We often experience "spinning problems" with our liquids (mostly
Bruker) NMR spectrometers. Often these faults are corrected by swabbing
the bore of the upper stack, particularly in the spinner stator region.
Sometimes this doesn't help, and simply replacing the spinner gets the
sample spinning again. We have accumulated a significant collection of
"bad" Bruker spinners over the years in this way. Last week we
experienced an odd behavior on our AC-250: When the spinning air was
turned on, the rate went 0-1-2-3-2-1-0 on the readout. Odd. Yet from
the appearance of the swept lock signal (ringing) or the lock level, the
sample was indeed spinning. Putting a scope to the appropriate test
point on the B-SN 18 shim supply (where sample spinning is controlled)
showed the spin detector signal had an amplitude of about 0.1 V p-p, at
a rate that showed the sample spinning at about 40 Hz despite an
indicated rate of 0 Hz. This amplitude is evidently below the threshold
for the comparator that squares up the spinning signal and converts it
to logic levels. With a new spinner, the analog signal voltage was ~2 V
p-p and the spinning rate displayed agreed with the 20 Hz setpoint.
Bruker spinners use a strip of reflective tape (3M Scotchlite?) with 4
black marks on it around the top to alternately reflect and absorb the
light from a light emitting diode, sended by a phototransistor. The
reflective part of the tape, and the black areas, looked about the same
on the "bad" spinner as on the one that worked.
According to the Motorola data book, most opto-emitters work in the
near IR (850nm and 940nm are common). My theory is that over time the
base of the reflective tape deteriorates and becomes strongly absorbing
in the near IR. Since the reflective material (glass or plastic beads)
is below the tape base, not much light gets reflected and the spinner no
longer gives a correct indication of its rotation. I took the offending
spinner, stripped off the tape, replaced it with a strip of white lab
tape (Time Tape brand), and marked it up with a black sharpie. VOILA!
Now proper spinning rate indication and ~2 V p-p sensor voltage, just
like with a new spinner with the proper reflective tape! All of our
"bad" spinners showed low signal voltage and all were "fixed" by
replacing the tape strips as described.
Probably I have just come to a belated realization of something all
of you already knew. It was just surprising to me that these tape
strips that looked nice and reflective upon visible inspection could be
the problem. That, and that one should pay attention to the lock signal
and not take the spin rate indication at face value!
Ben Bangerter
Received on Mon Sep 08 2003 - 14:36:46 MST