Conversion Dynode Research Articles

Analysis and Speciation of Lanthanoides by ICP-MS

AbstractInductively coupled plasma mass spectrometry (ICP-MS) is based on formation of positively charged atomic ions in a high-frequency inductively coupled Argon plasma at atmospheric pressure. The ions are extracted and transferred from the plasma source into a mass analyzer operated at high vacuum via an interface equipped with a sampling and a skimmer cone. The ions are separated in the mass analyzer according to their charge to mass ratio. The ions are converted at a conversion dynode and are detected by use of a secondary electron multiplier or a Faraday cup.From an analytical point of view, ICP-MS is a well-established method for multi-elemental analysis in particular for elements at trace- and ultra-trace levels. Furthermore, methods based on ICP-MS offer simple quantification concepts, for which usually (liquid) standards are applied, low matrix effects compared to other conventional analytical techniques, and relative limits of detection (LODs) in the low pg g−1range and absolute LODs down to the attomol range. For these applications, ICP-MS excels by a high sensitivity which is independent of the molecular structure and a wide linear dynamic range. It has found acceptance in various application areas and during the last decade ICP-MS is also more and more applied for detection of rare earth elements particularly in the life sciences.Due to the fact that all molecules introduced into the high temperature of the plasma in the ion source were completely dissociated and broken down into atoms, which are subsequently ionized, all elemental species information is completely lost. However, if the different species are separated before they enter the plasma by using adequate fractionation or separation techniques, then ICP-MS can be used as a very sensitive element-specific detector. We will discuss this feature of ICP-MS in this chapter in more detail at hand of the speciation of gadolinium-containing contrast agents.

High-Mass MALDI-MS Using Ion Conversion Dynode Detectors: Influence of the Conversion Voltage on Sensitivity and Spectral Quality

With the development of special ion conversion dynode (ICD) detectors for high-mass matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), the mass-to-charge ratio is no longer a limiting factor. Although these detectors have been successfully used in the past, there is lack of understanding of the basic processes in the detector. We present a systematic study to investigate the performance of such an ICD detector and separate the contributions of the MALDI process from the ones of the ion-to-secondary ion and the secondary ion-to-electron conversions. The performance was evaluated as a function of the voltages applied to the conversion dynodes and the sample amount utilized, and we found that the detector reflects the MALDI process correctly: limitations such as sensitivity or deviations from the expected signal intensity ratios originate from the MALDI process itself and not from the detector.

A high dynamic range pulse counting detection system for mass spectrometry

A high dynamic range pulse counting system has been developed that demonstrates an ability to operate at up to 2e8 counts per second (cps) on a triple quadrupole mass spectrometer. Previous pulse counting detection systems have typically been limited to about 1e7 cps at the upper end of the systems dynamic range. Modifications to the detection electronics and dead time correction algorithm are described in this paper. A high gain transimpedance amplifier is employed that allows a multi-channel electron multiplier to be operated at a significantly lower bias potential than in previous pulse counting systems. The system utilises a high-energy conversion dynode, a multi-channel electron multiplier, a high gain transimpedance amplifier, non-paralysing detection electronics and a modified dead time correction algorithm. Modification of the dead time correction algorithm is necessary due to a characteristic of the pulse counting electronics. A pulse counting detection system with the capability to count at ion arrival rates of up to 2e8 cps is described. This is shown to provide a linear dynamic range of nearly five orders of magnitude for a sample of aprazolam with concentrations ranging from 0.0006970 ng/mL to 3333 ng/mL while monitoring the m/z 309.1 → m/z 205.2 transition. This represents an upward extension of the detector's linear dynamic range of about two orders of magnitude. A new high dynamic range pulse counting system has been developed demonstrating the ability to operate at up to 2e8 cps on a triple quadrupole mass spectrometer. This provides an upward extension of the detector's linear dynamic range by about two orders of magnitude over previous pulse counting systems.

