Brian Millier

Inter-elemental selectivity, spectra and computer-generated specificity of some main-group elements in the flame photometric detector☆

Abstract Inter-elemental selectivities (ratios of emission intensities) of some important main-group elements—B, Ge, Sn, Pb, N, P, As, Sb, S and Se—have been measured in a filter-less flame photometric detector (FPD) under one common set of conditions. In cases of unknown, unassigned or doubtful spectral distributions— e.g. from B, Pb, N and Sb—luminescences were recorded directly from the detector under analytical operating conditions. Despite the detectors dependence on broad, low-resolution spectra that frequently overlap, a computer algorithm using dual-channel data allowed specific (= infinitely selective) chromatograms to be recorded for any FPD-active element. The spectral requirements of this method, which is based on the conditional access (CONDAC) of slope ratios, were minimal: one optical filter permitted a single computer-stored run to produce several CONDAC chromatograms. Each of these was specific in the sense that it showed only the peaks of one particular element.

Noise, filters and detection limits

Abstract The “noise” of Chromatographic baselines has been investigated in regard to the detector, the nature and extent of filtering or smoothing, and the methodologies of qualitative and quantitative assessment: all in order to clarify the role such factors play in the determination and interconversion of some common types of detection limits. This study scrutinizes baselines from the flame photometric detector in single-channel continuous and ten-channel multiplexing versions; it also examines baselines from flame ionization and electron-capture detectors. It makes use of finite impulse response and non-weighted moving-average digital smoothing, as well as three-pole analog filtering. Baseline fluctuations are quantified by the standard deviation derived from the common root-mean-square (RMS) calculation, or from the less common least-squares Gaussian fit; peak-to-peak noise (Np-p) is estimated by procedures including or excluding presumed outliers. Individual results are expressed as the ratio of Np-p measurement and RMS calculation performed on the same data set. A wide variety of such ratios are then assembled from different detectors, filters, and smoothing conditions. They prove conclusively that -contrary to common belief—the conversion factor between the two types of measurements does vary: usually between 4 and 10, but occasionally even farther. Consequently, the conversion factor between the corresponding two types of detection limits varies as well. The Np-p/RMS ratio depends largely on the detector-of-origin, its condition, and the extent to which noise has been filtered. In contrast, the nature and sophistication of the filter hardly matters: either for the Np-p/RMS ratio or for the practical detection limit. This is because the slow undulations characteristic of heavily filtered baselines represent —at least in the detectors we used-dampened fast noise rather than aboriginally slow noise. Corresponding computer simulations, based on amplitudinally random noise smoothed by stationary boxcar or non-weighted moving-average filters, produce results strikingly similar to actual baselines. Simulated fast RMS noise correlates, as expected, with the square root (log-log slope = 1 2 ) of the filters time constant. The corresponding slopes for experimental noise are usually close to 1 2 as well. Most importantly, though, the simulated Np-p/RMS ratio varies strongly with the extent of smoothing -thus mimicking and thereby explaining the behavior of the experimental ratio.

Multichannel chromatography and on-line spectra from a flame photometric detector

A rotating variable interference filter has been incorporated into a flame photometric detector to acquire and simultaneously display low-resolution spectra that are diagnostic for eluting peaks. In its present form this approach yields ten chromatograms (for ten different wavelength segments) that are stored in the computer for subsequent data manipulation such as baseline correction and curve smoothing; three-dimensional display; peak and noise diagnostics; and magnification, subtraction, or elemental correlation of chromatograms. A third, spectral dimension is thus added to the conventional two, chromatographic dimensions of retention time and analyte intensity. The three-dimensional performance is at present accompanied by an order-of-magnitude loss in detection limit and linear range. Typical one-segment (i.e. 5% dwell time) elemental detectabilities, at S/Np-p = 2, are 7 · 10−13 g P/s (via HPO*), 3 · 10−11 g S/s (S2*), 7 · 10−12 g Ge/s (GeH*) and 4 · 10−12 g Ru/s.

High‐resolution, high‐sensitivity ac calorimeter

A high‐resolution, high‐sensitivity, automated ac calorimeter capable of heat capacity measurements on very small samples (m<20 mg) with a temperature resolution of a few mK in the ac mode is described. This calorimeter also can be operated in the relaxation mode to provide absolute heat capacity values with precision and accuracy of around 2%. A microprocessor was employed to generate a stable oscillatory heating signal and to control the heater power, in order to improve the sensitivity of the measurement over other designs. A new very sensitive and miniature temperature probe (a film flake of a thermistor material), which increased measurement resolution and minimized the heat contribution of the addenda, also was used. This calorimeter was tested by measuring the heat capacity of gadolinium over its ferromagnetic phase transition. The results agree well with the literature data giving, however, substantially better resolution of the heat capacity in the critical region.

