先進(jìn)材料表征技術(shù)-ii

1、Advanced Technology for Materials Characterization(Part IV),By Dr. Yejun Qiu,Department of Materials Science and EngineeringSpring, 2011.,(9 credit hours),Hyphenated technology of GC-MS; Surface area and pore structur

2、e analysis.,(II) GC-MS,Mass spectrometry is a powerful analytical technique for forming gas-phase ions from intact, neutral molecules and subsequently determining their molecular masses. All mass spectrometers have thre

3、e essential components: an ion source, a mass analyzer, and a detector. Ions are produced from the sample in the ion source using a specific ionization method. The ions are separated in the mass analyzer based on their m

4、ass-to-charge (m/z) ratios and then detected, usually by an electron multiplier. The data system produces a mass spectrum, which is a plot of ion abundance versus m/z. Except in the case of electrospray mass spectrometry

5、 (where the ion source is at atmospheric pressure), the ion source, mass analyzer, and detector are usually situated inside a high-vacuum chamber (pressure between 10?8 and 10?4 Torr).,What is MS?,For example: for the io

6、n C7H72+, m/z equals 45.5.,Classification:,Organic mass spectrometer (Quadrupole MS, Ion-trap MS, Time of flight MS, Magnetic MS); Inorganic mass spectrometer (Spark source MS 火花源, Ion microprobe MS, Laser probe MS, etc

7、.); Isotope mass spectrometer; Gas analysis mass spectrometers (C13 breath mass spectrometer, Helium-leak apparatus ).,Mass spectrometry’s characteristics have raised it to an outstanding position among analytical meth

8、ods: unequalled sensitivity, detection limits, speed and diversity of its applications. In analytical chemistry, the most recent applications are mostly oriented towards biochemical problems, such as proteome, metabol

9、ome, high throughput in drug discovery and metabolism, and so on. Other analytical applications are routinely applied in pollution control, food control, forensic science, natural products or process monitoring. Othe

10、r applications include atomic physics, reaction physics, reaction kinetics, geochronology, inorganic chemical analysis, ion–molecule reactions, determination of thermodynamic parameters (△G?f, Ka, etc.), and many others.

11、,Applications of MS:,In 1898, W. Wien, by the use of very powerful magnetic fields, proved that the positive rays were deflected by magnetic forces. In 1897 the mass-to-charge ratio m/e of the electron was first measur

12、ed by J. J. Thomson. (J. J. Thomson, was a British physicist and Nobel laureate. He is credited for the discovery of the electron and of isotopes, and the invention of the mass spectrometer. Thomson was awarded the 1906

13、Nobel Prize in Physics.) In 1991 the thomson (symbol: Th) was proposed as a unit to measure mass-to-charge ratio in mass spectrometry. In 1918, A.J. Dempster reported the construction of an electron bombardment ion sou

14、rce mass spectrometer. Francis William Aston won the Nobel prize in Chemistry in 1922. "For his discovery, by means of his mass spectrograph, of isotopes, in a large number of non-radioactive elements, and for his

15、enunciation of the Whole Number Rule. In 1942, commercial MS occurred.,History:,MS does not belong to the region of spectrum; Mass-spectrogram is not related to the wavelength of electromagnetic wave and the change of

16、 physical quantity in molecules; Mass-spectrogram is related to molecular structure.,Characteristics:,Small amount of sample, high sensitivity, and rapid analysis speed. MS can provide the information of molecular wei

17、ght, and thus determine the molecular formula, which is very important for the identification of compounds.,Advantages:,Basic Principles of MS:,The first step in the mass spectrometric analysis of compounds is the produc

18、tion of gasphase ions of the compound, for example by electron ionization (電子電離 ):,When a high energy electron collides with a molecule it often ionizes it by knocking away one of the molecular electrons (either bonding

19、or non-bonding). This leaves behind a molecular ion (colored red in the diagram). Residual energy from the collision may cause the molecular ion to fragment into neutral pieces (colored green) and smaller fragment ions (

20、colored pink and orange). The molecular ion is a radical cation, but the fragment ions may either be radical cations (pink) or carbocations (orange), depending on the nature of the neutral fragment.,(碳陽(yáng)離子),(自由基陽(yáng)離子),Mass

21、spectrum of methanol by electron ionization, presented as a graph and as a table.,In either presentation, the most intense peak is called the base peak and is arbitrarily assigned the relative abundance of 100%. The abun

22、dances of all the other peaks are given their proportionate values, as percentages of the base peak. Many existing publications label the y axis of the mass spectrum as number of ions, ion counts or relative intensity. B

23、ut the term relative abundance is better used to refer to the number of ions in the mass spectra.,Base peak,,(基峰),Most of the positive ions have a charge corresponding to the loss of only one electron. For large molecule

