Halo ion trap mass spectrometer

We describe a novel radio frequency ion trap mass analyzer based on toroidal trapping geometry and microfabrication technology. The device, called the halo ion trap, consists of two parallel ceramic plates, the facing surfaces of which are imprinted with sets of concentric ring electrodes. Radii of the imprinted rings range from 5 to 12 mm, and the spacing between the plates is 4 mm. Unlike conventional ion traps, in which hyperbolic metal electrodes establish equipotential boundary conditions, electric fields in the halo ion trap are established by applying different radio frequency potentials to each ring. The potential on each ring can be independently optimized to provide the best trapping field. The halo ion trap features an open structure, allowing easy access for in situ ionization. The toroidal geometry provides a large trapping and analyzing volume, increasing the number of ions that can be stored and reducing the effects of space-charge on mass analysis. Preliminary mass spectra show resolution (m/Deltam) of 60-75 when the trap is operated at 1.9 MHz and 500 Vp-p.     

Ion soft landing using a rectilinear ion trap mass spectrometer

A new ion soft landing instrument has been built for the controlled deposition of mass selected polyatomic ions. The instrument has been operated with an electrospray ionization source; its major components are an electrodynamic ion funnel to reduce ion loss, a 90-degree bent square quadrupole that prevents deposition of fast neutral molecules onto the landing surface, and a novel rectilinear ion trap (RIT) mass analyzer. The ion trap is elongated (inner dimensions: 8 mm x 10 mm x 10 cm). Three methods of mass analysis have been implemented. (i) A conventional mass-selective instability scan with radial resonance ejection can provide a complete mass spectrum. (ii) The RIT can also be operated as a continuous rf/dc mass filter for isolation and subsequent soft landing of ions of the desired m/ z value. (iii) The 90-degree bent square quadrupole can also be used as a continuous rf/dc mass filter. The mass resolution (50% definition) of the RIT in the trapping mode (radial ion ejection) is approximately 550. Ions from various test mixtures have been mass selected and collected on fluorinated self-assembled monolayers on gold substrates, as verified by analysis of the surface rinses. Desorption electrospray ionization (DESI) has been used to confirm intact deposition of [Val (5)]-Angiotensin I on a surface. Nonmass selective currents up to 1.1 nA and mass-selected currents of up to 500 pA have been collected at the landing surface using continuous rf/dc filtering with the RIT. A quantitative analysis of rinsed surfaces showed that the overall solution-to-solution soft landing yields are between 0.2 and 0.4%. Similar experiments were performed with rf/dc isolation of both arginine and lysine from a mixture using the bent square quadrupole in the rf/dc mode. The unconventional continuous mass selection methods maximize soft landing yields, while still allowing the simple acquisition of full mass spectra.     

A quadrupole ion trap with cylindrical geometry operated in the mass-selective instability mode

A cylindrical geometry ion trap is used to record mass spectra in the mass-selective instability mode. The geometry of the cylindrical ion trap has been optimized to maximize the quadrupole field component relative to the higher-order field content through field calculations using the Poisson/Superfish code and through experimental variation of the electrode structure. The results correspond well with predictions of the calculations. The trap has been used to record mass spectra with better than unit mass resolution, high sensitivity, and a mass/charge range of ～600 Th. Multistage (MS(3)) experiments have been performed, and the Mathieu stability region has been experimentally mapped. The performance of this device compares satisfactorily with that of the hyperbolic ion trap.     

Rectilinear ion trap mass spectrometer with atmospheric pressure interface and electrospray ionization source

