Mass spectrometry experiments

The substituted radical cations [Me2S.. SMe2], [Et2S. .SEt2] and [Et2S.. SMe2] (Fig. 4) have been studied by lilies, McKee and co-workers using a combined experimental/theoretical approach [127-129]. Mass spectrometry experiments on the gas-phase association reactions... 

The molar mass (MM) of any substance is the mass of one mole of that substance. As described in Section 2-1. each isotope of a particular element has a different mass. Therefore, the mass of one mole of any isotope has a unique value, its isotopic moiar mass. This characteristic molar mass can be found by multiplying the mass of one atom of that isotope by Avogadro s number. For example, mass spectrometry experiments reveal that one atom of carbon-13 has a mass of 2.15928 X 10- g, from which we can calculate the isotopic molar mass of... 

In mass spectrometers, ions are analysed according to the ml7. (mass-to-charge) value and not to the mass. While there are many possible combinations of technologies associated with a mass-spectrometry experiment, relatively few forms of mass analysis predominate. They include linear multipoles, such as the quadrupole mass filter, time-of-flight mass spectrometry, ion trapping forms of mass spectrometry, including the quadrupole ion trap and Fourier-transform ion-cyclotron resonance, and sector mass spectrometry. Hybrid instruments intend to combine the strengths of the component analysers. 

Table 2.1 Information obtainable by mass spectrometry Experiment Information...

In soft ionization methods the excess energy deposited onto the ionized molecule is very small and stable even-electron ions are formed. This leads to easy determination of the molecular weight of the analyte, but as fragmentation is absent or it occurs to a very low extent, structural information is missing in the mass spectrum. However, one can obtain structural information by causing ion fragmentation out of the source by means of tandem mass spectrometry experiments (see below). 

The orange-red [S3N] anion is obtained by the addition of triphenylphosphine to a solution of an [S4N] salt in acetonitrile.69 It can be isolated as a salt in combination with a [Ph4As]+ or [N(PPh3)2]+ cation. The vibrational spectra suggest an unbranched [SNSS] arrangement of atoms (20). Mass spectrometry experiments support the SNSS connectivity in the gas phase.75... 

Ironically, our current plans call for the reverse linkage of the above enrichment procedures. That is, we shall use an electromagnetic isotope separator to enrich argon isotopes for a mass spectrometry experiment, and we shall enrich radiocarbon via thermal diffusion for improved mini-gas proportional counting. 

C-Acyl nitroso compounds are highly reactive species and no examples of stable and isolable C-acyl nitroso compounds have been reported. Charge-reversal and neutraHzation-reionization mass spectrometry experiments provided the first spectroscopic evidence for the existence of acyl nitroso compounds in the gas phase [30,... 

Willse, A., Belcher, A.M., Preti, G., Wahl, J.H., Thresher, M., Yang, P., Yamazaki, K. and Beauchamp, G.K. (2005) Identification of major histocompatibility complex-regulated body odorants by statistical analysis of a comparative gas chromatography/mass spectrometry experiment. Anal. Chem. 77, 2348-2361. 

The appearance energy (formerly known as appearance potential) is a widely used concept in threshold mass spectrometry experiments, which involve measuring the minimum energy required to cause a certain process. However, there are a number of theoretical and practical problems associated with the determination of reliable values of H o(A+/AB). In the following paragraphs we summarize the discussion of this subject made by the groups of Traeger for photoionization [64,65] and Holmes for electron impact [66]. 

N. V. Lokshin, L.R. Chuprikov, A.V. Discrimination Effects in Inoi anic lon-Cluste Detection by SEM in Mass Spectrometry Experiments. Rapid Commun. Mass Spectrom. 1990,4,9-12. 

Instrnments combining several analyzers in sequential order are very common. This combination allows mass spectrometry and mass spectrometry experiments (MS/MS) to be carried out. Modern MS/MS includes many different experiments designed to generate substructural information or to qnantitate componnds at trace levels. A triple quadru-pole mass spectrometer allows one to obtain a daughter ion mass spec-trnm resnlting from the decomposition of a parent ion selected in the first qnadrnpole. The MS/MS experiments using an FTICR or ion trap, however, are carried ont in a time-resolved manner rather than by spatial resolntion. 

