Analytical thermal desorption

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Analytical thermal desorption, known within the analytical chemistry community simply as "thermal desorption" (TD), is a technique that concentrates volatile organic compounds (VOCs) in gas streams prior to injection into a gas chromatograph (GC). It can be used to lower the detection limits of GC methods, and can improve chromatographic performance by reducing peak widths. [1]

Contents

History

Analytical thermal desorption originated in the mid-1970s as an adaptation to the injection procedure for GC. Injector liners were packed with a compound able to adsorb organic compounds, used to sample air or gas, and then dropped into the inlet of the GC. This principle was first widely employed for occupational monitoring, in the form of personal badge-type monitors containing a removable charcoal strip. [2] These offered the advantage of being amenable to analysis without a separate solvent-extraction step.

Also developed in the 1970s was a method by which volatiles in the air were collected by diffusion onto tubes packed with a sorbent, which was then heated to release the volatiles into the GC system. These were first introduced for monitoring sulfur dioxide [3] and nitrogen dioxide, [4] but the analyte scope later widened as the sorbents became more advanced. Another early method (closely related to the modern purge-and-trap procedure) involved passing a stream of gas through a water sample to release the volatiles, which were again collected on a sorbent-packed tube. [5]

Such axial-type samplers, which later became known as 'sorbent tubes', were laid out as an industry standard in the late 1970s, by Working Group 5 (WG5) of the UK Health & Safety's Committee on Analytical Requirements (HSE CAR). The tubes they outlined were 3+1⁄2 inches long with an outer diameter of 1⁄4 inch, and were first employed in Perkin Elmer's ATD-50 instrument. [6]

At the same time, WG5 specified various basic functionality requirements for thermal desorption, and in the years since then, a number of improvements have been made to instrumentation for thermal desorption, including two-stage operation (see below), splitting and re-collection of samples, improved trap-cooling technology, standard system checks, and automation.

Principles

Thermal desorption fundamentally involves collecting volatile organic compounds onto a sorbent, and then heating this sorbent in a flow of gas to release the compounds and concentrate them into a smaller volume.

Early thermal desorbers used just single-stage operation, whereby the volatiles collected on a sorbent tube were released by heating the tube in a flow of gas, from where they passed directly into the GC.

Modern thermal desorbers can also accommodate two-stage operation, whereby the gas stream from the sorbent tube (typically 100–200 mL) is collected on a narrower tube integral to the thermal desorber, called the focusing trap or cold trap. Heating this trap releases the analytes once again, but this time in an even smaller volume of gas (typically 100–200 μL), resulting in improved sensitivity and better GC peak shape. [1]

Modern thermal desorbers can accommodate both single-stage and two-stage operation, although single-stage operation is now usually carried out using the focusing trap to collect the analytes, rather than a sorbent tube.

It is normal for the focusing trap to be held at or below room temperature, although a temperature no lower than 0 °C is sufficient for all but the most volatile analytes. Higher trap temperatures also reduce the amount of water condensing inside the trap (when transferred to the GC column, water can reduce the quality of the chromatography).

Sampling configurations

A wide variety of sampling configurations are used for thermal desorption, depending on the application. The most popular are listed below.

Single-stage thermal desorption

This involves sampling direct onto the focusing trap of the thermal desorber. It is generally used for situations where the analytes are too volatile to be retained on sorbent tubes.

Two-stage thermal desorption

This involves sampling first onto a sorbent tube. The most widely used tubes are those following the pattern laid out by WG5 (see above). After sampling (for which a variety of accessories are available), the tube is desorbed to transfer the analytes to the focusing trap before the second desorption stage transfers them to the GC. The greater sensitivity of this method has made it increasingly popular for sampling dilute gas streams, or in exploratory work where the target atmosphere is unknown.

Sorbents

The sorbent tube and the focusing trap may be packed with one or more sorbents. The type and number of sorbents depends on a number of factors including the sampling setup, the analyte volatility range, analyte concentration, and the humidity of the sample. [9] [10]

One of the most versatile and popular sorbents for thermal desorption is poly(2,6-diphenyl-p-phenylene oxide), known by its trademark Tenax. [11]

Analyte range

Depending upon the sampling technique and the analytical conditions, thermal desorption can be used to reliably sample analytes ranging in volatility from ethane to about tetracontane (n-C40H82). Incompatible compounds include:

Applications

Applications of thermal desorption were originally restricted to occupational health monitoring, but have since extended to cover a much wider range. Some of the most important are mentioned below – where available, examples of early reports, and more recent citations (including those of widely used standard methods) have been given:

Related Research Articles

In chemical analysis, chromatography is a laboratory technique for the separation of a mixture into its components. The mixture is dissolved in a fluid solvent called the mobile phase, which carries it through a system on which a material called the stationary phase is fixed. Because the different constituents of the mixture tend to have different affinities for the stationary phase and are retained for different lengths of time depending on their interactions with its surface sites, the constituents travel at different apparent velocities in the mobile fluid, causing them to separate. The separation is based on the differential partitioning between the mobile and the stationary phases. Subtle differences in a compound's partition coefficient result in differential retention on the stationary phase and thus affect the separation.

