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G M Hieftje

Publications and source records attributed to G M Hieftje.

18 recordsLinked to original sources
PubMed2005

Read-noise characterization of focal plane array detectors via mean-variance analysis.

Mean-variance analysis is described as a method for characterization of the read-noise and gain of focal plane array (FPA) detectors, including charge-coupled devices (CCDs), charge-injection devices (CIDs), and complementary metal-oxide-semiconductor (CMOS) multiplexers (infrared arrays). Practical FPA detector characterization is outlined. The nondestructive readout capability available in some CIDs and FPA devices is discussed as a means for signal-to-noise ratio improvement. Derivations of the equations are fully presented to unify understanding of this method by the spectroscopic community.

Algorithms↗
PubMed2001

Standardless semiquantitative analysis of metals using single-shot laser ablation inductively coupled plasma time-of-flight mass spectrometry.

A method has been developed that allows the accurate, standardless measurement of the elemental composition of metal samples from single laser ablation (LA) pulses. This technique provides a fast, low-sample-consumption means for the characterization of samples having a range of matrixes. The method directly compares adjusted elemental signals with the total mass spectrometric signal to produce relative percent composition information. Three mathematical techniques were used to determine the accuracy and precision of single-shot LA measurement. Comparison of the techniques showed that a linear regression calculation, which plots individual elemental signals as a function of the summed signal for all elements in the sample on a point-by-point basis during a laser ablation transient proved superior. The simultaneous extraction capability of time-of-flight mass spectrometry permits the sampling of all analytes from any temporal position within the transient laser ablation pulse, thereby reducing quantitation error. A typical concentration dynamic range of 3 orders of magnitude, from 0.1 to 100%, was achieved. However, by measuring low-abundance isotopes for matrix elements, the dynamic range of the technique was extended to 4 orders of magnitude. The new technique is largely immune to sample matrix effects commonly experienced in laser ablation. By performing a complete elemental analysis from a single ablation pulse, high spatial resolution should be achieved.

Journal Article↗
PubMed1999

Use of analyte-modulated modal power distribution in multimode optical fibers for simultaneous single-wavelength evanescent-wave refractometry and spectrometry.

A new method is described for the simultaneous determination of absorbance and refractive index of a sample medium. The method is based on measurement of the analyte-modulated modal power distribution (MPD) in a multimode waveguide. In turn, the MPD is quantified by the far-field spatial pattern and intensity of light, i.e., the Fraunhofer diffraction pattern (registered on a CCD camera), that emerges from a multimode optical fiber. Operationally, light that is sent down the fiber interacts with the surrounding analyte-containing medium by means of the evanescent wave at the fiber boundary. The light flux in the propagating beam and the internal reflection angles within the fiber are both affected by optical absorption connected with the analyte and by the refractive index of the analyte-containing medium. In turn, these angles are reflected in the angular divergence of the beam as it leaves the fiber. As a result, the Fraunhofer diffraction pattern of that beam yields two parameters that can, together, be used to deduce refractive index and absorbance. This MPD based detection offers important advantages over traditional evanescent-wave detection strategies which rely on recording only the total transmitted optical power or its lost fraction. First, simultaneous determination of sample refractive index and absorbance is possible at a single probe wavelength. Second, the sensitivity of refractometric and absorption measurements can be controlled simply, either by adjusting the distance between the end face of the fiber and the CCD detector or by monitoring selected modal groups at the fiber output. As a demonstration of these capabilities, several weakly absorbing solutions were examined, with refractive indices in the range from 1.3330 to 1.4553 and with absorption coefficients in the range 0-16 cm-1. The new detection strategy is likely to be important in applications in which sample coloration varies and when it is necessary to compensate for variations in the refractive index of a sample.

Absorption↗
PubMed1999

Preliminary investigation of electrothermal vaporization sample introduction for inductively coupled plasma time-of-flight mass spectrometry.

The coupling of an electrothermal vaporization (ETV) apparatus to an inductively coupled plasma time-of-flight mass spectrometer (ICP-TOFMS) is described. The ability of the ICP-TOFMS to produce complete elemental mass spectra at high repetition rates is experimentally demonstrated. A signal-averaging data acquisition board is employed to rapidly record complete elemental spectra throughout the vaporization stage of the ETV temperature cycle; a solution containing 34 elements is analyzed. The reduction of both molecular and atomic isobaric interferences through the temperature program of the furnace is demonstrated. Isobaric overlaps among the isotopes of cadmium, tin, and indium are resolved by exploiting differences in the vaporization characteristics of the elements. Figures of merit for the system are defined with several different data acquisition schemes capable of operating at the high repetition rate of the TOF instrument. With the use of both ion counting and a boxcar averager, the dynamic range is shown to be linear over a range of at least 6 orders of magnitude. A pair of boxcar averagers are used to measure the isotope ratio for silver with a precision of 1.9% RSD, despite a cycle-to-cycle precision of 19% RSD. Detection limits of 10-80 fg are calculated for seven elements, based upon a 10-microL injection.

