Mass spectrometer principle

In recent years several new instruments have been developed based on different mass-spectrometer principles. Two different categories of ICP-MS instruments are currently commercially available low-resolution instruments (using either QMS, ITMS or ToF-MS) and focusing high-resolution instruments (DFS, FTMS). Selected specifications for these two categories are shown in Table 8.63. Both the quadrupole-based and the double-focusing instruments allow a sequential multielement measurement, whereas ICP-ToFMS allows... 

Some Important Points in Mass Spectrometry Mass Spectrometer Principle of Operation Interpretation of Some Spectra Mass Spectrum of Benzene, of 2,2,4 Trimethyl Pentane 265... 

The principles of operation of quadnipole mass spectrometers were first described in the late 1950s by Wolfgang Paul who shared the 1989 Nobel Prize in Physics for this development. The equations governing the motion of an ion in a quadnipole field are quite complex and it is not the scope of the present article to provide the reader with a complete treatment. Rather, the basic principles of operation will be described, the reader being referred to several excellent sources for more complete infonnation [13, H and 15]. 

Figure Bl.25.8. The principle of SIMS Primary ions with an energy between 0.5 and 10 keV cause a collisional cascade below the surface of the sample. Some of the branches end at the surface and stimulate the emission of neutrals and ions. In SIMS, the secondary ions are detected directly with a mass spectrometer.

The basic principles of fast-atom bombardment (FAB) and liquid-phase secondary ion mass spectrometry (LSIMS) are discussed only briefly here because a fuller description appears in Chapter 4. This chapter focuses on the use of FAB/LSIMS as part of an interface between a liquid chromatograph (LC) and a mass spectrometer (MS), although some theory is presented. 

Nebulizers are used to introduce analyte solutions as an aerosol spray into a mass spectrometer. For use with plasma torches, it is necessary to produce a fine spray and to remove as much solvent as possible before the aerosol reaches the flame of the torch. Various designs of nebulizer are available, but most work on the principle of interacting gas and liquid streams or the use of ultrasonic devices to cause droplet formation. For nebulization applications in thermospray, APCI, and electrospray, see Chapters 8 and 11. 

The effort to clarify and articulate the principles of spectrometry as simply as possible appears to have struck a welcome response among those who use mass spectrometers. The original series was not advertised, but, once it had been discovered outside the Micromass organization, there were inquiries as to how it could be purchased. From the start. Micromass offered the series free in a ring-binder format and later also provided it free on CD ROM and then on the Internet. Updating of the first CD led to over 600 requests for it on the day after its release had been announced on the Internet. Partly because of this response — but mainly because users or potential users frequently like to have a traditional reference book — it was decided to publish the series in this present book form. 

Many physical principles can be employed by different sensor vendors to obtain the same measurement. For example, in 1995, 49 vendors were listed for hydrogen sensing (2). Represented among the sensor systems are mass spectrometers, gas chromatographs, electrochemical cells, thermal... 

Atomic masses calculated in this manner, using data obtained with a mass spectrometer can in principle be precise to seven or eight significant figures. The accuracy of tabulated atomic masses is limited mostly by variations in natural abundances. Sulfur is an interesting case in point. It consists largely of two isotopes, fiS and fgS. The abundance of sulfur-34 varies from about 4.18% in sulfur deposits in Texas and Louisiana to 4.34% in volcanic sulfur from Italy. This leads to an uncertainty of 0.006 amu in the atomic mass of sulfur. 

Principles and Characteristics A mass spectrometer consists of various components which are necessary for the formation of ions from molecules, and for their separation and detection (Fig. 6.1). Miniaturisation of MS represents a strategic technology. 

Principles and Characteristics Mass spectrometry can provide the accurate mass determination in a direct measurement mode. For a properly calibrated mass spectrometer the mass accuracy should be expected to be good to at least 0.1 Da. Accurate mass measurements can be made at any resolution (resolution matters only when separating masses). For polymer/additive deformulation the nominal molecular weight of an analyte, as determined with an accuracy of 0.1 Da from the mass spectrum, is generally insufficient to characterise the sample, in view of the small mass differences in commercial additives. With the thousands of additives, it is obvious that the same nominal mass often corresponds to quite a number of possible additive types, e.g. NPG dibenzoate, Tinuvin 312, Uvistat 247, Flexricin P-1, isobutylpalmitate and fumaric acid for m = 312 Da see also Table 6.7 for m = 268 Da. Accurate mass measurements are most often made in El mode, since the sensitivity is high, and reference mass peaks are readily available (using various fluorinated reference materials). Accurate mass measurements can also be made in Cl... 

Principles and Characteristics Ionisation processes are the basis for mass-spectrometric detection. Each of the ionisation techniques occupies its own position in mass spectrometry. The optimum performance of any ionisation method (and therefore the result) will depend critically on the characteristics and reliability of the mass spectrometer. Ionisation may occur in the gas, liquid or condensed phase, and may be either hard or soft , i.e. with or without extensive... 

