Mass spectrometry capabilities

The increasing accessibility of bench-top LC-MS systems to researchers of all disciplines, combined with the tandem and high-resolution mass spectrometry capabilities of such instruments, will only increase the number of applications to which LC-MS can be directed. The examples documented in this chapter illustrate some of the diversity and power of the techniques, including analytical applications for known analytes in various matrices, metabolomic analysis, the tentative structural identification of novel compounds, and the screening of extracts for minor, and perhaps novel, components of the alkaloidal profile of plants. 

In order to reveal molecular ions, we moved to desorption/chemical ionization mass spectrometry. This direct chemical ionization (DCI), initially described by Me Lafferty in 1973 68 , was widely used and improved recently with the aim of decreasing sharply thermal degradation of samples under vaporization and to extend mass spectrometry capability both to poorly volatile and to very fragile compounds having high molecular masses 69-74). 

Quadrupole ion trap mass analyzers merge the trapping characteristics of the ICR with the physical principles of the linear quadrupole mass analyzer. Quadrupole ion traps produce time-dependent spectra with excellent sensitivity and tandem mass spectrometry capabilities, but unlike the ICR they provide these ion trapping characteristics with physically smaller and considerably less expensive instrumentation, giving them a reputation as a powerful and accessible tool for both qualitative and quantitative mass spectrometry [43-47]. The capability of quadrupole ion traps to be configured with either internal and external ionization sources has expanded their utility for modem analytical applications [48—52]. 

When combined with tandem mass spectrometry, capable of selectively detecting a few analytes from the many that could be present, this approach provides for unsurpassed analytical selectivity for difficult chemical problems such as the study of drug—receptor binding [36] or the separation of complex mixtures of proteins or peptides [37]. The detection approach can be implemented in either on- or off-line formats. Alternatively, the purified antibody can be immobilized on a matrix-assisted laser-desorption ionization probe to allow direct application and characterization of a liquid sample containing the target molecule [38]. 

Campbell, J.M., Collins, B.A., and Douglas, DJ. (1998). A Linear Ion Trap Time-of-flight System with Tandem Mass Spectrometry Capabilities, Rapid Commun. Mass Spectrom., 12,1463-1474. 

Campbell, J.M. Collings, B.A. Douglas, D.J. A new linear ion trap time-of-fiight system with tandem mass spectrometry capabilities. Rapid Commun. Mass Spectrom. 1998, 12, 1463-1474. 

Marabini, A. M., Passariello, B., and Barbaro, M. (1992). Inductively coupled plasma-mass spectrometry Capabilities and applications. Microchem.J. 46(3), 302-312. 

The result is the ability to couple TLC separations with the high resolution, high mass accuracy, and tandem mass spectrometry capabilities of modern mass spectrometers. It is important to note that the sensitivity of these TLC MALDI methods varies with the laser used. Since the IR lasers have a larger interaction volume with the sample, they showed improved sensitivity compared to UV-MALDI. ... 

There are otlier teclmiques for mass separation such as tire quadmpole mass filter and Wien filter. Anotlier mass spectrometry teclmique is based on ion chromatography, which is also capable of measuring tire shapes of clusters [30, 31]. In tills metliod, cluster ions of a given mass are injected into a drift tube witli well-defined entrance and exit slits and filled witli an inert gas. The clusters drift tlirough tills tube under a weak electric potential. Since the... 

Tandem mass spectrometry or ms/ms was first introduced in the 1970s and gained rapid acceptance in the analytical community. The technique has been used for stmcture elucidation of unknowns (26) and has the abiUty to provide sensitive and selective analysis of complex mixtures with minimal sample clean-up (27). Developments in the mid-1980s advancing the popularity of ms/ms included the availabiUty of powerhil data systems capable of controlling the ms/ms experiment and the viabiUty of soft ionisation techniques which essentially yield only molecular ion species. 

The analytical techniques covered in this chapter are typically used to measure trace-level elemental or molecular contaminants or dopants on surfaces, in thin films or bulk materials, or at interfaces. Several are also capable of providing quantitative measurements of major and minor components, though other analytical techniques, such as XRF, RBS, and EPMA, are more commonly used because of their better accuracy and reproducibility. Eight of the analytical techniques covered in this chapter use mass spectrometry to detect the trace-level components, while the ninth uses optical emission. All the techniques are destructive, involving the removal of some material from the sample, but many different methods are employed to remove material and introduce it into the analyzer. 

Other technique—for example, dynamic secondary ion mass spectrometry or forward recoil spectrometry—that rely on mass differences can use the same type of substitution to provide contrast. However, for hydrocarbon materials these methods attain a depth resolution of approximately 13 nm and 80 nm, respectively. For many problems in complex fluids and in polymers this resolution is too poor to extract critical information. Consequently, neutron reflectivity substantially extends the depth resolution capabilities of these methods and has led, in recent years, to key information not accessible by the other techniques. 

