LC/MS coupling

Another development arising from FAB has been its transformation from a static to a dynamic technique, with a continuous flow of a solution traveling from a reservoir through a capillary to the probe tip. Samples are injected either directly or through a liquid chromatography (LC) column. The technique is known as dynamic or continuous flow FAB/LSIMS and provides a convenient direct LC/MS coupling for the on-line analysis of mixtures (Figure 40.2). 

Samples containing mixtures of peptides can be analyzed directly by electrospray. Alternatively, the peptides can be separated and analyzed by LC/MS coupling techniques such as electrospray or atmospheric pressure chemical ionization (APCI). 

Table 7.46 shows the LC-FTIR interface detection limits. Detection limits approaching those for GC-FHR light-pipe interfaces have been reported for flow-cell HPLC-FTIR when IR-transparent mobile phases are employed. For both the moving-belt and thermospray LC-MS couplings the detection limits are in the ng range. Selective evaporation consisting of fraction collection followed by DRIFT identification achieves a detection limit of 100 ng. 

LC-MS interfaces generally produce ions with a relatively wide energy and spatial distribution. Table 7.49 lists the main LC-MS interface types. The most important types of contemporary LC-MS interfaces are direct inlet systems PB, TSP, API, ICPI and MIP (the latter two for plasma source detection, cf. Section 7.3.3.5). Three main types of LC-MS coupling systems are usually distinguished ... 

Table 7.63 Summary of interface types and chemistry in LC-MS couplings...

The ESI-MS is nowadays the most commonly used LC-MS coupling device for the analysis of LAS and SPC. But also for the analysis of these anionic analytes the most serious drawbacks of this ionisation technique are matrix effects. A crucial role is played by mobile phases containing relatively high salt concentrations or ion-pair reagents that lead to signal instability or even, in some cases, to plugging of the orifice plate. For successful removal of alkaline salts, a suppressor was incorporated between the LC column and the mass spectrometer [24]. With this set-up it was possible to reduce a spiked sodium concentration of 15 mg L-1 in the HPLC eluent before the suppressor by more than 99.8% at the entry into the MS. 

Arpino, P. J., and Cuiochon, G. (1979). LC/MS coupling. Anal. Chem. 51, 682A-701A. 

During LC-MS analyses, the simultaneous monitoring of the absorption spectrum of the eluate is recommended. The combination of LC-MS coupled with a photodiode array detector can provide additional information regarding the chemical structure of unknown degradation products. The ability of the photodiode array detector to monitor all wavelengths simultaneously assures that any analate eluting within the selected scan range will be monitored at its maximum sensitivity. 

J. D. Henion, Micro LC-MS Coupling, in P. Kucera (Ed.), Microcolumn High-Performance Liquid Chromatography, 1984, Elsevier, Amsterdam, p. 260. 

P.J. Arpino, J.P. Bounine, M. Dedieu, G. Guiochon, Optimization of the instrumental parameters of a combined LC-MS, coupled by an interface for DLL IV. A new desolvation chamber for droplet focusing or townsend discharge ionization, J. Chromatogr., 271 (1983)43. 

Vaporize compounds of varying volatility. This is accomplished in the inlet system. Introduction of the sample is done by direct insertion probe, reservoir inlet, or following a chromatographic separation (GC, HPLC, and CE). As mentioned earlier, to introduce the LC flow to the mass spectrometer on-line, we need an appropriate interface. Development of appropriate interfaces was the utmost for evolution of the LC-MS coupling. 

However, despite the great advantages it offers, LC-MS coupling has some limitations, apart from the interfacing need Incompatibility with some of the nonvolatile buffers and other mobile phase additives. Hence, phosphates, ion-pairing agents, and amine modifiers are replaced by ammonium acetate, ammonium formate, and so forth. 

In general, the results obtained with this method are comparable to the methods presented in this section (in-beam Cl, FD, PDMS, etc.) [101]. Here again, this mode is consistent with LC/MS coupling, as developed by Hardin and Vestal [97] and utilized for the first time in studies of sucrose, the peptide Gly-Trp, guanosine, Na2ATP, etc. 

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. 

Summarizing, the difficulties in LC/MS coupling can be related to the following aspects ... 

The extraction and chromatography of polar aromatic sulfonates are usually based on ion-pair formation, traditionally with tetraalkylammonium cations,because of their acidity. However, tetraalkylammonium cations are not suited for LC-MS coupling as they are virtually nonvolatile and tend to form adducts that complicate MS detection. ... 

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