Further Reading
7.3 Inductively Coupled Plasma–Mass Spectrometry
7.3.1 Instrumentation
A mass spectrometer consists of an ion source, a mass analyzer and a mass detector.
In ICP-MS technique, the ion source is the argon plasma.
7.3.1.1 Mass Analyzers
ICP-MS is the combination of an argon plasma as the ion source and a mass spec- trometer. Most mass spectrometers have been designed to determine molecules and thus are based on formation of molecular ions. There are many ways for formation and ionization of molecules in vapour form to be detected. The working principles of mass spectrometers require an operation in evacuated medium. In most of other mass spectrometers, ionization is carried out in vacuum. However, ambient condi- tions are required to form and sustain argon plasma. Therefore, the atomic ions mostly with a charge of ⫹1 are formed in argon plasma at ambient pressure condi- tions and these should then be transported into a mass spectrometer operating in vac- uum. Preservation of a high vacuum is difficult and any opening to accept ions from outside will cause a leak that will prevent proper evacuation. This technical problem has been a center of attention especially in the early years of the development of ICP-MS. The solution is uses an interface where the ions are first accepted to a sec- tion with a pressure that is between the ambient pressure and that of a high vacuum.
A typical interface between argon plasma ion source and a quadrupole mass spec- trometer is shown in Figure 7.10. The upper end of the plasma is in contact with the sampling conein such a way that the flame-like tail is diverted out; this section is rather cooler as it contacts with ambient air and is not suitable analytically. A part of plasma products from a useful zone is allowed to enter the sampling cone into the
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Figure 7.9 Schematic representation of the concept of abundance sensitivity
region of intermediate pressure. A second entrance is provided by skimmer cone into the high vacuum region where quadrupole mass analyzer (QMA) and detector are located. Both of these cones are made of nickel or platinum that are resistant to cor- rosion; they also have high heat conductance so that efficient cooling is feasible.
Samples with high solid content may cause accumulation at orifices of both cones adversely affecting sensitivity; orifices have a diameter of about 1.0 mm.
The mostly used mass analyzers are magnetic sector analyzer,quadrupole mass filters and time-of-flight (TOF) systems. ICP-MS instruments with magnetic sector analyzer can reach very high resolving powers. Presently, the use of these instru- ments is not as common as ICP-MS instruments with quadrupole analyzers.
Magnetic sector analyzer use a permanent electromagnet to cause the ion beam to travel a circular path, the species are spatially separated depending on their m/zval- ues. Species with a selected m/zvalue are directed through an exit slit to detector. In this respect, this type of mass analyzer operates in a similar way to a monochroma- tor that disperses the light into its wavelength components on a focal plane. By vary- ing the current in the electromagnet, the magnetic field is also varied and species with different m/zvalues can be scanned on the exit slit.
QMA, on the contrary, behaves as a variable narrow band filter for species with different m/zvalues. The system consists of related electronics and four metal rods with a diameter of about 1 cm and a length of about 20 cm; these are made from stainless steel or molybdenum; most models are coated with a ceramic layer to pre- vent corrosion. Working principle is given in Figure 7.11. Four cylindrical metal rods are used as electrodes to allow the species with only a specified narrow range of m/z value. This is accomplished by applying DC voltage and RF voltage to rods in vary- ing combinations. DC voltage is positive for two rods and negative in other two as shown in Figure 7.11. RF voltages applied to these pairs have same amplitude but different signs. The DC and RF voltage values are selected in such a way that in the corridor formed among these rods, the selected ions are directed to detector. While the selected ions with proper m/zvalue have a stable trajectory to pass the corridor, the others have unstable trajectories; these are either directed outside or collide to rods and are eliminated. Two of these rods behave as a low-pass mass filter, while Figure 7.10 Schematic representation of an interface between argon plasma and a quadru- pole mass spectrometer. AP, Argon plasma; SAC, sampling cone; SKC, skimmer cone; QMA, quadrupole mass analyzer
the other two operate as the high-pass mass filter; a narrow band of m/zspecies are accelerated. As the DC and RF voltages are varied properly, scanning for different m/zvalues is accomplished. Using a quadrupole analyzer, scanning can be done in about 100 ms for a range of 1–240 for m/z values. In real operating conditions, however, several minutes are required to determine about 25 elements.
There are several measuring modes in ICP-MS instruments with QMA. One tech- nique is scanningover m/zvalues. Single scan can be used for a fast pre-determina- tion of matrix components so that strategies for interference minimization can be developed. Multiple scans at the cost of longer time can be used to obtain signals with better S/Nvalues. Another mode of measurement is the hopping mode; this is to be used for samples of known matrix. In this mode, the time is allocated only for the measurements at the m/zvalue(s) of interest. Since the other (non-analyte) sec- tions of mass spectrum is not measured, an evaluation of matrix components is not possible using this mode.
TOF-ICP-MS instruments are based on accelerating ions in a tube of about 1 m.
The ions with lower m/zarrive at the detector at the end of the tube in a shorter time as compared to ions with higher m/zvalues. Therefore, the mass spectrum is obtained on the basis of signals from ionic species vs. time that corresponds to a certain m/z value. Since about 40 µs is required for scanning the whole m/zvalues, TOF is prac- tically a simultaneous multi-element analyzer while QMA is a fast scanner. This is particularly useful when several elements are to be determined in a very small amount of sample causing just a transient signal. TOF-ICP-MS instruments are available for element determinations; however, their use is not yet as common as QMA.
7.3.1.2 Detectors
Electron multipliers are most commonly used in ICP-MS. The principle of ion detection is similar to that of PMT that was described in Chapter 5. The ions strike a sensitive metal plate that will then emit several electrons. The emitted electrons strike the next dynode that is held at a more positive voltage than the first one.
Using several dynodes at successively more positive voltages, signal amplification is accomplished; these instruments have discrete dynodes. The detector may have a single continuous dynode, where the voltage gets progressively more positive along
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Figure 7.11 Working principle of quadrupole mass analyzer. (m), (m⫹1) and (m−1) are m/z values that are separated
the length of a curved surface inside a horn-shaped detector. Electron multipliers can provide a signal amplification of 105–108.
Among the other less commonly used detectors are Faraday cup collector, photo- graphic plates and scintillation-type transducers.