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Direct analysis in real time

Direct analysis in real time () is an ambient ionization technique for that enables the rapid, non-contact analysis of gases, liquids, solids, and surfaces under and open-air conditions without requiring or chromatographic separation. Invented in 2003 by Robert B. Cody, James A. Laramée, and H. Dupont Durst at USA, Inc., and first commercialized in 2005, DART utilizes a stream of excited neutral atoms, typically , generated via a gas discharge, which interact with atmospheric species to produce ions from analytes through Penning ionization and subsequent reactions. It is one of the pioneering ambient mass spectrometry sources that operate at ground potential without radioactive components. DART's versatility allows direct desorption and ionization from complex matrices, making it suitable for real-time monitoring across diverse applications, including forensics, pharmaceuticals, , , and preservation, where it non-destructively analyzes low levels of organic materials in artifacts. When coupled with high-resolution mass analyzers like time-of-flight (TOF), it provides accurate mass measurements and structural information. Its open-air design minimizes sample carryover and matrix effects compared to traditional methods.

History and Development

Invention and Early Research

Development of Direct Analysis in Real Time () began in late at USA, Inc., initiated by Robert B. Cody and James A. Laramee, with contributions from H. Dupont Durst, to create a safe, nonradioactive alternative to the nickel-63 sources commonly used in (IMS) for detecting agents and toxic industrial chemicals. The motivation stemmed from regulatory and safety concerns surrounding radioactive materials in portable detection devices, aiming to enable rapid, ambient-pressure without the hazards of beta emitters. The for the was filed in April 2003. Key early experiments focused on generating metastable atoms for , leveraging their 19.8 eV excited states to interact with atmospheric and analytes. In spring 2003, the first prototype was assembled and tested using a high-resolution time-of-flight mass spectrometer ( AccuTOF-LC), successfully detecting trace vapors such as those from glue and acetone across distances of several meters in open air. Early testing at the Edgewood Chemical Biological Center demonstrated DART's efficacy for surface analysis of chemical agents like , achieving low detection limits. The foundational proof-of-concept was detailed in the initial publication by Cody, Laramee, and Durst in 2005, marking DART as one of the earliest ambient ionization techniques developed shortly after (DESI) in December 2004. This work, publicly introduced at the January 2005 ASMS Sanibel Conference, addressed critical challenges, including the need for non-contact, preparation-free analysis at ground potential under ambient conditions, thereby expanding mass spectrometry's applicability to diverse sample types without radioactive components or high voltages.

Commercialization and Key Milestones

The U.S. Patent 6,949,741 for the design was granted on September 27, 2005, to inventors Robert B. Cody and James A. Laramee and assigned to USA, Inc. This patent covered a non-radioactive method using a carrier gas with metastable excited-state species to ionize analytes without sample preparation. launched the first commercial system in 2005 as an add-on for its AccuTOF time-of-flight mass spectrometers, enabling direct analysis of solids, liquids, and gases at ambient conditions. By 2010, DART technology had expanded to other major vendors, including LECO for its time-of-flight instruments and Waters for integration with systems like the Xevo TQ MS, broadening its availability for routine laboratory use. Key milestones in 's development include its initial integration with high-resolution in 2008, as demonstrated in applications combining DART with GC-TOF-MS for of complex mixtures like soft drinks. In the , advancements focused on portability, with mobile forensic laboratories incorporating DART systems for on-site by 2020 and prototypes for handheld configurations emerging around 2023 to support field-based detection of contaminants and drugs. The adoption of has driven substantial growth in , exceeding 2,000 publications by 2025, reflecting its versatility across disciplines. A notable surge occurred post-2020 in applications, particularly for detection in grains and other commodities, as highlighted in 2024 reviews emphasizing rapid, non-destructive screening methods. Significant events include the publication of the book Direct Analysis in Real Time Mass Spectrometry: Principles and Practices of DART-MS edited by Jürgen H. Gross in 2016, which consolidated foundational techniques and emerging applications. In 2025, advances in technology were showcased at the , including tools for automated identification and to improve accuracy in complex sample analysis.

