NIST library

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... 

Figure 2.19 Comparison between the electron ionization mass spectrum obtained in my laboratory (a) and that reported in the NIST library 35] (b) of nonacosan 15 one...

Today, GC-MS (see Section 4.1.1) is a golden standard for detection and quantification of drugs and poisons volatile under GC conditions, whereas nonvolatile compounds require LC-MS (see Section 4.1.2). The GC-MS technique is much more popular for identification purposes than LC-MS, because of the easy availability of the reference mass spectra for many xenobiotics and their derivatives, either in printed or computer form. The most popular libraries are the NIST library, which contains the mass spectra of 130,000 compounds, the Wiley Registry of Mass Spectral Data, which contains 390,000 reference spectra, and the Pfleger-Maurer-Weber library, with 6,300 mass spectra and other data, such as chromatographic retention indexes. 

Fig. 19.8. Electron ionization mass spectrum of toluene (top panel) from GC-MS analysis, and library search match (bottom panel) against the NIST library.

Fig. 21.11. Mass spectra of the unknown off-flavor compound after spectral subtraction from the co-eluting peak and the matching spectrum from the NIST library. (Redrawn/redrawn from J. Chromatogr., 351, R.A. Sanders, and T.R. Morsch, Ion profiling approach to detailed mixture comparison. Application to a polypropylene off-odor problem, 525-531, Copyright (1986) with permission from Elsevier.)...

FIA-MS-MS in parent or neutral loss mode on triple quad instruments can also be applied to screen mixtures of unknown compounds quite rapidly, so that compound classes can be recognised. Yet despite the information about molecular weight and the structural information by product ions, MS-data systems in their commercial form up to the mid-1990s provided no structural information for identification purposes in the form of libraries comparable with the NIST-library of El spectra in GC-MS analysis. It can be hoped that troubles arising out of the lack of computer-searchable library data for identification will be overcome with the gradual increase in trap applications in MS-MS mode. The situation in identification is set to change [36]. 

In Figure 8.14, the Cold El mass spectrum of corticosterone in methanol solution is shown in the upper trace, and is compared with the standard NIST 98 El library mass spectrum shown in the lower trace. Note the similarity of the library mass spectrum to that obtained with the SMB apparatus. All the major high mass ions of m/z 227, 251, 269, and 315 are with practically identical relative intensity and thus good library search results are enabled with the NIST library-matching factor of 829, and the reversed matching factor of 854% and 86.5% confidence level (probability) in corticosterone identification. In addition, the molecular ion at m/z 346 is now clearly observed while it is practically missing in the library (very small in the shown mass spectrum and absent in the other three replicate mass spectra). 

FIGURE 8.14 A comparison of the cold El mass spectrum of corticosterone obtained with the supersonic LC-EI-MS system and its fitted NIST library mass spectrum, including the NIST library-matching factors and the probability of identification. Note the enhanced molecular ion (m/z = 346) exhibited. The NIST library-matching factors and the probability of identification are included. Corticosterone was flow injected using a 200pL/min methanol solvent flow rate with a 1 ng/pL corticosterone sample concentration. (Reproduced from Amirav, A. et al, Rapid Commun. Mass Spectrom., 15, 811, 2001. Copyright 2001. With permission from Elsevier.)... 

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... 

To date, the etiology of the tumors has not been determined, though numerous avenues have been investigated. Initially, water and sediment samples were fractionated and subjected to gas chromatography-mass spectrometry. Obtained spectra were matched to an on-line NIST library of 40,000 known spectra. No significant matches were observed136. Simultaneously, EPA-approved method 800 for BITEX (benzene, toluenes, ethylbenzene and xylenes) analysis of water samples was negative as well. 

Mass spectrometers can readily be interfaced to GC (Figure 4.21). As the compounds leave the chromatograph, they are introduced into the mass spectrometer operating in a vacuum. The great majority of GC-MS are benchtop instruments with linear quadrupoles and EL Compound identification is performed by comparison of the spectrum with a data base for precise identification and confirmation. Huge El mass spectral libraries such as NIST Library or Wiley Library contain... 

