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NOAA's Particle Analysis by Laser Mass Spectrometry instrument aboard the NASA WB-57 high-altitude research aircraft.

Aerosol mass spectrometry is the application of mass spectrometry to aerosol particles. Mass spectrometry can be performed on collected particles (off-line) or particles introduced into the mass spectrometer in real time (on-line).

Introduction

Aerosol mass spectrometery was developed to tackle the problem analyzing atmospheric particulates, (or aerosol particles) which are defined as suspended solid and liquid particles with size range of 0.1 nm to 1000 μm in diameter. Aerosol particles are produce from natural and anthropogenic source, through a variety of different process that include combustion of fossil fuels and biomass, and wind-blown suspension. Analysis of aerosol particles is important because of there major impacts on the global climate change, visibility, regional air pollution and human health. Aerosol particles are very complex in structure and can contain thousand of different chemical compound within a single particle. Due to this complexity the instrumentation used to analysis these particulates must have the ability to separate based on size and in real-time provide information on there chemical composition. To meet these requirement for analysis, mass spectrometry instruments are used and they provide high sensitivity and the ability to detect a wide molecular mass range. Aersool mass spectrometery can be broken up into two categorizes; off-line and on-line.

off-line

Off-line is the older of the two and involves the chemical analysis of traditionally sampled aerosols collected on filters or with cascade impactors in the field and tested back in the lab. Once back in the lab the aerosol samples are analysis by the coupling mass spectrometer with pre-separation methods. The benefit of this method is a greater molecular and structural speciation, then the on-line due to separation ability. There are many different types of instrumentation used for the analysis due to various type and combinations of the ionization, separation, and mass detection methods. Not one combination is best for all samples, and as such depending on the need for analysis, different instrumentation is used.

The most common off-line instrument use the ionization method is electron ionization (EI) which is a hard ionization that utilized 70 eV to ionize the sample, which causes significant fragmentation that can be used in a library search to identify the compounds. For separation EI based mass spectromtry is usually coupled with gas chromatography (GC), where in GC the particles are separated by there boiling points and polarity, followed by solvent extraction of the samples collected on the filters. An alternative to solvent-based extraction for for particulates on filters is the use of thermal extraction (TE)-GC/MS, which utilizes oven interfaced with the GC inlet to vaporize the analyte and flow into the GC. This technique is more often used then solvent-based extraction, because of its better sensitivity, eliminates need for solvents, and can be fully automated. To increase the separation of the particles the GC can be coupled with a time of flight (TOF)-MS, which is a mass separation method that separates ions based on there size. Another method that utilizes EI is isotope ratio mass spectrometry (IR-MS), this instrumentation incorporates a magnetic sector analyzer and a faraday-collector detector array and separates ions based on there isotopic abundance. EI is a universal ionization method, but it does cause excessive fragmentation, and thus can be substituted with chemical ionization (CI) which is a much softer ionization method, and is often used to determine the molecular ion. One ionization method the utilizes CI is atmospheric-pressure chemical ionization (APCI). In APCI the ionization occurs at atmospheric pressure with ions produced by corona discharges on a solvent spray, and it is often coupled with high-performance liquid chromatography (HPLC) which provides quality determination of polar and ionic compounds in the collected atmospheric aerosols. The use of APCI allows for the sampling of the filters without the need of solvents for the extraction. The APCI is typically connected to a quadruple mass spectrometer.

Other ionization methods is often used for off-line mass spectrometer inductively coupled plasma (ICP). ICP is commonly used in the elemental analysis of trace metals, and can be used to determine the source of the particles and there health effects.

on-line

On-line mass spectrometry was develop to solve some of the limitations and problem that develop from off-line analysis, such as evaporation and chemical reactions in the filters during long analysis time. On-line instruments are also portable and allow for spatial variability to be examined. These portable instruments can be put on many different platforms such as boats, planes, and mobile platforms (e.g. car trailers). An example of this is in the picture at the beginning with the instrumentation attached to a plane. Like off-line, on-line mass spectrometry has many different type of instruments, which can be broken up into two types; instruments that measures the chemistry of the particle ensemble (bulk measurement) and those that measure the chemistry of individual particles (single-particle measurement). Thus based on analytical need different instrumentation is used in analysis the aerosol particles.

bulk measurement

Generally speaking bulk measurement instruments thermally vaporize the particles prior to ionization, and there are several different ways that the vaporization and ionization is performed. For the most part the main instrument that is used for bulk measurements is Aerodyne aerosol mass spectrometer (AMS).