Operating Principles

Ionization Mechanisms

Direct analysis in real time () ionization begins with a in or gas, where electrons bombard the gas atoms to produce long-lived metastable , such as excited atoms (He*) at energies of 19.8 and 17.6 . These metastable are carried by a gas stream at flow rates typically ranging from 2 to 5 L/min through a series of chambers and out into the open atmosphere. While is preferred for efficiency, can be used as a cost-effective alternative, though it may alter pathways. In the primary ionization process, known as Penning ionization, the metastable atoms transfer their internal energy to sample molecules (M) whose ionization energies are lower than that of the metastable species, resulting in the formation of radical molecular cations (M+•). For molecules with ionization energies exceeding the metastable energy, indirect chemical ionization pathways dominate; in positive ion mode, this involves proton transfer from species like ammonium ions (NH4+) derived from reactions with ambient water vapor, yielding protonated molecules ([M+H]+). A representative sequence begins with Penning ionization of atmospheric nitrogen by He*: \text{He}^* + \text{N}_2 \rightarrow \text{N}_2^{+•} + \text{He} + e^- followed by charge transfer and hydration steps, such as \text{N}_2^{+•} + \text{H}_2\text{O} \rightarrow \text{H}_2\text{O}^{+•} + \text{N}_2, \text{H}_2\text{O}^{+•} + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ + \text{OH}, ultimately forming protonating agents like H3O+ or NH4+ that react with the analyte. In negative ion mode, chemical ionization proceeds via electron transfer or proton abstraction, often involving superoxide anions (O2•-) generated from electrons produced in the discharge and captured by oxygen, which abstract protons to form [M-H]- or M- ions. Desorption of neutral sample molecules is facilitated by the heated gas stream, with temperatures adjustable from ambient to 550°C, promoting thermal vaporization without the need for solvents. A grid electrode, biased at 200-600 V, controls the transmission of ions into the mass spectrometer inlet while repelling neutrals. The operation of minimizes ion-molecule collisions that could cause fragmentation, favoring the production of intact molecular s over structurally informative but complex fragment patterns. This combination of Penning and pathways enables versatile, solvent-free analysis of diverse samples in open air.

Modes of Operation

Direct analysis in real time () mass spectrometry supports both positive and negative modes, which are selected based on the chemical properties of the analytes to optimize efficiency. The positive mode is used for analytes that can be protonated, such as or polar compounds, utilizing a helium discharge to produce metastable helium atoms that interact with ambient or , forming protonated species such as [M+H]⁺ or [M+NH₄]⁺ adducts. In this configuration, the grid voltage is maintained at a positive value, typically greater than 0 V (e.g., +250 V), to facilitate the transmission of cations to the mass spectrometer inlet. The negative ion mode is employed for acidic compounds or those capable of , such as nitroaromatics or carboxylic acids. It relies on oxygen doping of the carrier gas to generate anions (O₂⁻), which enable to form [M-H]⁻ ions or oxygen adducts like [M+O₂]⁻• through or attachment processes. The grid voltage is set to a negative value, typically less than 0 V (e.g., -250 V), to selectively transmit anions while repelling cations. Switching between positive and negative ion modes is accomplished by modifying the gas composition—such as introducing approximately 1% oxygen for negative mode operation—and reversing the of the disk and exit , without extinguishing the discharge plasma. This allows for seamless transitions, with typical acquisition times ranging from 1 to 5 seconds per mode, enabling high-throughput analysis of diverse sample types. DART sources can also be configured in transmission or reflection geometries to accommodate different sample formats. Transmission mode directs the ionizing gas beam through the sample, making it ideal for solids or porous materials like fabrics, where the beam penetrates to desorb and ionize analytes efficiently. In contrast, reflection mode positions the sample such that the beam impinges on the surface, with ions desorbed and directed back toward the inlet, which is better suited for opaque or dense substrates. Key operational parameters further tune performance, including temperature control of the perforated discharge tube, which can be adjusted from ambient to 550°C to enhance thermal desorption without degrading heat-sensitive compounds. The addition of dopant gases, such as methanol, promotes specific ion chemistry (e.g., enhanced protonation), improving sensitivity for targeted analytes by 10- to 100-fold in select applications.

