference substance for which accurate masses for its ions are already known, and the computer must have a stored table of these reference masses. The computer is programmed first to inspect the newly acquired data from the reference compound in comparison with its stored reference spectrum if all is well, the system then acquires data from the unknown substance. By comparison and interpolation techniques using the known reference... 

Electronic databases of the mass spectral fragmentation patterns of known molecules can be rapidly searched by computer. The pattern and intensity of fragments in the mass spectrum is characteristic of an individual compound so comparison of the experimental mass spectrum of a compound with those in a library can be used to positively identify it, if its spectrum has been recorded previously. 

Every organic chemical has a mass spectrum, which is a combination of ions with different masses and different intensities (abundances). To identify a compound, its mass spectrum is compared to the mass spectra of standards, analyzed under the same instrument settings, and to the EPA/National Institute for Standards and Technology (NIST) mass spectra library. The EPA/NIST library is stored in the database of the computer that operates the instrument. A comparison to the library spectra is possible only if there is consistency in the compound spectra generated by different GC/MS systems at hundreds of environmental laboratories. To achieve such consistency, the EPA methods for GC/MS analysis include the mass... 

Analysis method (1) retention time and comparison of the mass spectrum to an authentic sample or to a spectrum on the computer library (2) as in (1) plus preparative GC and analysis of H- and/or 13C-NMR spectra. 

The standard low-resolution mass spectrum (Fig. 30.3) is computer generated, which allows easy comparison with known spectra in a computer database for identification. The peak at the highest mass number is the molecular ion (M ), the mass of the molecule minus an electron. The peak at RA = 100%, the base peak, is the most abundant fragment in the spectrum and the computer automatically scales the spectrum to give the most abundant ion as 100%. The mass spectrum of a compound gives the following information about its chemical structure ... 

Idenlihcaiion of a species from iis mass spectrum involves a search of files of spectra for pure compounds until a match is found done manually, this process is lime consuming, but it can be accomplished quickly by using a computer. Here, the spectra of pure compounds, stored on a hard disk, are searched until spectra are found that arc sintilar to the analyte. Several thousand spectra can be scanned in a minute or less. Such a search usually produce.s several possible-compounds. Further comparison of the spectra by the scientist often makes identification possible. 

Alternatively, the scanned mass spectrum may be compared with a library of reference spectra, and many spectra of derivatives have been published for this purpose. This comparison step may be done by computer or manually using a variety of suitable reference publications. The comparison, when supplemented by the GC retention index, demonstrates the true power of GC-MS for the unambiguous identification of mixture components at trace levels. The choice of derivative may be influenced by that used in specialist libraries of reference spectra, e.g. methyl esters of organic acids. 

The use of gas chromatography-mass spectrometry (GC-MS) as an analytical technique is growing in importance. GC-MS is a powerful technique in which a gas chromatograph is coupled to a mass spectrometer that functions as the detector. If a sample is sufficiently volatile to be injected into a gas chromatograph, the mass spectrometer can detect each component in the sample and display its mass spectrum. The user can identify the substance by comparing its mass spectrum with the mass spectrum of a known substance. The instrument can also make this comparison internally by comparing the spectrum with spectra stored in its computer memory. 

With a GC-MS system, a mixture can be analyzed and results obtained that resemble very closely those shown in Figures 22.15 and 22.16. A library search on each component of the mixture can also be conducted. The data stations of most instruments contain a library of standard mass spectra in their computer memory. If the components are known compounds, they can be identified tentatively by a comparison of their mass spectrum with the spectra of compounds found in the computer library. In this way, a "hit list" can be generated that reports on the probability that the compound in the library matches the known substance. A typical printout from a GC-MS instrument will list probable compounds that fit the mass spectrum of the component, the names of the compounds, their CAS Nos. (see Technique 29, Section 29.11), and a "quality" or "confidence" number. This last number provides an estimate of how closely the mass spectrum of the component matches the mass spectrum of the substance in the computer library. 

Computer-controlled establishment of the mass scale and correction for mass defect is of great practical importance, particularly in quadrupole instruments. Mass calibration is customarily achieved by introducing a known compound whose spectrum is recognized by the computer, followed by comparison to an authentic spectrum stored in the memory. A very attractive alternate method of mass scale calibration is to utilize the compound of interest itself to establish the mass scale (assuming that the mass spectrum of the compound is known). 

When drugs and metabolites are analyzed in the pharmacokinetic studies or in patient monitoring, the mass spectrometer is indeed only a fancy detector, and one does not usually study the mass spectrum in detail. Here the role of the computer is perhaps even more important in semiroutine or routine analytical work the computer helps to detect the constituents of interest by selective ion monitoring, and performs quantification by correcting background contribution, measuring areas, making comparisons with internal standards, followed by calculations of absolute quantities... 

Vibrational properties can also be computed using the total energy formalism. Here, the atomic mass is required as input and the crystal is distorted to mimic a frozen-in lattice vibration. The total energy and forces on the atoms can be computed for the distorted configuration, and, through a comparison between the distorted and undistorted crystal, the lattice vibrational (or phonon) spectrum can be computed. Again, the agreement between theory and experiment is excellent. [22]... 

This PLOT separation allowed vapor-phase IR and mass spectral determinations to be made. Figure 7 shows a typical IR spectrum obtained from Peak X in the above FFAP chromatogram. The spectrum corresponds to a 6-sec scan with computer background correction and normalization. The spectral features correspond well with an alkyl phenolic structure and a comparison spectrum is shown in Figure 8 for 2,4-dimethylphenol eluted from the FFAP column under identical condi-... 

Mass spectra of chemical compounds have a high information content. This article describes computer-assisted methods for extracting information about chemical structures from low-resolution mass spectra. Comparison of the measured spectrum with the spectra of a database (library search) is the most used approach for the identification of unknowns. Different similarity criteria of mass spectra as well as strategies for the evaluation of hitlists are discussed. Mass spectra interpretation based on characteristic peaks (key ions) is critically reported. The method of mass spectra classification (recognition of substructures) has interesting capabilities for a systematic structure elucidation. This article is restricted to electron impact mass spectra of organic compounds and focuses on methods rather than on currently available software products or databases. 

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