Particle-beam interface sensitivity

The sensitivity of the particle-beam interface is dependent not only on the specific analyte but also on the experimental conditions employed. Detection limits are invariably higher than are desirable. 

The particle beam interface is an example of an HPLC/MS interface (see Fig. 16.14). In this interface, the mobile phase is vaporised in the form of a spray into a desolvation chamber before entering the area where the sample is concentrated by evaporation. Because solvent molecules are lighter, their angular dispersion is wider than that of the analyte, which can be transferred into the transfer capillary. This type of interface is slowly being abandoned in favour of others that have a greater sensitivity. 

A capillary-scale particle beam interface was used for the analysis of phenols in red wine by LC with MS detection. The interface allows very low mobile phase flows and sensitive detection of the analytes in complex matrices . 

Nowadays, interfacing of a liquid chromatograph or a capillary electrophoresis instrument with a mass spectrometer is used too, although technically more complex. The presence of water in elution solvents is an undesirable compound for the mass spectrometer. With HPLC, the use of micro-columns is desirable, to have very low flow rates. They are also well-suited with different ionization techniques for the analysis of high molecular-weight compounds. A rather old device, whose sensitivity is now judged to be very poor, is the particle beam interface... 

A comparison of various LC-MS systems for the analysis of complex mixtures of PAHs showed that (1) the moving belt interface was mechanically awkward and is compatible only with a limited range of mobile phases, (2) particle-beam interface had low sensitivity, and the response was nonlinear, (3) a heated nebulizer interface that uses atmospheric pressure chemical ionization (APCI) was the preferred procedure (Anacleto et al. 1995). 

MS is becoming the detection system of choice for LC by virtue of its flexibility and high selectivity for individual solutesHowever, LC-MS is always less sensitive than GC-MS as a result of the need to transfer the analytes from the liquid phase into a high-vacuum gas phase. Other limitations of LC-MS combination include the inability to use nonvolatile buffers, the narrow optimum range for eluent flow rate influence of the proportion of organic modifier on the sensitivity, and the narrow choice of ionization methods.Nevertheless, LC-MS has been widely accepted as an advantageous choice for the determination of carbamate pesticides in water matrices, which is more robust and flexible in the absence of derivatization. Thermospray and particle-beam interfaces are probably most commonly used for offline and online determination of carbamates in Atmospheric pressure sources such as... 

Ionisation methods investigated earlier, including thermospray, fast atom bombardment, and particle beam interfaces have been replaced by electrospray imiiza-tion (ESI) interfaces. Atmospheric pressure chemical ionisation (APCI) has also been applied but is less sensitive towards the PANOs. A combination of ESI with reversed-phase HPLC using acid mobile phases to protonate the PA molecules has most often been used [41]. 

A particle beam interface (PBI) better serves the purpose of LC-EI-MS [55-57]. The PBI removes the solvent by nebulization into an evacuated desolvation chamber from where the evolving microscopic sample particles are transferred into an El ion source via a jet separator (Fig. 5.14). Designs different from PBI have also been developed [57]. The PBI is comparatively robust and attained popularity in particular for low- to medium-polarity analytes, but has some drawbacks such as poor sensitivity especially with water-rich mobile phases, moderate linearity with polar compounds, and low tolerance for heat-sensitive compounds [58,59]. The most recent addition to LC-EI interfaces, also the simplest and most elegant solution, makes use of the very low liquid flow rates of nano-LC equipment [60,61]. The flow from a 30 pm i.d. fused silica capillary column is passed... 

The particle beam interface is very good for qualitative work because the 70 eV spectra contain a significant amount of structural information and can be searched against conventional mass spectra libraries. However, in the full scan mode the sensitivity is restricted to the low nanogram levels. 

As with GC, the combination of MS and MS/MS detection with LC adds an important confirmatory dimension to the analysis. Thermospray (TSP) and particle beam (PB) were two of the earlier interfaces for coupling LC and MS, but insufficient fragmentation resulted in a lack of structural information when using TSP, and insufficient sensitivity and an inability to ionize nonvolatile sample components hampered applications using PB. Today, atmospheric pressure ionization (API) dominates the LC/MS field for many environmental applications. The three major variants of API... 

