Effect of signal-to-noise ratio and number of data points upon precision in measurement of peak amplitude,position and width in fourier transform spectrometry

Chen, L., Cottrell, C.E. and Marshall, A.G., 1986. Effect of signal-to-noise ratio and number of data points upon precision in measurement of peak amplitude, position and width in Fourier transform spectrometry. Chemometrics and Intelligent Laboratory Systems, 1: 51–58.The theoretical precision in determining experimental spectral peak parameters (height, width, and position) should depend in a calculable way upon the peak shape, signal-to-noise ratio, and number of data points per line width. Expressions for precision in peak position and width for absorption-mode Lorentzian and Gaussian line shapes have been derived previously. We have extended the theory to include absorption-mode sinc as well as magnitude-mode Lorentzian and sinc line shapes, and have computed the predicted precision in peak amplitude as well as in position and width. Experimental (Fourier transform ion cyclotron resonance mass spectrometry and Fourier transform nuclear magnetic resonance spectrometry) precision for each of these parameters is found to be significantly poorer (e.g., factor of 5) than the theoretical predictions. The present results provide a direct test of the nature of noise (e.g., vertical vs. horizontal) in Fourier transform spectra, and suggest that experimental measurements of Fourier transform spectral line shape parameters may be much less precise than previously suspected.     

Improving precision in resonance ionization mass spectrometry: influence of laser bandwidth in uranium isotope ratio measurements

The use of broad bandwidth lasers with automated feedback control of wavelength was applied to the measurement of (235)U/(238)U ratios by resonance ionization mass spectrometry (RIMS) to decrease laser-induced isotopic fractionation. By broadening the bandwidth of the first laser in a three-color, three-photon ionization process from a bandwidth of 1.8 GHz to about 10 GHz, the variation in sequential relative isotope abundance measurements decreased from 10% to less than 0.5%. This procedure was demonstrated for the direct interrogation of uranium oxide targets with essentially no sample preparation.     

Isotope ratio measurements with elemental-mode electrospray mass spectrometry

The isotope ratio capabilities of an electrospray ionization source interfaced to a quadrupole mass spectrometer are described. With the instrument operated in the metal ion mode, isotope measurements of Ag, Tl, and Pb are conducted using elemental ions produced from 1 × 10(-)(4) M solutions of metal nitrates or acetates in methanol. For Ag and Tl, spray conditions are identified that produce spectra free of MH(+) ions. Unbiased Ag and Tl ratio measurements with precisions of ～0.2% RSD are readily attained. Further improvement in relative precision appears to be limited by temporal drift in the degree of mass discrimination imparted to the measurements by the mass spectrometer. Isotopic analysis of Pb is greatly complicated by significant yields of PbH(+) polyatomic ions.     

Evaluation and optimization of ion-current ratio measurements by selected-ion-monitoring mass spectrometry

Stable isotopically labeled compounds are regularly used as internal standards in quantitation and as tracers of in vivo metabolism. In both applications, the ratio of unlabeled to labeled analogues is determined from an ion-current ratio measured by a mass spectrometer. The precision of the ion-current ratio measurement defines the detection limit for quantitation and for tracer enrichment measurement. We have used standard models of noise to develop a method that evaluates ion-current ratio noise (i) that varies with the signal intensity and (ii) that is signal independent. This model produces a simple equation that defines the ion-current ratio precision using constants that can be evaluated empirically from the measurement of two ion-current ratios from a single standard measured multiple times. We demonstrate that our approach can predict the effect of signal intensity, ion-current ratio magnitude, and internal standard or tracer choice on the measurement precision. The standard deviations predicted by our method are shown to equal standard deviations of samples measured experimentally. This method allows a simple evaluation of a mass spectrometry system and can define the precision of new quantitation and tracer methods.     

Calculable coaxial resistors for precision measurements

Coaxial straight-wire resistors have been constructed with the purpose of improving comparisons between resistors, capacitors, and inductors in the audio-frequency range. The design is based on the principle that a coaxial line with a cylindrical shield can be described by relatively simple equations for the real and imaginary parts of the impedance. The resistors, with values at and above 100 Ω will be used as transfer standards for characterization of the frequency dependence of standard resistors and of the quantum Hall resistance in the audio frequency range     

Cryogenic microwave amplifiers for precision measurements

The noise performance of two cryogenic HEMT amplifiers has been studied. The effective noise temperature for each amplifier is shown to be close to the 6 K thermodynamic temperature before a power threshold of about -70 dBm is achieved.     

Performance analysis of rectangular quadrature amplitude modulation with combined arbitrary transmit antenna selection and receive maximal ratio combining in nakagami-m fading

The average symbol error probability (SEP) performance of arbitrary rectangular quadrature amplitude modulation in the context of arbitrarily ordered transmit antenna selection and receive maximal ratio combining diversity system is analysed. The channel gains are assumed to follow Nakagami-m fading distribution with in general arbitrary fading parameters. Exact expressions for the average SEP performance are derived for the general case of unequal in-phase and quadrature decision distances as well as distinct in-phase and quadrature modulation orders. The results generalise many previous case studies, and can be used to investigate the impact of various diversity-combining schemes and different modulation and channel parameters on the system average SEP performance.     