<span class="mw-page-title-main">Mass spectrometry</span> Analytical technique based on determining mass to charge ratio of ions

Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a mass spectrum, a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is used in many different fields and is applied to pure samples as well as complex mixtures.

<span class="mw-page-title-main">Ion source</span> Device that creates charged atoms and molecules (ions)

An ion source is a device that creates atomic and molecular ions. Ion sources are used to form ions for mass spectrometers, optical emission spectrometers, particle accelerators, ion implanters and ion engines.

<span class="mw-page-title-main">Electron ionization</span> Ionization technique

Electron ionization is an ionization method in which energetic electrons interact with solid or gas phase atoms or molecules to produce ions. EI was one of the first ionization techniques developed for mass spectrometry. However, this method is still a popular ionization technique. This technique is considered a hard ionization method, since it uses highly energetic electrons to produce ions. This leads to extensive fragmentation, which can be helpful for structure determination of unknown compounds. EI is the most useful for organic compounds which have a molecular weight below 600. Also, several other thermally stable and volatile compounds in solid, liquid and gas states can be detected with the use of this technique when coupled with various separation methods.

<span class="mw-page-title-main">Gas chromatography</span> Type of chromatography

Gas chromatography (GC) is a common type of chromatography used in analytical chemistry for separating and analyzing compounds that can be vaporized without decomposition. Typical uses of GC include testing the purity of a particular substance, or separating the different components of a mixture. In preparative chromatography, GC can be used to prepare pure compounds from a mixture.

<span class="mw-page-title-main">Gas chromatography–mass spectrometry</span> Analytical method

Gas chromatography–mass spectrometry (GC–MS) is an analytical method that combines the features of gas-chromatography and mass spectrometry to identify different substances within a test sample. Applications of GC–MS include drug detection, fire investigation, environmental analysis, explosives investigation, food and flavor analysis, and identification of unknown samples, including that of material samples obtained from planet Mars during probe missions as early as the 1970s. GC–MS can also be used in airport security to detect substances in luggage or on human beings. Additionally, it can identify trace elements in materials that were previously thought to have disintegrated beyond identification. Like liquid chromatography–mass spectrometry, it allows analysis and detection even of tiny amounts of a substance.

<span class="mw-page-title-main">Liquid chromatography–mass spectrometry</span> Analytical chemistry technique

Liquid chromatography–mass spectrometry (LC–MS) is an analytical chemistry technique that combines the physical separation capabilities of liquid chromatography with the mass analysis capabilities of mass spectrometry (MS). Coupled chromatography – MS systems are popular in chemical analysis because the individual capabilities of each technique are enhanced synergistically. While liquid chromatography separates mixtures with multiple components, mass spectrometry provides spectral information that may help to identify each separated component. MS is not only sensitive, but provides selective detection, relieving the need for complete chromatographic separation. LC–MS is also appropriate for metabolomics because of its good coverage of a wide range of chemicals. This tandem technique can be used to analyze biochemical, organic, and inorganic compounds commonly found in complex samples of environmental and biological origin. Therefore, LC–MS may be applied in a wide range of sectors including biotechnology, environment monitoring, food processing, and pharmaceutical, agrochemical, and cosmetic industries. Since the early 2000s, LC–MS has also begun to be used in clinical applications.

<span class="mw-page-title-main">Solid-phase extraction</span> Process to separate compounds by properties

Solid-phase extraction (SPE) is a solid-liquid extractive technique, by which compounds that are dissolved or suspended in a liquid mixture are separated, isolated or purified, from other compounds in this mixture, according to their physical and chemical properties. Analytical laboratories use solid phase extraction to concentrate and purify samples for analysis. Solid phase extraction can be used to isolate analytes of interest from a wide variety of matrices, including urine, blood, water, beverages, soil, and animal tissue.

