Analyzing Trace Metal Concentrations in Bird Feathers using Laser Ablation

Bird feathers are metabolically inert tissues that contain concentrations of trace elements. The exact concentration and makeup reflect the signature of the local environment in which the feather was grown and can help to identify birds’ geographical origins.

Arkansas State University and Water Rock Life Laboratory conducted a test using Laser Ablation–Inductively Coupled Plasma–Mass Spectrometry (LA-ICP-MS) to study these trace metal concentrations in bird feathers. LA-ICP-MS is preferred over microwave digestion and ICP-OES due to reduced interferences that could negatively affect signal intensities. Studies have shown better ablation pits and improved spatial resolution at lower wavelengths, which confirm that the interaction of the laser with the sample is wavelength dependent. Certain biological materials such as otoliths and shells ablate better by the 213nm as opposed to 266nm. However, little is known whether bird feathers ablate better by the shorter wavelength.

Experiments using different spot sizes for ablation and measuring signal intensities using ICP-MS compared the ablation characteristics of a 266nm Nd:YAG laser and a 213nm laser on bird feathers. The results using the 213 nm laser ablation indicate signal intensities, which exhibit a much higher degree of temporal stability when compared to those generated using the 266 nm laser.

Among the highlights of the study are the following:

• Nd:YAG lasers are the most widely used laser source for LA-ICP-MS,
• Generating the Nd:YAG fourth harmonic requires two harmonic generators; the first that doubles the frequency and converts the fundamental to a wavelength of 532nm, and the second which doubles the 532nm beam to a 150μm, 100μm and 50μm wavelength of 266nm. The 5th harmonic is generated by sum frequency mixing the 1st and 4th harmonic (or the 2nd and 3rd) using a BBO 5th harmonic generator (Jackson 2001).
• Better ablation pits and hence improved spatial resolution observed at T200 lower wavelengths indicate that interaction of the laser with the sample is 10000000 wavelength dependent for some matrices (Durrant & Ward 2005, Guillong Horn & Günther 2001). 
• Poor absorption by transparent materials is overcome with the use of 100000 shorter wavelength which deliver higher energy photons and increase absorption; shorter the wavelength, greater the energy absorbed.
• For analysis of difficult matrices such as feathers (for which an appropriate matrix matched standard is not available), good absorption efficiency and 100 long, stable signal durations are essential for multi-elemental measurements.
• Certain biological materials such as otoliths and shells ablate better by the 10 213nm as opposed to 266 nm.
• Studies are needed to determine whether feathers ablate better by the 213nm or the 266nm.

Methods

The bird feather samples, collected under state and federal permits, were cut to lengths of 3-4cms and fixed on the Laser Ablation sample chamber using sticky tape. The testing was conducted using 266nm and 213nm CETAC Technologies Laser Ablation systems coupled with PerkinElmer CRCII ICP-MS. Feathers were ablated using the following spot sizes: 50 μm, 100 μm, 150 μm, 200 μm and signal intensities. Spot sizes below 50 μm were avoided because of poor signal intensity.

Signal stability and intensity was poorer at 50μm than at 200μm despite the type of laser. Poor signal stability can be partially attributed to the non-uniform distribution of elements in the sample.

For laser operating conditions, charts and images, additional discussion points, limitations of samples, and results for the various ablation spot sizes, download the CETAC Application Note.