Chemical ionisation mass spectra

Produces chemical ionisation mass spectra (M + H)+ at atmospheric pressure... 

Stephanou [38] identified non ionic surfactants in non saline water by gas chromatography coupled with chemical ionisation mass spectrometry. Tertiary octylphenol and lauryl alcohol ethoxylates were qualitatively detected using their chemical ionisation mass spectra and the results used to compare the chemical ionisation and electron impact mass spectrometry techniques. 

Chemical ionisation mass spectra (CIMS NHJ as ionisation reactant) can yield satisfactory spectra from underivatised peptides and field-desorption ionisation has been shown to give intense [M+ 1]+ ions from otherwise involatile peptide derivatives (for example, CH3—CO—Gly—Arg—Arg—Gly—OCH3 Buehler el al., 1974), but less sequence information is gained in these mild ionisation procedures because less fragmentation occurs and there are relatively few peaks in the mass spectrum. MALDF - matrix-assisted laser desorption ionisation MS - is an acronym that is encountered in recent literature for this ionisation technique. 

Chemical ionisation mass spectra obtained with argon and methane reagent gases are presented in Figures 9.9(b) and (c), respectively. These spectra were obtained to... 

Electron impact and chemical ionisation mass spectra of [M(CO).(n -allyl)]... 

Since the background ion current of the positive ion methane chemical ionisation mass spectrum in the region of 10-60 m/z was intense, no attempt was made to monitor the evolution of hydrogen chloride ( H Cl, m/z 36). Protonated molecular ions were observed for the major fragments with the exception of methylnaphthalene, methylanthracene, and ethylanthracene. The nominal structures for the fragments that result from the thermal degradation of the polyene are shown in Table 4.1. 

The value of chemical ionisation mass spectrometry as a complementary technique to conventional electron impact mass spectrometry has been illustrated by Fales et al., ° who investigated the behaviour of representatives of nine of the major alkaloid families, including homatropine. In C.I. mass spectrometry the quasi-molecular ion [QM i.e. (M -I- 1) ] is invariably more abundant than is the molecular ion in E.I. mass spectrometry. Thus, with methane as reactant gas, homatropine shows a moderately strong quasi-molecular ion, and an ion at mje 258 owing to QM — HjO. In the E.I. mass spectrum, homatropine shows a very small molecular ion, and no ion at mje 258 thus the presence of a hydroxy-group passes unnoticed. ... 

The chemical ionisation mass spectral data indicate that intramolecular exchange reactions predominate in the primary thermal fragmentation process of polyethylene oxalate resulting in the formation of cyclic oligomers. These products are not stable in the electron ionisation (El) mode and are therefore not directly observed in the El mass spectrum. 

Microprobe laser desorption laser ionisation mass spectrometry (/xL2MS) is used to provide spatial resolution and identification of organic molecules across a meteorite sample. Tracking the chemical composition across the surface of the meteorite requires a full mass spectrum to be measured every 10 p,m across the surface. The molecules must be desorbed from the surface with minimal disruption to their chemical structure to prevent fragmentation so that the mass spectrum consists principally of parent ions. Ideally, the conventional electron bombardment ionisation technique can be replaced with an ionisation that is selective to the carbonaceous species of interest to simplify the mass spectrum. Most information will be obtained if small samples are used so that sensitivity levels should be lower than attomolar (10—18 M) fewer than 1000 molecules can be detected and above all it must be certain that the molecules came from the sample and are not introduced by the instrument itself. 

A typical REMPl at 266 nm mass spectrum is shown Fig. 15.22, obtained by roasting 80 g of Arabica coffee at 225 °C. The laserpower density was adjusted to 10 -10 W/cm in order to avoid non-resonant ionisation processes. The spectrum contains predominantly molecular ions. Chemical assignment of the ion peaks was based on three distinct pieces of information the literature on coffee flavour compounds [204], the mass as observed in TOFMS and optical absorption properties. With this information, many volatiles observed in Fig. 15.22 were unambiguously identified. 

Figure 16.17—Chemical ionisation. This figure shows reactions occurring when methane is used as a reagent gas. The last equation represents the reaction that occurs when isobutane is used. Because of the high pressure used, the intensity of ions from the reagent gas is high, thus the mass spectrum is not scanned below 50 Da.