Flow limitations restrict application of the DFI interface for pSFC-MS coupling. pSFC-DFI-MS with electron-capture negative ionisation (ECNI) has been reported [421], The flow-rate of eluent associated with pSFC (either analytical scale - 4.6 mm i.d. - or microbore scale 1-2 mm, i.d.) renders this technique more compatible with other LC-MS interfaces, notably TSP and PB. There are few reports on workable pSFC-TSP-MS couplings that have solved real analytical problems. Two interfaces have been used for pSFC-EI-MS the moving-belt (MB) [422] and particle-beam (PB) interfaces [408]. pSFC-MB-MS suffers from mechanical complexity of the interface decomposition of thermally labile analytes problems with quantitative transfer of nonvolatile analytes and poor sensitivity (low ng range). The PB interface is mechanically simpler but requires complex optimisation and poor mass transfer to the ion source results in a limited sensitivity. Table 7.39 lists the main characteristics of pSFC-PB-MS. Jedrzejewski... 

Table 7.53 shows the main characteristics of LC-PB-MS. Of all LC-MS interface methods, LC-PB-MS comes closest to GC-MS (Scheme 7.7). The particle beam is an acceptable choice in cases where sensitivity, volatility and analyte polarity are not an issue. Usually, the function of UV is added to LC-PB-MS this allows the investigation of peak homogeneity. Drawbacks of LC-PB-MS are the low sensitivity and the nonlinearity... 

Yinon et al. (228) used an HPLC interfaced with a triple-quadrupole mass spectrometer by means of a particle beam for the identification of several azo dyes. Characterization of the dyes was achieved by observing typical fragment ions formed by cleavage of the N-C and C-N bond on either side of the azo linkage and/or cleavage of the N=N double bond with the transfer of two hydrogen atoms to form an amine. Sensitivity was observed to be two to three orders of magnitude worse than with thermospray ionization. 

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... 

The particle beam LC/FT-IR spectrometry interface can also be used for peptide and protein HPLC experiments to provide another degree of structural characterization that is not possible with other detection techniques. Infrared absorption is sensitive to both specific amino acid functionalities and secondary structure. (5, 6) Secondary structure information is contained in the amide I, II, and III absorption bands which arise from delocalized vibrations of the peptide backbone. (7) The amide I band is recognized as the most structurally sensitive of the amide bands. The amide I band in proteins is intrinsically broad as it is composed of multiple underlying absorption bands due to the presence of multiple secondary structure elements. Infrared analysis provides secondary structure details for proteins, while for peptides, residual secondary structure details and amino acid functionalities can be observed. The particle beam (PB) LC/FT-IR spectrometry interface is a low temperature and pressure solvent elimination apparatus which serves to restrict the conformational motions of a protein while in flight. (8,12) The desolvated protein is deposited on an infrared transparent substrate and analyzed with the use of an FT-IR microscope. The PB LC/FT-IR spectrometric technique is an off-line method in that the spectral analysis is conducted after chromatographic analysis. It has been demonstrated that desolvated proteins retain the conformation that they possessed prior to introduction into the PB interface. (8) The ability of the particle beam to determine the conformational state of chromatographically analyzed proteins has recently been demonstrated. (9, 10) As with the ESI interface, the low flow rates required with the use of narrow- or microbore HPLC columns are compatible with the PB interface. 

The matrix effect can be avoided by using MS. This technique is characterized by being highly selective and sensitive and, in addition, it offers spectral information which permits the unequivocal identification of target compounds. LC-MS with thermospray (TSP) and particle beam (PB) interfaces have been widely... 

These efforts are centered around the use of techniques including enzymatic hydrolysis [5,6,7], physical or chemical degradation [8,9], and monodisperse aerosol generation interface (particle beam) for HPLC/MS [10,11] to solve specific problems. This paper discusses the implementation of several HPLC/MS methods which offer a combination of sensitivity and specificity for compounds such as peptides, pharmaceuticals, and pesticides in complex matrices. 

Particle beam ionisation, however, for a short time, became the interface of preference, since its spectra are similar to those electron impact (El) spectra listed in the NIST-Ubrary, and so sustain any identification of unknown compounds. Its sensitivity, though, was quite unsatisfactory. 

Currently, the main breakthrough in environmental analysis is observed in the application of LC-MS and LC-MS/MS techniques. One of the obstacles to routine analytical applications of LC-MS had been the unavailability of rugged and reliable LC-MS interfaces. The development of atmospheric pressure ionization (API) overcame such limitations as poor structural information or sensitivity seen with thermospray (TSP) or particle-beam (PB). API is used as a generic term for soft ionization obtained by different interface/ionization types, such as APCI and electrospray (ESI) that operate under atmospheric pressure conditions. Today, LC-MS has become a routine analytical tool, allowing the detection of polar and nonvolatile compounds not amenable to GC analysis. 

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