Instrumentation

Source Components

The ion source features a core (DBD) region where is excited to form metastable species for . This region consists of an axially segmented tube housing a needle maintained at a of 1000–5000 V and a current of approximately 2 mA, generating a between the needle and a grounded counter . The tube is typically constructed from or to withstand the conditions. The gas inlet supplies or through a flow controller, with typical flow rates of 1.5–3 L/min for to produce long-lived metastable at 19.8 , or lower rates (e.g., 50 mL/min) for specialized variants. Optional dopant ports allow introduction of additives like or oxygen to modify modes, enhancing selectivity for specific analytes. At the exit, a perforated disk serves as a grounded counter , followed by a grid biased at +250 V for positive-ion mode or -250 V for negative-ion mode to direct the and prevent recombination of oppositely charged . A , often resistive coils, surrounds the assembly to heat the gas stream up to 350 °C (with the heater itself reaching 550 °C), facilitating desorption from solid or liquid samples without . Safety features include no direct exposure of high voltages or radioactive materials to the sample area, minimizing risk and cross-contamination. The design also incorporates shielding to contain UV emissions from the discharge and interlocks to detect gas leaks, ensuring safe operation during ambient analysis. Variants of the DART source include thermal desorption configurations such as transmission-mode (TD-DART), thermal desorption (IRTD-DART) with temperatures up to 600 °C, and junction heated thermal desorption (JHTD-DART) reaching 750 °C for enhanced volatile compound release. Open-source and low-cost designs, such as 3D-printed low-temperature probes inspired by principles, have been developed for under $100 using accessible materials like 3D-printed insulators and off-the-shelf electrodes, enabling custom assembly for research applications. Compact portable units integrate DBD components with battery-powered flow controllers for field-deployable .

Interface and Integration

The is positioned such that there is a 5-20 mm open atmosphere gap between the source exit and the mass spectrometer inlet, allowing ions generated at to drift into the instrument via a grounded skimmer featuring a 0.5-1 mm , facilitated by differential pumping to maintain integrity. This configuration enables efficient ion transmission from ambient conditions without enclosing the sample, distinguishing from traditional vacuum-based sources. Within the vacuum interface, ions are focused using components such as an S-lens in systems or heated capillary inlets in instruments, achieving a rapid pressure drop from atmospheric 760 to approximately 10^{-5} in the mass analyzer to support high-resolution detection. These elements ensure collimated beams, minimizing losses during the transition to high . DART sources couple directly to various mass analyzers, including time-of-flight (TOF), , and systems, often via commercial adapters that align the ion stream with the instrument inlet. For integration with separation techniques, post-column adapters enable DART ionization of liquid chromatography/ (LC/HPLC) effluents, while specialized setups allow effluent ionization from (GC) columns, expanding DART's utility in hyphenated workflows. In (), scans plates in linear mode at speeds of 1-10 cm/min, enabling spatially resolved analysis of separated compounds directly from the stationary phase. () integration has advanced since 2013 through sheathless sprayer interfaces, such as coaxial tips, which deliver CE eluate into the plasma stream for sensitive, low-flow analyte detection without diluting sheaths. Recent developments include portable systems integrated with (IMS) for on-site forensic analysis, as demonstrated in 2024 field-deployable setups for rapid screening of opioids and other trace substances, enhancing decision-making in applications. As of 2025, commercial advancements such as Bruker's EVOQ DART-TQ+ triple quadrupole system have simplified workflows for routine testing with reduced solvent use.