The protonation reaction leading to MH + depends on the proton affinity of M. Because hydrocarbons of type RH have a low proton affinity, the R+ ion is often seen in the spectrum. Since proton transfer in chemical ionisation can lead to either M + H + or M — H 1, uncertainties can exist as to the true mass of the molecular ion. 

The chemical ionisation (Cl) mass spectrum Fig. 3, was recorded on a Finnigan 4000 Mass Spectrometer with ion source pressure 0.3 Torr, ion source temperature 150°C, emission current 300 yA, electron energy 100 eV using methane as a reagent gas. The electron impact (El) mass spectrum Fig. 4, was recorded on Varian MAT 311 Spectrometer, with an ion source pressure 10 6 Torr, ion source temperature 180, emission current 300 yA and electron energy of 70 eV. 

The most important piece of information which may be obtained from a mass spectrum is the molecular weight. However, certain classes of compounds do not show molecular ions and in other cases it is not always possible to identify unequivocally the molecular ion. Therefore a family of so called soft ionisation techniques has been developed. These generate a molecular ion or quasi-molecular ion and fragmentation is kept to a minimum. The most commonly used technique is chemical ionisation (Cl)... 

It is preferable to have a mass spectrum which shows the molecular ion of the compound and for this reason soft ionisation techniques such as chemical ionization (Cl), fast atom bombardment (FAB) and electrospray are now widely used instead of or as well as electron impact (El) ionization. 

Amoxicillin trihydrate did not give a useful mass spectrum by the conventional electron impact ionisation technique, even with a specialised in beam procedure which gave spectra from several other penicillins [26], Laser desorption [27] and desorption chemical ionisation [28] both gave pseudomolecular ions and the latter technique also gave significant fragmentation. 

The presence of an /V-alkylatcd terminal amino-acid residue (/V,TV-dimethyl-valine), deduced from the mass spectrum, means that chemical sequencing (Edman degradation) is ruled out. The sequence followed from the interpretation of the mass spectrum. The usual site of ionisation, the C-terminal carbonyl group, was the starting point for initial speculation on interpretation of the mass spectrum to solve the sequence. The positions of valine, /V-methylvaline, proline, hydroxyvaline (i.e. 2-... 

The differences in potential energy between the various species formed during Cl can also yield odd effects. The protonated molecule formed by ammonia chemical ionisation of acetyl salicylic acid is so energetic that it decomposes losing water, in some sources, whilst the ammoniated cluster ion is stable. Thus the spectrum shows ions at m/z 163 (M+H -H20) and 198 (M+NH4" ), but no ion at the expected mass m/z 181 (M+H ). Similar effects can be seen using APCI and thermospray, both hot sources where thermal energy can be absorbed to generate thermal decomposition of the intact molecule. Electrospray... 

Figure 1.16 Electroionisation and chemical ionisation spectra of polystyrene (a) Partial El spectrum of polystyrene. The solid lines are the trimer spectrnm and the nnmbers in parenthesis give the first number of monomer units present and second the assigned monoisotropic mass. The numbers in parenthesis give the evaporation temperature in kelvin. The dashed lines are fragment sequence peaks. Reproduced with permission from H.R. Udseth, Analytical Chemistry, 1981, 53, 29. 1981,...

The surface mass spectrum characterises the surface chemical structure. The spectral intensities can be used to determine the relative surface concentrations of the different surface species. Both positive and negative ion detection modes are possible in SIMS, as in all mass spectrometry techniques. A comparison of the positive and negative ion spectra can often substantially improve the analysis of the results. In SIMS, the charged fraction of the secondary particle flux is very small (10 ). Moreover, the number of sputtered ions per incident primary ion (i.e. the secondary-ion yield) is matrix dependent. With such yield variations direct quantification of surface species based on the number of desorbed secondary ions (i.e. from the SIMS data) is generally impossible [123]. Wucher et al. [143] have recently described a method to determine the secondary ion formation probability, i.e. the ionisation probability of sputtered particles in a direct and quantitative manner. 

---