El ionization

Verification of the molecular weight of thiirene dioxides by mass spectrometry, employing the conventional electron-impact (El) ionization method, has been unsuccessful due to the absence or insignificant intensity of molecular ion peaks in their mass spectra. The base peak is rather characteristic, however, and corresponds to the formation of the disubstituted acetylene ion by loss of sulfur dioxide91 (equation 3). 

Since 1960 mass spectrometry has always been an important tool to investigate the molecular composition of sulfur vapor, sulfur melts, and the solid dlotropes [201]. Mostly spectra obtained by electron impact (El) ionization have been reported, except for one study in which the main species present in sulfur vapor (S2-Ss) were studied by photoionization mass spectrometry [202]. The following ionization potentials were reported (in eV) [202] ... 

Under special conditions sulfur cations with up to 56 atoms have been observed [209]. Evaporation of liquid sulfur and cooling the vapor in an atmosphere of a cold buffer gas (He) at low pressures followed by adiabatic expansion into the vacuum of a mass spectrometer and El ionization produced mass spectra of clusters of sulfur molecules with m/e ratios up to ca. 1800. The intensity pattern shows that the species (Ss)h are most abundant n = 1-7) followed by (Sy)(S8)n-i clusters and (S6)(Ss)h-i clusters. The latter have the same mass as (Sy)2(S8) -2 clusters see Fig. 34. Thus, the composition of the clusters reflects the composition of hquid sulfur near the melting point which contains Sg, Sy and Se molecules as the majority species [34, 210]. 

Undoubtedly, the technique most suited to tackle polyatomic multichannel reactions is the crossed molecular beam (CMB) scattering technique with mass spectrometric detection and time-of-flight (TOF) analysis. This technique, based on universal electron-impact (El) ionization coupled with a quadrupole mass filter for mass selection, has been central in the investigation of the dynamics of bimolecular reactions during the past 35 years.1,9-11 El ionization affords, in principle, a universal detection method for all possible reaction products of even a complex reaction exhibiting multiple reaction pathways. Although the technique is not usually able to provide state-resolved information, especially on a polyatomic... 

However, a shortcoming with the VUV photoionization approach is that absolute PI cross-sections are very often not known, and therefore branching ratios cannot be estimated. As matter of fact, studies of photodissociation processes by soft PI using synchrotron light are usually accompanied by measurements carried out using classic (hard) El ionization, where much data have to be taken at all possible fragment masses in order to estimate branching ratios.14-16,20... 

In our laboratory we have recently implemented this detection method, that we call soft El ionization.31-34 It is analogous to soft PI by synchrotron radiation, but has the bonus that one can also derive branching ratios, a very important piece of information when studying multichannel reactions, and this affords an attractive alternative to the use of PI by a synchrotron source. 

We will first examine the effect of soft El ionization on the mass spectra of the background (detector closed) and during an experiment (detector open... 

Measurements of product angular and TOF distributions using soft El ionization will be discussed with reference to some multichannel reactions of 0(3P) and C(3P) with unsaturated hydrocarbons (see Secs. 3 and 4). 

We will now discuss some very recent applications of the soft El ionization method for product detection in CMB experiments. We will first deal with two polyatomic reactions of ground state oxygen atoms with unsaturated hydrocarbons (acetylene and ethylene) these reactions are characterized by multiple reaction pathways and are of great relevance, besides being from a fundamental point of view, in combustion and atmospheric chemistry. 

Product Angular and TOF Distributions Using Soft El Ionization... 

Detection of the CH2 counter-fragment at m/e = 14 is also problematic in classic CMB experiments employing hard El ionization, because of the... 

Indeed, by using soft El ionization, we have been able to unambiguously detect products from all five reaction pathways (2a)-(2e), determine their branching ratio and characterize their dynamics.34 Here we discuss some of the results that we have obtained on this reaction, which well exemplify the power of soft El ionization. First of all, from measurements of the El efficiency curves at various to/e ratios (15, 42, and 43), we have found that the parent ion at m/e = 43 (CH2CHO+, corresponding to one of the main reaction channels, the vinoxy radical,) is not stable, so measurements of angular and TOF distributions were carried out at m/e = 42. Incidentally, from the El ionization efficiency curve at m/e = 42 we have obtained some direct information on the IE of the vinoxy radical, for which no such information was available till now. The IE should be <11 eV. 

We would like to stress how, for instance, the detection of methylene from the reaction channel (2e) was only possible because of the use of soft El ionization. In fact, because the AE of Cl I.) from C2H4 is 18eV,72 by using an electron energy of 17 eV it was possible to remove completely the... 

These unsolved problems can be addressed by detailed CMB experiments, which use soft El ionization for product detection and possibly also very low collision energies. In fact, by measuring the El efficiency curves for the C3H products it may be possible to obtain some information on which isomer is actually formed, because the IE of C-C3H is expected to be... 

The MM-1 is a linear quadrupole-based mass spectrometer with electron impact (El) ionization.4 An ion pump provides the required vacuum. By today s standards, its size (0.34m3/12.2ft3) and weight (177kg/3901bs) are... 

For amino acid analysis the labeled protein needs to be hydrolyzed and derivatized. Most commonly the hydrolysis is performed in 6 M HC1, and the amino acids are converted into their isopropyl ester and pentafluoropropanamide derivatives (Fig. 1.1) before GC/ MS analysis. The molecular ion is not always visible after standard electron impact (El) ionization, and the fragment after loss of the carboisopropoxy group is the highest observable peak. This leaves m/e=175 plus the mass of the amino acid side chain, from which the degree of labeling can be directly deduced. 

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