Analytical Performance

Mass Spectra Characteristics

Direct analysis in real time () mass spectra typically exhibit a simple profile dominated by intact molecular or quasimolecular ions, such as protonated molecules [M+H]⁺ in positive-ion mode and deprotonated molecules [M-H]⁻ in negative-ion mode, with minimal in-source fragmentation under ambient conditions. This soft behavior arises from the gas-phase proton transfer or mechanisms, producing spectra that emphasize molecular weight information over structural fragments. The -to-charge (m/z) commonly spans 50–2000 Da when coupled to time-of-flight (TOF) mass analyzers, accommodating a broad array of small to medium-sized organic and inorganic analytes. Adduct ions frequently appear, particularly [M+NH₄]⁺ in positive mode due to trace present in ambient air, which competes with and shifts the observed m/z by +18. patterns in these spectra provide diagnostic clues for , as the even-electron nature of the ions preserves natural isotopic abundances for accurate formula assignment. For instance, in high-resolution DART-TOF spectra, (PEG) standards display distinct isotopic clusters for [M+NH₄]⁺ ions, confirming polymer chain lengths. Background ions are generally low in intensity but include persistent peaks at m/z 18 from H₂O⁺, m/z 32 from O₂⁺•, and m/z 44 from N₂O⁺• or CO₂⁺•, originating from atmospheric gases and interacting with the ionized or . These can be effectively subtracted using spectral processing software to reveal signals. High-resolution delivers accuracy below 5 , enabling precise identification; for example, yields [M-H]⁻ at measured m/z 313.1321 (theoretical 313.1334, error 4 ) in negative-ion mode. Similarly, the explosive can be detected with sub-ppm accuracy in negative mode on systems, often as ions. Potential artifacts include thermal degradation products for heat-sensitive analytes, such as fragment ions from or polymers exposed to the heated gas stream (typically 200–500 ), which may appear as lower m/z peaks indicative of bond cleavage. In complex mixtures, matrix effects can suppress molecular signals or promote unintended fragmentation, complicating interpretation without prior sample optimization. These characteristics underscore DART's utility for rapid, qualitative while highlighting the need for controlled operational parameters like gas and orifice voltage to minimize artifacts.

Sensitivity and Quantification

Direct analysis in real time (DART-MS) typically achieves limits of detection in the range of 1-10 ng for many analytes, such as 5 ng for and its metabolites on swipe materials. These limits can vary significantly depending on the sample matrix, with more complex matrices often requiring optimization to reach sub-ng . Post-2020 advancements, particularly when coupling DART with high-resolution (HRMS), have pushed detection limits to the picogram (pg) level, for example, 15-30 pg for certain explosives and 100 pg for deposited analytes on fabrics. The linearity of DART-MS responses supports over a of 3-4 orders of magnitude, enabling reliable curves for analytes spanning low ng to µg levels. is commonly performed using internal standards, such as deuterated analogs, to account for ionization variability and matrix effects, yielding correlation coefficients (R²) greater than 0.99. Quantitative strategies in DART-MS emphasize mass spectrometry () for enhanced accuracy, where stable isotope-labeled standards minimize errors from ion suppression. Peak area integration is facilitated by software platforms like Agilent MassHunter, which processes spectral data for precise quantification. Reproducibility of these measurements typically achieves relative standard deviations () of ±10-20%, with improvements to under 15% RSD possible through desorption configurations. Several operational factors influence DART-MS sensitivity and quantification. Optimizing gas temperature, often in the range of 200-500°C depending on volatility, enhances desorption and efficiency without thermal degradation. Maintaining the sample-to-orifice distance at 5-10 mm maximizes transmission while minimizing atmospheric dilution. suppression, which can reduce analyte signals by up to several orders of magnitude due to competitive , is mitigated through modifications like adjustable extraction cones that improve ion focusing and selectivity. Recent developments as of 2025 incorporate (AI) algorithms for automated quantification in real-time DART-MS data streams, particularly for complex mixtures like new psychoactive substances, enabling rapid peak and reduced manual intervention. These AI-driven approaches, including models for identification and , have improved throughput and accuracy in forensic and environmental analyses.

