Ion currents

This method relies on the simple principle that the flow of ions into an electrolyte-filled micropipette as it nears a surface is dependent on the distance between the sample and the mouth of the pipette [211] (figure B 1.19.40). The probe height can then be used to maintain a constant current flow (of ions) into the micropipette, and the technique fiinctions as a non-contact imaging method. Alternatively, the height can be held constant and the measured ion current used to generate the image. This latter approach has, for example, been used to probe ion flows tlirough chaimels in membranes. The lateral resolution obtainable by this method depends on the diameter of the micropipette. Values of 200 nm have been reported. 

Typically, PIXE measurements are perfonned in a vacuum of around 10 Pa, although they can be perfonned in air with some limitations. Ion currents needed are typically a few nanoamperes and current is nonnally not a limiting factor in applying the teclmique with a particle accelerator. This beam current also nonnally leads to no significant damage to samples in the process of analysis, offering a non-destmctive analytical method sensitive to trace element concentration levels. 

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. 

Clearly, the lower the ionization energy with respect to the work function, the greater is the proportion of ions to neutrals produced and the more sensitive the method. For this reason, the filaments used in analyses are those whose work functions provide the best yields of ions. The evaporated neutrals are lost to the vacuum system. With continued evaporation of ions and neutrals, eventually no more material remains on the filament and the ion current falls to zero. 

Diagram showing a flow of ions of m/z a, b, c, etc. traveling in bunches toward the front face of a microchannel array. After each ion strikes the inside of any one microchannel, a cascade of electrons is produced and moves toward the back end of the microchannel, where they are collected on a metal plate. This flow of electrons from the microchannel plate constitutes the current produced by the incoming ions (often called the ion current but actually a flow of electrons). The ion.s of m/z a, b, c, etc. are separated in time and reach the front of the microchannel collector array one set after another. The time at which the resulting electron current flows is proportional to V m/z). 

There is potential confusion in the use of the word array in mass spectrometry. Historically, array has been used to describe an assemblage of small single-point ion detectors (elements), each of which acts as a separate ion current generator. Thus, arrival of ions in one of the array elements generates an ion current specifically from that element. An ion of any given m/z value is collected by one of the elements of the array. An ion of different m/z value is collected by another element. Ions of different m/z value are dispersed in space over the face of the array, and the ions are detected by m/z value at different elements (Figure 30.4). 

The flow of positive ions is normally passed into a suitable ion analyzer in order to separate them according to tn/z value. For this surface ion emission process, z is aiways equal to 1, and, therefore, tn/z = m. The flow of ions at each m/z value generates an ion current that is used to measure the abundances of the ions. 

Organics produce no useful positive ions, but the ions produced by inorganic samples are remarkably free from background interference, and the resulting mass spectra are relatively simple. The ion currents derived from the positive sample ions at each m/z value, being free from background ions, represent an accurate measure of the amount of each element. 

Components of a mixture emerging from a liquid chromatographic column are dissolved in the eluting solvent, and this solution is the one directed across the target, as described above. Thus, as the components reach the target, they produce ions. These ions are recorded by the spectrometer as an ion current. 

The passage of a component of a mixture over the atom gun target area is accompanied by first a rise and then a fall in the ion current, and a graph of ion yield against time is an approximately triangularshaped peak. 

A graph or chart of ion current (y-axis) vs. time (x-axis) is therefore a succession of peaks corresponding to components eluting from the chromatographic column. This chart is called a total km current (TIC) chromatogram. 

Ions arrive at one end of each element of a multipoint collector and trigger a cascade of electrons, which moves toward the opposite end and is detected electronically. The resulting electric current corresponds to the ion current. 

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). 

Once the mass spectral information has been acquired, various software programs can be employed to print out a complete or partial spectrum, a raw or normalized spectrum, a total ion current (TIC) chromatogram, a mass chromatogram, accurate mass data, and metastable or MS/MS spectra. 

Mass resonant analyzer. A mass analyzer for mass-dependent resonant-energy transfer and measurement of the resonance frequency, power, or ion current of the resonant ions. 

Selected-ion monitoring (SIM). Describes the operation of a mass spectrometer in which the ion currents at one (or several) selected m/z values are recorded, rather than the entire mass spectrum. The use of the terms multiple-ion detection (MID), multiple-ion (peak) monitoring (MPM), and mass fragmentography are not recommended. 

Faraday cup (or cylinder) collector. A hollow collector, open at one end and closed at the other, used to measure the ion current associated with an ion beam. 

Photographic plate recording. The recording of ion currents (usually associated with ion beams that have been spatially separated by m/z values) by allowing them to strike a photographic plate, which is subsequently developed. 

Total ion current (TIC), (a) After mass analysis the sum of all the separate ion currents carried by the different ions contributing to the spectrum, (b) Before mass analysis the sum of all the separate ion currents for ions of the same sign. 

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