<span class="mw-page-title-main">Atmospheric-pressure chemical ionization</span> Ionization method

Atmospheric pressure chemical ionization (APCI) is an ionization method used in mass spectrometry which utilizes gas-phase ion-molecule reactions at atmospheric pressure (105 Pa), commonly coupled with high-performance liquid chromatography (HPLC). APCI is a soft ionization method similar to chemical ionization where primary ions are produced on a solvent spray. The main usage of APCI is for polar and relatively less polar thermally stable compounds with molecular weight less than 1500 Da. The application of APCI with HPLC has gained a large popularity in trace analysis detection such as steroids, pesticides and also in pharmacology for drug metabolites.

Sorbent tubes are the most widely used collection media for sampling hazardous gases and vapors in air, mostly as it relates to Industrial hygiene. They were developed by the US National Institute for Occupational Safety and Health (NIOSH) for air quality testing of workers. Sorbent Tubes are available from CARO Analytical Services, SKC Inc., 7Solutions BV, Uniphos Ltd., SKC Ltd, Zefon International, Sigma-Aldrich/Supelco and Markes International. SKC Inc. manufactured the first commercially available sorbent tubes. XAD2 Tubes.

Solid phase microextraction, or SPME, is a solid phase extraction sampling technique that involves the use of a fiber coated with an extracting phase, that can be a liquid (polymer) or a solid (sorbent), which extracts different kinds of analytes from different kinds of media, that can be in liquid or gas phase. The quantity of analyte extracted by the fibre is proportional to its concentration in the sample as long as equilibrium is reached or, in case of short time pre-equilibrium, with help of convection or agitation.

In mass spectrometry, direct analysis in real time (DART) is an ion source that produces electronically or vibronically excited-state species from gases such as helium, argon, or nitrogen that ionize atmospheric molecules or dopant molecules. The ions generated from atmospheric or dopant molecules undergo ion-molecule reactions with the sample molecules to produce analyte ions. Analytes with low ionization energy may be ionized directly. The DART ionization process can produce positive or negative ions depending on the potential applied to the exit electrode.

<span class="mw-page-title-main">Thermospray</span>

Thermospray is a soft ionization source by which a solvent flow of liquid sample passes through a very thin heated column to become a spray of fine liquid droplets. As a form of atmospheric pressure ionization in mass spectrometry these droplets are then ionized via a low-current discharge electrode to create a solvent ion plasma. A repeller then directs these charged particles through the skimmer and acceleration region to introduce the aerosolized sample to a mass spectrometer. It is particularly useful in liquid chromatography-mass spectrometry (LC-MS).

Sample preparation for mass spectrometry is used for the optimization of a sample for analysis in a mass spectrometer (MS). Each ionization method has certain factors that must be considered for that method to be successful, such as volume, concentration, sample phase, and composition of the analyte solution. Quite possibly the most important consideration in sample preparation is knowing what phase the sample must be in for analysis to be successful. In some cases the analyte itself must be purified before entering the ion source. In other situations, the matrix, or everything in the solution surrounding the analyte, is the most important factor to consider and adjust. Often, sample preparation itself for mass spectrometry can be avoided by coupling mass spectrometry to a chromatography method, or some other form of separation before entering the mass spectrometer. In some cases, the analyte itself must be adjusted so that analysis is possible, such as in protein mass spectrometry, where usually the protein of interest is cleaved into peptides before analysis, either by in-gel digestion or by proteolysis in solution.

<span class="mw-page-title-main">Two-dimensional chromatography</span>

Two-dimensional chromatography is a type of chromatographic technique in which the injected sample is separated by passing through two different separation stages. Two different chromatographic columns are connected in sequence, and the effluent from the first system is transferred onto the second column. Typically the second column has a different separation mechanism, so that bands that are poorly resolved from the first column may be completely separated in the second column. Alternately, the two columns might run at different temperatures. During the second stage of separation the rate at which the separation occurs must be faster than the first stage, since there is still only a single detector. The plane surface is amenable to sequential development in two directions using two different solvents.

<span class="mw-page-title-main">Proton-transfer-reaction mass spectrometry</span>

Proton-transfer-reaction mass spectrometry (PTR-MS) is an analytical chemistry technique that uses gas phase hydronium reagent ions which are produced in an ion source. PTR-MS is used for online monitoring of volatile organic compounds (VOCs) in ambient air and was developed in 1995 by scientists at the Institut für Ionenphysik at the Leopold-Franzens University in Innsbruck, Austria. A PTR-MS instrument consists of an ion source that is directly connected to a drift tube and an analyzing system. Commercially available PTR-MS instruments have a response time of about 100 ms and reach a detection limit in the single digit pptv or even ppqv region. Established fields of application are environmental research, food and flavor science, biological research, medicine, security, cleanroom monitoring, etc.

