Specialized MS techniques

To obtain the mass spectrum of a compound present in the ion source the mass scan unit requires activation. It remains for the operator to decide at which point this scan is taken and, in GC-MS, which GC peaks need to be scanned. Hites and Biemann (1970) introduced the principle of cyclic or repetitive 

In the analysis of organic acid extracts it has the advantage of storing data on the complete sample for extensive comparison with other extracts. It obviates the occasional erroneous decision to scan only selected peaks which may be the only ones of initial interest, but subsequently transpire to have limited use. This may have particular value when it is not possible to re-run the sample because of either its small size or deterioration. Fig. 5.6 shows the computer output of a repetitively scanned derivatized extract of organic acids. The summed total ion current in each scan is plotted against the scan number and from this chromatogram scans can be selected for which a full mass spectral plot is required. Examples of such spectra are shown in Fig. 5.7. 

In addition to this major advantage of comprehensive data collection, repetitive scanning techniques form the basis of other approaches to the analysis of multicomponent mixtures. Most common of these is the selected ion plot in which the block of mass spectral data from a repetitive scan run is inspected for particular ions. These are then represented as a plot of ion intensity versus scan number (see Figs. 5.8 and 5.9). In these figures, ions characteristic of spectra of 4-hydroxyphenylacetic acid and pentonic acids are plotted, and their positions in the chromatogram can be clearly seen. As the pentonic acid isomers are eluted under the peaks of hippuric and citric acids, their detection in composite spectra is not always definitive. 

Selected ion plots may also be of value to detect a small amount of a compound in the presence of large amounts of other compounds. In favourable instances, ions characteristic of the compound which appear insignificant on a relative intensity basis against compound background ions, can show up clearly. 

In quantitative procedures the selected ion response from the sample is measured in relation to that of a known amount of a carefully chosen internal standard. The latter is added at the earliest possible stage in the sample pretreatment to account for extraction and chemical losses. In general the most accurate analysis is achieved with a stable isotopically labelled form of the sample compound as internal standard and a number of examples of assays for organic acids has been reported (e.g. Gorden et /., 1974 Fri et ai, 1974 Langenbeck et al, 1978b). 

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In the synthesis of all concave acids and bases, a difunctionalized molecule A-A was cyclized with a difunctionalized bridge component B-B. Because telo- and polymerizations are the main side reactions [29] the isolated macrocycles need not be the expected [1 -I- 1] addition products, the (-A-AB-B-)i cycles, [n - - n] Telomers with the general structure (-A-AB-B-) are also possible. These molecules have identical elemental analyses and similar IR and NMR data. Therefore the mass spectral analyses of the macrocycles are very important because this is the only method which can tell [1 -t- 1] and [2 -t- 2] addition products apart. Due to the high molecular weight of the concave acids and bases, special MS techniques were necessary in some cases [30]. In the case of the macrocyclic diamine 7 [R = NEt2, X = CH2(CH20CH2)2CH2], a [2 -t- 2] addition product could be isolated and characterized besides the desired [1 + 1] product [12a]. 

Coupling the column from the GC to a mass spectrometer provides a very powerful combination, GC-MS, which can identify and quantify almost all the compounds in a complex mixture, such as an essential oil or perfume, by reference to libraries of mass spectra of known compounds. Careful investigation of the mass spectrum can be used deductively to determine a possible structure for an unknown material using fragmentation theories to identify sub-structural components of the molecule. Recent developments in benchtop mass spectrometers have brought a range of specialized MS techniques into the realm of GC-MS machines techniques such as chemical ionization and MS-MS are now available, which provide more information on individual sample components and allow better identification of unknown compounds. 

This was determined through isolation and characterization of PCB bound biopolymers and monomers. The greatest binding was observed in RNA followed by protein and DNA, respectively, and binding occurs in tissues other than liver as well. This binding is likely to be covalent and the result of metabolic activation, but proof of this awaits further chemical characterization of the isolated materials. Field desorption mass spectrometry (MS) and other specialized MS techniques should be useful in characterizing such adducts (16). 

When faced with the MS analysis of a moderately labile sample, derivatization followed by conventional MS (El or Cl) often proved to be more practical than the specialized MS techniques. The relatively low cost and subsequent availability of this approach served to bolster the development of sensitive derivatization techniques for use in probe and gas chromatography-mass spectrometry (GC-MS) experiments. The gains in volatility and thermal stability available through derivatization substantially extended the applicability of mass spectrometry. However, as sample molecular weight and thermal instability increased, derivatization was no longer able to provide the increased p>erformance observed at low mass (< 1500 daltons). This problem was further compounded by the net increase in sample molecular weight associated with most derivatization procedures. 

