Home built mass spectrometer

[Jan Ohlsson]

Hi John, I use a sweep of the acceleration voltage for the X-axis. I did think about sweeping the magnetic field at first, but I did not go down that road, for the reasons you mention. So the magnetic field is supplied by neodymium magnets.

[John Futter]

Ok thanks for the update on how you are doing it
The double peaks might be from not having a load on your acceleration voltage so that what is commanded to the power supply is not the actual voltage due little load and some stray capacitance in your system keeping the actual acceleration voltage higher than what you think. Work out what your max accel voltage is and load the accel psu to half of its current capability with a resistor to earth, and do your scans much slower to allow the voltage to track correctly especially if you are using coaxial cables to supply your acceleration voltage from the PSU to the accel stage ( remember about 100pF per meter)

[Jan Ohlsson]

Thanks John, that could very well be the case. Will look into that.

[ChristofferBraestrup]

Great work!

This might be the highest resolution mass spectra I've seen on a DIY instrument!

Sweeping a magnetic field is difficult, and requires a lot of heavy iron and coiling to get a uniform field, I can understand using permanent magnets.

Have you considered a quadrupole mass analyzer? The geometry of the parallel rods is tricky, but afterwards it should be pretty easy to operate.

-An ion shutter could be used to remove the return sweep peaks. Another option is to try and do a VERY slow ramp from min to max, over many minutes. That might also give good resolution.

[Jan Ohlsson]

Hi Christoffer, thanks for the kind remarks!
I have changed to a slower sweep now (15 s per sweep), as you suggest, which does not cause double peaks any more. It is still quick enough for helium leak detection. But the triangle wave still causes some differences in peak height on forward and reverse sweeps, so I will go for a sawtooth waveform as soon as I get delivery of an oscillator capable of that.

Regards,
Jan

[Jan Ohlsson]

Yes, the quadropole. I thought abut it, but the demand for absolute parallell rods are beyond my machining and mechanical skills...

Jan