Optimization of confined direct analysis in real time mass spectrometry (DART-MS)

Abstract

Direct analysis in real time mass spectrometry (DART-MS) is seeing increased use in many fields, including forensic science, environmental monitoring, food safety, and healthcare. With increased use, novel configurations of the system have been created to either aid in detection of traditionally difficult compounds or surfaces, provide a more reproducible analysis, and/or chemically image surfaces. This work focuses on increasing the fundamental understanding of one configuration, where the DART ionization gas is confined in a junction, such as with thermal desorption (TD) DART-MS. Using five representative compounds and a suite of visualization tools, the role of the DART ionization gas, Vapur flow rate, gas back pressure, and exit grid voltage were examined to better understand both the chemical and physical processes occurring inside the confined configuration. The use of nitrogen as a DART ionization gas was found to be more beneficial than helium because of enhanced mixing with the analyte vapors, providing a more reproducible response. Lower Vapur flow rates were also found to be advantageous as they increased the analyte residence time in the junction, thus increasing the probability of its ionization. Operation at even lower Vapur flow rates was achieved by modifying the junction to restrict the DART gas flow. The DART exit grid voltage and gas back pressure had little observed impact on analyte response. These results provide the foundation to better understand and identify best practices for using a confined DART-MS configuration.

Introduction

Direct analysis in real time mass spectrometry (DART-MS), one of the many ambient ionization mass spectrometry systems, is a commonly researched and implemented tool in the fields of forensic science,¹ food safety,² explosives detection,³ and pharmecuticals.⁴ While the majority of the research surrounding DART-MS has focused on understanding its limitations and best practices for a wide range of compound properties, a significant body of work is being developed to understand the fundamentals of DART ionization pathways and sampling protocols. Ionization pathways have been investigated since the inception of DART⁵ and a number of studies have agreed upon Penning ionization of water as the predominant ionization pathway.^6,7 Ionization mechanisms for nitrogen DART have also been proposed in the literature.^7,8 The use of nitrogen as the DART ionization gas has seen increased attention recently, causing a renewed interest in understanding the ionization mechanism, as evidenced by work from Su et al.⁹ and An et al.¹⁰

While most the fundamental work surrounding DART-MS has been focused on understanding ionization mechanisms, significant work has also looked into the effects of sample placement and sample introduction on instrument response. Work by Harris et al.¹¹ investigated the ideal placement of a glass microcapillary for the analysis of nerve agents and found correlations between analyte concentration and sample placement that yielded increased responses and minimal ion suppression from secondary species. Modeling of ion transport in DART-like experiments has also been completed, which again highlighted optimal sample placement positions and the role of electric fields in increasing ion mobility.¹² Other work has utilized schlieren imaging¹³ or neon doping¹⁴ to better visualize the DART gas stream for the purposes of understanding sample interaction or surface sampling.

Much of this fundamental work was been completed using the traditional transmission mode configuration or an off-axis scanning mode configuration, however, several other sampling configurations have been developed. Confinement of the DART gas stream is one of the main modifications and is used in conjunction with a thermal desorption (TD) unit (TD-DART-MS) that allows for wipe-sample collection and subsequent analysis of drugs of abuse^15–17 and explosives.^10,18 A confined configuration has also been used for the analysis of volatile organic compounds from fruits.¹⁹ A similar approach, using a heating element capable of much higher temperatures has also been used for the analysis of paints.²⁰ Confinement of the DART gas has been shown to increase inter-sample reproducibility, enhance sensitivity, and allow for the detection of thermally non-labile compounds that traditionally are not amenable to DART analysis.¹⁸

This work aims to expand the existing body of literature on DART-MS fundamentals to include the confined DART gas configuration by detailing an investigation of the various front-end parameters utilizing a TD-DART-MS configuration consisting of a Vapur interface, a glass T-junction, and a thermal desorption unit. A better physical understanding of the confined DART configuration was obtained through coupling mass spectral measurements with schlieren imaging and high-speed videography. The representative compounds were chosen to span a range of polarities, molecular weights, and the major ionization pathways. Parameters that were investigated included DART ionization gas, Vapur flow rate, grid electrode, and DART gas back pressure. Methods to enhance analyte signal through modifications to the junction were also explored.