AMS

Schematic of the Aerodyne aerosol mass spectrometer (AMS)

The Aerodyne AMS is the most common real-time aerosol mass spectrometer analysis of size-resolved mass concentration of non-refractory components (Ex. organics, sulfate, nitrate, and ammonium). The term non-refractory is assigned to species that evaporate rapidly at 600 °C under vacuum conditions (e.g. organic matter, NH₄NO₃ and (NH₄)₂SO₄. The schematic of a typical AMS is showed in the figure to the right. The Aerodyne AMS is made up of three section; aerosol inlet, the particle sizing chamber, and the particle composition detection section. The aerosol inlet has a flow limiting entrance orifice that is around 100 um in diameter. Once in the chamber the sample goes through aerodynamic focusing lens system, which consist of several orifice lenses that are mount in sequence of decreasing inner diameter. The lens focus the particles into a narrow particle beam. As the particle beam exist the last aerodynamic lens in the sequence, the particles are accelerated towards a heated tungsten element(600 °C) by the applied vacuum. At the tungsten element non-refaractory components of the particle beam are flash vaporized,and are ionized by EI. The ionized sample now travels through the particle sizing chamber where the particle aerodynamic diameter is measured. First a mechanical chopper is used to modulate the particle beam, then using both the fixed length of the tube and the time-resolved detection at the end the particles velocity can be determined. Using the velocity and the particles diameter is obtained. The particle composition detection section can consist of of either a quadruple (Q), time-of-flight (ToF), or high-resolution (HR)-ToF mass analyzer.

single-particle measurements

Generally speaking single-particle measurement instruments desorb particles one at a time using a pulsed laser. The process is called laser desorption/ionization (LDI) and is the primary ionization method used for single-particle measurements. The main advantage of using LDI over thermal desorption, is the ability to analyze both non-refractory and refractory (e.g., mineral dust, soot) components of atmospheric aerosols. The most common of these instruments is the aerosol time-of-flight mass spectrometer (ATOFMS).

ATOFMS

Schematic of aerosol time-of-flight mass spectrometer (ATOFMS)

The ATOFMS allows for the determination of mixing state, or distribution of chemical species, within individual particles. These mixing states are important in the determination of climate and health impact of aerosols. The schematic of a typical ATOFMS is shown to the right. The overall structure of ATOF instruments is; sampling, sizing, and the mass analyzer region. The inlet system is similar to the AMS by using the same aerodynamic focusing lens, but it has smaller orifices because of it analysis single particles. In the sizing region particle passes through the first continuous solid state laser that generates a initial pulse of scattered light. Then the particle passes through the second laser that is orthogonal to the first and produces a pulse of scattered light. The light is detected by a Photomultiplier (PMT) that is matched up to each laser. Using the transit times between the two detected pulse and the fixed distance the velocity and size of each particle is calculated. Next the particles travel through to the mass analyzer region where it is ionized by the LDI. Once ionized the positive ions are accelerated towards the positive TOF section and the negative ions are accelerated towards the negative TOF section where they are detected.

Application

Aerosol mass spectrometer has numerous applications in the both the lab and field, and provides measurements that are needed for studying aerosols and atmospheric chemistry, emissions sources, human exposure to pollutants, radiative transfer and cloud microphysics. Most of these studies have utilized the mobility of the AMS and has been fielded in urban, remote, rural, marine, and forested environments around the world. AMS have also been deployed in mobile platforms such as ships, mobile laboratories, and aircraft.

For future work AMS is being used to study cloud formation and can be used to improve predictions of climate, atmospheric composition, and meteorology. Along with atmospheric applications AMS have the potential of diagnosing disease from the analysis of aerosols in exhaled breath. developments in AMS is vastly need for the development and enhancements of aerosolized pharmaceutical drugs.

See also

• Laser microprobe mass spectrometer
• Particulate matter sampler
• Aerosol impaction
• Particle size analysis
• Centre for Atmospheric Science (http://www.cas.manchester.ac.uk/resactivities/aerosol/topics/massspec/)
• TOF-AMS resources (http://cires1.colorado.edu/jimenez-group/wiki/index.php/ToF-AMS_Main)
• Q-AMS resources (http://cires1.colorado.edu/jimenez-group/QAMSResources/)

• List of publications using all versions of the AMS (http://cires1.colorado.edu/jimenez/ams-papers.html)
• Glossary of AMS terms (http://cires1.colorado.edu/jimenez-group/wiki/index.php/FAQ_AMS_Glossary)

Further reading

• Hartonen, Kari; Laitinen, Totti; Riekkola, Marja-Liisa (2011). "Current instrumentation for aerosol mass spectrometry". TrAC Trends in Analytical Chemistry. 30 (9): 1486–1496. doi:10.1016/j.trac.2011.06.007. ISSN 0165-9936.

Category:Mass spectrometry Category:Aerosols