Conversion Dynode Research Articles

The detection of large molecules in matrix‐assisted UV‐laser desorption

AbstractMechanisms related to high‐mass ion detection were studied using matrix‐assisted UV laser desorption for ion generation and a tandem (MS/MS) time‐of‐flight mass spectrometer for the analysis of secondary species formed at the conversion dynodes. Secondary‐electron emission was found to be less pronounced with increasing molecular weight. Instead small secondary ions generated by collision of primary ions with the conversion dynodes appeared to be the dominant process for the detection of ions of masses up to several hundred thousand Da. The use of microchannel plate detectors has been shown to become less effective for high molecular weight applications. Mass spectra of secondary ions emitted from a conversion dynode bombarded with keV primary ions have been recorded. Regardless of the composition of the primary molecules, the secondary mass spectra were similar. Sputtering of surface adsorbates is assumed to be the major mechanism of secondary ion formation, rather than surface induced dissociation or fragmentation of the primary ions.

Low background-rate detector for 40-keV ions using a conversion dynode and a microchannel-plate electron multiplier to reject low-energy ions, electrons, and photons

We have developed and tested an inexpensive ion detector with dark-count rates below 10−4 s−1 and excellent background rejection. Each 40-keV ion that strikes a grazing-incidence Al2O3 conversion dynode ejects a few tens of secondary electrons, which enter separate pores of a microchannel plate and are independently multiplied to saturated pulse height. The pulses are summed to produce an output signal that is proportional to the number of secondary electrons. This permits pulse-height discrimination against backgrounds of low-energy ions, electrons, and photons. We have successfully tested the detector with C+, Na+, K+, Rb+, and Cs+ at 5–40 keV, and with 36-keV C− and CH−. It should detect ions and neutrals of all species, at energies above 5–10 keV, with good efficiency and excellent background discrimination. Counting efficiency and background discrimination improve with higher ion energy. The detector can be operated at pressures up to at least 2×10−7 Torr and be exposed to air repeatedly without requiring reactivation. The maximum ion count rate is 3×106 s−1 in pulse-counting mode and 6×109 s−1 in current-integrating mode. Detection of 40-keV ions at rates below 10−2 s−1 was required in development of a 10-cm radius 40-keV cyclotron dedicated to high-sensitivity radioisotope dating by accelerator mass spectrometry (AMS). In the cyclotron, the detector suppresses to 6×10−4 s−1 the background count rate from the presence of low-energy ions, photons, and electrons.

252Cf plasma desorption mass spectrometry using a Mamyrin reflectron in a low voltage regime

A 252Cf plasma desorption mass spectrometer was constructed using a linear Mamyrin reflectron in a low voltage regime with acceleration potentials ranging from +40 to +500 V. The Mamyrin reflectron is an electrostatic reflector which allows ions with slightly greater velocities to travel longer flight paths, resulting in velocity focusing. This design includes a novel electron conversion dynode ion detection system. The conversion dynode consists of an aluminum plate coated with CsI maintained at a high negative potential. The reflected ions strike the dynode and electrons are ejected which travel the flight path, pass through the reflectron, and are detected by a channel electron multiplier array. Experimental results demonstrate the successful performance of the detector system and the establishment of the velocity focusing conditions at several acceleration voltages.

後段加速を用いた正，負両イオン用高感度検出器

A high sensitive detection system for heavy ions of low energy was described. In order to improve the conversion efficiency from positive or negative heavy ions, to electrons at the conversion dynode of the electron multiplier, the whole assembly of detection sysytem (electron multiplier, d. c. amplifier, high voltage power supply for the electron multiplier and power supply for the d. c. amplifier and A-D converter) were isolated from the ground and were kept at -10kV potential for the positive ion detection and +10kV for the negative ion detection. The d. c. signal from the d. c. amplifier was converted to 12-bits digital signal by the A-D converter and was sent to the ground side by light-emitting-diodes (LED). It was received by photo-transistors and converted back to a d. c. signal by a D-A converter. The conversion cycle of the A-D converter was 8 μsec. With this detection sysytem, the high sensitivity for negative heavy ions was obtained.

A spurious [CH5]− ion: Problems with negative chemical ionization quadrupole instrument

AbstractIt is shown that in a quadrupole mass spectrometer set up for negative chemical ionization, positive ions can be formed between the ionization chamber and the analyser both by electron impact and by chemical ionization. These ions will be registered together with negative ions formed in the ion source if the instrument is equipped with a positive and a negative conversion dynode and may reach the secondary electron multiplier even if the negative conversion dynode is disconnected.