A rectilinear ion trap (RIT) mass analyzer was incorporated into a mass spectrometer fitted with an electrospray ionization source and an atmospheric pressure interface. The RIT mass spectrometer, which was assembled in two different configurations, was used for the study of biological compounds, for which performance data are given. A variety of techniques, including the use of a balanced rf, elevated background gas pressure, automatic gain control, and resonance ejection waveforms with dynamically adjusted amplitude, were applied to enhance performance. The capabilities of the instrument were characterized using proteins, peptides, and pharmaceutical drugs. Unit resolution and an accuracy of better than m/z 0.2 was achieved for mass-to-charge (m/z) ratios up to 2000 Th at a scan rate of approximately 3000 amu/(charge.s) while reduced scan rates gave greater resolution and peak widths of less than m/z 0.5 over the same range. The mass discrimination in trapping externally generated ions was characterized over the range m/z 190-2000 and an optimized low mass cutoff value of m/z 120-140 was found to give equal trapping efficiencies over the entire range. The radial detection efficiency was measured as a function of m/z ratio and found to rise from 35% at low m/z values to more than 90% for ions of m/z 1800. The way in which the ion trapping capacity depends on the dc trapping potential was investigated by measuring the mass shift due to space charge effects, and it was shown that low trapping potentials minimize space charge effects by increasing the useful volume of the device. The collision-induced dissociation (CID) capabilities of the RIT instrument were evaluated by measuring isolation efficiency as a function of mass resolution as well as measuring peptide CID efficiencies. Overall CID efficiencies of more than 60% were easily reached, while isolation of an ion with unit resolution at m/z 524 was achieved with high rejection (>95%) of the adjacent ions. The overall analytical capabilities of the ESI-RIT instrument were demonstrated with the analysis of a mixture of pharmaceutical compounds using multiple-stage mass spectrometry.     

A miniature cylindrical quadrupole ion trap: simulation and experiment

A cylindrical quadrupole ion trap (r(0) = 2.5 mm, z(0) = 2.88 mm, ～(1)/(64) of the volume of commercial hyperbolic ion traps) has been constructed, its geometry optimized, and its performance examined in the mass-selective instability scan mode. Spectra of ionized perfluorotributylamine and o-dichlorobenzene show a resolution (m/Δm, 50% valley definition) of ～100. The instrument has been coupled to a membrane introduction system to test its applicability for on-line reaction monitoring and to determine detection limits. Simulations using the ion trap simulation program are used to explore the effects of geometry on performance and to validate the experimental results.     

离子在矩形波四极场中的运动与四极场离子阱质谱的数字化操作

矩形波驱动的四极场同样可用于对离子进行质量分析和存储。本文推导了离子在矩形波四极场中的运动规律,并以常用于正弦波四极场的马绍方程参量a,q来表出离子的稳定性。运用赝势阱概念探讨了离子在这种四极场中的本征振动,推导了本征振动频率的近似公式。在理论研究的基础上,提出了数字化离子阱质谱的概念,并运用离子光学模拟,研究了实际离子阱几何结构下的共振出射以及数字化质量扫描的可行性。     

Multiplexed rectilinear ion trap mass spectrometer for high-throughput analysis

A multichannel mass spectrometer based on the rectilinear ion trap (RIT) analyzer was designed and constructed for simultaneous high-throughput analysis of multiple samples. The instrument features four parallel ion source/mass analyzer/detector channels assembled in a single vacuum chamber and operated using a common set of control electronics, including a single rf amplifier and transformer coil. This multiplexed RIT mass spectrometer employs an array of four millimeter-sized ion traps (x(o) = 5.0 mm and y(o) = 4.0 mm, where x(o) and y(o) are the half-distances in the x and y dimensions, respectively). Mass spectra are acquired from four different samples simultaneously. The available mass/charge range is m/z 15-510 with excellent linearity of the mass calibration (R2 = 0.999 999). The peak width is less than 0.3 mass/charge units at m/z 146, corresponding to a resolution of approximately 500. Simultaneous MS/MS of ions due to four compounds (3-fluoroanisole, 4-fluoroanisole, 2-fluorobenzyl alcohol, 2,6-dimethylcyclohexanone) with the same nominal molecular radical cation but distinctive fragmentation patterns was demonstrated. Isolation and fragmentation efficiencies were approximately 25 and approximately 75%, respectively, measured in the typical case of the molecular radical cation of acetophenone. Preacquisition differential data were obtained by real-time subtraction of the ion signals from two channels of the multiplexed mass spectrometer. The differential experiment presented offers proof of principle of comparative mass spectra in high-throughput screening applications while reducing data storage requirements.     

Ion trap/ion mobility/quadrupole/time-of-flight mass spectrometry for peptide mixture analysis

An ion trap/ion mobility/quadrupole/time-of-flight mass spectrometer has been developed for the analysis of peptide mixtures. In this approach, a mixture of peptides is electrosprayed into the gas phase. The mixture of ions that is created is accumulated in an ion trap and periodically injected into a drift tube where ions separate according to differences in gas-phase ion mobilities. Upon exiting the drift tube, ions enter a quadrupole mass filter where a specific mass-to-charge (m/z) ratio can be selected prior to collisional activation in an octopole collision cell. Parent and fragment ions that exit the collision cell are analyzed using a reflectron geometry time-of-flight mass spectrometer. The overall configuration allows different species to be selected according to their mobilities and m/z ratios prior to collision-induced dissociation and final MS analysis. A key parameter in these studies is the pressure of the target gas in the collision cell. Above a critical pressure, the well-defined mobility separation degrades. The approach is demonstrated by examining a mixture of tryptic digest peptides of ubiquitin.     

