Ion detector

To obtain a mass spectrum, ions need to be converted into a usable signal by a detector. The simplest form of ion detection is a photographic plate or a Faraday cup for the direct measurement of the charge. In a Faraday cup the induced current is generated by an ion which hits the surface of a dynode and emits 

In this type of detector the electrons are accelerated down the channel producing additional electrons to the output signal. The created cascade of electrons results in a measurable current at the end of the detector [77]. 

Channel electron multipliers (CEM) are fabricated from lead-silica glass (Fig. 1.32) and can have curved or straight forms. In a channel electron multiplier, when the charged particles (positive or negative) hit the surface of the electrode, electrons are produced from the surface which then generate the current. 

The lifetime of channel electron multipliers is ca. 1-2 years. Neutrals or photons hitting the detector also increase the noise of the detection. 

A further widely used multiplier is the photon multiplier. In this case the ions (positive or negative) elicit secondary ions formed by a conversion dynode, which are further accelerated towards a phosphorescent screen where they undergo conversion into photons detected by a photomultiplier (Fig. 1.34). 

There are two strategies that can be adopted to try and measure such low current levels (1) use a very sensitive current measuring device (a picoammeter or, when suitably configured, an electrometer) or (2) devise some means to amplify the current, thus making it easier to 

This TOF-MS can operate in two modes, the so-called V and W modes. The V mode is equivalent to a standard single reflectron ion pathway whereas the W mode is essentially two consecutive reflectron pathways, such that the ion trajectory traces out a W-shape. The resolution in the W mode is even higher than the 7000 achieved in the V mode but with a substantial decrease in ion transmission, which lowers the instrument sensitivity. 

---

For either the in-line or hybrid analyzers, the ions injected into the TOF section must all begin their flight down the TOF tube at the same instant if arrival times of ions at a detector are to be used to measure m/z values (see Chapter 26, TOF Ion Optics ). For the hybrid TOF instruments, the ion detector is usually a microchannel plate ion counter (see Chapter 30, Comparison of Multipoint Collectors (Detectors) of Ions Arrays and MicroChannel Plates ). 

An AutoSpec-TOF mass spectrometer has a magnetic sector and an electron multiplier ion detector for carrying out one type of mass spectrometry plus a TOF analyzer with a microchannel plate multipoint ion collector for another type of mass spectrometry. Either analyzer can be used separately, or the two can be run in tandem (Figure 20.4). 

A further important property of the two instruments concerns the nature of any ion sources used with them. Magnetic-sector instruments work best with a continuous ion beam produced with an electron ionization or chemical ionization source. Sources that produce pulses of ions, such as with laser desorption or radioactive (Californium) sources, are not compatible with the need for a continuous beam. However, these pulsed sources are ideal for the TOF analyzer because, in such a system, ions of all m/z values must begin their flight to the ion detector at the same instant in... 

Ion detectors can be separated into two classes those that detect the arrival of all ions sequentially at one point (point ion collector) and those that detect the arrival of all ions simultaneously along a plane (array collector). This chapter discusses point collectors (detectors), while Chapter 29 focuses on array collectors (detectors). 

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. 

There are two common occasions when rapid measurement is preferable. The first is with ionization sources using laser desorption or radionuclides. A pulse of ions is produced in a very short interval of time, often of the order of a few nanoseconds. If the mass spectrometer takes 1 sec to attempt to scan the range of ions produced, then clearly there will be no ions left by the time the scan has completed more than a few nanoseconds (ion traps excluded). If a point ion detector were to be used for this type of pulsed ionization, then after the beginning of the scan no more ions would reach the collector because there would not be any left The array collector overcomes this difficulty by detecting the ions produced all at the same instant. 

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). 

Ion trajectory through a conventional (EB) sector instrument, showing three field-free regions in relation to the sectors, the source, and the ion detector. 

A true baseline output from an ion detector is electrically noisy and, if recorded as such, the noise would appear as a great many small (unwanted) peaks. By creating an artificial baseline at a voltage just above the noise, the small peaks are eliminated and only the desired signal is recorded. It is important not to set the artificial baseline voltage too high, since this would eliminate too much of the required peak. 

Thus, ions are produced, deflected in a magnetic field, then focused in an electric field, and finally detected by an electron multiplier or other ion detector. 

