Analytical methods mass spectrometers

A connnon feature of all mass spectrometers is the need to generate ions. Over the years a variety of ion sources have been developed. The physical chemistry and chemical physics communities have generally worked on gaseous and/or relatively volatile samples and thus have relied extensively on the two traditional ionization methods, electron ionization (El) and photoionization (PI). Other ionization sources, developed principally for analytical work, have recently started to be used in physical chemistry research. These include fast-atom bombardment (FAB), matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ES). 

Quantitative mass spectrometry, also used for pharmaceutical appHcations, involves the use of isotopicaHy labeled internal standards for method calibration and the calculation of percent recoveries (9). Maximum sensitivity is obtained when the mass spectrometer is set to monitor only a few ions, which are characteristic of the target compounds to be quantified, a procedure known as the selected ion monitoring mode (sim). When chlorinated species are to be detected, then two ions from the isotopic envelope can be monitored, and confirmation of the target compound can be based not only on the gc retention time and the mass, but on the ratio of the two ion abundances being close to the theoretically expected value. The spectrometer cycles through the ions in the shortest possible time. This avoids compromising the chromatographic resolution of the gc, because even after extraction the sample contains many compounds in addition to the analyte. To increase sensitivity, some methods use sample concentration techniques. 

Each type of mass spectrometer has its associated advantages and disadvantages. Quadrupole-based systems offer a fairly simple ion optics design that provides a certain degree of flexibility with respect to instrument configuration. For example, quadrupole mass filters are often found in hybrid systems, that is, coupled with another surface analytical method, such as electron spectroscopy for chemical analysis or scanning Auger spectroscopy. 

Although GC/MS is the most widely used analytical method that combines a chromatographic separation with the identification power of mass spectrometry, it is not the only one. Chemists have coupled mass spectrometers to most of the instruments that are used to separate mixtures. Perhaps the ultimate is mass spectrometry/mass spectrometry (MS/MS), in which one mass spectrometer generates and separates the molecular ions of the components of a mixture and a second mass spectrometer examines their fragmentation patterns ... 

Electrospray ionization mass spectrometry (ESI-MS) is an analytical method for mass determination of ionized molecules. It is a commonly used method for soft ionization of peptides and proteins in quadmpole, ion-trap, or time-of-flight mass spectrometers. The ionization is performed by application of a high voltage to a stream of liquid emitted from a capillaty. The highly charged droplets are shrunk and the resulting peptide or protein ions are sampled and separated by the mass spectrometer. 

When optimum experimental conditions have been obtained, all of the mobile phase is removed before the analyte(s) are introduced into the mass spectrometer for ionization. As a consequence, with certain limitations, it is possible to choose the ionization method to be used to provide the analytical information required. This is in contrast to the other LC-MS interfaces which are confined to particular forms of ionization because of the way in which they work. The moving belt can therefore provide both electron and chemical ionization spectra, yielding both structural and molecular weight information. 

In contrast to most other ionization methods, the majority of ions produced by electrospray are multiply charged. This is of great significance as the mass spectrometer measures the m/z (mass-to-charge) ratio of an ion and the mass range of an instrument may therefore be effectively extended by a factor equivalent to the number of charges residing on the analyte molecule, i.e. an ion of m/z 1000 with... 

In many cases when methods involve internal or external standards, the solutions used to construct the calibration graph are made up in pure solvents and the signal intensities obtained will not reflect any interaction of the analyte and internal standard with the matrix found in unknown samples or the effect that the matrix may have on the performance of the mass spectrometer. One way of overcoming this is to make up the calibration standards in solutions thought to reflect the matrix in which the samples are found. The major limitation of this is that the composition of the matrix may well vary widely and there can be no guarantee that the matrix effects found in the sample to be determined are identical to those in the calibration standards. 

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. 

The method for chloroacetanilide soil metabolites in water determines concentrations of ethanesulfonic acid (ESA) and oxanilic acid (OXA) metabolites of alachlor, acetochlor, and metolachlor in surface water and groundwater samples by direct aqueous injection LC/MS/MS. After injection, compounds are separated by reversed-phase HPLC and introduced into the mass spectrometer with a TurboIonSpray atmospheric pressure ionization (API) interface. Using direct aqueous injection without prior SPE and/or concentration minimizes losses and greatly simplifies the analytical procedure. Standard addition experiments can be used to check for matrix effects. With multiple-reaction monitoring in the negative electrospray ionization mode, LC/MS/MS provides superior specificity and sensitivity compared with conventional liquid chromatography/mass spectrometry (LC/MS) or liquid chromatography/ultraviolet detection (LC/UV), and the need for a confirmatory method is eliminated. In summary,... 

Analytical standards are prepared for two purposes for fortifying control matrices to determine the analytical accuracy and for calibrating the response of the analyte in the mass spectrometer detector. The purity of all standards must be verified before preparation of the stock solutions. All standards should be refrigerated (2-10 °C) in clean amber-glass bottles with foil/Tefion-lined screw-caps. The absolute volume of the standard solutions may be varied at the discretion of the analyst, as long as the correct proportions of the solute and solvent are maintained. Calibrate the analytical balance before weighing any analytical standard material for this method. 

Currently, HPLC/fiuorescence is still the most common technique for the determination of residues of oxime carbamates. With the introduction of ESI and APCI MS interfaces, HPLC/MS analysis for oxime carbamates in various sample matrices has become widespread. However, for a rapid, sensitive, and specific analysis of biological and environmental samples, HPLC/MS/MS is preferred to HPLC/MS and HPLC/fiuorescence. With time, improved and affordable triple-quadrupole mass spectrometers will be available in more analytical laboratories. With stricter regulatory requirements, e.g., highly specific and conclusive methods with lower LOQ, HPLC/MS/MS will be a method of choice for oxime carbamates and their metabolites. 

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