Femtosecond Laser Desorption Postionization MS vs ToF-SIMS Imaging for Uncovering Biomarkers Buried in Geological Samples

Abstract

The study of lipid molecular fossils by traditional biomarker analysis requires bulk sample crushing, followed by solvent extraction, and then the analysis of the extract by gas chromatography-mass spectrometry (GC−MS). This traditional analysis mixes all organic compounds in the sample regardless of their origins, with a loss of information on the spatial distribution of organic molecules within the sample. These shortcomings can be overcome using the chemical mapping of intact samples. Spectroscopic techniques such as UV fluorescence or Raman spectroscopy, laser ablation inductively coupled plasma mass spectrometry, and time-of-flight secondary ion mass spectrometry (ToF-SIMS) are among those elemental and molecular mapping techniques. This study employed femtosecond (fs) laser ablation combined with single-photon ionization, a method called fs-laser desorption postionization mass spectrometry (fs-LDPI-MS). A pulsed ∼75 fs, 800 nm laser was used to ablate the geological sample, which was then photoionized after a few microseconds by a pulsed 7.9 eV vacuum ultraviolet laser. An organic carbon-rich geological sample was used for this study to map hydrocarbon biomarkers in sediments that were previously studied by GC−MS. The petrography of this sample was examined by optical and fluorescence microscopy. It is demonstrated here that fs-LDPI-MS combined with petrography for multimodal imaging can expose buried compounds within the sample via in situ layer removal. When used in conjunction with traditional organic geochemical analysis, this method has the potential to determine the spatial distribution of organic biomarkers in geological material. Finally, fs-LDPI-MS imaging data are compared with ToF-SIMS imaging that is commonly used for such studies.

Mass Spectrometry Imaging of Organic Biomarkers in Geological Samples

“…MS imaging can overcome these shortcomings by identifying organic biomarkers within intact geological samples and correlating such MS data with petrographic and fluorescence images. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) imaging is currently the premier method for identifying organic biomarkers within geological samples [4], so ToF-SIMS is compared here with a novel strategy for MS imaging: femtosecond laser desorption postionization mass spectrometry (fs-LDPI-MS) [6]. An entirely custom-built fs-LDPI-MS instrument utilizes 800 nm, ~75 fs pulses for ultrafast laser ablation of neutral organic molecules from a thin prepared slice of a geological sample, followed by postionization using 157 nm (7.9 eV) laser pulses and subsequent detection of ions by a time-of-flight mass analyzer [5,6].…”

“…Time-of-flight secondary ion mass spectrometry (ToF-SIMS) imaging is currently the premier method for identifying organic biomarkers within geological samples [4], so ToF-SIMS is compared here with a novel strategy for MS imaging: femtosecond laser desorption postionization mass spectrometry (fs-LDPI-MS) [6]. An entirely custom-built fs-LDPI-MS instrument utilizes 800 nm, ~75 fs pulses for ultrafast laser ablation of neutral organic molecules from a thin prepared slice of a geological sample, followed by postionization using 157 nm (7.9 eV) laser pulses and subsequent detection of ions by a time-of-flight mass analyzer [5,6]. A TOF.SIMS 5 NCS instrument (IONTOF GmbH, Münster, Germany) was used for the ToF-SIMS data collection: Samples were sputter-cleaned using a 20 keV Ar 2200 gas cluster ion beam to expose areas of 800 µm × 800 µm then areas of 500 µm × 500 µm were analyzed with 25 keV Bi 3 + primary ions [6].…”

“…An entirely custom-built fs-LDPI-MS instrument utilizes 800 nm, ~75 fs pulses for ultrafast laser ablation of neutral organic molecules from a thin prepared slice of a geological sample, followed by postionization using 157 nm (7.9 eV) laser pulses and subsequent detection of ions by a time-of-flight mass analyzer [5,6]. A TOF.SIMS 5 NCS instrument (IONTOF GmbH, Münster, Germany) was used for the ToF-SIMS data collection: Samples were sputter-cleaned using a 20 keV Ar 2200 gas cluster ion beam to expose areas of 800 µm × 800 µm then areas of 500 µm × 500 µm were analyzed with 25 keV Bi 3 + primary ions [6]. An ultraclean sample preparation method was developed to avoid organic contamination of geological samples [6].…”

“…A TOF.SIMS 5 NCS instrument (IONTOF GmbH, Münster, Germany) was used for the ToF-SIMS data collection: Samples were sputter-cleaned using a 20 keV Ar 2200 gas cluster ion beam to expose areas of 800 µm × 800 µm then areas of 500 µm × 500 µm were analyzed with 25 keV Bi 3 + primary ions [6]. An ultraclean sample preparation method was developed to avoid organic contamination of geological samples [6]. The entire procedure was demonstrated on a thin slice of the kerogen-rich Oxford Clay formation (UK) [6].…”

“…An ultraclean sample preparation method was developed to avoid organic contamination of geological samples [6]. The entire procedure was demonstrated on a thin slice of the kerogen-rich Oxford Clay formation (UK) [6].…”