Mass Spectrometry

1. Mass Spectrometry 质 谱

2. 4.1 Introduction • Mass spectrometry (MS) is one of the most versatile and powerful tools of chemistry analysis. It provides quantitative information of atoms or molecules and qualitative information about molecules (such as the molecular weight and valuable information about the molecular formula), using few nanomoles of the sample (as little as 10-12g, 10-15 moles for a compound of mass 1000 Daltons).

3. It also provides the most structural information that can confirm a structure derived from NMR and IR spectroscopy. Mass spectrometry fundamentally differs from other forms of spectroscopy (such as UV-vis, infrared, NMR, etc.) because electromagnetic radiation is not involved. The mass spectrometer works by using magnetic and electric fields to exert forces on charged particles (ions) in a vacuum. Therefore, a compound must be charged or ionized to be analyzed by a mass spectrometer.

4. Furthermore, the ions must be introduced in the gas phase into the vacuum system of the mass spectrometer. The gas phase ions are sorted in the mass analyzer according to their mass-to-charge (m/z) ratios and then collected by a detector. In the detector the ion flux is converted to a proportional electrical current. The data system records the magnitude of these electrical signals as a function of m/z and converts this information into a mass spectrum.

5. 质谱图 横坐标：质荷比(m/z) 纵坐标：相对丰度(最强峰强度定为100%) 苯乙酮的质谱图

6. 4.2 质谱仪与质谱分析原理 离子源 质量分析器 检测器 进样系统 1.单聚焦 2.双聚焦 3.飞行时间 4.四极杆 1.气体扩散 2.直接进样 3.气相色谱 1.电子轰击 2.化学电离 3.场致电离 4.激光 1.电子倍增管 2.渠道式电子 倍增器阵列 质谱仪需要在高真空下工作：离子源（10-3 10 -5 Pa） 质量分析器（10 -6 Pa） (1)大量氧会烧坏离子源的灯丝；(2)用作加速离子的几千伏高压会引起放电；(3)引起额外的离子－分子反应，改变裂解模型，谱图复杂化。

7. Figure 4.1 The scheme of electron-impact ion source and ion-accelerating system

8. + : R1 + : R2 : R3 + : R4 + + : e (M-R2)+ (M-R1)+ Mass Spectrum M+ (M-R3)+ 离子源 Electron Ionization (EI)

9. m/z 43 m/z 56

10. 优点：1）稳定, 质谱图再现性好，便于计算机检索及比较； 2）离子碎片多，可提供较多的分子结构信息。 缺点：1）样品必须易于气化； 2）当样品分子稳定性不高时，分子离子峰的强度低，甚至不存在分子离子峰。

11. 电子 + + 试样分子XH（M） 气体分子CH4 (M+1)+; (M+17)+; (M+29)+; 化学电离源（Chemical Ionization，CI）: 离子室内的反应气（甲烷等；10~100Pa，样品的103~105倍），电子（100~240eV）轰击，产生离子，再与试样分离碰撞，产生准分子离子（最强峰）。 谱图简单；不适用难挥发试样； 准分子离子 CH4  CH4+、CH3+； CH4 CH5+、C2H5+ XH  XH2+、XHCH5+XHC2H5+、X+；

12. EI源 邻苯二甲酸二辛酯的质谱图 CI源 (甲烷) CI源 (异丁烷)

13. Magnetic sectors • This equation shows that the m/z of the ions that reach the detector can be varied by changing either H or V.

14. Table 4-1 Exact masses of some common isotopes and simple molecular species, with the atomic mass of 12C taken to be exactly 12 Resolving power（分辨率） 低分辨MS R<1000 高分辨MS R≧10000

15. 三重四级杆质谱

16. Detectors检测器 • After passage through a mass analyzer, resolved ion beams sequentially strike a detector. There are many types of detectors, but the electron multiplier, either single or multichannel, is most commonly used. 1）电子倍增管15～18级；可测出10-17A微弱电流； 2）渠道式电子倍增器阵列

17. 4.3 Determination of the Molecular Formula by Mass Spectrometry • Consider a molecular ion with a mass of 44. This approximate molecular weight might correspond to C3H8 (propane), C2H4O (acetaldehyde), CO2, or CN2H4. Each of these molecular formulas corresponds to a different exact mass.

18. Table 4-2 Exact mass for C3H8 (propane), C2H4O (acetaldehyde), CO2, or CN2H4 利用HRMS仪给出精确分子量，以推出分子式。 例1. HRMS仪测定精确质量为166.0630（0.006） 166.0570~166.0690

19. Use of Heavier isotope Peaks(LRMS) • Most elements do not consist of a single isotope but contain heavier isotopes in varying amounts. These heavier isotopes give rise to small peaks at higher mass numbers than the major M+ molecular ion peak. A peak that is one mass unit heavier than the M+ peak is called the M+1 peak;two units heavier, the M+2 peak; and so on.