Radiative Ion–Ion Neutralization: A New Gas-Phase Atmospheric Pressure Ion Transduction Mechanism

All atmospheric pressure ion detectors, including photo ionization detectors, flame ionization detectors, electron capture detectors, and ion mobility spectrometers, utilize Faraday plate designs in which ionic charge is collected and amplified. The sensitivity of these Faraday plate ion detectors are limited by thermal (Johnson) noise in the associated electronics. Thus approximately 10(6) ions per second are required for a minimal detection. This is not the case for ion detection under vacuum conditions where secondary electron multipliers (SEMs) can be used. SEMs produce a cascade of approximately 10(6) electrons per ion impinging on the conversion dynode. Similarly, photomultiplier tubes (PMTs) can generate approximately 10(6) electrons per photon. Unlike SEMs, however, PMTs are evacuated and sealed so that they are commonly used under atmospheric pressure conditions. This paper describes an atmospheric pressure ion detector based on coupling a PMT with light emitted from ion-ion neutralization reactions. The normal Faraday plate collector electrode was replaced with an electrode "needle" used to concentrate the anions as they were drawn to the tip of the needle by a strong focusing electric field. Light was emitted near the surface of the electrode when analyte ions were neutralized with cations produced from the anode. Although radiative-ion-ion recombination has been previously reported, this is the first time ions from separate ionization sources have been combined to produce light. The light from this radiative-ion-ion-neutralization (RIIN) was detected using a photon multiplier such that an ion mobility spectrum was obtained by monitoring the light emitted from mobility separated ions. An IMS spectrum of nitroglycerin (NG) was obtained utilizing RIIN for tranducing the mobility separated ions into an analytical signal. The implications of this novel ion transduction method are the potential for counting ions at atmospheric pressure and for obtaining ion specific emission spectra for mobility separated ions.

A simplified toroidal ion trap mass analyzer

A radiofrequency toroidal ion trap mass analyzer comprised of four cylindrical electrodes is described. The ion trap consists of two RF electrodes and an AC electrode that surround a central electrode. Mass-analyzed ions are ejected through a slit in the central cylinder. A conversion dynode and electron multiplier are contained within the central cylinder for ion detection. The orientation of the RF and AC electrodes allows for inward-radial ejection of ions, removing the need for ion collection optics. Resolution of the toluene m/z 91 ion was determined for both a forward mass selective instability scan as well as for a reverse scan with resonant ejection. These experiments were performed in parallel with the stretching and compressing of the RF ring electrode spacing. Experimental resolution (Δm) values compare well with previous toroidal ion traps. The stability diagram for the toluene m/z 91 ion is also reported. The diagram possesses a slightly asymmetric profile due to asymmetry of the trapping dimensions as well as the curvature of the toroidal trapping region. Finally, the tandem (MS2) mass spectrum of iso-butylbenzene is reported.

Detection of Explosives as Negative Ions Directly from Surfaces Using a Miniature Mass Spectrometer

A miniature mass spectrometer was modified by incorporating a conversion dynode detector system and the appropriate electronics to allow the detection of negatively charged ions. The system was fitted with a discontinuous atmospheric pressure interface to allow external ionization by desorption electrospray ionization (DESI). It was used to identify the explosives 2,4,6-trinitrotoluene (TNT), 2,4,6-trinitrophenyl-N-methylnitramine (Tetryl), and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX) present in trace amounts on surfaces (500 pg/cm(2) to 1 microg/cm(2)) both individually and as components of mixtures. Detection of explosives was demonstrated in the presence of an interfering matrix. A large surface (5 cm x15 cm) on which 1 microg/cm(2) samples of TNT, Tetryl, and HMX had been spotted randomly was interrogated in 22 s in the full scan mode, and signals characteristic of each of the explosives were observed in the DESI mass spectrum.