Materials and methods

Chemicals and materials

This work investigated the response of five compounds: cocaine, anthracene, reserpine, 1,3,5-trinitro-1,3,5-triazinane (RDX), and xylitol which were chosen to represent common compound classes for TD-DART-MS analysis. Cocaine was chosen as a representative polar compound which forms a protonated molecule. Anthracene was chosen as a non-polar compound which also forms a protonated molecule. Reserpine was chosen as a representative high molecular weight compound. RDX and xylitol were chosen because they form negative ions: RDX forms nitrate adducts and xylitol forms deprotonated molecules.

Cocaine and RDX were purchased as 1 mg mL⁻¹ methanolic solutions (Cerilliant, Round Rock, TX, USA), while anthracene was purchased as a 1 mg mL⁻¹ solution in acetone from Supelco (Bellefonte, PA, USA). Reserpine and xylitol were purchased as solids (Millipore-Sigma St Louis, MO, USA) and dissolved in Chromasolv-grade methanol (Millipore-Sigma). All solutions were further diluted in methanol to reach the desired concentration (10 μg mL⁻¹ or 50 μg mL⁻¹). For analysis, samples were pipetted (1 μL) onto polytetrafluoroethylene (PTFE) coated fiberglass wipes (DSA Detection, North Andover, MA, USA) and allowed to dry before insertion into the thermal desorption unit. Table 1 provides additional information for the compounds including the m/z values that were monitored and the masses of material that were deposited onto the wipes.

Table 1.

Information for the five representative compounds investigated in this study

Compound	Molecular formula	Monoisotopic mass (Da)	Peak of interest (m/z)	Mass per wipe (ng)
Cocaine	C17H21NO4	303.147	304.155 [M + H]+	10
Anthracene	C14H10	178.078	179.086 [M + H]+	50
Reserpine	C33H40N2O9	608.273	609.281 [M + H]+	50
RDX	C3H6N6O6	222.035	284.023 [M + NO3]−	10
Xylitol	C5H12O5	152.068	151.061 [M − H]−	50

TD-DART-MS

All analyses were completed on a JEOL JMS-T100LP time-of-flight mass spectrometer (JEOL USA, Peabody, MA, USA) coupled using a DART SVP ion source (IonSense, Saugus, MA, USA) and a custom-made thermal desorption unit (Fig. 1). The custom-built thermal desorption unit was mounted to a glass T-junction which was mounted to the Vapur interface (IonSense). The glass T-junction had an outer diameter of 6.35 mm, and inner diameter of 3.18 mm, an overall length (DART to Vapur) of 100 mm with a 30 mm arm located 20 mm from the DART side. The junction was flared at the DART end, to an inner diameter of 12.7 mm, to aid in positioning of the DART source. An air gap between the DART source and T-junction of approximately 5 mm was used. The Vapur interface was used, in conjunction with an auxiliary vacuum pump (VacuuBrand MZ2C, Essex, CT, USA), to pull vapor created in the thermal desorber through the T-junction and towards the mass spectrometer. The flow rate through the Vapur interface was measured using a mass flow meter (Omega FMA1823A, Norwalk, CT, USA) that was installed between the Vapur interface and the ball valve (Easy Read EF 20 S, Deltrol, Bellwood, IL, USA). More details on the configuration can be found elsewhere.¹⁶ A thermal desorber temperature of 275 °C was used throughout the study. The Vapur flow rate, DART gas temperature, and DART exit grid voltage were varied for specific experiments but, unless otherwise noted, 4 L min⁻¹, 400 °C, and ±50 V were used. Under these conditions, the temperature inside the glass T-junction at the junction was approximately 160 °C. Both nitrogen and helium (99.999% purity) were used for analysis. Flow rates of nitrogen and helium through the DART source were measured at 2.5 L min⁻¹ and 2.9 L min⁻¹, respectively. When switching between gases sufficient time was given to allow for flow stabilization before measurements were taken. Relevant mass spectrometer settings included an orifice 1 voltage of ±10 V, ring lens and orifice 2 voltage of ±5 V, a peaks voltage of ±1000 V, a 120 °C orifice temperature, a detector voltage of ±2300 V, and a mass range of m/z 130 to m/z 650 at 2 scan per s.

Fig. 1.

Schematic of the TD-DART-MS configuration.

Visualization tools

High-speed videography coupled with light scattering flow visualization was used to visualize the gas flow inside the T-junction of the confined DART. A Photron APS-RX high-speed video camera (Toyko, Japan) imaged theatrical fog flowing inside the T-junction at 1000 frames per second. The theatrical fog was produced by a custom-built miniature fog machine. Fiber optic illumination with adjustable gooseneck lighting (Motic, British Columbia, Canada) was used to scatter light from the fog for visualization.

