Dimethyl Methylphosphonate Vapor Research Articles

Adsorption of DMMP and water vapor on granular activated carbon: An experimental and molecular simulation study

The adsorption behavior of Dimethyl methylphosphonate (DMMP), as a simulant of sarin nerve agent, especially in the presence of water vapor, is crucial for the design of efficient chemical protection equipment. Experimental results reveal DMMP's adsorption isotherm on granular activated carbon (GAC) as a typical type I. The adsorption process was a spontaneous and exothermic physical adsorption process, and the adsorption mechanism was in accordance with Dubinin's micropore filling theory. In the relative humidity range of 10 % ∼ 90 %, the isotherm remained I-type, and the effect of water vapor on the equilibrium adsorption capacity and apparent adsorption rate was weak. Molecular simulations also show that DMMP adsorption on GAC follows a type I isotherm, primarily driven by van der Waals forces of physical adsorption. The adsorption potential energy between DMMP and GAC surpasses that of water molecules, resulting in negligible effects from water vapor on DMMP adsorption equilibrium. Meanwhile, water molecules had minimal effect on the diffusion of DMMP molecules in the GAC pores, and the positional distribution of DMMP multilayer adsorption was consistent under different relative humidity conditions. Furthermore, the presence of polar functional groups such as hydroxyl groups significantly affects the adsorption equilibrium of DMMP.

The Application of Microfibrous Entrapped Activated Carbon Composite Material for the Sarin Simulant Dimethyl Methylphosphonate Adsorption.

Granular activated carbon (GAC) has proven to be an effective adsorbent for removing the chemical warfare agent sarin (GB) and simulants like Dimethyl methylphosphonate (DMMP). However, it comes with certain limitations, including inadequate contact efficiency, notable mass transfer resistance, and lower bed utilization efficiency. This study synthesized steel fiber-entrapped activated carbon composites (SFEACs), which exhibited a maximum adsorption capacity of 285.3 mg/g at 303 K. Compared with the packed bed (PB) filled with GAC, while the adsorption capacity of SFEACS decreased, there was a substantial increase in the adsorption mass transfer rate. These SFEACs were combined with GAC to create a structural fixed bed (SFB), which demonstrated excellent performance in DMMP removal. Under identical experimental conditions, the DMMP breakthrough curve of SFB exhibited a steeper profile compared to the packed bed (PB) filled with GAC at the same bed height, and the breakthrough time against DMMP vapor could be extended by 13.8%. Furthermore, the adsorption rate constant of the Yoon-Nelson model increased by more than 17.6%, and the unused bed length, according to the Wheeler-Jonas model, decreased by more than 14%.

Water-Accelerated Transport: Vapor-Phase Nerve Agent Simulant Delivery within a Catalytic Zirconium Metal-Organic Framework as a Function of Relative Humidity.

Zirconium-based metal-organic frameworks (MOFs) are candidate materials for effective nerve agent detoxification due to their thermo- and water stability as well as high density of catalytic Zr sites. However, as high-porosity materials, most of the active sites of Zr-MOFs can only be accessed by diffusion into the crystal interior. Therefore, the transport of nerve agents in nanopores is an important factor in the catalytic performance of Zr-MOFs. Here, we investigated the transport process and mechanism of a vapor-phase nerve agent simulant, dimethyl methyl phosphonate (DMMP), through a representative Zr-MOF, NU-1008, under practical conditions of varying humidity. Confocal Raman microscopy was used to monitor the transport of DMMP vapor through individual NU-1008 crystallites, where the relative humidity (RH) of the environment was tuned to understand the impact of water. Counterintuitively, water in the MOF channels, instead of blocking DMMP transport, assists DMMP diffusion; indeed, the transport diffusivity (Dt) of DMMP in NU-1008 is one order of magnitude higher at 70% than 0% RH. To understand the mechanism, magic angle spinning NMR and molecular dynamics simulations were performed and suggested that high water content in the channels prevents DMMP from hydrogen-bonding with the nodes, allowing for faster diffusion of DMMP in the channels. The simulated self-diffusivity (Ds) of DMMP is observed to be concentration-dependent. At low loading of DMMP, Ds is higher at 70% RH than 0% RH, while at high loadings the trend reverses due to the DMMP aggregation in water and the reduction of free volume in channels.

