Flame sampling mass spectrometer

Instrumental methods have become more sophisticated to face these challenges. In particular, Westmoreland and Cool have developed a flame-sampling mass spectrometer that has provided several revelations in terms of relevant molecular intermediates in combustion. " Their setup couples a laminar flat-flame burner to a mass spectrometer. This burner can be moved along the axis of the molecular beam to obtain spatial and temporal profiles of common flame intermediates. By using a highly tunable synchrotron radiation source, isomeric information on selected mass peaks can be obtained. This experiment represents a huge step forward in the utility of MS in combustion studies lack of isomer characterization had previously prevented a full accounting of the reaction species and pathways. 

Figure k. 13 9" ion profiles using flame sampling mass spectrometer technique in C2H2/O2 flame (2.1 kPa, 50 cm s" unburned velocity). Critical threshold, 2.5. 

Flames offer a fertile field for studying ion-molecule reactions in the temperature range of 1000-4000°K, and at pressures of 1-760 Torr or higher. Because of the complexities of the system, it is necessary to use several different tools, including special ion-sampling mass spectrometers. 

If samples are introduced continuously, then the measurement of isotope ratios can also be continuous as long as sample is flowing into the flame, and several m/z ratios can be examined with almost any kind of mass spectrometer,... 

The special problems for vaUdation presented by chiral separations can be even more burdensome for gc because most methods of detection (eg, flame ionization detection or electron capture detection) in gc destroy the sample. Even when nondestmctive detection (eg, thermal conductivity) is used, individual peak collection is generally more difficult than in Ic or tic. Thus, off-line chiroptical analysis is not usually an option. Eortunately, gc can be readily coupled to a mass spectrometer and is routinely used to vaUdate a chiral separation. 

Confirmation of the formation of the radicals during combustion reactions has been made by inuoducing a sample of dre flames into a mass spectrometer. The sample is withdrawn from a turbulent flame which is formed into a thin column, by admitting a sample of the flame to the spectrometer drrough a piidrole orifice, usually of diameter a few tenths of a millimetre. An alternative procedure which has been successful in identifying the presence of radicals, such as CHO, has been the use of laser-induced fluorescence. 

The part that marries the plasma to the mass spectrometer in ICPMS is the interfacial region. This is where the 6000° C argon plasma couples to the mass spectrometer. The interface must transport ions from the atmospheric pressure of the plasma to the 10 bar pressures within the mass spectrometer. This is accomplished using an expansion chamber with an intermediate pressure. The expansion chamber consists of two cones, a sample cone upon which the plasma flame impinges and a skimmer cone. The region between these is continuously pumped. 

The extracted fractions were esterified with either BF3-MeOH reagent or diazomethane and analyzed by GLC. Gas liquid chromatography (GLC) was conducted with a Perkin-Elmer Sigma 3 equipped with flame ionization detector. Separations were obtained on a Hewlett Packard 12 m x 0.2 mm i.d. capillary column coated with methyl silicon fluid (OV-101). The temperature was maintained at 80°C for 2 min then programmed from 80 to 220°C at 8°C/min. The injector temperature was 250°C. Mass spectra were obtained on a Hewlett Packard model 5995 GC-MS mass spectrometer, equipped with a 15 m fused silica capillary column coated with 5% phenyl methyl silicone fluid. Spectra were obtained for major peaks in the sample and compared with a library of spectra of authentic compounds. 

The combustion efficiency of the benzene was beyond 99.999% even at an overall equivalence ratio of 1.0 (including the entrainment air which was 30% of the total air flow). With the controller off, the flame was extremely sooty and, in fact, sooting as the quartz tube would be blackened and the mass spectrometer sampling probe clogged in a matter of seconds, and soot was sucked into the scrubber system. This comparison between controller off and controller on conditions with benzene fuel is extremely dramatic and shows the efficacy of active combustion control in vortices to eliminating soot from diffusion flames. 

