Coorg R. Prasad

Desorption/ionization of biomolecules from aqueous solutions at atmospheric pressure using an infrared laser at 3 μm

A new atmospheric pressure (AP) infrared (IR) matrix-assisted laser desorption/ionization (MALDI) ion source was developed and interfaced with a Thermo Finnigan LCQ ion trap mass spectrometer. The source utilized a miniature all-solid-state optical parametric oscillator (OPO)-based IR laser system tunable in the λ = 1.5–4 μm spectral range and a nitrogen ultraviolet (UV) laser (λ = 337 nm) for use in comparative studies. The system demonstrated comparable performance at 3 μm and 337 ran wavelengths if UV matrices were used. However, AP IR-MALDI using a 3 μm wavelength showed good performance with a much broader choice of matrices including glycerol and liquid water. AP IR-MALDI mass spectra of peptides in the mass range up to 2000 Da were obtained directly from aqueous solutions at atmospheric conditions for the first time. A potential use of the new AP IR-MALDI ion source includes direct MS analysis of biological cells and tissues in a normal atmospheric environment as well as on-line coupling of mass spectrometers with liquid separation techniques.

Compact tunable Cr:LiSAF laser for infrared matrix-assisted laser desorption/ionization.

A tunable Cr:LiSAF laser-pumped optical parametric oscillator was used for mid-infrared matrix-assisted laser desorption/ionization (MALDI) mass spectrometry experiments. The mass spectra of substance P, bovine insulin and pd(T)10 in the 2.88-2.96 microns range showed excellent single shot signal quality (signal-to-noise-ratio, resolution) but poor shot-to-shot reproducibility. The reproducibility is expected to improve with more stable laser design. No correlation was found between the absorption spectrum of the matrix and the MALDI response.

Matrix-assisted laser desorption/ionization time-of-flight analysis of Bacillus spores using a 2.94 µm infrared laser

The performance of infrared (2.94 microm) and ultraviolet (337 nm) lasers were compared for analysis of purified spores of B. subtilis, B. cereus and B. globigii on a four-inch end-cap reflectron time-of-flight instrument. Infrared matrix-assisted laser desorption/ionization (IR-MALDI) mass spectra of these microorganisms displayed a larger number of biomarker peaks above m/z 4000, compared with UV-MALDI. Biomarker peaks were observed at higher m/z values with the IR laser.

Tunable, Narrow Linewidth, High Repetition Frequency Ce:LiCAF Lasers Pumped by the Fourth Harmonic of a Diode-Pumped Nd:YLF Laser for Ozone DIAL Measurements

Ozone plays a crucially important role in all aspects of human life, although it is only a trace gas present in the middle and low atmosphere. Variations in ozone concentration in the stratosphere have an affect on the protection of the earth’s biosphere from the harmful portion of the Sun’s ultraviolet rays. Tropospheric ozone initiates the formation of photochemical smog and in high concentrations is harmful to human health and vegetation. Also ozone has a significant influence on the earth radiation budget. Human activities have produced adverse effects on atmospheric ozone distribution, which it left unchecked could lead to catastrophic changes to the biosphere . Hence the continuous measurement of ozone with good spatial resolution over large regions of the globe is an important scientific goal. A remote sensing technique for the monitoring of ozone concentration based on differential absorption lidar (DIAL) has been established as a method providing rapid and precise time and spatial resolutions [Browell, 1989, Richter, 1997]. Ozone absorbs strongly in the UV over the 240 – 340 nm region and also in the IR at near 9.6 μm. A two-wavelength differential absorption technique in the UV is commonly used for ozone measurement. After obtaining the lidar signals at two neighboring wavelengths (onand off-line), the differential absorption due to ozone is obtained by taking the ratio of the two signals to eliminate the contribution to extinction from scattering commen to both signales. Since the ozone absorption in UV exhibits a smooth band structure, the separation between the onand offline wavelengths is required to be a few nanometers. A number of ground-based [Profitt & Langford, 1997] and aircraft-based DIAL [Richter et al.,1997] systems for monitoring ozone concentrations in the planetary boundary layer, the free troposphere and the stratosphere have been developed by research groups all over the world [McGee et al, 1995, Mc Dermit et al,1995, Carswell et al,1991, Sunesson, et al,1994]. Most of the ground-based ozone DIAL instruments utilize large excimer gas lasers and Raman wavelength shifters, or flashlamp pumped frequency tripled and quadrupled Nd:YAG lasers and dye lasers, which are large complex systems requiring considerable

Tunable IR differential absorption lidar for remote sensing of chemicals

Standoff sensors for rapid remote detection of chemical emissions from either clandestine chemical production sites, chemical and biological warfare agents, concealed internal combustion engine emissions or rocket propellants from missiles are required for several DoD applications. The differential absorption lidar (DIAL) operating in the infrared wavelengths has established itself as a very effective tool for rapidly detecting many of the chemicals, with sufficient sensitivity with a range of several kilometers. The wavelengths required for this task lie within the atmospheric window regions 3 to 5 micrometers and 8 to 12 micrometers . We are currently developing a differential absorption lidar (DIAL) tunable in the 3 to 5 micrometers range for detecting low concentrations of chemical species with high sensitivity (5 ppb) and accuracy (error < 10%) measurements for greater than 5 km range. We have successfully established the feasibility of an innovative frequency agile laser source which is the crucial component of the infrared DIAL. A diode-pumped ytterbium YAG laser was built for pumping and rapidly tuning an optical parametric oscillator (OPO) over the mid-infra red region. Good performance (5 mJ/pulse) of the laser and low threshold wide infra red tuning of OPO (2.2 - 3.1 micrometers ) were demonstrated. The simulated performance of the topographical IR-DIAL showed that 5 ppb concentration can be measured at 5 km range with a 35 cm telescope.

