Ion Abundance Measurement

The choice of a particular type of gas discharge for quantitative studies of ion-molecule reactions is essential if useful information is to be obtained from ion abundance measurements. Generally, two types of systems have been used to study ion-molecule reactions. The pulsed afterglow technique has been used successfully by Fite et al. (3) and Sayers et al. (1) to obtain information on several exothermic reactions including simple charge transfer processes important in upper atmosphere chemistry. The use of a continuous d.c. discharge was initiated in our laboratories and has been successful in both exothermic and endothermic ion-molecule reactions which occur widely within these systems. 

It has been shown that loss of a hydroxyl radical from metastable p-nitromethyl benzoate ions involves abstraction of an ortho hydrogen by the nitro group [474]. From metastable ion abundance measurements on the 3-d, and 2-dj compounds, an average isotope effect /OH//OD of 1.41 was determined. On the basis of an intermolecular isotope effect, it has been further suggested that this loss of a hydroxyl radical from the p-nitromethyl benzoate ion is also preceded by hydrogen transfer from the methyl group to the ring [474], The ratio (M— OH)+/M+ for the d3 -isomer was found to be less than half the same ratio for the unlabelled isomer. 

A conventional mass spectrometer was used to measure ion abtmdance ratios of the diligand fragments [Fe(6511702)2] which were formed during electron-impact ionization. Sample isotopic enrichment levels were obtained from standard curves that related ion abundance ratios to enrichment levels. Tracer concentration was calculated from the values for total iron content and enrichment level. The relative standard deviation for the ion abundance measurement was less than 2%. Recovery of tracers from spiked fecal samples ranged from 90% to 104%. The method was used to analyze samples collected from a human study. Iron availability from breakfast meals was determined in 6 yo mg women by giving 7 mg of in apple juice on one... 

The aim of this study was to further explore the potential and limitations of using stable iron isotopes as tracers and EI-MS in absorption studies. Procedures were developed for preparing iron acetylacetonate from both blood and fecal samples for mass spectrometric analysis. The precision and accuracy of ion abundance measurements were evaluated. In vivo use of stable iron isotope tracers was tested with a human study in which 54pe and 57pe were given orally and absorption was estimated with the fecal monitoring and hemoglobin incorporation methods. 

Figure 3 Memory effect for the ion abundance measurement of m/e 252, expressed as a percentage deviation of the measured ratios from that of the last replicate for each enrichment level when enriched standards were analyzed from low to high enrichments.

It has been shown that loss of a hydroxyl radical from metastable p-nitromethyl benzoate ions involves abstraction of an ortho hydrogen by the nitro group [474]. From metastable ion abundance measurements on the 3-di and 2-di compounds, an average isotope effect/qh/fon... 

Changing the magnetic or electric fields that effect separation causes ions of different m/z values to reach the collector. On-line computer systems produce sets of mass and abundance values directly by measuring the field and ion current corresponding to each peak. These systems can record complete mass spectra several times per second, which is especially valuable for GC/MS (see below). However, the scan rate affects the accuracy of ion-abundance measurement, which is important for deducing elemental compositions from isotope ratios check the performance of your instrument with known samples to be sure you have not sacrificed this accuracy unnecessarily. 

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) 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. 

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). 

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. 

Only the knowledge of relative useful ion yields and isotopic abundances is required to calculate elemental composition from the relative ion current measurements. The useful ion yield is the number of ions x detected relative to the number of atoms of element xsputtered. The measured relative ion current of two isotopes is... 

The earlier stable isotope dilution mass spectrographic work was accomplished with a thermal ion mass spectrometer which had been specifically designed for isotope abundance measurements. However, Leipziger [829] demonstrated that the spark source mass spectrometer could also be used satisfactorily for this purpose. Although it did not possess the excellent precision of the thermal unit, Paulsen and coworkers [830] pointed out that it did have a number of important advantages. 

Fletcher IR, Maggi AL, Rosman KJR, McNaughton NJ (1997b) Isotopic abundance measurements of K and Ca using a wide-dispersion multi-collector mass spectrometer and low-fractionation ionisation techniques. Int J Mass Spectrom Ion Proc 163(1-2) 1-17... 

The most intense peak of a mass spectrum is called base peak. In most representations of mass spectral data the intensity of the base peak is normalized to 100 % relative intensity. This largely helps to make mass spectra more easily comparable. The normalization can be done because the relative intensities are independent from the absolute ion abundances registered by the detector. However, there is an upper limit for the number of ions and neutrals per volume inside the ion source where the appearance of spectra will significantly change due to autoprotonation (Chap. 7). In the older literature, spectra were sometimes normalized relative to the sum of all intensities measured, e.g., denoted as % Lions, or the intensities were reported normalized to the sum of all intensities above a certain m/z, e.g., above m/z 40 (% L 4o)-... 

McKeegan KD (1987) Ion microprobe measurements of H, C, O, Mg, and Si isotopic abundances in individual interplenary dust particles. Ph D Thesis, Washington University, St. Louis, Missouri... 

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