Results & discussion

Initial optimization through a partial factorial design of experiments (DOE)

Initial studies to identify the important parameters that influence analyte response using the confined DART-MS configuration were completed by employing a two-level four-factor partial factorial (2^4-1) design of experiments (DOE). The parameters, and levels, that were studied included: DART exit grid voltage (±50 V and ±300 V), Vapur flow rate (3 L min⁻¹ and 8 L min⁻¹), DART gas temperature (100 °C and 400 °C), and DART source gas (helium and nitrogen). The geometry of the T-junction was not modified in this work as it was previously identified that a short DART-to-intersection distance and a long intersection-to-MS distance is desired to minimize the loss of metastable species and maximize the time for analyte ionization to occur.¹⁶ The study, which required eight different settings (Table S1^†), was completed for all five representative compounds and the order in which the settings were analyzedwas randomized. Five replicate wipes for each compound at each setting were measured. The results of the study can be found in Fig. S1.^† Given the variation in compounds chosen, the most influential factors were found to be compound dependent. Therefore, all properties, except for DART gas stream temperature, were studied in detail. A DART gas stream temperature of 400 °C was used for the remaining experiments to prevent condensation of the analyte on the glass junction.

The role of the DART ionization gas and Vapur flow rate

The goal of these studies was to support the analytical results with an understanding of the physical phenomena at work in the system. Traditional DART-MS analysis employs helium, due to its higher energy metastable (19.8 eV for He²² and 6.2 eV for N₂ ²³), which can lead to orders-of-magnitude higher signal for most analytes because of its ability to directly ionize water molecules through Penning ionization.²¹ Results from the DOE (Fig. S1^†), showed increased signal intensity for cocaine, anthracene, and RDX when using nitrogen as the DART ionization gas. To further investigate the perceived benefits of nitrogen, as well as the role of the Vapur flow rate, all five compounds were analyzed using helium and nitrogen as the DART ionization gas across a range of Vapur flow rates, from 1.5 L min⁻¹ to 8 L min⁻¹. Five replicates of each compound at each flow rate and DART ionization gas combination were taken, and the peak areas from the respective extracted ion chronographs (EICs) were measured. Valve dial positions for the respective flow rates are provided in Table S2.^†

The results of this study, shown in Fig. 2, demonstrate the correlation between DART ionization gas and Vapur flow rate. The lowest Vapur flow rate from the DOE experiments, 3 L min⁻¹, was chosen because it was the minimum flow rate at which reproducible analyte signal was obtained when using nitrogen as the DART ionization gas. However, analyte responses were still obtained when helium flow rates lower than 3 L min⁻¹ were used. Helium Vapur flow rates of 2 L min⁻¹ to 2.5 L min⁻¹ produced an analyte response higher than at 3 L min⁻¹. The analyte response when using helium at these lower Vapur flow rates was, apart from the response for RDX, higher than any response obtained when using nitrogen. At Vapur flow rates above 3 L min⁻¹ there was little difference in the average peak areas between the two DART ionization gases, except for RDX. An overall decrease in signal was also observed with increasing flow rate. The decrease in signal with increasing Vapur flow rates may have been caused by the decreased residence time of the analyte in the junction, effectively minimizing the interaction time and probability of ionization. Increased flow may have also increased the probability of adduct break-up. The residence times at 3 L min⁻¹ and 8 L min⁻¹ were estimated to be 16 ms and 6 ms, respectively.

Fig. 2.

Analyte signal as a function of Vapur flow rate (x-axis) and DART gas (series) for (A) cocaine, (B) anthracene, (C) reserpine, (D) RDX, and (E) xylitol. The lighter datapoints correspond to helium gas while the darker datapoints correspond to nitrogen gas. Error bars represent the standard deviation of five replicate measurements. Note the dual axis on the RDX plot (D) demonstrating the significant difference in intensity.

To understand the differences in analyte response using Vapur flow rates at and below 3 L min⁻¹, schlieren imaging of the TD-DART front-end was completed. To simplify the observation, and eliminate the interference of dissipating heat, no temperature was applied to the thermal desorber and the DART source was operated at only 100 °C. Visualization of the two DART ionization gases at these low flow rates, video stills of which are shown in Fig. 3 (full videos in ESI^†), provided deeper insight into the analytical observations. When using either helium or nitrogen at a Vapur flow rate of 1.5 L min⁻¹ consumption of the entire DART gas by the Vapur pump was not possible which caused the gas to be pushed outward from the DART-junction interface and out the thermal desorber (Fig. 3A and C). At this lower flow rate, it was not possible for the analyte vapor to make it through the junction and into the MS because of the counter-flow caused by the ionization gas.