An important breakthrough in HTS ee assays came from the group of Reetz in late 1990, with the introduction of mass spectroscopy (MS)-based procedures [90]. These methods use special asymmetrically isotope-labeled compounds. Enzymatic transformations of these compounds usually lead to two pseudoenantiomeric compounds whose relative concentration can be estimated using MS techniques. 

This review will first concentrate on the unimolecular gas-phase chemistry of diene and polyene ions, mainly cationic but also anionic species, including some of their alicyclic and triply unsaturated isomers, where appropriate. Well-established methodology, such as electron ionization (El) and chemical ionization (Cl), combined with MS/MS techniques in particular cases will be discussed, but also some special techniques which offer further potential to distinguish isomers will be mentioned. On this basis, selected examples on the bimolecular gas-phase ion chemistry of dienes and polyenes will be presented in order to illustrate the great potential of this field for further fundamental and applied research. A special section of this chapter will be devoted to shed some light on the present knowledge concerning the gas-phase derivatization of dienes and polyenes. A further section compiles some selected aspects of mass spectrometry of terpenoids and carotenoids. 

As noted earlier, certain techniques such as colligative methods, light-scattering photometry, special mass spectrometry (MS) techniques, and ultracentrifugation allow the calculation of specific or absolute molecular weights. Under certain conditions some of these also allow the calculation of the MWD. 

MALDI-MS was developed for the analysis of nonvolatile samples and was heralded as an exciting new MS technique for the identification of materials with special use in the identification of polymers. It has fulfilled this promise to only a limited extent. While it has become a well-used and essential tool for biochemists in exploring mainly nucleic acids and proteins, it has been only sparsely employed by synthetic polymer chemists. This is because of lack of congruency between the requirements of MALDI-MS and most synthetic polymers. 

The number of detectors that are sensitive and selective enough to be applied online with LC is limited because the solvents used are not compatible, as in the case of immunochemical detection after reversed- or normal-phase LC. The technology of coupling is still under development and not yet available in a large number of laboratories not specialized in techniques such as LC-MS. Therefore, LC separations are frequently followed by offline detection. Confirmatory analysis of suspected liquid chromatographic peaks can be made possible by coupling liquid chromatography with mass spectrometry. Atmospheric-pressure chemical ionization LC-MS has been employed for the identification of six steroid hormones in bovine tissues (448). 

Structural elucidation of natural macromolecules is an important step in understanding the relationships between the chemical properties of a biomolecule and its biological function. The techniques used in organic structure determination (NMR, IR, UV, and MS) are quite useful when applied to biomolecules, but the unique nature of natural molecules also requires the application of specialized chemical techniques. Proteins, polysaccharides, and nucleic acids are polymeric materials, each composed of hundreds or sometimes thousands of monomeric units (amino acids, monosaccharides, and nucleotides, respectively). But there is only a limited number of these types of units from which the biomolecules are synthesized. For example, only 20 different amino acids are found in proteins but these different amino acids may appear several times in the same protein molecule. Therefore, the structure of... 

The HPLC/MS technique has been only recently initiated into environmental applications. It is considered a non-routine application, and is conducted mostly by the laboratories specializing in pesticide analysis. 

More sophisticated mass spectrometric methods have been found in the electrospray (ES-MS) and plasma desorption (PD-MS) techniques which have successfully been applied directly to nonvolatile pteridines. A small peak can be detected with folic acid at m/z = 441 together with the mono-, di-, and trisodium species. The dihydro- and tetrahydro derivatives also give the expected results <83Mi 718-05). Fast atom bombardment (FAB) works also with folic acid in special matrices and is another tool for structural studies <83MI 718-08). Even in a molar mixture of 5-methyl-5,6,7,8-tetrahydropterin and tris(pentane-2,4-dionato)iron(III), the metal-pterin complex could be detected by ES-MS <92HCA1955>. 

