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Table 6 Summary of methods discussed for the assessment of changes on the molecular level

From: A review of analytical methods for assessing preservation in waterlogged archaeological wood and their application in practice

Technique Summary Advantages Disadvantages Assessment parameters Sample requirements
Infrared Spectroscopy (FTIR) 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)
Pyrolysis gas chromatography 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)
2D 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)
Raman spectroscopy 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
ICP-AES 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
EGA-MS 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
  1. * Denotes that although conserved samples can be analysed, the conservation history of the object must be known to allow correction of the data