Multiplexed Four-Channel Rectilinear Ion Trap Mass Spectrometer

A four-channel multiplexed mass spectrometer with rectilinear ion trap (RIT) mass analyzers was designed, constructed, and characterized. The system consists of four parallel atmospheric pressure ion (API) sources, four RIT mass analyzers, four sets of ion optical elements, and four conversion dynode detectors. The complete instrument is housed in a single vacuum manifold with a common vacuum system. It has a relatively small footprint, and costs and complexity were minimized and controls simplified by sharing the electronics and control modules among different channels. Each channel of the instrument can be operated in either positive or negative ion mode with a choice of ionization methods to improve the information content from an experiment. Also, the instrument is equipped with simultaneous data acquisition capabilities from all four channels, but the use of a common RF electronics system limits the degree to which the analyzer channels can be scanned independently. The instrument was characterized over the mass/charge range of 150 to 1300 Th. Mass misassignments in different ion traps because of machining and assembly tolerances were avoided by the application of supplementary direct current signals to each mass analyzer to correct mass offsets. A multiplexed automatic gain control (AGC) scheme was developed to control the ion population in each of the traps independently. These two features allow tandem mass spectrometry to be performed with an isolation window of 1 Th so trapping identical ions in all four channels. There are two principal modes of operation. In one, the same sample is analyzed in all four channels using different ionization methods to increase the information content of the analysis. In the other mode of operation, different samples are analyzed in all four channels with the same ionization method, so providing higher throughput. These capabilities were demonstrated by examining lipids produced by Escherichia coli and complex mixtures containing drugs of abuse.

Epitope mapping on bovine prion protein using chemical cross‐linking and mass spectrometry

An analytical strategy for the analysis of antigen epitopes by chemical cross-linking and mass spectrometry is demonstrated. The information of antigen peptides involved in the binding to an antibody can be obtained by monitoring the antigen peptides modified by a partially hydrolyzed cross-linker in the absence and in the presence of an antibody. This approach was shown to be efficient for characterization of the epitope on bovine prion protein bPrP(25-241) specifically recognized by a monoclonal antibody, 3E7 (mAb3E7), with only a small amount of sample (200 picomoles) needed. After cross-linking of the specific immuno complex, a matrix-assisted laser desorption/ionization (MALDI) mass spectrometer equipped with an ion conversion dynode (ICD) high-mass detector was used to optimize the amount of cross-linked complex formed at 202 kDa before proteolytic digestion. To identify the cross-linked peptides after proteolysis without ambiguity, isotope-labeled cross-linkers, disuccinimidyl suberate (DSS-d0/d12) and disuccinimidyl glutarate (DSG-d0/d6), together with high-resolution Fourier transform ion-cyclotron resonance mass spectrometry (FTICR-MS) were used. As a result, a complete fading of the peak intensities corresponding to the peptides representing the epitope was observed when bPrP/mAb3E7 complexes were formed.

Bipolar Ion Detector Based on Sequential Conversion Reactions

This work demonstrates the feasibility of a novel scintillation detector with greater detection efficiency than that of chevron-type microchannel plate (MCP) detectors. The detection mechanism involves sequential conversion reactions induced by ion-surface impacts. Identical detection conditions can be utilized to monitor both positive and negative ions in mass spectrometers. The proposed detector comprises an ion beam guiding device, a negatively biased washer-shaped conversion dynode, and an aluminum-coated scintillation detector. The beam guide changes the electric field around the washer-shaped conversion dynode, and it allows the primary and secondary ions to propagate toward the scintillation phosphor and the conversion dynode, respectively. The detection is achieved by the detection of electron-induced luminescence on a phosphor. The amplification efficiency of this bipolar ion detector increases as the conversion dynode voltage increases. For ions with a mass-to-charge ratio of up to 90 000, the sensitivity of the BID is 1.4-14.4 times that of the MCP. Further improvement of the sensitivity can be achieved by increasing the conversion dynode voltage or the ion acceleration voltage. Results of this study demonstrate that this detector is a promising alternative for efficient ion detection.

