Detector

FIGURE 4 Separation of mass fragments with magnetic sector instrument. Ions (M+, M+, M +, ...) are accelerated out of the ion source into the flight tube of the magnetic sector instrument, whose magnetic field, perpendicular to the trajectories of the ions, forces the charged particles to move in circular orbit. Only those ions (M-) with a given m/z value will, at a given acceleration and magnetic strength, become stable and exit the magnetic mass separator to be detected, whereas the unstable ones (M+ M 2, ...) will stick to the wall of the flight tube.

when ions travel in a beam of an oscillating electric field, their trajectories become bent. The quadrupole mass filter is made up by four parallel rods, about 10 cm long and 1 cm in diameter, which in a cross section are arranged to form a square box with a gap in between the four bars. The fragments are allowed to travel in this space along the rods, where they are accelerated in the oscillating electric field set up by dc and ac currents applied to the rods. At a certain ac and dc potential, ions with a specific m/z value will become stable and oscillate in a fixed path through the quadrupole to reach the detector to be recorded. The fragments with m/z values that do not suit the applied potentials will become unstable and, as in the magnetic sector instrument, will not reach the detector but will stick to the rods.

3. Ion Trap Detector

The working principle of a third type of mass filter, the ion trap detector (ITD), is illustrated in Fig. 6. It operates like the quadrupole based on the concept that the trajectories of ions, traveling in an oscillating electric field, become influenced by the wave frequency. Dissimilar to the quadrupole filter, however, the ion separation of the ITD occurs in a closed cavity, where the dc and ac currents applied to a ring electrode and an end cap of the cell set up the electric field. The molecules enter the cell in which they are ionized by an electron beam, and under the control of the given electric field the ions are forced to move in an orbit within the space of the cell. When the ac or dc potential is changed, the motion of some ions becomes unstable,

FIGURE 5 Separation of mass fragments with quadrupole instrument. Ions (M

... ) are accelerated out of the ion source into the space between four parallel rods with an oscillating electric field. At a certain ac and dc potential, ions with a specific m/z (M* ) value will become stable and oscillate in a fixed path through the quadrupole to reach the detector to be recorded, whereas the unstable ones (Mj , M2 , ...) will stick to the rods.

FIGURE 5 Separation of mass fragments with quadrupole instrument. Ions (M

... ) are accelerated out of the ion source into the space between four parallel rods with an oscillating electric field. At a certain ac and dc potential, ions with a specific m/z (M* ) value will become stable and oscillate in a fixed path through the quadrupole to reach the detector to be recorded, whereas the unstable ones (Mj , M2 , ...) will stick to the rods.

and 50% of these are then ejected from the cell through a hole in its bottom to be recorded by a detector. Unlike the quadrupole or magnetic sector instrument, in which the separation and detection of an ion occur in a continues process, the ITD operates in two steps separated in time: ion accumulation and mass analysis. The ITD also

FIGURE 6 Separation of mass fragments with ion trap detector. Ions (M*, Mj , M2 , ...) are introduced into a closed cavity and, under the control of the applied electric field, the ions are forced to move in an orbit within the space of the cell. When the ac or dc potential is changed, the motion of ions with given m/z values becomes unstable (M* ), and they are then ejected from the cell through a hole in its bottom to be recorded by a detector.

FIGURE 6 Separation of mass fragments with ion trap detector. Ions (M*, Mj , M2 , ...) are introduced into a closed cavity and, under the control of the applied electric field, the ions are forced to move in an orbit within the space of the cell. When the ac or dc potential is changed, the motion of ions with given m/z values becomes unstable (M* ), and they are then ejected from the cell through a hole in its bottom to be recorded by a detector.

differs from the beam-type scanning MS by detecting the unstable ions, whereas the magnetic sector or quadruple instrument scans the stable ones, conditions that may offer rather different analytical possibilities.

