Mass spectrometry common types

Another type of ion is formed almost uniquely by the electrospray inlet/ion source which makes this technique so valuable for examining substances such as proteins that have large relative molecular mass. Measurement of m/z ratios usually gives a direct measure of mass for most mass spectrometry because z = 1 and so m/z = m/1 = m. Values of z greater than one are unusual. However, for electrospray, values of z greater than one (often much greater), are quite coimnonplace. For example, instead of the [M + H]+ ions common in simple Cl, ions in electrospray can be [M + n-H]- where n can be anything from 1 to about 30. 

GC/MS has been employed by Demeter et al. (1978) to quantitatively detect low-ppb levels of a- and P-endosulfan in human serum, urine, and liver. This technique could not separate a- and P-isomers, and limited sensitivity confined its use to toxicological analysis following exposures to high levels of endosulfan. More recently, Le Bel and Williams (1986) and Williams et al. (1988) employed GC/MS to confirm qualitatively the presence of a-endosulfan in adipose tissue previously analyzed quantitatively by GC/ECD. These studies indicate that GC/MS is not as sensitive as GC/ECD. Mariani et al. (1995) have used GC in conjunction with negative ion chemical ionization mass spectrometry to determine alpha- and beta-endosulfan in plasma and brain samples with limits of detection reported to be 5 ppb in each matrix. Details of commonly used analytical methods for several types of biological media are presented in Table 6-1. 

The most common final separation techniques used for agrochemicals are GC and LC. A variety of detection methods are used for GC such as electron capture detection (BCD), nitrogen-phosphorus detection (NPD), flame photometric detection (FPD) and mass spectrometry (MS). For LC, typical detection methods are ultraviolet (UV) detection, fluorescence detection or, increasingly, different types of MS. The excellent selectivity and sensitivity of LC/MS/MS instruments results in simplified analytical methodology (e.g., less cleanup, smaller sample weight and smaller aliquots of the extract). As a result, this state-of-the-art technique is becoming the detection method of choice in many residue analytical laboratories. 

The investigations directed at the synthesis of thymine-substituted polymers demonstrate that the type of functional groups displayed by nucleic acid bases are compatible with ROMP. Moreover, the application of MALDI-TOF mass spectrometry to the analysis of these polymers adds to the battery of tools available for the characterization of ROMP and its products. The utility of this approach for the creation of molecules with the desired biological properties, however, is still undetermined. It is unknown whether these thymine-substituted polymers can hybridize with nucleic acids. Moreover, ROMP does not provide a simple solution to the controlled synthesis of materials that display specific sequences composed of all five common nucleic acid bases. Nevertheless, the demonstration that metathesis reactions can be conducted with such substrates suggests that perhaps neobiopolymers that function as nucleic acid analogs can be synthesized by such processes. 

Analytical instrumental methods are commonly referred to by abbreviations using the first letters of the method s name (see Table 15.1). Thus, GC always refers to gas chromatography and MS to mass spectrometry. When these abbreviations are used, there is commonly no indication as to the type of GC (i.e., capillary, packed column, gas-solid, gas-liquid) being used. Likewise, no indication of the MS ionization method being used (i.e., El or Cl) is given. 

While competitive methods to determine KIE s are free from errors due to differences in reaction conditions (impurities, temperature, pH, etc.) they do require access to equipment that allows high precision measurements of isotope ratios. The selection of an appropriate analytical technique depends on the type of the isotope and its location in the molecule. For studies with stable isotopes the most commonly used technique (and usually the most appropriate) is isotope ratio mass spectrometry (IRMS). 

In many cases, the analytical tasks are simply to detect and quantify a specific known analyte. Examples include the detection and quantification of commonly used buffer components (e.g., Tris, acetate, citrate, MES, propylene glycol, etc.). These simple tasks can readily be accomplished by using a standard one-dimensional NMR method. In other situations, the analytical tasks may involve identifying unknown compounds. This type of task usually requires homonuclear and heteronuclear two-dimensional NMR experiments, such as COSY, TOCSY, NOESY, HSQC, HMBC, etc. The identification of unknown molecules may also require additional information from other analytical methods, such as mass spectrometry, UV-Vis spectroscopy, and IR spectroscopy.14... 

A wide number of sensor types have been described in the literature, from optical to mass spectrometry-based devices, but the sensors most commonly used in artificial tongues are electrochemical. 

Individual VOC. The term VOC is commonly used to describe speciated measurements of individual organics. The almost universal approach to the identification and measurement of individual VOC is GC with either FID or mass spectrometry (MS). GC-MS is used to establish the identity of a particular compound through the combination of retention times and mass spectra and, of course, can also be used for quantification. However, for a given type of air mass, GC-FID is commonly used for more extensive quantitative measurements after the individual peaks have been identified. For reviews of various aspects of sampling and measurement of VOC in air, see Westberg and Zimmerman (1993), Apel et al. (1994), Klemp et al. (1994), Sacks and Akard (1994), and Dewulf and Van Langen-hove (1997). 

Tandem mass spectrometers are just one type of mass spectrometer and there are many abbreviations and terms that are used commonly. Tandem mass spectrometers have been given many shorten names and abbreviations from tandem mass to TMS (many say they use these terms because of the challenge of pronouncing spectrometry). Unfortunately, these are incorrect abbreviations and it is essential to use tandem mass spectrometry with proper terms (TMS is and abbreviation for trimeth-ylsilyl derivatives used in gas chromatography/mass spectrometry). The acceptable shortened name or abbreviation is tandem MS (where mass spectrometry is abbreviated and it removes the challenging spectrometry pronunciation) or MS/MS [ 1 ]. 

