Aerosol mass spectrometry is the application of mass spectrometry for aerosol particles. Aerosol particles are defined as solid and liquid particles suspended in a gas (air), with a size range of 3 to 100 m. Aerosol particles are produced from natural and anthropogenic sources, through different processes that include; wind blown suspension, and burning of fossil fuels and biomass. Analysis of aerosol particles is important because of its major impact on 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 in one particle. Because of the complexity of this instrumentation used to analyze these particles must have the ability to separate by size and in real-time provide information about its chemical composition. To meet these requirements for analysis, mass spectrometry instrumentation is used and they provide high sensitivity and the ability to detect wide molecular mass ranges. Aerosol mass spectrometry can be divided into two categories; off-line and on-line. Off-line mass spectrometry is performed on collected particles. On-line mass spectrometry is performed on particles that are introduced in real time.
Video Aerosol mass spectrometry
History
Analysis of particles in the atmosphere is a topic that can be traced back to the early literature, in ancient Rome there was a record of dirty air complaints. Another example of aerosol's initial discussion is in London (1273) and coal burning ban, due to particulate air pollution it produces. Throughout history it can be seen that there is a clear need for the ability to collect and analyze aerosol particles. Unfortunately science and aerosol measurements did not really form until the second half of the 19th century.
The first concept of airborne particle was hypothesized by H. Becquerel in 1847, in a condensation core experiment. This hypothesis was confirmed in a later experiment by Coulier in 1875. John Aitken (meteorologist) (1839-1919) took the Coulier and Becquerel concept further by experimenting between 1880-1890 which shows the fundamental role of dust particles in the formation of clouds and fog.. John Aitken is regarded as the founder of atmospheric aerosol science and aerosol measurement techniques. The Aitken method for aerosol analysis consists of particle counting and measurements, performed using microscopic methods. It consists of particles collected on a plate and then calculated and measured by a microscope. By using a refractive index of transparent particles, the particles can be identified.
Starting the aerosol measurement of the 1920s became a more common place because of the negative health effects of industrial aerosols and dust began to be recognized by health organizations. The main concern now is the increased incidence of silicosis in industry and minerals. The main method for this measurement is based on Aitken's simple microscopic method. It was not until 1960 that the aerosol measurement method began to become more complex and involved the advancement of technology and instrumentation of the time.
Along with advancements in instrumentation after the 1960s came an increase in filters and there was used for aerosol sampling. These come along with the invention and application of polycarbonate filters, also called Nuclepore filters or NPFs. This development is important for the field especially for off-line methods, because to get representative sample measurements and analysis, you must have the ability to collect, store, and transport samples without disturbing the physical and chemical state of the particles.
The on-line aerosol measurement method takes longer than off-line to be developed and refined. It was not until 1973 with Davis that developed and patented a single particle mass spectrometry (RTSPMS) instrument real-time. This arrangement is very similar to the current AMS system, with samples being introduced through small steel capillaries to the ion source region. The sample will ionize after plugging the hot rhenium filament. The resulting ion is separated in the magnetic sector and detected by the electron multiplier. This method can only ionize elements with ionization potential under the filament working function (~ 8 eV), usually alkali and alkaline earth metals. This instrument produces unit resolution up to a mass-to-charge ratio of 115. The RTSPMS instrument has a transmission/particle detection efficiency of 0.2-0.3%. Davis uses the RTSPMS instrument to study samples from calibration aerosols, ambient laboratory air, and aerosol sources. Most of his studies where focused on inorganic salts are made in the laboratory. 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 car emissions. This development is the first step towards a modern, on-line instrument today.
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 has direct inlet mass spectrometry (DIMS) also known as mass-particle inlet spectrometry (PIMS). The PIMS instrument is the first to have a deferential direct-injected inlet composed of stainless steel capillaries, followed by a skimmer and conical kollimator that focuses the sample into a continuous emission of particles into the ionization region. This type of inlet system is a modern online aerosol spectrometer instrument used today. In 1982 Sinha and Fredlander developed particle analysis with mass spectrometry (PAMS), this method was the first to incorporate particle optical detection followed by laser desorption/ionization (LDI) in RTSPMS technique. Prior to this point all RTSPMS methods use surface desorption/ionization (SDI) consisting of a heated metal that ionises the sample. The LDI method involves a sample being hit with a continuous wave, in which the particles absorb the photon, and are subjected to desorption and ionization by the same pulse. LDI has several advantages over SDI for single particle mass spectrometry on-line, since since its development it has become the main ionization method for RTSPMS. The last major step in the development of RTSPMS was in 1994 by Kimberly A. Prather. Prather developed a time-of-flight mass spectrometry (ATOFMS) aerosol, this method is the first to allow simultaneous measurement of the size and composition of a single air particle. This technique differs from previous methods that instead of using unreliable methods using light scatter signal intensity to measure particle size, this method uses two laser systems that allow for aerodynamic size.
