Ferromagnetic contamination of ultra-low-field-NMR sample containers. Quantification of the problem and possible solutions
Graphical abstract
Introduction
Most of the magnetic resonance (MR) experiments aimed at spectroscopic measurements or at imaging (MRI) suffer from distortions of the magnetic field. Hence, particular care is necessary to prevent field disturbance induced in the vicinity of the sample by parts of the apparatus, including the sample itself.
In conventional (high field) NMR experiments, field distortions causing line broadening or misshaping (in spectroscopy) as well as image artifacts (in MRI) arise most commonly from susceptibility (see e.g. Refs. [1], [2] and references therein) and conductance [3], [4], but also from ferromagnetic terms [2].
In earth-field [5], ultra-low-field (ULF) [6], [7], and zero-field [8] MR apparatuses, the ferromagnetic terms may play an important – potentially dominant – role, and their presence can be detected directly by the same sensor that measures the MR signal [9], [10]. This aspect is the focus of the present work. In particular, we study, characterize and analyze spurious effects occurring in an ULF-NMR apparatus, originating from ferromagnetic contamination of polymeric sample containers (cartridges). These cartridges contain samples for a remote-polarization NMR experiment that uses an optical atomic magnetometer (OAM) as a high-sensitivity, non-inductive detector [11], [12], [13].
Magnetic detectors based on OAMs are an interesting class of sensors that rival with the top-sensitivity ones based on superconducting quantum-interference devices (SQUIDs), compared to which they have advantages in terms of maintenance cost, practicality (no cryogenics needed), and of robustness with respect to strong fields. In some implementations, OAMs have a broadband response, extending to static signals. This feature is here exploited to detect simultaneously both the DC signal due to ferromagnetic impurities and the time-dependent signal generated by nuclear precession.
This work is about the characterization of spurious ferromagnetic remanence of the cartridges and of the material used for their production. Besides magnetometric measurements, surface analysis based on X-ray fluorescence spectroscopy (XRFS) and Time-of-Flight Secondary-Ion Mass-Spectrometry (ToF-SIMS) have been performed. In addition, several approaches attempted to remove or reduce the contamination are described and discussed, drawing conclusions about their effectiveness.
Detection and analysis of contamination by ferromagnetic impurities, as well as tiny ferromagnetic behaviour due to specific phenomena, are topics of interest among a wide community, spanning from material science [14], [15], to semiconductor technology [16], nanotechnology [17], and medicine [18]. A recent paper addresses the problem of characterizing and counteracting ferromagnetic contamination in a variety of metal-oxide substrates, in which weak extrinsic ferromagnetic behaviour is observed [19], also evidencing possible artifacts due to the measurement procedure.
Works devoted to investigate extremely weak ferromagnetic response in nano-structures [17], [15] or diluted dopants [14] commonly make use of state-of-art magnetometers (typically SQUIDs) and study the saturation level and the hysteresis that characterize the material. On the other hand our measurements are more tightly focused to the implications of spurious ferromagnetism in ULF-NMR apparatuses and are mainly concerned with the remanence.
The paper is organized as follows: the Section 2 provides a brief description of the OAM used, of the setup making it suited to detect ULF-NMR signals, and of the instrumentation used to chemically analyze the surface contaminants; the Section 3 shows the effects caused by the spurious magnetization of the containers and complementary methodologies used to evaluate and characterize the contamination; the Sections 4 Preventing surface contamination: non-ferromagnetic tools, 5 Removing surface contamination: cleaning methods provide the results obtained by the application of methods aimed at preventing/removing the ferromagnetic contamination, and the information inferred about the bulk or surface localization of the problem. The Section 6 concludes the paper, providing a summary of the observations and of the results as well as a discussion about possible implications and perspectives.
Section snippets
Magnetometer
The magnetometric setup is designed to measure NMR signals from samples that have been previously magnetized in a field at the Tesla level, generated by a permanent-magnet Halbach array [20]. The nuclear precession occurs in a ULF (at micro-Tesla level, corresponding to proton Larmor frequencies of the order of 100 Hz), so to require non-inductive detection. The apparatus contains an OAM [21] that detects in situ the NMR signal, a system for pneumatic sample transfer [22] from the
Evidence of contamination
Ferromagnetic contaminants on the surface and/or in the polymer bulk, cause a remanence that, after the premagnetization in the Halbach array, makes the cartridge produce a nearly dipolar field on its exterior and, internally, a field in average antiparallel to the magnetization and scarcely homogeneous. We are dealing with a spurious remanence that is extremely weak, so to produce only perturbative effects.
Concerning the mentioned dipole approximation, an estimation of the dipolar term and of
Preventing surface contamination: non-ferromagnetic tools
The surface contamination most probably occurs at the machining stage. To prove that, titanium tools have been crafted and used at the lathing stage to avoid direct transfer of contaminants from the blade to the PEEK surface.
In this way two additional evidences emerged proving that surface contamination occurs during the manufacturing process. The first one is indirect: polymeric samples machined with titanium tools instead of ordinary HSS tools give much weaker DMFV. The second evidence is
Removing surface contamination: cleaning methods
An alternative procedure to reduce the surface contamination is the use of mechanical or chemical methods to remove residues from HSS-machining.
Conclusion
The problem of residual permanent magnetization of polymeric sample containers used in a remote-detection ULF-NMR experiment has been studied by means of magnetometric measurement and chemical analyses of the polymer surface.
Some degree of magnetic remanence has been pointed out, and has been attributed to polymer surface contamination occurring during the container production, and (at a much lower extent, as to have negligible consequences in our applications) to bulk contamination of the
CRediT authorship contribution statement
Giuseppe Bevilacqua: Conceptualization, Methodology, Formal analysis, Investigation. Valerio Biancalana: Conceptualization, Methodology, Validation, Investigation, Data curation, Writing - original draft, Writing - review & editing, Project administration. Marco Consumi: Methodology, Validation, Investigation, Resources, Data curation. Yordanka Dancheva: Conceptualization, Methodology, Investigation, Writing - original draft. Claudio Rossi: Conceptualization, Methodology, Validation,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors are pleased to thank Dr. Tommaso Lisini from SirsLab at DIISM, for providing the 3D printed samples, and Dr. Sophie Versavaud – Victrex Technical Support for the kind and effective assistance.
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