Mass Spectrometry↗
PubMed1998

Determination of halogenated hydrocarbons by helium microwave plasma torch time-of-flight mass spectrometry coupled to gas chromatography.

A helium microwave plasma torch (MPT) was coupled to time-of-flight mass spectrometry (TOFMS) for the detection of halogenated hydrocarbons separated by capillary gas chromatography (GC). The GC-MPT-TOFMS system offered excellent stability over the course of the experiments and avoided mass spectral peak distortions caused by spectral skew. In the initial studies, empirical formulas based on the halogen-to-carbon ratio were predicted utilizing a flow cell apparatus. The MPT proved to be very robust and could handle large amounts of organic vapor. Results from this study indicate that, for both aromatic and aliphatic halogenated hydrocarbons, the ratios of carbon to chlorine signals correlate well (r = 0.994) with the ones expected from their chemical composition. This study was later extended to include chromatographic separation. For a series of homologous aliphatic halogenated hydrocarbons, a correlation coefficient of 0.999 was obtained for both peak heights and peak areas obtained from a single chromatogram. A novel Nichrome wire-heated transfer line was developed to ensure that the capillary column was heated efficiently from the GC oven to the MPT and then through the length of the MPT up to the microwave plasma itself. No appreciable peak broadening and no detectable memory effects were associated with the heated transfer line. The GC-MPT-TOFMS system offered equal sensitivity for I, Br, and Cl. Absolute detection limits for the halogenated hydrocarbons ranged from 160 to 330 fg, constituting an improvement by a factor of 5-35 over earlier results obtained with MIPs supported in a TM010 cavity and combined with quadrupole-based mass spectrometry. In addition, the effect of molecular gases on the MPT performance was investigated. Up to about 1% (v/v) of either oxygen or hydrogen in the central channel helium flow attenuated the signal levels for both carbon and chlorine, with the larger loss seen in the chlorine signal.

Gas Chromatography-Mass Spectrometry↗
PubMed1998

Optical time-of-flight chemical detection: absorption-modulated fluorescence for spatially resolved analyte mapping in a bidirectional distributed fiber-optic sensor.

A continuous chemically sensitive optical fiber is used with optical time-of-flight chemical detection (OTOF-CD) for spatially resolved analyte mapping. To enhance signal levels and to improve their reproducibility, two novel principles for signal generation and processing are introduced. In the first, the fluorescene of an analyte-insensitive fluorophore is monitored as a function of the evanescent wave absorption of an analyte-sensitive indicator. The resulting signal levels are well above those encountered in optical time domain reflectometry methods that rely upon backscattering for spatially resolved detection. As a result, the method could significantly expand the range of species that can be detected with absorption reagents used in OTOF sensors. The second method raises signal-to-noise ratios by 3-4.5-fold for measurements made at the far ends of the sensing fiber. It functions by sending probe laser pulses into and monitoring their return sequentially from both ends of the sensing fiber. Because the two pulses provide complementary information, only the first half of each of the collected waveforms is used for analyte quantitation. The introduced concepts were experimentally verified with a distributed sensor constructed from a 40-m-long continuous chemically sensitive optical fiber. This sensing element was produced by immobilization of an ammonia-sensitive absorbing reagent (phenol red) and an analyte-insensitive fluorophore (rhodamine 640) into the original silicone cladding of the plastic-clad silica fiber.

Fiber Optic Technology↗
PubMed1998

Cerenkov radiation as a UV and visible light source for time-resolved fluorescence.

We demonstrate the first use of Cerenkov radiation for the measurement of fluorescence lifetimes. Relativistic beta particles from the nuclear decay of 90 Sr and 90Y generate a spectral continuum in a quartz waveguide. Light flashes of < 100-ps duration are delivered simultaneously to a sample cell and a reference photomultiplier. A simple, digitally based cross-correlation signal processor allows extraction of the sample fluorescence decay kinetics without distortions which can result from the random excitation pulse sequence. We characterize both the pulse duration and the pulse intensity of the light that is emitted from the waveguide. Although the excitation intensity is very weak, we demonstrate that accurate lifetime measurements are possible with only a few hundred seconds of integration time. Tests on a variety of compounds illustrate the utility of the light source throughout the UV and blue regions of the spectrum. We also discuss future design improvements and potential applications of this new approach to time-resolved fluorescence.