Ambient mass spectrometry with a handheld mass spectrometer at high pressure

The first coupling of atmospheric pressure ionization methods, electrospray ionization (ESI) and desorption electrospray ionization (DESI), to a miniature hand-held mass spectrometer is reported. The instrument employs a rectilinear ion trap (RIT) mass analyzer and is battery-operated, hand-portable, and rugged (total system: 10 kg, 0.014 m(3), 75 W power consumption). The mass spectrometer was fitted with an atmospheric inlet, consisting of a 10 cm x 127 microm inner diameter stainless steel capillary tube which was used to introduce gas into the vacuum chamber at 13 mL/min. The operating pressure was 15 mTorr. Ions, generated by the atmospheric pressure ion source, were directed by the inlet along the axis of the ion trap, entering through an aperture in the dc-biased end plate, which was also operated as an ion gate. ESI and DESI sources were used to generate ions; ESI-MS analysis of an aqueous mixture of drugs yielded detection limits in the low parts-per-billion range. Signal response was linear over more than 3 orders of magnitude. Tandem mass spectrometry experiments were used to identify components of this mixture. ESI was also applied to the analysis of peptides and in this case multiply charged species were observed for compounds of molecular weight up to 1200 Da. Cocaine samples deposited or already present on different surfaces, including currency, were rapidly analyzed in situ by DESI. A geometry-independent version of the DESI ion source was also coupled to the miniature mass spectrometer. These results demonstrate that atmospheric pressure ionization can be implemented on simple portable mass spectrometry systems.     

A tandem ion trap/ion mobility spectrometer

A tandem quadrupole ion trap/ion mobility spectrometer (QIT/IMS) has been constructed for structural analysis based on the gas-phase mobilities of mass-selected ions. The instrument combines the ion accumulation, manipulation, and mass-selection capabilities of a modified ion trap mass spectrometer with gas-phase electrophoretic separation in a custom-built ion mobility drift cell. The quadrupole ion trap may be operated as a conventional mass spectrometer, with ion detection using an off-axis dynode/multiplier arrangement, or as an ion source for the IMS drift cell. In the latter case, pulses of ions are ejected from the trap and transferred to the drift cell where mobility in the presence of helium buffer gas is determined by the collision cross section of the ion. Ions traversing the drift cell are detected by an in-line electron multiplier and the data processed with a multichannel scaler. Preliminary data are presented on instrumental performance characteristics and the application of QIT/ IMS to structural and conformational studies of aromatic ions and protonated amine/crown ether noncovalent complexes generated via ion/molecule reactions in the ion trap.     

Handheld rectilinear ion trap mass spectrometer

A shoebox-sized, 10-kg, handheld mass spectrometer, Mini 10, based on a rectilinear ion trap mass analyzer has been designed, built, and characterized. This instrument has evolved from a decade-long experimental and simulation program in mass spectrometer miniaturization. The rectilinear ion trap has a simplified geometry and high trapping capacity, and when used with a miniature and ruggedized pumping system, it allows chemical analysis while the instrument is being carried. Compact electronics, including an air core RF drive coil, were developed to control the instrument and to record mass spectra. The instrument runs on battery power, consuming less than 70 W, similar to a laptop computer. Wired and wireless networking capabilities are implemented. The instrument gives unit resolution and a mass range of over m/z 500. Tandem mass spectrometry capabilities are implemented using collision-induced dissociation, and they are used to provide confirmation of chemical structure during in situ analysis. Continuous monitoring of air and solution samples is demonstrated, and a limit of detection of 50 ppb was obtained for toluene vapor in air and for an aqueous naphthalene solution using membrane sample introduction.     

