Design of an open path near-infrared diode laser sensor: application to oxygen, water, and carbon dioxide vapor detection

An open path diode laser sensor was constructed with near-infrared diode lasers and two-tone frequency-modulation spectroscopy. The sensor incorporates several novel features (such as digital signal-processing algorithms, a computerized line-locking routine, and discontinuous wavelength scanning) that are important in a field instrument. The sensor was used to monitor oxygen, water, and carbon dioxide in the near-infrared spectral range. For oxygen, an absorbance detection sensitivity of 2 × 10(-6) in a 10-Hz bandwidth was demonstrated with a GaALAs laser at 760.56 nm. The stability of the sensor was 0.1% over a period of 10 h when an absorbance of 6 × 10(-3) was monitored.     

Resonant cavity gas-phase polarimeter

A high-sensitivity polarimeter is demonstrated for application to gas-phase chirality measurement. This device is based on the physics of the eigenstates of a Fabry-Perot cavity, permitting improvement in the sensitivity with respect to the usual polarimeters. Typical measurements of rotations of 50 (±1) × 10(-)(5)° induced by the optical activity of (R)-(+)-limonene and (S)-(-)-limonene in the vapor phase are shown. A noise level corresponding to a rotation of 10(-)(6)° is experimentally demonstrated. Application to the polarimetric monitoring of an enantiomer mixing racemization of limonene in the gas phase is also presented.     

Xanthamide fluorescent dyes

Two derivatives of fluorescein, termed "xanthamides," were prepared from fluorescein, an inexpensive dye. Relative to fluorescein, which contains a 6-phenolic OH and a 2'-carboxyl, the first derivative (5) contains a carboxymethyl ether at the 6-position and a secondary amide of dimethylamine at the 2'-position. The second derivative (8) contains a corresponding 6-methyl ether and a secondary amide of isonipecotic acid at the 2'-position. Thus, both derivatives contain a single carboxyl group, making them monofunctional. Especially 8 is much more photostable (about 10 times) than either fluorescein or BODIPY FL dye when exposed to ordinary light (tungsten light bulb). When tested for relative response in a capillary electrophoresis instrument fitted with an argon ion laser detector (488 nm) and a broad band emission filter, 8 was found to be 4-fold less bright than fluorescein. Both xanthamides, consistent with prior literature on 6-O-alkylated fluorescein, exhibit relatively pH-independent fluorescence (pH 4-10 was tested here). Because they possess two absorbance maximums, the xanthamides can be excited over a broader wavelength range than fluorescein or BODIPY. These collective properties of the xanthamides will make them advantageous over fluorescein and BODIPY dyes for some applications.     

High-pressure stopped-flow spectrometer for kinetic studies of fast reactions by absorbance and fluorescence detection

The development of a stopped-flow instrument that operates over a temperature range of -40 to +100 °C and up to 200 MPa is described. The system has been designed so that measurements can be performed in absorbance and fluorescence modes simultaneously, without dismantling the unit. It can easily be combined with an optical system of a conventional ambient pressure setup by using light guides. Optimum optical performance and a wide operating wavelength range (220-850 nm) are achieved as the light is not passing through the pressurizing fluid. A special design for the pistons has been developed; thus, the apparatus has proven to be leak-free, even under extreme conditions (high pressure, low temperature, various solvents). The dead time of the system is found to be less than 2 ms at 298 K and is pressure independent up to 200 MPa. We examined the kinetics for the formation of the Mg(2+)-8-hydroxyquinoline chelate in aqueous solutions at pH 8.0 in order to develop a convenient alternative test method for high-pressure stopped-flow spectrometers with absorption and fluorescence detection.     

