Mass Spectrometry in IB and OCR Chemistry | IB OCR 化学：质谱考点精讲

📚 Mass Spectrometry in IB and OCR Chemistry | IB OCR 化学：质谱考点精讲

Mass spectrometry (MS) is a powerful analytical technique used to determine the relative atomic mass of an element and the relative molecular mass of a compound, as well as to provide structural information through fragmentation patterns. Both IB and OCR chemistry specifications place strong emphasis on interpreting mass spectra, calculating relative atomic masses from isotopic abundances, and understanding the principles behind the spectrometer, including time-of-flight (TOF) instruments.

质谱是一种强大的分析技术，用于测定元素的相对原子质量和化合物的相对分子质量，并通过碎片化模式提供结构信息。IB 和 OCR 化学大纲都非常注重解读质谱图、根据同位素丰度计算相对原子质量，以及理解质谱仪的原理，包括飞行时间质谱仪。

1. Introduction to Mass Spectrometry | 质谱简介

Mass spectrometry detects the mass-to-charge ratio (m/z) of ions. The sample is first vaporised and ionised, then the resulting ions are separated according to their m/z values and detected. The output is a mass spectrum showing the relative abundance of each ion against its m/z ratio.

质谱法检测离子的质荷比 (m/z)。样品首先气化并电离，然后生成的离子根据 m/z 值进行分离并检测。输出是质谱图，显示每种离子的相对丰度与其 m/z 比的关系。

In IB and OCR exams, you must be able to explain the stages of a mass spectrometer, interpret spectra for organic and inorganic substances, and perform calculations involving isotopes.

在 IB 和 OCR 考试中，必须能够解释质谱仪的各阶段、解读有机物和无机物的谱图，并进行涉及同位素的计算。

2. Basic Components and Operation | 基本组成与操作

A typical mass spectrometer consists of an inlet system, an ion source, a mass analyser and a detector, all operating under high vacuum. The vacuum prevents collisions between ions and air molecules that would interfere with their paths.

典型的质谱仪由进样系统、离子源、质量分析器和检测器组成，全部在高真空下运行。真空可防止离子与空气分子发生碰撞，以免干扰其运行路径。

The four key stages are: vaporisation, ionisation, acceleration and deflection (or separation), and detection. In older magnetic sector instruments, ions are deflected by a magnetic field; modern TOF instruments use an electric field and a flight tube.

四个关键阶段为：气化、电离、加速和偏转（或分离）以及检测。在较旧的磁扇形仪器中，离子由磁场偏转；现代的 TOF 仪器则使用电场和飞行管。

3. Ionisation: Electron Impact (EI) and Electrospray Ionisation (ESI) | 电离：电子轰击与电喷雾电离

In electron impact ionisation, the vaporised sample is bombarded with high-energy electrons. This knocks out one electron from a molecule, forming a radical cation M⁺·, which can then fragment. EI is used for small, volatile organic molecules.

在电子轰击电离中，气化的样品受到高能电子的轰击。这会从分子上打出一个电子，生成自由基阳离子 M⁺·，该离子随后可能碎裂。EI 用于小的、挥发性的有机分子。

Electrospray ionisation (ESI) is a softer technique often used for large biomolecules. The sample is dissolved in a volatile solvent and sprayed through a fine needle at a high voltage, producing charged droplets that evaporate to give multiply charged ions like [M + nH]ⁿ⁺. IB and OCR may reference ESI when discussing modern applications.

电喷雾电离是一种较温和的技术，常用于大生物分子。样品溶解在挥发性溶剂中，通过细针头在高压下喷雾，产生带电液滴，蒸发后得到多电荷离子，例如 [M + nH]ⁿ⁺。IB 和 OCR 在讨论现代应用时可能会提及 ESI。

4. Acceleration, Deflection and Detection in Traditional MS | 传统质谱中的加速、偏转与检测

After ionisation, positive ions are accelerated through an electric field with potential difference V, gaining kinetic energy KE = zeV, where z is the charge number. In a magnetic sector instrument, the ion beam then enters a curved magnetic field that deflects the ions; heavier ions with lower charge are deflected less, so ions separate by m/z.

