Image Credit: Vital Diagnostics | GPE Scientific Limited
ESR/EPR tubes hold samples in ESR or EPR spectrometers. They are thin glass walled tubes, uniformly thick, that produce low background noise.
Electron spin resonance (ESR), also referred to as electron paramagnetic resonance (EPR), is a direct method for measuring the absorption of microwave radiation corresponding to the energy splitting of an unpaired electron when placed in magnetic field. One type of species that contain unpaired electrons is free radicals. Free radicals are reactive molecules with unpaired electrons. They cause the oxidative breakdown of oils, food products, and play a role in the biochemisty of disease.
The ESR spectrum of a free radical is the simplest of all forms of spectroscopy. If an external magnetic field is no present, the two electron spin states (spin up and spin down) are degenerate. The degeneracy of the electron spin states characterized by the quantum number, ms =+/- 1/2, is lifted by the use of a magnetic field. Transitions between the electron spin levels are induced by radiation at the matching frequency. When a magnetic field is induced, atoms with unpaired electrons spin either in the same direction (spin up) or in the opposite direction (spin down) of the applied field. The alignments are directly proportional to the applied magnetic field strength. This is called the Zeeman Effect. An un-paired electron interacts with its environment, and the details of ESR spectra depend on the nature of those interactions. The readings can provide information on structural and dynamic information, even from the chemical or physical process, without influencing the process itself.
The standard size for ESR/EPR tubes is a 4mm OD (outer diameter). However, varying sizes are available and can typically be accommodated by the instrument. However, 4mm and 5mm OD sizes are the ONLY sizes suitable for cryogenic experiments. OD sizes include 2mm, 3mm, 4mm, 5mm, and 10mm, as well as some smaller (thin walled) tube sizes for samples with weak signals.
The defining characteristic of EPR tubes is that they are made out of quartz. This is in contrast to NMR tubes, which are made out of Pyrex; this material may contain iron that may or may not, depending on the sample, contribute error to the EPR signal. Types of quartz materials include natural quartz and Suprasil® quartz. Suprasil® quartz is a UV grade quartz material recommended for UV catalyzed reactions, and provides greater background noise reduction than natural quartz for applications requiring higher signal sensitivity.