Mass-spectroscopy

Mass spectrometry analyzes the chemical composition of a sample based on a mass spectrum. A mass spectrum is an intensity vs. mass-to-charge ratio (usually referred to as m/z) plot representing the constituent component profile of the sample. The following paragraph briefly explains the process of generating a mass spectrum. 

The mass spectrometer was developed by J.J. Thompson. In obtaining the mass spectrum of neon, he found a large signal at 20 amu, but also a small signal at 22 amu. He first assumed it was an impurity. So, he kept repeating the experiment, each time more and more carefully purifying the sample, but the small signal at 22 amu remained constant. What Thompson discovered was the existence of isotopes. Isotopes have the same atomic number but different mass number (same Z, different A), or the same number of protons and different number of neutrons. 

Mass spectroscopy is a useful technique for the characterization of dendrimers because it can be used to determine relative molar mass. Also, from the fragmentation pattern, the details of the monomer assembly in the branches can be confirmed. A variety of mass spectroscopic techniques have been used for this, including electron impact, fast atom bombardment and matrix-assisted laser desorption ionization (MALDI) mass spectroscopy. 

In general, dendrimers show sharp M+ signals at the expected values. This confirms the overall dendrimer structure. Mass spectra may be complicated by the presence of small peaks corresponding to two- or three-molecule aggregates bearing a single charge and this phenomenon is 

Mass spectroscopy adds further knowledge to the structural analysis of coal by allowing calculation of ring distribution as well as identification of individual molecular ions (Herod et al., 1991). For example, gas chromatography-mass spectroscopy as well as pyrolysis gas chromatography-mass spectroscopy of coal has enabled low-molecular-weight benzenes, phenols, and naphthalenes to be 

The use of a pyrolysis technique masks the precise manner in which the thermal products are located within the coal matrix, that is, whether or not they were trapped ( caged ) in the coal or whether they were an integral part of the organic strnctnre of the coal. However, the identification of these materials does offer valuable information abont the geochemical origin of the coal as well as some skeletal information about the coal (Chapter 3). 

Application of this technique to the identification of methyl esters of the organic acids obtained by the controlled oxidation of bituminous coal allowed the more volatile benzene carboxylic acid esters to be identified (Studier et al., 1978). These were esters of benzene tetracarboxylic acid, tere-phthalic acid, toluic acid, and benzoic acid. Decarboxylation of the total acid mixture was shown to afford benzene, toluene, Cj-benzenes (i.e., ethylbenzene or xylenes), Cj-benzenes, butylbenzenes, Cj-benzenes, Cybenzenes, naphthalene, methylnaphthalene, C2-naphthalene, biphenyl, methylbi-phenyl, C3-biphenyl, indane, methylindane, Cj-indane, phenanthrene, and fluorene. 

It was concluded that these nuclei occur in bituminous coal but are linked by more readily oxi-dizable structures. An examination of the solvent extracts of coal has produced evidence for the presence of short methylene chains which were part of neither an aliphatic chain nor an alicyclic ring. It was noted that a series of tetralin derivatives (or indane derivatives) and higher kata-condensed aromatics were prevalent in the extracts of this same coal. 

High-resolution mass spectroscopic analyses of pyridine extracts from reduced and untreated coals support the concept that ether linkages exist in the coal and are split during hydrogenation and that hydroaromatic compounds can be formed by addition of hydrogen to the aromatic nuclei. 

The ion masses obtained for a compound CgH2oSi2 [634] were 73 (frequency factor 100%) and 153 (frequency factor 30%) These indicate the structural formula [(H3C)3Si]2C = CHj, 1,1-bis(trimethylsilyl)ethene. The mass 73 corresponds to a 

The pressure of the sample in the ion source of a mass spectrometer is usually about 10 5 mm, and, under these conditions, buildup of fragments to give significant peaks with mje greater than M+ is rare. One exception to this is the formation of (M + 1)+ peaks resulting from transfer of a hydrogen atom from M to M+. The relative intensities of such (M + 1)+ peaks are usually sensitive to the sample pressure and may be identified in this way. 

With the molecular weight available from the M+ peak with reasonable certainty, the next step is to determine the molecular formula. If the resolution of the instrument is sufficiently high, quite exact masses can be measured, which means that ions with mje values differing by one part in 50,000 can be distinguished. At this resolution it is possible to determine the elemental composition of each ion from its exact m/e value (see Exercise 9-44). 

