Fourier transform infrared spectroscopy as a non-destructive method for analysing herbarium specimens

Abstract

Dried plant specimens stored in herbaria are an untapped treasure chest of information on environmental conditions, plant evolution and change over many hundreds of years. Owing to their delicate nature and irreplaceability, there is limited access for analysis to these sensitive samples, particularly where chemical data are obtained using destructive techniques. Fourier transform infrared (FTIR) spectroscopy is a chemical analysis technique which can be applied non-destructively to understand chemical bonding information and, therefore, functional groups within the sample. This provides the potential for understanding geographical, spatial and species-specific variation in plant biochemistry. Here, we demonstrate the use of mid-FTIR microspectroscopy for the chemical analysis of Drosera rotundifolia herbarium specimens, which were collected 100 years apart from different locations. Principal component and hierarchical clustering analysis enabled differentiation between three main regions on the plant (lamina, tentacle stalk and tentacle head), and between the different specimens. Lipids and protein spectral regions were particularly sensitive differentiators of plant tissues. Differences between the different sets of specimens were smaller. This study demonstrates that relevant information can be extracted from herbarium specimens using FTIR, with little impact on the specimens. FTIR, therefore, has the potential to be a powerful tool to unlock historic information within herbaria.

Keywords: Fourier transform infrared spectroscopy; herbarium; hierarchical cluster analysis; non-destructive; principal component analysis.

Figure 1.

(a) Brightfield image showing different regions of the specimens investigated. Scale bar 100 mm. (b) Absorbance spectra for different tissue types in different samples groups for ‘Old’ and ‘Young’ specimens. Presented are normalized absorbance across the full spectral range. Solid lines represent the mean spectrum for each group, translucent lines are the spectra for each point within a tissue. (c) HCA dendrograms separating between specimens (left) and tissue type (right).

Figure 2.

PCA scores and loadings plots for (a–c) the whole spectra analysis (4000–650 cm⁻¹) and (d–f) the lipid specific region (3000–2800 cm⁻¹). PCA coordinates on PC axes 1, 2 and 3 are presented for each measured point for different tissue types and different sample categories (details in table 1). PCA loadings plots present the contribution of each wavelength to each PC.