Abundance of ions

Nd in samples. Unfortunately, mass spectrometry is not a selective technique. A mass spectrum provides information about the abundance of ions with a given mass. It cannot distinguish, however, between different ions with the same mass. Consequently, the choice of TIMS required developing a procedure for separating the tracer from the aerosol particulates. 

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. 

All mass spectrometers analyze ions for their mass-to-charge ratios (m/z values) by separating the individual m/z values and then recording the numbers (abundance) of ions at each m/z value to give a mass spectrum. Quadrupoles allow ions of different m/z values to pass sequentially e.g., ions at m/z 100, 101, 102 will pass one after the other through the quadrupole assembly so that first m/z 100 is passed, then 101, then 102 (or vice versa), and so on. Therefore, the ion collector (or detector) at the end of the quadrupole assembly needs only to cover one point or focus for a whole spectrum to be scanned over a period of time (Figure 28.1a). This type of point detector records ion arrivals in a time domain, not a spatial one. 

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. 

Before measurement it must be decided exactly which isotopes are to be compared. For oxygen, it is usually the ratio of 0 to 0, and for hydrogen it is H to H. Such isotope ratios are measured by the mass spectrometer. For example, examination of a sample of a carbonaceous compound provides abundances of ions at two m/z values, one related to C and one to C (it could be at m/z 45 and COj at m/z 44). By convention, the heavier isotope is always compared with the lighter isotope. The ratio of isotopes is given the symbol R (Figure 48.1). 

A mass spectrum is a chart showing on the x-axis the mass of each ion (M , M , M, etc.) and on the y-axis the number (abundance) of ions at each mass. 

If solid samples are vaporized quickly, then the sample enters the flame as a small plug and the elements must be measured over a short period of time. This mode is useful for high sensitivity because the entire sample passes through the flame in a short time. (The abundances of ions appear as a sharp peak on the output.) If samples are introduced continuously, then ultimate sensitivity may be reduced, but isotope ratios can be determined continuously to provide high accuracy. 

The mass spectrum gives the abundances of ions for different times of arrival at the detector. Since the times are proportional to the square root of the m/z values, it is simple to convert the arrival times into m/z values. 

After acceleration through an electric field, ions pass (drift) along a straight length of analyzer under vacuum and reach a detector after a time that depends on the square root of their m/z values. The mass spectrum is a record of the abundances of ions and the times (converted to m/z) they have taken to traverse the analyzer. TOP mass spectrometry is valuable for its fast response time, especially for substances of high mass that have been ionized or selected in pulses. 

The flow of electric current marks the arrival of ions, and its magnitude marks the abundance of ions arriving at a given m/z value. 

The strength of the ion current relates to the number of ions per second arriving at the collector plate, and a mass spectrum can be regarded as a snapshot of the current taken over a definite period of time. Because of the finite time taken to produce a mass spectrum, it is a record of the abundances of ions (often mistakenly called intensities of ions). 

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. 

The mass spectrum of a compound is typically presented as a bar graph with masses (m/z values) on the x axis and intensity, or relative abundance of ions of a given m/z striking the detector, on the y axis. The tallest peak, assigned an intensity of 100%, is called the base peak, and the peak that corresponds to the unfragmented cation radical is called the parent peak or the molecular ion (M+). Figure 12.2 shows the mass spectrum of propane. 

Mass spectrometer (MS) An instrument used to analyze ions according to their mass-to-charge ratios to determine the abundance of ions. 

The principle of mass action explains the relationship between concentration and complexation. The abundance of ion pairs in aqueous solution is controlled by... 

Mass Spectrometer An instrument that measures the m/z values and relative abundances of ions. See also discussion in entry m/z. 

Mass spectrometry measures the abundance of ions versus their m/z ratio, and it is common practice to use the ratio /mH//mD = kn/ku as a direct measure of the isotope effect. The typical procedure for determining isotope effects from intensity ratios... 

Mass spectrometry is a sensitive analytical technique which is able to quantify known analytes and to identify unknown molecules at the picomoles or femto-moles level. A fundamental requirement is that atoms or molecules are ionized and analyzed as gas phase ions which are characterized by their mass (m) and charge (z). A mass spectrometer is an instrument which measures precisely the abundance of molecules which have been converted to ions. In a mass spectrum m/z is used as the dimensionless quantity that is an independent variable. There is still some ambiguity how the x-axis of the mass spectrum should be defined. Mass to charge ratio should not lo longer be used because the quantity measured is not the quotient of the ion s mass to its electric charge. Also, the use of the Thomson unit (Th) is considered obsolete [15, 16]. Typically, a mass spectrometer is formed by the following components (i) a sample introduction device (direct probe inlet, liquid interface), (ii) a source to produce ions, (iii) one or several mass analyzers, (iv) a detector to measure the abundance of ions, (v) a computerized system for data treatment (Fig. 1.1). 

Jorgensen, T. J. D., Hvelplund, P., Andersen, J. U., Roepstorff, P. Tandem mass spectrometry of specific vs. nonspecific noncovalent complexes of vancomycin antibiotics and peptide ligands. Int J Mass Spectrom 2002, 219, 659-670. Tahallah, N., Pinkse, M., Maier, C. S., Heck, A. J. The effect of the source pressure on the abundance of ions of noncovalent protein assemblies in an electrospray ionization orthogonal time-of-fiight instrument. Rapid... 

The magnetic scan is synchronised with the x-axis of a recorder and calibrated to appear as mass number (strictly m/e). The amplified current from the ion collector gives the relative abundance of ions on the y-axis. The signals are usually pre-processed by a computer that assigns a relative abundance of 100% to the strongest peak (base peak). 

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