<span class="mw-page-title-main">Desorption atmospheric pressure photoionization</span>

Desorption atmospheric pressure photoionization (DAPPI) is an ambient ionization technique for mass spectrometry that uses hot solvent vapor for desorption in conjunction with photoionization. Ambient Ionization techniques allow for direct analysis of samples without pretreatment. The direct analysis technique, such as DAPPI, eliminates the extraction steps seen in most nontraditional samples. DAPPI can be used to analyze bulkier samples, such as, tablets, powders, resins, plants, and tissues. The first step of this technique utilizes a jet of hot solvent vapor. The hot jet thermally desorbs the sample from a surface. The vaporized sample is then ionized by the vacuum ultraviolet light and consequently sampled into a mass spectrometer. DAPPI can detect a range of both polar and non-polar compounds, but is most sensitive when analyzing neutral or non-polar compounds. This technique also offers a selective and soft ionization for highly conjugated compounds.

Comprehensive two-dimensional gas chromatography, or GC×GC, is a multidimensional gas chromatography technique that was originally described in 1984 by J. Calvin Giddings and first successfully implemented in 1991 by John Phillips and his student Zaiyou Liu.

<span class="mw-page-title-main">Surface-assisted laser desorption/ionization</span>

Surface-assisted laser desorption/ionization (SALDI) is a soft laser desorption technique used for mass spectrometry analysis of biomolecules, polymers, and small organic molecules. In its first embodiment Koichi Tanaka used a cobalt/glycerol liquid matrix and subsequent applications included a graphite/glycerol liquid matrix as well as a solid surface of porous silicon. The porous silicon represents the first matrix-free SALDI surface analysis allowing for facile detection of intact molecular ions, these porous silicon surfaces also facilitated the analysis of small molecules at the yoctomole level. At present laser desorption/ionization methods using other inorganic matrices such as nanomaterials are often regarded as SALDI variants. As an example, silicon nanowires as well as Titania nanotube arrays (NTA) have been used as substrates to detect small molecules. SALDI is used to detect proteins and protein-protein complexes. A related method named "ambient SALDI" - which is a combination of conventional SALDI with ambient mass spectrometry incorporating the direct analysis real time (DART) ion source has also been demonstrated. SALDI is considered one of the most important techniques in MS and has many applications.

Headspace gas chromatography uses headspace gas—from the top or "head" of a sealed container containing a liquid or solid brought to equilibrium—injected directly onto a gas chromatographic column for separation and analysis. In this process, only the most volatile substances make it to the column. The technique is commonly applied to the analysis of polymers, food and beverages, blood alcohol levels, environmental variables, cosmetics, and pharmaceutical ingredients.