Detection of large cluster ions by time-of-flight mass spectrometry with a conversion dynode

A linear time-of-flight mass spectrometer (TOF-MS) has been constructed to measure mass distribution of negatively-charged clusters formed in the hot filament diamond chemical vapor deposition (CVD) process and in the metal evaporation. A conversion dynode made of CsI was employed to enhance detection efficiency of the large clusters greater than 10 000 amu. An atomic peak from the tungsten evaporation was used for the mass calibration of the carbon clusters. The carbon clusters produced in the CVD diamond process was loosely bound so that they were easily fragmented during passage through accelerating electric fields. The mass distribution measured with the TOF-MS system showed much less abundance of large clusters than those measured previously with the energy analyzer and Wien filter.

Precise in situ measurements of isotopic abundances with pulse counting of sputtered ions

Ion counting with an electron multiplier (EM) is necessary when sputtered ions are used for in situ and precise isotopic abundance measurements (10 −4 $(1\sigma)$ ) on small sample volume (about 100 μ m 3 for pure silicon). Measurements were performed on silicon samples bombarded with Cs + ions by extracting negative secondary monatomic Si − ions. Pulse-height distributions ( PHD ) and isotopic ratios were used as diagnostic tools for repeatability studies. Repeatability could be greatly improved by determining the optimal position of the impact area on the conversion dynode and by addressing each isotopic beam properly focused on this area (adaptive optics). A simplified model based on Poisson's laws was developed to fit PHD s and allowed us to calculate quantum detection efficiencies versus thresholds. EM isotopic discriminations were determined with the resulting semiempirical algorithm so as to reconstruct the lost information and get data independent of threshold setting. To reach consistent results, quasi-simultaneous arrivals ( QSA ) on the conversion dynode had to be assumed and modelled using direct ionisation yields Si − / Cs + at different collection efficiencies. The QSA corrected data fitted well on the terrestrial isotopic fractionation line. Dead time uncertainties and possible emission non-linear isotopic fractionation processes were examined. PHD s from other elements and polyatomic ions were also discussed.

Surface Modification Using a Commercial Triple Quadrupole Mass Spectrometer

We report instrumental modifications to a commercial mass spectrometer that allow surface modification experiments to be performed using low-energy (electronvolt range) mass-selected ion beams. The design of the detector housing allows placement of the surface on the ion optical axis and some distance beyond the off-axis detector. Manipulation of the potentials applied to the final lens, detector housing, conversion dynode, and electron multiplier allow the ions to pass through the detector housing and impinge upon the surface without loss of the normal mode of detector operation. Ex situ analysis of the modified surface is performed using a home-built multisector mass spectrometer. The ability to modify organic thin films is demonstrated by a number of soft landing and surface modification experiments including (i) soft landing of (CH3)2SiNCS+ ions formed from trimethylsilyl isothiocyanate upon a fluorinated self-assembled monolayer (F-SAM) surface, (ii) soft landing and dissociative soft landing of the pseudomolecular cation of triphenylpyrylium tetrafluoroborate, viz. the triphenylpyrylium cation, upon an F-SAM surface, (iii) dissociative soft landing of 35ClCH2(CH3)2SiOSi(CH3)2+ formed from 1,3-bis(chloromethyl)disiloxane upon an F-SAM surface, (iv) surface passivation by reaction of the trimethylsilyl cation, Si(CH3)3+, with a hydroxyl-terminated self-assembled monolayer (OH-SAM), and (v) transhalogenation by reaction of CCl3+ (m/z 119) with an F-SAM surface.