MASERATI: a RocketBorne tunable diode laser absorption spectrometer

The MASERATI (middle-atmosphere spectrometric experiment on rockets for analysis of trace-gas influences) instrument is, to our knowledge, the first rocket-borne tunable diode laser absorption spectrometer that was developed for in situ measurements of trace gases in the middle atmosphere. Infrared absorption spectroscopy with lead salt diode lasers is applied to measure water vapor and carbon dioxide in the altitude range from 50 to 90 km and 120 km, respectively. The laser beams are directed into an open multiple-pass absorption setup (total path length 31.7 m) that is mounted on top of a sounding rocket and that is directly exposed to ambient air. The two species are sampled alternately with a sampling time of 7.37 ms, each corresponding to an altitude resolution of approximately 15 m. Frequency-modulation and lock-in techniques are used to achieve high sensitivity. Tests in the laboratory have shown that the instrument is capable of detecting a very small relative absorbance of 10(-4)-10(-5) when integrating spectra for 1 s. The instrument is designed and qualified to resist the mechanical stress occurring during the start of a sounding rocket and to be operational during the cruising phase of the flight when accelerations are very small. Two almost identical versions of the MASERATI instrument were built and were launched on sounding rockets from the And?ya Rocket Range (69 degrees N) in northern Norway on 12 October 1997 and on 31 January 1998. The good technical performance of the instruments during these flights has demonstrated that MASERATI is indeed a new suitable tool to perform rocket-borne in situ measurements in the upper atmosphere.     

Direct solar spectral irradiance and transmittance measurements from 350 to 2500 nm

A radiometrically stable, commercially available spectroradiometer was used in conjunction with a simple, custom-designed telescope to make spectrally continuous measurements of solar spectral transmittance and directly transmitted solar spectral irradiance. The wavelength range of the instrument is 350-2500 nm and the resolution is 3-11.7 nm. Laboratory radiometric calibrations show the instrument to be stable to better than 1.0% over a nine-month period. The instrument and telescope are highly portable, can be set up in a matter of minutes, and can be operated by one person. A method of absolute radiometric calibration that can be tied to published top-of-the-atmosphere (TOA) solar spectra in valid Langley channels as well as regions of strong molecular absorption is also presented. High-altitude Langley plot calibration experiments indicate that this technique is limited ultimately by the current uncertainties in the TOA solar spectra, approximately 2-3%. Example comparisons of measured and modtran-modeled direct solar irradiance show that the model can be parameterized to agree with measurements over the large majority of the wavelength range to the 3% level for the two example cases shown. Side-by-side comparisons with a filter-based solar radiometer are in excellent agreement, with a mean absolute difference of tau = 0.0036 for eight overlapping wavelengths over three experiment days.     

Instrument for long-path spectral extinction measurements in air: application to sizing of airborne particles

A novel instrument that is capable of taking spectral extinction measurements over long optical paths (approximately 1-100 m) in the UV, visible, and IR ranges is described. The instrument is fully automated, and the extinction spectrum is acquired in almost real time (approximately 5-10 s) with a resolution of ~3 nm. Its sensitivity and accuracy were estimated by tests carried out in a clean room that showed that, for optical paths between 50 and 100 m, the extinction coefficient can be detected at levels of ~10(-5) m(-1). Tests carried out on calibrated latex particles showed that, when it was combined with an appropriate inversion method, the technique could be profitably applied to characterize airborne particulate distributions. By carrying out measurements over optical paths of ~100 m, the instrument is also capable of detecting extinction coefficients that are due to aerosol concentrations well below the limits imposed by the European Economic Community for atmospheric pollution (150 mug/m(3)). Scaled over optical paths of ~10 m, the limit imposed for particle emissions from industrial plants (10 mg/m(3)) can also be detected sensitively.     

Continuous automated measurement of hexavalent chromium in airborne particulate matter

An automated continuous instrument for the collection and measurement of aerosol Cr(VI) is described. The system alternately collects the sample on one of two glass fiber filters. After 15-min sample collection on one filter, the sampling switches over to the second filter. The freshly sampled filter is washed for 8.5 min, and the washings are preconcentrated on a minicolumn packed with anion exchange resin. The washed filter is dried with filtered hot air for the next 6.5 min so that it is ready for sampling at the end of the 15-min cycle. The preconcentrated Cr(VI) on the column is eluted with 0.1 M sodium perchlorate and then reacted with sym-1,5-diphenylcarbazide prior to absorbance detection with a light emitting diode-based dedicated flow-through absorbance detector. The detection limit (S/N = 3) is 5 ng of Cr(VI)/m3, orders of magnitude lower than current regulatory requirements. The instrument operates unattended over long periods. In continuous round-the-clock operation, filter replacement frequency is every 24-72 h, depending on dust loading. The system is portable for facile field deployment and permits fully automated rapid determinations at a very low analysis cost per sample.     

