Process for interpreting mass spectra of unknown samples
??To analyze the mass spectrum of unknown samples, roughly follow the following procedures:
1??. Analyze the molecular ion region
①. Mark the mass-to-charge ratio of each peak, paying special attention to the peaks in the high mass-to-charge ratio area.
②. Identify molecular ion peaks. First, assume a molecular ion peak in the high mass-to-charge ratio region, determine whether the relationship between the assumed molecular ion peak and adjacent fragment ion peaks is reasonable, and then determine whether it complies with the nitrogen law. If both are consistent, it can be considered as a molecular ion peak.
③. Analyze the relative intensity ratio of isotope peak clusters and the Dm value between peaks to determine whether the compound contains elements such as C1, Br, S, Si, and elements without isotopes such as F, P, and I.
④. Derive the molecular formula and calculate the degree of unsaturation. The precise molecular weight is determined by a high-resolution mass spectrometer or the molecular formula is calculated from the relative intensities of clusters of isotope peaks. If both are difficult to achieve, it can be deduced from the missing fragments of the molecular ion peak and the main fragment ions, or combined with other methods.
⑤. Understand the information of molecular structure from the relative intensity of molecular ion peaks. The relative intensity of the molecular ion peak is determined by the structure of the molecule. The greater the structural stability, the greater the relative intensity.
2??. Resolve fragment ions
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①. Understand possible structural information from characteristic ion peaks and missing neutral fragments.
?. If a series of CnH2n+1 peaks appear in the mass spectrum, the compound may contain long-chain alkyl groups.
?. If weak fragment ions such as m/z 77, 66, 65, 51, 40, 39 appear or partially appear, it indicates that the compound contains a phenyl group.
?. If m/z 91 or 105 is a base peak or a strong peak, it indicates that the compound contains benzyl or benzoyl group.
1. If the base peak or strong peak in the mass spectrum appears in the middle of the mass-to-charge ratio, and there are few other fragment ion peaks, the compound may have a stable structure of two parts, connected by weak bonds that are easily broken.
②. Comprehensively analyze all the information obtained above, combine the molecular formula and degree of unsaturation, and propose the possible structure of the compound.
③. Analyze the fragmentation mechanism of the deduced possible structure to see if it is consistent with the mass spectrum, determine its structure, and further explain the mass spectrum, or compare it with the standard spectrum, or cooperate with other spectra (1H NMR, 13CNMR, IR), Confirm structure.