Individual Mass Analyzers

The term Q/TOF is used to describe a type of hybrid mass spectrometer system in which a quadrupole analyzer (Q) is used in conjunction with a time-of-flight analyzer (TOP). The use of two analyzers together (hybridized) provides distinct advantages that cannot be achieved by either analyzer individually. In the Q/TOF, the quadrupole is used in one of two modes to select the ions to be examined, and the TOF analyzer measures the actual mass spectrum. Hexapole assemblies are also used to help collimate the ion beams. The hybrid orthogonal Q/TOF instrument is illustrated in Figure 23.1. 

The choice of a mass spectrometer to fulfill any particular task must take into account the nature of the substances to be examined, the degree of separation required for mixtures, the types of ion source and inlet systems, and the types of mass analyzer. Once these individual requirements have been defined, it is much easier to discriminate among the numerous commercial instruments that are available. Once suitable mass spectrometers have been identified, it is then often a case of balancing capital and running costs, reUability, ea.se of routine use, after-sales service, and manufacturer reputation. 

Figure 2.5. Tandem mass spectrometry. A. A peptide mixture is electrosprayed into the mass spectrometer. Individual peptides from the mixture are isolated (circled peptide) and fragmented. B. The fragments from the peptide are mass analyzed to obtain sequence information. The fragments obtained are derived from the N or C terminus of the peptide and are designated "b" or "y" ions, respectively. The spectrum shown indicates peptides that differ in size by the amino acids shown.

Test methods that analyze individual compounds (e.g., benzene-toluene-ethylbenzene-xylene mixtures and PAHs) are generally applied to detect the presence of an additive or to provide concentration data needed to estimate environmental and health risks that are associated with individual compounds. Common constituent measurement techniques include gas chromatography with second-column confirmation, gas chromatography with multiple selective detectors, and gas chromatography with mass spectrometry detection (GC/MS) (EPA 8240). 

The polymer sample (35 mg) was pyrolyzed in a quartz cell which was directly attached to the inlet flange of a quadrupole mass spectrometer. Gases evolved from the pol3raier compound were dynamically sampled via a 1.0-mm diameter orifice, formed into a modulated molecular beam, and mass analyzed. Information was obtained on the total yield of volatile products, product composition, and individual product yields as a function of temperature. 

Traditionally, particles have been collected and then analyzed for the distribution of mass and chemical composition. Various size ranges, or bins, have been used, ranging from simple cutoffs at 10 /im, for example, to multibin analyses in which particles in six or more size ranges are collected and analyzed individually. Such approaches have produced the vast majority of the data in the literature, and the techniques used are summarized briefly in the following sections. 

ESI tandem MS stands for electro spray ionization mass spectrometry performed in multistage. This technique is conducted based on the production of multiply charged ions from proteins and peptides. In this technique, ionization procedure is carried out within the instrument. Three types of mass analyzers are used individually or in combination. 

More ambitious attempts at measuring the heterogeneity of the atmospheric aerosol have been undertaken as well. Single-particle analysis by mass spectrometry was demonstrated by Sinha and co-workers (31, 32). In this technique, an aerosol sample is introduced into a vacuum chamber in the form of a particle beam. The particles are injected into a Knudsen cell oven, where they undergo many collisions with the cell wall and are ultimately vaporized and ionized. The ions are then mass-analyzed with a quad-rupole or sector mass spectrometer. So that individual particles can be analyzed, the flux of particles into the Knudsen cell is limited so that coincidence errors are minimized. Ion pulses from individual particles allow the determination of the amount of the species being analyzed in the particular particle. The sensitivity of the technique is limited. For sodium, the detection... 

It is often important to examine and analyze individual cells.3 For example, large numbers of single blood cells can be tested for the presence of specific antigenic determinants that arise by mutation. This permits assessment of the frequency of these mutations.b The complex chemical processing of neuropeptides can be studied on the contents of a single neuron (Chapter 30) using mass spectrometry.c... 

The information obtained from the detector is used to generate the mass spectrum. The mass spectrum is a plot of the intensity of the individual mass-analyzed ions plotted as a function of m/z. Usually the most intense ion, termed the base peak, is given a relative abundance of 100% and the rest of the ions in the mass spectrum are normalized to this intensity. Figure 5.1 shows the El mass spectrum of 2-methoxy-4-vinyl phenol (molecular weight 150 Daltons). The base peak is the molecular ion at m/z 150, a radical... 

Finally, one concept that must be included in assessing quantitation by HRMS is the effective scan rate of the system. Quadrupole and time of flight mass analyzer are capable of rapid scan rates for SRM-type quantitation, with individual dwell times (quad) or scans (TOF) at 10-50 milliseconds possible. This permits acquisition of numerous data points across a chromatographic peak, which is critical for accurate and precise quantitation. Mass resolution is unaffected by changes in dwell time/scan... 

The on-line hyphenation of CEC and MS has several potentially challenging instrumental aspects which complicate the successful combination of these two techniques. The first arises due to the absence of a CEC column outlet electrolyte reservoir and the need to achieve electrical continuity for the CEC system, and, in the case of ESI, also for the ESI ion source. Another consideration is the requirement to efficiently remove the mobile phase and simultaneously generate gas phase ions from the analyte, which have to be transferred with high efficiency into the vacuum of the mass analyzer. Because of this situation, numerous designs have been advanced which solve to various degrees these related problems and are discussed individually below. 

In order to compensate for variations during sample analysis (e.g. thermal instabilities, variability in flow rate and also electronic instability in the mass analyzer), samples are usually analyzed together with an internal standard, which is always added to the sample in the same amount. All measured peak areas or peak heights can be normalized on the signal of the internal standard, which helps to eliminate fluctuations during the individual measurement. 

A mass spectrometer analyzes the masses of individual molecules, not the weighted average mass of a group of molecules, so the whole-number masses of the most common Individual isotopes must be used to calculate the mass of the molecular ion. Thus, the mass of the molecular ion for CH4 should be 16. As a result, the mass spectrum of CH4 shows a line for the molecular ion—the parent peak or M peak—at miz =16. 

There are three main types of mass analyzers in ESTMS-MS instruments triple quadrupole, ion traps, and quadrupole-time-of-flight (Q-TOF). There are several differences between the mass analyzers in MALDI-TOF and in ESI-MS-MS. Unlike in MALDI-TOF-MS, in ESTMS-MS two mass analyzers are used in tandem to increase the sensitivity of the technique. The peptide ions produced by the ESI sources are carried to the first mass analyzer and only peptides of a set miz ratio are selected. The selected ions are then carried to a collision cell where they are subjected to additional fragmentation to produce smaller amino acid ions using a process called as collision induced dissociation (CID). The CID process employs inert gases such as argon for the dissociation of peptides. These smaller amino acid ions are then resolved in the second mass analyzer before sending to the detector. This process essentially enables highly sensitive detection of actual amino acid sequence of the peptides based on the mIz ratios of individual amino acids. 

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