Fast Response-Recovery and High Selectivity Chemicapacitive Detection of a Nerve Agent Simulant Vapor

Early detection of chemical warfare agents (CWAs) is critical in minimizing the exposure to chemical threats. This study presents a fast response-recovery chemicapacitive sensor (chemicapacitor) for a nerve agent simulant, dimethyl methylphosphonate (DMMP), with high selectivity and sensitivity. Chemicapacitors with interdigitated electrodes were fabricated on a SiO2/Si wafer by aligning single-walled carbon nanotubes (SW-CNTs) coated with polyhedral oligomeric silsesquioxane-supported 1,1,1,3,3,3-hexafluoro-2-propanol (POSS-HFIP) receptors. The stable, nano-sized three-dimensional structure with multiple terminal alcohol groups played a crucial role as a high-performance receptor via efficient hydrogen-bonding interaction with the CWA simulant. The response and recovery times of the fabricated chemicapacitors were estimated to be 13 and 88 s, respectively, outperforming chemiresistive sensors in terms of response-recovery dynamics. The capacitive responses were obtained at varying DMMP vapor concentrations, ranging from 25 to 150 ppm, and they exhibited superior sensitivity compared to receptor-free sensor devices. The concentration-dependent sensitivity was well-fitted with the Langmuir isotherm model, indicating that the sensing mechanism is based on the adsorption/desorption process. In addition, excellent selectivity was realized by introducing different toxic molecules (sulfur dioxide, ammonia, and ethylene oxide) and a blood agent (cyanogen chloride), where the fabricated POSS-HFIP/SW-CNTs chemicapacitor selectively responded to the DMMP vapor. The limit-of-detection was calculated to be 0.70 ppm. The proposed POSS-HFIP/SW-CNTs chemicapacitor demonstrated rapid response-recovery characteristics (with improved selectivity towards DMMP), suggesting its potential in reducing casualties or injuries by early identification of CWAs.

Gas sensing of organophosphorous compounds with III–V semiconductor plasmonics

Chemical warfare agents including nerve agents, e.g., sarin, are part of the organophosphorous compounds group known for their extreme lethal potency towards humans. With increasing risks of exposure to these molecules, there is a need for sensitive and selective detection. Infrared absorption spectroscopy is a powerful technique for the identification of molecules by probing their vibro-rotational molecular resonances. However, infrared light wavelength and molecule absorption cross-section dimensions mismatch, significantly hampering light-matter interaction especially at low concentrations, therefore lowering the sensitivity. To overcome this challenge, we propose a rapid chemistry-free III–V semiconductors InAsSb plasmonic sensor working in the mid-infrared with ribbon-shape nano-antennas associated to a confined enhanced electric field at the nanoscale. Exploiting well-known Salisbury perfect absorber structure coupled to an epsilon-near-zero layer benefit both spectral and spatial overlap, two required conditions for surface-enhanced infrared spectroscopy. Finite difference time domain and rigorous coupled-wave analysis coupled to Drude formalism were performed to design the sensor. The plasmonic sensor was exposed to hundreds of ppm-level concentration vapors of dimethyl methylphosphonate, known as DMMP, a commonly used sarin simulant. We determined that DMMP bound to the sensor native oxide to form a 5 Å estimated monolayer. The sensor response results from the coupling between surface plasmons and DMMP molecular vibrations upon adsorption at resonances frequency. Further simulations confirmed experimental results. This work demonstrates the application of III–V semiconductor plasmonics for gas sensing of complex molecules exploiting surface-enhanced infrared absorption.

Photonic Liquid Crystal Polymer Absorbent for Immobilization and Detection of Gaseous Nerve Agent Simulants

Detection and sequestration of chemical warfare agents (CWAs), such as poisonous organophosphates, are highly desirable for both personal security and environmental protection. However, both sensing and absorption in a single device have been rarely reported. In this study, we describe a photonic absorbent based on a cholesteric liquid crystal polymer as a dual sensing and decontamination device for gas-type CWAs. Dimethyl methylphosphonate (DMMP) was used as a simulant compound. A blue reflective photonic polymer was fabricated that was able to detect DMMP vapor through absorption. Hydrogen bond interactions between DMMP and mesogenic carboxylic groups of the polymer allow selectivity and capture. A distinct optical change of the film from blue to bright green indicates the absorption of DMMP vapor molecules and confirms when full absorption of the polymer is achieved. The diffusion of DMMP vapor into the material was observed by the formation of a sharp boundary between swollen and unswollen material, as evidenced by scanning electron microscopy images and the structural color changes. In ambient conditions, DMMP molecules are retained in the photonic absorbent without release to the environment. Heating above approximately 60 °C releases the absorbed DMMP, leading to a reusable optical device. These results confirm the ability of photonic polymers to sense and immobilize dangerous vapor, paving the way for the realization of simple, battery-free optical devices able to simultaneously warn and protect.

