Particle-beam interface performance

The particle-beam interface gives optimiun performance at flow rates of between 0.1 and 0.5 mlmin . These rates are directly compatible with 2 mm... 

The performance of the particle-beam interface deteriorates as the percentage of water in the HPLC mobile phase increases. 

Various transport type interfaces, such as SFC-MB-MS and SFC-PB-MS, have been developed. The particle-beam interface eliminates most of the mobile phase using a two-stage momentum separator with the moving-belt interface, the column effluent is deposited on a belt, which is heated to evaporate the mobile phase. These interfaces allow the chromatograph and the mass spectrometer to operate independently. By depositing the analyte on a belt, the flow-rate and composition of the mobile phase can be altered without regard to a deterioration in the system s performance within practical limits. Both El and Cl spectra can be obtained. Moving-belt SFE-SFC-MS" has been described. 

El is performed in a high-vacuum ion source (typically < 10 Pa) intermolecular collisions are avoided in this way. As a result, El mass spectra are highly reproducible. Extensive collections of standardized El mass spectra are available [3-4], also for on-line computer evaluation. An important limitation of El is the necessity to present the analyte as a vaponr, which excludes the use of El in the study of nonvolatile and thermally labile compounds. El is widely applied in GC-MS [5]. In LC-MS, its applicability is hmited to the particle-beam interface and the moving-belt interface. 

Many papers on the LC-MS analysis of pesticides and related compounds deal with the characterization of interface and ionization performance, the improvement of detection limits by variation of experimental conditions, and the information content of the mass spectra. As far as ESI and APCI ate concerned, this type of information is reviewed for various pesticide classes in this section (see Ch. 4.7.4 for results with thermospray and Ch. 5.6.1 with particle-beam interfacing). 

In the past 10 years, the manner in which LC-MS analysis is performed has significantly changed. While in the past it was necessary to choose the most appropriate LC-MS interface for a particular application from a list of five possibilities, e.g., the moving-belt interface, the direct-liquid introduction interface, the thermospray interface, the particle-beam interface, and the continuous-flow fast-atom bombardment interface, today all LC-MS technologies are based on API. The two most important... 

Ventura, R., Nadal,T., Alcalde, R, and Segura, . (1993) Determination of mesocarb metabolites by high-performance liquid chromatography with UV detection and with mass spectrometry using a particle-beam interface. Journal of Chromatography, 647,203-210. 

The LC/MS analyses were performed with either thermospray ionization (TSI) or particle beam (PB) interfaces. These systems successfully analyzed the labile, polar, or higher mass compounds, whereas a complementary gas chromatography (GC)/MS system was used for volatile compounds. The LC/MS system proved to be widely applicable to a range of chemically diverse compounds. The TSI and PB systems were both successful for 80-90% of the compounds analyzed. Automated, open-access LC/MS analyses performed well because sample throughput was expected to reach 250,000 in 1995. This throughput corresponds to approximately 1000 samples per day. 

The two stage momentum separator used in this interface is shown schematically in Figure 3 coupled to the combination Thermospray/EI source. This device is conceptually similar to those used in other MAGIC (2) or particle beam (3,4) interfaces. However, since most of the solvent vapor is removed in the gas diffusion cell, this separator is required primarily to remove sufficient helium to allow the standard MS pumping system to achieve the good vacuum required for El operation. The performance of this device can be optimized much more readily for separating helium from macroscopic particles than when copious quantities of condensible vapors are present as in the more conventional particle beam systems. 

Molecular beam experiments performed with -1.5 nm gold particles supported on an MgO(lOO) surface annealed in UHV at 973 K [171] showed that, after exposure to an O2 beam then pumping, no CO2 was produced during CO pulse titration, indicating that O2 does not dissociate on gold before reaction with adsorbed CO. The authors are more in favor of a mechanism at the interface, as proposed... 

By employing a laser for the photoionization (not to be confused with laser desorption/ ionization, where a laser is irradiating a surface, see Section 2.1.21) both sensitivity and selectivity are considerably enhanced. In 1970 the first mass spectrometric analysis of laser photoionized molecular species, namely H2, was performed [54]. Two years later selective two-step photoionization was used to ionize mbidium [55]. Multiphoton ionization mass spectrometry (MPI-MS) was demonstrated in the late 1970s [56—58]. The combination of tunable lasers and MS into a multidimensional analysis tool proved to be a very useful way to investigate excitation and dissociation processes, as well as to obtain mass spectrometric data [59-62]. Because of the pulsed nature of most MPI sources TOF analyzers are preferred, but in combination with continuous wave lasers quadrupole analyzers have been utilized [63]. MPI is performed on species already in the gas phase. The analyte delivery system depends on the application and can be, for example, a GC interface, thermal evaporation from a surface, secondary neutrals from a particle impact event (see Section 2.1.18), or molecular beams that are introduced through a spray interface. There is a multitude of different source geometries. 

Figure 4 Sketch of a TIRM. The incident beam is totally reflected at the glass-fluid interface, creating an evanescent wave, which penetrates into the fluid. The intensity of the scattered light from a colloidal particle, which performs Brownian motions along the z direction, that is, normai to the surface, will fluctuate according to the particle position in the evanescent wave. The intensity trace is recorded with a photomultiplier and converted to histogram of intensities, from which the probability density of separation distances and finally the potential is calculated. Optionally, the probe particle can be laterally...

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