Dependence of detector dead time on analyte mass number in inductively coupled plasma mass spectrometry

ICP mass spectrometry is being increasingly used for isotopic analysis and for element determination by means of isotope dilution. For accurate isotope ratio determination, both the mass discrimination and the detector dead time have to be appropriately corrected for. For a Finnigan-MAT Element and a Perkin-Elmer SCIEX Elan 5000 ICP mass spectrometer, the constancy of the detector dead time across the mass range was systematically investigated. For the Element, which is equipped with a conversion dynode and a secondary electron multiplier with discrete dynodes, the dead time was observed to be independent of the analyte mass number. For the Elan 5000, however, which is equipped with a Channeltron—continuous dynode—electron multiplier, the detector dead time was observed to be strongly mass-dependent. The decreasing gain of Channeltron electron multipliers with increasing ion mass may be at the origin of the behaviour established. This observation has to be taken into account when highly accurate isotope ratio measurements are required and hence the detector dead time has to be properly determined and corrected for.

On-line sampling and intact mass analysis of nanometer-size aerosols via time-of-flight high-mass spectrometry

A device capable of continuously sampling and mass analyzing aerosols in the 1–10 nm diameter size range (masses up to 1 MDa) at part per billion (ppb, 10 10 cm −3) concentrations is described. Small aliquots of an aerosol flowing at atmospheric pressure are sampled into a time-of-flight mass spectrometer (TOFMS) via a pulsed molecular beam valve at 10–50 Hz. The aerosol molecular beam is singly and nondestructively ionized with light from an ultraviolet excimer laser and accelerated across a 20 kV electrostatic field. Ionized particle packets are separated in mass during a free flight and re-accelerated across an additional 30 kV into a stainless steel conversion dynode plate. Signals from ejected electrons and negative ion fragments resulting from the particle-dynode collisions are amplified in a dual microchannel plate detector, digitized, and stored in a fast transient recorder. Sampling of He flow streams bearing benzene, fullerenes, as well as Na, Mg, and CsI particles (nanocrystals) is demonstrated. In addition, the gas-phase reaction between a pre-formed Na nanocrystal and alkane thiols is monitored in real-time.

An ion-to-photon conversion detector for mass spectrometry

An ion-to-photon conversion detector (IPD) for time-of-flight mass spectrometry was studied and tested with ions produced by matrix-assisted laser desorption-ionization. The detector consisted of a conversion surface located at the end of the drift tube of a time-of-flight mass spectrometer and, behind it, a head-on photomultiplier tube. Fluorescent organic scintillator materials like Bu-PBD [2-(4- t-buthylphenyl)-5-(4-biphenylyl)-1,3,4-oxidiazole] were found to be the most efficient converters of those materials tested. Similar mass resolutions were found with the ion-to-photo detector and standard microchannel plates in a linear time-of-flight instrument. The background noise of the IPD was more intense than with microchannel plates. Slow unfocused ions are suspected to contribute to this noise. Test analytes as large as 70 000 Da could be measured with the IPD. Even with no secondary particle conversion surface in front of the IPD, masses up to approximately 20 000 Da may be more efficiently detected with the IPD than the MCP. For higher masses, a conversion dynode should be considered for increased signal.

Isotopic fractionation in secondary ionization mass spectrometry

AbstractWe have studied isotopic fractionation effects which occur during the analysis of oxygen isotope ratios by secondary ionization mass spectrometry. Variable instrumental isotopic fractionation has been well documented in the past as making reliable ion microprobe analyses of oxygen isotopes elusive. We report here techniques for minimizing these effects by careful optimization of an Isolab 54 ion microprobe and a method of integrating the secondary‐ion beam to eliminate a major source of fractionation caused by unequal focusing of isotopes on the source slit of the mass spectrometer. A further improvement has been the installation of new ion detectors for measuring 18O− and 17O− secondary ion beams in which the ions strike a conversion dynode (Conversion dynode system—CDS) to release electrons into a channeltron. The CDS detectors have a uniform, high and stable gain which, coupled with the beam integration techniques referred to above, yield reproducible oxygen isotope ratios on both conducting and insulating mineral standards (⩽1.2‰ on 18O/16O during a day and <2‰ over periods of days and weeks).