A "screened" electrostatic ion trap for enhanced mass resolution, mass accuracy, reproducibility, and upper mass limit in Fourier transform ion cyclotron resonance mass spectrometry

Until now, it was thought that the optimal static electromagnetic ion trap for Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry should be designed to produce a quadrupolar electrical potential, for which the ion cyclotron frequency is independent of the ion's preexcitation location within the trap. However, a quadrupolar potential results in a transverse (to the magnetic field) electric field that increases linearly with distance from the center of the trap. That radially linear electric field shifts the observed ICR frequency, increases the ICR orbital radius, and ultimately limits the highest mass-to-charge ratio ion that can be contained within the trap. In this paper, we propose a new static electromagnetic ion "trap" in which grounded screens placed just inside the usual "trapping" plates produce a good approximation to a "particle-in-a-box" potential (rather than the quadrupolar "harmonic oscillator" potential). SIMION calculations confirm that the electric potential of the screened trap is near zero almost everywhere within the trap. For our screened orthorhombic (2.5 in. X 2 in. X 2 in.) trap, the experimental ICR frequency shift due to trapping voltage is reduced by a factor of approximately 100, and the experimental variation of ICR frequency with ICR radius is reduced by a factor of approximately 10 compared to a conventional (unscreened) 2-in. cubic ion trap.(ABSTRACT TRUNCATED AT 250 WORDS)     

矩形离子阱质量分析过程的数值模拟

传统的四极离子阱质量分析器中离子的运动可用二阶线性微分方程——Mathieu方程的解来描述。根据Mathieu稳定图所反映的离子运动特性,可实现离子的质谱分析。但Mathieu方程对平板电极的矩形离子阱描述不再准确适用,本文通过数值模拟的方法,依次对矩形离子阱质量分析器内电场分布、离子运动轨迹、离子阱的Mathieu稳定图以及标准工作模式下获得的质谱图进行了模拟和分析。     

VUV single-photon ionization ion trap time-of-flight mass spectrometer for on-line, real-time monitoring of chlorinated organic compounds in waste incineration flue gas

In this work, a sensitive and robust vacuum ultra-violet (VUV) single-photon ionization (SPI) ion trap time-of-flight mass spectrometer (VUV-SPI-IT-TOFMS) for on-line, realtime monitoring of chlorinated organic compounds in waste incineration flue gas has been newly developed. The fragment-free SPI technique with 121.6-nm VUV lamp irradiated by a microwave generator and the quadrupole ion trap to accumulate and select analyte ions were combined with a reflectron time-of-flight mass spectrometer to detect chlorinated organic compounds at trace level. This measuring system was tuned up to detect dioxins precursors with the aim at an application to monitoring trace level toxic substances in flue gases from incinerator furnaces. As a result, this technology has made it possible to analyze trichlorobenzene (T3CB), a dioxin precursor, in 18 s with a sensitivity of 80 ng/m3-N (10 pptv) using the selective accumulation of analyte substances and separation of interfering substances in the ion trap. Moreover, the first field test of the continuous monitoring T3CB in an actual waste incineration flue gas had been done for 7 months. The results show that this system has an exceeding robust performance and is able to maintain the high sensitivity in analyzing T3CB for long months of operation.     

Sampling wand for an ion trap mass spectrometer

A new sampling wand concept for ion trap mass spectrometers equipped with discontinuous atmospheric pressure interfaces (DAPI) has been implemented. The ion trap/DAPI combination facilitates the operation of miniature mass spectrometers equipped with ambient ionization sources. However, in the new implementation, instead of transferring ions pneumatically from a distant source, the mass analyzer and DAPI are separated from the main body of the mass spectrometer and installed at the end of a 1.2 m long wand. During ion introduction, ions are captured in the ion trap while the gas in which they are contained passes through the probe and is pumped away. The larger vacuum volume due to the extended wand improves the mass analysis sensitivity. The wand was tested using a modified hand-held ion trap mass spectrometer without additional power or pumping being required. Improved sensitivity was obtained as demonstrated with nano-electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), and low temperature plasma (LTP) probe analysis of liquid, gaseous, and solid samples, respectively.     

Monitoring structural changes of proteins in an ion trap over approximately 10-200 ms: unfolding transitions in cytochrome c ions.

A new technique for studying the time dependence of conformational changes of gas-phase protein ions is described. In this approach, a short pulse of electrosprayed protein ions is introduced into an ion trap and stored. After a defined time period, the distribution of ions is ejected from the trap into an ion mobility/time-of-flight mass spectrometer. Combined measurements of mobilities and flight times in the mass spectrometer provide information about the abundances of different conformer types and charge-state distributions. By varying the storage time in the trap, it is possible to monitor changes in ion conformation that occur over extended time periods (approximately 10-200 ms). The method is demonstrated by examining changes in cytochrome c ion conformations for the +7 to +10 charge states.     

