A Novel Optical Module for Detection and Sizing of Particles in a Single Particle Mass Spectrometer - OptiPart-MS

Aerosol particles are key components of atmospheric pollution; they scatter and absorb solar radiation and so have a major influence on the radiative balance of the atmospheric column; and they act as sites for both cloud droplet formation in liquid clouds and as surfaces for the formation of ice in colder conditions, playing a key role in glaciation and hence the onset of precipitation. There are major uncertainties associated with all of these processes and a number of which surround how the chemical components are combined within single particles.

To address such questions it is important that rapid measurement methods are available that can observe the chemical composition of single particles so that the mixing of chemical components across a whole particle population can be determined. Single particle mass spectrometry (SPMS) provides one of the few ways of analysing the chemical composition of single particles in real time. In SPMS, ambient aerosol is entrained into a vacuum region, the particles are detected, usually by light scattering and their size is measured either by their aerodynamic flight time between two lasers separated by a known distance or by the scattering intensity and this information is used to trigger a high powered laser that ablates the particles and ionizes their fragments at the entrance to a mass spectrometer. The SPMS system must not suffer biases in particle detection as a function of size; and particle sizing must be accurate and robust. The detection efficiency must be sufficient for it to be used to address atmospherically relevant problems such as ice nucleation formation or to be operated from an aircraft.

We will develop a novel particle detection system for an SPMS system based around the combination of multiple lasers of different, non-resonant wavelengths into a single beam which our modelling work shows addresses the above criteria. This approach uses new technology that was originally developed for the telecoms industry but has only recently become available for use with high powered laser systems required for this application. The relationship between scattered light intensity and particle size when using a single scattering angle and single wavelength laser light is complex and this limits the use of scattered light for detection and sizing of particles. Previously, this has been overcome using light collection mirrors that are bulky and cannot be easily housed close to the ablation laser, leading to reductions in detection efficiency. Our proposed system will detect particles across the size range covered by the inlet of the current instrument, and can be easily incorporated into the laser ablation region, greatly improving the detection efficiency.

There are two possibilities for particle sizing with a multi-wavelength system, one involves using two lasers separated by a fixed distance to measure the flight time of the particles; the other is to use the scattering intensity to measure the optical size. The first of these will give an accurate measure of size, free from biases, however, it means that particle detection may reduce since the particle beam is divergent. The second system will greatly improve detection efficiency though we need to demonstrate that our model simulations correctly predict a monotonic relationship between scattered light intensity and particle size.

Our modular approach will allow both of these approaches to be assessed using the same components and an assessment made of the optimum system for our purposes. In doing so we aim to deliver an optical design for an SPMS that can be used in airborne and laboratory studies relevant to atmospheric science that require chemical characterisation of particles with very low number concentrations, and provide an approach for non-invasive optical detection of particles that has a wide number of applications both across atmospheric science and in other fields.

Who might benefit from this research?

Mass Spectrometer Manufacturers: Mass spectrometry (MS) is a highly competitive industry with forecast global instrument sales in excess of $4.6bn for 2015. Hyphenated liquid chromatography - mass spectrometry (LC/MS) is a routine application in the biotechnology, pharmaceutical, food safety and environmental market sectors. LC/MS typically uses electrospray ionisation (ESI) where the sample is converted from the liquid to the gas phase in an aerosol jet. In addition, there have been many recent developments in techniques that sample the ambient environment directly such as ASAP, REIMS, and DART. Instrument manufacturers are keen to understand the size distribution of the aerosol within the source region of the mass spectrometer in order to optimise ion formation, ion transmission and differential vacuum pumping.

Aerosol Instrument Manufacturers: To date there are very few commercial devices that can non-intrusively count and size airborne particles in the size range 0.5 to 2.5um; existing techniques for analysing sprays involve photographing particles passing through a light curtain or phase Doppler anemometry, neither of which is capable of detecting particles in this size range. A multi wavelength laser based optical detection system with a compact modular design would be an extremely valuable addition to the present market and one that a number of manufacturers of aerosol measurement instruments are likely to wish to explore.

How might they benefit from this research?

Mass Spectrometer Manufacturers: A system for non-invasive particle detection would be of great value in the development of MS systems and have possible market opportunities in some applications. Unlike traditional optical particle sizers, a multi-wavelength system would not involve using a concave mirror to collect the scattered light, so the system would not interfere with the normal operating conditions of the instrument. We have had a number of discussions with instrument manufacturers who are interested in being able to sense and size particles within mass spectrometers for a range of applications which include analysis of particles suspended in air for the food, defence and pharmaceutical industries. Waters Corporation, a leading manufacturer of mass spectrometers based in the North West region, have stated an interest in the potential outcomes of these developments. We intend to host a half day discussion with Waters technology team in month 12 to identify specific applications of our optical system.

Aerosol Instrument Manufacturers:
While there are a number of commercial devices that are capable of sizing particles between 0.5 and 2.5um, optical resonances that occur when mono-wavelength light interacts with particles of these sizes means different sizes of particles produce similar scattered light intensities. This in turn places a fundamental limitation on the resolution of these instruments. Currently, we are only aware of one commercial device that uses an incoherent multi-wavelength white light source to size particles in this range. The application of state-of-the-art multi-wavelength laser system will deliver significantly more radiant flux than a white light source and will be capable of delivering scattered light from particles whose intensity can be monotonically related to the particle size. There are a number of applications across a range of industries including aerospace, automotive, forensic analysis, petrochemicals, pharmaceuticals, chemicals and product development that could all benefit from detecting and sizing particles in this size range. We will aim to set up meetings with the instrument manufacturers with whom we already have strong relationships and explore further development of this system.

It is probable that the optical modules developed in this work have commercial impact and we will explore this through UMIP (University of Manchester Intellectual Property).