4. Tandem Mass Spectrometer

In tandem mass spectrometry (MS/MS), two mass spectrometers and a collision chamber are hooked up in series. Early devices utilized two magnetic sector instruments, but because these instruments became bulky and high priced, modern tandem devices are nearly all based on the use of quadrupole or ITD apparatus. The operating scheme of MS/MS is shown in Fig. 7. An advantage of MS/MS is that it provides high selectivity and extra information about the nature of an analyte, as well as reduced chemical background noise, which results in an increase in the signal-to-noise ratio for a detected peak. Even though this method, as judged from the number of scientific papers reported, does not seem to be in common use for foren-sics today, MS/MS will probably become the method of choice in the years to come.

D. Mass Focusing

The MS test can be run in the full scan mode or with selected ion monitoring (SIM). In the full scan mode, the mass analysis covers a range of m/z values, whereas one or a limited number of m/z values are selected for the exam with SIM. When picking between the two options, gas (helium) in

MASS SELECTOR 1 COLLISION CHAMBER MASS SELECTOR 2

FIGURE 7 Tandem mass separator. Sample ions enter mass selector 1, where the parent ions are separated. The selected ions next enter the collision chamber, where they collide with gas molecules to form the daughter ions, which are finally separated in the mass selector 2 and expelled for detection. Either magnetic sector or quadrupole mass analyzers or both types mixed can make up a tandem mass spectrometer. A single ion trap can also function as a tandem mass spectrometer performing the same processes as described above in the same location but in consecutive steps.

FIGURE 7 Tandem mass separator. Sample ions enter mass selector 1, where the parent ions are separated. The selected ions next enter the collision chamber, where they collide with gas molecules to form the daughter ions, which are finally separated in the mass selector 2 and expelled for detection. Either magnetic sector or quadrupole mass analyzers or both types mixed can make up a tandem mass spectrometer. A single ion trap can also function as a tandem mass spectrometer performing the same processes as described above in the same location but in consecutive steps.

operators of the magnetic sector instrument or the quadrupole have to consider to what extent they are willing to trade sensitivity for selectivity or vice versa and whether they are searching for the general unknown or for a suspected agent. The reason for this is that the window of the stable m/z values is sequentially swept across the entire m/z range of interest. The ratio of the transmitted window width to the width of the entire m/z range (i.e., the duty cycle) is in most scanning tests only a fraction of 1%. More than 99% of the ions from the target agents are lost. A duty cycle of 100% is possible with a beam-type instrument, but only when run in a nonscanning mode, as with SIM.

With the ITD, on the other hand, deciding whether to work in the full scan or SIM mode becomes less crucial. As described earlier, the ITD monitors the ions with unstable trajectories in two serial steps. Because these operating steps are separated in time, the yield of detectable ions will become high and rather independent on the scan range. Use of the ITD allows scanning with a high sensitivity the entire mass range that covers a substance group of interest. Given that forensic scientists often do not know what to look for and therefore need a search method with high sensitivity, the ITD should perhaps best meet such demands. A drawback of the ITD is that its sensitivity is more dependent on interfering substances than the beam-type scanning mass spectrometer; the ITD sensitivity thus drops with increasing amounts of impurities that may be co-eluted with the analytes during the chromatographic separation. The generation of somewhat distorted mass spectra at high analyte concentrations, giving rise to enhanced [M + 1]+ peaks, is another ITD problem.

In addition to the positive ions formed during the ion-ization of a molecule by EI or CI, negative ions are also produced and, by changing the electric field of the mass spectrometer, these can be monitored. At certain instances, such as when the target substances have a high affinity for electrons, negative ion monitoring can be extremely useful, mainly because of the high sensitivity that can be achieved. The approach of using negative ion monitoring has been particularly fruitful for the analysis of halogenated drug substances, which have been detected at 100- to 1000-fold higher sensitivity than when tested by the positive ion monitoring.

Relaxation Audio Sounds Relaxation

Relaxation Audio Sounds Relaxation

This is an audio all about guiding you to relaxation. This is a Relaxation Audio Sounds with sounds called Relaxation.

Get My Free MP3 Audio


Post a comment