Polarization, relaxation and unrestrictedly linear response in a bipolar, constant-frequency electron-capture detector

Abstract To investigate bipolar constant-frequency regimes in the electron-capture detector (ECD), a “tripulser” was built. The tripulser was able to generate unit sequences of up to three pulses, individually defined as to width amplitude and relative position, with 600 ns to 1 s and 0 to 250 V definition ranges. On a commercial 63Ni two-chamber ECD (Tracor), the high-frequency region of bipolar pulsing (ca. 10 to 100 kHz) was explored. The detector showed clear polarization-relaxation (PR) effects within time spans (on the order of 10−5 s) that were commensurate with the theoretical mobility of electrons. Speculative evidence was found to suggest that PR kinetics, as driven by particular bipolar pulse sequences, resulted in changes to the (heterogeneous) charged-particle distribution and effectively allowed higher than usual concentrations of electrons (and cations) to exist in the ECD. Based on this evidence, a bipolar, constant-frequency drive was developed that, when tested on the Tracor ECD, showed good analytical performance. Most important (and in contrast to the behavior of any other unipolar constant-frequency mode) the bipolar (Tracor) ECD yielded strictly linear calibration curves—starting from the detection limit (5 · 10−18 mol/s of α-1,2,3,4,5,6-hexachlorocyclohexane at S/Np-p=2), over three orders of magnitude, all the way to an amount of analyte that totally exhausted the baseline current.

Phase-shift-free subtraction chromatograms from a dual-channel detector

Compared to single-channel analysis, dual-channel correlational chromatography can dramatically improve selectivity. Yet the extent of improvement is often limited by phase shifts due to the slow conventional electrometer/filter instrumentation used prior to signal deduction. Since fast amplification/correlation circuitry can avoid these phase shifts, a suitable electronic module with a millisecond time constant was constructed. In a demonstration experiment, it improved the linear selectivity of a chromium vs. a ruthenium compound to theoretical limits, i.e. by well over three orders of magnitude.

Time-integrated spectra from a flame photometric detector

A rotating, variable-wavelength interference filter has been used to acquire spectra from peak or baseline of a dual-channel, high-sensitivity flame photometric detector. On its spectral channel, the device monitors 100 data points from 400 to 700 nm ten times a second, and ensemble-averages them over seconds to (if needed) hours of acquisition time. The spectra can be smoothed, adjusted for background, corrected for filter transmission and photocathode profiles, and plotted with energy or photon ordinates. The extent of spectral fluctuation suggests the emergence of flicker noise at high analyte concentrations.

Dual-channel response ratios from an integrative algorithm

Abstract An integrative algorithm has been developed, and compared with existing differential algorithms, for automatically determining the response ratios of peaks from a dual-channel flame photometric detector. The comparison was carried out using high and low, and constant and variable concentrations of an organosulfur test compound; under different degrees of solvent quenching and at two sets of detector flow conditions; and with and without digital filtering. The new integral algorithm performed as well as and, particularly in the presence of strong noise, significantly better than the existing differential ones. Typically, the response ratios of good peaks varied by 2 to 3% R.S.D. for different, and by ca. 0.5% R.S.D. for similar concentrations; the former, larger variation owing to previously unnoticed spectral changes. Different algorithms, working on single, large and well-smoothed peaks, varied among themselves by typically less than 1% R.S.D. The integral response ratios were displayed on the screen in graphic form and simultaneously printed in numeric form. At the discretion of the operator, they could also be printed in the form of a scalable “response-ratio chromatogram”, with or without the constituent signal traces.

Computer-controlled instrumentation for accurate mass determination by peak matching in mass spectrometry

Abstract Peak matching, although a tedious and somewhat subjective process, has long been used with sector mass spectrometers for accurate mass determination. Computer-assisted control and data acquisition can remove the subjectivity and make the process faster and less tedious. The circuits for data acquisition and sweep generation and the data analysis programs that provide these advantages are described. These can also be used, with minor modification, for peak ratio determinations.

Constant-current a.c. electron-capture detector☆

Abstract The constant-current drive is the most popular mode of polarization for the conventional (unipolar) electron-capture detector: in this mode, an increase in analyte produces an increase in frequency. In contrast, when the electron-capture detector is operated under a constant-current constraint in the bipolar a.c. mode, an increase in analyte produces a decrease in frequency in the, roughly, 104 to 105 Hz region (as well as an increase in frequency in the, roughly, 102 to 103 Hz region). Both constant-current mechanisms — similar to the mechanism of the conventional unipolar detector — rely on the increasing withdrawal of electrons from their reactions with increasing concentrations of analyte molecules and carrier cations.