Maps Aerosol mass spectrometry
Off-line
Off-line is a method that is older than on-line and involves chemical analysis of a traditionally collected aerosol sample on a filter or with a cascade collector (shown on the right) in the field and re-analyzed in the laboratory. Multilevel predators collect particles as they cross a series of impaction plates, and separate them by size. The aerosol samples were analyzed by coupling the pre-separation method with mass spectrometry. The benefit of this method relative to on-line sampling is a larger molecular and structural speciation. The larger molecular and structural spectra are due to pre-separation. There are different types of instrumentation used for analysis because of the different types and combinations of ionization, separation, and mass detection methods. Not one combination is best for all samples, and therefore depends on the needs of the analysis, different instrumentation is used.
The most common ionization method used for off-line instruments is electron ionization (EI) which is a hard ionization technique that uses 70 eV to ionize the sample, leading to significant fragmentation that can be used in library search to identify compounds. The method of separation typically combined with EI is gas chromatography (GC), wherein GC particles are separated by their boiling point and polarity, followed by solvent extraction from the samples collected on the filter. An alternative to solvent-based extraction for particulates in filters is the use of thermal extraction (TE) -GC/MS, which uses an oven connected with the GC inlet to evaporate the sample analyte and into the GC inlet. This technique is more commonly used then solvent-based extraction, because of better sensitivity, eliminates the need for solvents, and can be fully automated. To increase the separation of GC particles can be combined with the flying time (TOF) -MS, which is a method of separation of the mass that separates the ions by size. Another method that uses IE is the mass-isotope mass spectrometry (IR-MS) ratio. This instrumentation combines magnetic sector analyzers and faraday collector detector arrays and separates ions based on their isotopic abundance. The abundance of carbon isotopes, hydrogen, nitrogen, and oxygen of the abundance of isotopes becomes enriched or exhausted locally through various atmospheric processes. This information helps in determining the source of aerosols and their interactions.
EI is a universal ionization method, but it causes excessive fragmentation, and thus can be replaced by chemical ionization (CI) which is a much more refined ionization method, and is often used to determine molecular ions. One of the ionization methods using CI is atmospheric pressure chemical ionisation (APCI). In APCI, ionisation occurs at atmospheric pressure with ions produced by the release of the corona at the solvent spray, and is often combined with high performance liquid chromatography (HPLC) which provides the determination of the quality of polar and ionic compounds in a collected atmospheric aerosol. Use of APCI allows filter sampling without the need for solvents for extraction. APCI is usually connected to a quadruple mass spectrometer.
Other ionization methods are often used for inductively inductively multiplied inductive plasma spectrometers (ICP). ICP is commonly used in the analysis of trace metal elements, and can be used to determine the particle source and there are health effects.
There are also various soft ionisation techniques available to assess the molecular composition of aerosol particles in more detail, such as electrospray ionization, resulting in less fragmentation of compounds in the aerosol. These techniques are only useful when combined with high-resolution or ultra-high mass spectrometers, such as FTICR-MS or Orbitrap, because very high resolution is required to distinguish between high amounts of existing compounds.
On-line
On-line mass spectrometry was developed to solve some of the limitations and problems that evolved from off-line analysis, such as evaporation and chemical reaction of particles in the filter over a long analysis time. On-line Mass spectrometry solves this problem by being applied to collect and analyze aerosol particles instantaneously. The on-line instrument is highly portable and allows for spatial variability to be checked. These portable instruments can be placed on various platforms such as boats, planes, and mobile platforms (eg car trailers). This example is in the picture at the beginning with the instrumentation attached to the aircraft. Like off-line, on-line mass spectrometry has many different types of instruments, which can be split into two types; instruments that measure the chemistry of the particle ensemble (mass measurements) and instruments that measure the chemistry of individual particles (single particle measurements). So based on the different analytical needs of instrumentation used in aerosol particle analysis.
Bulk size
In general, thermal measurement instruments thermally evaporate particles prior to ionization, and there are several different ways that evaporation and ionization are performed. The main instrument used for bulk measurement is Aerodyne aerosol mass spectrometer (AMS).
Aerosol mass spectrometer
The Aerodyne AMS provides real-time aerosol mass spectrometry analysis of mass concentrations that solved the size of non-refractory components (Ex. Organic, sulphate, nitrate, and ammonium). The term non-refractory is assigned to species that evaporate rapidly at temperatures of 600 ° C under vacuum conditions (eg organic matter, NH 4 NO 3 and (NH 4 ) 2 SO 4 The typical AMS scheme is shown in the figure on the right.The Aerodyne AMS consists of three parts: Aerosol inlet, particle size space, and particle detection chamber The aerosol entry channel has an orifice inlet about 100 μm in diameter Once inside the sample chamber passes through an aerodynamic focus lens system, consisting of several orifices which are successive mounts that lower the inner diameter The lens focuses the particles into the rays narrow particle.