The combination of chromatography and mass spectrometry (MS) is a subject that has attracted much interest over the last forty years or so. The combination of gas chromatography (GC) with mass spectrometry (GC-MS) was first reported in 1958 and made available commercially in 1967. Since then, it has become increasingly utilized and is probably the most widely used hyphenated or tandem technique, as such combinations are often known. The acceptance of GC-MS as a routine technique has in no small part been due to the fact that interfaces have been available for both packed and capillary columns which allow the vast majority of compounds amenable to separation by gas chromatography to be transferred efficiently to the mass spectrometer. Compounds amenable to analysis by GC need to be both volatile, at the temperatures used to achieve separation, and thermally stable, i.e. the same requirements needed to produce mass spectra from an analyte using either electron (El) or chemical ionization (Cl) (see Chapter 3). In simple terms, therefore, virtually all compounds that pass through a GC column can be ionized and the full analytical capabilities of the mass spectrometer utilized. 

The power of mass spectrometry lies in the fact that the mass spectra of many compounds are sufficiently specific to allow their identification with a high degree of confidence, if not with complete certainty. If the analyte of interest is encountered as part of a mixture, however, the mass spectrum obtained will contain ions from all of the compounds present and, particularly if the analyte of interest is a minor component of that mixture, identification with any degree of certainty is made much more difficult, if not impossible. The combination of the separation capability of chromatography to allow pure compounds to be introduced into the mass spectrometer with the identification capability of the mass spectrometer is clearly therefore advantageous, particularly as many compounds with similar or identical retention characteristics have quite different mass spectra and can therefore be differentiated. This extra specificity allows quantitation to be carried out which, with chromatography alone, would not be possible. 

Mass spectrometry (see Chapter 3) is capable of providing molecular weight and structural information from picogram amounts of material and to provide selectivity by allowing the monitoring of ions or ion decompositions characteristic of a single analyte of interest. These are the ideal characteristics of both a qualitative and a quantitative detector. 

Before considering these fonr scan modes in detail, it is worthwhile considering the types of instrnment that have MS-MS capability because, as two stages of mass spectrometry are involved, not all systems will provide this facility. 

Reliable analytical methods are available for determination of many volatile nitrosamines at concentrations of 0.1 to 10 ppb in a variety of environmental and biological samples. Most methods employ distillation, extraction, an optional cleanup step, concentration, and final separation by gas chromatography (GC). Use of the highly specific Thermal Energy Analyzer (TEA) as a GC detector affords simplification of sample handling and cleanup without sacrifice of selectivity or sensitivity. Mass spectrometry (MS) is usually employed to confirm the identity of nitrosamines. Utilization of the mass spectrometer s capability to provide quantitative data affords additional confirmatory evidence and quantitative confirmation should be a required criterion of environmental sample analysis. Artifactual formation of nitrosamines continues to be a problem, especially at low levels (0.1 to 1 ppb), and precautions must be taken, such as addition of sulfamic acid or other nitrosation inhibitors. The efficacy of measures for prevention of artifactual nitrosamine formation should be evaluated in each type of sample examined. 

Specifically for triazines in water, multi-residue methods incorporating SPE and LC/MS/MS will soon be available that are capable of measuring numerous parent compounds and all their relevant degradates (including the hydroxytriazines) in one analysis. Continued increases in liquid chromatography/atmospheric pressure ionization tandem mass spectrometry (LC/API-MS/MS) sensitivity will lead to methods requiring no aqueous sample preparation at all, and portions of water samples will be injected directly into the LC column. The use of SPE and GC or LC coupled with MS and MS/MS systems will also be applied routinely to the analysis of more complex sample matrices such as soil and crop and animal tissues. However, the analyte(s) must first be removed from the sample matrix, and additional research is needed to develop more efficient extraction procedures. Increased selectivity during extraction also simplifies the sample purification requirements prior to injection. Certainly, miniaturization of all aspects of the analysis (sample extraction, purification, and instrumentation) will continue, and some of this may involve SEE, subcritical and microwave extraction, sonication, others or even combinations of these techniques for the initial isolation of the analyte(s) from the bulk of the sample matrix. 

In the case of the low abundance of some compounds, there are difficulties with signal overlap. To overcome these difficulties, there have been developments involving NMR hyphenation with techniques such as HPLC and mass spectrometry. In LC/NMR methods of analysis, NMR is used as the detector following LC separation and this technique is capable of detecting low concentrations in the nanogram range. This technique has been reported for the detection and identification of flavanoids in fruit juices and the characterization of sugars in wine [17]. 

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