Secondary‐ion generation from large keV molecular primary ions incident on a stainless‐steel dynode

AbstractPositive and negative secondary ions from large (up to 40 KDa) peptide and protein primary ions incident at 5–40 keV collisional energies on a stainless‐steel (mesh) conversion dynode have been investigated in a tandem (reflectron) time‐of‐flight arrangement. Negative secondary‐ion spectra consisted mainly of H− and C2H−, C2H ions; positive secondary‐ion spectra were more complex and contained prominent signals due to H+, Na+ and K+, and further signals at m/z. 28, 41, 43, 45, 73 up to m/z 326. The secondary mass spectra were qualitatively very similar amongst the various peptide primaries and not different from those of other organic projectiles such as, for example, coronene or cyclodextrin. With increasing mass of the primary ion, ratios of the secondary‐ion abundances changed drastically in the negative secondary‐ion spectra (increase of higher m/z signals, decrease of H−). Since most of the positive secondary ions could be attributed to a silicon‐organic surface contamination of the dynode (vacuum grease), it is concluded that the formation of such secondary ions is due to sputtering rather than surface‐induced dissociation.

A position-sensitive microchannel-plate detector for 27-keV ions

We have improved a low background-rate microchannel-plate detector for 15 to 27 keV ions by using a strip anode. This detector uses an aluminum oxide conversion dynode which is impacted by the ions at grazing incidence. A number of secondary electrons are liberated from the dynode and pulled into the microchannel plate which is operated in saturation. The charges are summed by the strip anode to produce an output signal proportional to both the number of the secondary electrons and the number of the strips. From analysing the two-dimensional spectrum it is indicated that the background rate is 2 × 10−4 s−1 at 80% counting efficiency, or 8 × 10−5 s−1 at 50% for 27 keV 40Ar ions.

Optimized conversion dynode positioning for a sector mass spectrometer operated in normal, linked scan, or low accelerating voltage modes

The simple, correct positioning of the conversion dynode in a common sector type of mass spectrometer has been shown to increase the signal obtained in a variety of experiments by a factor of at least 3–5. In addition, with incorrect positioning, small vibrations of 6.8 μm are suggested to lead to 10% signal fluctuations. No degradation of any instrument performance has been observed with the changes made to allow optimization of the position of the conversion dynode. Details of the procedure necessary to implement this technique are given. The benefits of increasing the conversion dynode potential from 5 to 10 kV, for enhanced sensitivity and its effect on conversion dynode positioning, are also discussed. Careful positioning of the conversion dynode in a sector type mass spectrometer yields several advantages. The enhancement is best applied in low accelerating voltage or linked scan modes of operation.

Study of clusters by mass spectrometry

A review of mass spectrometric methods used to study different kinds of clusters is presented. These methods have proved to be indispensable in the cluster field owing to their great flexibility, sensitivity and wide dynamic range. This reviews presents new mass spectrometric methods and techniques such as quadrupole analysers and RF lenses, different ion-trap geometries, time-of-flight (TOF) methods including the reflectron geometry). A variety of ionization methods, both for the production of positive and negative ions, with different amounts of released excess energy, are discussed in detail. Special attention is given to different degree of post-ionization fragmentaqtion and metastability of ions produced by the respective ionization methods. The ability to follow the metastable decay processes in rather different time-windows is also one of the salient features of the mass spectrometric methods reviewed here, involving both classical and new geometries. Detection of very large cluster ions by conventional methods is not always effective, and alternative methods like post-acceleration and using a conversion dynode in front of the multiplier will be included here. Moreover, mass spectrometric methods offer the possibility of producing beams of mass-selected cluster ions necessary for the study of a whole line of process like optical spectrometry, photodetachment, photofragmentation, photoionization, electron beam ionization, and charge-exchange processes. Examples of these processes are also discussed in this review.