Lube-Tech
β (-1/2). For NMR analysis, the spins are oriented using an external magnetic field. By irradiating the sample with radio frequency (hν1
), the spins absorb (a) or emit
(e) the energy and subsequently change their original orientation (Figure 1). [10]
Figure 1: Nuclei Spin Energy Levels. [10]
Modern NMR spectrometers typically rely on the “pulse” principle, meaning that all nuclei of one type are simultaneously stimulated by the action of a high frequency pulse inducing resonances. These are directly related to the vicinities in situ. surrounding them. The resonances are detected as free induction decay (FID) which is converted into the NMR spectra by the action of Fourier Transformation. [10]
The magnetic field in modern NMR instruments is generated through super conducting magnets using liquid Helium as coolant. Thus, magnetic fields of 23.5 T with a 1H resonance frequency of 1 GHz are available today. Furthermore, modern instruments allow the observation of different nuclei and the possibility of multidimensional NMR experiments, showing direct spin-spin couplings between neighboring nuclei, facilitating molecule structure solving in R&D facilities. [16]
The concentration of the NMR active nuclei isotope is also of great relevance. A high concentration thereof leads to superior signal to noise ratio, yielding evaluable information and to acceptable measurement timing. [Lit] Usually several thousand high frequency pulses are used during the acquisition of an NMR spectra. Before the sample is subjected to a new
32 LUBE MAGAZINE NO.176 AUGUST 2023
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.147 page 3
pulse, the full relaxation of the spins must be ensured. [10] This may be affected by the viscosity of the lubricant. Thus, dilution with a NMR specific solvent is applied, leading to low viscosity of the resulting solution, ensuring spin relaxation within a reasonable timeframe. [15] [17]
As indicated above, the observed resonance of the nuclei is directly influenced by its environment. Thus, different neighboring structures lead to altered resonances. Additionally, only the resonance is specifically linked to the observed core, leaving other parts of the molecule mostly aside. Therefore, the resulting information is more precise as for other analytical methods where the whole molecule is brought into resonance, regardless of the contained elements (i.e. FTIR).
Thus, NMR allows the deliberate observation of different elements and their neighbouring chemical structures. This is used to observe the changes within the additive molecules in comparison with fresh oil references. It is a very versatile method as the specific observation of elements and structures is possible without disturbances of other compounds present in the lubricant mixture as described. [18]
Table 1: 31
P-NMR Resonances of common AW/EP Additives.[19]
It is known that AW and EP additives are structurally altered during their use. [8] [19] As their structures change, it is possible to track the concentration of the original and the aged AW/EP molecules (Figure 2).
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64
