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Aroma is an important characteristic in the acceptance of beef by consumers, and preference is often culturally dependent. Traditional grading of aroma quality has been carried out using sensory analysis of very limited numbers of samples, due to the inherent costs and the lack of appropriate technologies to replace the human olfactory system. Since meat aroma is derived from various volatile organic compounds (VOCs) that impart favorable or unfavorable qualities, there is potential to apply selected ion flow tube mass spectrometry (SIFT--MS) to facilitate wider scale, economic grading of carcasses. SIFT--MS is a direct analysis technique that can provide both high sample throughput and selective analysis of the chemically diverse volatiles that contribute to the aroma. This paper describes an investigation of the applicability of SIFT--MS to beef grading. Classifications of beef aroma were provided by a trained sensory panel. SIFT--MS was used to analyze the same samples for aroma volatiles. Application of multivariate statistical analysis to the combined data set demonstrated that SIFT--MS discriminates premium quality beef from eight sensory defects, and, therefore, could be applied as an instrumental grading tool obviating sensory panel grading., There are several factors that contribute to the acceptability of a beef cut to the consumer, but most important is its flavor. Certain volatile organic compounds (VOCs) impart favorable or [...]

The rate coefficients and branching ratios of 15 chemical warfare agent precursor and surrogate compounds reacting with [H.sub.3][O.sup.+], N[O.sup.+], and [O.sub.2.sup.+] have been measured in the laboratory using the selected ion flow tube (SIFT) technique. Measurement of the relevant kinetic parameters for these agents has enabled quantitative monitoring using the SIFT-MS analytical technique. Thirteen of the 15 compounds studied were found to have real-time detection limits in the parts-per-trillion-by-volume concentration range when measured on a standard commercial Voicel00 instrument, with specific compounds having detection limits below 100 parts-per-trillion-by-volume. 10.1021/ac901486c

Selected ion flow tube mass spectrometry (SIFT-MS) is a technique that offers real-time alternatives to existing methods for monitoring hazardous air pollutants (HAPs) in the environment using chemical ionization. The use of this technique requires knowledge of the kinetic parameters of the reagent ions [H.sub.3][O.sup.+], N[O.sup.+], and [O.sub.2.sup.+] that are most commonly used. We report here measurements with these reagent ions of kinetic parameters for 17 HAP molecules ranging from 1,1-dichloroethene to nitrobenzene. From these data, limits of quantitation are established for all 17 compounds on a commercial SIFT-MS instrument and are found to be well below the time-weighted averages required by legislating bodies for workplace conditions.

The rate coefficients of the ion-molecule reactions between [H.sub.3][O.sup.+], N[O.sup.+], [O.sub.2.sup.+], and phosphine were determined using a selected ion flow tube. Using these data, the selected ion flow tube mass spectrometry (SIFT-MS) method was applied to the real-time measurement of phosphine in nitrogen without sample preparation down to concentrations in the mid parts per trillion range. This is the first reported measurement using SIFT-MS in the parts per trillion range. Linear dependencies on concentration were found from 190 ppt to the ppm range, and the limit of detection for a 10-s scan was 190 ppt (0.27 pg/mL).

A fast, efficient, real-time method for the quantitative analysis of the peroxide explosive, TATP, is described. The method utilizes rapid ion-molecule reactions of chemical reagent ions with the vapor above solid TATP. The reactions of three reagent ions ([H.sub.3][O.sup.+], [O.sub.2.sup.-]-, N[O.sup.+]) were examined. Although all three ions exhibited a near-collision-rate reaction with TATP, only N[O.sup.+] showed product ions that provide unequivocal evidence for a TATP-based explosive. The limit of detection of TATP in the gas phase is 10 ppb (4 x [10.sup.10] mol [L.sup.-1]).

Atomic oxygen reactions with nitrile cations, protonated nitriles and protonated carbon oxides are examined.

An ion flow tube operating at room temperature was used to examine the reactions of a series of CmHn+ ions, where m = 1-6 and n = 0-6, with atomic and molecular nitrogen. Molecular nitrogen was found to be relatively unreactive to CmHn+ ions due to the strong nitrogen-nitrogen bond, which seems to the electrostatic in nature. In contrast, reactions between atomic nitrogen and CmHn+ are common with many displaying several reaction channels.

Different reactivities with various neutral reagents in a selected-ion flow tube (SIFT) differentiated the CCCN+ and cyclic C3N+ isomeric form of the C3N+ ion, produced by electron impact on HC3N and C4N2 and as a product in various ion-molecule reactions. Collision-rate reactions with majority of the neutral molecules examined were evident for the most reactive isomer, CCN+, which was the key form of the ion from all sources of induction.

The study of the ion-molecule reactions of 17 alkyl esters reacting with the common selection ion flow tube mass spectrometry (SIFT-MS) reagent ions, [H.sub.3][O.sup.+], [H.sub.3][O.sup.+]*[H.sub.2]O (n = 1,2,3), N[O.sub.+], and [O.sub.2.] is reported. The rearrangement of a protonated alkyl ester to form a parent protonated carboxylic acid is found to occur via an intramolecular rearrangement in the gas phase.