A simple and inexpensive modification of a finnigan 3300 quadrupole mass spectrometer for negative ion detection

The incorporation of a conversion dynode, positioned in the available space between the ion exit hole of the quadrupole and the entrance of the electron multiplier permits the detection of negative ions when this dynode is held at a high positive potential (4 kV). The system makes use of the original electron multiplier and does not require additional electrical feedthroughs into the vacuum system. The conversion dynode is also used for positive ion detection. It is then kept at −4 kV and helps to increase the gain for high mass ions, to reduce mass discrimination and to improve the signal to noise ratio.

Electron Multiplier Pulse-Height Distributions for keV Ions

Pulse height distributions of the output of a secondary electron multiplier for various kinds of ions in the keV region were measured and analyzed to determine the true detection efficiencies for these ions. A high sensitivity mass spectrometer was used for the measurement at residual gas pressures around 10-9 Torr. An Allen-type secondary electron multiplier of 12-stage Cu-Be dynodes connected to a low-noise charge-sensitive preamplifier was used for detecting keV ions. It is assumed in the analysis that the pulse height distribution can be obtained from Polya statistics. The pulse height distributions for the ions of which secondary electron yields were markedly different made it possible to estimate the pulse height distribution for the single electron emitted from the conversion dynode. It was made clear that the statistics of the secondary electron emission by the light ions, e. g. H+, H2+ and He+, was different from that by the heavy ions, e. g. Ar+, CO+ and CO2+.

Detection of Single Ions by Fulse Counting: Application to Ion Microprobe Mass Analyzer

The need to detect low intensity beams of resolved ions with keV energy is encountered frequently in mass spectrometry. It is most readily accomplished by using some form of pulse-counting detector in which ions are allowed to strike a target surface (conversion dynode) where each ion releases a pulse of low energy secondary electrons. The secondary electrons undergo further amplification in an electron multiplier or scintillator type of ion detector which is specially designed to detect single ions. Ion-to-electron conversion, amplification of the secondary electrons, efficiency of ion detection by pulse counting and dead-time corrections to observed random counting rates are all statistical processes which must be understood so that observed data can be corrected properly to obtain precise and accurate measurements. These processes are reviewed, along with some of the basic properties of ion-induced secondary electron emission from an amorphous Al2O3thin film target. Ion detection in the ion microprobe is interpreted in light of these basic considerations, and several mass spectra are given for electron multiplier dynode surfaces.

The Secondary-emission Electron Multiplier Tube Used on a Mass Spectrometer and its Amplification of Ion Currents

Abstract For the purpose of measuring very small ion currents by the mass spectrometer, a secondary electron multiplier, using the dynodes of copper-beryllium, has been constructed and installed. The amplification by the electron multiplier of various supplied voltages to the conversion dynode was measured for inert gas ions, and the amplification factor and its variation with the supplied voltage to the conversion dynode was compared with both the activated and the electro-polished dynode. For the electron multiplier using the electro-polished dynodes, the amplification was roughly inversely proportional to the square root of the ion mass, while for the activated dynode this was not the case. The average yield of the secondary electrons impacted by electrons on the copper-beryllium dynodes was determined, using the values of γi which had been estimated by extrapolation of the data obtained in a previous study. The increase of the amplification factors of the electron multiplier due to the dynode activation was explained on the basis of an increase in γi and \barδ due to dynode activation, an increase to be attributed to the formation of beryllium oxide layers. In this study, an amplification factor of a 105 order for inert gas ions has been obtained with the electron multiplier constructed with dynodes made from activated copper-beryllium. By using this electron multiplier, ion currents of 10 −19 amperes are easily detectable with a mass spectrometer using a conventional D. C. amplifier.

The Study on a Secondary Electron Multiplier Tube(II)

A 12-stage Cu-Be electron multiplier tube was constructed and mounted in the high vacuum mass spectrometer at Osaka University. The following improvements for the construction, comparing with the similar tube previously reported, were made: 1)plane electrodes for deflecting the ion beam injecting the conversion dynode of multiplier were located behind the defining slit, 2)The several dynodes near the top of the tube were covered by a permalloy cylinder for shielding stray magnetic field of the magnet. The output current of multiplier was measured by a vibrating reed electrometer. As the results of the preliminary investigation of multiplier characteristics, the shape and height of mass peaks were found to be considerably affected by the voltage supplying on the deflecting electrodes and the proper voltage depended on the ion energy. The stability of multiplier gain(∼6×103)was fairly sufficient for measuring isotope ratio. For example, the measured 20Ne/22Ne ratio was 10. 563±0.006(not corrected for mass discrimination). At the ultimate pressure in the mass spectrometer, the background mass spectra up to m/e∼150 were recorded and some hydrocarbon fragment peaks corresponding to 10-16-10-18 amps. were found in C2-C7 region. And mass peaks below 10-16 amps. show very fluctuated top, but such statistical fluctuation could be smoothed out by using the amplifying system with sufficiently time constant and low speed scanning. Under such condition, the ion current down to 1-2×10-19 amps. could be detected the noise level of amplifying system.kn-abstract=