The Atmospheric-Pressure Chemical Ionization Interface

The electrospray interface, described in the previous section, enables mass spectra to be obtained from highly polar and ionic compounds. 

The processes leading to the ionization of the analyte at atmospheric pressure are similar to those described previously in relation to CI (see Section 3.2.2 above) in that ions, produced by the interaction of the electrons with the surrounding gas, undergo a number of reactions leading to the generation of reactive ions which interact with the analyte molecules present. 

The reagent species in the positive-ion mode may be considered to be proto-nated solvent ions, and in the negative ion mode O2-, its hydrates and clusters. 

As with conventional Cl, this is a very mild form of ionization leading to molecular species with little or no fragmentation, i.e. (M + H)+ and (M — H) . This is not, however, always the case. The use of chromatographic modifiers may change the composition of the Cl plasma to such a state that, as in Cl and thermospray, other ions may be formed, e.g. the presence of ammonium acetate may lead to (M + NH4) 1 and (M + CH3COO) ions in the positive- and negative-ion modes, respectively. The chemistry of the analyte may also have an effect, as has been discussed for ESI, with, for example, the spectra of fullerenes extracted from soot particles yielding an M+ molecular species [16]. 

In addition to the formation of these ions of direct analytical utility, APCI leads to the formation of ion clusters involving solvent molecules. Since these tend to make interpretation more difficult, they need to be removed and this may be accomplished either by the use of a curtain gas or by cone-voltage fragmentation (see Section 4.7.4 above) which is also applicable to APCI. 

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However, only a portion of the eluent actually provides ions that enter the mass spectrometer. A diagram of the atmospheric pressure chemical ionization interface is shown in figure 28. 

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. 

In the following chapters, the basic principles of HPLC and MS, in as far as they relate to the LC-MS combination, will be discussed and seven of the most important types of interface which have been made available commercially will be considered. Particular attention will be paid to the electrospray and atmospheric-pressure chemical ionization interfaces as these are the ones most widely used today. The use of LC-MS for identification and quantitation will be described and appropriate applications will be discussed. 

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. 

Figure 22-19 (a) Atmospheric pressure chemical ionization interface between a liquid chromatography column and a mass spectrometer. A fine aerosol Is produced by the nebulizing gas flow and the heater. The electric discharge from the corona needle creates gaseous ions from the analyte. [Adapted from E. C. Huang, T. Wachs, J. J. Conboy, and J. D. Henion, Atmospheric Pressure Ionization Mass Spectrometry," Anal. Chem. 1990,62,713A ] (b) Atmospheric pressure chemical ionization probe. [Courtesy Shimadzu Scientific Instruments, Columbia, MD.J... 

Various hyphenated LC-MS-based assays, using either the electrospray ionization (ESI) or the atmospheric pressure chemical ionization (APCI) interface, have been developed and applied in the clinical setting in order to support pharmacokinetic (PK), pharmacogenetic-pharmacokinetic (PG-PK), and pharmacokinetic-pharmacodynamic (PK-PD) studies in BC patients under tamoxifen therapy (Table 3). 

Another popular and efficient inlet system for the LC/MS combination is the atmospheric pressure chemical ionization process. This system has some similarity to the electrospray interface and can also cope with flow rates of up to 2 ml/min. and thus the total column eluent can be utilized without splitting the flow. 

Complementary to ESP and ISP interfaces, with respect to the analyte polarity, is the atmospheric pressure chemical ionization (APCI) interface equipped with a heated nebulizer. This is a powerful interface for both structural confirmation and quantitative analysis. 

The introduction of the thermospray interface in the mid to late 1980s provided the first efficient LC-MS connecting technique. With the relatively new interface techniques of electrospray interface and the complementary atmospheric pressure chemical ionization interface (APCI), the full potential of the LC-MS system can now be achieved. 

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. 

Mass spectrometry with its excellent sensitivity is emerging as one of the most powerful analytical techniques.16 Its importance was recognized by the awarding of the 2002 Nobel Prize in Chemistry to John B. Fenn and Koichi Tanaka for their research in mass spectrometric methods for biomolecules. The primary difficulties of combining LC and MS have been the interface, and the ionization of analytes in a stream of condensed liquids and transfer of ions into the high vacuum inside the mass spectrometry. Two common LC/MS interfaces are the electrospray ionization (ESI) and the atmospheric pressure chemical ionization (APCI). Figure 4.13a shows a schematic diagram of an... 

As an alternate ionization method to ESI, the atmospheric pressure chemical ionization (APCI) has been applied to the MEKC-MS system. In MEKC-APCTMS, an SDS micellar solution can be introduced directly into the interface without a severe decrease in MS intensity. For highly sensitive analysis of environmental pollutants, an application study of sweeping to MEKC hyphenated with MS using an APCI interface has been reported. 

The first online coupling of TLC and MS for the analysis of carotenoids was applied for detection of neoxanthin, violaxanthin, lutein, and P-carotene in spinach extract. Separation on Cjg RP HPTLC plate was followed by extraction of the carotenoids by means of CAMAG TLC-MS interface. After the elution of compounds from the sorbent with methanol, 1% acetic acid was added to the effluent to augment the ionization in the atmospheric pressure chemical ionization (APCI) source, which operated in positive mode [25]. 

Mass spectrometric detection allows analysis of most nonionic surfactants without deriva-tization. Thermospray, electrospray, or atmospheric pressure chemical ionization interfaces permit direct introduction of the effluent of the LC into the MS and make the MS a very selective detector for nonionics. Quasimolecular ions are produced for each discrete compound, so that the HPLC system is not required to separate both by degree of ethoxylation and by alkyl character. A relatively simple HPLC separation, coupled with MS anal-... 

It was also the first of a number of interfaces, with the others being electrospray and atmospheric-pressure chemical ionization, in which ionization is effected directly from solution within the interface itself, i.e. the mass spectrometer was not nsed to prodnce ions from the analyte simply to separate them according to their m/z ratios. 

Particular emphasis has been placed upon electrospray and atmospheric-pressure chemical ionization (APCI) which, in addition to being the currently most widely used interfaces, are ionization techniques in their own right. 

As with GC/MS, LC/MS offers the possibility of unequivocal confirmation of analyte identity and accurate quantiation. Similarly, both quadrupole and ion-trap instruments are commercially available. However, the responses of different analytes are extremely dependent on the type of interface used to remove the mobile phase and to introduce the target analytes into the mass spectrometer. For pesticide residue analyses, the most popular interfaces are electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI). Both negative and positive ionization can be used as applicable to produce characteristically abundant ions. 

McLoughlin, D.A., Olah, T.V., and Gilbert, J.D. 1997. A direct technique for the simultaneous determination of 10 drug candidates in plasma by liquid chromatography/atmospheric pressure chemical ionization mass spectrometry interfaced to a Prospekt solid-phase extraction system. J. Pharm. Biomed. Anal. 15 1893. 

In atmospheric pressure chemical ionization (APCI) ion-molecule reactions occurring at atmospheric pressure are employed to generate the ions, i.e., it represents a high-pressure version of conventional chemical ionization (Cl, Chap. 7). The Cl plasma is maintained by a corona discharge between a needle and the spray chamber serving as the counter electrode. The ions are transferred into the mass analyzer by use of the same type of vacuum interface as employed in ESI. Therefore, ESI ion sources can easily be switched to APCI instead of an ESI sprayer, a unit comprising a heated pneumatic nebulizer and the spray chamber with the needle electrode are put in front of the orifice, while the atmospheric pressure-to-vacuum interface remains unchanged. [48,138]... 

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