Resolving power of TOF increases with longer drift time (mass/delt(mass) = (t/delt(t) using the kinetic energy ((1/2)*mv^2) and potential energy(z*e*V) ). t is the centroid of drift time. delt(t) is the wdith of the distribution. m is mass, v velocity, z charge of the ion, e elementary charge and V electric field applied with known strength.
With increasing drift length, the t term gets bigger, increasing the resolving power. But it is well known that increasing the distance of TOF compromises the transmission.
Is that because there is more chance for diffusion of ions? (Considering that both shorter and longer lengths are under vaccuum as they should..)
There are several factors that could result in ion losses. The “perfect focusing” multi-turn ion optics developed at Osaka University and used in the JEOL SpiralTOF and InfiTOF permit very long flight paths with no losses due to ion optic transmission. The product of beam divergence, kinetic energy and spatial foxus is a constant, so you can’t focus all three at the same time. Multi-turn ion optics get around this limitation by bringing different combinations of 2 out of the 3 parameters into focus at different times. Thus ion optic transmission is essentially 100%. However, loss in signal can still occur because of ion/neutral collisions or unimolecular decomposition. Ion internal energy and vacuum are important considerations. There are published references to multi-turn ion optic design.
Mr. Hh,
Try with the following model equation for diffusion:
Experimental and theoretical mass spectrometric quantification of diffusion parameters and 3D structural determination of ions of L-tryptophyl-l-tryptophan in electrospray ionization conditions in positive operation mode, Bojidarka Ivanova and Michael Spiteller, Journal of Molecular Structure, 1173, 2018, 848-864.
It is based on stochastic dynamics and is exact model. It is validated using independent both experimentala nd theoretical quantum chemical methods.
And because of we aim at a universal application of our model; we need a statistical representativeness, in context MS methods. Towards compounds belonging to different classes of chemicals we already have collected a relatively representative set of comounds. But towrards methods, we have tested the concept using CID, ESI, APCI, MALDI as ionization methods and Orbitrap and quadrupole ion trap analyzers. It would be useful to share application to TOF and ICR analyzers, together with other soft ionization approaches.
Hi Nav,
if you increase the length of your flying tube, then you also increase the probability that your charged molecules collide with air molecules, discharge themselves and are then no longer detectable - in other words your transmission decreases. I have listed the mean free path length for various mass spectrometers (source: Willoughby, Sheehan, Mitrovitch, A global view of LC/MS, 2nd edition, Page 490):
Pressure (torr) Mean free path
FTMS < 10^-8 5 km
Sector < 10^-6 50 m
TOF < 10^-6 50 m
Quadrupole < 10^-4 50 cm
Ion Trap < 10^-4 50 cm
Regards
Markus
P.S.: Gradually broadening of ion packet along the velocity axis reduced the peak height to a certain extent.
Hi Nav,
also the geometry of the focalisation and flight plays a role. We have a Bruker Micro-TOF-Q-II and there the ions must fly a parabel form. On their half way the repeller electrode changes the direktion of the ions. The jet of ions will of course broader with distance. But the geometry is calculated by each producer to get best results. The Mikro-TOF is equipped with a mikro-channnel -plate as detector to get a large area of detection. Our resolution is limited by the A/D-conveter with 8 Bit at 2.4 GHz. Newer systems have already much faster A/D-converters.
Considering only the time distribution is not enough (delt(t) = witdh of the distribution). The main initial width comes from the kinetic energy distribution i.e. the velocity distribution. This is compensated in the reflectron (the fast ions go a longer path, and are refocused when going back.) I think that collisions with residual gases are not the main cause of ion spread or loss. An ion packet may undergo "space charge" effect, by repulsion of the ions of the same charge. This will enlarge the ion packet (broadening) and reduce the resolution and finally the number of ions.
Hi Nav,
" But it is well known that increasing the distance of TOF compromises the transmission."
As usual, "it is well known" does not prove anything. Increasing TOF path length in multi-reflecting TOFs does not compromise transmission. True, there is other issue. You did not mention that dt may increase (and it does) with distance as well. For conventional TOF distances it is nearly constant, defined by initial phase volume of ions (thermal spread, "turn-around time", you might hear about it) and detector dead time. For long paths it is important to consider various aberrations of ion beam, that lead to rise of dt. That is why people use complicated designs of reflectors (ion mirrors). Of course for multiple reflections mirrors should have no grids. The grids are the main factor limiting transmission in conventional TOFs.
I am not sure, what do you mean by diffusion. People normally think about ion mobility, using this term (see answer from Bojidarka). If you mean the beam divergence - that is correct.
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