9 Separation and Purification. Identification of Organic Compounds by Spectroscopic Techniques 

Exercise 9-44 Explain how a mass spectrometer, capable of distinguishing between ions with m/e values differing by one part in 50,000, could be used to tell whether an ion of mass 29 is C2H5 or CHO . 

Many mass spectrometers in routine use are incapable of resolving ions with m/e values that differ by less than one mass unit. In this event, the determination of elemental composition is not always straightforward. However, elemental composition can be determined by the method of isotope abundance. We will illustrate this with the following simple example. 

This technique uses the fact that particles of different masses will respond differently to electrical and magnetic forces. The lighter particles move more easily than the heavier ones. 

A very small quantity (as little as a microgram, 10-6 g) of a sample material to be analysed is heated to convert it to its vapour. It is then subjected to a stream of fast moving electrons. This has the effect of charging the fragments of materials present in the sample, forming ions  

If the sample is of an organic origin (including human samples), the fast moving electrons can cause the molecules present to fragment at various places in their 

This technique is about 1000 times more sensitive than infrared and magnetic resonance spectroscopy, outlined later. It can also be used to detect different masses of radioactive and nonradioactive isotopes of elements. It can distinguish between the carbon 12 and 13 isotopes and also the oxygen 16 and 18 isotopes. By 

Mass spectroscopy has been extensively used in the following situations  

When exposed to sufficient energy, a molecule may lose an electron to form a cation-radical, which then may undergo fragmentation of bonds. These processes make mass spectroscopy (ms) a useful tool for structure proof. Very small concentrations of the parent molecules (RS), in the vapor state, are ionized by a beam of energetic electrons (e ), 

The standard against which all peak intensities in a given mass spectrogram are measured is the most intense peak, called the base peak, which is arbitrarily assigned the value 100. If few parent molecules fragment—not a typical situation—the parent cation will furnish the base peak. 

The masses and possible structures of fragment cations, especially the more stable ones, are clues to the structure of the original molecule. However, rearrangement of cations complicates the interpretation. 

Mass spectra, like other spectra, are unique properties used to identify known and unknown compounds. 

The electron in the highest-energy MO is most likely to be lost. The relative energies for electrons are n 7T a (Problem 12.3). In a compound with no n or tt electrons, the electron most likely comes from the highest-energy cr bond. 

Problem 12.31 (a) What molecular formulas containing only C and H can be assigned to a cation with m/e equal 

Mass spectrometry is an analytical method of choice for identification of volatile compounds and has been employed in investigation of thermolysis and acidolysis mechanisms of chemical amplification resists [96, 121, 122]. This technique has been also utilized in screening of resists systems, especially Si-containing 193 nm bilayer resists,for outgassing [438,439]. MALDI-TOF mass spectroscopy has been applied to characterization of dendritic resist polymers. 

An advantage of this method is the good volatility and hence ready separation of medium sized poly-aminoalcohols by gas chromatography. A mixture of small peptides, obtained by enzymatic or chemical partial hydrolysis of a polypeptide, can be reduced and separated in this way before each fraction is submitted to 

Molecular ion and sequence peak ions of an acyl tripeptide methylester 

At about the same time, in 1963, it was reported that simple iV-acetyl-peptides (Heyns and Griitzmacher [29]) or iV-trifluoroacetyl peptide esters (F. Weygand et al. [30]) are cleaved under electron impact at the peptide bonds, giving rise mainly to ions corresponding to N-terminal parts of the chain. If such fission occurs at each peptide bond, a series of ions are formed all having the same N-acyl group. This is equal to a stepwise degradation of the chain from its carboxyl end (Fig. 13). 

Since N-trifluoroacetyl peptide methyl esters are suitable for separation by gas chromatography, the procedure just described can also be applied to peptide mixtures obtained through partial methanolysis of larger peptides. This method permitted the structure elucidation of cyclic peptides such as antamanide (p. 213) and cyclolinopeptide (p. 216) by Prox and Weygand [31]. 

In 1967 E. Lederer (Plate 28) and B.C. Das reported [32] that the volatility of acetyl-peptide esters can be increased by replacing all dissociable protons with methyl groups. This permethylation can be performed with methyl iodide in anhydrous dimethylsulfoxide in the presence of a strong base. In iV-methyl peptides the coherence caused by hydrogen bonds is overcome. Moreover, the fragmentation patterns are much simpler than the patterns found in N-acetyl peptide esters without permethylation. Fragmentation occurs almost exclusively at the peptide bonds and the mass spectra consist mainly of sequence peaks of high intensity (Fig. 14). 

