Melissa A Bodnar Willard1, Ruth Waddell Smith, Victoria L McGuffin. 1. Department of Chemistry, Michigan State University, East Lansing, MI, 48824, USA; Forensic Science Program, School of Criminal Justice, Michigan State University, East Lansing, MI, 48824, USA.
Abstract
RATIONALE: In many legal and regulatory applications, mass spectral comparison of an unknown or questioned sample to a reference standard or database is used for identification; however, no statistical confidence level or error rate is determined. Therefore, a simple and rapid method to establish the statistical equivalence of mass spectra is needed. METHODS: The standard deviation of the abundance at each m/z ratio was determined from replicate measurements or from a statistical model. These standard deviations were used in an unequal variance t-test to compare two spectra at every m/z ratio over the entire scan range. If determined to be statistically indistinguishable at every m/z ratio, the random-match probability (RMP) that the specific mass spectral fragmentation pattern occurred by chance was calculated. RESULTS: n-Alkane and alkylbenzene standards of varying concentrations were analyzed on the same instrument at different ionization voltages. Using the proposed method, replicate spectra were successfully associated at the 99.9% confidence level, with RMP values less than 10(-29). Despite the similarity in fragmentation patterns, spectra were distinguished from others in the homologous series. Moreover, the n-alkane spectra were appropriately associated to and discriminated from those in a standard reference database at the 99.9% confidence level. CONCLUSIONS: A simple and rapid method to assign statistical significance to the comparison of mass spectra was developed and validated. This method may be useful for legal and regulatory applications, such as the identification of controlled substances, environmental pollutants, and food and drug contaminants.
RATIONALE: In many legal and regulatory applications, mass spectral comparison of an unknown or questioned sample to a reference standard or database is used for identification; however, no statistical confidence level or error rate is determined. Therefore, a simple and rapid method to establish the statistical equivalence of mass spectra is needed. METHODS: The standard deviation of the abundance at each m/z ratio was determined from replicate measurements or from a statistical model. These standard deviations were used in an unequal variance t-test to compare two spectra at every m/z ratio over the entire scan range. If determined to be statistically indistinguishable at every m/z ratio, the random-match probability (RMP) that the specific mass spectral fragmentation pattern occurred by chance was calculated. RESULTS:n-Alkane and alkylbenzene standards of varying concentrations were analyzed on the same instrument at different ionization voltages. Using the proposed method, replicate spectra were successfully associated at the 99.9% confidence level, with RMP values less than 10(-29). Despite the similarity in fragmentation patterns, spectra were distinguished from others in the homologous series. Moreover, the n-alkane spectra were appropriately associated to and discriminated from those in a standard reference database at the 99.9% confidence level. CONCLUSIONS: A simple and rapid method to assign statistical significance to the comparison of mass spectra was developed and validated. This method may be useful for legal and regulatory applications, such as the identification of controlled substances, environmental pollutants, and food and drug contaminants.