Applications

Forensic and Security Analysis

Direct analysis in real time () has emerged as a valuable tool in forensic analysis, enabling the rapid detection of illicit substances such as opioids and through direct swabbing of surfaces without extensive . For instance, thermal desorption -MS (TD--MS) facilitates on-site or laboratory-based screening of and its analogues in seized samples, achieving detection limits in the nanogram range and allowing identification within seconds. This approach has been particularly effective for trace-level analysis of on various matrices, including powders and residues, supporting field-deployable protocols that minimize handling risks associated with potent opioids. Similarly, -MS has been validated for qualitative screening of in seized materials, often integrated with library matching for confirmatory identification, as demonstrated in evaluations of real-world . In 2022, portable -MS systems were tested in field environments, including security checkpoints akin to operations, to expedite seizure screening and enhance operational efficiency. In explosives detection, -MS provides non-contact screening capabilities for trace residues of compounds like (PETN) and cyclotrimethylenetrinitramine () on fabrics and other substrates, with analysis times under 10 seconds. Studies have shown that DART coupled with high-resolution , such as , effectively ionizes these nitro-based explosives in both positive and negative modes, enabling differentiation from interferents in complex matrices like post-blast debris. This rapid, ambient ionization method supports security applications by allowing direct analysis of swabs from suspicious packages or clothing, contributing to threat assessment in high-throughput scenarios. DART-MS also plays a role in , including the examination of seized inks for document authentication and the detection of in pharmaceutical preparations. For seized inks, DART-MS analyzes writing inks directly on paper substrates, preserving document integrity while providing molecular profiles for source comparison in questioned document cases. In pharmaceutical , it identifies cutting agents and contaminants in illicit tablets, aiding in the profiling of substances like opioids. Integration with (IMS) enhances portability, as seen in hybrid TD-DART-IMS systems for on-site drug and detection, with pilots by the Department of (DHS) in 2023 evaluating these for field use in threat screening. Notable case studies underscore DART-MS's forensic impact. In 2016, validation studies confirmed its efficacy for organic gunshot residue (OGSR) analysis, detecting propellant components like diphenylamine and nitroglycerin on swabs from shooting scenes, offering a complementary approach to traditional scanning electron microscopy. More recently, in 2024, DART-MS applications in border security have focused on synthetic cannabinoids, enabling rapid screening of seized herbal matrices and powders at ports of entry, with high-confidence library matches for compounds like MDMB-4en-PINACA. A key advantage in forensic workflows is DART-MS's minimal sample preparation, which reduces contamination risks and preserves the chain of custody by avoiding extraction steps that could compromise evidence integrity.

Food, Pharmaceutical, and Environmental Uses

Direct analysis in real time (DART-MS) has emerged as a valuable tool for applications, enabling rapid screening of contaminants without extensive sample preparation. In pesticide residue analysis, DART-MS facilitates the detection of multiple agrochemicals on and surfaces through direct swabbing, achieving limits of detection in the low parts-per-billion range for compounds like organophosphates and carbamates. For mycotoxins, such as aflatoxins in nuts and grains, DART-MS provides quantitative capabilities with high throughput, supporting by identifying and quantifying these toxins at trace levels directly from sample matrices. Recent studies have extended DART-MS to discriminate geographical origins of monofloral honeys, such as honeys from different regions, using high-resolution . A 2024 review highlights DART-MS's role in multi-class contaminant screening across food types, emphasizing its adaptability for on-site safety assessments. In pharmaceutical , DART-MS supports direct evaluation of active pharmaceutical ingredients () in solid like tablets, allowing for rapid identification of composition without dissolution or extraction steps. This technique generates characteristic [M+H]+ ions for , enabling verification of potency and uniformity in commercial products. For detection, DART-MS has been validated by the U.S. (FDA) in protocols for screening suspect tablets, such as those containing designer benzodiazepines, where it distinguishes authentic formulations from falsified ones based on molecular profiles in under a minute per sample. An FDA evaluation demonstrated DART-MS efficacy for identifying undeclared in suspect drug products with high accuracy. These applications underscore DART-MS's integration into regulatory workflows for ensuring drug integrity. Environmental monitoring benefits from DART-MS's ability to perform direct sampling and of pollutants in complex matrices. For per- and polyfluoroalkyl substances () in water, DART-MS coupled with arrows achieves rapid screening and quantification across diverse PFAS classes, with limits of detection below 1 ng/mL for legacy compounds like PFOA, supporting environmental compliance testing. In cosmetics-related environmental , DART-MS identifies organic UV filters such as benzophenones and cinnamates in cosmetic products. For soil contaminants, DART-MS enables on-site direct sampling of polycyclic aromatic hydrocarbons (PAHs), detecting parent and alkylated PAHs in wildfire-impacted or petroleum-contaminated soils through surface desorption, with high-resolution modes resolving isomers for accurate source attribution. Biomedical applications of -MS extend to non-invasive monitoring via exhaled breath and swabs, capturing metabolites for diagnostics. sources have been adapted for analysis of breath volatiles to detect potential biomarkers. In swab analysis, -MS profiles drug residues and endogenous compounds from surface wipes, facilitating non-invasive for compliance in clinical settings. Studies have refined methods for breath exhalome profiling to identify metabolites with high-resolution . Emerging uses of DART-MS focus on in , particularly for volatile organic compounds () that influence product safety and environmental footprint. Recent 2024-2025 reviews emphasize DART-MS for direct interrogation of packaging materials, identifying migrants like plasticizers and antioxidants from polymers without , to evaluate with eco-friendly standards. This approach supports rapid assessment of VOC emissions in biodegradable alternatives, promoting greener supply chains by detecting low-level leachables that could affect food quality.

