Table 6 Summary of methods discussed for the assessment of changes on the molecular level
Technique | Summary | Advantages | Disadvantages | Assessment parameters | Sample requirements |
|---|---|---|---|---|---|
Provides information about relative abundance of functional groups based on the wavelength of IR light absorbed | Can be portable; fast; cheap; accessible; increasingly common; can be non-destructive; no sample preparation; direct surface analysis; large existing body of literature meaning that data can be compared between studies; suitable for chemometric studies | Complex data leading to inconsistencies in data interpretation; peak overlap and alteration of archaeological wood makes assignment difficult; only semi-quantitative; a very small part of the sample is assessed; does not give detailed assessment of degraded components; can underestimate lignin content | L:C ratios are calculated by comparing heights or areas of peaks related to cellulose (e.g. 898 or 1375 cm−1) to those for lignin (1505 or 1596 cm−1) Lignin decay can be assessed comparing heights or areas of peaks related to lignin functional groups (1230–1260 cm−1) to lignin aromatic structure (1505 cm−1) Oxidation and hydrolysis observed by changes in peak shape (at 1260–1280 cm−1 and 950–1150 cm−1) Loss of hemicellulose by loss of peak at 1738 cm−1 | Dry, waterlogged or conserved sample (NB presence of water may obscure polymeric peaks); non-destructive (small indentations may be made); sample may need to be cut to fit into instrument; < 10 mg (if sub-sampling is done) | |
Sample is burnt in the absence of oxygen, breaking it down into small sub-units which are then separated by GC and detected by either FID or MS | Small sample size; gives information on degradation products as well as intact polymer; reproducible; minimal sample preparation; products are easily identifiable (MS); analysis is highly quantitative (FID); allows detailed lignin characterisation | Derivatisation steps are recommended; instruments are not widely available; needs to be compared against libraries or standards (FID); different response ratios for different compounds (MS); good background knowledge and expertise needed to interpret data; slow analysis (> 40 min); instruments are expensive | L:C ratios calculated by comparing intensity of cellulose related peaks to lignin Loss of methoxy groups from lignin signified by increased guaiacyl, 1,2-benzenediol and phenol Presence of oxidation products indicate lignin decay; quantified by an increase in the acid: aldehyde ratio Increased concentration of short-chain compounds signifies lignin decay | Dry or conserved* sample; destructive; approx. 100 µg sample required | |
NMR spectroscopy (13C, 1H, 31P) | Uses the magnetism of nuclei to determine the chemical environment of target nuclei; a fast-evolving field with increasing range of applications; sample sizes and analysis times vary depending on the information needed | Provides detailed information about structural changes; lack of sample preparation gives a more direct analysis (13C); analysis probes into the depth of a sample; examines bonds between sub-units; portable versions available | Samples must be in solution, requiring harsh preparation (1H, 31P and 2D); lack of availability of instruments and expertise; expense of instruments; complexity of spectra; spectra influenced by contaminants; cannot detect oxygen containing groups (13C); better resolution is achieved with larger amounts of sample | Increased abundance of β-O-4 linkages in relation to the methoxy groups signifies degradation (13C) L:C ratios calculated by comparing intensity of cellulose related peaks to lignin (13C, 1H, 31P, 2D) Increased concentrations of phenol and acids signify decay (31P) Comprehensive assessment of degradation mechanisms (2D) | 13C: Dry or conserved* sample; destructive; approx. 4.7 mg 1H, 31P and 2D: Dry or conserved* sample; sample is solubilised; destructive; > 7 mg (larger sample likely required due to complex solubilisation process) |
2DÂ NMR spectroscopy | Probe both the 13C and 1H nuclei in one experiment | Allows identification of additional structural features | Highly complex spectra; long experiment times (up to several days) | ||
 X-ray fluorescence | Elemental composition; can scan an entire surface, e.g. a core taken from a wooden object | Non-destructive or small sample sizes; easy data interpretation; wide availability | May not detect low concentration contaminants | Quantitative analysis of a wide range of elements, allowing assessment of inorganic content | Dry, waterlogged or conserved sample; non-destructive |
 X-ray diffraction | X-ray techniques penetrate a sample, providing an analysis of the bulk; information relates to long range internal structure | Small sample sizes; gives detailed structural information about inorganic inclusions; analyses a larger area than many other techniques | Lack of availability of synchrotron instruments (X-ray absorption); exact location of decay difficult to elucidate; may not detect low concentration contaminants | A decrease in cellulose crystallinity signifies decay (XRD) | Dry or conserved; non-destructive (but limited by instrument size, and milling samples may improve sensitivity) |
 X-ray absorption | Highly quantitative analysis of inorganic content, including charge states (X-ray absorption) | ||||
Characteristic spectrum from scattered light; complementary information to FTIR | Non-destructive; fast analysis times; can detect inorganic components; less affected by presence of water than FTIR is | Not as familiar or widely available as FTIR; not very sensitive | Cellulose: lignin composition by comparing peak heights at 1100–1150 cm−1 (cellulose) and 1600-1650 cm−1 (lignin) Wood crystallinity indicated by band at 93 cm−1 Presence of inorganic inclusions | Dry, waterlogged or conserved sample; non-destructive | |
An advanced method of elemental analysis | Highly quantitative and highly sensitive | Less widely available than some EA techniques (e.g. SEM-EDX); no structural information given | Highly quantitative analysis of inorganic content | Dry or conserved* sample; destructive; approx. 5–10 mg sample required | |
The gas evolved from burning a sample is detected and analysed by MS | Small sample size; gives information on degradation products as well as intact polymer; minimal sample preparation; ideal for analysing conserved material | Not widespread or familiar; high cost of instrument and ongoing maintenance; peaks from consolidants can overlap with polymeric signals | As for py-GC, with additional information regarding conservation consolidants | Dry or conserved* sample; destructive; approx. 100 µg sample required |