A theoretical model of the electron capture detector

Summary This paper reports a theoretical model of the ECD detector. The model presented here can be used to examine the influence of pulse parameters on the current and signal characteristics of the detector. On the basis of this model it was found that a space charge is created in the detector when it is supplied with pulse voltage. Due to the electric potential generated by the space charge, in the time between the pulses the electrons and negative ions move towards the detector electrodes. The ionization current of the detector is the sum of the electron current flowing to the anode under the influence of the supplied pulse voltage and the current flowing under the space charge potential in the time between the pulses. It was also found that the detector signal is the sum of the differences between those two currents caused by introducing the sample molecules to the detector. The model was tested for a detector with different electrode configurations which worked at temperature of 300 K or 573 K and which was supplied with nitrogen or Ar+10% CH₄ as the carrier gas.     

The pulsed glow discharge as an elemental ion source

A pulsed glow discharge, rather than a conventional constant dc voltage discharge, is used as an ion source for a quadrupole mass spectrometer. Both sputter yield and ion signal are enhanced by using the pulsed system because of an increase in the voltage necessary to maintain a constant average current at the cathode over the pulse period. Irregularities are seen in the pulse spectrum that appear as rapid surges in the ion signal for both sputtered and contaminant gas species. These peaks appear at the beginning of the pulse for gaseous species but are limited to the postpulse period for sputtered species. Differences in the signal forms allow for the discrimination against selected types of ion signals by using narrow data collection gates placed over different portions of the pulse period.     

Determination of the lipid bilayer breakdown voltage by means of linear rising signal

Electroporation is characterized by formation of structural changes within the cell membrane, which are caused by the presence of electrical field. It is believed that "pores" are mostly formed in lipid bilayer structure; if so, planar lipid bilayer represents a suitable model for experimental and theoretical studies of cell membrane electroporation. The breakdown voltage of the lipid bilayer is usually determined by repeatedly applying a rectangular voltage pulse. The amplitude of the voltage pulse is incremented in small steps until the breakdown of the bilayer is obtained. Using such a protocol each bilayer is exposed to a voltage pulse many times and the number of applied voltage pulses is not known in advance. Such a pre-treatment of the lipid bilayer affects its stability and consequently the breakdown voltage of the lipid bilayer. The aim of this study is to examine an alternative approach for determination of the lipid bilayer breakdown voltage by linear rising voltage signal. Different slopes of linear rising signal have been used in our experiments (POPC lipids; folding method for forming in the salt solution of 100 mM KCl). The breakdown voltage depends on the slope of the linear rising signal. Results show that gently sloping voltage signal electroporates the lipid bilayer at a lower voltage then steep voltage signal. Linear rising signal with gentle slope can be considered as having longer pre-treatment of the lipid bilayer; thus, the corresponding breakdown voltage is lower. With decreasing the slope of linear rising signal, minimal breakdown voltage for specific lipid bilayer can be determined. Based on our results, we suggest determination of lipid bilayer breakdown voltage by linear rising signal. Better reproducibility and lower scattering are obtained due to the fact that each bilayer is exposed to electroporation treatment only once. Moreover, minimal breakdown voltage for specific lipid bilayer can be determined.     

Optimizing electron-capture detector performance for gas chromatographic analysis of polychlorinated biphenyls

Summary A modified simplex technique has been applied to optimize the performance of an electron-capture detector under constant-current and constant-frequency pulse modes for the gas chromatographic analysis of polychlorinated biphenyls. Optimal performance was obtained by adjusting the four experimental parameters of pulse voltage, detector temperature, make-up gas flow rate, and reference current (or pulse frequency) using the signal-to-noise ratio of the eluting peak as the criterion. The results show that constant-current pulse mode has better sensitivity and dynamic range whereas the constant-frequency pulse mode has the lower detection limit. Subsequent principal component factor analysis indicated that similar performance is achievable using only three parameters.     