MRI provides quantitative measurements of concentration and molecular mobility as functions of time and, more importantly, as functions of position (i.e. depth) in the coating. A pixel resolution of about 9 pm can be obtained. MRI, with its capability of determining molecular mobiUty and concentration as functions of depth in a coating, clearly has enormous potential in the field of coatings research. 

The fidl power of mass analysis emerges from higher resolution (millidalton level) because each nuclide has a different mass defect (i.e. difference between the exact mass and the nominal mass) is of mass 12.000 00 Da, H is of mass 

00725 Da, is of mass 15.9949 Da, etc. Thus every different elemental composition, CcHaOcN Sj. .., has a different mass, so that the chemical formula of a 

Routine structural approaches with MS include the identification of polymers, which is typically achieved by confirmation of the primary structure via measurement of the molecular mass. 

MS can be combined with traditional multidimensional separation techniques that significantly improve the selectivity and specificity of the experimental approach for use in increasingly complex polymer applications. 

---

SIMS Secondary-ion mass spectroscopy [106, 166-168] (L-SIMS liquids) [169, 170] Ionized surface atoms are ejected by impact of -1 keV ions and analyzed by mass spectroscopy Surface composition... 

SIMS Secondary Ion mass spectroscopy A beam of low-energy Ions Impinges on a surface, penetrates the sample and loses energy In a series of Inelastic collisions with the target atoms leading to emission of secondary Ions. Surface composition, reaction mechanism, depth profiles... 

Thermal electrocyclizations of perhalogenated 1,3-butadienes yield perhalogenated cyclobutenes which can be solvolysed to 3,4-dihydroxy-3-cydobutene-l,2-dione ( squaric acid") and its derivatives (G. Maahs, 1966 H. Knorr, 1978 A.H. Schmidt, 1978). Double CO extrusion from fused cyclobutenediones has been used to produce cycloalkynes, e.g., benzyne from benzocyclobutenedione by irradiation in an argon matrix (O.L. Chapman, 1973) and cyc/o-Ci8, cyclo-Cn, etc. by laser desorption mass spectroscopy of appropriate precursors (see section 4.9.8). 

The preparation and spectroscopic properties (infrared, ultraviolet, NMR) of iV-alkoxycarbonyl-N -(2-thiazolyl)thioureas (268) have been studied by the Nagano group (78, 264). These compounds react with bromine in acetic acid or chloroform to give 2--alkoxycarbonylimino-thiazolo[3,2-h]thiadiazolines (Scheme 162), whose structures were established by mass spectroscopy, infrared, NMR, and reactivity patterns (481). 

The section on Spectroscopy has been expanded to include ultraviolet-visible spectroscopy, fluorescence, Raman spectroscopy, and mass spectroscopy. Retained sections have been thoroughly revised in particular, the tables on electronic emission and atomic absorption spectroscopy, nuclear magnetic resonance, and infrared spectroscopy. Detection limits are listed for the elements when using flame emission, flame atomic absorption, electrothermal atomic absorption, argon ICP, and flame atomic fluorescence. Nuclear magnetic resonance embraces tables for the nuclear properties of the elements, proton chemical shifts and coupling constants, and similar material for carbon-13, boron-11, nitrogen-15, fluorine-19, silicon-29, and phosphorus-31. 

Two other techniques that depend only on base SI units are coulometry and isotope-dilution mass spectrometry. Coulometry is discussed in Chapter 11. Isotope-dilution mass spectroscopy is beyond the scope of an introductory text, however, the list of suggested readings includes a useful reference. 

Taylor, H.E., Inductively Coupled Plasma-Mass Spectroscopy, Academic Press, New York, 2000. 

Most of the experimental information concerning copolymer microstructure has been obtained by physical methods based on modern instrumental methods. Techniques such as ultraviolet (UV), visible, and infrared (IR) spectroscopy, NMR spectroscopy, and mass spectroscopy have all been used to good advantage in this type of research. Advances in instrumentation and computer interfacing combine to make these physical methods particularly suitable to answer the question we pose With what frequency do particular sequences of repeat units occur in a copolymer. 

Acrylonitrile has been characterized using infrared, Raman, and ultraviolet spectroscopies, electron diffraction, and mass spectroscopy (10—18). 

Polyester composition can be determined by hydrolytic depolymerization followed by gas chromatography (28) to analyze for monomers, comonomers, oligomers, and other components including side-reaction products (ie, DEG, vinyl groups, aldehydes), plasticizers, and finishes. Mass spectroscopy and infrared spectroscopy can provide valuable composition information, including end group analysis (47,101,102). X-ray fluorescence is commonly used to determine metals content of polymers, from sources including catalysts, delusterants, or tracer materials added for fiber identification purposes (28,102,103). 