Table I contains a list of some of the compounds that have been submitted to this type of analysis. The recovery data is intended to be illustrative only since recoveries depend strongly on several important method variables. Recoveries are expressed as a percentage of the amount added to organic free water. The purge time was 11-15 minutes with helium or nitrogen, the purge rate was 20 ml/minute at ambient temperature, and the trap was Tenax followed by Silica Gel. Data from the 5 ml sample was obtained with a custom made purging device and either flame ionization, microcoulo-metric, or electrolytic conductivity GC detectors. Data from the 25 ml sample was obtained with a Tekmar commercial liquid sample concentrator and a mass spectrometer GC detector using CRMS.

Sampling and Analysis. A frozen slice of bread was cut in pieces and stacked in an enlarged sample flask of an aroma isolation apparatus according to MacLeod and Ames (74). Volatile compounds were trapped on Tenax TA and afterwards thermally desorbed and cold trap injected in a Carlo Erba GC 6000 vega equipped with a Supelcowax 10 capillary column (60 m x 0.25 mm i.d.) and a flame ionisation detector. Similar GC conditions were used for GC-MS identification of volatile compounds by dr. M.A. Posthumus (Dept. Organic Chemistry, VG MM7070F mass spectrometer at 70 eV El, 75). 

The most general purpose detector for open tubular chromatography is a mass spectrometer. Flame ionization is probably the most popular detector, but it mainly responds to hydrocarbons and Table 24-5 shows that it is not as sensitive as electron capture, nitrogen-phosphorus, or chemiluminescence detectors. The flame ionization detector requires the sample to contain SlO ppm of each analyte for split injection. The thermal conductivity detector responds to all classes of compounds, but it is not sensitive enough for high-resolution, narrow-bore, open tubular columns. 

In sampling with probes where only stable species can be determined, radicals will affect the analysis if their concentration is comparable to any stable species. In the methane flame, the formaldehyde measured by a mass spectrometer is partially due to methyl radical (Fll). 

TPR of the samples in flowing He or H2 were performed in a Pyrex flow system which was also used for catalytic reactions. Acid properties of the samples were probed by TPD of NH3 preadsorbed at RT. The analysis of gaseous products was made by an on-line mass spectrometer or a thermal conductivity detector. Reactions of n-hexane in the presence of excess H2 were carried out at 623 K and atmospheric pressure. A saturator immersed in a constant temperature bath at 273 K was used to produce a reacting mixture of 6% n-hexane in H2. Reaction products were analyzed by an online gas chromatograph (HP-5890A) equipped with a flame ionization detector and an AT-1 (Alltech) capillary column. 

The final stage of the residue analysis procedures involves the chromatographic separation and instrumental determination. Where chromatographic properties of some food residues are affected by sample matrix, calibration solutions should be prepared in sample matrix. The choice of instrument depends on the physicochemical properties of the analyte(s) and the sensitivity required. As the majority of residues are relatively volatile, GC has proved to be an excellent technique for pesticides and drug residues determination and is by far the most widely used. Thermal conductivity, flame ionization, and, in certain applications, electron capture and nitrogen phosphorus detectors (NPD) were popular in GC analysis. In current residue GC methods, the universality, selectivity, and specificity of the mass spectrometer (MS) in combination with electron-impact ionization (El) is by far preferred. 

An uncooled 1/4-in. O.D. quartz probe with a 70 micron I.D. tip was used to withdraw samples from the flames. The pressure differential between the flame and the inside of the probe was approximately 70 1. The flame samples were analyzed on a Finnigan Model 1015 quadrupole mass spectrometer. Argon was substituted for nitrogen as a diluent in the flames to allow better CO analysis with the mass spectrometer. 

Gas chromatography (GC) is another widely used analytical technique for phytochemical determination. Similar to HPLC, GC requires sample preparation, which may include lipid extraction and/or extraction of phytochemicals. Once the sample is prepared, it enters the inlet system, flows through the column, and then reaches the detector. In the case of phytochemical analysis, the detector is often a flame ionization detector, which is suitable for all organic particles, or more commonly, the sample passes through the column directly to a mass spectrometer, which serves as the detector. 

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