Aerosol mass spectrometry

Aerosol mass spectrometry is the application of mass spectrometry to aerosol particles. Aerosol particles are defined as solid and liquid particles suspended in a gas (air), with size range of 3 nm to 100 μm in diameter. Aerosol particles are produced from natural and anthropogenic sources, through a variety of different processes that include; wind-blown suspension, and combustion of fossil fuels and biomass. Analysis of aerosol particles is important because of their major impacts on the global climate change, visibility, regional air pollution and human health. Aerosol particles are very complex in structure and can contain thousands of different chemical compounds within a single particle. Due to this complexity the instrumentation used to analysis these particles must have the ability to separate based on size and in real-time provide information on their chemical composition. To meet these requirements for analysis, mass spectrometry instrumentation is used and they provide high sensitivity and the ability to detect a wide molecular mass range. Aerosol mass spectrometry can be divided into two categorizes; off-line and on-line. Off-line mass spectrometry is performed on collected particles. On-line mass spectrometry is performed on particles introduced in real time. Aerosol mass spectrometry is the application of mass spectrometry to aerosol particles. Aerosol particles are defined as solid and liquid particles suspended in a gas (air), with size range of 3 nm to 100 μm in diameter. Aerosol particles are produced from natural and anthropogenic sources, through a variety of different processes that include; wind-blown suspension, and combustion of fossil fuels and biomass. Analysis of aerosol particles is important because of their major impacts on the global climate change, visibility, regional air pollution and human health. Aerosol particles are very complex in structure and can contain thousands of different chemical compounds within a single particle. Due to this complexity the instrumentation used to analysis these particles must have the ability to separate based on size and in real-time provide information on their chemical composition. To meet these requirements for analysis, mass spectrometry instrumentation is used and they provide high sensitivity and the ability to detect a wide molecular mass range. Aerosol mass spectrometry can be divided into two categorizes; off-line and on-line. Off-line mass spectrometry is performed on collected particles. On-line mass spectrometry is performed on particles introduced in real time. The analysis of particles in atmosphere is a topic that can be traced back to early literature, in ancient Rome there are records of complaints of foul air. Another example of early discussion of aerosols was in London (1273) and the prohibition of coal burning, because of the particulate air pollution that it was producing. Throughout history it can be seen that there was a clear need for the ability to collect and analyzing aerosol particles. Unfortunately aerosol science and measurement wasn't really established until the second half of the 19th century. The first concept of particles in the air was hypothesized by H. Becquerel in 1847, in his condensation nuclei experiment. This hypothesis was confirmed in later experiments by Coulier in 1875. John Aitken (meteorologist) (1839-1919) took Coulier and Becquerel concept even further with experiments between 1880- 1890 that demonstrated the fundamental role of dust particles in the formation of clouds and fogs. John Aitken is considered the founder of atmospheric aerosol science and aerosol measurements techniques. Aitken method for aerosol analysis consisted of particle counting and sizing, which was performed using microscopic methods. This consisted of particles being collected on plates and then being counted and sized with a microscope. Using refractive index of transparent particles, the particle could be identified. Beginning in the 1920s aerosol measurements became more common place because the negative health effects of industrial aerosols and dust were starting to be recognized by health organization. The major concern at this time was the increase in incidents of silicosis in industry and mine workers. The main methods for these measurements where based off Aitken’s simple microscopic method. It wasn't until the 1960s that aerosol measurement methods started to get more complex and involve the technological and instrumentation advancements of the time. Along with the advancement in instrumentation after the 1960s came the improvement of filters and there use for sampling aerosols. This came along with the invention and application of polycarbonate filters, also called Nuclepore filters or NPFs. This development was important to the field especially to off-line methods, because to obtain a representative measurement and analysis of a sample, you must have the ability to collect, store, and transport sample without disturbing the physical and chemical state of the particles. On-line aerosol measurements methods took a little longer than off-line to be developed and perfected. It wasn't till 1973 with Davis who developed and patented of the real-time single particle mass spectrometry (RTSPMS) instrument. The setup is quite similar to today’s AMS system, with the sample being introduced through a small steel capillary into the ion source region. The sample would ionize after striking a hot rhenium filament. The resulting ions were separated in a magnetic sector and detected by an electron multiplier. The method could only ionize elements with ionization potentials below the work function of the filament (~8 eV), typically alkali and alkaline earth metal. The instrument did yield unit resolution up to a mass-to-charge ratio of 115. The RTSPMS instrument had a particle transmission/detection efficiency of 0.2-0.3%. Davis used the RTSPMS instrument to study samples from calibration aerosols, ambient laboratory air, and aerosols sources. Majority of his studies where focused on inorganic salts created in lab. In Davis's analysis of ambient air, he found a significant increase in lead at the end of the day, which was concluded to be due to automobile emissions. This development was the first step towards, today's modern on-line instruments. The next major development in technological improvement that came out of the 1970s was in 1976 by Stoffel with the development of a magnetic sector RTSPMS technique that had a direct-inlet mass spectrometry (DIMS) also known as particle-inlet mass spectrometry (PIMS). The PIMS instrument was the first to have a deferentially-pumped direct inlet that consists of a stainless steel capillary, followed by a skimmer and conical collimator that focuses the sample into a particle beam that goes on to the ionization region. This type of inlet system is what modern on-line aerosol mass spectrometer instruments use today. In 1982 Sinha and Fredlander developed the particle analysis by mass spectrometry (PAMS), this method was the first to incorporate the optical detection of particles followed by laser desorption/ionization (LDI) in a RTSPMS technique. Prior to this point all RTSPMS methods used surface desorption/ionization (SDI) which consist of a heated metal that ionized the samples. The LDI method involves the sample being hit with a continuous wave, where the particle absorbs photons, and undergoes both desorption and ionization by the same pulse. LDI has several advantages over SDI for on-line single particle mass spectrometry, as such since its development it has been the primary ionization method for RTSPMS.The last major step in RTSPMS development was in 1994 by Kimberly A. Prather. Prather developed the aerosol time-of-flight mass spectrometry (ATOFMS), this method was the first that allow for simultaneous measurement of size and composition of single airborne particle. This techniques was different then previous methods in that instead of using the unreliable method of using light scattering signal intensity to measure particle size, this method uses a two laser system that allows for aerodynamic sizing. Off-line is an older method than on-line and involves the chemical analysis of sampled aerosols collected traditionally on filters or with cascade impactors (shown to the right) in the field and analyzed back in the lab. Cascade impactors collects particles as they transverse a series of impaction plates, and separate them based on size. The aerosol samples are analyzed by the coupling of pre-separation methods with mass spectrometry. The benefit of this method relative to on-line sampling is greater molecular and structural speciation. The greater molecular and structural speciation is due to the pre-separation. 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 commonly used ionization method for off-line instrument is electron ionization (EI) which is a hard ionization technique that utilized 70 eV to ionize the sample, which causes significant fragmentation that can be used in a library search to identify the compounds. The separation method that EI is usually coupled with is gas chromatography (GC), where in GC the particles are separated by their boiling points and polarity, followed by solvent extraction of the samples collected on the filters. An alternative to solvent-based extraction 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 of the sample and into the GC inlet. 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 their 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 their isotopic abundance. Isotopic abundance of carbon, hydrogen, nitrogen, and oxygen isotopic abundance become locally enriched or depleted through a variety of atmospheric processes. This information helps in determining the source of the aerosols and the interaction it has had.