Measurements of the magnetic susceptibility of solid He 3 are complicated by the nature of the coupling of the spin system to the lattice. This coupling is examined at temperatures down to 0.1 °K, magnetic fields up to about 2 kG, and molar volumes between 18.5 and 24 cm 3 using a continuous-wave nmr apparatus. For pure He 3 , the three-bath model of Garwin and Landesman gives a good description of the way in which the spins achieve equilibrium with the lattice. By measuring the various relaxation times, values of the exchange integral are derived by several different methods with good internal consistency. Values of the exchangebath specific heat very close to the calculated values are observed. We have measured values of the exchangelattice relaxation time over the same wide range of temperature and molar volume and find values very different from the results of Garwin and Reich. This and their large discrepancy between observed and calculated exchange-bath specific heats we believe to be due to the presence of about 1% He 4 in their samples. In the bcc phase, over certain temperature regions, diffusion appears to be the dominant coupling process, and values of the self-diffusion coefficient are calculated which suggest that the diffusion process is thermally activated at values down to at least 10 -16 cm 2 sec -1 . Experiments on samples containing 0.5% He 4 show that the presence of small amounts of He 4 impurity affects very strongly certain parameters in the threebath model, and concentrations as low as 0.05% are detectable by their effect on the exchange-bath specific heat. Also in the impure samples a new long relaxation time appears below 0.7°K, which we believe was incorrectly identified as the exchange-lattice relaxation time by Garwin and Reich and which was the cause of nonequilibrium between spins and lattice in the magnetic susceptibility experiments of Adams, Meyer, and Fairbank.

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Rationale Lignin occurs in a broad range of forms, e.g., native as the main support for plant walls, and processed, for which its structure depends on the nature of the industrial isolation method, such as in paper production or in biorefineries. Due to the variety of lignin sources, there is no unified agreement on the structure of lignin or even its molecular weight (MW). Methods The focus of this review is on the application of atmospheric pressure ionization methods to lignin analysis by mass spectrometry (MS), namely electrospray ionization (ESI) or direct analysis in real‐time (DART). Specific parameters affecting ionization including electrolytes and solvents are discussed. Results The main challenge for MW determination of lignin is its heteropolymer character as well as the mass range limitations of MS instrumentation. To date, only a few studies have successfully used the mass range above m/z 1500. We present the advantage of ESI in generating multiply charged ions, allowing for a further increase in the mass range of deconvoluted mass spectra. While some methods such as DART do not address the mass range problem, they may serve as excellent imaging tools suitable for structural characterization of lignin. Conclusions A literature review presents the recent accomplishments in lignin MS analysis by atmospheric pressure ionization techniques. Although significant breakthroughs have been made, it is essential to further improve the operating conditions and validate the methods for a broader range of feedstocks with the results being confirmed using other methods.

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