Trace moisture detection using continuous-wave cavity ring-down spectroscopy

We have developed an instrument to measure trace concentrations of small hydride species in gases using continuous-wave cavity ring-down spectroscopy with near-infrared diode laser excitation. An rms baseline equivalent absorbance of 9.2 x 10(-11) cm(-1)/square root(n) is found, where n is the number of ring-down transients. When the 1396.376-nm absorption line of water is used, this corresponds to a noise equivalent moisture concentration in nitrogen gas of 68 pptv/square root(n). Water vapor concentration is detected over a range extending from 3 to 1000 ppbv and found to depend linearly on the concentration as determined by a calibrated commercial moisture sensor.     

Fast membrane osmometer as alternative to freezing point and vapor pressure osmometry

Osmometry is an essential technique for solution analysis and the investigation of chemical and biological phenomena. Commercially available osmometers rely on the measurements of freezing point, vapor pressure, and osmotic pressure of solutions. Although vapor pressure osmometry (VPO) and freezing point osmometry (FPO) can perform rapid and inexpensive measurements, they are indirect techniques, which rely on thermodynamic assumptions, which limit their applicability. While membrane osmometry (MO) provides a potentially unlimited direct measurement of osmotic pressure and solution osmolality, the conventional technique is often time-consuming and difficult to operate. In the present work, a novel membrane osmometer is presented. The instrument significantly reduces the conventional MO measurement time and is not subject to the limitations of VPO and FPO. For this paper, the osmotic pressure of aqueous sucrose solutions was collected in a molality range 0-5.5, by way of demonstration of the new instrument. When compared with data found in the literature, the experimental data were generally in good agreement. However, differences among results from the three techniques were observed.     

Fiber-optic-graded-index-lens absorbance sensor with wavelength-scanning capability

A single optical-fiber absorbance sensor that contains a graded-index lens is described. The sensor can be tailored for a desired broad wavelength region, path length, and size. Laboratory evaluations of sensors of varying sizes and path lengths are presented. The sensors, even without the added expense of optical coatings, report true absorbance spectra for the 420-850-nm wavelength region with a linear response over a wide absorbance range. Direct comparison with several other sensor configurations shows that the graded-index-lens-based sensor has a high optical efficiency. Potential applications of the sensor include absorbance measurements at hazardous or remote sites and in vivo medical applications.     

Chemical ionization mass spectrometer instrument for the measurement of tropospheric HO2 and RO2

Laboratory characterizations of the peroxy radical chemical ionization mass spectrometer (PerCIMS) instrument have been performed. The instrument functions by drawing ambient air through a 50-microm-diameter orifice into an inlet held at low pressure. Peroxy radicals (HO2 and RO2) within this air are detected by amplified chemical conversion into a unique ion (HSO4-) via the chemistry initiated by the addition of NO and SO2 to the inlet. HSO4- ions are then quantified by a quadrupole filter mass spectrometer. PerCIMS provides measurements of the sum of peroxy radicals, HO2 + RO2 (HOxROx mode), or the HO2 component only (HO2 mode), achieved through the control of concentration of NO and SO2 added to the instrument. The characterization and response of this instrument have been evaluated through modeling of inlet chemistry and laboratory experiments and have also been demonstrated through successful deployment during field campaigns. The performance of PerCIMS with respect to calibration pressure and relative humidity is reported, as are the sensitivities of the instrument to organic peroxy radicals with different hydrocarbon groups. These data show PerCIMS to be a practical field instrument for the fast and accurate evaluation of the concentration of peroxy radicals over a variety of atmospheric conditions. The estimated accuracy of the derived [HOxROx] concentrations is +/- 35% (at the 95% confidence interval), while [HO2] measurements have accuracies of +/- 41% (at the 95% confidence interval). Typical precision of measurements well above the detection limit is 10%, and typical detection limits are 1 x 10(7) radicals cm(-3) for 15-s averaging times.     

Cavity-enhanced quantum-cascade laser-based instrument for carbon monoxide measurements

An autonomous instrument based on off-axis integrated cavity output spectroscopy has been developed and successfully deployed for measurements of carbon monoxide in the troposphere and tropopause onboard a NASA DC-8 aircraft. The instrument (Carbon Monoxide Gas Analyzer) consists of a measurement cell comprised of two high-reflectivity mirrors, a continuous-wave quantum-cascade laser, gas sampling system, control and data-acquisition electronics, and data-analysis software. CO measurements were determined from high-resolution CO absorption line shapes obtained by tuning the laser wavelength over the R(7) transition of the fundamental vibration band near 2172.8 cm(-1). The instrument reports CO mixing ratio (mole fraction) at a 1-Hz rate based on measured absorption, gas temperature, and pressure using Beer's Law. During several flights in May-June 2004 and January 2005 that reached altitudes of 41,000 ft (12.5 km), the instrument recorded CO values with a precision of 0.2 ppbv (1-s averaging time) and an accuracy limited by the reference CO gas cylinder (uncertainty < 1.0%). Despite moderate turbulence and measurements of particulate-laden airflows, the instrument operated consistently and did not require any maintenance, mirror cleaning, or optical realignment during the flights.     

