Garett M. Leskowitz

Force-detected magnetic resonance without field gradients

A novel method of nuclear magnetic resonance (NMR) is described which promises to be preferable to known general methods at sample length scales below approximately 100 microm. Its advantages stem from the seemingly paradoxical combination of a homogeneous static magnetic field and detection of a mechanical force between a spin-bearing sample and a magnet assembly. In contrast to other methods of force-detected nuclear magnetic resonance (FDNMR), the method is characterized by better observation of magnetization, enhanced resolution, and no gradient (BOOMERANG), and it is generally applicable with respect to sample composition, pulse sequence, and magnetic field strength. Further advantages of portability and low cost stem from the small instrument volume and mass and promise to extend the use of NMR to new applications and environments. A sensitivity analysis, relevant to spectroscopy or imaging, quantifies the advantage of BOOMERANG relative to magnetic induction using microcoils and to FDNMR methods that rely on large gradients of the magnetic field at the sample.

Evanescent-wave spectroscopy down infrared transmitting optical fibers

The evanescent-wave spectrum of a sample surrounding the core of an optical fiber is a complex function of the optical constants of the media involved as well as the geometry of the sensing fiber. We develop a simple theory for evanescent-wave absorption in the weak absorption limit where we show that the absorbance of a length of sensor fiber may be related linearly to the bulk sample absorption coefficient. We present experimental data that verifies the observed scaling between the evanescent-wave absorbance and the bulk absorption coefficient for an isopropanol sample. The application of evanescent-wave spectroscopy with different sensor fiber materials is discussed, along with experimental and theoretical data for the enhancement of evanescent-wave spectroscopy using tapered fibers. Finally we discuss the results of a numerical series of calculations based on the exact ray paths of radiation within the fiber and the fundamental theory of ATR absorption at an interface assuming a plane wave approximation. In the more complex theory the evanescent-wave absorption coefficient is a decreasing function of the bulk absorption coefficient.

Fourier transform infrared (FTIR) fiber optic monitoring of composites during cure in an autoclave

Real time in situ monitoring of the chemical states of epoxy resins were investigated during cure in an autoclave using infrared evanescent spectroscopy. Fiber evanescent sensors were developed which may be sandwiched between the plies of the prepreg sample. In this work a short length of sapphire fiber as the sensor cell portion of the fiber probe was utilized. Heavy metal fluoride glass (HMFG) optical fiber cables were designed for connecting the FTIR spectrometer to the sensor fiber within the autoclave. The sapphire fibers have outstanding mechanical properties (Youngs Modulus 65 = Mpsi) and outstanding thermal properties (T. = 2000°C) which should permit their use as an embedded link in all thermoset composites. The system is capable of operation at a temperature of 250°C (482 °F) for periods up to 8 hours without major changes to the fiber transmission. A discussion of the selection of suitable sensor fibers, the construction of a fiber optic interface, and the interpretation of in situ infrared spectra of the curing process is pre-sented.

MEMS-based force-detected nuclear magnetic resonance spectrometer for in situ planetary exploration

NMR is the most widely used spectroscopic technique for characterization of molecular structures and reactions. NMR is especially powerful in detecting the presence of water and distinguishing between arbitrary physisorbed and chemisorbed states. This ability is particularly important in the search for extraterrestrial life on planets such as Mars, where there are strong indications that liquid water exists or has existed previously. Conventional NMR technology based on magnetic induction is unsuitable for remote applications because these instruments are heavy and require excessive power. We recently demonstrated a new NMR technology based on magnetic force detection that is suitable for NMR analysis in remote environments and on small samples. This paper details our efforts toward realizing this technology on small scales using a MEMS approach. Applications of this new MEMS-based NMR method to planetary exploration are discussed.

Applications of IR-transmitting optical fiber in the chemical industry

Infrared transmitting heavy metal fluoride optical fiber has been used to separate an FTIR analyzer from a remote measurement point. Several types of remote sensors have been developed for species concentration measurements. Remote transmission cells connected to fiber cables have been used for the measurement of spectra of liquids and gases. Evanescent wave probes have been developed to obtain spectra in highly absorbing and highly scattering media. Remote spectra taken with an FTIR fiber-optic analyzer in the 8000 - 2000 cm1 spectral region are presented. A calculation of detectability limits for these species based on the measured data will be presented. A discussion of sensor multiplexing applied to remote fiber optic FTIR spectroscopy will be given.

MEMS-Based Micro Instruments for In-Situ Planetary Exploration

NASAs planetary exploration strategy is primarily targeted to the detection of extant or extinct signs of life. Thus, the agency is moving towards more in-situ landed missions as evidenced by the recent, successful demonstration of twin Mars Exploration Rovers. Also, future robotic exploration platforms are expected to evolve towards sophisticated analytical laboratories composed of multi-instrument suites. MEMS technology is very attractive for in-situ planetary exploration because of the promise of a diverse and capable set of advanced, low mass and low-power devices and instruments. At JPL, we are exploiting this diversity of MEMS for the development of a new class of miniaturized instruments for planetary exploration. In particular, two examples of this approach are the development of an Electron Luminescence X-ray Spectrometer (ELXS), and a Force-Detected Nuclear Magnetic Resonance (FDNMR) Spectrometer. The ELXS is a compact (< 1 kg) electron-beam based microinstrument that can determine the chemical composition of samples in air via electron-excited x-ray fluorescence and cathodoluminescence. The enabling technology is a 200-nm-thick, MEMS-fabricated silicon nitride membrane that encapsulates the evacuated electron column while yet being thin enough to allow electron transmission into the ambient atmosphere. The MEMS FDNMR spectrometer, at 2-mm diameter, will be the smallest NMR spectrometer in the world. The significant innovation in this technology is the ability to immerse the sample in a homogenous, uniform magnetic field required for high-resolution NMR spectroscopy. The NMR signal is detected using the principle of modulated dipole-dipole interaction between the samples nuclear magnetic moment and a 60-micron-diameter detector magnet. Finally, the future development path for both of these technologies, culminating ultimately in infusion into space missions, is discussed.

Multiplexed sensor systems in quantitative FTIR process spectroscopy

A series of FT-IR spectrometer based remote sensing systems have been developed taking advantage of the new technology of IR transmitting optical fibers. The systems may be used to monitor the chemical composition of solid, liquid and gas phase samples. An array of remote sensors may be interfaced to a single FT-IR spectrometer through a multi-fiber launch module. An optical channel selector (OCS) allows the sensors to be addressed with a single opto-mechanically multiplexed detector system. Remote collimated beam sensors have been developed for web monitoring and liquid and gas phase sensing. An optimized multi-detector web monitoring system has been developed for moving web sensing on optically dttuse webs. Quantitative data will be presented for a number of remote spectroscopic measurements.

Fiber-Optic Chemical Sensing With Infrared-Transmitting Optical Fiber

Infrared transmitting heavy metal fluoride optical fiber has been used to separate a Fourier-transform infrared (FTIR) spectrometer from a remote measurement point. Several types of remote sensors have been developed for concentration measurements. Remote transmission cells connected to fiber cables have been used to measure near-infrared spectra of liquids and gases. An evanescent-wave probe for obtaining spectra of highly scattering samples has been developed. Fiber-optic FTIR may be used to solve many problems in process monitoring and control.