Functional electrospray ionization mass spectromete

The basic function of the mass spectrometer is to measure the mass-to-charge ratios of analyte ions, and the various designs of mass spectrometers have been described in detail in the literature. The HPLC-MS system has four main components consisting of a sample inlet, an ion source, a mass analyzer, and finally an ion detector. The sample introduction system vaporizes the HPLC column effluent. The ion source produces ions from the neutral analyte molecules in the vapor phase. Several designs of ion sources have been used over the past years including electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), thermospray ionization (TSP), continuous flow fast atom bombardment (FAB), and atmospheric pressure photoionization (APPI). The inductively coupled plasma (ICP) is a hard ionization source and is used specifically for the detection of metals and metals in adducts or in organometallic compounds. Generally, ICP-MS is used for elemental speciation analysis with HPLC, which has been described elsewhere in... 

Under many experimental conditions, the mass spectrometer functions as a mass-sensitive detector, while in others, with LC-MS using electrospray ionization being a good example, it can behave as a concentration-sensitive detector. The reasons for this behaviour are beyond the scope of this present book (interested readers should consult the text by Cole [8]) but reinforce the need to ensure that adequate calibration and standardization procedures are incorporated into any quantitative methodology to ensure the validity of any results obtained. 

Mass spectrometers that use electrospray ionization (ESI) do not function well if the eluent contains low volatility salts. This is a major concern when an ion-exchange column is used as a first-dimension column and the salt concentration is used to modulate the retention in this column. In this case, another valve can be connected between the second-dimension column and the detector so that any salt from the second-dimension elution process that is either unretained or weakly retained can be diverted prior to feeding zones to the mass spectrometer. 

Figure 1 Components of a Mass Spectrometer and the Names of Those Commonly Used to Perform the Function of Ionization (Electrospray or MALDI), Analysis (Quadrupoles or Time-of-Flight), and Detection (Electron Multiplier)...

Kramer R. W., Kujawinski E. B., ZangX., Green-Church K. B., Jones R. B., Freitas M. A., and Hatcher P. G. (2001) Studies of the structure of humic substances by electrospray ionization coupled to a quadrupole-time of flight (QQ-TOF) mass spectrometer. In Humic Substances Structures, Models and Functions (eds. E. A. Ghabbour and G. Davies). The Royal Society of Chemistry, Cambridge, England, pp. 95-107. 

Mass spectrometers operate at high vacuum (Section 2.5), thus they can only analyze samples that are in the vapor state. Equally importantly, the neutral analyte molecules must be converted into ions. The functions of sample introduction systems are to produce vapors from samples (or reduce the pressure of gaseous samples) and to introduce a sufficient quantity of the sample into the ion source in such a way that its composition represents that of the original sample. It is important to note that the concept of sample introduction followed by ionization has changed with the development of recent techniques where the sample introduction and ionization process occur simultaneously. These techniques include atmospheric pressure ionization (API), particularly electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI). 

Electrospray ionization works best with preformed ions in solution and when preformed ions are separated from their counterions. In 1991 Blades et al. reported on the electrophoretic nature of electrospray, in which the charge balance requires the conversion of ions into electrons. Therefore, oxidation occurs at the needle (Fig. 8.5) and the interface of the mass spectrometer acts as a counterelectrode. Electrospray is particularly suitable for the analysis of inorganic ions and molecules, which have acidic or basic functional groups. Organic molecules are generally observed as protonated or deprotonated... 

Figure 5.38 Electrospray ionization peak areas measured as a function of flow rate for a solution of diltiazem injected (no HPLC column) into a flow of 50 50 acetonitrile water with 1 % acetic acid. A true mass flow dependent detector is predicted (Equation [4.5], Appendix 4.1) to yield peak areas independent of flow rate but this is clearly not observed in (a). A concentration dependent detector should give peak areas proportional to flow rate , and this is observed in (b) at lower flow rates but the peak area response falls below the extrapolated prediction as flow rate increases. Thus in this flow rate range the ESI mass spectrometer did not behave in accord with either of these idealized models.

To become an analytically useful mass spectrometer, the above combination still needs to be interfaced to an ion source. Tlie prototype implementation of the orbitrap made use of laser desorption/ionization in the immediate vicinity of the orbitrap [205], but soon, an external electrospray ion source followed [214]. For highest versatility, the orbitrap has been attached to the rear of a fully functional... 

Carnitine and its esters (see [1]) cannot be introduced to the mass spectrometer by gas chromatography, as they incorporate quaternary amine functions and will decompose in the attempt. Fast atom bombardment (FAB) and electrospray ionization (ESI) can use the formal charge on the quaternary amine function to advantage, as carnitine and its esters are very easily desorbed from glycerol on the FAB probe and from aerosol sprays in ESI. Eigure 7A illustrates the use of EAB in the quantitation of carnitine and its esters excreted in the urine of a patient presenting with a severe dicarboxylic aciduria associated with medium-chain acyl-CoA dehydrogenase (MCAD) deficiency. 

The mass spectrometry section (Chapter 8) has been completely revised and expanded in this edition, starting with more detailed discussion of a mass spectrometer s components. All of the common ionization methods are covered, including chemical ionization (Cl), fast-atom bombardment (FAB), matrix-assisted laser desorption ionization (MALDl), and electrospray techniques. Different types of mass analyzers are desalbed as well. Fragmentation in mass spectrometry is discussed in greater detail, and several additional fiagmentation mechanisms for common functional groups are illustrated. Numerous new mass spectra examples are also included. 

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