If, just before the ion beam reaches the ion detector, a pusher electrode is used alongside it to deflect the beam at right angles (orthogonal) to its original direction into the flight tube of a time-of-flight sector (TOP analyzer), the m/z values can be measured by the TOP section. 

For particular magnitudes and frequencies of the electric fields, only ions of selected mass can pass (filter) through the assembly to reach an ion detector. 

Ions of a given m/z value are collected at one of the small point ion detectors ions of larger or smaller m/z values are collected at other point collectors placed on either side. 

In its simplest form, a mass spectrometer is an instmment that measures the mass-to-charge ratios ml of ions formed when a sample is ionized by one of a number of different ionization methods (1). If some of the sample molecules are singly ionized and reach the ion detector without fragmenting, then the ml ratio of these ions gives a direct measurement of the molecular weight. The first instmment for positive ray analysis was built by Thompson (2) in 1913 to show the existence of isotopic forms of the stable elements. Later, mass spectrometers were used for precision measurements of ionic mass and abundances (3,4). 

FIGURE B.5 A mass spectrometer is used to measure the masses of atoms. As the strength of the magnetic field is changed, the path of the accelerated ions moves from A to C. When the path is at B, the ion detector sends a signal to the recorder. The mass of the ion is proportional to the strength of the magnetic field needed to move the beam into position. 

QL = quadrupole lens, EC = displaceable electrometer collector, Ml, M2 = magnetic sectors, and Dl, D2 = ion detectors... 

A scintillation ion detector, described in detail elsewhere (41), detected virtually every ion which entered the detector chamber. Pulse counting techniques were used. 

Conditions apparatus, Hewlett-Packard HP5890 equipped with an HP5972 mass-selective ion detector (quadruple) column, PTE-5 (30 m x 0.25-mm i.d.) with 0.25- am film thickness column temperature, 50 °C (1 min), increased at 20 °C min to 150 °C(5 min) and then at 4 °Cmin to 280 °C (30 min) inlet and detector (GC/MS transfer line) temperature, 250 and 280 °C, respectively gas flow rate, He carrier gas ImLmin" injection method, splitless mode solvent delay, 3 min electron ionization voltage, 70eV scan rate, 1.5 scanss scanned-mass range, m/z 50-550. The retention times of benfluralin, pendimethalin and trifluralin are 15.2, 25.1 and... 

Sample inlet Source (ion production) Mass analyser Ion detector... 

Figure 4.2 is a block diagram that illustrates the principle of the SIMS technique. The apparatus includes a primary ion source, a vacuum chamber where the objects under study are placed, a mass analyser and a secondary ion detector. 

The analyser will always be preceded by some form of collection optics, and followed by an ion detector (usually a channel electron multiplier which converts ions into electron showers). There are three types of analyser for use in SIMS spectrometers, the magnetic sector instrument, the quadrupole analyser and time-of -flight (TOF) systems. 

Figure 2.3. A. Mass spectrometer consisting of an ionization source, a mass analyzer and an ion detector. The mass analyzer shown is a time-of -flight (TOF) mass spectrometer. Mass-to-charge (m/z) ratios are determined hy measuring the amount of time it takes an ion to reach the detector. B. Tandem mass spectrometer consisting of an ion source, a first mass analyzer, a collision cell, a second mass analyzer and a detector. The first mass analyzer is used to choose a particular peptide ion to send to the collision cell where the peptide is fragmented. The mass of the spectrum of fragments is determined in the second mass analyzer and is diagnostic of the amino acid sequence of the peptide. Figure adapted from Yates III (2000).

Mass spectrometers measure the mass-to-charge ratio (m/z) of ions. They consist of an ionization source that converts molecules into gas-phase ions and a mass analyzer coupled to an ion detector to determine the m/z ratio of the ion (Yates III, 2000). A mass analyzer uses a physical property such as time-of-flight (TOF) to separate ions of a particular m/z value that then strike the detector (Fig. 2.3). The magnitude of the current that is produced at the detector as a function of time is used to determine the m/z value of the ion. While mass spectrometers have been used for many years for chemistry applications, it was the development of reproducible techniques to create ions of large molecules that made the method appropriate for proteomics. 

The apparatus consists of a pulsed molecular beam, a pulsed ultraviolet (UV) photolysis laser beam, a pulsed vacuum ultraviolet (VUV) probe laser beam, a mass spectrometer, and a two-dimensional ion detector. The schematic diagram is shown in Fig. 1. 

---