The rays now move through a particle size space where the aerodynamic diameter of the particles is measured. The particle size space is made of a flying tube maintained at (~ 10-5 torr). The entrance of the flight tube is a mechanical helicopter used to modulate the particle beam, then using both fixed tube length and the time-settled detection at the end of the particle speed can be determined. Using the velocity and particle diameter obtained. When the particle beam comes out of the fly jar, it enters the particle detection chamber. In this section the particles collide with heated tungsten elements (~ 600 Â ° C). In this tungsten element the non-refractory component of the flash particle file is evaporated, and then ionized by EI. After ionization, samples can be analyzed by quadruple (Q), time-of-flight (ToF), or High-resolution (HR) -ToF mass analysis.
Single particle measurement
In general, single particle measurement instruments absorb particles one by one using a pulsed laser. This process is called laser desorption/ionization (LDI) and is the main ionization method used for the measurement of single particles. The main advantage of using LDI over thermal desorption, is the ability to analyze non-refractory and refractory components (eg, mineral dust, soot) from atmospheric aerosols. The most common of these instruments is the aerosol time mass spectrometer (AToFMS).
Aerosol flight time spectrometer
AToFMS allows the determination of mixing state, or distribution of chemical species, in individual particles. This mixing situation is important in determining climate impact and aerosol health. A typical AToFMS scheme is shown to the right. The overall structure of the ATOF instruments is; sampling, sizing, and mass analysis areas. The inlet system is similar to the AMS using the same aerodynamic focus lens, but has smaller holes because of its analysis of single particles. In particles the size of the area passes through the first continuous solid state laser that produces an initial scattered light pulse. Then the particles pass through the second laser which is orthogonal to the first and produces a diffused light pulse. Light is detected by photomultiplier (PMT) matched with each laser. Using the transit time between two detected pulses and a fixed distance, the speed and size of each particle is calculated. Subsequently the particles move through to the mass analyzer area where it is ionized by a pulsed LDI laser, which is time to hit the particles as it reaches the center of the ion extraction region. Once ionized, the positive ions are accelerated towards the positive ToF section and the negative ions accelerated toward the negative ToF part where they are detected.
Apps
The field of aerosol science and measurement, especially aerosol mass spectrometry has grown considerably over the last few decades. Growth in part due to instrument flexibility, it has the ability to analyze particle size and chemical composition, and perform bulk and particle measurements. The flexibility of the aerosol mass spectrometer allows it to be used for applications both in the laboratory and in the field. Over the years aerosol mass spectrometers have been used for anything from determining emission sources, human exposure to pollutants, radiation transfer and cloud microphysics. Most of these studies have used AMS mobility and have been deployed in urban, remote, rural, marine, and forest environments around the world. AMS has also been deployed on mobile platforms such as ships, mobile laboratories, and airplanes.
A recent study of emissions in 2014 was conducted by two NASA, DC-8 and P-3B research planes, complemented by aerosol instrumentation (AMS). The aircraft was sent to conduct an atmospheric sample analysis of oil sands mining and an improvement facility near Ft. McMurray, Alberta, Canada. The purpose of this study was to test the emissions from the facility, and determine if they met the requirements. The result of this study is that compared to the estimated annual forest fire emissions in Canada, the oil sands facility is a small source of aerosol quantities, aerosol mass, particulate organic matter, and carbon black.
Aerosol mass spectrometry has also found its way into the field of pharmaceutical aerosol analysis, due to its ability to provide particle size measurement and chemical composition in real-time. Persons suffering from chronic respiratory diseases usually receive their treatment through the use of either dose-measured dose inhalers (pMDI) or dry powder inhalers (DPI). In both methods the drug is delivered directly to the lungs through inhalation. In recent years, inhaled products have been available which provide two types of drug in a single dose. Studies have shown that two drug inhalers provide a better clinical effect beyond those achieved when both drugs are given simultaneously from two separate inhalers. It was determined using AToFMS that the inhaled particles in the DPI product and the pMDI product consisted of a mutually-linked active pharmaceutical ingredient, which is the reason behind the increased effect of the two drug inhalers.
See also
- Microprobe laser mass spectrometer
- Sampler of particulate material
- Aerosol impotence
- Analysis of particle size
References
Further reading
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Hartonen, Kari; Laitinen, Totti; Riekkola, Marja-Liisa (2011). "The current instrument for aerosol mass spectrometry". Trac Trends in Analytical Chemistry . 30 (9): 1486-1496. doi: 10.1016/j.trac.2011.06.007. ISSN 0165-9936.
External links
- Atmospheric Science Center
- TOF-AMS resources
- Q-AMS resources
- List of publications using all versions of AMS
- Glossary of AMS
Source of the article : Wikipedia
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