Sources of uncertainty in isotope ratio measurements by inductively coupled plasma mass spectrometry

A model is presented describing the effects of dead time and mass bias correction factor uncertainties, flicker noise, and counting statistics on isotope ratio measurement precision using inductively coupled plasma mass spectrometry (ICPMS) with a single collector. Noise spectral analysis is exploited to enable estimation of the flicker noise parameters. For the instrument used, the flicker noise component exhibited a fairly weak frequency (t) dependence (is proportional to f -0.33+/-0.12), but was directly proportional to the total number of counts, Q. As white noise, determined by counting statistics, is given by Q0.5, the isotope ratio measurement uncertainties will actually cease to improve when Q exceeds a certain threshold. This would suggest that flicker noise could become the limiting factor for the precision with which isotope ratios can be determined by ICPMS. However, under most experimental conditions, uncertainties associated with mass discrimination and dead time correction factors are decisive. For ratios up to approximately 22 (115In/113In), optimum major isotope count rates are generally below 0.3 MHz, for which precision in the mass discrimination factor is limiting. The model derived could be used as a starting point for determining optimum conditions and understanding the limitations of single-collector ICPMS for precise isotope ratio measurements.     

Low blank isotope ratio measurements of rhenium, osmium, and platinum using tantalum filaments with negative thermal ionization mass spectrometry

Platinum is most commonly used as a filament for Re and Os isotopic measurements, but it contains impurities of Re and Os. Tantalum is low in platinum group elements (PGE) and in Re, but it is not used for negative thermal ionization mass spectrometry because of high electron emission and high reactivity with O(2). High thermal electron emission from Ta distorts the preoptimized ion source optics. In addition, Ta consumes O(2), leaving little for samples, but O(2) is essential for isotopic ratio measurements of PGE and Re as they are measured as negatively charged oxides, such as OsO(3)(-) and PtO(2)(-). These problems are solved by prebaking a filament to remove tantalum oxides before sample loading, keeping relatively high filament temperatures and high O(2) pressures (P(O)((2))) during the sample run, and lowering the potential difference between the filament and the draw-out plate. At P(O)((2)) of ～1 × 10(-)(5) Torr in the source, strong (>10 V) stable (>6 h) peaks of ReO(4)(-), OsO(3)(-), and PtO(2)(-) are obtained at 750 °C for Re, 850 °C for Pt, and over 900 °C for Os. Accurate isotopic ratio measurements of Re, Os, and Pt at picogram levels are possible using Ta filaments.     

Factors controlling precision and accuracy in isotope-ratio-monitoring mass spectrometry

The performance of systems in which picomole quantities of sample are mixed with a carrier gas and passed through an isotope-ratio mass spectrometer system was examined experimentally and theoretically. Two different mass spectrometers were used, both having electron-impact ion sources and Faraday cup collector systems. One had an accelerating potential of 10kV and accepted 0.2 mL of He/min, producing, under those conditions, a maximum efficiency of 1 CO2 molecular ion collected per 700 molecules introduced. Comparable figures for the second instrument were 3 kV, 0.5 mL of He/min, and 14000 molecules/ion. Signal pathways were adjusted so that response times were <200 ms. Sample-related ion currents appeared as peaks with widths of 3-30 s. Isotope ratios were determined by comparison to signals produced by standard gases. In spite of rapid variations in signals, observed levels of performance were within a factor of 2 of shot-noise limits. For the 10-kV instrument, sample requirements for standard deviations of 0.1 and 0.5% were 45 and 1.7 pmol, respectively. Comparable requirements for the 3-kV instrument were 900 and 36 pmol. Drifts in instrumental characteristics were adequately neutralized when standards were observed at 20-min intervals. For the 10-kV instrument, computed isotopic compositions were independent of sample size and signal strength over the ranges examined. Nonlinearities of <0.04%/V were observed for the 3-kV system. Procedures for observation and subtraction of background ion currents were examined experimentally and theoretically. For sample/ background ratios varying from >10 to 0.3, precision is expected and observed to decrease approximately 2-fold and to depend only weakly on the precision with which background ion currents have been measured.     

Selective post-IFFT amplitude randomising for peak-to-average power ratio reduction in orthogonal frequency-division multiplexing-based systems

The inherent high peak-to-average power ratio (PAPR) of multicarrier transmission, such as orthogonal frequency-division multiplexing (OFDM) or discrete multi-tone (DMT), can lead to a significant degradation in the transmission power efficiency which is unacceptable especially in battery-powered terminals. Among the most popular PAPR reduction techniques proposed in the literature is the selective mapping (SLM) technique which has been shown to offer PAPR reductions of several decibels. However, the SLM technique requires invoking the inverse fast fourier transform (IFFT) process several times per transmitted OFDM block which increases the system's complexity and hence may result in long latencies and high power consumption. The authors propose a new low complexity post-IFFT PAPR reduction technique that can outperform the SLM technique in terms of PAPR reduction while its operational complexity is orders of magnitude less than that of SLM technique.     

Nonparametric tests and the stationarity of signal amplitude and time measurements

Nonparametric tests have been applied to the stationarity of measurements for pulse edge length measurements, with the object of determining the time taken for a stroboscopic oscilloscope to transfer from the dynamic mode to the static one. Translated from Izmeritel'naya Tekhnika, No. 11, pp. 3–6, November, 2008.