24、s, multiply charged ions also can be obtained. Ions are separated and detected according to the mass-to-charge ratio. The total charge of the ions will be represented by q, the electron charge by e and the number of char

25、ges of the ions by z:,The x axis of the mass spectrum that represents the mass-to-charge ratio is commonly labelled m/z. When m is given as the relative mass and z as the charge number, both of which are unitless, m/z is

26、 used to denote a dimensionless quantity.,Most of the ions formed in a mass spectrometer have a single charge, so the m/z value is equivalent to mass itself.,Generally in mass spectrometry, the charge is indicated in mul

27、tiples of the elementary charge (元電荷) or charge of one electron in absolute value (1 e=1.602177×10?19 C) and the mass is indicated in atomic mass units (1 u=1.660540×10?27 kg, 原子質(zhì)量單位). As already mentioned, the

28、 physical property that is measured in mass spectrometry is the mass-to-charge ratio. When the mass is expressed in atomic mass units (u) and the charge in elementary charge units (e) then the mass-to-charge ratio has u/

29、e as dimensions. For simplicity, a new unit, the Thomson, with symbol Th, has been proposed. The fundamental definition for this unit is:,Ions provide information concerning the nature and the structure of their precurso

30、r molecule. In the spectrum of a pure compound, the molecular ion, if present, appears at the highest value of m/z (followed by ions containing heavier isotopes) and gives the molecular mass of the compound. The term mol

31、ecular ion (分子離子) refers in chemistry to an ion corresponding to a complete molecule regarding occupied valences.,Example: CH3OH. The molecular ion appears at m/z 32 in the spectrum of methanol, where the peak at m/z 33

32、is due to the presence of the 13C isotope, with an intensity that is 1.1% of that of the m/z 32 peak. In the same spectrum, the peak at m/z 15 indicates the presence of a methyl group. The difference between 32 and 15, t

33、hat is 17, is characteristic of the loss of a neutral mass of 17Da by the molecular ion and is typical of a hydroxyl group. In the same spectrum, the peak at m/z 16 could formally correspond to ions CH4?+, O+ or even CH3

34、OH2+, because they all have m/z values equal to 16 at low resolution. However, O+ is unlikely to occur, and a doubly charged ion for such a small molecule is not stable enough to be observed.,Mass spectra of carbon dioxi

35、de, propane and cyclopropane. The molecules of these compounds are similar in size, CO2 and C3H8 both have a nominal mass of 44 Da, and C3H6 has a mass of 42 Da. The molecular ion is the strongest ion in the spectra of C

36、O2 and C3H6, and it is moderately strong in propane. The unit mass resolution is readily apparent in these spectra (note the separation of ions having m/z=39, 40, 41 and 42 in the cyclopropane spectrum). Even though thes

37、e compounds are very similar in size, it is a simple matter to identify them from their individual mass spectra.,Even with simple compounds like these, it should be noted that it is rarely possible to explain the origin

38、of all the fragment ions in a spectrum. Also, the structure of most fragment ions is seldom known with certainty.,molecular ion, CO2,molecular ion,molecular ion,fragment ions CO,fragment ions, O,ethyl cation,(乙基陽(yáng)離子),Sinc

39、e a molecule of carbon dioxide is composed of only three atoms, its mass spectrum is very simple. The molecular ion is also the base peak, and the only fragment ions are CO (m/z=28) and O (m/z=16). The molecular ion of

40、 propane also has m/z=44, but it is not the most abundant ion in the spectrum. Cleavage of a carbon-carbon bond gives methyl and ethyl fragments, one of which is a carbocation and the other a radical. Both distributions

41、are observed, but the larger ethyl cation (m/z=29) is the most abundant, possibly because its size affords greater charge dispersal. A similar bond cleavage in cyclopropane does not give two fragments, so the molecular

42、 ion is stronger than in propane, and is in fact responsible for the base peak. Loss of a hydrogen atom, either before or after ring opening, produces the stable allyl cation (m/z=41). The third strongest ion in the spec

43、trum has m/z=39 (C3H3). Its structure is uncertain. The small m/z=39 ion in propane and the absence of a m/z=29 ion in cyclopropane are particularly significant in distinguishing these hydrocarbons.,MS Equipment:,The thr

44、ee essential functions of a mass spectrometer, and the associated components, are: A small sample is ionized, usually to cations by loss of an electron.   The ions are sorted and separated according to their mass

45、and charge.   The separated ions are then measured, and the results displayed on a chart.,Ion Source,Mass Analyzer,Detector,,,,The heart of the spectrometer!,Ions are very reactive and short-lived, their formation

46、and manipulation must be conducted in a vacuum.,A sample inlet, for example a gas chromatograph or a direct insertion probe.,A mass spectrometer should always perform the following processes:,1. Produce ions from the sam