Triphenylphosphine oxide [791-28-6], C gH OP, and triphenyl phosphate [115-86-6], C gH O P, as model phosphoms flame retardants were shown by mass spectroscopy to break down in a flame to give small molecular species such as PO, HPO2, and P2 (33—35). The rate-controlling hydrogen atom concentration in the flame was shown spectroscopically to be reduced when these phosphoms species were present, indicating the existence of a vapor-phase mechanism. 

Analytical Procedures. Oxygen difluoride may be determined conveniently by quantitative appHcation of k, nmr, and mass spectroscopy. Purity may also be assessed by vapor pressure measurements. Wet-chemical analyses can be conducted either by digestion with excess NaOH, followed by measurement of the excess base (2) and the fluoride ion (48,49), or by reaction with acidified KI solution, followed by measurement of the Hberated I2 (4). 

Other Fluorosulfanes. Difluorotrisulfane [31517-17-6] FSSSF, and difluorotetrasulfane [31517-18-7] FSSSSF, have been identified as the constituents of the yellow oil obtained when sulfur vapor reacts with AgF. Their existence was demonstrated by nmr and mass spectroscopy (134,135). 

Oxygen and nitrogen also are deterrnined by conductivity or chromatographic techniques following a hot vacuum extraction or inert-gas fusion of hafnium with a noble metal (25,26). Nitrogen also may be deterrnined by the Kjeldahl technique (19). Phosphoms is determined by phosphine evolution and flame-emission detection. Chloride is determined indirecdy by atomic absorption or x-ray spectroscopy, or at higher levels by a selective-ion electrode. Fluoride can be determined similarly (27,28). Uranium and U-235 have been determined by inductively coupled plasma mass spectroscopy (29). 

The preferred quantitative deterrnination of traces of acetylene is gas chromatography, which permits an accurate analysis of quantities much less than 1 ppm. This procedure has been highly developed for air poUution studies (88) (see Airpollution control methods). Other physical methods, such as infrared and mass spectroscopy, have been widely used to determine acetylene in various mixtures. 

Acetylene Derived from Hydrocarbons The analysis of purified hydrocarbon-derived acetylene is primarily concerned with the determination of other unsaturated hydrocarbons and iaert gases. Besides chemical analysis, physical analytical methods are employed such as gas chromatography, ir, uv, and mass spectroscopy. In iadustrial practice, gas chromatography is the most widely used tool for the analysis of acetylene. Satisfactory separation of acetylene from its impurities can be achieved usiag 50—80 mesh Porapak N programmed from 50—100°C at 4°C per minute. 

Mass Spectroscopy. A coUection of 125,000 spectra is maintained at Cornell University and is avaUable from John WUey Sons, Inc. (New York) on CD-ROM or magnetic tape. The spectra can be evaluated using a quaHty index algorithm (63,76). Software for use with the magnetic tape version to match unknowns is distributed by Cornell (77). The coUection contains aU avaUable spectral information, including isotopicaUy labeled derivatives, partial spectra, and multiple spectra of a single compound. 

Mass Spectroscopy Society of Japan ms 6,000 JICST-JOIS ... 

Spectroscopic methods for the deterrnination of impurities in niobium include the older arc and spark emission procedures (53) along with newer inductively coupled plasma source optical emission methods (54). Some work has been done using inductively coupled mass spectroscopy to determine impurities in niobium (55,56). X-ray fluorescence analysis, a widely used method for niobium analysis, is used for routine work by niobium concentrates producers (57,58). Paying careful attention to matrix effects, precision and accuracy of x-ray fluorescence analyses are at least equal to those of the gravimetric and ion-exchange methods. 

G. Beck, Elemental Mnalysis byMrgon Plasma Mass Spectroscopy, Teledyne Wab Chang Corp., Albany, Oieg., 1988. 

The principal techniques for determining the microstmcture of phenoHc resins include mass spectroscopy, proton, and C-nmr spectroscopy, as well as gc, Ic, and gpc. The softening and curing processes of phenoHc resins are effectively studied by using thermal and mechanical techniques, such as tga, dsc, and dynamic mechanical analysis (dma). Infrared (ir) and electron spectroscopy are also employed. 

V. Loon, Plasma Source Mass Spectroscopy, CRC Press Inc., Boca Raton, Fla., 1994. 

---