离子富集脉冲电源对提高飞行时间质谱仪灵敏度的影响

研究自制离子富集脉冲电源对飞行时间质谱仪灵敏度的提高效果。离子富集脉冲电源包括脉冲信号产生模块、放大隔离模块、开关驱动模块、高压电路模块和负载阻抗匹配模块,输出一路中压脉冲信号,两路对称高压脉冲信号。利用自制电喷雾电离源及激光解析电离源与飞行时间质谱仪联用,调试中压脉冲信号。结果显示,应用离子富集脉冲电源后仪器对不同样品的检测灵敏度可提高6~24倍。     

A helium ionization detector with a thermionic electron emitter for gas chromatography

The principal circuit of a helium ionization detector with a thermionic electron emitter (He-IDTEE) is described. The detector works at atmospheric pressure. The characteristics of the detector were studied. The difference between the voltammetric characteristics of He-IDTEE and that of a helium ionization detector with a radioactive source was shown. The dependences of the analytical signal, background signal, and noise on an accelerating voltage were analyzed. The dependences of the signal-to-noise ratio on the accelerating voltage were studied at different cathode temperatures. The optimal working conditions of the detector were selected. The detection threshold and linear dynamic range were measured. For isobutane, they were 3 × 10^?9 vol % and 2–4 orders of magnitude, respectively.     

Detection of metal ions without interference from dissolved oxygen by reverse pulse amperometry in flow-injection systems and liquid chromatography

Reverse pulse amperometry (r.p.a.) is used to monitor fluid streams for the presence of electroactive metal ions using a mercury electrode without the requirement of deaeration. The technique is based on the application of an unsymmetrical square wave with a large negative initial potential for deposition of the metal ion followed by a positive potential pulse for anodic stripping of the metal. The analytical signal is measured during the stripping process at a potential where dissolved oxygen is not reduced. R.p.a. is applied at a polarographic detector in flow-injection systems and ion-exchange chromatography for detection of several ions of transition and heavy metals in aqueous solutions.     

Development of an X-ray detector using surface plasmon resonance

A new X-ray detector using surface plasmon resonance (SPR) is proposed. The detector consists of a prism coated with a thin metal film and semiconductor film. Optical laser pulse induces SPR condition on the metal surface, and synchronized X-ray pulse which is absorbed into the semiconductor film can be detected by measuring the change of the resonance condition of the surface plasmon. The expected time and spatial resolution of this detector is better than that of conventional X-ray detectors by combining this SPR measurement with ultra-short laser pulse as the probe beam. Our preliminary investigation using Au and ZnSe coated prism implies this scheme works well as the detector for the ultra-short X-ray pulse.     

Electrical breakdown of human erythrocytes: a technique for the study of electro-haemolysis

This paper describes a technique suitable for investigating the electromechanical breakdown properties of erythrocyte cells. The cells were exposed to square wave electric pulses of precise duration and voltage. The erythrocytes were suspended in normal isotonic saline between two opposing platinum electrodes. A red LED light source and photodiode detector system were positioned orthogonally to the electrodes to record changes in the light transmission that occur immediately after applying an electric pulse. The light transmitted through the electrically treated erythrocyte suspension could be monitored continuously. Experiments were conducted to explore the inter-relationship between the critical voltage and pulse length for haemolysis. Human blood taken from "healthy" donors underwent haemolysis at a critical field strength of 304 kV/m for a 5 micros pulse and 292 kV/m for a 50 micros pulse. The relationship of critical pulse length and critical voltage for the blood samples was found to be inversely linear.     

A facile light-emitting-diode induced fluorescence detector coupled to an integrated microfluidic device for microchip electrophoresis

In this paper, a compact and inexpensive light emitting diode induced fluorescence (LED-IF) detector with simplified optical configuration was developed and assembled in an integrated microfluidic device for microscale electrophoresis. The facile detector mainly consisted of an LED, a focusing pinhole, an emission filter and a photodiode, and was encapsulated in the upper layer of an aluminum alloy device with two layers. At the bottom layer, integrated circuit (IC) was assembled to manipulate the voltage for sample injection and separation, LED emission and signal amplifying. A high-power LED with fan-shaped heat sink was used as excitation source. The excitation light was focused by a 1.1 mm diameter pinhole fabricated in a thin piece of silver foil, and the obtained sensitivity was about 3 times as high as that using electrode plate. Other important parameters including LED driven current, fluorescence collection angle and detection distance have also been investigated. Under optimal conditions, considerable high-response of 0.09 fmol and 0.18 fmol mass detection limits at 0.37 nL injection volume for sodium fluorescein (SF) and FITC was achieved, respectively. This device has been successfully employed to separate penicillamine (PA) enantiomers. Due to such significant features as low-cost, integration, miniaturization, and ease of commercialization, the presented microfluidic device may hold great promise for clinical diagnostics and bioanalytical applications.     

