Ions, abundance

The mass spectrum of benzene is relatively simple and illustrates some of the mfor matron that mass spectrometry provides The most intense peak m the mass spectrum is called the base peak and is assigned a relative intensity of 100 Ion abundances are pro portional to peak intensities and are reported as intensities relative to the base peak The base peak m the mass spectrum of benzene corresponds to the molecular ion (M" ) at miz = 78... 

The formation of a simple El mass spectrum from a number (p) of molecules (M) interacting with electrons (ep. Peak 1 represents M , the molecular ion, the ion of greatest mass (abundance q). Peaks 2, 3 represent A+, B. two fragment ions (abundances r, s). Peak 2 is also the largest and, therefore, the base peak. 

The ion current resulting from collection of the mass-separated ions provides a measure of the numbers of ions at each m/z value (the ion abundances). Note that for this ionization method, all ions have only a single positive charge, z = 1, so that m/z = m, which means that masses are obtained directly from the measured m/z values. Thus, after the thermal ionization process, m/z values and abundances of ions are measured. The accurate measurement of relative ion abundances provides highly accurate isotope ratios. This aspect is developed more fully below. 

Figure 7.9 shows a schematic representation of this effect, in which the ratio of the two isotopes changes with time. To obtain an accurate estimate of the ratio of ion abundances, it is better if the relative ion yields decrease linearly (Figure 7.9) which can be achieved by adjusting the filament temperature continuously to obtain the desired linear response. An almost constant response for the isotope ratio can be obtained by slow evaporation of the sample, viz., by keeping the filament temperature as low as is consistent with sufficient sensitivity of detection (Figure 7.9). 

All mass spectrometers analyze ions for their mass-to-charge ratios (m/z values) and simultaneously for the abundances of ions at any given m/z value. By separating the ions according to m/z and measuring the ion abundances, a mass spectrum is obtained. 

The major advantage of array detectors over point ion detectors lies in their ability to measure a range of m/z values and the corresponding ion abundances all at one time, rather than sequentially. For example, suppose it takes 10 msec to measure one m/z value and the associated number of ions (abundance). To measure 100 such ions sequentially with a point ion detector would necessitate 1000 msec (1 sec) for the array detector, the time is still 10 msec because all ions arrive at the same time. Therefore, when it is important to be able to measure a range of ion m/z values in a short space of time, the array detector is advantageous. 

Ions in a TOF analyzer are temporally separated according to mass. Thus, at the detector all ions of any one mass arrive at one particular time, and all ions of other masses arrive at a different times. Apart from measuring times of arrival, the TDC device must be able to measure the numbers of ions at any one m/z value to obtain ion abundances. Generally, in TOF instruments, many pulses of ions are sent to the detector per second. It is not unusual to record 30,000 spectra per minute. Of course, each spectmm contains few ions, and a final mass spectrum requires addition of all 30,000 spectra to obtain a representative result. 

Each bin is connected to a memory location in a computer so that each event can be stored additively over a period of time. All the totaled events are used to produce a histogram, which records ion event times versus the number of times any one event occurs (Figure 31.5).With a sufficiently large number of events, these histograms can be rounded to give peaks, representing ion m/z values (from the arrival times) and ion abundances (from the number of events). As noted above, for TOP instruments, ion arrival times translate into m/z values, and, therefore, the time and abundance chart becomes mathematically an m/z and abundance chart viz., a normal mass spectrum is produced. 

A mass spectrum is a chart of ion abundances versus m/z values. It is shown above that the TDC measures ion arrival times, which are converted directly into m/z values. Notionally, the number of ions arriving at the detector at any one m/z value is equal to the number of events recorded (one... 

A mass spectrum consists of peaks corresponding to ions. The position of a peak on the x-axis is proportional to its mass (strictly, its m/z value), while the height of the peak on the y-axis gives the number of ions (abundances) at a particular m/z. The ions giving rise to the spectrum are formed in an ion source and are passed through an analyzer for measurement of m/z and into a detector for measurement of abundance (Figure 32.1). 

Metastable ions yield valuable information on fragmentation in mass spectrometry, providing insight into molecular structure. In electron ionization, metastable ions appear naturally along with the much more abundant normal ions. Abundances of metastable ions can be enhanced by collisionally induced decomposition. 

Schematic diagram of a mass spectrometer. After insertion of a sampie (A), it is ionized, the ions are separated according to m/z value, and the numbers of ions (abundances) at each m/z value are plotted against m/z to give the mass spectrum of A. By studying the mass spectrum, A can be identified,...

