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So I know the answer to this question varies widely across mass spec techniques, masses, and, of course, budgets, but my question is about the best case scenario for all of these variables (although I'm not really looking at proteins or anything of that size). I realize that the concept of depletion spectroscopy relies heavily on very sensitive mass spectrometers, and I'm curious about the lower limits of signal detection for such methods. Thus, I'm interested in the sensitivity, not resolution ("mass resolving power"), which seems to be a statistic that is difficult to find for most of the systems that I've looked at. Some related follow-up questions are what mass spectrometry methods can and cannot be used with depletion spectroscopy and what methods/manufacturers are the gold standard in the field of mass spec?

Thanks in advance for your answers.

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Hello David, I found some depletion detectors, useful in CCDs for IR and maybe in ion detectors. But no depletion spectroscopy. Could You give some links, please? –  Georg Jan 19 '11 at 15:40
    
Sorry, it seems depletion spectroscopy is a less ubiquitous technique than I thought. This paper uses the technique, and much of helium droplet isolation spectroscopy (review articles here and here) is based on the depletion spectroscopy technique. It is also often used with molecular beam spectroscopic experiments. –  David Hollman Jan 20 '11 at 14:16
    
@David Hollmann, From what I can read in the abstract, there is IR-Spectroscopy. (Maybe in those helium droplets?). In my opinion "spectroscopy" is a question of a spectrometer which resolves wavelenghts, masses or velocities etc. The way of probe preparation or detection is not a reason for a new "spectroscopy". Of course "inventors" or makers/sellers of such apparatus have other opinions :=( –  Georg Jan 24 '11 at 15:10
    
@Georg: I agree with your assessment of the use of the term "spectroscopy," in that it is usually used in terms like IR spectroscopy, UV-Vis spectroscopy, X-ray absorbance spectroscopy, etc. However, most papers I've read refer to the "category of spectroscopies" that measure spectroscopic information by measuring depletion of something in a molecular beam of some sort as "depletion spectroscopy" (as opposed to, for instance, the "categories" of absorption spectroscopy, fluorescence spectroscopy, or raman spectroscopy, all of which span multiple wavelength ranges). –  David Hollman Jan 24 '11 at 16:38
    
So, that "depletion" is the analogon to absorption for molecular beams? Would You say emission/absorption UV/vis spectroscopy or absorption only, leaving to the reader what is absorbed? All to be said, can be said clearly (or similar) :=) –  Georg Jan 24 '11 at 16:47

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Since there are no specialists in depletion mass spectrometry, I will try to answer in a more general way.

With depletion spectroscopy you look at a small variation of a large signal so what you need is not high sensitivity but signal stability and high dynamic range of the detector. I assume that you have a continuous stream of ions, otherwise pulse to pulse fluctuations of ion concentrations will be the major limiting factor. Probably, you won't need superb mass resolution so you would want to buy a quadrupole mass spectrometer - these are cheap, compact and there are plenty of companies that make them.

Dynamic range depends on the ion detector and since you would be looking for a wide dynamic range and stability with time, the very best choice is a simple Faraday cup. You can be sure that, whatever your experiment is, the sensitivity will be limited not by the detector but by fluctuations of your signal - most likely, by how stable your depleting factor is.

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Can't fluctuations in signal be remedied by integrating over a long enough time span? Unless the fluctuations are non-random (which is a bigger problem anyway), won't the differences cancel out? –  David Hollman Jan 27 '11 at 16:17
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That helps, of course, but signal to noise ratio increases only as a $\sqrt{t}$. You will integrate as much as you can but there is still a limit. The first Faraday cup controller I found on the net has readout noise below 0.1% (at 200 kHz readout rate) and it will be also reduced by integration. You have not described your experiment but it is unlikely that your experimental conditions would be stable down to 0.1%. –  gigacyan Jan 27 '11 at 21:14

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