Molecular ion intensity

Molecular ion The molecular ion intensity decreases with increasing molecular weight, but is still detectable through CUl although the ion abundance is low (0.1%). 

Molecular ion The presence of sulfur can be detected by the 34S isotope (4.4%) and the large mass defect of sulfur in accurate mass measurements. In primary aliphatic thiols, the molecular ion intensities range from 5-100% of the base peak. 

The molecular ion is slightly more intense in the mass spectra of secondary alcohols than in tertiary alcohols, but even in secondary alcohols, the molecular ion intensity is very small. 

The presence of chlorine and/or bromine is easily detected by their characteristic isotopic patterns (see Appendix 11). As in many aliphatic compounds, the abundance of the molecular ion decreases as the size of the R group increases. For example, in the El mass spectra of methyl chloride and ethyl chloride, the molecular ion intensities are high, whereas in compounds with larger R groups such as butyl chloride, the molecular ion peak is relatively small or nonexistent. 

The molecular ion intensity decreases with increased branching, therefore the molecular ion peak may be nonexistent. The loss of 15 Daltons from the molecular ion indicates a methyl side chain. The mass spectra of branched alkanes are dominated by the tendency for fragmentation at the branch points, and hence are difficult to interpret. 

Alkylbenzenes have molecular ions at the following m/z values 92, 106, 120, 148, and so forth. The molecular ion intensity decreases with increasing alkyl chain length, but can be detected up to at least Cifi. Characteristic fragment ions are m/z 39, 50, 51, 52, 63, 65, 76, 77, and 91. 

Figure 3 The molecular ion intensity of Rhodamine 6-G (determined by 252-Cf-PDMS) as a function of solution concentration.

Contributions from molecular ions are typically more difficult to correct quantitatively because there may be many molecular ions important over a short mass range the molecular ion intensity varies, depending on the sample matrix and the intensities may vary over time more dramatically than elemental ion signals. Multivariant methods including multiple linear regression [150,151], principal component analysis [152], and multicomponent analysis [153] have been used. Improvements in detection limits by up to two orders of magnitude... 

Shorter pulse excitation has been shown to be more efficient for producing a molecular ion for the case of C6o [20,21], Ceo was ionized by pulses from 5 ps to 9 fs. The shortest pulse of 9 fs has been shown to produce Cg(j" (z = 1) without fragmentation. Metal CO complexes also showed smaller yields of fragmentation by shorter pulase excitation in two different pulse widths of 50 and 110 fs [16]. The same tendency has been observed in ionization of 2,3-dimethyl-l,3-butadiene. The relative molecular ion intensity increased as the decrease in the pulse width in a range from 1000 to 15 fs [4,13],... 

The ratios of the molecular ion intensities of all charge states ( 1VF+) to the total ions (Total) are plotted in Fig. 2.10. The total ions are the sum of molecular and fragment ions. The open circles indicate the results at 1.4 pm and the squares represent those observed at 0.8 pm. The ratio is 1.0 at 0.2-0.3 x 1014 Wcm-2 in other words, the molecular ions that consist of singly and doubly charged ions account for 100% of the ions for the case... 

Fig. 2.10. The ratio of the molecular ion intensities to the total ions of dibenzo-p-dioxin vs. laser intensity at two excitation wavelengths filled squares ( ) 0.8-pm excitation. Open circles (o) 1.4-pm excitation...

Fig. 2.11. 1,3,6 -Trichlorodibenzo-p-dioxin was ionized by 130-fs pulses at 1.4 pm The molecular ion intensity, M+, was the highest. M2+ indicates the doubly charged molecular ion...

The weight percents of the individual homologs in each specific-Z series (carbon-number distributions) were calculated from LV/EI/MS molecular-ion intensities assuming constant mole sensitivities for each specific-Z series. An invalid factor was inadvertently used in the previous conversion of the LV/EI/MS carbon-number distributions for the asphaltene neutral fraction to carbon-number distributions based on the total liquid. Consequently, the entries in the LV/EI/MS carbon-number distributions for Z(H), Z(O), and Z(S) asphaltene neutral aromatic compounds in References 35 and 47, the total weight percentages of these specific-Z series in References 35, 47, and 48, and the sums of these latter weight percentages reported in all these references should be multiplied by 0.892. 

