Mass Spectrometry Analyzer

Table 2.4 Mass spectrometry analyzers and their features...

Proteomics proteomics, where mass spectrometry analyzes peptides derived from... 

The ToF mass analyzer is the fastest, has a broad m/z range, is one of most sensitive mass spectrometry analyzers available, and is well-suited to imaging applications. In ToF mass analysis, gas-phase ions produced by a pulsed ion source are accelerated in a high vacuum by an electric field. After being accelerated, the ions enter a field-free region between the ion source and detector at a vdocily related to their m/z ratio, with lower-m/z ions reaching the detector earlier than higher-m/z ions. 

Fenn and co-workers described electrospray ionization for mass spectrometry analyzing large biomolecules. ESI greatly enhanced MS ability for analysis of proteins or peptides. The mechanism of the ESI source is relatively simple. In an electrospray ionization source, the solution of analytes is nebulized into fine droplets via a capillary tube under a high electric field. The positive charges are accumulated on the surface of the droplets in this field. Later, because of evaporation of droplets, the surface coulombic forces exceed the surface tension and the droplets are dissociated into smaller droplets. This process continues until nanometer-sized droplets are formed. In this way, the ions pass from the source into the mass analyzer, whereas the bulk solvent is pumped away by a vacuum system. The stability limit (Rayleigh limit) of droplets is determined by the Coulomb forces of the accumulated positive... 

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. 

Before ehding this presentation on mass spectrometry, we should cite the existence of spectrometers for which the method of sorting ions coming from the source is different from the magnetic sector. These are mainly quadripolar analyzers and, to a lesser degree, analyzers measuring the ion s time of flight. 

In many applications in mass spectrometry (MS), the sample to be analyzed is present as a solution in a solvent, such as methanol or acetonitrile, or an aqueous one, as with body fluids. The solution may be an effluent from a liquid chromatography (LC) column. In any case, a solution flows into the front end of a mass spectrometer, but before it can provide a mass spectrum, the bulk of the solvent must be removed without losing the sample (solute). If the solvent is not removed, then its vaporization as it enters the ion source would produce a large increase in pressure and stop the spectrometer from working. At the same time that the solvent is removed, the dissolved sample must be retained so that its mass spectrum can be measured. There are several means of effecting this differentiation between carrier solvent and the solute of interest, and thermospray is just one of them. Plasmaspray is a variant of thermospray in which the basic method of solvent removal is the same, but the number of ions obtained is enhanced (see below). 

If a sample solution is introduced into the center of the plasma, the constituent molecules are bombarded by the energetic atoms, ions, electrons, and even photons from the plasma itself. Under these vigorous conditions, sample molecules are both ionized and fragmented repeatedly until only their constituent elemental atoms or ions survive. The ions are drawn off into a mass analyzer for measurement of abundances and mJz values. Plasma torches provide a powerful method for introducing and ionizing a wide range of sample types into a mass spectrometer (inductively coupled plasma mass spectrometry, ICP/MS). 

An AutoSpec-TOF mass spectrometer has a magnetic sector and an electron multiplier ion detector for carrying out one type of mass spectrometry plus a TOF analyzer with a microchannel plate multipoint ion collector for another type of mass spectrometry. Either analyzer can be used separately, or the two can be run in tandem (Figure 20.4). 

The previous discussion has centered on how to obtain as much molecular mass and chemical structure information as possible from a given sample. However, there are many uses of mass spectrometry where precise isotope ratios are needed and total molecular mass information is unimportant. For accurate measurement of isotope ratio, the sample can be vaporized and then directed into a plasma torch. The sample can be a gas or a solution that is vaporized to form an aerosol, or it can be a solid that is vaporized to an aerosol by laser ablation. Whatever method is used to vaporize the sample, it is then swept into the flame of a plasma torch. Operating at temperatures of about 5000 K and containing large numbers of gas ions and electrons, the plasma completely fragments all substances into ionized atoms within a few milliseconds. The ionized atoms are then passed into a mass analyzer for measurement of their atomic mass and abundance of isotopes. Even intractable substances such as glass, ceramics, rock, and bone can be examined directly by this technique. 

After acceleration through an electric field, ions pass (drift) along a straight length of analyzer under vacuum and reach a detector after a time that depends on the square root of their m/z values. The mass spectrum is a record of the abundances of ions and the times (converted to m/z) they have taken to traverse the analyzer. TOP mass spectrometry is valuable for its fast response time, especially for substances of high mass that have been ionized or selected in pulses. 

Laser-desorption mass spectrometry (LDMS) or matrix-assisted laser desorption ionization (MALDI) coupled to a time-of-flight analyzer produces protonated or deprotonated molecular ion clusters for peptides and proteins up to masses of several thousand. 

Peptides and proteins can be analyzed by mass spectrometry. Molecular mass information can be obtained particularly well by MALDI and ESI. 

The combined techniques of gas chromatography/mass spectrometry (gc/ms) are highly effective in identifying the composition of various gc peaks. The individual peaks enter a mass spectrometer in which they are analyzed for parent ion and fragmentation patterns, and the individual components of certain resoles are completely resolved. 

The main advantages of the ms/ms systems are related to the sensitivity and selectivity they provide. Two mass analyzers in tandem significantly enhance selectivity. Thus samples in very complex matrices can be characterized quickly with Htde or no sample clean-up. Direct introduction of samples such as coca leaves or urine into an ms or even a gc/lc/ms system requires a clean-up step that is not needed in tandem mass spectrometry (28,29). Adding the sensitivity of the electron multiplier to this type of selectivity makes ms/ms a powerhil analytical tool, indeed. It should be noted that introduction of very complex materials increases the frequency of ion source cleaning compared to single-stage instmments where sample clean-up is done first. 

Hypochlorous acid reacts similarly to CI2O and cannot be distinguished from CI2O by wet analysis. Gaseous mixtures of CI2, CI2O, and HOCl can be analyzed by mass spectrometry (66,67), by in (68), or by uv spectrophotometry (9,66,69). Gas chromatography can be used to analyze mixtures of air, CI2, and CI2O (54). 

One of the reasons for lack offlterature was probably because environmental analysis depends heavily on gas chromatography/mass spectrometry, which is not suitable for most dyes because of their lack of volatility (254). However, significant progress is being made in analyzing nonvolatile dyes by newer mass spectral methods such as fast atom bombardment (EAB), desorption chemical ionization, thermospray ionization, etc. 

Solid-phase microextraction (SPME) was used for headspace sampling. The FFA were extracted from the headspace with PA, Car/PDMS, and CW/DVB fibers. It was examined whether addition of salt (NaCl) and decreasing the pH by addition of sulphuric acid (H SO ) increased the sensitivity. FFA were analyzed using gas chromatography coupled to mass spectrometry in selected ion monitoring. 

A liquid chromatography-mass spectrometry (LC-MS) method that can quantitatively analyze urinar y normal and modified nucleosides in less than 30 min with a good resolution and sufficient sensitivity has been developed. Nineteen kinds of normal and modified nucleosides were determined in urine samples from 10 healthy persons and 18 breast cancer patients. Compounds were separ ated on a reverse phase Kromasil C18 column (2.1 mm I.D.) by isocratic elution mode using 20 mg/1 ammonium acetate - acetonitrile (97 3 % v/v) at 200 p.l/min. A higher sensitivity was obtained in positive atmospheric pressure chemical ionization mode APCI(-i-). 

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