Laser-induced fluorescence of PO-photofragments of dimethyl methylphosphonate.

The paper presents the results of calculating the absorption spectrum of a phosphorus monoxide (PO) molecule corresponding to the A2Σ+(v'=0)-X2Π(v''=0) transition. The efficiency of excitation of the PO molecule is estimated as a function of the spectral parameters of the laser radiation. The positions of the fluorescence bands of PO are calculated. It is shown that the use of excitation wavelengths near the bandheads of the (P22+Q12) and P12 branches of the A2Σ+(v'=0)-X2Π3/2(v''=0) band provides spectral separation of the (0, 1) γ-band of the LIF of PO and vibrational-rotational Raman spectrum of atmospheric oxygen. The spectral responses of dimethyl methylphosphonate vapor in air under the action of KrF-laser radiation at a wavelength of 247.78 nm have been experimentally studied. In the wavelength range 252-260 nm, the (0, 1) γ-band of the LIF of PO-fragments and the vibrational-rotational band of spontaneous Raman scattering on oxygen molecules are unambiguously interpreted. It is shown that the results of calculations of the shape and position of the fluorescence spectra are in good agreement with the experimental data.

Vapor Sorption-Desorption Phenomena of HD and GB Simulants from Polyurethane Thin Films on Aluminum Oxide via a Quartz Crystal Microbalance.

Protection and decontamination of surfaces after exposure to chemical warfare agents (CWAs) are of considerable interest to the homeland defense and battlespace operation communities. In this work, polyurethane was spin-coated onto aluminum oxide quartz crystal microbalance (QCM) sensors. Polyurethane film thickness was varied by altering the concentration of the polymer/chloroform solution used for spin-coating. Atomic force microscopy confirmed the formation of smooth, homogeneous films on the QCM sensor surface. Aluminum oxide QCM sensors coated with polyurethane were exposed to saturated vapors of dichloropentane (DCP), a mustard gas (HD) simulant, and dimethyl methylphosphonate (DMMP), a sarin gas (GB) simulant, and the mass uptake, diffusion coefficient, volume fraction, and partition coefficient of the simulant in the film were determined from QCM data. Results showed that both DCP and DMMP readily sorbed into the films although the mass uptake of DCP was greater than that of DMMP owing to DCP’s higher vapor pressure. Additionally, the CWA simulant uptake increased with polyurethane film thickness. Sorption diffusion coefficients were 1 × 10–13 cm2/s and 1 × 10–12 cm2/s for DCP and DMMP vapor, respectively. Simulant desorption was also measured and showed that some DMMP remained in the film/substrate system, while DCP sorption was fully reversible. Reversible desorption for both CWA simulants was relatively quick and independent over the range of film thicknesses studied, with average desorption diffusion coefficients of 2 × 10–9 cm2/s and 1 × 10–11 cm2/s for DCP and DMMP, respectively. Collectively, this study is expected to inform protection and decontamination strategies of equipment and structures upon exposure to CWAs.

High-performance p-hexafluoroisopropanol phenyl functionalized multi-walled carbon nanotube film on surface acoustic wave device for organophosphorus vapor detection

A delay line-type surface acoustic wave (SAW) gas sensor based on p-hexafluoroisopropanol phenyl (HFIPPH) functionalized multi-walled carbon nanotube (MWCNT) film is developed to detect organophosphorus dimethyl methylphosphonate (DMMP) vapor (a simulant of chemical nerve agent sarin). Inspired by the transfer process of Cu-based graphene, a uniform and size-controllable HFIPPH-MWCNT film is successfully prepared on the SAW device via a wet-etching transfer method. For the first time, we use the method of measuring the change of the sensor’s insertion loss to achieve the detection of ultra-low concentration DMMP vapor. The designed sensor exhibits a fast response/recovery time about 3 s/50 s, and a low detection limit of 0.1 ppm. Additionally, the stability and selectivity of the sensor and the influence of humidity on its response are evaluated through experiments. The acoustoelectric effect is proved to be the sensing mechanism of the sensor insertion loss response.