Each component was initially identified by the GC retention index and the NIST library connected to the QP-5050 mass spectrometer, as described in previous papers. (10,11) A JNM-LA400 spectrometer (Jeol, Tokyo) was employed for recording the C-NMR spectra, CDCI3 being used as a solvent. 

Figure 6 Comparison of GC-MS and GCxGC-ToF MS in the characterisation of rural aerosols. Spectral quality improvement of alloaromadendrene extracted ion chromatogram and mass spectrum obtained by (a) GC-MS and (b) GCxGC-ToF MS and from (c) NIST library spectrum [32].

The measurement of the conductivity properties of liquids and gases is less straightforward than for solids. Measuring the characteristics of saturated mixtures presents a greater level of difficulty. The first place to look for thermal conductivity data is in the NIST library http //webbook.nist.gov/chemistry/fluid/. However, the values of many fluids remain under investigation. For example, (Marsh et al., 2002) published new experimental measurements for propane from 86 K to 600 K at pressures as high as 70 MPa in 2002. 

The ions used for the identification and quantification of each compound were chosen according to the literature and NIST library. The resulting retention times of the analysed compounds and of the internal standards are presented in Table 2. 

Fig. 4. Mass spectrum of the peak with the retention time of 10.72 min. From the NIST library search, the compoimd was matched with 2-phenylethanol (phenylethyl alcohol) with 932 similar index (SI).

NIST Libraries of Peptide Tandem Mass Spectra... 

The second example here was similar to the first in many ways (indeed, one oxygen is replaced with a NH group). However, there are some differences in this example that warranted a more detailed look. This unknown eluted at 25.4 minutes in the chromatogram from the same mutagenic subfraction of the blue rayon extract. The results of the NIST library search are summarized in Table 9.3. 

Unknown 2. Furthermore, the match between the unknown and standard spectrum was 947 / 949 for the match and reverse match, respectively, when performing a NIST library search, which indicated a very good match between the spectra. Similarly, the LC-MS retention time also matched, with the m/z[M - H] = 146.0254 (4 ppm error) detected at 4.85 minutes. Thus, phthalimide was confirmed as the identity of Unknown 2. Similar to Unknown 1 however, phthalimide showed no activity in the Ames test for mutagenicity. Thus, although this was another successful example of CASE, the toxicity confirmation for the EDA remained incomplete. 

Identification of volatile compounds in strawberries and sliced bread headspace was performed in full scan mode (m/z 30-550). Carvacrol and thymol were identified by a combination of the NIST mass spectral library and gas chromatographic retention times of standard compounds. The rest of volatiles were tentatively identified by their GC/MS spectra. In this sense, the compounds having 90% similarity with spectra in the NIST library were not taken into consideration. Chromatographic responses of detected volatile compoimds (peak area cormts) were monitored for comparative measurements of each compotmd in the studied samples. 

MS, identification by comparison with mass spectrum stored in the NIST library RT, identification by comparison with retention time of authentic reference compounds KI, agreement with literature data. 

Table 2.47 Average similarity match values depending on the data-acquisition rate of TOP mass spectra versus NIST library for disulfoton and diazinon.

The NIST library is available with a new version of the NIST Mass Spectral (MS) Program (v.2.0g) and the enhanced versions of MS Interpreter and AMDIS, the mass spectral interpretation tools with thermodynamics-based interpretation of fragmentation and chemical substructure analysis. The binary format has not changed from the 2002 version, although several new files have been added that associate equivalent compounds and link individual compounds to the RI library. Raw data files are provided in both an SDFile format (structure and data together) as well in earlier formats. The SDFile format holds the chemical structure as a MOLFile and the data in a simple ASCII format. The NIST MS Search Program is also part of many commercial instrumental GC-MS software suites. 

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