Fig. 3.

Video stills from schlieren imaging of the TD-DART front-end to show the DART ionization gas as a function of increasing Vapur flow rate. Examples of helium at 1.5 L min⁻¹ (A), helium at 3 L min⁻¹ (B), nitrogen at 1.5 L min⁻¹, and (C) nitrogen 3.0 L min⁻¹ (D) are shown. Red arrows in A and C represent visualized helium (A) and nitrogen (C) leaking from the junction and desorber. The corresponding videos can be found in ESI (Videos S1 and S2^†).

At slightly higher Vapur flow rates, the pump was able to handle the DART ionization gas flow which generated entrainment of air through the thermal desorber. When using helium, this occurred between 2.5 L min⁻¹ and 3 L min⁻¹, as shown in Fig. 2B. However, with nitrogen, a small amount of gas was still pushed outward through the desorber at 2.5 L min⁻¹ and a portion of the gas was also being pushed out backwards at the DART-junction interface. By 3 L min⁻¹, the pump was able to handle the DART ionization gas and pull air through the thermal desorber to transport analyte vapor to the MS.

A clear benefit of increased signal when using helium was observed at low Vapur flow rates with no major difference in absolute analyte response was at flow rates above 3 L min⁻¹, except for RDX. While the response between the two gases was similar, significant differences were observed in sample to sample reproducibility as well as chronographic response. For these measures, use of nitrogen produced more reproducible (Fig. 2) and skewed normal (Fig. 4) peaks when compared to helium. Additionally, use of helium consistently produced a total ion chronograph (TIC) where peaks corresponding to sample insertion were indiscernible from background (due to a higher abundance of background peaks in the blank spectrum) (Fig. S2^†), whereas nitrogen produced readily identifiable peaks in the TIC (due to a less abundant background spectrum). This may be an important consideration when non-targeted analyses are completed as an extracted chronograph of a known mass may not be obtained.

Fig. 4.

(A) Extracted ion chronograph (EIC) of the cocaine base peak across different Vapur flow rates using nitrogen as the DART ionization gas. (B) EIC of cocaine across different Vapur flow rates using helium as the DART ionization gas.

Visualization tools were again employed to investigate the cause behind the observed decline in reproducibility and peak shape when using helium. To accomplish this, the TD-DART front-end, with the thermal desorber and insulation removed, was visualized using high speed videography and theatrical fog. The fog was introduced into the junction through the port typically occupied by the thermal desorber, allowing for visualization of the interaction between the DART ionization gas and simulated analyte vapor (in this case, theatrical fog). Differences when using nitrogen and helium were readily apparent with this visualization technique. When nitrogen was employed as the ionization gas substantial mixing between the analyte vapor and DART ionization gas was observed at the junction, evidenced by near instantaneous dilution of the theatrical fog (video still in Fig. 5, ES Videos S3 and S4^†). Alternatively, the use of helium as the ionization gas showed minimal to no mixing between the two gases, with the analyte vapor (represented by theatrical fog) traveling along the walls of the junction to the mass spectrometer, separated from the DART ionization gas.

Fig. 5.

Video stills highlighting the interaction of the DART ionization gas with the analyte vapor (theatrical fog) when using (A) nitrogen and (B) helium. DART gas and entrained air enter the tee on the right, then mix with the analyte vapor stream coming from the desorber. The corresponding high-speed videos showing these interactions in greater detail can be found in the ESI.^† Stills were created using a Vapur flow rate of 3 L min⁻¹.

The observed differences were a direct result of the gas properties (density) and resulting flow regimes upstream of the junction. These differences yielded a nearly order of magnitude difference in the Reynolds number. Though dependent to some extent on the Vapur flow rate, the constant DART gas flow rate resulted in Reynolds numbers well within transition to turbulent regime for nitrogen. As this DART stream flow interacts with the entrained thermal desorber flow and analyte vapor, chaotic mixing occurs (Fig. 5A and ESI Videos S3 and S4^†). The chaotic mixing greatly reduces distances between neutral vapor and DART-generated metastable species and ions. Contrastingly, the helium DART gas remains well within the laminar regime across all flow rates experienced here. This flow profile simply confined the incoming air and analyte vapor from the thermal desorber along the side of the tube (Fig. 5B). Mixing between the ion and metastable stream from the DART source and the neutral analyte vapor from the thermal desorber was greatly reduced and limited to diffusion and shear dispersion at the interface. Relative to nitrogen, this provided a much lower probability of interaction with the DART gas and ions, effectively suppressing the analyte response. This also contributed to the lower reproducibility of signal using helium, as mixing with the analyte vapor was significantly more sporadic.