Optimization of pulse processing parameters for HPGe gamma-ray spectroscopy systems used in extreme count rate conditions and wide count rate ranges

The need to perform gamma-ray spectroscopy measurements at high count rates with HPGe detectors is more common than many believe. Examples exist in safeguards, radiochemistry, nuclear medicine, and neutron activation analysis. In other applications wide dynamic ranges in count rate may be encountered, for example samples taken after a nuclear accident are counted on a system normally used for environmental monitoring. In a real situation, it may not be possible to reduce count-rates by increasing the distance or using collimators. The challenge is to obtain the “best” data possible in the given measurement situation. “Best” is a combination of statistical (number of counts) and spectral quality (peak width and position) considerations over a wide range of count rates. The development of multichannel analyzers (MCA) using digital signal processing (DSP) has made possible a much wider range of values for shaping times as well as the processing of the detector signal in various ways to improve performance with pulse-by-pulse adjustments. The pulse processing time is directly related to the shaping time. The throughput is related to the pulse processing time and the duration of the detector signal. Longer shaping times generally produce better peak resolution. However, the longer shaping times mean larger dead times and lower throughput. The ability to select the best compromise between throughput and resolution is possible with DSP MCAs. In addition, the dead-time-per-pulse can be reduced by changing the digital filter without significant impact on the full-width at half-maximum. To evaluate the improvements and to suggest an approach to optimization of system performance, a small and a large GEM (p-type) coaxial HPGe detector were selected for measurements to determine the performance at various input count rates and wide range of rise times and flattops in the DSPEC 50 MCA. Results will be presented for the throughput measured at dead times from 30 to 99.9 % with and without the use of the ORTEC Enhanced Throughput Mode.     

Influence of Polarity and Rise Time of Pulse Voltage Waveforms on Diesel Particulate Matter Removal Using an Uneven Dielectric Barrier Discharge Reactor

The influence of polarity and rise time of the pulse voltage on the removal of particulate matter (PM) emitted from a diesel engine was investigated using a dielectric barrier discharge reactor. Four kinds of pulse voltage waveforms (positive, negative, positive–negative and negative–positive) were used. It was found that the energy efficiency for PM removal is just a function of energy injection and that there are no obvious influences on PM removal and energy efficiency within the voltage waveforms except the negative pulse voltage of a peak voltage below 8 kV. A comparison of PM removals using various kinds of pulse voltage waveforms and different types of plasma reactors is given.     

A MULTICHANNEL TRANSIENT SPECTRUM ANALYSER FOR ABSORPTION CHANGES MEASUREMENT WITH ONE MICROSECOND RESOLUTION

A custom-designed photometer for recording flashlight-induced transient absorbance spectra with microsecond time resolution in the wavelength range from 350 to 800 nm with 0.5 nm resolution is reported. The apparatus is equipped with two 600 ns half-width pulse (output = 400 mJ) tunable dye lasers for double-flash analysis and an optical multichannel analyser having a silicon intensified target detector. Measuring light from a tungsten-halogen lamp is passed through the sample, dispersed into a spectrum and focused on the head of the detector. A transient spectrum is measured over a range of 330 nm simultaneously by applying high voltage to the detector for the desired interval. The delay from the actinic flash to applying the high voltage can be controlled electronically. Distortion caused by scattering of the actinic flash under optimal conditions is negligible within 1.5 is after the flash peak. Detector linearity reaches about 10⁴ and a noise level of less than ±0.005A can be achieved without data accumulation. This apparatus is applicable not only to transparent samples but to turbid ones with 100 μs resolution.     

X-ray imaging during plasma-source ion implantation

Plasma source ion implantation (PSII) is a technique for modifying stafaces that places the object to he modified directly into a plasma and then negatively pulse biases the object so as to implant positive ions. If the voltage is high enough, X-rays can he generated by electrons that are also accelerated by the pulse. This work describes techniques for imaging and characterizing the X-rays A pinhole camera was used to image the X-rays being emitted as electrons collided with surfaces in the chamber. The images show that X-rays are generated at the chamber walls and near the target. The time dependence of these X-rays during each pulse was examined using a PIN diode X-ray detector. Then, using another X-ray sensor and pulseheight analyzer, the spectra of the emitted X-rays was determined. The object is to relate the X-ray intensity and spectrum to the temporal and spatial values of the implantation dose so that it may he used as a process monitor and a control sensor.     

Matrix-assisted laser desorption ionization with a magnetic mass spectrometer.

