Ionization source, atmospheric-pressure inlet

The ion guides are frequently used to transmit ions from an atmospheric-pressure inlet/source system (electrospray ionization, atmospheric-pressure chemical ionization) into the vacuum region of an m/z analyzer. 

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... 

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). 

One of the first successful techniques for selectively removing solvent from a solution without losing the dissolved solute was to add the solution dropwise to a moving continuous belt. The drops of solution on the belt were heated sufficiently to evaporate the solvent, and the residual solute on the belt was carried into a normal El (electron ionization) or Cl (chemical ionization) ion source, where it was heated more strongly so that it in turn volatilized and could be ionized. However, the moving-belt system had some mechanical problems and could be temperamental. The more recent, less-mechanical inlets such as electrospray have displaced it. The electrospray inlet should be compared with the atmospheric-pressure chemical ionization (APCI) inlet, which is described in Chapter 9. 

The term nebulizer is used generally as a description for any spraying device, such as the hair spray mentioned above. It is normally applied to any means of forming an aerosol spray in which a volume of liquid is broken into a mist of vapor and small droplets and possibly even solid matter. There is a variety of nebulizer designs for transporting a solution of analyte in droplet form to a plasma torch in ICP/MS and to the inlet/ionization sources used in electrospray and mass spectrometry (ES/MS) and atmospheric-pressure chemical ionization and mass spectrometry (APCI/MS). 

Electrospray is both an atmospheric-pressure (API) liquid inlet system for a mass spectrometer, and, at the same time, it is an ionization source. 

Schmidt et al. [111] developed an atmospheric pressure laser ionization (APLI) source based on REMPI in pulsed gas expansions close to the inlet nozzle orifice (at high... 

A new ionization method called desorption electrospray ionization (DESI) was described by Cooks and his co-workers in 2004 [86]. This direct probe exposure method based on ESI can be used on samples under ambient conditions with no preparation. The principle is illustrated in Figure 1.36. An ionized stream of solvent that is produced by an ESI source is sprayed on the surface of the analysed sample. The exact mechanism is not yet established, but it seems that the charged droplets and ions of solvent desorb and extract some sample material and bounce to the inlet capillary of an atmospheric pressure interface of a mass spectrometer. The fact is that samples of peptides or proteins produce multiply charged ions, strongly suggesting dissolution of the analyte in the charged droplet. Furthermore, the solution that is sprayed can be selected to optimize the signal or selectively to ionize particular compounds. 

It might be noted at this stage that some mass spectrometer inlets are also ionization sources. For example, with electrospray ionization (ES) and atmospheric pressure chemical ionization (APCI), the inlet systems themselves also provide the ions needed for mass spectrometry. In these cases, the method of introducing the sample becomes the method of ionization, and the two are not independent. This consideration can be important. For example, electrospray produces abundant... 

A block diagram of a typical molecular-mass spectrometer is shown in Figure 31-10. Sample molecules enter the mass spectrometer through an inlet system. In the case of GC, the sample is in the form of a vapor, and the inlet must interface between the atmospheric pressure GC system and the low-pressure (10 to 10 torn) mass spectrometer system. An elaborate vacuum system is needed to maintain the low pressure. In the mass spectrometer, sample molecules enter an ionization source, which ionizes the sample. The ionization sources for molecular mass spec-... 

Earlier implementation of SFC-MS followed the evolution of both HPLC-MS and GC-MS interfaces [11,21,23-26], As the API interfaces of HPLC-MS became mainstream analytical techniques in recent decades, they were also quickly employed for SFC-MS [21,23,26-37], The atmospheric pressure chemical ionization (APCI) [27,33] and electrospray ionization (ESI) [36,37] sources are the most popular API interfaces for SFC-MS systems and allow for direct introduction of the effluent to the inlet of the mass spectrometer (Table 9.1). In some cases, the commercial API sources used for HPLC-MS system were proven to be applicable to the SFC-MS system with no modification [11,21,38-41], However, some modification in the SFC-MS interface may be desired for SFC to achieve stable operation and enhanced ionization [22], The ideal interfaces for SFC-MS would provide uniform pulse free flow, maintain chromatographic integrity, and ionize a wide range of analytes. 

A new generation of mass spectrometer inlets allow for direct sampling of a substrate under ambient conditions. Theoretically, this eliminates the need for any sample preparation. Examples include direct analysis in real time (DART) and desorption electrospray ionization (DESI), as well as desorption atmospheric-pressure chemical ionization (DAPCI) and atmospheric solids analysis probe (ASAP). These techniques utilize a source of energy interacting directly with a sample surface at ambient pressure, causing molecules of interest to desorb, ionize, and be sampled by a mass spectrometer. 

Next to ESI, APCI is an alternative, especially for less polar compounds. The APCI interface consists of a different solvent inlet probe, the so-called heated nebulizer, and a discharge electrode, but uses exactly the same atmospheric-pressure ion source and vacuum transition region as in ESI (see Figure 1). In a heated nebulizer, the LC column effluent is first pneumatically nebulized into an aerosol, which subsequently is evaporated in a heated vaporizer zone. Solvent and analyte vapor meet the discharge needle, where a potential of a few kilovolts initiates a continuous discharge that results in the ionization of solvent molecules, which in turn ionize the analyte molecules by gas-phase ion-molecule reactions (common chemical ionization processes). 

In the case of processes occurring in a reactor, the molecules can be delivered directly via transfer line systems or picked up by carrier gases to the inlet of a mass spectrometer. If the reaction occurs in ambient conditions, the molecules can be sampled by atmospheric pressure ion sources. If the ionization, transmission, and detection efficiencies of the reactants and the products of Equation 10.6 are available, their concentrations can be estimated from... 

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