This chapter provides some insight into the chemistry of a number of commonly used polymeric sorbents. Particular focus is placed on the chemical identification of contaminants typically associated with each of the following types of polymeric sorbents Amberlite XAD resins, Ambersorb XE resins, and PUF. Emphasis is placed on the chemical speciation of solvent-extractable organic contaminants present in a number of these sorbents as received from the manufacturer. Both qualitative and quantitative data on a micrograms-per-gram (parts-per-million) basis are provided as determined by combined gas chromatography-mass spectrometry (GC-MS). 

Studies of distonic ion radicals have been performed in recent years with an emphasis on theoretical approaches. From the experimental point of view, the presence of ionic moieties makes free radicals, which would not normally be investigated by mass spectrometry, amenable to detection in the gas phase. A lot of experiments were carried out to prove their existence and to observe their behavior in mass spectrometers see reviews (Kenttaemaa 1994 Hammerum 1988) and, for example, one recent experimental work (Polce Wesdemiotis 1996). At the next stage, syntheses of distonic ion radical organic salts stable under common conditions will likely be developed. These salts would be used to create magnetic, conductive, and other materials of practical use. In a chemical sense, the especial strength of distonic organic ion radicals is that they can, in principle, enter reactions of the ionic type at the charged center and reactions of the radical type at the radical center. 

To achieve the optimum reversed-phase LC separation, one needs to explore variables such as the analyte chemistry, mobile-phase composition (solvent type, solvent composition, pH, and additives), column composition, column particle size, and column temperature. For pharmaceutical analysis using mass spectrometry, the chemistry of an analyte is rarely changed beyond manipulation of the mobile phase pH, and even there options are limited. Volatile pH modifiers (buffers) are still preferred for LC-MS, and concentrations of these modifiers are kept low. Relatively simply mobile phases consisting of water, acetonitrile, and either formic acid (0.1% v/v), ammonium acetate (1-20 mM), or both have been common. 

Developments in mass spectrometry technology, together with the availability of extensive DNA and protein sequence databases and software tools for data mining, has made possible rapid and sensitive mass spectrometry-based procedures for protein identification. Two basic types of mass spectrometers are commonly used for this purpose Matrix-assisted laser desorption/ionization (MALDI)-time-of-flight (TOF) mass spectrometry (MS) and electrospray ionization (ESI)-MS. MALDI-TOF instruments are now quite common in biochemistry laboratories and are very simple to use, requiring no special training. ESI instruments, usually coupled to capillary/nanoLC systems, are more complex and require expert operators. We will therefore focus on the use of MALDI-... 

Mass spectrometric techniques are based on the measurement or counting of ions produced at high temperatures. An ion can be identified on the basis of its mass-to-charge ratio (m/z), characteristic of a certain isotope. In addition, quantification is based on the dependence between the number of ions and the concentration of a given isotope in the sample. Mass spectrometers consist of an ion source, a mass analyzer, and an ion detector. The ion source is typically the basis for the different types of mass spectrometric techniques. Plasmas are the most common ion sources for Mass spectrometric elemental determinations, and it is mass spectrometry (MS) using this ion source that will now be described. Complete details of this technique can be found in published monographs.29,30... 

They are still the workhorses of coupled mass spectrometric applications, as they are relatively simple to run and service, relatively inexpensive (for a mass spectrometer), and provide unit mass resolution and scanning speeds up to approximately 10,000 amu/s. This even allows for simultaneous scan/ selected ion monitoring (SIM) operation, in which one part of the data acquisition time is used to scan an entire spectrum, whereas the other part is used to record the intensities of selected ions, thus providing both qualitative information and sensitive quantitation. They are thus suitable for many GC-MS and liquid chromatography-mass spectrometry (LC-MS) applications. In contrast to GC-MS with electron impact (El) ionization, however, LC-MS provides only limited structural information as a consequence of the soft ionization techniques commonly used with LC-MS instruments [electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI)]. Because of this limitation, other types of mass spectrometers are increasingly gaining in importance for LC-MS. 

The spectroscopic methods chosen to characterize a particular compound will depend on the character of that compound as well as the type of information to be gained. For example, neutral clusters are readily characterized by mass spectrometry, whereas this method has been of little use for characterizing anionic clusters. Likewise, the subject of interest is the location of a hydride ligand or the site of phosphine or phosphite substitution, the most versatile tools are NMR spectroscopy and X-ray diffraction. We cannot discuss all the common characterization techniques in detail, but we shall highlight a few of the more important features. 

There are two main types of internal standards. The first ones are stable isotope labeled (SIL) internal standards. They are compounds in which several atoms in the analytes are replaced by their respective stable isotopes, such as deuterium (2H, D or d), 13C, 15N, or 170. Labeling with the first three isotopes are most common, particularly labeling with deuterium (due to less difficulty in synthesis and therefore less expensive). For examples, raloxifene-d4-6-glucuronide was used as the internal standard for the determination of raloxifene-6-glucuronide [5] and 1, 2, 3, 4-13C4 estrone (PCJEl) was used as the internal standard for estrone (El) [6], The usage of stable isotope labeled internal standards in quantitative LC-MS or GC-MS analysis is often termed as isotope dilution mass spectrometry (IDMS) [7],... 

The components in a mixture separate in the column and exit from the column at different times (retention times). As they exit, the detector registers the event and causes the event to be recorded as a peak on the chromatogram. A wide range of detector types are available and include ultraviolet adsorption, refractive index, thermal conductivity, flame ionization, fluorescence, electrochemical, electron capture, thermal energy analyzer, nitrogen-phosphorus. Other less common detectors include infrared, mass spectrometry, nuclear magnetic resonance, atomic absorption, plasma emission. 

---