Follow the Circuitry Road Part II: The little proton signal that could

Welcome back!  If you recall from our last blog, we followed a bench-top NMR electronics blueprint to highlight the technology that allows the generation of an excitation wave and tailors it to nutate our hydrogen nuclei in our sample.  Now, keeping with our goal of demystifying the technology of an incredible benchtop NMR, we will now take a look at what happens with our sample from the excitation wave, and how the resulting NMR signal propagates through the rest of the circuitry and is collected in such a way that we obtain a spectrum!  Let’s start where we left off:

Tuning Capacitor and RF Inductor

First, if a light bulb went off in your head when you read the “M” probe label for the Matching Capacitor in our previous blog, then you surely deduced that the “M” and “T” dials on your high-field instrument correspond to these Matching and Tuning Capacitors: these are the dials that you probably remember taking a considerable amount of time and effort to calibrate just right.

Might I mention that bench-top NMRs like the NMReady™-60 are designed to eliminate such tinkering practices and drastically reduce your instrument set-up and maintenance time, but more on that later…

It is also important to know that the Tuning Capacitor is also variable, but for a different reason than the Matching Capacitor: it is made variable in order to tune to a specific frequency depending on the nuclei in question (remember γ).

But first, allow me to side step for a moment and divert your attention to the Tuning Capacitor/rf Inductor circuit system.  This system takes advantage of another fundamental property of electromagnetism: the relation of the capacitance (C) and inductance (L) of a circuit.  These values in said circuit create what is called aresonance circuit which builds up a resonance frequency that is much more powerful and oscillates in the circuit longer than it normally would.  This frequency can be calculated in the below equation for a resonance or “tank” circuit:

This is ideal for strengthening the newly acquired FID signal.  Each time this signal oscillates in the probe circuit (oscillating along the B1 field lines noted above in blue), it continues across the matching capacitor and proceeds unhindered past the Second Crossed Diode (which is “off” for this low voltage signal) towards the Pre-Amp.

The Pre-Amp

Now we’re on the homestretch.  We have our FID signal, but remember that it’s weak, considerably so: it consists of the 15 MHz Larmor frequency from most protons, plus the NMR signal from the slight excess of nutated protons processing at a frequency just near that value.  Even after the boost from the resonant circuit, this signal carries a very low voltage (i.e. on the order of 1 mV).  This is amplified by a factor of 1000 to reach 1V potential so that it can be matched with that long forgotten red wire at the Phase Detector.

Band Pass Filter

This filter acts as a secondary means to exclude noise from the proceeding signal and only allow a small “band” of frequencies encompassing the 15 MHz signal and some room for the NMR signal.

Phase Detector

The Phase Detector is another double balance mixer (like the Gate described above).  It is fed by two inputs: the continuous FID (15 MHz + NMR signal); and the continuous, pure 15 MHz excitation wave originally generated at the beginning of our journey.  Since these are two continuous waves, this double balance mixer will output two signals through one outlet, which will be the sum and difference of these two signals: the 30 MHz + NMR signal, and the NMR signal alone (of very low frequency difference).  Of course, we already know the Larmor frequency for unaltered 1H, and we don’t usually need that information.  All we need is the NMR signal that indicates a deviance from said frequency.

Low Pass Filter

This is a final filter that blocks the higher frequency (30 MHz) and only allows the NMR signal to pass, and then from there the signal is routed to a computer with software that digitizes the signal, then Fourier Transforms the data from a time domain to a frequency domain, and…Voila!

The result is a beautifully resolved and highly informative NMR spectrum for your learning and researching pleasure!

We have finally processed and acquired our long-awaited NMR signal.  Looking back to our first blog, we started by inserting an unknown sample into what we could have called a “black box” of instrumentation (though now we know better!) and we end up with a nice, clean, and informative spectrum.  To explain this in gross mathematical terms:

 

And there you have it – this incredible journey, all just to acquire a small, analog NMR signal.  My hope is that you are now much more acquainted with the impressive technology that allows a bench-top NMR to exist in the scientific field.

Now that I’ve shown you how to build one, you have two options:  the first is to try and do it yourself (which I’m quite confident you can now do), albeit with much troubleshooting and second-tries, OR you can leave it to the experts of bench-top NMR to bring an NMReady-60 right to your lab bench!  Check out all of the features it offers including a touch-screen interface, automatic shimming settings, and a sleek and portable design; all of which can be brought to you and made a great addition to your laboratory!