Moving belt interface HPLC

Reference has been made to the problems associated with the presence of highly involatile analytes. Many buffers used in HPLC are inorganic and thus involatile and these tend to compromise the use of the interface, in particular with respect to snagging of the belt in the tunnel seals. The problem of inorganic buffers is not one confined to the moving-belt interface and, unless post-column extraction is to be used, those developing HPLC methods for use with mass spectrometry are advised to utilize relatively volatile buffers, such as ammonium acetate, if at all possible. 

Arguably the ultimate LC-MS interface would be one that provides El spectra, i.e. a spectrum from which structural information can be extracted by using famihar methodology, and this was one of the great advantages of the moving-belt interface. There is, however, an incompatibility between the types of compound separated by HPLC and the way in which electron ionization is achieved and therefore such an interface has restricted capability, as previously discussed with respect to the moving-belt interface (see Section 4.2 above). 

Spray deposition A method used to apply HPLC eluate in later versions of the moving-belt interface to provide a uniform layer of mobile phase on the belt and thus minimize the production of droplets. 

H. Schauenburg, H. Schlitt, and H. Knoppel, Assessment of a Moving Belt Type HPLC-MS Interface with Respect to Its Use in Organic Water Pollution Analysis , EUR 7623, Commission of the European Communities, Brussels, p. 193 (1982). 

Perhaps the most mechanically complex solution ever developed for uniting HPLC with mass spectrometry was the moving belt interface [54]. The heart of this system was a mechanically driven continuous belt (analogous to an escalator or moving walkway) to which the HPLC eluent was applied. The majority of the mobile phase was evaporated by a heat source (ideally hot enough to vaporize the solvents but not to... 

The solvent elimination problem became less of a problem with the commercialization of microbore columns. Hayes et al. (54) studied gradient HPLC-MS using microbore columns and a moving-belt interface. The heart of the system was the spray deposition device designed to be compatible with microbore-column flow rates. Nebulization of the eluent was found to be applicable to a variety of mobile-phase compositions and thus was readily compatible with gradient elution. Figure 13 shows a comparison of UV detection with that obtained with the HPLC-MS system. Applications of this system were demonstrated on water from coal gasification processes. 

Moving belt interface. This solute(s) is deposited after HPLC onto a moving belt, HPLC solvents being flash evaporated. The belt transports... 

SEC suffers from poor resolution and low sensitivity [5], while GC is limited by the high molecular weight and polar nature of many antioxidants and light stabilisers, which are designed to be reactive and so decompose when exposed to heat [9]. HPLC the most widely used instrumental method also has limitations [10-12]. HPLC lacks a simple sensitive universal detector that is compatible with all liquid mobile phases. UV or fluorescence detectors, which are commonly used, require that additives have a chromophoric moiety, while the universal refractive index detector only functions under isocratic conditions. As a result, Vargo and Olson have coupled HPLC with mass spectrometry (MS) for this type of application by using a moving belt interface [13]. 

The use of mass spectroscopy and HPLC offers great potential for the quantification and identification of polymer additives. However, its adoption has been fraught with practical problems, created by the very nature of the eluent from the chromatograph. One possible solution to this problem is the moving-belt interface, in which the eluent is sprayed on to a circulating belt and the solvent evaporated in stages, so that finally only the analyte is carried into the mass spectrometer. This approach has been successful in the separation and identification of some polymer additives [60]. However, it can only be used for Cl and El mass spectroscopy, and the interface can only be operated with mobile phases containing less than 50% water. Newer interfaces such as thermospray and particle beam offer potential in this area. 

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. 

Different methods are used to tackle these problems [10-13], Some of these coupling methods, such as moving-belt coupling or the particle beam (PB) interface, are based on the selective vaporization of the elution solvent before it enters the spectrometer source. Other methods such as direct liquid introduction (DLI) [14] or continuous flow FAB (CF-FAB) rely on reducing the flow of the liquid that is introduced into the interface in order to obtain a flow that can be directly pumped into the source. In order to achieve this it must be reduced to one-twentieth of the value calculated above, that is 5 pi min. These flows are obtained from HPLC capillary columns or from a flow split at the outlet of classical HPLC columns. Finally, a series of HPLC/MS coupling methods such as thermospray (TSP), electrospray (ESI), atmospheric pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI) can tolerate flow rates of about 1 ml min 1 without requiring a flow split. Introducing the eluent entirely into the interface increases the detection sensitivity of these methods. ESI can accept flow rates from 10 nl min-1 levels to... 

However, the great improvement to solve the possibility of coupling HPLC to MS was the development of interfaces that allow the elimination of the solvent and the vaporization of the solute before this can be analyzed in the mass spectrometer. Several interfaces are now available, such as continuous flow FAB (CF-FAB), thermospray (TSP), moving belt (MB), direct liquid introduction (LDI), and atmospheric pressure ionization (API) in the field of API, three different, but fundamentally similar, techniques are available ion evaporation, electrospray, and ion spray. In each case, the removal of... 

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