Atmospheric-pressure-chemical-ionization operation

S. Lacorte and D. Barcelo, Determination of parts per trillion levels of organophospho-rus pesticides in groundwater by automated on-line liquid- solid extraction followed by liquid chr omatography/atmospheric pressure chemical ionization mass spectrometry using positive and negative ion modes of operation . Anal. Chem. 68 2464- 2470 (1996). 

The pump must provide stable flow rates from between 10 ttlmin and 2 mlmin with the LC-MS requirement dependent upon the interface being used and the diameter of the HPLC column. For example, the electrospray interface, when used with a microbore HPLC column, operates at the bottom end of this range, while with a conventional 4.6 mm column such an interface usually operates towards the top end of the range, as does the atmospheric-pressure chemical ionization (APCI) interface. The flow rate requirements of the different interfaces are discussed in the appropriate section of Chapter 4. 

Some combinations of IC with MS detection are available, including ICP-MS (element-specific detection), particle beam MS and MS with atmospheric pressure ionization (API) operated in either electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI) modes that give information on molecular ions or adducts. 

An atmospheric pressure chemical ionization (APCI) interface is generally considered extremely easy to optimize and operate. This is perhaps best proven by the fact that hardly any optimization of the interface parameters is reported... 

LC-MS, as a technique, is very much dependent upon ionization (and ion vaporization) techniques that are suited to LC conditions, i.e. techniques where a relatively large solvent flow can be accommodated, which restricts us to just two ionization methods electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCl). Both techniques are very similar in their modes of operation (see Section 5.2.1), relying on the formation of a spray from a solvent flow at atmospheric pressure, and hence they are ideally suited to use in LC-MS applications. 

In 1995, Taylor and co-workers also described the use of an open-access LC/MS system for routine structure confirmation, featuring atmospheric pressure chemical ionization (APCI). This system featured dual personal computers (PCs) for automated instrument control and sample log-in. A system-PC is responsible for running the Windows NT for Workgroups operating system and interfaces with the network for instrument control. A separate log-in PC, isolated from the LC/MS system, is used by the synthetic chemist to enter details about the samples. The analyst prepares the sample in an autosampler vial in one of several solvent options. The system specifies where to place the sample vial in the autosampler, and following analysis with a standard method, spectra are automatically processed and printed without any chemist intervention. 

They are still the workhorses of coupled mass spectrometric applications, as they are relatively simple to run and service, relatively inexpensive (for a mass spectrometer), and provide unit mass resolution and scanning speeds up to approximately 10,000 amu/s. This even allows for simultaneous scan/ selected ion monitoring (SIM) operation, in which one part of the data acquisition time is used to scan an entire spectrum, whereas the other part is used to record the intensities of selected ions, thus providing both qualitative information and sensitive quantitation. They are thus suitable for many GC-MS and liquid chromatography-mass spectrometry (LC-MS) applications. In contrast to GC-MS with electron impact (El) ionization, however, LC-MS provides only limited structural information as a consequence of the soft ionization techniques commonly used with LC-MS instruments [electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI)]. Because of this limitation, other types of mass spectrometers are increasingly gaining in importance for LC-MS. 

The instrumentation and interfaces that had been used up to 1998 in CWC-related LC/MS analysis were summarized previously (4). At that time, sources that operate at atmospheric pressure, using electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI), were displacing their predecessors that used thermospray ionization or continuous flow fast atom bombardment. Atmospheric pressure ionization (API), either ESI or APCI, is now the method of choice in CWC-related analysis and will be the focus of this current review. A small number of recent applications involving alternative types of ionization are also included. For earlier applications of LC/MS to chemical weapons (CW) analysis, using thermospray and other ionization methods, the reader is referred to our previous review (4). The other major trend has been the increasing availability and ease of use of less-expensive bench-top quadrupole and... 

Capillary electrokinetic chromatography (CEKC) with ESI-MS requires either the use of additives that do not significantly impact the ESI process or a method for their removal prior to the electrospray. Although this problem has not yet been completely solved, recent reports have suggested that considered choices of surfactant type and reduction of electro-osmotic flow (EOF) and surfactant in the capillary can decrease problems. Because most analytes that benefit from the CEKC mode of operation can be effectively addressed by the interface of other separations methods with MS, more emphasis has until now been placed upon interfacing with other CE modes. For small-molecule CE analysis, in which micellar and inclusion complex systems are commonly used, atmospheric pressure chemical ionization (APCI) may provide a useful alternative to ESI, as it is not as greatly affected by involatile salts and additives. 

API interfaces have two available modes for operation, electrospray ionization (ESI) and atmospheric-pressure chemical ionization (APCI). The mechanistic aspects of ESI [10-12] and APCI [13] have been well covered in the literature. ESI, regarded as the softer, more versatile of the two methods, is able to ionize extremely polar/nonvolatile molecules, sometimes difficult for APCI. APCI mass spectra often contain fragment ions from the analyte due to the... 

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. 

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. 

Atmospheric pressure chemical ionization (Bruins, 1991) was developed starting from the assumption that the yield of a gas-phase reaction depends not only on the partial pressure of the two reactants, but also on the total pressure of the reaction environment. For this reason, the passage from the operative pressure of 0.1-1 Torr, present inside a classical Cl source, to atmospheric pressure would, in principle, lead to a relevant increase in ion production, which consequently leads to a relevant sensitivity increase. Furthermore, the presence of air at atmospheric pressure can play a positive role in promoting ionization processes. 

