Between LC and MS

By way of illustration, very simple spectra for four substances (A, B, C, D) are shown (a) separately and (b) mixed in unequal proportions. The mixture spectrum is virtually impossible to decode if A, B, C, D are not known beforehand to be present. 

In the earliest interface, a continuous moving belt (loop) was used onto which the liquid emerging from the chromatographic column was placed as a succession of drops. As the belt moved along, the drops were heated at a low temperature to evaporate the solvent and leave behind any mixture components. Finally, the dried components were carried into the ion source, where they were heated strongly to volatilize them, after which they were ionized. 

This method is still in use but is not described in this book because it has been superseded by more recent developments, such as particle beam and electrospray. These newer techniques have no moving parts, are quite robust, and can handle a wide variety of compound types. Chapters 8 through 13 describe these newer ionization techniques, including electrospray, atmospheric pressure ionization, plasmaspray, thermospray, dynamic fast-atom bombardment (FAB), and particle beam. 

It is worth noting that some of these methods are both an inlet system to the mass spectrometer and an ion source at the same time and are not used with conventional ion sources. Thus, with electrospray, the process of removing the liquid phase from the column eluant also produces ions of any emerging mixture components, and these are passed straight to the mass spectrometer analyzer no separate ion source is needed. The particle beam method is different in that the liquid phase is removed, and any residual mixture components are passed into a conventional ion source (often electron ionization). 

Finally, note that the ions produced by the combined inlet and ion sources, such as electrospray, plasmaspray, and dynamic FAB, are normally molecular or quasi-molecular ions, and there is little or none of the fragmentation that is so useful for structural work and for identifying compounds through a library search. While production of only a single type of molecular ion may be useful for obtaining the relative molecular mass of a substance or for revealing the complexity of a mixture, it is often not useful when identification needs to be done, as with most general analyses. Therefore, 

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The (almost) fundamental incompatibihty between LC and MS was nicely pictured by Arpino [1] in Figure 3.1. 

Reducing the pathway length between LC and MS starts already with setting up your UHPLC instrumentation. The conventional LC setup typically follows a top-down path of your mobile phase (Figure 1.7a) with the solvent bottles on top, the stack sequentially contains the degasser, the pump, the autosampler, the column thermostat, and finally the detector(s) downstream. Most of all commercial mass... 

The most common interfaces between LC and MS used today are electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI). Compared with ESI, APCI requires the analytes to be thermally stable and may yield multiple fragmentation ions. For these reasons, APCI is often used when a particular analyte is not ionized well with ESI. ESI should be used for LC-MS analysis of pantothenic acid since pantothenic add is ionized very well under ESI. 

LC coupled with MS LC-MS has been applied in the determination of BPA, OP, and NP in the environment. The advantage of this analytical method is the capability of directly determining nonvolatile or polar compoimds by using ESI or APCl techniques as an interface between LC and MS [61,113]. 

Tomlinson and Caruso [28] also performed the speciation of Cr(III) and (VI) using a Dionex AS-11 anion-exchange microbore column and 6 mM 2,6-PDCA-8.6 mill lithium hydroxide mobile phase. A thermospray source was used as the interface between LC and ICP-MS. Absolute limits of detection were at the pg level for both species using this instrument assembly. 

The most obvious and therefore oldest way of coupling LC and MS is by means of a capillary between the LC colunm and the MS ion source. This approach, the capillary inlet interface, was pioneered and theoretically described by Tal roze et al. [1-3] in the early seventies a similar approach was used by the group of McLafferty [4-5]. A variety of laboratory-built capillary inlet interfaces have been described by several research groups [6-12]. A typical example is given in Figure 4.1 [7]. 

Initially, considerable attention was paid to the selection of the best solvent composition. Nowadays, it is readily understood that the operation of LC-MS implies compromises in the performance of both LC and MS. A particular mobile-phase composition might be ideal in terms of analyte ionization, but if this mobile phase yields infinite retention or no retention at all, it cannot be applied. A free selection of the solvent composition is not possible. Always, a compromise mnst be fonnd between LC separation and MS ionization. 

After improvements made to the concentric design to enhance ion transmission to the MS, Kapron et al. demonstrated the use of EAIMS as an interface between liquid chromatography (LC) and MS. - The online EAIMS device improved the relative accuracy and precision of drug analysis by removing metabolite interferences before entrance to the MS. 

Figure 9.14 shows the results of a commercial instrument using FAIMS as an ion filter between the LC and MS. In this figure, the LC on the right was an assay... 

Recently, the linear DMS has been interfaced to a MS -type instrument and is also commercially available for use as an ion filter between LC and ESI-MS. The primary benefit is the elimination of solvent ions that interfere with detection. As with the FAIMS interface, the DMS interface is expected to increase the signal-to-noise ratio of MS detection... 

Because LC and MS have both evolved rapidly over the last ten years, the majority of recent mass spectrometers is compatible with fast chromatography, as demonstrated in Figure 4.1, which makes MS(/MS) the detector of choice for many UHPLC applications. In this chapter, the compatibility between fast-LC and MS... 

The basic principles of fast-atom bombardment (FAB) and liquid-phase secondary ion mass spectrometry (LSIMS) are discussed only briefly here because a fuller description appears in Chapter 4. This chapter focuses on the use of FAB/LSIMS as part of an interface between a liquid chromatograph (LC) and a mass spectrometer (MS), although some theory is presented. 

Trypsin digests of 17 proteins were carried out and the peptides produced then studied by using LC-MS-MS. In summary, these produced between one and six peptides which gave some sequence information by MS-MS. Of these peptides, the number of amino acid residues sequenced ranged from three to thirteen. 

Figure 5.67 Reconstructed ion chromatograms for Idoxifene and internal standard (ds-Idoxifene using LC-ToF-MS for (a) double-blank human plasma extract, (b) extract of blank human plasma containing internal standard (IS), and (c) control-blank human plasma spiked with Idoxifene at 5 gml , the LOQ of the method. Reprinted from 7. Chromatogr., B, 757, Comparison between liquid chromatography-time-of-flight mass spectrometry and selected-reaction monitoring liquid chromatography-mass spectrometry for quantitative determination of Idoxifene in human plasma , Zhang, H. and Henion, J., 151-159, Copyright (2001), with permission from Elsevier Science.

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