Accumulation mass spectrum

Figure 21.7 Comparison of mass spectra obtained from rat brain. Optical observation of microspotted tissue sections employing spray-droplet (a), droplet (b), and spraycoating (c) methods. Scale bar, 1.0 mm. White squares (a-c) represent the cortex (A, d) and the medulla (B, e) of the cerebellum region, respectively. Accumulated mass spectra collected from each region are shown (d, e). In each spectrum, asterisks represent major unique signals for spectra using the spray-droplet method. The number of detected signals in the mass range of 2000 < m/z < 30,000 from each region is shown (f). Reprinted with permission from Sugiura et al.7...

Figure 5.12 GALDI mass spectrum of shellac (from methanol solution). Peaks at m/z—570 are related to esters of aliphatic hydroxy acids with sesquiterpenoid carboxylic acids (see text and Table 5.4). Signals marked with crosses are contaminants in the spectrometer that accumulated over time (m/z 413, 469, and 507) peaks marked ( ) are contaminating graphite clusters from the matrix (m/z 264, 276, 288).

The ion-trap mass spectrometer uses three electrodes to trap ions in a small volume. The mass analyzer consists of a ring electrode separating two hemispherical electrodes. A mass spectrum is obtained by changing the electrode voltages to eject the ions from the trap. The advantages of the ion-trap mass spectrometer include compact size and the ability to trap and accumulate ions thus increasing the signal-to-noise ratio of a measurement [534,535,551, 553]. 

For some mass spectrometers, such as TOF mass spectrometers, for reasons of data transfer speed and data storage capacity, the data acquisition system needs to accumulate data for a period of time in the summing memory, and forward the accumulated data to the data system. Each spectrum is added to the sum of the previous spectra so that a continuous summation process takes place. This type of data acquisition system is called digital signal averaging (DSA) or integrating transient recorder (ITR). Figure 3.8 illustrates the principle of mass spectrum acquisition with this type of system. 

A further important aspect of the abundances of ions is their relationship to the initial structure of the sample molecule. It is often possible to deduce this structure, or elements of it, by comparison of the MS fragmentation reactions of related compounds. Examples of mass spectra are shown in Figures 1.6, 1.12 and 1.13. The reasoning used in the interpretation of a mass spectrum is based on the accumulated knowledge from the rationalisation of fragmentation mechanisms of known compounds and supported by labelling studies and the accurate mass measurement of ions. Detailed information of such mechanisms can be found in specific textbooks [14] and throughout the literature. 

Fast spectrum-acquisition capabilities Unlike quadrapole or ion-trap instruments, the acquisition of a mass spectrum is not based on scanning. Typical pulse rates of TOF instrument are 20-50 kHz. Spectra from different ion-introduction events are accumulated, resulting in improved S/N due to the averaging of random noise. The resulting mass spectra are stored at 4-10 spectra/s. The TOF is an integrating rather than a scanning instrument. 

Electrospray ionization is classihed as a soft ionization technique. It produces molecular-weight information and very little, if any, fragmentation of the analyte ion, unless induced in the vacuum region of the mass analyzer. The number of charges accumulated by an analyte ion is proportional to its number of basic or acidic sites. The spray polarity and conditions, solution pH and nature, as well as solute concentration will all effect the charge state distribution observed in the mass spectrum. Multiple charging of an analyte ion en-... 

While a spectrum interpretation can be a very involved and tedious task while dealing with a new structure, many spectra need not be interpreted from the first principles . Once a spectrum has been recorded, it is now soon included into one of the growing libraries of mass-spectral information. Many thousands of mass spectra from biological and environmentally important substances have been acquired. A computer search for such compounds is a relatively straightforward task. A mass spectrum from a particular sample can be compared to the reference spectra that have been accumulated over the years. The individuals specializing in certain compound types may also have their own mass-spectral libraries. 

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