While the increased mixing between analyte and DART gas using nitrogen explained the ability to achieve increased signals for most compounds, it did not directly explain why RDX was observed to have a substantially higher response using nitrogen, and helium consistently produced a higher response when analyzing xylitol (Fig. 2D and E). The cause of these observations was attributed to the background ions produced using the different DART ionization gases (Fig. S3^†). The background spectrum for nitrogen in negative mode showed the near-complete absence of all ions other than nitrate. Conversely, when helium was employed, significant peaks at m/z 60 ([CO₃]⁻) and m/z 61 ([HCO₃]⁻) were observed, with the nitrate signal (m/z 62) present at only 6.5% of the base peak (m/z 60). The dominant production of nitrate ions when nitrogen gas was used likely aided in adduct formation for RDX and inhibited the deprotonation of xylitol. While initially this may seem determinantal, it does open the possibility of tailoring the DART parameters to detect certain compounds. For instance, if explosives were targeted, the use of nitrogen as the DART gas would allow for preferential ionization of the explosives, which typically form nitrate adducts, potentially minimizing the effects of background ions, many of which form deprotonated molecules. The background spectra for positive mode (Fig. S4^†) was identical for nitrogen and helium, except for variations in absolute intensity.

Potential strategies for increasing gas mixing

The results of the previous two studies highlighted the benefits of mixing the DART gas with the analyte vapor to increase signal. The energy of the helium metastable is greater, leading to more efficient ionization; therefore, two modifications to the T-junction were made to determine if enhanced mixing could be achieved. The first strategy involved the use of a 3-D printed inline mixer placed inside the junction, between the DART inlet and the intersection, to try to passively mix the helium prior to interaction with the analyte vapor (Fig. S5^†). While this modification was successful at increasing mixing (ESI Video S5^†), the increased mixing and interaction with surfaces caused complete quenching of the helium metastable species. This configuration was not studied further due to a lack of mass spectral signal.

The second strategy used a glass T-junction modified to include dimples in the section between the DART source and the intersection (Fig. S5^†). It should be noted that in order to include the dimples within the junction, the respective portion of the junction had to be extended from 20 mm to 35 mm. Theatrical fog and high-speed videography were again used to visualize the intersection of the dimpled T-junction (ESI Video S6^†). No noticeable improvement in mixing between the analyte vapor and DART gas was observed using this configuation with helium, though it still allowed for mixing when using nitrogen. It was observed that this configuration allowed for entrainment of the analyte vapors into the junction at lower Vapur flow rates than the normal T-junction (ESI Video S7^†), likely due to the dimples acting as restrictions on the DART gas stream. While this may not be relevant in a laboratory setting, use of a restricted tube size for the DART gas stream may be beneficial in fieldable instruments as it may lower power requirements for the Vapur pump and may lower gas consumption.

This configuration did not quench the metastable species, allowing for mass spectral analysis, and therefore was evaluated to see if it provided a better analytical result than the normal junction. The response of cocaine, RDX, and xylitol were measured across a range of Vapur flow rates between 1.5 L min⁻¹ and 8 L min⁻¹ using both helium and nitrogen as the DART ionization gas. Generally, little to no measurable improvement in signal was observed by incorporating the dimpled junction, as shown in Fig. 6. Potential evidence of increased mixing was observed through comparison of the EICs of the analytes to those obtained using the normal T-junction. Use of the dimpled T-junction (Fig. 6D) produced smoother more skewed normal responses when using helium. This type of response may be beneficial if quantitative analysis is desired. The role of the dimples acting as a restrictor on the DART ionization gas was evidenced by the ability to obtain any, or higher, analyte signal at the low end of the Vapur flow rate range where the normal junction did not elicit a response for RDX and xylitol.

Fig. 6.

Comparison of the response obtained using the dimpled T-junction and the normal T-junction for (A) cocaine, (B) RDX, and (C) xylitol across the range of Vapur flow rates. Error bars represent the standard deviation of five replicate measurements. A comparison of the xylitol EIC using the different junctions and DART ionization gases is also shown (D).

