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10/22/97 10:03 'x`916 362 n947 SPARGER x]003/003 <br /> the two peak heights. Otherwise, str�tural isomers are <br /> identified as isomeric pairs. <br /> 7.6.1.1.5 Identification is hampered when sample <br /> components are not resolved chromatographically and produce <br /> mass spectra containing ions contributed by more than one <br /> analyte. When gas chromatographic peaks obviously represent <br /> more than one sample component (i .e. , a broadened peak with <br /> shoulder(s) or a valley between two or more maxima), <br /> appropriate selection of analyte spectra and background <br /> spectra is important. Examination of extracted ion current <br /> profiles of appropriate ions can aid in the selection of <br /> spectra, and in qualitative identification of compounds. When <br /> analytes coelute (i .e. , only one chromatographic peak is <br /> apparent), the identification criteria can be met, but each <br /> analyte spectrum will contain extraneous ions contributed by <br /> the coeluting compound. <br /> 7.6.1.2 For samples containing components not associated <br /> with the calibration standards, a library search may be made for the <br /> purpose of tentative identification. The necessity to perform this <br /> type of identification will be determined by the type of analyses <br /> being conducted. Guidelines for making tentative identification are: <br /> (1) Relative intensities of major ions in the reference <br /> spectrum (ions > 10% of the most abundant ion) should <br /> be present in the sample spectrum. <br /> (2) The relative intensities of the major ions should agree <br /> within + 20%. (Example: For an ion with an abundance <br /> s of 50% in the standard spectrum, the corresponding <br /> sample ion abundance must be between 30 and 70%) . <br /> (3) Molecular ions present in the reference spectrum should <br /> be present in the sample spectrum. <br /> (4) Ions present in the sample spectrum but not in the <br /> reference spectrum should be reviewed for possible <br /> background contamination or presence of coeluting <br /> compounds. <br /> (5) Ions present in the reference spectrum but not in the <br /> sample spectrum should be reviewed for possible <br /> subtraction from the sample spectrum because of <br /> background contamination or coeluting peaks. Data <br /> system library reduction programs can sometimes create <br /> these discrepancies. <br /> Computer generated library search routines should not use <br /> normalization routines that would misrepresent the library or unknown <br /> spectra when compared to each other. Only after visual comparison <br /> of sample with the nearest library searches will the mass spectral <br /> interpretation specialist assign a tentative identification: . <br /> (• '.'j 8260A - 23 Revision 1 <br /> ti_J September 1994 <br />