47、ple in the ionization source.2. Separate these ions according to their mass-to-charge ratio in the mass analyser.3. Eventually, fragment the selected ions and analyze the fragments in a second analyser. (得到碎裂的離子譜—質(zhì)量分析離

48、子動(dòng)能譜)4. Detect the ions emerging from the last analyser and measure their abundance with the detector that converts the ions into electrical signals.5. Process the signals from the detector that are transmitted to the

49、computer and control the instrument through feedback.,Ion source:,Electron Ionization, EI; (電子電離) Chemical Ionization , CI; (化學(xué)電離) Fast Atomic bombardment, FAB; (快速原子轟擊) Electron spray Ionization, ESI; (電噴霧電離) A

50、tmospheric pressure chemical Ionization, APCI. (大氣壓化學(xué)電離),Electron Ionization (EI),Schematic view of the ion source based on electron impact ionization and the quadrupole mass filter typically found in a GC-MS instrument

51、.,Electron ionization is an ionization method in which energetic electrons interact with gas phase atoms or molecules to produce ions. This technique is widely used in mass spectrometry, particularly for gases and volati

52、le organic molecules.,The following gas phase reaction describes the electron ionization process:M + e- → M+· + 2 e-where M is the analyte molecule being ionized, e? is the electron and M+? is the resulting ion.

53、,Mass analyzer,The ions can be separated by charge-mass ratio; Five basic mass analyzers:1) Magnetic-sector mass spectrometer; (磁質(zhì)譜)2) Quadrupole (Q); (四極桿)3) Ion trap (IT); (離子陷阱)4) Time of flight (TOF);

54、 (飛行時(shí)間)5) Fourier transform ion cyclotron resonance (FT-ICR).(傅里葉變換離子回旋共振),Quadrupole (Q): As the name implies, it consists of 4 circular rods, set parallel to each other. Ions are separated in a quadrupole based on t

55、he stability of their trajectories (軌道) in the oscillating electric fields (振蕩電場(chǎng)) that are applied to the rods.,Characteristic: cheap, small volume, easy to operate, fast scanning rate, no hysteresis, suitable to be used

56、 in GC-MS and LC-MS. But its resolution is not very high.,Each opposing rod pair is connected together electrically, and a radio frequency (RF,無(wú)線(xiàn)電頻率) voltage is applied between one pair of rods and the other. A direct cu

57、rrent voltage is then superimposed on the RF voltage. Ions travel down the quadrupole between the rods. Only ions of a certain mass-to-charge ratio m/z will reach the detector for a given ratio of voltages: other ions ha

58、ve unstable trajectories and will collide with the rods. This permits selection of an ion with a particular m/z or allows the operator to scan for a range of m/z-values by continuously varying the applied voltage. Mathem

59、atically this can be modeled with the help of the Hill differential equation.,Principle:,,Time of flight (TOF):,The time-of-flight mass analyzer is a pseudo-simultaneous detection type device, which can accomplish a full

60、 mass spectrum (2-260 amu) in approximately 30µs.,How does TOF work?,In this technique, ions are subjected to a pulsed electric field which, ideally, imparts the same kinetic energy (KE) on all ions in the packet. T

61、hese ions are then directed to a field free region (i.e., drift tube, 漂移管) where the differences in velocities spatially separate ions of differing m/z. The KE attained by each ion will be product of the ion's charge

62、 (q), the electric field (E), and the distance (sa) under which the ion is subject to the electric field. This expression can be equated to the expression for KE based on ion mass (m), and ion velocity (v) to yield:,Rear

63、ranging Equation 1, the time for an ion to traverse traverse the drift tube, td, can be determined by Equation 2:,where D is the distance traversed before striking the detector.,,GC-MS:,GC-MS is the combined technique fo

64、r the determination of organic compounds. The GC-MS can provide data in two domains. A reconstructed total ion current chromatogram is generated which provides quantitative information. Also a mass spectrum is available

65、at any point in the chromatogram which provides the qualitative information.,Combination modes： Offline mode; (離線(xiàn)模式) Online mode; (在線(xiàn)模式) Embedded mode. (嵌入模式),Hardware interface of GC-MS:,(硬件接口),A GC/MS connect

66、or:,The main problem lies in the difference of pressure: 100 kPa for GC, while 10-3 Pa for MS.,by-pass type,,Total ion chromatograms (TIC):,A plot of the total ion signal in each of a series of mass spectra that are reco

67、rded as a function of chromatographic retention time. Care should be taken to avoid confusion between the abbreviations for total ion current (總離子流) and total ion chromatogram.,(總離子流圖),Examples of application of GC-MS:,E

68、xample 1. GC-MS Screening Procedure for the Presence of Diethylene Glycol (二甘醇) and Ethylene Glycol (乙二醇) in Toothpaste (牙膏).,This procedure was developed to examine toothpaste for the presence of Diethylene Glycol (and