Electron-transfer processes play many very important roles in chemistry and biology. Because the present work is focused on electron-transfer events occurring within positively charged gas-phase peptides as they occur in ETD and ECD mass spectrometry experiments, it is not appropriate or feasible to review the myriad of other places electron-transfer reactions occur in chemistry. Chapter 10 of the graduate level textbook by Schatz and Ratner [12] gives a nice introduction to the main kinds of electron-transfer events that chemists usually study as well as to the theoretical underpinnings. They also give, at the end of Chapter 10, several literature references to selected seminal papers on these subjects. 

The analytically important features of Fourier transform ion cyclotron resonance (FT/ICR) mass spectrometry (1) have recently been reviewed (2-9) ultrahigh mass resolution (>1,000,000 at m/z. < 200) with accurate mass measurement even 1n gas chromatography/mass spectrometry experiments sensitive detection of low-volatility samples due to 1,000-fold lower source pressure than in other mass spectrometers versatile Ion sources (electron impact (El), self-chemical ionization (self-Cl), laser desorption (LD), secondary ionization (e.g., Cs+-bombardment), fast atom bombardment (FAB), and plasma desorption (e.g., 252cf fission) trapped-ion capability for study of ion-molecule reaction connectivities, kinetics, equilibria, and energetics and mass spectrometry/mass spectrometry (MS/MS) with a single mass analyzer and dual collision chamber. 

Even though theoretical studies have identified a number of potentially metastable structures [1, 2], the experimentally observed nitrogen compounds are still few. The azide anion, N3, wai. first synthesized in 1890 by Curtius [3]. Christie and coworkers have since 1999 reported the preparation and isolation of Ns"1" together with several different counter ions [4, 5]. A few other species, such as N3, Ns+, N4+, and N6, have been observed only as gaseous or matrix-isolated ions or radicals [6-12], We recently reported the detection of cyclic N5 in a mass spectrometry experiment [13], This observation has later been verified by Christie et. al. in a more elaborate study [14], The experimental preparation and detection of an open-chain N4 molecule was reported in 2002 by Cacace et al [15]. This species is expected to be unstable towards bimolecular decomposition and also too low in energy to be of any greater interest as a HEDM. 

The assembly of molecular ions formed in a mass spectrometry experiment typically contains ions with different amounts of internal energy. A distribution function P( ) can be defined, provided internal energy is randomised and reaction is slow compared with the ionization and randomisation processes. P( ) must be appropriately normalised. It will be assumed here that /fmax P(E)dE is equal to the total number (per time) of molecular ions formed without restriction as to internal energy content. For a single decomposition, eqn. (3) becomes... 

In the most common tandem mass spectrometry experiment a first analyser is used to isolate a precursor ion, which then undergoes spontaneously or by some activation a fragmentation to yield product ions and neutral fragments ... 

Basically, a tandem mass spectrometer can be conceived in two ways performing tandem mass spectrometry in space by the coupling of two physically distinct instruments, or in time by performing an appropriate sequence of events in an ion storage device. Thus there are two main categories of instruments that allow tandem mass spectrometry experiments tandem mass spectrometers in space or in time. 

Similar mass spectrometry experiments with pure human and mouse transferrin (Li et al., 2008c), and with human kinesin showed that the OP label was consistently on tyrosine (Table 56.1). Studies with human plasma identified OP labeling on tyrosine in apolipoprotein and alpha-2-glyco-protein. Aggressive treatment of hiunan albumin with FP-biotin and chlorpyrifos oxon led to identification of seven OP-labeled tyrosines (Ding et al, 2008). Finally, we found that synthetic peptides made a covalent bond with DFP, chlorpyrifos oxon, and dichlorvos (Table 56.1). Mass spectrometry conclusively proved that the OP was attached to tyrosine. 

Types of Mass Spectrometry Experiment for Organic Chemists... 

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