The positive-ion electron-ionization spectra of BFB and DFTPP must exhibit molecular and specified fragment ions, the relative abundances of which must fall within a predefined range. Ion abundance criteria for BFB and DFTPP are shown in Table 41.1. 

If digital voltage readings (V1-V9) are taken at time intervals (At = 0.0001 sec in the example of Figure 44.4), then the area of the true peak (dotted) can be (mathematically) closely approximated to give ion abundance and, similarly, the time (tj to the center of gravity (centroid) of the peak can be determined, thereby giving the m/z value. 

When a mass spectrum has been acquired by the spectrometer/computer system, it is already in digital form as m/z values versus peak heights (ion abundances), and it is a simple matter for the computer to compare each spectrum in the library with that of the unknown until it finds a match. The shortened search is carried out first, and the computer reports the best fits or matches between the unknown and spectra in the library. A search of even 60,000 to 70,000 spectra takes only a few seconds, particularly if transputers are used, thus saving the operator a great deal of time. Even a partial match can be valuable because, although the required structure may not have been found in the library, it is more than likely that some of the library compounds will have stractural pieces that can be recognized from a partial fit and so provide information on at least part of the structure of the unknown. 

Figure 48.2 shows that a compares the ratio of atom or ion abundances for two isotopes in each of two samples. If a is not equal to 1, then the isotopes in one sample must have a different ratio from those in the other. If isotopes behave chemically almost identically on a universal stage,... 

For several reasons — including the complete breakdown of sample into its substituent elements in the plasma and the use of an unreactive monatomic plasma gas (argon) — background interferences in the resulting mass spectra are of little importance. Since there are no or very few background overlaps with sample ions, very precise measurements of sample ion abundances can be made, which facilitate the determination of precise isotope ratios. 

Once inside the hot plasma, which is at a temperature of about 8000 K and contains large numbers of energetic electrons and ions, the sample molecules are broken down into their constituent elements, which appear as ions. The ions are transported into a mass analyzer such as a quadrupole or a time-of-flight instrument for measurement of m/z values and ion abundances. 

Gases and volatile materials can be swept into the center of an argon plasma flame, where they are fragmented into ions of their constituent elements. The m/z values of ions give important information for identification of the elemental composition of a sample, and precise measurement of ion abundances is used to provide accurate isotope ratios. 

A chart showing the number of ions (abundance) arriving at the collector and their respective m/z values is a mass spectrum. 

Thus, a mass spectrum records ion abundances in one dimension. In the second dimension, it records m/z ratios. The mass spectrum is a record of m/z values of ions and their abundances. 

This problem is known as dead time. To offset this effect, an algorithm is used to adjust the actual number of events into a true number of events. Since the numbers of ions represent ion abundances, the correction adjusts only abundances of ions before a mass spectrum is printed. 

This assembly of ions (M+, F/,. .., F +) in the source is analyzed by the mass spectrometer to give a mass (m/z) versus ion abundance chart, i.e., a mass spectrum. 

This reduction in information is achieved by a preprocessor, which uses the digital voltages corresponding to an ion peak to estimate the peak area (ion abundance) and centroid (mean arrival time of peak, equivalent to m/z value) these two pieces of information — plus a flag to identify the peak — are stored. 

Quantitative mass spectrometry, also used for pharmaceutical appHcations, involves the use of isotopicaHy labeled internal standards for method calibration and the calculation of percent recoveries (9). Maximum sensitivity is obtained when the mass spectrometer is set to monitor only a few ions, which are characteristic of the target compounds to be quantified, a procedure known as the selected ion monitoring mode (sim). When chlorinated species are to be detected, then two ions from the isotopic envelope can be monitored, and confirmation of the target compound can be based not only on the gc retention time and the mass, but on the ratio of the two ion abundances being close to the theoretically expected value. The spectrometer cycles through the ions in the shortest possible time. This avoids compromising the chromatographic resolution of the gc, because even after extraction the sample contains many compounds in addition to the analyte. To increase sensitivity, some methods use sample concentration techniques. 

Electron impact fragmentation studies on 1,2-benzisoxazoles and benzoxazole indicate that isomerization takes place before degradation. Shape analysis and metastable ion abundances in the mass spectra indicate that isomerization to o-cyanophenols occurred prior to degradation by loss of CO or NCH (75BSB207). 

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