Zooming in on plumes to improve mass spectroscopy

hank.hogan@photonics.com

By using a high-speed camera to study plumes created by a tunable infrared laser, researchers at Louisiana State University have revealed some of the mechanisms involved in material ablation. The plume study technique could someday result in better mass spectrometer performance, said research team leader and chemistry professor Kermit K. Murray, and that could help life sciences and materials research.

When studying biomolecules and other macromolecules, investigators frequently make use of matrix-assisted laser desorption ionization. In this method, a laser pulse ablates material, and a mass spectrometer samples the ions in the plume.

Shown is a plume captured 24 µs after a 2.94-µm laser shot struck glycerol. By studying how such plumes evolve over time, researchers can improve how much material is ionized and thus becomes suitable for mass spectrometry. Courtesy of Kermit K. Murray, Louisiana State University.

The problem is that most of the material removed is electrically neutral and, therefore, undetectable. “If we can adjust the ablation conditions to maximize the conversion of the sample into ions, we will improve the performance of the mass spectrometer,” Murray said.

In trying to understand just what is going on in a plume, researchers have long turned to high-speed photography. Murray and graduate student Xing Fan added a twist to this idea. They combined a fast camera with a tunable mid-infrared laser, constructing it from an optical parametric oscillator and an Nd:YAG laser from Continuum of Santa Clara, Calif. This setup allowed them to adjust the pulse wavelength from 1.4 to 4.0 µm and to study how that changed material removal.

In their experiments, the researchers fired 5-ns pulses from this mid-IR source into a glycerol sample sitting on a stainless steel target. For illumination, they used an excimer KrF laser from Lambda Physik of Göttingen, Germany, triggering it after the first laser shot. As a result, the 248-nm, 8-ns pulses from this second laser illuminated the plume at specific times. To capture the plume’s appearance, the duo used a CMOS digital camera from Vitana Co. of Ottawa.

Using this setup, they obtained one image per laser shot. Murray noted that an ideal camera would record a video from a single shot. However, the technology doesn’t yet exist to strobe and record images at the necessary speed, he said.

Thus, reconstructing the plume evolution required that the researchers take multiple shots. They did so for wavelengths from 2.7 to 3.5 µm, finding that the greatest material ablation and the longest plume duration took place near 3.0 µm. That, they noted in the Jan. 28, 2009, issue of Journal of Physical Chemistry, corresponds to the stretch absorption of OH, a molecular constituent of glycerol. Modeling calculations suggested that the material removal was driven by a stress-confined phase explosion.

Murray said that tuning the wavelength and energy will allow optimization of laser desorption mass spectrometry techniques, especially those performed under ambient conditions. Maximizing the laser’s effectiveness is important to various research areas, he said. “One of the most promising applications is tissue imaging, where the ability to remove the biomolecules from within the sample is critical to success.”