分子离子峰Molecular ion

分子离子峰通常是谱图中质荷比最大的峰（同位素峰除外），但不一定是最强峰（基峰）。判断依据：①氮规则——含偶数个氮（包括零个）的有机分子分子量为偶数，含奇数个氮为奇数；②与相邻碎片的质量差应合理，不应出现 3–14、21–25 等不合理的丢失；③同位素峰的相对强度应符合分子式（如 M+1 峰约为 1.1% × 碳原子数）。

The molecular ion is usually the highest-m/z peak apart from isotope peaks, but not necessarily the most intense (the base peak). It is recognised by the nitrogen rule (organic molecules with an even number of nitrogens, including none, have even molecular masses, and with an odd number odd ones); by sensible mass differences to nearby fragments (losses of 3–14 or 21–25 are implausible); and by isotope peaks whose intensities match the formula, such as M+1 at about 1.1% per carbon atom.

分子离子峰的强度与分子结构有关：芳香化合物 > 共轭烯烃 > 环状化合物 > 酮 > 直链烷烃 > 醚 > 酯 > 胺 > 酸 > 醇 > 支链烷烃。醇、高度支化的烷烃在电子轰击下分子离子峰很弱甚至不出现，此时可改用化学电离、电喷雾等软电离方法得到准分子离子峰来确定分子量。

Molecular-ion intensity depends on structure, decreasing roughly aromatics > conjugated alkenes > cyclic compounds > ketones > straight-chain alkanes > ethers > esters > amines > acids > alcohols > branched alkanes. Alcohols and highly branched alkanes give very weak or no molecular ions under EI, so soft ionisation such as CI or ESI is used instead to obtain quasi-molecular ions and the molecular weight.

含氯化合物有 M 和 M+2 峰（约 3:1），含溴化合物有 M 和 M+2 峰（约 1:1），含硫有较明显的 M+2 峰（约 4.4%）。高分辨质谱测得精确质量后，可直接推算元素组成，例如 m/z 122.0368 对应 C₇H₆O₂（苯甲酸）而不是 C₈H₁₀O（122.0732）。

Chlorine compounds show M and M+2 peaks in about 3:1, bromine compounds about 1:1, and sulfur a noticeable M+2 of about 4.4%. With an exact mass from high-resolution MS the elemental composition follows directly: m/z 122.0368 is C₇H₆O₂ (benzoic acid), not C₈H₁₀O (122.0732).