NMR Glossary#

A practical reference for the NMR terms you'll encounter in TQT Nuclei. Each entry explains what the term means, where it appears in the app, and when it matters.

Acquisition time

How long the receiver collects signal data after a pulse sequence event. Longer acquisition captures more of the signal decay but also picks up more noise. Set this in the ACQUISITION control panel or in experiment presets.

Apodization

A mathematical window function applied to time-domain data before FFT. It reduces spectral artifacts (ringing) at the cost of slightly broadened peaks. Configure in SETTINGS → Advanced → Digital Processing.

CPMG (Carr-Purcell-Meiboom-Gill)

A pulse sequence that measures T2 relaxation time by generating a train of spin echoes. It compensates for magnetic field inhomogeneities better than a single Hahn-Echo. Available as a preset on the ACQUISITION page and the T2 page.

Tip: More echoes give a more accurate T2 curve, but very long echo trains can accumulate errors. Start with the default and adjust from there.

Dead time (ring-down time)

The delay after a radiofrequency pulse before the receiver starts listening. During this period, the electronics are still "ringing" from the pulse. Too short and you record hardware noise; too long and you miss early signal. Tune it in the HOME → DEAD TIME tab.

Decimation

Skipping a set number of p180 pulses in a CPMG echo train. This lets you sample later echoes without recording every single one — useful for samples with long T2 times. Set in the T2 page or CPMG sequence parameters.

Dwell time

The time interval between consecutive data points during acquisition (typically 0.5 µs). This is set by the hardware and cannot be changed by the user. Visible in SETTINGS → Advanced.

Echo time (τ)

The time delay between the excitation pulse (p90) and the refocusing pulse (p180) in an echo sequence. It determines the spacing between echoes and affects which part of the T2 decay you measure. Set in experiment presets or custom sequences.

FID (Free Induction Decay)

The signal emitted by excited nuclei immediately after a single radiofrequency pulse. It decays as nuclei lose coherence. FID is the simplest NMR measurement and is often used for initial calibration. Available as a preset on the ACQUISITION page.

FIR filter (Finite Impulse Response)

A digital filter applied to the raw signal to reduce noise. Choose between Gauss (smoother) and Sync (sharper cutoff) types in SETTINGS → Advanced. You can also bypass filtering entirely if you need raw data.

Frequency / Larmor frequency

The resonant frequency of the nuclei in the magnetic field. The device must be tuned to this frequency to detect signal. Adjust in HOME → FREQUENCY tab or SETTINGS → Device.

Tip: If you're getting no signal, the frequency is the first thing to check. Use the auto-tune feature to find it automatically.

Hahn-Echo

A two-pulse sequence (90°–τ–180°) that produces a single spin echo. Simpler than CPMG and useful for measuring T2 when you don't need a full echo train. Available as a preset on the ACQUISITION page.

Inversion recovery

A pulse sequence (180°–τ–90°) used to measure T1 relaxation time. The 180° pulse inverts the magnetization, then you wait a variable time τ and measure how much it has recovered. Used by the T1 page preset.

Nutation

The process of sweeping pulse duration to find the optimal flip angle (p90). During nutation, you measure signal amplitude at different pulse lengths to find the maximum. Performed in the HOME → DURATION tab.

Phase cycling

A technique where the phase of RF pulses is varied across multiple scans to cancel artifacts and improve signal quality. Enable it in the ACQUISITION control panel. When active, the number of scans must be a multiple of 4.

Pulse duration (p90, p180, p270, p360)

The length of the radiofrequency pulse in microseconds, determining how far the nuclear magnetization is tipped. p90 tips it 90° (maximum signal), p180 tips it 180° (full inversion). These values are sample-dependent — find them using the HOME → DURATION tab.

Tip: Always re-tune pulse durations when changing samples or after significant temperature changes.

Repetition delay

The wait time between consecutive scans, allowing the magnetization to fully recover. Should be at least 5× the T1 time of your sample. The software sets this automatically based on the anticipated T1 value, but you can override it in SETTINGS → Device.

Signal vs. Spectrum

Signal shows the raw time-domain data (amplitude over time). Spectrum shows the frequency-domain data after FFT (amplitude over frequency). Toggle between them on the ACQUISITION page using the SIGNAL / SPECTRUM switch.

T1 (spin-lattice relaxation time)

The time constant for nuclear magnetization to recover along the magnetic field direction after being disturbed. Longer T1 means the sample takes longer to return to equilibrium between scans. Measured on the T1 page using inversion recovery.

Practical meaning: T1 affects how long you need to wait between scans. A sample with T1 = 2 seconds needs at least 10 seconds between scans for accurate results.

T2 (spin-spin relaxation time)

The time constant for the decay of the transverse (perpendicular) magnetization after excitation. Shorter T2 means the signal fades faster. Measured on the T2 page using CPMG.

Practical meaning: T2 tells you about molecular mobility. Liquids have long T2 (slow decay), solids have short T2 (fast decay). Water at room temperature: T2 ≈ 2-3 seconds.

Zero filling

Adding zeros to the end of time-domain data before FFT. This doesn't add real information but interpolates between frequency points, making spectra look smoother and easier to read. Configure in SETTINGS → Advanced → Digital Processing.