Ionization methods, mass photoionization

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). 

In this chapter we have discussed the successful implementation in our laboratory, for the first time, of the soft (i.e. low energy) electron-impact ionization method for product detection in crossed molecular beams reactive scattering experiments with mass spectrometric detection. Analogous to the approach of soft photoionization by tunable VUV synchrotron radiation,... 

D. R. Robb, T. R. Covey, and A. P. Bruins. Atmospheric Pressure Photoionization An Ionization Method for Liquid Chromatography-Mass Spectrometry. Anal. Chem., 72(2000) 3653-3659. 

A third source of error is associated with the fragmentation pattern caused by dissociation of the molecular ions formed in the source region of the spectrometer. Under severe conditions these processes may proceed with substantial isotopic fractionation, and this obscures the measurements of isotopic composition at the collector. To some extent careful standardization of the instrumental conditions may ensure that errors from fragmentation are systematic, and thus cancel (at least to some extent). Alternatively, softer ionization methods can be used to prevent most or all of the fragmentation. The bottom spectrum in Fig. 7.7 illustrates this approach it shows the mass spectrum of chlorobenzene obtained by photoionization. Only the parent molecular ions are observed. It should be kept in mind, however, that softer ionization usually yields smaller ion currents and consequently statistical counting errors increase. 

Robb, D. B., Covey, T. R., and Bruins, A. P. (2000). Atmospheric pressure photoionization an ionization method for liquid chromatography-mass spectrometry. Anal. Chem. 72, 3653 — 3659. 

Atmospheric pressure photoionization (APPI) was recently introduced in the world of atmospheric pressure ionization techniques to analyze nonpolar molecules that are not efficiently ionized either by ESI or by APCI. Photoionization (PI) was already exploited some 30 years ago as a detection method for GC and LC, but only in recent times it has been used as an ionization method for mass spectrometry [52],... 

Analytical methods have been developed to measure benzene levels in exhaled breath, blood, and various body tissues. The primary method of analyzing for benzene in exhaled breath, body fluids and tissues is gas chromatography (GC) coupled with either flame ionization detection (FID), photoionization detection (PID), or mass spectrometry (MS). Rigorous sample collection and preparation methods must be followed when analyzing for benzene to prevent contamination of the sample. A summary of commonly used methods of measuring benzene in biological samples is presented in Table 6-1. 

Electrospray is surely the ionization method most widely employed for the liquid chromatography (LC)-MS coupling (Cappiello, 2007). The possibility of performing ionization at atmospheric pressure [also obtained in the case of atmospheric pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI), allows the direct analysis of analyte solutions. However, some problems arise from the intrinsically different operative conditions of the two analytical methods. First, there are the high-vacuum conditions that must be present at the mass analyzer level. Second, the mass spectrometers generally exhibit a low tolerance for the nonvolatile mobile-phase components, usually employed in LC conditions to achieve high chromatographic resolution. 

Robb DB, Covey TR, Bruins AP. Atmospheric pressure photoionization An ionization method for liquid chromatography—mass spectrometry. Anal Chem 2000 72 (15) 3653-3659. 

Atmospheric pressure ionization (API) techniques encompass a range of techniques in which ionization occurs external to the mass spectrometer vacuum. Ionization can be achieved by a variety of methods, including photoionization, corona discharge at the tip of a needle, or by the use of radioisotopes such as Ni. 

In the mid-1960s, the availability of tunable ultraviolet radiation laboratory sources led to the replacement of electron beams with photon sources. Rare gas resonance lamps producing continuum sources of radiation that could then be passed through monochromators allowed the field of photoionization mass spectrometry to develop. The determination of photoion yield as a function of ionization energy, the photoionization efficiency curve, led to determination of ionization potentials with accuracies exceeding those of electron impact methods. Modern photoionization experiments often utilize laser or synchrotron light sources with narrow bandwidths and may employ collimated molecular beam sources that reduce the effects of... 

For these classes of substances, other methods have been developed, such as the coupling of ESI with an electrochemical cell [20-31], the coordination ion-spray [31-46], or the dissociative electron-capture ionization [37-41]. The APPI or the dopant-assisted (DA) APPI presented by Syage et al. [42, 43] and Robb et al. [44,45], respectively, are relatively new methods for photoionization (PI) of nonpolar substances by means of vacuum ultraviolet (VUV) radiation. Both techniques are based on photoionization, which is also used in ion mobility mass spectrometry [46-49] and in the photoionization detector (PID) [50- 52]. 

Robb, D.B. and Bruins, A.P. (2001). Atmospheric pressure photoionization (APPI) A new ionization method for liquid chromatography - mass spectrometry. Patent No. W00133605. 

In terms of the hardware, TRMS methods described in this book use most common types of ion sources and analyzers. Electrospray ionization (ESI), electron ionization (El), atmospheric pressure chemical ionization (APCI), or photoionization systems, and their modified versions, are all widely used in TRMS measurements. The newly developed atmospheric pressure ionization schemes such as desorption electrospray ionization (DESI) and Venturi easy ambient sonic-spray ionization (V-EASI) have already found applications in this area. Mass analyzers constitute the biggest and the most costly part of MS hardware. Few laboratories can afford purchasing different types of mass spectrometers for use in diverse applications. Therefore, the choice of mass spectrometer for TRMS is not always dictated by the optimum specifications of the instrument but its availability. Fortunately, many real-time measurements can be conducted using different mass analyzers equipped with atmospheric pressure inlets - with better or worse results. For example, triple quadrupole mass spectrometers excel at quantitative capabilities however, in many cases, popular ion trap (IT)-MS instruments can be used instead. On the other hand, applications of TRMS in fundamental studies often require a particular type of instrument (e.g., Fourier transform ion cyclotron resonance mass spectrometer for photodissociation studies on trapped ions). 

Photoionization Photoionization provides an alternative method of ion production for mass spectral analysis. Recent reports by Uchimura and coworkers have demonstrated that shortening the photoionization UV laser pulse from 260 to 60 fs leads to a substantial increase in the TATP molecular ion at m/z 222 [44,45]. At a pulse energy of 11 pJ, the molecular ion was observed to be 15% of the intensity of the m/z 43 base peak in the spectrum. The short pulse ionization method was successfully interfaced with GC [45]. 

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