Hydrogen transfer is a major channel in the ionized C (sub 2) H (sub 2)(super +)/H (sub 2) CO and H (sub 2) CO (super +)/C (sub 2) H (sub 2) reaction systems, producing C (sub 2) H (sub 4)(super +) + CO. Similarly, H(sub 2) transfer from H(sub 2)CO is observed in reactions of the ions C(sub 3)H(sub 3)(super +) (linear), CH(sub 3-)CCH(super +), H(sub 2)CCCH(sub 2)(super +), C(sub 3)H(sub 5)(super +), and HCCCCH(super +) with formaldehyde, resulting in hydrogenated hydrocarbon ions. The reactions where either H(sub 2)(super +) or H(sub 2) is transferred can provide significant pathways for the hydrogenation of unsaturated hydrocarbons in interstellar clouds.

A new solution to the master equation relating the rate coefficients for unimolecular, recombination (association) and chemical activation reactions, incorporating weak collision effects, is presented. The solution establishes conditions for the validity of the commonly used procedure of relating the recombination rate coefficient, throughout the falloff regime, to the reverse single-channel unimolecular rate coefficient via the equilibrium constant. In addition, a relationship between the rate coefficient for stabilization in a chemical activation reaction and the reverse multichannel unimolecular dissociation rate coefficient is derived. This result, in conjunction with recently developed methods for fully incorporating angular momentum conservation into the solution of the master equation for unimolecular dissociation, enables both angular momentum and weak collision effects to be accurately incorporated into the solution of the master equation for chemical activation reactions in the falloff regime. Application of this method to a typical ion/molecule chemical activation reaction, that of CH+3 with NH3, illustrates the importance of weak collision and angular momentum effects in this system. [ABSTRACT FROM AUTHOR]

Two new techniques are presented for calculating the pressure dependence of ion/molecule association rates: (i) a semiclassical method of incorporating the effect of the hindered dipole rotation into an RRKM calculation of the microscopic rate coefficients k(E,J), and (ii) a solution for the master equation for unimolecular dissociation and recombination reactions, which incorporates angular momentum (J) conservation and is applicable when the moments of inertia of reactant and activated complex differ by a large amount. These techniques provide the optimal currently available means for calculating the pressure dependence of rate coefficients for ion–molecule reactions, which are highly sensitive to J-conservation effects. The method may be used for reliable estimates and fitting of experimental fall-off data. The new technique shows that Troe’s solution of the low-pressure J-conserving master equation is accurate for the nonequilibrium population distribution but overestimates (by up to a factor of 2) the rate coefficient; extensions to Troe’s method to provide more accurate analytical approximations are derived. Application is presented to the association of CH3+ with HCN. The experimental pressure dependence, which cannot be fitted without proper accounting for J conservation, is fitted using the new treatment with values of ∼125 cm-1 for the average downward energy transfer for both rotational and internal energies. The new J-conservation method is also applicable to reactions of neutrals where the moment of inertia of the activated complex is more than six times that of the stable reactant. [ABSTRACT FROM AUTHOR]

The reaction of the methyl cation with hydrogen cyanide is revisited. We have confidence that we have resolved a long standing apparent contradiction of experimental results. A literature history is presented along with one new experiment and a re-examination of an old experiment. In this present work it is shown that all of the previous studies had made consistent observations. Yet, each of the previous studies failed to observe all of the information present. The methyl cation does react with HCN by radiative association, a fact which had been in doubt. The product ions formed in the two-body and three-body processes react differently with HCN. The collisionally stabilized association product formed by a three-body mechanism, does not react with HCN and is readily detected in the experiments. The radiatively stabilized association product, formed by a slow two-body reaction, is not detected because it reacts with HCN by a fast proton transfer reaction forming the protonated HCN ion. Previous studies either ‘‘lost’’ this product in the extremely large protonated HCN signal that is always present when HCN is used, or discounted it for various reasons. We have been able to show by ion cyclotron resonance (ICR) techniques (both FT-ICR and tandem ICR-dempster-ICR) that the radiative association product does react with the HCN to form the protonated HCN ion. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The ion–molecule association system (CH+3/CH3CN) has been reexamined by the ion cyclotron double resonance technique. An experimental distribution of lifetimes has been measured for the collision complex (CH3CNCH+3)* formed in the association reaction between CH+3 and CH3CN. The experimental mean lifetime of the association complex formed within the ICR cell was 140 μs. A theoretical examination of the distribution of complex lifetimes using an RRKM model was also undertaken. The matrix of lifetimes for the various values of the total energy of the system (E) and the total angular momentum of the system (J) was obtained. This information was used to visualize the canonical ensemble of collision complexes in the ICR experiment in terms of their lifetimes. Once the distribution of lifetimes predicted by the model was modified to conform to experimental constraints, it was found to give a good approximation of the lifetime distribution determined experimentally. As a result of the new measurements of the complex lifetimes, we report absolute values of the collisional stabilization efficiencies. We also report rate coefficients for unimolecular dissociation and radiative relaxation. [ABSTRACT FROM AUTHOR]

A flowing afterglow-selected ion flow drift tube is used to calculate the rate coefficients and product ion dispersions for reactions of H3O(super +), N2H(super +), and H3(super +) with a range of alkanes, alkenes, alkynes and aromatic hydrocarbons. The results indicate that in hydrogen-dominated planetary and interstellar conditions the reactions of H(sub 3(super +) can transform C2-C6 hydrocarbons to smaller and less saturated molecules.