A prototype portable instrument employing micro-preconcentrator and FBAR sensor for the detection of chemical warfare agents

The presence of chemical warfare agents (CWAs) in the environment is a serious threat to human safety, but there are many problems with the currently available detection methods for CWAs. For example, gas chromatography–mass spectrometry cannot be used for in-field detection owing to the rather large size of the equipment required, while commercial sensors have the disadvantages of low sensitivity and poor selectivity. Here, we develop a portable gas sensing instrument for CWA detection that consists of a MEMS-fabricated micro-preconcentrator (μPC) and a film bulk acoustic resonator (FBAR) gas sensor. The μPC is coated with a nanoporous metal–organic framework material to enrich the target, while the FBAR provides rapid detection without the need for extra carrier gas. Dimethyl methylphosphonate (DMMP), a simulant of the chemical warfare agent sarin, is used to test the performance of the instrument. Experimental results show that the μPC provides effective sample pretreatment, while the FBAR gas sensor has good sensitivity to DMMP vapor. The combination of μPC and FBAR in one instrument gives full play to their respective advantages, reducing the limit of detection of the analyte. Moreover, both the μPC and the FBAR are fabricated using a CMOS-compatible approach, and the prototype instrument is compact in size with high portability and thus has potential for application to in-field detection of CWAs.

Blocking chemical warfare agent simulants by graphene oxide/polymer multilayer membrane based on hydrogen bonding and size sieving effect

Chemical warfare agents (CWAs) are toxic materials that cause death by contact with the skin or by respiration. Although studies on detoxification of CWAs have been intensively conducted, studies that block CWAs permeation are rare. In this study, for blocking CWAs, a multilayer thin film composed of linear polyethylenimine (LPEI) and graphene oxide (GO) is simply prepared through a spray-assisted Layer-by-Layer (LbL) assembly process. LPEI could change its morphology dependent on pH, which is known as a representative hydrogen donor and acceptor. By controlling the shape of the polymer chain, a heterogenous film could have a loose or dense inner structure. CWAs mainly move through diffusion and have hydrogen bonding sites. Therefore, the heterogeneous film can limit CWAs movement based on controlling pathways and hydrogen bonds within the film. The protective effect of this membrane is investigated using dimethyl methylphosphonate (DMMP), a nerve gas simulant. DMMP vapor transmittance rate (DVTR) and N2 permeance of LPEI/GO are 67.91 g/m2 day and 34,293.04 GPU. It means that the protection efficiency is 72.65%. Although this membrane has a thin thickness (100 nm), it shows a high protective effect with good breathability. And water/DMMP selectivity of the membrane is 66.63. Since this multilayer membrane shows efficient protection performance with a simple preparation method, it has a high potential for applications such as protective suits and masks.

Hierarchical Co3O4 decorated nitrogen-doped graphene oxide nanosheets for energy storage and gas sensing applications

Nano-sized cobalt oxide decorated nitrogen-doped graphene oxide (Co3O4@NGO) composite was produced by a feasible and cost-effective hydrothermal route for electrochemical supercapacitors and gas sensor applications. The composite materials formation was ascertained by Raman spectroscopy, X-ray diffraction, and X-ray photo electron spectroscopy analyses. Field emission scanning electron microscopy (FE-SEM) and field emission transmission electron microscopy (FE-TEM) results explored the controlled nanoscale-sized sheet-like morphology for the prepared composite materials. Electrochemical storage properties were studied by cyclic voltammetry (CV), galvanostatic charge–discharge process (GCD), and electrochemical impedance spectroscopy analyses using three-electrode configuration with 3 M KOH electrolyte. The observed results showed ~466 F/g specific capacitance at a current density of 1 A/g for Co3O4@NGO composite structure with the capacity retention of 96 % after 5000 cycles. Further, the synthesized Co3O4@NGO composite revealed improved detection response, cyclability, and linearity for dimethyl methyl phosphonate vapor gas sensing. The synthesized composite also demonstrated excellent selectivity, stability, sensitivity, and rapid response time.