Cross-correlation algorithm for calculation of peptide molecular weight from tandem mass spectra

We have developed an approach to identify the molecular weight of a peptide ion directly from its corresponding tandem mass spectrum using a cross-correlation function. We have shown that the monoisotopic molecular weight can be calculated for approximately 90% of tandem mass spectra identified from tryptic digests of complex protein mixtures. The accuracy of the calculated monoisotopic masses was dependent on the resolution and mass accuracy of the spectra analyzed, but was typically <0.25 amu for linear ion trap mass spectra. The ability to calculate accurate monoisotopic molecular weights for low-resolution ion trap data should significantly improve both the speed and performance of database searches for which typical mass accuracies of approximately 3 amu are employed. In addition, this strategy can also be used to identify the precursor ion for tandem mass spectra acquired using large ion selection windows in data-independent collision-activated dissociation and has the potential to identify multiplexed tandem mass spectra.     

Axial ion focusing in a miniature linear ion trap

A novel miniature linear ion trap with a total length of 19 mm and a quadrupole rod length of 15 mm has been fabricated to enable ion focusing in the axial plane (between the end caps). Each end cap includes an inwardly projecting tubular section, which prevents dc fringe fields from penetrating to the center of the miniature linear ion trap and aids in ion extraction. Axial focusing of ion packets to dimensions of less than 1 mm through collisional cooling is predicted and demonstrated in the miniature linear ion trap. Due to this demonstrated collisional cooling, narrow kinetic energy distributions are also illustrated on batch ion extraction as might be useful for ion transfer to enable subsequent mass analysis.     

Detection of chemical warfare-related species on complex aerosol particles deposited on surfaces using an ion trap-based aerosol mass spectrometer

A new type of aerosol mass spectrometer was developed by minimal modification of an existing commercial ion trap to analyze the semivolatile components of aerosols in real time. An aerodynamic lens-based inlet system created a well-collimated particle beam that impacted into the heated ionization volume of the commercial ion trap mass spectrometer. The semivolatile components of the aerosols were thermally vaporized and ionized by electron impact or chemical ionization in the source. The nascent ions were extracted and injected into the ion trap for mass analysis. The utility of this instrument was demonstrated by identifying semivolatile analytes in complex aerosols. This study is part of an ongoing effort to develop methods for identifying chemical species related to CW agent exposure. Our efforts focused on detection of CW-related species doped on omnipresent aerosols such as house dust particles vacuumed from various surfaces found in any office building. The doped aerosols were sampled directly into the inlet of our mass spectrometer from the vacuumed particle stream. The semivolatile analytes were deposited on house dust and identified by positive ion chemical ionization mass spectrometry up to 2.5 h after deposition. Our results suggest that the observed semivolatile species may have been chemisorbed on some of the particle surfaces in submonolayer concentrations and may remain hours after deposition. This research suggests that identification of trace CW agent-related species should be feasible by this technique.     

When should we change the definition of the second?

The microwave caesium (Cs) atomic clock has formed an enduring basis for the second in the International System of Units (SI) over the last few decades. The advent of laser cooling has underpinned the development of cold Cs fountain clocks, which now achieve frequency uncertainties of approximately 5×10(-16). Since 2000, optical atomic clock research has quickened considerably, and now challenges Cs fountain clock performance. This has been suitably shown by recent results for the aluminium Al(+) quantum logic clock, where a fractional frequency inaccuracy below 10(-17) has been reported. A number of optical clock systems now achieve or exceed the performance of the Cs fountain primary standards used to realize the SI second, raising the issues of whether, how and when to redefine it. Optical clocks comprise frequency-stabilized lasers probing very weak absorptions either in a single cold ion confined in an electromagnetic trap or in an ensemble of cold atoms trapped within an optical lattice. In both cases, different species are under consideration as possible redefinition candidates. In this paper, I consider options for redefinition, contrast the performance of various trapped ion and optical lattice systems, and point to potential limiting environmental factors, such as magnetic, electric and light fields, collisions and gravity, together with the challenge of making remote comparisons of optical frequencies between standards laboratories worldwide.