References

  1. 1 2 E. Woolfenden, Thermal desorption for gas chromatography, in: Gas Chromatography, ed. C.F. Poole, Elsevier, 2012, chapter 10, pp. 235–289; Analytical thermal desorption: History, technical aspects and application range, Thermal Desorption Technical Support Note 12, Markes International, April 2012, http://www.markes.com/Downloads/Application-notes.aspx.
  2. 1 2 Lautenberger, W.J.; Kring, E.V.; Morello, J.A. (1980). "A new personal badge monitor for organic vapors". American Industrial Hygiene Association Journal. 1980 (41): 737–747. doi:10.1080/15298668091425581. PMID   7435378.
  3. ↑ Palmes, E.D.; Gunnison, A.F. (1973). "Personal monitoring device for gaseous contaminants". American Industrial Hygiene Association Journal. 34 (2): 78–81. doi:10.1080/0002889738506810. PMID   4197577.
  4. ↑ Palmes, E.D.; Gunnison, A.F.; DiMattio, J.; Tomczyk, C. (1976). "Personal sampler for nitrogen dioxide". American Industrial Hygiene Association Journal. 37 (10): 570–577. doi:10.1080/0002889768507522. PMID   983946.
  5. ↑ Badings, H.T.; Cooper, R.P.M. (1985). "Automatic system for rapid analysis of volatile compounds by purge-and-cold-trapping/capillary gas chromatography". Journal of High Resolution Chromatography and Chromatography Communications. 8 (11): 755–763. doi:10.1002/jhrc.1240081111.
  6. ↑ J. Kristensson, The use of ATD-50 system with fused silica capillaries in dynamic headspace analysis, in: Analysis of volatiles, ed. P. Schreier, De Gruyter, 1984, pp. 109-120.
  7. ↑ Vas, G.; Vékey, K. (2004). "Solid-phase microextraction: a powerful sample preparation tool prior to mass spectrometric analysis". Journal of Mass Spectrometry. 2004 (39): 233–254. doi:10.1002/jms.606. PMID   15039931.
  8. ↑ A.-L. Sunesson, Passive sampling in combination with thermal desorption and gas chromatography as a tool for assessment of chemical exposure, in: Comprehensive Analytical Chemistry, Volume 48: Passive Sampling Techniques in Environmental Monitoring, ed. R. Greenwood, G. Mills and B. Vrana, Elsevier, 2007.
  9. ↑ Woolfenden, E. (2010). "Sorbent-based sampling methods for volatile and semi-volatile organic compounds in air. Part 1: Sorbent-based air monitoring options". Journal of Chromatography A. 1217 (16): 2674–2684. doi:10.1016/j.chroma.2009.12.042. PMID   20106481.
  10. ↑ Woolfenden, E. (2010). "Sorbent-based sampling methods for volatile and semi-volatile organic compounds in air. Part 2. Sorbent selection and other aspects of optimizing air monitoring methods". Journal of Chromatography A. 1217 (16): 2685–2694. doi:10.1016/j.chroma.2010.01.015. PMID   20106482.
  11. ↑ Zlatkis, A.; Lichtenstein, A.; Tishbee, A. (1973). "Concentration and analysis of trace volatile organics in gases and biological fluids with a new solid adsorbent". Chromatographia. 6 (2): 67–70. doi:10.1007/BF02270540. S2CID   95423469.
  12. ↑ Bruner, F.; Ciccioli, P.; Nardo, F. Di (1974). "Use of graphitized carbon black in environmental analysis". Journal of Chromatography. 99: 661–672. doi:10.1016/s0021-9673(00)90893-8. PMID   4422759.
  13. ↑ Pankow, J.F.; Isabelle, L.M.; Hewetson, J.P.; Cherry, J.A. (1984). "A syringe and cartridge method for down-hole sampling for trace organics in ground water". Ground Water. 22 (3): 330–339. doi:10.1111/j.1745-6584.1984.tb01405.x.
  14. ↑ US EPA Compendium Method TO-17: Determination of volatile organic compounds in ambient air using active sampling onto sorbent tubes, US Environmental Protection Agency, January 1999, PDF
  15. ↑ W.R. Betz, S.G. Maroldo, G.D. Wachob and M.C. Firth, Characterization of carbon molecular sieves and activated charcoal for use in airborne contaminant sampling, American Industrial Hygiene Association Journal, 1989, 50: 181–187.
  16. ↑ Houldsworth, H.B.; O'Sullivan, J.; Musgrave, N. (1982). "Passive monitors for the determination of personal nitrous oxide exposure levels". Anaesthesia. 37 (4): 467–468. doi: 10.1111/j.1365-2044.1982.tb01175.x . PMID   7081695. S2CID   37132681.
  17. ↑ MDHS 80, Laboratory method using diffusive solid sorbent tubes, thermal desorption and gas chromatography, UK Health & Safety Executive, August 1995, PDF
  18. ↑ Grote, A.A.; Kennedy, E.R. "Workplace monitoring for VOCs using thermal desorption-GC-MS". Journal of Environmental Monitoring. 2002 (4): 679–684.
  19. ↑ E. Woolfenden, Standardized methods for testing emissions of organic vapors from building products to indoor air, in: Organic Indoor Air Pollutants (2nd edn), ed. T. Salthammer and E. Uhde, Wiley-VCH, 2009, chapter 6, http://eu.wiley.com/WileyCDA/WileyTitle/productCd-3527312676.html.
  20. ↑ Method VDA 278: Thermal Desorption Analysis of Organic Emissions for the Characterization of Non-Metallic Materials for Automobiles, October 2011, http://www.vda.de/en/publikationen/publikationen_downloads/detail.php?id=1027.
  21. ↑ Kessler, A.; Baldwin, I.T. (2001). "Defensive function of herbivore-induced plant volatile emissions in nature". Science. 291 (5511): 2141–2144. doi:10.1126/science.291.5511.2141. PMID   11251117.
  22. ↑ Manolis, A. (1983). "The diagnostic potential of breath analysis". Clinical Chemistry. 29: 5–15. doi:10.1093/clinchem/29.1.5. PMID   6336681.
  23. ↑ E. Woolfenden, Flavour and fragrance profiling by ATD/GC, Laboratory Equipment Digest, April 1989, pp. 23–25.
  24. ↑ Kelly, L.; Woolfenden, E.A. "Enhanced GC-MS aroma profiling using thermal desorption technologies". Separation Science. 2008 (1): 16–23.
  25. ↑ Workplace chemical monitoring: Monitoring considerations, in: Occupational Health and Workplace Monitoring at Chemical Agent Disposal Facilities, Board on Army Science and Technology (National Research Council), 2001, chapter 2.
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