Hi Siveen,
I think it's important to differentiate between theory, current status, and potential future practice.
The mass window of the Helios is about 76-210 (about 130 mass units). So, in *theory*, you could use up to 130 different probes.
In *practice*, some of these mass channels will never be usable for probe-labeling. For example, if you look at isotope lists (eg,
http://www.chem.ualberta.ca/~massspec/a ... _abund.pdf ):
Ar-dimer: 40 x 2 = 80
Kr (gas): 83
Xe (gas): 129, 131
There will *never* be probes on those channels because they're masses occupied *solely* by gases, and/or Ar-dimer which is too high a background to ever have a probe on it.
The *current status* has probes (chelators or otherwise) for Y, Ru, Rh, Pd, Ag, Cd, In, Te, I, La, Ce, Pr, Nd, Sm, Gd, Eu, Tb, Tm, Dy, Ho, Er, Yb, Lu, Ir, Pt, Os, and Bi.
There are a couple papers which have used Au, Re, Cs, Nb, Zr, and Ta.
But, that still leaves several elements like Sn, Pb, and Mo which are in the mass window, but haven't been formed into a stable probe (yet?). As illustrated last year by the Cadmium channels, the nice thing about mass cytometry is that once you can form a stable probe with one isotope of an element, you automatically can form a stable probe with *all* isotopes of the element that exist in nature and that you can obtain at sufficiently high purity.
So, for "future practice", give the chemists time, and there will hopefully be more channels opening up (assuming they don't overlap with current channels from other elements, of course).
Mike