Gas Chromatography - Tandem Techniques - Gas Chromatography IR Spectroscopy (GC/IR) Systems > Page 41



Figure 28. Pole Arrangement for the Quadrapole and Ion Trap Mass Spectrometers

Figure 28. Pole Arrangement for the Quadrapole and Ion Trap Mass Spectrometers

It was shown in figure 18, that the quadrapole spectrometer contains four rod electrodes. The ion trap mass spectrometer (figure 21) has a quite different electrode arrangement, which consists of three cylindrically symmetrical electrodes comprised of two end caps and a ring. The pole system can be made very small; the opposite internal electrode faces being only 2 cm apart. Each electrode has accurately machined hyperbolic internal faces. In a similar manner to the quadrapole spectrometer, an rf voltage together with an additional DC voltage is applied to the ring and the end caps are grounded.


In the same way as the quadrapole mass spectrometer, the rf voltage causes rapid reversals of field direction, so any ions are alternately accelerated and decelerated in the axial direction and vice versa in the radial direction. There are operating parameters, (a), and (q), that define the conditions of oscillation which are analogous to those for the quadrapole mass spectrometer but, in this case, (ro) is the internal radius of the ring electrode. As already stated, the ion trap is small and (ro) is typically only about 1 cm. At a given voltage, ions of a specific mass range are held oscillating in the trap. Initially, the electron beam is used to produce ions and after a given time the beam is turned off. All the ions, (except those selected by the magnitude of the applied rf voltage) are lost to the walls of the trap, and the remainder, continue oscillating within the trap. The potential of the applied rf voltage is then increased, and the ions sequentially assume unstable trajectories and leave the trap via the aperture to the sensor. The ions exit the trap in order of their increasing m/z values. The early ion trap mass spectrometers were not very efficient, but it was found by introducing traces of helium into the ion trap stabilized the system and significantly improved the quality of the spectra. This improvement was explained on the basis of ion–helium collisions that reduced the energy of the ions and allowed them to concentrate in the center of the trap. The spectra produced have proved to be quite satisfactory for solute identification by comparison with reference spectra. However, the spectrum produced for a given substance will probably differ considerably from that produced by the normal quadrapole mass spectrometer.



The Time of Flight Mass Spectrometer


The time of flight mass spectrometer was invented many years ago but, due to the factors controlling resolution not being clearly recognized and also due to certain design defects that occurred in the first models, it exhibited limited performance and was rapidly eclipsed by other developing mass spectrometer techniques. However, with improved design, modern fabrication methods and the introduction of Fourier transform techniques, the performance has been vastly improved. As a result, there has been a resurgence of interest in this particular form of mass spectrometry. A diagram of the time of flight mass spectrometer is shown in figure 29.



In a time of flight mass spectrometer the following relationship holds,



where (t) is the time taken for the ion to travel a distance (L)
(V) is the accelerating voltage applied to the ion,
and (L) is the distance traveled by the ion to the ion sensor.


Figure 29. The Time of Flight Mass Spectrometer


It follows, that for a given system, the mass of the ion is directly proportional to the square of the transit time to the sensor. The sample is volatilized (or passed as a vapor) into the space between the first and second electrodes and a burst of electrons (over a period of about a microsecond) is allowed to produce ions. An extraction potential (E) is then applied for another short time period which, as those further from the second electrode will experience a greater force than those closer to the second electrode, will result in the ions being focused.



After focusing, an accelerating potential (V) is applied for a much shorter period than that used for ion production (ca 100 nsec) so that all the ions in the source are accelerated virtually simultaneously. The ions then pass through the third electrode into the drift zone and are eventually collected by the sensor electrode. The time of flight mass spectrometer is not employed extensively in gas chromatography/mass spectroscopy combination systems as it is more commonly used to examine high molecular weight materials

Many analysts that use GC/Mass Spectrometer combined systems are neither specialists in gas chromatography or mass spectrometry and may need the support of experienced gas chromatographers or mass spectroscopists for particularly challenging samples. For those who wish to study mass spectrometry further, an excellent discussion on general organic mass spectrometry is given in Practical Organic Mass Spectrometry edited by Chapman (9).


Gas Chromatography IR Spectroscopy (GC/IR) Systems


IR spectra were initially obtained off-line, by condensing the eluted solute in a cooled trap, making into a 'mull', or pressing into an alkali halide pellet and the spectrum obtained using standard techniques. Collection of a solute by condensation, however, can be difficult as, due to the very low concentrations at which each solute is eluted, the partial pressure of the condensed material is often similar to its partial pressure as it leaves the GC column. An efficient method to collect the solute is to use argon as the carrier gas, and condense the argon and the solute simultaneously in a tube cooled with liquid nitrogen. The trapping efficiency can also be improved by trapping the solute on an adsorbent contained in a short length of packed tube and regenerated in a stream of hot gas or by solvent extraction.

The first fully automated on-line GC/IR system was that developed by Scott et al. (10). Each eluted solute was adsorbed in a cooled packed tube, and then thermally regenerated into an infrared vapor cell. Subsequent to the IR spectrum being obtained, a small sample of the vapor was drawn from the IR cell into a low-resolution mass spectrometer and the mass spectrum was also taken.


This system was not a tandem system but, in fact, the first triplet instrument to be reported (GC/IR/MS). The layout of the pneumatic system of the triplet instrument is shown in figure 30. The procedure for analyzing a peak was as follows. As the peak started to elute it was sensed by the detector and the exit carrier gas diverted through the IR cell into a packed trap which concentrated the peak onto the front of the trap packing. After peak elution was complete, the flow of carrier gas was stopped and the solute regenerated back into the IR cell by heating the trap in a secondary stream of nitrogen.



Figure 30. Diagram of an Automatic GC/IR Tandem System