Advantages and Limitations

Key Strengths

Direct analysis in real time (DART) mass spectrometry offers rapid analysis times, typically completing sample ionization and spectral acquisition in seconds to minutes, eliminating the need for time-consuming extraction or chromatographic separation steps common in traditional techniques. This speed enables real-time monitoring and screening, making it ideal for scenarios requiring immediate results, such as on-site forensic evaluations or high-volume quality control. The technique's versatility allows direct of diverse sample types—including solids, liquids, and gases—under ambient conditions without significant alteration of the sample matrix, reducing the risk of artifacts from preparation or vacuum environments. can analyze materials on various surfaces, from biological tissues to industrial substrates, and detects a broad range of compounds, including polar and nonpolar molecules that may be challenging for other methods. Minimal is a core strength, as facilitates noncontact, ambient-pressure sampling directly from surfaces or headspaces, often requiring no solvents, wipes, or derivatization, which minimizes contamination risks and preserves sample integrity. This direct approach supports in-situ analysis, particularly with portable configurations paired with compact analyzers. DART enhances safety by avoiding the use of radioactive sources, high voltages at the sample site, or hazardous solvents, operating instead with inert gases like at ground potential. Its compatibility with further enables high-throughput workflows, such as analyzing hundreds of samples per day in screening applications like pharmaceutical , where operational costs remain low due to reduced consumables and labor. For instance, in assessments, DART has demonstrated efficient screening of contaminants across large batches.

Challenges and Future Directions

One significant challenge in DART-MS is the influence of matrix effects, which can reduce analytical accuracy in complex samples such as adulterated drugs or matrices by causing ion suppression or competitive ization. This issue is particularly pronounced in forensic applications, where heterogeneous substrates like fabrics or residues interfere with direct sampling. DART-MS is primarily suited for volatile and semi-volatile analytes, limiting its applicability to non-volatile compounds that require additional desorption techniques for effective ionization. Quantitative analysis remains semi-quantitative at best, with variability arising from inconsistent sample positioning and lack of internal standards, hindering precise measurements without calibration. Sensitivity in DART-MS typically achieves detection limits in the ppb to ng range for many analytes, but it falls short of ultra-trace femtogram levels attainable with electrospray ionization (ESI) for polar or non-volatile species. Additionally, the high gas temperatures (up to 550°C) employed for desorption can cause thermal degradation of heat-sensitive biomolecules, such as steroids or peptides. Post-2020 developments have addressed some outdated limitations, including integrations with portable mass spectrometers for field deployment, enabling on-site of seized drugs without extensive infrastructure. Emerging enhancements, such as models for spectral classification using , improve data interpretation in complex mixtures like bacterial VOCs. Future directions include hybrid ionization sources combining DART with ESI or other techniques to expand analyte coverage and enhance selectivity for low-volatility compounds, with recent hyphenated systems demonstrating improved performance in multi-technique workflows. efforts are advancing portable DART-MS units for integration with drones or handheld devices, facilitating remote environmental or . Expanded spectral databases, such as those for drugs and inks hosted by NIST, are being developed to support automated identification and forensic validation. Research gaps persist in protocols for forensic , including uniform method validation across instruments. To promote sustainability, alternatives to , such as or as ionization gases, are under investigation for their efficiency and reduced environmental impact, with showing comparable ionization for many analytes.

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