LC-MS using atmospheric pressure ionization (LC-API-MS) has dramatically changed the analytical methods used to detect polar pollutants in water. Most API mass spectrometers offer two interfaces electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI), both of which can be operated in positive and negative ion mode. ESI transfers ions from solution into the gas phase, whereas APCI ionizes in the gas phase. Analytes occurring as ions in solution may be best analyzed by ESI, while nonionic analytes may be well suited for APCI. What must always be taken into consideration is the relations between analyte properties and the chosen method of chromatographic separation. 

HPLC with ESI produces mostly molecular ions and few to no molecular fragments. This limits the use of LC-MS in the differentiation of isomers. Two atmospheric pressure ionization (API) interfaces allow for the formation of molecular ion, [PAH]" , yet in general, derivatization and additives are required to induce fragmentation. ESI is an interface that transfers ions from the mobile phase into the gaseous phase for introduction into the mass spectrometer so that atmospheric pressure chemical ionization (APCI) can cause ionization of chemical species in the gaseous phase. Both techniques can be operated in the positive- or negative-ion mode. Reports exist for the ionization of PAHs by both techniques, although there are few actual applications to the analysis of real environmental samples. 

The criteria for optimum performance of atmospheric pressure chemical ionization and electrospray are different. For electrospray, it is important to control the flow rate and composition of the sheath-flow liquid, the mobile phase flow rate, and the dimensions of the electrospray and sheath-flow capillaries, as well as their relative position with respect to each other. Sample ionization with carbon dioxide alone does not occur in the absence of sheath-flow liquid. For atmospheric pressure chemical ionization, apart from those factors that affect the performance of the nebulizer, the flow dynamics throughout the interface and temperature in the ionization region, are the most important parameters. Moistened air is sometimes used as a makeup gas to maintain stable operation of the atmospheric pressure chemical ionization interface with HaO ions supplementing solvent ions in the reaction gas plasma. The ionization efficiency of both methods is similar to liquid chromatography. 

There are two major types of ionization sources that operate at atmospheric pressure, electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCl). A modified version of the ESI source is the ion spray source. These sources are described in detail in Chapter 13, Section 13.1.6.1, because they are used to interface EC with MS for the separation and mass spectrometric analysis of mixtures of nonvolatile high molecular weight compounds, especially in the fields of pharmaceutical chemistry, biochemistry, and clinical biomonitoring. ESI will be described briefly so that its use may be demonstrated but more detail will be found in Chapter 13. 

Finally, the Appendix shows schematic representations of the principle of operation of some of the ionization processes presented. " Among these are the theory of ESI, atmospheric pressure chemical ionization (APCI), and atmospheric pressure photoionization (APPI). Also shown are schematics of the ionizers and typical experimental conditions for the APCI and APPI sources as well as that of an ESI-APCl mixed source. It should be noted that these schematics are for sources that are interfaced with a mass spectrometer but are similar to IMS interfaces. 

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). 

The only other LC detection technique used is MS. Atmospheric pressure chemical ionization and ESI interfaces are also employed in many extract analyses. These are highly sensitive and stable. However, although the MS detectors offer a high selectivity and sensitivity, due to their high cost and more complicated operation and maintenance they remain a method of choice for clinical studies of the metabolites of natural constituents. 

Atmospheric pressure ionization (API) has been mainly used for the ionization of phenolic compounds, applying either electrospray (ESI) or atmospheric pressure chemical ionization (APCI), as it can be observed in Table 16.7. However, ESI is more often used to ionize the different families of phenolic compounds. Moreover, APCI and ESI can be operated under both negative and positive ion modes. Although positive ionization mode [47,93,102,103] was used for detection of various phenolic compounds, it was found that negative ionization mode [94,95,101,104,107] was excellent for phenolic compounds analysis. In this sense, the combination of both polarities in the same method provided good results [75,92,99,100] for the simultaneous determination of several families of compounds. Phenolic acids [75,94,95,101,103,107], flavonols [75,94,95,99,101,103,107], flavonones [99,103] flavanols [92,101,103,104], and flavones [75,103] were often detected in negative ion mode, although some of these families were also detected in positive mode [75,99,100,102]. Anthocyanidins [47,100], coumarins [102], and isoflavones [75,93,102] were detected in positive mode. 

Suppression of ionization efficiency is important when the total ionizing capability of the ionization technique is limited, so that there is a competition for ionization among compounds that are present in the ion source simultaneously. In principle such a saturation effect must be operative for all ionization techniques, but in practice it is most important for electrospray ionization (Section 5.3.6), slightly less important for atmospheric pressure chemical ionization (Section 5.3.4), atmospheric pressure photoionization (Section 5.3.5) and matrix assisted laser desorption ionization (Section 5.2.2) it does not appear to be problematic under commonly used conditions for electron ionization and chemical ionization (Section 5.2.1) or thermospray (Section 5.3.2). Enhancement of ionization efficiency for an analyte by a co-eluting compound is less commonly observed and is, in general, not well understood. 

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