Microscopy-based mass measurement of a single whole virus in a cylindrical ion trap.

This paper reports the first mass determination of whole viruses with sizes in the range of 80 300 nm using a miniature cylindrical ion trap. The trap is unique in that its endcap electrodes were made of transparent, electrically conducting glass plates, allowing collection of more than 10% of scattered laser light from a single trapped viral ion produced by laser-induced acoustic desorption. Three viruses have been successfully examined in this study: vaccinia virus, grouper iridovirus, and recombinant human adenovirus. Our results suggest a broad and promising application of this new technology to viral systems. Viruses are the simplest life forms on our planet, consisting of only DNA (or RNA) and a protein shell. After the prokaryotes, viruses are the second most common type of organism. In our oceans, they are the most common life form. In order to gain a better understanding of the structure and characteristics of these genetically varied little organisms, it would be highly useful to be able to determine their masses and how much these vary within a given population. This work demonstrates that it is possible to use very gentle ionization techniques and a miniaturized ion trap of our own devising to very accurately analyze the masses of individual, intact viruses. Previous methods for determining the masses of viruses had an error rate of ±15 %, which made them too inaccurate to ensure the resolution of small differences in mass. This work presents a new concept to attain higher precision. In order to determine their mass, viruses must first be converted to the gas phase, given an electric charge, and accelerated in an electric field. However, this process must leave the viruses intact. We chose to use a very gentle method known as LIAD (laser-induced acoustic desorption). In this method, the virus particles are released from the sample by laser-induced sound waves. They are then caught in an ion trap, which holds charged particles by means of its special geometry and the superposition of a direct and alternating electric field. Once trapped, the virus particles are ready for mass determination (Figure 1). Laser light is beamed into the ion trap. If a particle is present, it scatters the light. The scattered light can be detected through the transparent surfaces of the ion trap. A portion of the light is sent to a CCD camera, which records the flight path of the trapped particle. The rest of the light goes to a measuring device that precisely analyzes the scattering signal. The scattered light is different from the initial light beam because the virus particle in the 1 Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan 2 Institute of Cellular and Organismic Biology, Academia Sinica, Taipei, Taiwan 3 Institute of Biomedical Sciences, Academia Sinica, Taipei, Taiwan Zongxiu Nie, Yan-Kai Tzeng, *Huan-Cheng Chang, Yan-Yang Lin, Chi-Yao Chang, Chia-Ming Chang, and Mi-Hua Tao