Trace Level Analysis of Nerve Agent Simulant DMMP With Silicon Nanowire FET Sensor

In this paper, we propose a new strategy for the sensitive and specific detections of dimethyl methylphosphonate (DMMP), as a simulant of nerve agent Sarin. Our silicon nanowire based sensor is modified with carboxyphenylboronic acid (CPBA) to demonstrate great efficiency. The response time is less than 100 seconds to recognize the DMMP, which is a key ability for on-site monitoring. The sensor exhibits a good reversibility during the repeated measurements between DMMP vapor and air. The sensor still performs well after being exposed in high concentration of DMMP vapor with excellent recovery for 3 times. The sensor is demonstrated to have a linear behavior in detecting DMMP within the concentration of 13~65 ppm in gaseous phase with the limit of detection (LOD) of 100 ppb. It is expected that the proposed sensor enables the testing system accurate, rapid, sensitive and reversible for on-site detection of nerve agents in the future.

Trace Detection of Nerve Agent Simulant in the Fuel Vapour Environment using Metal Oxide Surface Acoustic Wave E Nose

Nerve agents are often used at the military warfront, where diesel is a very common interferant. In the present work, a group of surface acoustic wave (SAW) sensors, called E-Nose with dissimilar sensing layers is developed for the recognition of the mixture of diesel and dimethyl methylphosphonate (DMMP) vapors. The exposure of DMMP and diesel vapors is kept at ppb and ppm levels respectively. Varied response patterns of DMMP and diesel vapors were obtained by SAW E-nose. Principal component analysis (PCA) has been used to extract features from the response curves of SAW sensors. Artificial Neural Network pattern recognition has been implemented to identify the precise detection of DMMP vapors in the binary mixture of DMMP and diesel. The effect of pre-processing (using PCA) the raw data before feeding it to artificial neural network is also studied.

Reactive adsorption and photodegradation of soman and dimethyl methylphosphonate on TiO2/nanodiamond composites

We report on the synthesis and characterization of TiO2/Nanodiamond (ND) nanocomposites for rapid decontamination of chemical warfare agents (CWA) and toxic industrial compounds prepared by a simple water-based, low-temperature method using urea as a homogeneous precipitating agent. The excellent water-compatibility of NDs promoted their good dispersion within the TiO2 matrix resulting in intergrown TiO2/ND nanostructures. NDs with an abundance of oxygen-containing surface moieties increased the porosity of the composites resulting in their three times more efficient spontaneous degradation of the CWA soman in solution compared to pure TiO2. In situ DRIFT spectroscopy revealed the enhanced reactive adsorption and solar light photodecomposition of dimethyl methyl phosphonate vapor on TiO2/ND. The charge transfer across TiO2/ND interfaces that hinder recombination of photo-excited electron-hole pairs was inferred from surface potential measurements. The results indicate that well-dispersed NDs forming heterojunctions together with their high porosity contribute to the reactive properties of the nanocomposites.

Room temperature decomposition of dimethyl methylphosphonate on cuprous oxide yields atomic phosphorus

Organophosphorus compounds have a wide range of reactivity depending on the coordination and oxidation state of the P atom. Due to their toxicity, certain organophosphonates are used as pesticides, herbicides, and, notoriously, as chemical nerve agents. The organophosphonate decomposition mechanism depends strongly on the substrate material, which motivates careful analysis of a range of potential materials for decontamination and sensing applications. Here, we present a study of the decomposition of dimethyl methylphosphonate (DMMP), a simulant molecule for the nerve agent sarin, on polycrystalline cuprous oxide surfaces (Cu2O). We use ambient pressure X-ray photoelectron spectroscopy (APXPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy to monitor the pressure-dependent surface chemistry during exposure to DMMP vapor. At room temperature, we observe extensive degradation of DMMP, which includes the formation of reduced P products on the sample surface. We identify five unique P-containing species in the P 2p APXPS spectra, including atomic P and a surface-bound phosphinate [OPH(OCH3)OCu]. We propose a DMMP decomposition pathway whereby accumulation of this phosphinate species leads to an autocatalytic reaction on the Cu2O surface which yields reduced P species. After DMMP exposure, heating the Cu2O sample to 265 °C leads to complete removal of all carbonaceous species and accumulation of a phosphate layer with stoichiometry similar to P2O5. These results introduce a new mechanism of room-temperature DMMP decomposition on metal oxides, which could be leveraged for the design of tailored metal oxide materials for catalyzing organophosphonate decomposition.