Matrix-assisted laser desorption ionization has been carried out with a high mass double-focusing magnetic mass spectrometer. The pulsed ion signal, generated by irradiation of the sample (substance P, ubiquitin, and cytochrome c) embedded in 2,5-dihydroxybenzoic acid with a XeF excimer laser (353 nm, 12 ns pulse, 10-160 Hz), was recorded with an integrating array detector. Good resolution of 2600 (full width at half maximum) and high sensitivity (a few pmol) were obtained. The loss of small neutral fragments was observed, supporting the notion that the peak broadening observed in the time-of-flight mass spectrometers commonly used for this ionization mode is due to such metastable decomposition.     

Systematic examination of the signal area precision of a single quadrupole enhanced low mass option (ELMO) MSQ [corrected] mass spectrometer

To examine the precision of the signal area response of an enhanced low mass option (ELMO) MSQmass spectrometer, operated in the negative electrospray ionization (ESI) mode, extended tests were performed, using flow injection analysis mass spectrometry (FIA-MS). Analytes were nitrate, nitrite, malonic acid, and D,L-mandelic acid. Composition and concentration of injected samples, application of an ASRS anion suppressor and of the cone wash unit, methanol addition to the FIA flow medium, and the voltage bias of the hexapole transfer lens were test variables. Individual test cycles comprised up to 90 injections, processed within 20 h. With a few exceptions the signal response tended to decline over time leading to a loss of more than 80% of the initial signal area in extreme cases. A hexapole radio-frequency (RF) voltage bias of -0.3 V led to an overall low detector response and to high losses of sensitivity over time. Other correlations between the insufficient signal reproducibility and FIA-MS operating conditions could not be established. The test scheme gave hints how to localize the cause of the mass spectrometer malfunction. The repetition of the test scheme after remedying the detected electronic default demonstrated that relative standard deviations less than 5% can be achieved for a sequence of 30 injections if methanol is added to the FIA flow medium and if a suppressor is used. Based on these findings a recommendation is formulated to supplement current test schemes for instrument performance verification by a detector response precision criterion.     

Controlling low-rate signal path microdischarge for an ultra-low-background proportional counter

Pacific Northwest National Laboratory (PNNL) has developed an ultra-low-background proportional counter (ULBPC) made of high purity copper. These detectors are part of an ultra-low-background counting system (ULBCS) in the newly constructed shallow underground laboratory at PNNL (at a depth of ~30 m water-equivalent). To control backgrounds, the current preamplifier electronics are located outside the ULBCS shielding. Thus the signal from the detector travels through ~1 m of cable and is potentially susceptible to high voltage microdischarge and other sources of electronic noise. Based on initial successful tests, commercial cables and connectors were used for this critical signal path. Subsequent testing across different batches of commercial cables and connectors, however, showed unwanted (but still low) rates of microdischarge noise. To control this noise source, two approaches were pursued: first, to carefully validate cables, connectors, and other commercial components in this critical signal path, making modifications where necessary; second, to develop a custom low-noise, low-background preamplifier that can be integrated with the ULBPC and thus remove most commercial components from the critical signal path. This integrated preamplifier approach is based on the Amptek A250 low-noise charge-integrating preamplifier module. The initial microdischarge signals observed are presented and characterized according to the suspected source. Each of the approaches for mitigation is described, and the results from both are compared with each other and with the original performance seen with commercial cables and connectors.     

Kinetics of atomic force microscope-based scanned probe oxidation on an octadecylated silicon(111) surface

Atomic force microscope (AFM)-based scanned probe oxidation (SPO) nanolithography has been carried out on an octadecyl-terminated Si(111) surface to create dot-array patterns under ambient conditions in contact mode. The kinetics investigations indicate that this SPO process involves three stages. Within the steadily growing stage, the height of oxide dots increases logarithmically with pulse duration and linearly with pulse voltage. The lateral size of oxide dots tends to vary in a similar way. Our experiments show that a direct-log kinetic model is more applicable than a power-of-time law model for the SPO process on an alkylated silicon in demonstrating the dependence of oxide thickness on voltage exposure time within a relatively wide range. In contrast with the SPO on the octodecysilated SiO2/silicon surface, this process can be realized by a lower voltage with a shorter exposure time, which will be of great benefit to the fabrication of integrated nanometer-sized electronic devices on silicon-based substrates. This study demonstrates that the alkylated silicon is a new promising substrate material for silicon-based nanolithography.