Portable digital lidar: a compact stand-off bioagent aerosol sensor

Remote detection of biological warfare agents (BWA) is crucial for providing early warning to ensure maximum survivability of personnel in the battlefield and other sensitive areas. Although the current generation of stand- off aerosol and fluorescence lidars have demonstrated stand- off detection and identification of BWA, their large size and cost make them difficult for field use. We have introduced a new eye-safe portable digital lidar (PDL) technique based on digital detection that achieves orders of magnitude reduction in the size, cost and complexity over the conventional lidar, while providing an equal or better sensitivity and range. Excellent performance has been obtained with two of our PDL sensors during two bio-aerosol measurement campaigns carried out at Dugway Proving Grounds. In the JFT 4.5 (Oct 98) tests, high aerosol sensitivity of 300 ppl of biosimulant particles at up to 3 km was demonstrated with an eye-safe wavelength (523nm) aerosol micro PDL that utilized a 8 inch telescope, <10(mu) J/pulse energy at 2.5kHz, photon counting digital detection and 2 sec averaging. For the JBREWS DFT (June 99) tests an eye-safe two wavelengths (523nm and 1.047mum) horizontally scanned, aerosol micro PDL with the same 8 inch telescope was utilized. With this lidar, high sensitivity, preliminary differentiation between natural and unusual clouds, and the ability to track the aerosol cloud location, their wind speed and direction were also demonstrated. Lidar simulations of both PDL and conventional analog detection have been performed. Based on these model calculations and experimental results an analysis and comparison of the inherent capabilities of two types of systems is given.

A Tunable, Narrow Linewidth, 1 kHz Ce:LiCAF Laser with 46% Efficiency

A Ce:LiCAF laser having the highest known conversion efficiency of 46% is described. This laser has a narrow linewidth of (0.2 - 0.3 nm), a1 kHz pulse repetition frequency, and is tunable in the 281-315 nm wavelength region. The laser is pumped by the fourth harmonic of a diode-pumped Nd:YLF laser and delivers up to 0.85 J/pulse of output energy. The obtained laser efficiency is close to theoretical maximal efficiency of this laser. The laser is developed for use with an ozone DIAL.

Compact, diode-pumped Yb:YAG laser with combination acousto-optic and passive Q-switch for LIDAR applications

We have developed a Yb:YAG laser equipped with both an acousto-optic and passive Q-switches. With this setup, we obtained 1 mJ/pulse energy and a pulse width of ~7 ns, at a repetition rate of 2 kHz.

A mid-IR DIAL system using interband cascade laser diodes

A compact, portable mid-IR differential absorption lidar system was built using Interband Cascade Lasers operating at 3.38 ¿m and 3.54 ¿m and its operation was demonstrated by measuring absorption of vapor phase ethanol.

Measurement of atmospheric formaldehyde profiles with a laser-induced fluorescence lidar

Formaldehyde is a trace species that plays a key role in atmospheric chemistry. It is an important indicator of nonmethane volatile organic compound emissions. Also, it is a key reactive intermediate formed during the photochemical oxidation in the troposphere. Because the lifetime of formaldehyde in the atmosphere is fairly short (several hours), its presence signals hydrocarbon emission areas. The importance of measuring formaldehyde concentrations has been recognized by the National Academys Decadal Survey and two of NASAs forthcoming missions the GEO-CAPE and GACM target its measurement. There are several techniques some of which are highly sensitive (detection limit ~ 50 parts-per-trillion) for in-situ measurement of formaldehyde and many reported atmospheric measurements. However there appear to be no reported standoff lidar techniques for range resolved measurements of atmospheric formaldehyde profiles. In this paper, we describe a formaldehyde lidar profiler based on differential laser induced fluorescence technique. The UV absorption band in the 352 - 357nm is well suited for laser excitation with frequency tripled Neodymium lasers and measuring the strong fluorescence in the 390 - 500nm region. Preliminary nighttime measurements of formaldehyde were demonstrated with a lidar using a commercial Nd:YAG laser (354.7 nm) with a rather large linewidth (~.02 nm). The measured sensitivity was ~1 ppb at 1 km with 100 meters range resolution even with this non-optimized system. In this paper we describe our approach for increasing the sensitivity by many orders and for daytime operation by improving the laser parameters (power and linewidth) and optimizing the receiver.