Basic Techniques in Wood
Chemistry
Objective: obtain a better feel for the "tools
of the trade."
- Important Methods
- Separation Schemes
- Chromatography (HPLC and GC)
- Spectroscopy (UV, IR, MS, NMR)
- Introduction
- How do we know what we have when we look at a piece of wood,
a wood extract, or a purified cellulose, hemicellulose or lignin?
It's not that easy. However, there are several very useful tools
that chemists rely upon when characterizing the chemical constitution
of a material. Outlined below are the main tools of the trade.
- Separation
- The first thing you have to do is separate out the substance
that you are interested in. A general scheme to separate out
the components of wood using solvents, acids and bases is shown
below:
- In this scheme, lignin is obtained as a degradation product
and is not intact. The polysaccharides are relatively unmodified
although their DPs will be shorter and the hemicellulose acetate
groups will be lost. If one wanted a relatively intact lignin,
one typically prepares an extractive-free wood meal (grinding
followed by solvent extraction) and then ball mill to mechanically
degrade the wood structure to the point that some of the lignin
can be solubilized in a solution of dioxane/water. Newer techniques
for improved lignin yield utilize an enzymatic digestion of the
polysaccharides prior to extraction with dioxane/water. This
opens up the matrix to better extract out the soluble lignin.
Both techniques will modify the lignin to some degree.
- Analysis
- Let's say we have separated out are components and wish to
investigate structure. What tools are available to do this? The
most useful techniques rely on chromatographic and spectroscopic
techniques. Chromatography relies
on the fact that compounds have different polarities and/or shapes.
When they are placed in contact with a solid material, it will
interact with this material at a level which depends upon the
polarity and/or size of the solid surface. The pdf file below
entitled Basics of HPLC provides an introduction to the technique
and the file entitled Example Chromatograms describes its use
in investigating lignin-tannin bonds in wood. Gas chromatography
works essentially the same way although the carrier "solvent"
is now a gas like nitrogen. For this technique to work, the sample
has to be somewhat volatile.
-
- Spectroscopy is also a useful technique. Ultraviolet spectroscopy
(UV) relies on the ability of certain compounds to absorb UV
light. This can be quantitated and the technique is often used
to detect compounds eluting from an HPLC column. Infrared
spectroscopy (IR) is a technique that provides an indication
of functional groups. Bonds will stretch and vibrate at specific
IR frequencies and this can be detected and quantitated in an
IR instrument.
-
- By far the most useful spectroscopic techniques are mass
spectroscopy (MS) and nuclear magnetic resonance spectroscopy
(NMR). Mass spectroscopy involves volatilizing a compound
and essentially breaking it apart with a sledgehammer. The molecule
will cleave in specific ways due to the functional groups it
contains. These molecular pieces can be detected based solely
on their molecular weight. When compared to a database you can
piece the molecule back together and certain structures can often
be generated or ruled out. NMR relies on the fact that nuclei
in each atom will tend to align when placed in a magnetic field.
If I perturb their alignment with radiofrequency, the nuclei
absorb the energy and move to a higher energy level. An NMR spectrometer
monitors how they relax back to the aligned state. The rate at
which they do that depends upon the local environment within
the molecule. Since most environments are different the relaxation
is different. What results is a spectrum which is a fingerprint
specific for each atom. Hydrogen and carbon NMR are the most
useful techiques in wood chemistry, and with a database in hand
one can often generate a structure from the NMR spectra recorded.
This is a powerful but expensive technique with the most sophisticated
commercially available spectrometers costing about $3 million.
To see the power of NMR in determining some fundamental aspects
of lignin biosynthesis, you can download the pdf files available
from the Dairy
Forage Research Center Web Site in Madison, WI.
-
- Separation
Scheme.
- Basics
of HPLC.
- Example
Chromatograms.
-
- Back to the
Wood 3434 Information Page