Library Search Results

When the search algorithm compares the unknown spectrum to each library spectrum, a large number of hit quality indices will be calculated. Sorting through these numbers could be a difficult task, but fortunately FTIR search software programs 

FIGURE 3.30 A library search report. The Correlation search algorithm was used, and the HQI for a perfect match is 100. 

Typically, an FTIR search report will include a table listing the best matches along with their HQIs and the library where they were found. Additionally, the spectra of several of the best matches will be displayed. It is vital that the information in a search report be interpreted properly. The author has seen FTIR users perform a search, look at the best match in the search report, and declare that s the answer without looking at the HQI or the spectra thanselves. This is a recipe for disaster. 

What then does the HQI mean Unfortunately, different software vendors use different HQI indexing systems. Values of 0,1,10,100, and 1000 have all been used to denote a perfect match, and in some software packages different search algorithms use different numbering systans So, you need to read the manual for your FTIR software package to determine what the HQI values reaUy mean. That being said. 

---

The three closest matches from spectral library searching are shown below each sample spectrum. The library search results indicate that the clear outer film is an aromatic polyester. This is most likely PET or an ethylene terephthalate/ethylene isophthalate copolymer. No significant spectral... 

Of course one may employ automated library searches ( library percent reports ) to check for compound identities, but algorithms for library matching are not infallible, and mass spectral libraries are not exhaustive, thus some compounds of interest will likely not be identified. Additional dilemmas are presented by mere reliance on retention times and library percent reports to ascertain the presence of common or unique peaks from among multiple mass spectral data files. As illustrated in Table 2.1, the TICs from the GC-MS of urine from four elephants evidence a peak at essentially the same retention time, but the library search results are inconclusive as to their common identity or lack thereof. As will be seen below, our novel macros can assist in making such decisions for a large number of peaks. 

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). 

O. D. Sparkman, Evaluating Electron Ionization Mass Spectral Library Search Results, J. Am. Soc. Mass Spectrom. 1996, 7, 313. 

Searching the spectrum of an unknown chemical against a spectral library is a routine method used to identify chemicals. Most of the commercial infrared instruments include library search software that has several search algorithms to choose from. The search algorithm can sometimes have a strong effect on the library search result. This is due to the different ways the actual comparison between the spectra is done. Especially when the library and the unknown spectra have been measured differently (e.g. using solid KBr disk and cryodeposition GC/FTIR), the... 

Figure 1.22. GC-MS-computer total ionization-retention index plot obtained from a urine extract from a comatose patient. Labelled peaks indicate library search results and the identification of methadone, methaqualone and their metabolites (reproduced from reference...

A) from one of the peaks in the Fig. 3 extractables profile, along with a best fit library search result... 

Fig. 2 El mass spectra from an unknown residual solvent in a sample of drug substance (see the chromatogram in Fig. 1), and the best fit computerized library search result (butyl acetate).

Gas Chromatography-Mass Spectrometry. A VG Trio-2 mass spectrometer was directly coupled with the Hewlett-Packard 589 R gas chromatograph, equipped with a 30 m x 0.32 mm DB-WAX capillary column (J and W. Scientific bonded polyethylene glycol phase). The carrier gas was He at 2.3 ml/min. The injector and transfer temperature was programmed as follows 30 C (2 min isothermal), to 38°C at l°C/min, then to 180°C at 2°C/min. The instrument was operated in the electron-impact mode at an ionization voltage of 70eV. Quantitative analysis by peak area, plots of chromatograms, and library search results were obtained. 

Figure 7-13 shows the multi-ion chromatogram generated from the full scan and the library search result of a 20-ng LSD standard which was derivatized with BSTFA. 

Fig. 7-13. Scan mode chromatogram and library search result of 20 ng/ml LSD/TMS (absolute) measured in full scan...

In the internal ionization mode, ions are formed, stored, and excited resonantly to the point of ejection inside the ion trap in the presence of neutral sample molecules. Primary fragment ions confined in the ion trap experience the flow of neutral sample molecules through the ion trap for a period of time that varies from cfl 2 ms to ca 200 ms. The longer is this period of time, the greater is the probability of the occurrence of undesirable ion/molecnle reactions this process is known as self-chemical ionization, or self-Cl [20], Typically, self-CI will create [M -H H] + ions due to proton transfer. However, adduct ion peaks may be formed when an alkyl group is transferred. The self-CI process may affect library-search results of those compounds because the majority of mass spectra in mass-spectral libraries were acquired with quadru-pole mass filters or magnetic sector mass spectrometers wherein self-CI does not... 

Identifications based primarily on evaluation of library search results. Other isomers are possible. 

Fig. 1. (a) Mass list, (b) Total ion current of the survey scan mode, (c) Underlying Q3-only enhanced mass spectrum (EMS) at 9.2 min, (d) Extracted ion chromatogram of the compound with / 278.0, (e) Enhanced product ion spectrum of the selected compound in a forensic blood sample, (f) list of search constraints, (g) unknown spectrum window, (h) first library hit window, (i) second library hit window, ( ) library search results. 

Figure 331 NIST library search result of a co-elution spectrum. The hit list upper left is sorted by SI match value. The RSI match value with 812 is high, SI value 74 low due to additional masses in the unknown spectrum (upper left) which are not in the library reference (bottom left). The probability with 91.8% is very high, indicating a unique spectrum with significant difference to the next search hit. The difference spectrum (upper right) can be retrieved for search by spectrum subtraction in the chromatogram.

Figure 4.15 Library search result using the NIST 98 library.

Figure 4.85 Acquired mass spectra and library search results for the coeluting compounds of Figure 4.86 top acquired spectrum from data file in Figure 4.86...

FIG U RE 3.31 The library search result for a mixture of cyclohexane and benzene. Note that cyclohexane is the best match (correlation algorithm, 0.0 is a perfect match). 

---