Principles and Characteristics Continuous-flow (or dynamic) FAB/FTB [102] and frit FAB/F1B [103] offer a means of introducing samples in solution into a continuous flow of solvent which terminates at the modified FAB/FIB probe tip, and they extend the applicability of FAB. Samples are injected through a conventional HPLC injection valve, or solutions are simply drawn in by the high vacuum in the ionisation source of the mass spectrometer. These very similar techniques are particularly amenable to coupling with HPLC columns, and ionisation of the sample is unchanged with respect to conventional FAB and FIB/LSIMS. 

Principles and Characteristics Thermospray ionisation (TSP) involves introduction of a relatively high flow (0.2-2mLmin ) of solvent into the ion source of a mass spectrometer, and is therefore suitable as an interface for HPLC-MS, using standard bore columns. A vaporiser probe (essentially a resistively heated capillary tube of about 100 xm i.d.) acts as a transfer line for taking solvent and solute into the source. The source is heated to prevent condensation of the solvent, and the temperature of the capillary is chosen so as to ensure vaporisation of the solvent. In this way, a vapour jet is generated, which contains small, electrically charged droplets if the solvent is at least partially aqueous and... 

Principles and Characteristics Analytical multistage mass spectrometry (MSn) relies on the ability to activate and dissociate ions generated in the ion source in order to identify or obtain structural information about an unknown compound and to analyse mixtures by exploiting two or more mass-separating steps. A basic instrument for the currently most used form, tandem mass spectrometry (MS/MS), consists of a combination of two mass analysers with a reaction region between them. While a variety of instrument set-ups can be used in MS/MS, there is a single basic concept involved the measurement of the m/z of ions before and after a reaction in the mass spectrometer the reaction involves a change in mass and can be represented as ... 

The principle of MS/MS for direct analysis of a multicomponent system is shown in Figure 6.18, in which the first mass spectrometer (MS I) operates with soft ionisation (FI, FD, Cl, LD), and thus produces an ensemble of molecular ions (M + H+, M — H+, or adducts). For identification of molecule ABC only ABC+ is allowed to enter an interface or fragmentation zone for excitation by collisional activation, laser radiation or surface-induced dissociation. Within the time of one vibration (10-13s), ABC+ dissociates into fragments characterising the original molecule. They are separated and detected by MS II [226]. Soft ionisation with FI/FD produces low ion yields, which may be insufficient for MS/MS LVEI (typically at 20 V) can be an alternative. Complete analysis of a multicomponent system is carried out in some 20 min. 

Principles and Characteristics Mass-spectral analysis methods may be either indirect or direct. Indirect mass-spectral analysis usually requires some pretreatment (normally extraction and separation) of the material, to separate the organic additives from the polymers and inorganic fillers. The mass spectrometer is then used as a detector. Direct mass-spectrometric methods have to compete with separation techniques such as GC, LC and SFC that are more commonly used for quantitative analysis of polymer additives. The principal advantage of direct mass-spectrometric examination of compounded polymers (or their extracts) is speed of analysis. However, quite often more information can be... 

Principles and Characteristics Ion mobility spectrometry (IMS) is an instrumental technique for the detection and characterisation of organic compounds as vapours at atmospheric pressure. Modern analytical IMS was created at the end of the 1960s from studies on ion-molecule chemistry with mass spectrometers and from ionisation detectors for vapour monitoring. An ion mobility spectrometer (or plasma chromatograph in the original termininology) was first produced in 1970 [272],... 

Principles and Characteristics Extraction or dissolution methods are usually followed by a separation technique prior to subsequent analysis or detection. While coupling of a sample preparation and a chromatographic separation technique is well established (Section 7.1), hyphenation to spectroscopic analysis is more novel and limited. By elimination of the chromatographic column from the sequence precol-umn-column-postcolumn, essentially a chemical sensor remains which ensures short total analysis times (1-2 min). Examples are headspace analysis via a sampling valve or direct injection of vapours into a mass spectrometer (TD-MS see also Section 6.4). In... 

Principles and Characteristics The most commonly used and widely known technique coupled to gas chromatography is mass spectroscopy. In early couplings, with packed GC columns being the major problem encountered, elimination of the carrier gas at relatively high flow-rates (25mLmin 1), resulted in a significant loss of sensitivity. With the advent of open tubular devices, the column flow could be passed directly into the mass spectrometer. GC-MS is typically... 

Principles and Characteristics With sample spots in a developed thin-layer chromatogram, there are no constraints on the operation of the mass spectrometer. Depending on the analytical information required, either low- or high-resolution mass-spectral data can be recorded, and both positive- and negative-ion mass spectra can be obtained from the same sample spot. 

Principles and Characteristics The main reasons for hyphenating MS to CE are the almost universal nature of the detector, its sensitivity and the structural information obtainable, including assessment of peak purity and identity. As CE is a liquid-phase separation technique, coupling to the mass spectrometer can be achieved by means of (modified) LC-MS interfaces. Because of the low flow-rates applied in CE, i.e. typically below lOOnLmin-1, a special coupling device is required to couple CE and the LC-MS interface. Three such devices have been developed, namely a... 

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