The role of gas flow rate and back pressure

The drive towards creating field deployable ambient ionization mass spectrometry systems is of interest to several sectors including forensic science, homeland security, environmental science, and healthcare. Implementation of DART based systems in this realm would require a substantial reduction in the gas requirements. The benefit of nitrogen over helium using a confined TD-DART-MS platform addresses part of this need, allowing for the use of a nitrogen generator instead of gas cylinders.

A study into whether decreased back pressures would hinder analyte response was completed by analyzing the response of cocaine, RDX, and xylitol using nitrogen as the DART ionization gas at back pressures between 138 kPa (20 psi) and 552 kPa (80 psi). While the manufacturer suggested back pressure was 517 kPa (75 psi), the DART source could be operated at as low as 138 kPa (20 psi) (Fig. 7) with no negative effect on analyte response. The average response of cocaine was nearly identical at 138 kPa (20 psi) and 552 kPa (80 psi) (2.9 × 10⁵ counts and 3.1 × 10⁵ counts, respectively) and the lower pressure did not cause decreased reproducibility (16% and 12% relative standard deviation for replicate measurements, respectively). Similar observations were also found for xylitol and RDX as well.

Fig. 7.

The effect of the DART ionization gas back pressure, when using nitrogen, to analyze cocaine (navy circle), RDX (orange diamond), and xylitol (blue triangle). The cocaine and xylitol data points are offset on the x-axis (−13.8 kPa/2 psi and +13.8 kPa/2 psi, respectively) for better presentation of the data. Error bars represent the standard deviation of five replicate measurements.

The role of the grid electrode

The final parameter that was investigated in this study was the DART exit grid voltage. The exit grid may act to prevention-electron recombination, propel ions towards the MS, and act as an electron producing surface for negative mode ionization.^5,6,21 Ion-electron recombination or movement towards the MS were not expected to be enhanced by increasing the exit grid voltage due to the active pull of the Vapur interface. To study this further the response of cocaine, RDX, and xylitol at increasing exit grid voltages was measured.

Similar to other work,²⁴ and consistent with the DOE results (Fig. S1^†), variations in the DART exit grid were not found to be significant, as shown in Fig. 8. While the response of all three compounds tended to decline with increasing exit voltage, the reduction in signal was never more than a factor of two. The reduction in signal was attributed to an overall reduction in ions produced, as evidenced by the lower intensity background spectra observed at higher exit grid voltages (Fig. S3 and S4^†). Changing the exit grid voltage did not affect the background species produced.

Fig. 8.

The effect of the DART exit grid voltage on the response of (A) cocaine (navy circle), RDX (orange diamond), and xylitol (blue triangle) and (B) anthracene (red square) and reserpine (purple diamond), when using nitrogen as the DART ionization gas. Error bars represent the standard deviation of five replicate measurements.

Conclusions

The combination of analytical measurements with visualization tools provided a unique, holistic, method of understanding the physical and chemical processes occurring in a confined DART-MS configuration. Unlike traditional DART, where analyte response is a function of how well the DART gas can ionize a sample or the environment around the sample, in a confined configuration, the interaction between DART gas and analyte vapor must also be considered. Using this type of configuration, it was possible to obtain similar analyte responses using helium or nitrogen, which is something traditional DART does not allow. The use of nitrogen as the ionization gas also allowed for the added benefit of increased inter-sample reproducibility because of enhanced mixing within the junction. The Vapur flow rate controlled the residence time within the junction, and slower residence times typically corresponded to higher analyte responses. The DART exit grid voltage and gas back pressure had little to no impact on analyte response.

While some of the considerations for a basic T-junction were established as a result of this work, it is obvious that additional advancements in signal response could be made through further refinement and understanding of the system. Strategies to increase the mixing and interaction of helium and entrained vapors within the junction, without enhancing the quenching of metastable species, would likely lead to enhancements in signal response. Further research into methods to restrict the DART gas flow may lead to the ability to operate at both lower DART gas and Vapur flow rates – a necessity for field deployment applications. Additionally, other areas that could be further investigated include the use of different materials for the T-junction, incorporation of dopants into the junction to aid in ionization and mixing of multiple DART gases. Other methods of sample introduction could also be examined. In order to fully understand the benefits of these types of modifications, however, it will be crucial to understand both the physical and chemical processes that are occurring.