W. E. Cooke

Magnetic Moment of the Proton in H2O in Bohr Magnetons

We have measured the ratio of the electron g-factor in hydrogen to the proton g-factor in a spherical sample of H2O by comparing the electron spin frequency, as measured with a hydrogen maser, to the proton spin flip frequency, as measured by NMR. The result is gj(H)/gp = 658.216 0091(69) [10 parts per billion (ppb)] at a temperature of 34.7°C. This yields a value for the proton moment in Bohr magnetons of μp/μB = 0.001 520 992 983(17) (11 ppb). Our result differs from the best previous value by 150 ppb.

Precision Enhancement of MALDI-TOF-MS Using High Resolution Peak Detection and Label-Free Alignment*

We have developed an automated procedure for aligning peaks in multiple TOF spectra that eliminates common timing errors and small variations in spectrometer output. Our method incorporates high‐resolution peak detection, re‐binning, and robust linear data fitting in the time domain. This procedure aligns label‐free (uncalibrated) peaks to minimize the variation in each peaks location from one spectrum to the next, while maintaining a high number of degrees of freedom. We apply our method to replicate pooled‐serum spectra from multiple laboratories and increase peak precision (t/σt) to values limited only by small random errors (with σt less than one time count in 89 out of 91 instances, 13 peaks in seven datasets). The resulting high precision allowed for an order of magnitude improvement in peak m/z reproducibility. We show that the CV for m/z is 0.01% (100 ppm) for 12 out of the 13 peaks that were observed in all datasets between 2995 and 9297 Da.

A Bayesian network approach to feature selection in mass spectrometry data

BackgroundTime-of-flight mass spectrometry (TOF-MS) has the potential to provide non-invasive, high-throughput screening for cancers and other serious diseases via detection of protein biomarkers in blood or other accessible biologic samples. Unfortunately, this potential has largely been unrealized to date due to the high variability of measurements, uncertainties in the distribution of proteins in a given population, and the difficulty of extracting repeatable diagnostic markers using current statistical tools. With studies consisting of perhaps only dozens of samples, and possibly hundreds of variables, overfitting is a serious complication. To overcome these difficulties, we have developed a Bayesian inductive method which uses model-independent methods of discovering relationships between spectral features. This method appears to efficiently discover network models which not only identify connections between the disease and key features, but also organizes relationships between features--and furthermore creates a stable classifier that categorizes new data at predicted error rates.ResultsThe method was applied to artificial data with known feature relationships and typical TOF-MS variability introduced, and was able to recover those relationships nearly perfectly. It was also applied to blood sera data from a 2004 leukemia study, and showed high stability of selected features under cross-validation. Verification of results using withheld data showed excellent predictive power. The method showed improvement over traditional techniques, and naturally incorporated measurement uncertainties. The relationships discovered between features allowed preliminary identification of a protein biomarker which was consistent with other cancer studies and later verified experimentally.ConclusionsThis method appears to avoid overfitting in biologic data and produce stable feature sets in a network model. The network structure provides additional information about the relationships among features that is useful to guide further biochemical analysis. In addition, when used to classify new data, these feature sets are far more consistent than those produced by many traditional techniques.

The detection of 300 °K blackbody radiation with Rydberg atoms

Using selective field ionization (SFI) of Na Rydberg atoms we have detected the 300 °K blackbody radiation which induces the Na transitions 17s→17p, 18s→18p, 17d→18p, and 17d→18f frequencies of 27.3, 22.8, 7.4, and 41.6 cm−1, respectively. Investigation of the probable collisional effects show that they are negligible at 300 °K and allow us to project temperatures of <25 °K for Rydberg‐atom long‐wave detectors.

Wavelength dependence of laser‐induced sputtering from the (111) surface of BaF2

Using blue tunable pulse laser radiation of low fluence, we have investigated laser‐induced sputtering from cleaved BaF2 (111) surfaces under ultrahigh vacuum conditions. Time correlated with the laser pulses the positive ions Ba+, Ba++, (BaF)+ and F+ were observed. Practically no negative ions were found. Neutral atomic fluorine (F 0) was desorbed abundantly. A pulse correlation of F 0 as well as the relative amount of F 0 and F+ could not be established at this stage. The emission yield of all positive ions as well as of F 0 was strongly wavelength dependent and showed a broad resonance around 2.9 eV.

Spectroscopic decay-rate measurements below the laser linewidth

When two levels can be strongly coupled by a pulsed laser, it is possible to measure the decay rate from the upper state by applying sufficient laser power to depletion broaden the transition. The depletion broadening is proportional to the square root of the decay rate times the total energy density of the laser pulse. With sufficient laser power, this technique can be used even if the decay rate is smaller than inhomogeneous broadenings or the laser linewidth. The technique is used here to measure autoionizing rates of some 6P((1/2))ns states.

Measurement of 1 D 2 → 1 F 3 microwave transitions in strontium Rydberg states using selective resonance ionization

A new technique for state-selective detection of highly excited atoms is reported. It is usable over a much wider range of quantum numbers than previously available methods and has excellent time resolution. The high-state selectivity is used to measure the 5s(n + 2)d,(1)D(2) ? 5snf, (1)F(3) microwave transitions in strontium for n = 38-40.

Automated assignment of ionization states in broad-mass matrix-assisted laser desorption/ionization spectra of protein mixtures

A computational technique is presented for the automated assignment of the multiple charge and multimer states (ionization states) in the time-of-flight (TOF) domain for matrix-assisted laser desorption/ionization (MALDI) spectra. Examples of the application of this technique include an improved, automatic calibration over the 2 to 70 kDa mass range and a reduced data redundancy after reconstruction of the molecular spectrum of only singly charged monomers. This method builds on our previously reported enhancement of broad-mass signal detection, and includes two steps: (1) an automated correction of the instrumental acquisition initial time delay, and (2) a recursive TOF detection of multiple charge states and singly charged multimers of molecular [MH](+) ions over the entire record range, based on MALDI methods. The technique is tested using calibration mixtures and pooled serum quality control samples acquired along with clinical study data. The described automated procedure improves the analysis and dimension reduction of MS data for comparative proteomics applications.

Saturation Effects in the Spatial and Energy Distributions in Short-pulse High-intensity Multiphoton Ionization

Abstract We have calculated the spatial and electron energy structure for short-pulse high-intensity multiphoton ionization taking into account saturation. Using an avoided-crossing picture, we have modelled multiphoton resonances induced by the high-intensity a.c. Stark effect. We have explicitly considered ionization due to the rising and falling edges of the laser pulse including saturation and ground-state depletion. Using only the laser-pulse duration as the adjustable parameter, our results show the dramatic modifications both to the spatial structure and to the photoelectron energy spectrum as saturation becomes important.

Boxcar controller for low‐repetition‐rate use

We present a simple digital circuit which will control a Princeton Applied Research Corporation (model No. 164) boxcar integrator and thereby allow full utilization of the linear sum mode. This provides for improved signal-to-noise ratios and shorter data acquisition times for short signals [10-50 ns occuring at moderate repetition rates (1-20 Hz)]. This provides significant data acquisition time savings for low-repetition-rate, high-time-resolution measurements. It also increases the signal-to-noise ratio in many circumstances.