The Determination of Lignin

In considering the various methods proposed for the determination of lignin, it is important to bear in mind that none can be regarded as totally satisfactory. This situation arises from the fact that lignin has yet to be isolated in a pure or unmodified condition. As a result there is no definitive structural formula of lignin currently available upon which to base the development of a rational and quantitative measurement or to assess accurately the validity of the methods currently available. In addition, many of the methods which are commonly applied to the determination of lignin have fundamental defects which seriously undermine their accuracy. For these reasons, values reported in the literature may not always provide a reliable indication of the true lignin content of a material. 

In organizing the subject matter for this chapter, it was found expedient to divide the presentation into two main subsections, one dealing with the analytical methods applicable to lignin when initially present as a component of a solid mixture and the other focusing on the determination of lignin in solution. 

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It is unclear how accurate this method is for the determination of lignin content in grasses, since a fraction of the lignin from grasses appears to be readily soluble in acidic solutions, such as used in this procedure. This would result in underestimation of the actual lignin content (Hatfield and Fukushima, 2005). 

Fukushima, R. S., and Hatfield, R. D., 2004, Comparison of the acetylbromide spectrophotometric method with other analytical lignin methods for the determination of lignin concentration in forage samples, J. Agric. Food Chem. 52 3713-3720. 

The methods listed above have the advantage of being applicable to the determination of lignin in the solid state and, being nondestructive, they obviate the risk of concurrent chemical modification. In addition, these analyses required very small amounts of sample. 

A number of spectral methods for the determination of lignin consist essentially of totally dissolving the sample in a suitable solvent and measuring the UV-absorbance of the solution at a wavelength characteristic for lignin, usually 280 nm. Among the solvents used to dissolve the lignocellulosic material... 

Another spectroscopic method for the determination of lignin in solution is that utilizing fluorescence spectrophotometry. This technique is reportedly capable of detecting lignin in concentrations two to three orders of magnitude... 

Berzins V (1965) A rapid procedure for the determination of kappa number Tappi 48 15-20 Berzins V (1966) Micro-kappa numbers Pulp Pap Mag Can 67 T206-T208 Bethge PO, Gran G, Ohlsson K-E (1952) Determination of lignin in chemical wood pulp I Principles and methods Sven Papperstidn 55 44-48 Bihkova A (1973) Verification of analytical methods for the determination of lignin in water Vodm Hospod 23(2) 44, Abstr Bull Inst Pap Chem 48 1245 Birkett MD, Gambino MJT (1989) Estimation of pulp kappa number with near-infrared spectroscopy Tappi J 72(9) 193-197... 

In the past, researchers have used different UV microscopic methods for the determination of lignin concentration. Two of the procedures are briefly described for softwoods. 

This procedure is followed for quantitative measurements of lignin in an unknown sample, such as for the determination of lignin concentration in water... 

Principles and methods. Sven Papperstidn 55 44-48 Bilikova A (1973) Verification of analytical methods for the determination of lignin in water. 

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