电离后，正离子通过电场加速，电势差为 V，获得的动能 KE = zeV，其中 z 为电荷数。在磁扇形仪器中，离子束随后进入弯曲磁场，磁场使离子偏转；质量较大、电荷较小的离子偏转较小，因此离子按 m/z 分离。

The radius of the ion path is given by r = mv²/(zeB). By scanning the magnetic field strength B, ions of different m/z reach the detector sequentially. The signal is amplified and displayed as relative abundance vs m/z.

离子路径半径由 r = mv²/(zeB) 给出。通过扫描磁场强度 B，不同 m/z 的离子依次到达检测器。信号经放大后以相对丰度对 m/z 的形式显示。

5. Time-of-Flight (TOF) Mass Spectrometry | 飞行时间质谱

TOF mass spectrometry is a major topic in both IB and OCR. Here, ions are accelerated by an electric field to the same kinetic energy: KE = zeV. Lighter ions reach a higher velocity and travel through a field-free drift tube faster than heavier ions.

飞行时间质谱是 IB 和 OCR 的一个重要专题。这种方法中，离子被电场加速至相同的动能：KE = zeV。较轻的离子获得更高的速度，通过无场漂移管的速度快于较重的离子。

The kinetic energy equation and time-of-flight relationship are essential:

KE = ½mv² and v = d / t

动能方程和飞行时间关系至关重要：

KE = ½mv² 和 v = d / t

Combining these gives t = d √(m/(2zeV)). Since d and V are constants, t is proportional to √(m/z). The detector records the arrival time of each ion, which is converted to m/z. Calculations often involve finding the mass of an ion from flight time, or comparing two ions.

联立可得 t = d √(m/(2zeV))。由于 d 和 V 是常数，t 正比于 √(m/z)。检测器记录每个离子的到达时间，并转换为 m/z。计算常涉及根据飞行时间求离子质量，或比较两种离子。

6. The Mass Spectrum: Key Features | 质谱图：关键特征

The mass spectrum is a plot of relative abundance (often stated as a percentage of the most abundant peak) against m/z. The highest peak is called the base peak, assigned an abundance of 100%. The molecular ion peak M⁺ corresponds to the unfragmented molecule and gives the relative molecular mass Mᵣ. (Strictly in EI it is M⁺·, but typically written M⁺.)

质谱图是相对丰度（通常表示为最高峰的百分比）对 m/z 的关系图。最高峰称为基峰，其丰度定为 100%。分子离子峰 M⁺ 对应于未碎裂的分子，给出相对分子质量 Mᵣ。（严格在 EI 中为 M⁺·，但通常写作 M⁺。）

Other peaks are fragment ions produced when the molecular ion breaks down. The peak at the highest m/z (excluding small isotopic contributions) is usually the molecular ion, unless it is too unstable to be observed. A small M+1 peak often appears due to the ¹³C isotope.

其他峰是分子离子碎裂产生的碎片离子。通常最高 m/z 处的峰（忽略同位素小峰）就是分子离子峰，除非它不稳定无法观测。常因 ¹³C 同位素出现一个小的 M+1 峰。

7. Isotopic Peaks and Relative Atomic Mass Calculations | 同位素峰与相对原子质量计算

Elements that have several naturally occurring isotopes produce multiple molecular ion peaks. For example, chlorine has two isotopes ³⁵Cl and ³⁷Cl in a 3:1 ratio, so a compound containing one chlorine atom shows M and M+2 peaks in a ~3:1 intensity ratio. Bromine gives ⁷⁹Br and ⁸¹Br in ~1:1 ratio, producing M and M+2 peaks of nearly equal height.

具有多种天然同位素的元素会产生多个分子离子峰。例如，氯有 ³⁵Cl 和 ³⁷Cl 两种同位素，丰度比约为 3:1，因此含一个氯原子的化合物会出现 M 和 M+2 峰，强度比约为 3:1。溴有 ⁷⁹Br 和 ⁸¹Br，比例约为 1:1，产生高度近等的 M 和 M+2 峰。

To calculate relative atomic mass Aᵣ from a mass spectrum, use the formula:

Aᵣ = (Σ mᵢ × abundanceᵢ) / Σ abundanceᵢ

根据质谱图计算相对原子质量 Aᵣ，使用公式：

Aᵣ = (Σ mᵢ × 丰度ᵢ) / Σ 丰度ᵢ

For example, the mass spectrum of boron shows peaks at m/z 10 (20% abundance) and 11 (80% abundance). Aᵣ(B) = (10×20 + 11×80)/(20+80) = 10.8. This type of calculation is a standard IB/OCR question.