FIGURE 12.12 Electrospray ionization mass spectra of bupropion (solid line) and [ Hgjhydroxy-bupropion (dotted line). Note that the protonated molecular ions (MH+, respectively, at mjz 256 and 262) exhibit characteristic chlorine isotope peaks that have 25% of the molecular ion intensity at m/z 258 and 264, due to the relative natural abundance of Cl and Cl. This is reflected also in the MH+-H2O ions at mjz 238 and 244. Data provided by R.L. Walsky and R.S. Obach, Pfizer, New York, NY. 

Two FD studies of sulfonic acids in the positive-ion mode have been published. Schiilten and Kiimmler11 examined a variety of sulfonic acids, including alkanesulfonic acids, substituted benzenesulfonic acids, naphthalenesulfonic acids and anthraqui-nonesulfonic acids. Their results can be summarized as follows (a) all the compounds gave high molecular ion intensities, normally with less intense [M + H] + ions, except... 

Fig. 6. A comparison of the absorbance of fractions eluting off an HPLC column (UV/VIS) with the total molecular ion intensities of the fractions (TIC). Separation C-18 reverse phase colrnnn with 90 10 v/v water/acetonitrile (both containing 0.1% HAC) to 10 90 v/v over 20 min. Detection UV absorbance at 254 nm total ion coimts (TIC) with ESI source, m/z scanned 350-2000 in positive ion mode (Esquire LC/MS Ion Trap Mass Spectrometer/ Bruker)...

As another example, aliphatic hydrocarbons contain a large number of bonds, all with nearly equal energies. The likelihood that at least one of the bonds will break is high, so molecular ion peaks from aliphatic hydrocarbons will be of low intensity if observed at all. Cyclic hydrocarbons, on the other hand, must cleave two carbon-carbon bonds before any loss of mass occurs (neglecting the relatively unimportant loss of a hydrogen atom), with the result of greatly increased molecular ion intensities. 

Heteroatoms in a molecular ion tend to promote fragmentation. Cleavage of the carbon-heteroatom bond and of the bond a to the heteroatom is common. n-Decanol, for example, has a relative molecular ion intensity nearly 1,000 times less than that of n-decane, and the expulsion of HCN from pyridine (Figure 2.58b) is considerably more facile than loss of C2H2 from benzene (Figure 2.58a), depicted by Equation 2.91. 

The first of these methods was by Cook s group and involved use of an ammonium chloride matrix. An increase in molecular ion intensity was observed along with reduction in fragmentation for sugars, polypeptides, nucleotides, and vitamins. The mechanism proposed was that large clusters of ions are sputtered initially, consisting of analyte ions surrounded by matrix ions. It is the formation of these clusters that results in increased yield of protonated species (by proton transfer from the ammonium ion) and softer ionization because of relaxation processes in the clusters. 

Addition of a stabilizing entity into a chain will dramatically change molecular ion intensity. The aromatic nucleus and other conjugate systems are particularly notable in this respect as the radical site may be dispersed by conjugation over an extended part of the ion. Stabilization is also found in compounds containing unsaturated functional groups such as carbonyl, nitro, nitrite, and sulfonyl. 

Molecular ion Intensity decreasing with increasing degree of branching. No M+ is observed in highly branched systems. 

Radio-frequency spark spectra from pelleted samples of oxides and other compounds contain high molecular ion intensities due to fragments of the inorganic compound. There are also high intensity molecular ions due to formation of almost any imaginable combination of individual atoms with atoms of the... 

Recorded kinetic energy distributions of secondary ions (secondary ion intensity versus emission energy) display a peak at around 2-5 eV emission energy whereupon they drop in intensity. Molecular ion intensities decrease more rapidly than those for atomic secondary ions. Indeed, the latter can extend out to several hundred eV. Decreased matrix effects are also noted for the higher emission energy secondary ions. 

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