Ion Current Detectors

The role of the detector, properly called an ion current detector, is to determine the abundance of the ions of different mk after they have been separated in, and emerge from, the mass analyzer. Detectors are also referred to as multipliers because they multiply (amplify) the very low current produced by the ions arriving at the detector surface. Ions traveling from the source through the analyzer to the detector constitute an ion current. The arrival of one ion per second corresponds to a current of 1.6 X 10 A. Ion currents in most mass spectrometers are in the 10 to 10 A range. As long as ions of a particular m/z continue to arrive at the detector, the small current can be amplified to increase the signal attributable to that miz value. 

The first electronic ion detector was the Faraday cup. It consists of a metal container in which ions arriving from the mass analyzer are discharged and generate a current. While it is simple and capable of very precise operation, the Faraday cup is used only in specific applications where sufficient sample is available because it cannot amplify the signal derived from the very small charge carried by each arriving ion i.e., the cup is a detector but not a multiplier. 

A combination of amplification steps is employed to improve detection sensitivity. First, the signal is amplified within the detector, and then there is subsequent electronic amplification, outside the vacuum chamber, followed by recording by the data system. Detectors that amplify the current associated with the arriving ions 

All types of multipliers (electron multiplier, continuous dynode, multichannel plate) work on the same principle. 

Ions arriving from the analyzer release electrons from a conversion dynode (except multichannel plate detector). 

---

In principle, it is also possible to use the scanning capability of Ql, using the TOF part only as a total ion current detector. However, this scan mode is not used, as all the above-mentioned advantages of using the TOF analyser are lost. 

Adding too much or too little IS can also limit the dynamic range of the assay, as the comparison of very large ion currents (detector saturation) with very small ion currents (poor ion statistics) will greatly increase the variance of the assay. A good guide is a threefold excess of the IS over the analyte but this may take a few trials to establish. 

Just as it is almost impossible to represent a typical EGD-EGA apparatus, it is equally difficult to describe the wide variety of detectors that have been employed- Using the techniques listed in Table 8.2, the type of detector employed for each method is listed in Table 8.5. As can be seen, there are a number of different types of detectors used, from simple thermal conductivity detectors to more sophisticated ion current detectors in mass spectrometry. It is, of course, impossible to discuss each one in detail here, although the complete apparatus is described in certain cases. 

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). 

Ionisation detectors. An important characteristic of the common carrier gases is that they behave as perfect insulators at normal temperatures and pressures. The increased conductivity due to the presence of a few charged molecules in the effluent from the column thus provides the high sensitivity which is a feature of the ionisation based detectors. Ionisation detectors in current use include the flame ionisation detector (FID), thermionic ionisation detector (TID), photoionisation detector (PID) and electron capture detector (ECD) each, of course, employing a different method to generate an ion current. The two most widely used ionisation detectors are, however, the FID and ECD and these are described below. 

Total-ion-current trace A plot of the total number of ions reaching the mass spectrometry detector as a function of analysis time. 

Microbeam scanning of the sample cross-section was performed with an external microbeam (in air), using a focused 4 MeV proton beam and a 50 pm thick Kapton foil at the vacuum-air interface, with a 5 mm diameter beam exit hole. The 2 mm thick slice of gel polymer sample was placed less than 100 pm from the exit foil, with the cross-section facing the Kapton foil. A HPGe y-ray detector was placed just behind the sample in order to achieve as large as possible detector solid angle. The ion current was kept below 100 pA in order to minimize damage to the sample. 

A stream-splitter may be used at the end of the column to allow the simultaneous detection of eluted components by destructive GC detectors such as an FID. An alternative approach is to monitor the total ion current (TIC) in the mass spectrometer which will vary in the same manner as the response of an FID. The total ion current is the sum of the currents generated by all the fragment ions of a particular compound and is proportional to the instantaneous concentration of that compound in the ionizing chamber of the mass spectrometer. By monitoring the ion current for a selected mass fragment (m/z) value characteristic of a particular compound or group of compounds, detection can be made very selective and often specific. Selected ion monitoring (SIM) is more sensitive than TIC and is therefore particularly useful in trace analysis. 

The quantification capability is normally limited by the detector and/or the ion source. The MCP that is often utilized in TOF instruments cannot fully handle the ion currents that are produced in MALDI and are often saturated to some extent. With other ion sources, such as SIMS, the detection system is less strained so the detector is less limiting. Instead the ion source will limit the quality in quantification. Magnetic sectors and also qudmpoles are more often utilized when quantification is important. 

---