Ion intensity

In a mass spectrometer, the molecules, in the gaseous state, are ionized and fragmented. The fragments are detected as a function of their mass-to-charge ratio, m/e. The graphical representation of the ion intensity as a function of m/e makes up the mass spectrogram as illustrated In Figure 3.1. 

The ion intensity expressed as the amount of electricity, is proportional to the following factors ... 

Knowing and 2 by studying reference components, it becomes possible to calculate from the measurements of ion intensities found on the mass spectrum. 

Figure Cl.4.13. Trap modulation experiment showing much greater deptli of ion intensity modulation (by more tlian one order of magnitude) tlian fluorescence or atom number modulation, demonstrating tliat excited atoms are not tire origin of tire associative ionizing collisions.

In GC-MS effluent from the column is introduced directly into the mass spectrometer s ionization chamber in a manner that eliminates the majority of the carrier gas. In the ionization chamber all molecules (remaining carrier gas, solvent, and solutes) are ionized, and the ions are separated by their mass-to-charge ratio. Because each solute undergoes a characteristic fragmentation into smaller ions, its mass spectrum of ion intensity as a function of mass-to-charge ratio provides qualitative information that can be used to identify the solute. 

A plot of ion intensity as a function of the ion s mass-to-charge ratios. 

Prior to the introduction of ESI, ms /ms studies of peptides were generally limited to molecules mol wt < 3500 (33). This limitation was a consequence of the rapid drop in precursor ion intensity from Isims ion sources with increasing mass, and the inefficiency of coUisional activation. Good... 

Figure 3a Unprocessed depth profile (secondary ion intensity versus sputtering time) of a silicon sample containing a boron ion implant.

Figure 1 shows a positive static SIMS spectrum (obtained using a quadrupole) for polyethylene over the mass range 0—200 amu. The data are plotted as secondary ion intensity on a linear y-axis as a function of their chaige-to-mass ratios (amu). This spectrum can be compared to a similar analysis from polystyrene seen in Figure 2. One can note easily the differences in fragmentation patterns between the...

Figure 5 Plot of positive CF3 secondary ion intensity versus ellipsometric thickness from a set of perfluoropolyether standards.

In a molded polymer blend, the surface morphology results from variations in composition between the surface and the bulk. Static SIMS was used to semiquan-titatively provide information on the surface chemistry on a polycarbonate (PC)/polybutylene terephthalate (PBT) blend. Samples of pure PC, pure PBT, and PC/PBT blends of known composition were prepared and analyzed using static SIMS. Fn ment peaks characteristic of the PC and PBT materials were identified. By measuring the SIMS intensities of these characteristic peaks from the PC/PBT blends, a typical working curve between secondary ion intensity and polymer blend composition was determined. A static SIMS analysis of the extruded surface of a blended polymer was performed. The peak intensities could then be compared with the known samples in the working curve to provide information about the relative amounts of PC and PBT on the actual surface. 

Ion intensities up to a count rate of 2 x 10 are measured using a secondary electron multiplier (SEM). When it becomes saturated above that value, it is necessary to switch to a Faraday cup. Its ion-current amplification must be adjusted to fit to the electron multiplier response. 

For measurement of local ion intensities in the direct imaging mode (see Fig. 3.19), amplification ensuring laterally uniform-single ion detection is necessary. Depending on the sensitivity of the detector a single or double channel plate is used. Two imaging devices are in use ... 

Fig. 3.21. SIMS spectra obtained from a high-speed steei. (A) primary ions OJ no secondary ion energy iimitation eiec-tropositive eiements are sensitive many moiecuie ions are visibie. (B) same conditions but 300 Voffset was used the moiecuie ion intensities are reduced significantiy. (C) Primary ions Cs" 300 Voffset was used therefore eiectronegative eiements are detected more sen-sit iveiy.

For detection of secondary ions a laterally resolving detector is necessary. In the first step a channel plate for amplification is used secondary electrons from the output of this device are accelerated either to a fluorescent screen or to a resistive anode. If a fluorescent screen is used the image is picked up by a CCD camera and summed frame by frame by use of a computer. The principal advantage of this system is unlimited secondary ion intensities, but compared with the digital detection of the resistive anode encoder the lateral and intensity linearity is not as well-defined. 

Practically it is more convenient to measure intensity ratios instead of absolute intensities. Thus, e.g., Cu may serve as a reference material, relative to which the ion intensities back-scattered from the atoms of the surface under consideration are measured ... 

By using a flame ionization detector (FID), most compounds having a bond of carbon and hydrogen can be measured. This detector was originally developed for gas chromatography and employs a sensitive electrometer that measures the change in ion intensity resulting from the combustion of air... 

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. 

The heart of the mass spectrometer is the mass analyzer, the function of which is to measure the mass-to-charge ratios of ions and provide a means of their identification. This is achieved by a combination of a dispersive action to separate the ions according to their m/e ratios and a focusing action to maximize the resolved ion intensities... 

The vapor pressure of Pu(g) in equilibrium with various phases was also determined from ion intensity measurements with the mass spectrometer. The pressure was calculated from the equation... 

Ion exchange technique at Rocky Flats 378 Ion intensity measurements, vapor... 

In Figure 4 the logarithm of the observed ion intensities was plotted as a function of the logarithm of the pressure in the collision chamber. As the intensity of a product ion of a certain order increases proportionally to the same power of the pressure, the curves in the diagram corresponding to primary, secondary, and tertiary ions are represented by straight lines of slopes equal to 1, 2, and 3, respectively. Measurements were performed with 11 incident ions with different recombina-... 

Figure 4. Logarithmic ion intensity-pressure graph of ethylene obtained by bombarding with H2S + of low kinetic energy...

Figure 1 shows the ion intensity vs. the voltage between the ionization chamber and the electron trap, the voltage between filament and chamber being held constant at 8 volts. The gas was methane, in which the secondary ions CH5 + and CH4 + are formed by the following processes ... 

Figure 2. Ionization efficiency curves in the Cermak-Herman operation of an ion source. Relative ion intensity normalized at 40 volts for CHA+ and CH +. Voltage between filament and ionization chamber constant at 8 volts...

The ion intensity is plotted vs. the velocity in units of the velocity V0 of the incident ion. The large peak at V0 is attributed to unreacted primary ions. This band shows a tail towards lower velocities since... 

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