Fluorescence↗
PubMed1998

Adapting selected nucleic acid ligands (aptamers) to biosensors.

A flexible biosensor has been developed that utilizes immobilized nucleic acid aptamers to specifically detect free nonlabeled non-nucleic acid targets such as proteins. In a model system, an anti-thrombin DNA aptamer was fluorescently labeled and covalently attached to a glass support. Thrombin in solution was selectively detected by following changes in the evanescent-wave-induced fluorescence anisotropy of the immobilized aptamer. The new biosensor can detect as little as 0.7 amol of thrombin in a 140-pL interrogated volume, has a dynamic range of 3 orders of magnitude, has an inter-sensing-element measurement precision of better than 4% RSD over the range 0-200 nM, and requires less than 10 min for sample analysis. The aptamer-sensor format is generalizable and should allow sensitive, selective, and fast determination of a wide range of analytes.

Anisotropy↗
PubMed1998

Near-ultraviolet evanescent-wave absorption sensor based on a multimode optical fiber.

Fiber-optic near-ultraviolet evanescent-wave sensors have been constructed, and their feasibility for practical applications has been demonstrated. The sensors, used for the detection of ozone near the 254-nm peak of the Hartley absorption band, were fabricated from coiled segments of low-cost multimode plastic-clad silica optical fibers. The sensing sections were produced alternatively by stripping only the protective jacket from the fiber to expose the gas-permeable silicone cladding or by stripping the jacket and the cladding to expose the bare-silica fiber core. Response characteristics are given, including sensitivity to ozone, reversibility, and aging effects. The useful lifetime was unacceptably short for the sensor that employed the bare-silica core, whereas the exposed-cladding sensor demonstrated good stability over the entire two-month period of investigation. The latter, more useful sensor demonstrated a linear response to ozone over the range 0.02-0.35 vol% and a reversible response with a time constant on the order of 1 min. Differences in ozone absorption spectra obtained in the transmission and evanescent-wave modes are discussed. Projected applications of the new exposed-cladding sensor include ozone determination in water-treatment processes and ozone production plants.

Absorption↗
PubMed1998

Optical time-of-flight chemical detection: spatially resolved analyte mapping with extended-length continuous chemically modified optical fibers.

We theoretically evaluate and experimentally verify a novel strategy for spatially resolved analyte mapping over extended remote areas. The approach combines a method for the fabrication of continuous extended-length sensors with optical time-of-flight chemical detection (OTOF-CD). The use of OTOF-CD makes it possible to locate the zones in the fiber where attenuation or fluorescence takes place, to determine the magnitude of these variations, and to relate the magnitude of the variations to the local concentration or concentrations of a single analyte or several analytes. Simulation experiments suggest that OTOF-CD should provide spatial resolution close to its theoretical limit by deconvolution of the returned wave form with all time-dependent experimental variables (laser pulse width, reagent fluorescence lifetime, etc.). The signal-processing technique should be useful for a wide variety of sensors based on absorption, refractive index, or statically and dynamically quenched fluorescence. Experimental results with a model system (a 48-m-long oxygen sensor) compare favorably with those predicted by numerical simulations. Possible experimental difficulties in the realization of these novel sensors are discussed as are ways to overcome them.

Adsorption↗
PubMed1997

Scintillator light source for chemical sensing in the near-ultraviolet.

A novel chemical sensor based on a light source composed of a radionuclide and a scintillator is experimentally evaluated. Proper selection of a radionuclide/scintillator combination permits fabrication of a practical light source emitting in the ultraviolet (UV). Such a UV light source is critical for chemical sensors which utilize UV-excitable chromophores or fluorophores. Unlike conventional gas-filled discharge lamps, the developed UV source is compact, inexpensive, simple in design, stable, and highly reliable, and it does not require an external power source. The utility of the new source was demonstrated through construction of sensors for oxygen. This application was selected for experimental evaluation of the new light source since oxygen sensors have been characterized well with conventional light sources. Although the scintillator light source is less intense than conventional sources, its excellent short- and long-term stability provides a reproducibility of fluorescence measurements of about 0.35% RSD. The stability of the scintillator light source suggests its utility in simple single-beam detection configurations.

