Presentation on theme: "Sanja Risticevic Chem 323 Poster Presentation Quadrupole Ion Trap Mass Spectrometry."— Presentation transcript:
Sanja Risticevic Chem 323 Poster Presentation Quadrupole Ion Trap Mass Spectrometry
Introduction Ion trap mass spectrometry has recently developed very rapidly It is a high performance technique and one of the leading tools in the chemistry and biochemistry fields It can be used for measurements of very high mass/charge ratios Ion trap mass spectrometry has high resolution capabilities and also excellent non-destructive broad-band Fourier transform capabilities
Advantages of using ion trap: High sensitivity Capable of high performance Compactness and mechanical simplicity Ion/Molecule reactions can be studied for mass-selected ions and the reaction time can be varied in the ion trap. Therefore, the kinetics and equilibrium of ion-molecule reactions can be studied High resolution for slow scans The resonance experiments are applicable in the study of ions that have high m/z ratios Fourier transform techniques provide non-destructive detection MS/MS experiments are possible (multiple stage mass spectrometry). In these experiments, individual ions can be examined in a mixture of ions. The ions of interest are isolated by their characteristic m/z values and they dissociate. The product ions are then analyzed in a second mass measurement step.
Ions are subjected to stabilizing and destabilizing forces applied by the field. The forces occur in three dimensions. To record the mass spectra, the quadrupole ion trap can be operated in the mass selective instability scan mode which is the usual mode of operation Ions of a given m/z value undergo stable motion in the trap A helium buffer gas is used to remove the kinetic energy from ions and cause them to occupy the central region of the trap The ion trap can hold up to 10 5 -10 6 ions before columbic repulsions reduce the mass resolution Ions are trapped in the system consisting of three electrodes which have hyperbolic surfaces The central electrode is the rotationally symmetrical ring electrode and it is located between two end-cap electrodes of the same cross-section
The diagram which illustrates the ion trap instrumentation. r 0 is the internal radius of the ring electrode and z 0 is the closest distance from the center to the end-cap electrodes. The electrodes are aligned and isolated using ceramic posts.
Potential for trapping ions The quadrupole potential surface is saddle-shaped when the phase of the voltage signal is positive The ion shown is on a potential downhill in the z-direction and is accelerated from the center of the device Potential used for trapping ions in the radial direction. The ion is accelerated away from the trapping center in the axial direction.
When the voltage field changes sign, this ion is accelerated toward the center of the trap. The ions are trapped in both the r and z directions. Potential used for trapping ions in the axial direction. The ion is accelerated towards the trap center.
Trapped ions have characteristic frequencies of oscillation known as secular frequencies. The frequency in the r-direction is half the frequency in the z-direction. An additional potential of frequency equal to the secular frequency causes ions to absorb more kinetic energy. Ions are activated in the z- direction when the signal is applied between the two end-cap electrodes. When the signal is strong, it is possible for the ions to be ejected from the trap in the z-direction. The population of ions in the trap can also be controlled since particular ions can be excited so that they dissociate or get ejected
Ions move along the q z axis until q z = 0.908. Here the ions become unstable in the normal mass-selective instability mode of operation and reach the boundary. If the supplementary resonance frequency can be varied, q z becomes a variable. Thus, q z can be lowered by modulating the ion motion at a chosen frequency. For this purpose, a dipolar electric field is applied across the end-cap electrodes. This is called the resonance ejection experiment which can be used to increase m/z. The ions of a particular m/z value pick up translational energy and exit the trap through a hole in the end-cap electrode. These ions exit the ion trap in the sequence of m/z values and reach an external detector.
The resonance experiment can also be used to allow the fragmentation of the specified ions only. Resonance experiments are usually performed by applying a mixture of frequencies. Thus, the ions of different m/z values are manipulated simultaneously. The technique used for this is called SWIFT (stored waveform inverse Fourier transform). This technique is also used for the ion population control. For example, in trace level analysis, the trap can be filled with the analyte ions only. Thus, the ions can be stored selectively which is very applicable in the ultra-trace-level analysis of volatile organic compounds. This analysis can be performed at levels as low as parts-per-quadrillion (pg/L). A Brief Word on Non-Destructive Ion Detection The population of a single ion can be measured multiple times Achieved by impulsive excitation of a group of trapped ions of different m/z values The ion image currents are induced on a small detector electrode. This electrode is isolated from the end-cap electrodes. The image currents are measured using a differential preamplifier, filter and amplifier. The image currents are then Fourier analyzed and the broad-band spectra can be obtained.
Mass Selective Instability Scan Ions of different m/z values arrive at the detector at different times When the voltage is increased across the ring electrode, the ions of high m/z are ejected. When the voltage is changed too fast, the loss of resolution can result. Therefore, the rate of voltage change should be slow to ensure the high resolution. A zoom-scan mode can be applied in these circumstances to provide a better study of the ions which have m/z<10 dalton/charge The high resolution of the instrument helps to resolve the isotopic forms of the multiply -charged ions The figure shows the zoom scan of the +4 charged state of rat interleukin-8. There is one-forth m/z unit difference between carbon isotopes. These carbon isotopes are different in mass by one dalton. This is electrospray ionization mass spectrum, but it shows how the zoom-scan mode of the ion trap can be useful in the isotopic study.
MS/MS Experiment The additional sequence of operations in the scan function is used. The ionization is followed by the selection of a parent ion. Thus, all other ions are ejected from the trap. The parent ion then undergoes a translational excitation. Excited ions collide with a helium buffer gas and dissociate. The resulting product ions are recorded by scanning the voltage. Thus, a second mass–analysis scan is performed. Applications: The enriched specificity is useful in the distinction of isomers, sequencing of biopolymers and analysis of complex mixtures The structural elucidation of complex molecules in the presence of mixtures. A compound can be fragmented and the resulting fragments can be further analyzed. Conclusions: The sensitivity and resolution of the ion trap are outstanding. Thus, an increasing number of analyses will be performed using ion trap mass spectrometry. The small size of the instrument, the low cost and the reasonable pressure requirements make this device one of the most powerful tools in the chemical analysis.
References: 1 http://www.currentseparations.com/issues 16-3/cs16-3c.pdf 2 Schalley,C.A.Modern Mass Spectrometry, volume 225, Springer, New York, 2003.