Fusion Message Board

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Subject: Re: clarification (Mass spec)
Date: Aug 13, 1:58 am
Poster: Dave Cooper

On Aug 13, 1:58 am, Dave Cooper wrote:

>I seem to recall a mention in "the Bell Jar" about a quadrupole mass spec being refurbished, or built from scratch, by a determined amateur. The theory is simple (isn't it always) but I suspect the practice is far from trivial.

I seem to also recall an article describing a simple "home built" quadrupole Mass Spectrometer, in which the drive electronics consisted of a good
Data Acquisition card...with several channels of A/D output. The combined ramping DC with AC that does the mass filtering, was produced by a simple computer algorithm.. and then output to a modest power transistor driver. For the smaller Quadrupoles, 20 cm long rods about 6 mm in diameter are used... and operate at voltages of about 1200- 1300 Volts, at about 3 MHz. The drive power is actually not large. The rods need to be stainless steel, with very good finish, straightness to about 25 um. (1 mil). Centerless ground SS stock would work well, I'm sure.

The really high grade MSD (Mass selective Detectors) HP's units, for example, use a hyperbolic cross section, ceramic rod assembly (all one piece ???) with a metallic coating. The filtering formulae indicate hyperbolic electrodes give the highest mass selectivity, but for small rod diameters, round works quite well.

That said, the detector is perhaps the most important part. This is true also for the refurbished units. Sometimes, you might get a real treasure.. when the electron multiplier gets accidently baked out.. and ruined.... and the RGA head gets tossed. most of it is repairable, and you don't need to be a genius.. just know what it is supposed to do.

Filaments are another possibly expensive item... our lab's used Thoriated Iridium, (about $150 - $200 per set) which could take a hit all the way to ambient pressure without failing.. (provided the pressure safety relays were right behind it, shutting things down)

The only repairable detectors would be the older discrete dynode multipliers which use Be-Cu anodes. These can be rebuilt I am told, at little cost. Channeltrons go when the impact site deteriorates and I don't think there is a practical way to refurbish them.


>> An older magnetic field sorter with multiple detectors might be better? Don't forget that the quadrupole essentialy scans over the range, only looking at a small area at any one time, so if your lone T or He3 happens to enter when the qpole is tuned to D or H, you've lost it. Much like a scanner radio looking for short burst signals... your probability of intercept is low.
>The mass spec approach has the advantage of providing a definitive answer to the stripping vs fusion question

Most modern Mass Specs can sit on a number of AMU's to monitor selected species directly. But the measurements are not really simutaneous. The minimum dwell time for a measurement in a quadrupole MS, is the time of flight at a few EV energy down the 20 cm or so to the detector -- probably no more than a few milliseconds. Then, on to the next species.

I think, as Jim has hinted at, the magnetic sector MS, is capable of simultaneous measurments... provided you could get the several detectors into the beam paths.

Think the separation of low concentration species from adjacent AMU high conc species is the main challenge.

Dave Cooper