Miniature Personal Ozone Monitor Based on UV Absorbance

A new portable instrument has been developed that offers rapid detection of ozone at the ppb level for personal exposure monitoring. The Personal Ozone Monitor is based on the EPA Federal Reference Method of UV absorbance and has the advantage of being small (10 × 7.6 × 3.8 cm), light-weight (0.3 kg), low power (2.9 watts), and battery-operated. The instrument can be worn by an individual during normal daily activities because it is unaffected by humidity, physical orientation, temperature, and vibration. In order to eliminate any significant interference from water vapor, Nafion(?) tubing was installed before the detection cell, and the optical path was lined with quartz. A precision of 1.5 ppbv and limit of detection of 4.5 ppbv (S/N = 3) was demonstrated with the instrument making measurements every 10 seconds.     

Detection of minute chemical species by principal-component analysis

A novel analytical technique based on the detection of minute bands in a mixture spectrum with the use of principal-component analysis (PCA) is presented. This new aspect of PCA indicates that overlapped spectra of some components can be separated with no a priori knowledge of the components when the absorbances of the components vary greatly. This technique can be used for the detection of minute chemical species. The concept was confirmed by computer simulations. In the simulations, abstract spectra (loading vectors) were successfully obtained, and the changes of the component absorbances were also successfully followed semiquantitatively by calculating their scores. The method developed with PCA was applied to the analysis of infrared reflection-absorption (RA) spectra to study molecular interaction mechanism between alkyl-deuterated dipalmitoylphosphatidylcholine (DPPC-d(62)) monolayer and sucrose. The samples were Langmuir-Blodgett (LB) films of the DPPC-d(62) monolayer that was prepared on a sucrose solution. The LB films consisted of the following phases: air/DPPC-d(62) + sucrose/sucrose/substrate (gold). The abstract spectra corresponding to "DPPC-d(62) + sucrose" and "sucrose" phases were successfully separated by PCA, and the absorbance change of sucrose in each phase was semiquantitatively calculated from the score. The absorbance change was experimentally confirmed with quartz-crystal microbalance (QCM) experiments. In addition, minute water molecules that remained in the LB films after drying were readily detected from an abstract spectrum, and their binding site was found to be the phospholipid moiety in the head group of DPPC-d(62).     

Phosphorescence of erythrosin B as a robust probe of molecular mobility in amorphous solid sucrose

Luminescence from the triplet probe erythrosin B (tetra-iodo fluorescein, Ery B) provides spectroscopic characteristics such as lifetime and emission energy that are sensitive to molecular mobility of the local environment in amorphous solids. This study investigated how variations in the local concentration of Ery B free acid as well as the presence of the dispersing solvent affect the spectroscopic measurements of solid matrix properties (the free acid of Ery B is poorly soluble in water and thus must be introduced via an organic solvent). The emission energy of Ery B from 5 to 100 degrees C in thin films of amorphous sucrose at various probe and solvent (N,N-dimethyl formamide, DMF) concentrations was determined using excitation at 500 nm and emission over the range 520-750 nm. The emission lifetime was determined over the same temperature range using a stretched exponential analysis of intensity decays collected using excitation at 530 nm and emission at 680 nm. Variations in the probe/sucrose mole ratio (concentration) over the range from 0.5 to 10 x 10(-4) and 10-fold variations in the amount of DMF used to disperse the probe did not affect the emission energy, the shape of the emission spectra, or the measured lifetimes of Ery B in amorphous sucrose. These results thus indicate that erythrosin B introduced into amorphous solids can provide a robust measure of the intrinsic mobility of the solid matrix that is relatively insensitive to final probe concentration or presence of residual solvent.     