Detection of Dimethyl Methylphosphonate (DMMP) Using Polyhedral Oligomeric Silsesquioxane (POSS)

The detection and monitoring of colorless and odorless chemical warfare agents (CWAs) has become important due to the increasing threat of terrorist activities. To enhance the sensitivity and selectivity of CWAs a number of sensing materials have been developed, including the widely used polyvinylidene fluoride (PVDF). However, PVDF is limited by its low sensitivity and selectivity for many CWAs. In this study, polyhedral oligomeric silsesquioxane (POSS) was used as a sensing material for dimethyl methylphosphonate (DMMP) a simulant of sarin nerve gas. Sensitivity, selectivity, and reusability were investigated with an AT-cut 5 MHz quartz crystal microbalance. At room temperature, POSS exhibited a strong response for DMMP vapor at different concentrations from 20 ppm to 120 ppm leading to fast chemical adsorption and desorption. To investigate selectivity the volatile organic compounds (VOCs) ethanol, water, toluene, ACN, methanol, and n-hexane were tested at a fixed flow rate. Targeted VOCs showed lower responses than DMMP as measured with a quartz crystal microbalance (QCM) sensor, demonstrating the high selectivity of the method. POSS can be considered a potentially useful material for sensing nerve simulants.

Round-patterned ZnO nanostructure coated with siloxane-based polymer for nerve agent detection

The alignment of zinc oxide (ZnO) nanostructures is expected to improve device sensitivities due to large surface areas which can be utilized to capture significant quantities of gas particles. In this study, we investigated patterned ZnO nanorods modified with polystyrene monolayers synthesized directly onto a quartz crystal microbalance (QCM) cell to increase the coating surface area of the sensing material. Also, we designed and synthesized a siloxane-based polymer (S1 polymer) as a sensing material. The patterned ZnO nanorods coated with S1 polymers were fabricated and used for the detection of dimethyl methylphosphonate (DMMP). The resonance frequency of QCM was shifted due to the adsorption and desorption of a compound at the surface of the modified electrodes. We have synthesized an S1 polymer that exhibited high sensitivity to DMMP. The patterned ZnO nanorods coated with the polymer also exhibited improved sensitivity due to an enhanced surface area capable of adsorbing more DMMP vapor.

Hybrid Nanocomposite-Based QCM Sensor for Detecting DMMP

Taking a hybrid nanocomposite of nitrogen doped multiwalled carbon nanotube/cobalt oxide nanoparticle/gold nanoparticle as a sensing material, we developed a quartz crystal microbalance-based nerve simulant sensor. Thesensor was simply fabricated by a drop casting of a mixture of the nanocomposite and ethanol on the top of gold electrode in a quartz crystal microbalance (QCM). Dimethyl methylphosphonate (DMMP) vapor as a targeted nervesimulant was generated by a bubbler system and modulated by two mass flow controllers. The frequency shifts taken from QCM was linearly responded to DMMP concentrations, ranging from 20 to 120 ppm. The adsorption rate of DMMP molecules was relatively high, while the desorption rate was low. It reveals a high sensitivity to DMMP vapor. To investigate the selectivity various VOCs including ethanol, water, toluene, methanol, and n-hexane were tested at 120 ppm of targeted VOCs concentration. Interestingly, the targeted VOCs other than DMMP were relatively low detected by the QCM sensor, implying remarkable selectivity. In this regards, the developed hybrid nanocomposite is a potential material to be used for sensing nerve simulants.

Sensitivity Enhancement Studies of SAW Vapor Sensor by Oscillator Tuning Using Varactor Diode

This paper reports the designing of surface acoustic wave (SAW) oscillator with varactor diode (or varicap) as a part of tuning element. A practical way to enhance sensitivity of SAW sensor oscillator is performed by tuning capacitance using varicap. The varactor diode is placed at three different locations of the oscillator circuit to realize three different configurations. The change in frequency with applied voltage to varactor diode is studied for all three configurations. Sensitivity study for the three configurations is carried out with di-methyl methyl phosphonate vapor. The sensitivity, stability, and noise of each configuration are analyzed. Using varicap, about six times enhancement in sensitivity (~1.15 kHz/ppm) could be achieved. A simple and additive electronic method of tuning is utilized to improve sensitivity as compared with other methods that involve major changes in the sensor.