碎片离子Fragmentation (mass spectrometry)

碎裂倾向于生成稳定的正离子和中性碎片：α-断裂——与杂原子（O、N、S）或羰基相邻的键断裂，生成共振稳定的鎓离子（如胺生成 CH₂=NH₂⁺，m/z 30）；苄基断裂——烷基苯生成卓鎓离子 C₇H₇⁺（m/z 91）；烯丙基断裂；支化烷烃在分支处断裂生成较稳定的叔或仲碳正离子；麦克拉弗蒂重排——含 γ-氢的羰基化合物经六元环过渡态转移 γ-氢并断裂 β-键，失去中性烯烃，如丁醛给出 m/z 44。常见中性丢失：M−15（CH₃）、M−18（H₂O，醇）、M−28（CO 或 C₂H₄）、M−29（CHO 或 C₂H₅）、M−45（COOH）。

Fragmentation favours stable cations and neutrals: α-cleavage next to a heteroatom (O, N, S) or carbonyl gives resonance-stabilised onium ions (amines give CH₂=NH₂⁺ at m/z 30); benzylic cleavage gives the tropylium ion C₇H₇⁺ at m/z 91 from alkylbenzenes; allylic cleavage; branched alkanes break at branch points to give more stable tertiary or secondary cations; and in the McLafferty rearrangement, carbonyl compounds with a γ-hydrogen transfer it through a six-membered transition state and cleave the β-bond, losing a neutral alkene, as butanal gives m/z 44. Common neutral losses are M−15 (CH₃), M−18 (H₂O, alcohols), M−28 (CO or C₂H₄), M−29 (CHO or C₂H₅) and M−45 (COOH).

1950 年代末，美国化学家弗雷德·麦克拉弗蒂系统研究了有机分子在电子轰击下的碎裂规律，1959 年提出以他命名的重排反应，并出版了经典教科书《质谱解析》。比曼用质谱推断生物碱和肽的结构。这些工作使质谱成为有机结构解析的常规工具。

In the late 1950s the American chemist Fred McLafferty studied how organic molecules fragment under electron impact, described the rearrangement that bears his name in 1959 and wrote the classic text Interpretation of Mass Spectra. Biemann used mass spectra to deduce structures of alkaloids and peptides. Their work made MS a routine tool of organic structure determination.

在串联质谱（MS/MS）中，选定一个前体离子，使其与惰性气体碰撞诱导解离（CID），再分析产物离子。多肽在 CID 中主要沿肽键断裂，生成 b 离子和 y 离子系列，相邻离子质量差对应氨基酸残基，由此可读出肽序列，这是蛋白质组学的基础。

In tandem MS (MS/MS) a chosen precursor ion is fragmented by collision-induced dissociation (CID) with inert gas and the product ions analysed. Peptides break mainly at peptide bonds under CID, giving series of b and y ions whose successive mass differences correspond to amino-acid residues, so the sequence can be read off; this is the basis of proteomics.