例如，硼的质谱显示 m/z 10（丰度 20%）和 11（丰度 80%）两峰。Aᵣ(B) = (10×20 + 11×80)/(20+80) = 10.8。这类计算是 IB/OCR 的常见题型。

8. Fragmentation in Organic Mass Spectrometry | 有机质谱中的碎片化

When an organic molecule is ionised by electron impact, the molecular ion M⁺· has excess internal energy and can break covalent bonds. The fragmentation pattern is characteristic of the compound’s structure. Cleavage tends to occur at weak bonds and forms more stable carbocations.

有机分子通过电子轰击电离时，分子离子 M⁺· 具有多余的内能，可导致共价键断裂。碎片化模式是化合物结构的特征。断裂倾向于发生在弱键处，并形成更稳定的碳正离子。

A mass spectrum tells us the relative molecular mass from the M⁺ peak and structural information from fragment ions. Candidates must be able to deduce possible structures by matching peak m/z values to common fragments. Always look for logical neutral losses, such as loss of H₂O (18), CO (28), or alkyl radicals.

质谱可提供从 M⁺ 峰得到的相对分子质量，以及从碎片离子得到的结构信息。考生必须能通过将峰 m/z 值与常见碎片匹配，推导出可能的结构。始终寻找合理的中性丢失，如丢失 H₂O (18)、CO (28) 或烷基自由基。

9. Characteristic Fragment Ions for Common Functional Groups | 常见官能团的特征碎片离子

The following table summarises typical fragment ions seen in IB and OCR spectra. Note that fragmentation can produce different ions from the same functional group, so a combination of peaks is used for identification.

下表总结了 IB 和 OCR 谱图中常见的典型碎片离子。注意同一官能团可产生不同的离子，因此借助峰的组合进行鉴别。

m/z	Possible fragment	常见碎片可能	Indication / 指示
15	CH₃⁺	CH₃⁺	Methyl end group / 末端甲基
29	C₂H₅⁺ or CHO⁺	C₂H₅⁺ 或 CHO⁺	Ethyl group / aldehyde / 乙基或醛
43	C₃H₇⁺ or CH₃CO⁺	C₃H₇⁺ 或 CH₃CO⁺	Propyl / acetyl / 丙基或乙酰基
57	C₄H₉⁺	C₄H₉⁺	Butyl group / 丁基
31	CH₂OH⁺	CH₂OH⁺	Primary alcohol / 伯醇
45	COOH⁺	COOH⁺	Carboxylic acid / 羧酸
77	C₆H₅⁺	C₆H₅⁺	Phenyl group / 苯基

In addition to these, neutral losses are diagnostic: a loss of 15 (CH₃·), 17 (OH·), 18 (H₂O), 28 (C₂H₄ or CO), 31 (OCH₃·) and 44 (CO₂) help identify halogenoalkanes, alcohols, aldehydes, esters and others.

除上述离子外，中性丢失具有诊断意义：丢失 15 (CH₃·)、17 (OH·)、18 (H₂O)、28 (C₂H₄ 或 CO)、31 (OCH₃·) 和 44 (CO₂) 有助于识别卤代烷、醇、醛、酯等。

10. High-Resolution Mass Spectrometry (HRMS) | 高分辨质谱

High-resolution mass spectrometry can measure m/z to several decimal places (e.g. four or five decimal places). This allows the determination of the exact molecular formula because different combinations of atoms have slightly different exact masses. For example, N₂ has monoisotopic mass 28.0061, CO has 27.9949, and C₂H₄ has 28.0313. A standard mass spectrometer would show all three at m/z 28, but HRMS can distinguish them.