Light↗
PubMed1997

Fluorescence lifetime measurement via a radionuclide-scintillation light source and analog cross correlation.

beta-Emitting 90Sr is used with a plastic scintillator to produce excitation-light pulses for fluorescence lifetime analysis. This light source is less expensive, more compact, and much more reliable than traditionally employed excitation sources such as lasers or pulsed flash lamps. The pulse train from this light source varies randomly in amplitude and time. Cross-correlation signal analysis is ideal for such a source because, unlike other time domain techniques, cross correlation takes complete advantage of its random nature. Here we report on the construction of an instrument and the methods employed to make fluorescence lifetime measurements via the new source and an analog correlation processor. Although the light intensity of the scintillator-based excitation source is comparatively low, an adequate signal level can be generated. The fluorescence lifetimes of three fluorophores are measured with a 1-mCi radionuclide to demonstrate a lifetime range from less than 1.5 to 28 ns. Long-lifetime measurements require an extra calibration step in order to compensate for delay cable energy loss. The light collection efficiency of the current instrument was found to be undesirably low; improvements in the instrument optics are suggested that will increase the collection efficiency and enhance the detection capability.

Fluorescence↗
PubMed1997

Space charge evaluation in a plasma-source mass spectrograph.

Space charge effects, and the matrix interferences they cause, are problems in inductively coupled plasma mass spectrometry (ICPMS). It has previously been observed that these deleterious space charge effects are not significantly present in sector-field instruments, a fact that has been attributed, but not demonstrated, to the high accelerating potentials they commonly employ. To examine the significance of space charge in our plasma-source mass spectrograph (which operates at only moderate accelerating potentials) and in other sector instruments, a graphite disk was placed approximately 7 cm behind the skimmer. An inductively coupled plasma was operated for 17 h while a 0.01 mM multielement solution was introduced. This disk was then analyzed by spatially resolved laser ablation ICP time-of-flight MS. Second vacuum-stage acceleration appears to be an important factor that governs the elemental distribution within the ion beam. The ion beam width at m/z 208 is one-third of its width at m/z 7 using an accelerating potential of 800 V; at an accelerating potential of 4000 V, the ion beam width does not vary with mass.

Mass Spectrometry↗
PubMed1995

Near-infrared measurement of relative and absolute humidity through detection of water adsorbed on a silica gel layer.

Near-IR spectroscopy is especially well suited to moisture determination because of the relatively high absorptivity of water compared to most other substances. In the present work, near-IR diffuse-transmittance spectroscopy is applied to the measurement of humidity via observation of adsorbed water on a high-performance thin-layer chromatography silica gel plate. The adsorbed water is detected through both ordinary absorption of radiation by water molecules and the increased scattering of near-IR radiation by the silica gel as it adsorbs more water. This technique shows promise as a highly accurate and sensitive humidity sensor with a time constant of less than 1 min. The addition of inorganic salts to the silica gel layer is shown to increase the response to water vapor. However, it also increases the time constant of the sensor. A miniature humidity sensor using a commercially available near-IR transmissive switch is also demonstrated. Possible interferences and hysteresis effects are investigated.

Adsorption↗
PubMed1990

Feasibility of using liquid crystals for the development of molecularly selective fiber-optic chemical sensors.

A new type of fiber-optic chemical sensor has been developed for the determination and differentiation of geometric isomers. The operation of the sensor is based upon the molecular-geometric-selective absorption of polynuclear aromatic hydrocarbons (PAH) on a liquid crystal. The selective interaction of a PAH with a liquid-crystal substrate causes quenching of the liquid-crystal fluorescence. Detection limits of such a device for PAH compounds approach 10(-10) mol/cm3; the sensor response time is about 2 min. Because the sensor is based upon physical absorption, it is reusable and reversible.

Fiber Optic Technology↗
PubMed1983

New techniques and tools for clinical chemistry.

In this paper are described and evaluated several new tools of potential use in clinical chemistry. The first, intended to minimize required sample volumes, is a device with which a total sample volume of 1 microL can be dispensed in the form of 1000 identical aliquots. Any number of such nanoliter aliquots can be taken if larger samples are needed. The second new tool is one for detecting anions or cations separated by ion chromatography. Unlike conventional conductometric detectors used in ion chromatography, the new system offers potential sensitivities in the sub-microgram per liter range and useful operating ranges up to 100 mg/L. The third tool is a scheme for background correction in atomic absorption spectrometry; the new technique requires no special auxiliary sources or double-beam optics. Finally, fluorescence time-decay curves and fluorescence lifetimes are shown to be able to overcome the effects of diffusional quenching and scattering resulting from turbidity of solutions in clinical fluorometry.

Chemistry, Clinical↗