Isolation of Processes To minimize cross-contamination and microbiological contamination, the manufacturer may develop special procedures for the isolation of processes. The level of facilities isolation depends on the types of products to be manufactured. For instance, steroids and sulfas require more isolation than over-the-counter (OTC) oral products [6], To minimize exposure of personnel to drug aerosols and loss of product, a sealed pressure vessel must be used to compound aerosol suspensions and emulsions [21], An example of cross-contamination with steroids was the controversial case of a topical drug manufactured for the treatment of skin diseases. Fligh-performance liquid chromatography/ultraviolet and mass spectrometry (FIPLC/UV, FIPLC/MS) techniques were used by the FDA for the detection of clobetasol propionate, a class 1 superpotent steroid, as an undeclared steroid in zinc pyrithione formulations. The product was forbidden and a warning was widely published [22],... 

In these products, simple analysis gives the average molar substitution (MS) per anhydroglucose unit of the cellulose rather than the DS. Specialized analytical techniques must be employed to ascertain the DS of the grafted derivatives. 

Occurrence and formation of TBC 30, the condensation product of serotonin with acetaldehyde, has received special attention by the Swedish group of Beck and colleagues. They found TBC 30 to be present in human urine and in animal tissues, in which it is often excreted in conjugated form (59). Analysis of tissue extracts using GC-MS techniques and chiral columns showed the compound to be present in rat brain and to be excreted in the urine of different animal species in unequal proportions of the enantiomers (60,61). Unequal proportions of deuterated TBC 30 enantiomers were found when rats were fed trideuterated ethanol (30). TBC 30 occurs in the urine of controls and alcoholics in comparable amounts and is excreted mostly in conjugated form (62). The methyl ether TBC 31 is a... 

In Table 1, the principle features of ICP-OES and ICP-MS techniques are presented. As can be observed, each technique presents specific capabilities and limitations, which determine their applications in the analysis of real-world samples. In the following sections, the basic principles of ICP-OES and ICP-MS are described with a special focus on the recent technological and methodological developments. An overview of their applications in inorganic and organometallic analysis is also presented. 

It is also worthwhile to mention that FF-TLC can be used as a clean-up method before HPLC. Special detection techniques as well as direct coupling methods (such as on-line coupling to GC-MS. digital autoradiography, etc.) may afso be used with FF-TLC. 

In order to obtain even simpler MS spectra that will counteract a number of drawbacks in the common El ionization procedures applied in Py-EI MS, several special ionization techniques were applied. One of them is Cl ionization, but also field ionization (FI), field desorption (FD), and photoionization (PI) were utilized to obtain simplified mass spectra for pyrolysates. Also, MS/MS techniques were utilized for the analysis in attempts to substitute at least in part for the need of a separation. 

Another MS technique used in connection to pyrolysis is MIMS (membrane introduction mass spectrometry). MIMS is in fact a special inlet for the mass spectrometer, where a membrane (usually silicone, non-polar) lets only certain molecule types enter the Ionization chamber of the MS. This allows, for example, direct analysis of certain volatile organic compounds from air. The system makes possible the coupling of atmospheric pyrolysis to a mass spectrometer [61a] allowing direct sampling of the pyrolysate. Other parts of the mass spectrometer do not need to be changed when using MIMS. 

For pesticides, a combination of GC-MS and LC-MS techniques is used to analyze quantities in the ppb range. Special detector systems such as ECD (electron capture detector) and AAS (atomic absorption spectroscopy) are used for detection and quantification of halogen and heavy metal content. 

Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and mass spectrometric (ICP-MS) techniques are modern, sophisticated, very sensitive, and generally quite expensive. These methods have been used successfully for the determination of metals in tobacco and tobacco smoke (20A90, 20A112). The success of these two methods can be affected by the metal-solvent matrix employed. In many cases preseparation procedures and other special handling techniques are required. Each method for analysis of metals in tobacco and tobacco smoke has its own advantages and challenges. 

A new technique that resembles the GC-MS technique described here is high-performance liquid chromatography-mass spectrometry (HPLC-MS). An HPLC instrument is coupled through a special interface to a mass spectrometer. The substances that elute from the HPLC column are detected by the mass spectrometer, and their mass spectra can be displayed, analyzed, and compared with standard spectra found in the computer library built into the instrument. 

In organic and inorganic chemistry, MS can be used to identify reaction products and byproducts. Impurities at concentrations as low as parts per trillion (ppt) can be detected MS is widely used for this purpose. In inorganic chemistry, with special inlet techniques, the elemental composition of materials as diverse as crystals and semiconductors can be determined. Reaction kinetics and ion-molecule reactions can be studied using MS. 

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