Molecular beam sampling

Molecular beam sample introduction (described in section (Bl.7.2)). followed by the orthogonal extraction of ions, results in improved resolution in TOP instruments over eflfrisive sources. The particles in the molecular beam typically have translational temperatures orthogonal to the beam path of only a few Kelvin. Thus, there is less concern with both the initial velocity of the ions once they are generated and with where in the ion source they are fonned (since the particles are originally confined to the beam path). 

Kamphus, M. et al., REMPI temperature measurement in molecular beam sampled low-pressure flames, Proc. Combust. Inst., 29,2627, 2002. 

D. L. Styris and D. A. Redfield, Mechanisms of Graphite Furnace Atomization of Aluminum by Molecular Beam Sampling Mass Spectrometry, Anal. Chem. 1987,59, 2891. 

The experimental apparatus consists of eight main parts an ultraviolet flashlamp capable of repetitive flashing at about 5 Hz, a purge flow reactor with either pinhole or molecular beam sampling, an ion source, a mass filter, an ion detector, pulse-counting electronics, computer data aquisition, and a vacuum system. A diagram of the apparatus is shown in Figure 1. 

R.A. Beyer, Molecular Beam Sampling Mass Spectrometry of High Heating Rate Pyrolysis. Description of Data Acquisition System and Pyrolysis of HMX in a Polyurethane Binder ,... 

Differential pumping is a technique which is widely applied when an interface must be established between systems in which very large pressure differences exist. An excellent example is the coupling of mass spectrometers (normal working pressure 10 5mbar or below) to higher pressure systems such as GC columns (in GC-MS GC columns at 1 bar) or flow tubes (at a few mbar) in kinetics studies or molecular beam sampling. 

The present work involves measurement of k in a 0.1 atmosphere, stoichiometric CH -Air flame. All experiments were conducted using 3 inch diameter water-cooled sintered copper burners. Data obtained in our study include (a) temperature profiles obtained by coated miniature thermocouples calibrated by sodium line reversal, (b) NO and composition profiles obtained using molecular beam sampling mass spectrometry and microprobe sampling with chemiluminescent analysis and (c) OH profiles obtained by absorption spectroscopy using an OH resonance lamp. Several flame studies (4) have demonstrated the applicability of partial equilibrium in the post reaction zone of low pressure flames and therefore the (OH) profile can be used to obtain the (0) profile with high accuracy. 

Other observations of the reaction of hydrazine and nitrogen tetroxide substantiate the production of non-equilibrium combustion products. Non-equilibrium product concentrations were found in combustion gases extracted from a small rocket combustion chamber through a molecular beam sampling device with direct mass spec-trometric analysis (31) (39). Under oxidizer rich conditions excessive amounts of nitric oxide were found under fuel rich conditions excessive amounts of ammonia were found. A correlation between the experimentally observed characteristic velocity and nitric oxide concentration exists (40). Related kinetic effects are postulated to account for the two stage flame observed in the burning of hydrazine droplets in nitrogen dioxide atmospheres (41) (42). 

In addition. Figure 2 presents a comparison between the voliune of heavy hydrocarbon molecules (neutral and ionized) measured by light absorption 14, 15) and the volume of soot particles measured by molecular beam sampling and electron microscopy (7, 11). These data show that enough heavy molecules exist to account for soot formation, thereby supporting the view that these molecules are probably intermediates of soot. 

Low pressure CH4/O2 flames. 429 All species, including OH, by mass spectrometry with molecular beam sampling. 

The mass spectrometer and its collimated molecular beam sampling system 6,12) have been especially designed for the study of reactive free radicals. 

Figure 1. Molecular beam sampling system of mass spectrometer...

Figure 4.2. Experimental set-up for kinetic studies of SiHj reactions by excimer laser photolysis in a tubular slow-flow reactor with time-resolved detection by molecular beam sampling, near-threshold ionization electron impact mass spectrometry...

The apparatus is a low-pressure flat-flame burner with a molecular-beam sampling instrument having a quartz probe and an on-line quad-rupole mass-spectrometer (Figure 1). The design features and measurement techniques are the same as those described elsewhere (1,10). 

When methylamine is allowed to react with hydrogen atoms the stable products are Hg, NH3, CH4, and CgHg, in the temperature range 473—683 K. However, e.s.r. and molecular beam sampling techniques have allowed the rate constant for the reaction ... 

Detection of fluorine atoms was by means of mass spectrometry with molecular beam sampling. The value of kf was reported to be 10 exp (—7Q0jT) cm mol s in the range 300 to 400 K. 

A more extensive investigation, using mass spectrometry with molecular beam sampling, and first order kinetic analysis for the measurement of —dln[03]/dr in the presence of excess [Br], gave ko= (7 1) X 10 cm mol s at 298 K. The same method has been used for the rate constant of the more rapid Cl + O3 reaction. 

Nitrogen. The reaction of PH3 in Ar with atomic N is accompanied by a pale green luminescence. The bands observed in the range 6000 to 2300 cm- were assigned to PH, PH2, PN, NJ, and NH. The final product is a solid [64]. The reaction of PH3 in the carrier gas He with N( S) atoms was found to be very slow. A rate constant of k 4.0 x10" cm molecule" s" at ambient temperature was determined in a flow tube by molecular-beam sampling mass spectrometry. This rate constant is compatible with the slight endothermicity (ArH298 = 4 kJ/mol) of the reaction PH3 + N- -PH2 + NH [36]. 

The ozonolysis of simple alkenes was studied in two different apparatus a 2 litre stirred tank reactor (for ethene, propene, fran.y-2-butene, butadiene, and isoprene) coupled via molecular beam sampling to a matrix isolation FTIR set-up [20, 21], and a 570 L spherical glass vessel "big sphere" (for ethene, 2-butene isomers, isobutene, and isoprene) where products were identified by Fl lR spectroscopy, GC and a scrubber sampling unit for analysis with HPLC (for peroxides) and IC (for organic acids). In the latter system, two extreme humidity conditions, one with 0.5 ppm and the other with 2 x 10" ppm (corresponding to ca. 60 % relative humidity at 298 K) were used, which are referred to as "dry" and "wet" conditions, respectively. Results of the studies performed in the "big sphere" are summarised here. 

---