高分辨质谱可以测量 m/z 至小数点后几位（例如四到五位小数）。这使得确定精确分子式成为可能，因为不同原子组合具有略微不同的精确质量。例如，N₂ 的单同位素质量为 28.0061，CO 为 27.9949，C₂H₄ 为 28.0313。标准质谱仪会在 m/z 28 显示这三者，但 HRMS 可加以区分。

IB expects you to understand that HRMS provides the molecular formula directly from the M⁺ peak exact mass. Be aware of the term ‘monoisotopic mass’ and that the most abundant isotope is used. Always relate this to the identification of unknown compounds.

IB 要求了解 HRMS 可直接从 M⁺ 峰的精确质量得到分子式。注意术语“单同位素质量”，并使用的是丰度最高的同位素。始终将此与未知化合物的鉴定联系起来。

11. Using Mass Spectrometry in Structure Elucidation | 质谱在结构解析中的应用

Mass spectrometry is rarely used in isolation for structure determination. It is combined with IR spectroscopy and NMR spectroscopy. The molecular ion gives the Mᵣ, and fragment peaks suggest substructures. For IB and OCR, you will often be given a mass spectrum along with other data and asked to propose a structure.

质谱很少单独用于结构测定，通常与红外光谱和核磁共振波谱结合使用。分子离子给出 Mᵣ，碎片峰提示子结构。在 IB 和 OCR 考试中，常给出质谱图及其他数据，要求提出结构式。

Steps to follow: (1) Identify M⁺, note the relative molecular mass. (2) Check for characteristic isotopic patterns (Cl, Br). (3) Use fragment peaks to deduce likely functional groups and connect to other spectral evidence. (4) Propose a consistent structure and check it against all peaks.

遵循以下步骤：(1) 识别 M⁺，记下相对分子质量。(2) 检查特征性同位素模式（Cl, Br）。(3) 利用碎片峰推断可能的官能团，并结合其他波谱证据。(4) 提出一致的结构，对照所有峰进行验证。

For example, if a compound has M⁺ at 74 and strong peaks at 59 and 43, the loss of 15 suggests a methyl group, and 43 could be CH₃CO⁺. This hints at a ketone or ester. Combine with an IR carbonyl peak at ~1715 cm⁻¹ to choose between them.

例如，某化合物 M⁺ 为 74，且有强峰 59 和 43，丢失 15 暗示一个甲基，43 可能是 CH₃CO⁺。这提示酮或酯。结合红外光谱中约 1715 cm⁻¹ 处的羰基峰，可在两者间做出选择。

12. Exam Technique and Common Pitfalls | 考试技巧与常见错误

Calculate relative atomic mass carefully using the abundance weighted average. Do not simply take the midpoint. Remember that the abundance can be given as percentages or as raw peak heights; the formula still works if you use consistent units.

计算相对原子质量时要仔细使用丰度加权平均，不要简单取中点。记住丰度可能以百分比或原始峰高给出；只要单位一致，公式同样适用。

In TOF questions, often you are given the time of flight of one ion and asked to find the mass of another or its m/z. Use t ∝ √(m/z), set up a ratio, and solve. Pay attention to charge state; watch for 2+ ions in ESI spectra.

在 TOF 题中，常给出一种离子的飞行时间，要求计算另一种离子的质量或其 m/z。利用 t ∝ √(m/z) 建立比例关系求解。注意电荷状态；留意 ESI 谱中的 2+ 离子。

A common mistake is to confuse the base peak with the molecular ion peak. The base peak is the most intense, while the molecular ion peak is usually the highest m/z value among significant peaks. Also, do not forget that oxygen and nitrogen can contribute to exact mass differences; practice HRMS determination of molecular formula.

一个常见错误是将基峰与分子离子峰混淆。基峰是丰度最高的峰，而分子离子峰通常是显著峰中 m/z 值最高的那个。此外，不要忘记氧和氮在精确质量上的差异；要多练习通过 HRMS 确定分子式。

Finally, always write the explicit equation for TOF derivations and show your working to gain partial credit, even if the final numeric answer is wrong.

最后，在 TOF 推导中始终写出明确的方程式并展示步骤，即使最终数值错误也能获得部分分数。

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Mass Spectrometry in IB and OCR Chemistry | IB OCR 化学：质谱考点精讲