Mass spectrum molecular ion

Mass spectrum Molecular ion at 150 base peak at 135, M - 15, is loss of methyl. 

Mass spectrum Molecular ion at 170 two prominent peaks are M - 15 (loss of methyl) and M - 43 (as we shall see, most likely the loss of acetyl, CH3CO). 

If the substrate (M) is more basic than NHj, then proton transfer occurs, but if it is less basic, then addition of NH4 occurs. Sometimes the basicity of M is such that both reactions occur, and the mass spectrum contains ions corresponding to both [M + H]+ and [M + NH4]. Sometimes the reagent gas ions can form quasi-molecular ions in which a proton has been removed from, rather than added to, the molecule (M), as shown in Figure 1.5c. In these cases, the quasi-molecular ions have one mass unit less than the true molecular mass. 

A single instrument — a hybrid of a quadrupole and a TOF analyzer — can measure a full mass spectrum of ions produced in an ion source. If these are molecular ions, their relative molecular mass is obtained. Alternatively, precursor ions can be selected for MS/MS to give a fragment-ion spectrum characteristic of the precursor ions chosen, which gives structural information about the original molecule. 

Although a typical mass spectrum contains ions of many different masses, the heaviest ion is generally due to the ionized molecule itself. By measuring the mass of this ion, the molecular mass of the molecule can be determined. The naphthalene sample discussed in the previous section, for example, gives rise to an intense peak at mass 128 amu in its spectrum, consistent with its molecular formula of Ci0H8 (Figure 3.10b). 

N—[2—(5,5-Diphenyl-2,4-imidazolidinedion-3-yl)ethyl]-7-acetoxy-1-napthalene sulfonamide This substituted sulfonamide has a molecular weight of 543 daltons, and in the positive LD mass spectrum two ions at m/z 566 and m/z 582, corresponding to attachment of sodium and potassium ions, respectively, were observed. No significant photodissociation pathway other than loss of the attached cation was seen for either of these two ions. In the negative ion LD mass spectrum, an ion at m/z 500 originating from loss of C HjO was observed. This ion photodissociated when irradiated by the (gated) cw laser to produce an ion at m/z 251 (see Figure 9) (Equation 6). 

The first step in the analysis of an unknown elastomer compound is to obtain and examine low-resolution survey MS from the material. These spectra cover a wide mass range (typically - 50-1000 Da) and give an overview of the sample composition. The most useful ionisation method for this is generally field ionisation, since the simplest possible spectrum is obtained. In the FI spectrum molecular ions are dominant, which facilitates the characterisation of the complex organic additive mixtures that are present in typical elastomer compounds. 

Figure 8, MALDI mass spectrum (negative-ion mode) of the acidic xylooligosaccharides obtained by endoxylanase treatment of a hexenuronoxylan isolated from alkaline peroxide bleached hardwood kraft pulp. The spectrum consists of two series of peaks assigned to the molecular ions [M-Hf of xylooligosaccharides with varying chain lengths and containing one hexenuronic acid (HexA) residue or one 4-O-methylglucuronic acid ( 4-0-...

While the practical application of IRMPD spectroscopy to mass-selected molecular ions had thus been shown, its widespread use as a structural tool in irai chemistry was impeded by the limited tunability of the CO2 laser and the absence of other useful laser sources featuring a high power and wide tunability across the IR spectrum. The interest in IRMPD spectroscopy of gaseous ions revived around the mm of the millennium, when IR free electron lasers (FELs) as well as novel high pulse-energy OPOs were coupled with ion storage tandem mass spectrometers. 

Division of Equation [3] by the measured mass-resolved molecular ion current 7 at an anode potential Vq, i.e. in the normal FI spectrum, yields the so-called normalized rate k(r) ... 

The major peaks on a mass spectrum representing ions lighter than the molecular ion are called... 

Time-of-flight mass spectrometers have been used as detectors in a wider variety of experiments tlian any other mass spectrometer. This is especially true of spectroscopic applications, many of which are discussed in this encyclopedia. Unlike the other instruments described in this chapter, the TOP mass spectrometer is usually used for one purpose, to acquire the mass spectrum of a compound. They caimot generally be used for the kinds of ion-molecule chemistry discussed in this chapter, or structural characterization experiments such as collision-induced dissociation. Plowever, they are easily used as detectors for spectroscopic applications such as multi-photoionization (for the spectroscopy of molecular excited states) [38], zero kinetic energy electron spectroscopy [39] (ZEKE, for the precise measurement of ionization energies) and comcidence measurements (such as photoelectron-photoion coincidence spectroscopy [40] for the measurement of ion fragmentation breakdown diagrams). 

The amino add analysis of all peptide chains on the resins indicated a ratio of Pro Val 6.6 6.0 (calcd. 6 6). The peptides were then cleaved from the resin with 30% HBr in acetic acid and chromatogra phed on sephadex LH-20 in 0.001 M HCl. 335 mg dodecapeptide was isolated. Hydrolysis followed by quantitative amino acid analysis gave a ratio of Pro Val - 6.0 5.6 (calcd. 6 6). Cycll2ation in DMF with Woodward s reagent K (see scheme below) yielded after purification 138 mg of needles of the desired cyc-lododecapeptide with one equiv of acetic add. The compound yielded a yellow adduct with potassium picrate, and here an analytically more acceptable ratio Pro Val of 1.03 1.00 (calcd. 1 1) was found. The mass spectrum contained a molecular ion peak. No other spectral measurements (lack of ORD, NMR) have been reported. For a thirty-six step synthesis in which each step may cause side-reaaions the characterization of the final product should, of course, be more elaborate. 

During the course of biochemical studies (138). the mass spectrum of 2-acetamidothiazole was recorded its main peaks are the molecular ion (m/e= 142, relative intensity = 26%) and fragments 100 (100), 58 (2. 5), and 43 (39). For 2-acetamido-5-bromothiazole the main peak results again from the loss of C2H2O by the molecular ion. 2-AcetyIacet-amido-4-methylthiazole (2S) exhibits significant loss of from the... 

A protomeric equilibrium favors the acetamido rather than the acetimido form (105, 121). The parent molecular ion has been reported to be absent in the mass spectrum of 2-acylaminothiazoles (130). 

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