20. Table 4-3 Isotopic Composition of Some Common Elements

21. That means that you would get a set of lines in the m/z = 160 region. The relative heights of the 158, 160 and 162 lines are in the ratio 1:2:1. 79Br: 50.50% 81Br: 49.50%

22. Now substituting the percent abundance of each isotope (79Br and 81Br) into the expansion: • Gives Which are the proportions of M:(M+1):(M+4), a triplet that is slightly distorted from a 1:2:1 pattern. When two elements with heavy isotopes are present, the binomial expansion (4.10) is used.

23. The Figure (b) and (c) spectra show that chlorine is also composed of two isotopes, the more abundant having a mass of 35 amu, and the minor isotope a mass 37 amu. The precise isotopic composition of chlorine and bromine is: 35Cl: 75.77% 37Cl: 24.23%

24. 6 4 1 0 8 1 1 0 C H C l 2 9 2 5 C H B r 4 9 2 5 2 7 2 8 6 6 2 9 5 1 9 3 8 1 9 5 7 9 C l B r 1 4 0 1 5 6 1 5 8 1 2 5 C H 1 4 2 2 5 1 2 7 1 0 5 7 7 5 1 7 1

25. Molecular ions (or fragment ions) containing various numbers of chlorine and/or bromine atoms therefore give rise to the patterns shown in Figure 4.8. Fig. 4.8 The abundances containing various numbers of chlorine and/or bromine atoms

26. For a compound CwHxOzNy, a simple formula allows one to calculate the percent of the heavy isotope contributions from a monoisotopic peak, PM, to the PM+1 peak:

27. Tables of abundance factors have been calculated by Beynon and Williams for all combinations of C, H, N, and O up to mass 500. For example, an isotopic cluster with most mass: 80 (M=100), 81 (M+1=5.13), 82 (M+2=0.10). For nominal mass 80, in the Beynon Table M+1 and M+2 values are listed below (Table 4-4). • It is quite evident that molecular formula must be C4H4N2, because its M+1 and M+2 are close to that in Beynon Table. Once the empirical formula is established with reasonable assurance, hypothetical molecular structures are written.

28. Table 4-4 The tabular data of Beynon for nominal mass 80

29. Three general rules aid in writing formulas. (1) If the molecular weight of a C, H, O, and N compound is even, so is the number of hydrogen atoms it contains; (2) If the molecular weight is divisible by four, the number of hydrogen atoms is also divisible by four. (3) When nitrogen is known to be present in any compound of C, H, O, As, P, S, Si, and the halogens that have an odd molecular weight, the number of nitrogen atoms must be odd.This rule is called nitrogen rule.

30. 例2：某化合物MS数据：M=181，PM%=100% P(M+1)%=14.68% P(M+2)%=0.97% 查[贝诺表] 根据“氮规则”：M=181，化合物分子式为(2)。

31. Example 3 • A molecular-ion cluster 150 (M=100), 151 (M+1=9.9), 152 (M+2=0.9). Here M+2 = 0.9 suggests absence of the other A+2 elements except for oxygen. The even numbered value for M means that either no or an even number of N atoms are present. According to M+1 = 1.1w + 0.36y, we have

32. While the M+2 = 0.006 w (w-1) + 0.2 z,we have This suggests this molecular formula of C9H10O2

33. Example 4 • The molecular-ion peak at m/z 151 is accompanied by isotopic peaks at m/z 152, 153 and 154. The intensity rations of M, M+1, M+2 and M+3 peaks are 100:10.4:32.1:2.9. The high M+2 value suggests the presence of a Cl atom. The odd numbered value for M means the presence of an odd number N atom. While the M+1 value requires around 9 carbon atoms, but in consideration of the limit of molecular weight can only allow 8 carbonatoms. Thus the molecular formula can be assigned as C8H6ClN.

34. 4.4 The Mass Spectra of Organic Compounds

35. McLafferty rearrangement • -Cleavage of a bond with -hydrogen rearrangement to form a cation radical and a neutral molecule is called McLafferty rearrangement.This rearrangement is characteristic of compounds containing a -hydrogen with respect to multiple bond. The general scheme is shown as following:

37. 1. The mass spectra of hydrocarbons1.1 The mass spectrum of alkane Figure 4.12 The mass spectrum for 1-pentane

38. Figure 4.12 The mass spectrum for 2-methylbutane

39. 1.2 The mass spectrum of alkenes

40. Figure 4.16 The spectra of 1-heptene (a) and 2-heptene (b)

42. 1.3 The mass spectra of alkynes

43. 1.4 The mass spectra of aromatic compounds 91 134 105

44. 2. The mass spectra of alcohols, phenol, ethers 2.1 The mass spectrum of alcohols Figure 4.24 The mass spectrum of 1-butanol

45. MS of 1-cetanol (a) MS of 1-cetene (b)

46. 2.2 The mass spectra of phenols

48. 2.3 The mass spectrum of ethers

49. 3. The mass spectra of aldehydes, ketones • 3.1 The mass spectra of aldehydes