Testing carbon sequestration site monitor instruments using a controlled carbon dioxide release facility

Two laser-based instruments for carbon sequestration site monitoring have been developed and tested at a controlled carbon dioxide (CO(2)) release facility. The first instrument uses a temperature tunable distributed feedback (DFB) diode laser capable of accessing the 2.0027-2.0042 microm spectral region that contains three CO(2) absorption lines and is used for aboveground atmospheric CO(2) concentration measurements. The second instrument also uses a temperature tunable DFB diode laser capable of accessing the 2.0032-2.0055 mum spectral region that contains five CO(2) absorption lines for underground CO(2) soil gas concentration measurements. The performance of these instruments for carbon sequestration site monitoring was studied using a newly developed controlled CO(2) release facility. A 0.3 ton CO(2)/day injection experiment was performed from 3-10 August 2007. The aboveground differential absorption instrument measured an average atmospheric CO(2) concentration of 618 parts per million (ppm) over the CO(2) injection site compared with an average background atmospheric CO(2) concentration of 448 ppm demonstrating this instrument's capability for carbon sequestration site monitoring. The underground differential absorption instrument measured a CO(2) soil gas concentration of 100,000 ppm during the CO(2) injection, a factor of 25 greater than the measured background CO(2) soil gas concentration of 4000 ppm demonstrating this instrument's capability for carbon sequestration site monitoring.     

A submersible autonomous sensor for spectrophotometric pH measurements of natural waters

An autonomous sensor for long-term in situ measurements of the pH of natural waters is described. The system is based upon spectrophotometric measurements of a mixture of sample and sulfonephthalein indicator. A simple plumbing design, using a small, low-power solenoid pump and valve, avoids the need for quantitative addition of indicator. A approximately 50-microL slug of indicator is pulled into the sample stream by the pump, and subsequent pumping and mixing results in a section of indicator and sample where absorbance measurements can be made. The design also permits direct determination of the indicator pH perturbation. Absorbances are recorded at three wavelengths (439, 579, and 735 nm) using a custom-built 1.7-cm path length fiber-optic flow cell. Solution blanks are obtained by periodically flushing the cell with sample. Field tests were performed in a local river over an 8-day period. The in situ accuracy, based on comparison with laboratory spectrophotometric pH measurements, was -0.003 pH unit (n = 16), similar to the measurement precision. No drift was observed during the 8-day period. The absorbance ratio used to calculate pH, in combination with a simple and robust optical design, imparts an inherent stability not achievable with conventional potentiometric methods, making the design feasible for long-term autonomous pH measurements.     

Automated broad tuning of difference frequency sources for spectroscopic studies

Transmission spectroscopy over large spectral ranges (>100?cm(-1)) generally requires a reference measurement to be taken separately from the sample scan. The ratio of the two measurements (i.e., the transmittance) is therefore susceptible to baseline changes that occur between the recording of the two spectra. The origins of relatively strong baseline changes (?1%) of a difference-frequency-generation-based laser spectrometer (tuning range 2900-3144?cm(-1), 150?μW average power) were investigated and a method for minimizing them by improving reproducibility and reducing measurement time is presented. The new method was tested for a gas mixture and the sensitivity for broad absorption features was determined as 5×10(-3) minimum measurable absorbance for a total scan duration of 70?min.     

Changes in the Radiometric sensitivity of SeaWiFS determined from lunar and solar-based measurements

We report on the lunar and solar measurements used to determine the changes in the radiometric sensitivity of the Sea-viewing Wide Field-of-view Sensor (SeaWiFS). Radiometric sensitivity is defined as the output from the instrument (or from one of the instrument bands) per unit spectral radiance at the instrument's input aperture. Knowledge of the long-term repeatability of the SeaWiFS measurements is crucial to maintaining the quality of the ocean scenes derived from measurements by the instrument. For SeaWiFS bands 1-6 (412-670 nm), the change in radiometric sensitivity is less than 0.2% for the period from November 1997 through November 1998. For band 7 (765 nm), the change is approximately 1.5% and for band 8 (865 nm) approximately 5%. The rates of change of bands 7 and 8, which were linear with time for the first eight months of lunar measurements, are now slowing. The scatter in the data points about the trend lines in this analysis is less than 0.3% for all eight SeaWiFS bands. These results are based on monthly measurements of the moon. Daily solar measurements using an onboard diffuser show that the radiometric sensitivities of the SeaWiFS bands have changed smoothly during the time intervals between lunar measurements. Because SeaWiFS measurements have continued past November 1998, the results presented here are considered as a snapshot of the instrument performance as of that date.