Massive particle measurement: Viruses and nanoparticles sized by rapid MS method

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thumbnail image: Massive particle measurement: Viruses and nanoparticles sized by rapid MS method

Heavyweight challenge

The masses of viruses and nanoparticles have been measured by a rapid mass spectrometric method and the mass distribution of a virus has been reported for the first time.

Mass spectrometry has evolved over the years to allow the analysis of larger and larger compounds so that the mass measurement of peptides and large proteins is now a routine operation. This was made possible largely by the advent of electrospray ionisation and matrix-assisted laser desorption/ionisation (MALDI), two techniques which helped to take the mass limit up to 1,000,000 Da (1 MDa) and beyond.

Getting far beyond this molecular mass milestone is more difficult but several research groups have managed to do so, with some success. One team from Taiwan developed a new technique in 2007 which allowed them to measure the molecular masses of viral particles up to 3.26 x 109 MDa. They went even higher with polystyrene particles of 2.38 x 1011 Da.

The new technique was laser-induced acoustic desorption (LIAD), which has now been improved by the same team so that they can take rapid measurements of heavy particles within 1 minute. Senior reporters Richie Chen from the National Taiwan University and Chung-Hsuan Chen from Academia Sinica, Taipei, demonstrated their refined system for the analysis of virus particles and polymeric nanoparticles.

Acoustic desorption

In LIAD, a laser is directed at the reverse side of a silicon wafer that is coated with sample on the opposite side. As the silicon surface is ablated, shock waves travel through the wafer to desorb the sample and produce ions by a mechanism which is not fully understood. The ions are directed into a quadrupole ion trap for trapping and detection.

Unlike MALDI, there is no sample matrix but many of the particles or cell components that are desorbed are charged, most with more than 1000 charges. This amount of charging enables the use of a charge detector. The combination of an ion trap mass spectrometer to detect the mass-to-charge ratios and the charge detector to measure the charge of each particle leads to the masses of the particles.

In order to speed up the analysis, a phase lock system was employed to improve the trapping efficiency of the ions and the laser was synchronised with the zero rf voltage in the trap. These modifications meant that the researchers could increase mass measurement speed by a factor of 10 compared with their previous incarnation, now taking less than 1 minute per analysis.

This is a marked improvement over electron microscopy, which has been used to calculate the masses of nanoparticles from their sizes, bettering the speed by several orders of magnitude. The performance of the modified system was illustrated by the analysis of polymeric nanoparticles and virus particles.

Virus and nanoparticle mass distributions

In the first instance, suspensions of standard spherical polystyrene particles of diameter 50, 100 and 900 nm were each dried on the silicon target for LIAD and analysis. For the 50-nm sample, the m/z values of the singly charged Mn+ ions were 4.12 x 107, 8.3 x 107, 1.25 x 108, 1.66 x 108 and 2.05 x 108 for n = 1-5, respectively. The results agreed with a molecular mass of 4.14 x 107 Da.

A section of the mass spectrum of the 900-nm sample revealed peaks corresponding to single particles with total positive charges ranging from 200 to 525, measured by the charge detector. The spectra were indicative of a mass distribution of 2%, compared with a declared value of about 0.5% from the manufacturer, giving a mass resolution for the mass spectrometer of 50.

The second demonstration of the mass spectrometer was carried out with particles of HIV which is a spherical enveloped virus of diameter 90-120 nm. The m/z values for the singly charged monomer to pentamer were 3.51 x 108, 7.12 x 108, 1.08 x 109, 1.46 x 109 and 1.76 x 109, respectively. The values of some multicharged clusters were also measured.

The data corresponded to a mass distribution of 10%, which is the first report of the mass distribution of a virus. The content of the HIV virion, including its RNA and proteins, is well established so it would be expected to have a narrow mass distribution. The researchers postulated that the variation might be due to a small amount of water that remained inside the virion.

These results were all obtained rapidly, in the region of 1 minute, and should aid the advancement of large particle measurement by mass spectrometry. The technique will be useful for nanoparticles characterisation and quality control, where differences in size can affect the properties. Other potential applications include the analysis of the plasma or tissue of infected people to determine the extent of viral infection.

Related Links

Analytical Chemistry 2012, 84, 4965-4969: "High-speed mass measurement of nanoparticle and virus"

Article by Steve Down

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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