A miniaturized matrix-assisted laser desorption/ionization time of flight mass spectrometer with mass-correlated acceleration focusing

Mass correlated acceleration (MCA) has now been integrated into a 4 in (10.2 cm) matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometer to achieve high resolving power across a broader mass range, without sacrificing detection of higher mass ions. The goal was to combine MCA with a custom-built miniaturized instrument such as those that might be used for field-portable applications. Unlike other pulsed extraction methods, MCA is not mass dependent and mass spectra can be achieved with a single tuning of instrument parameters. Additionally, the multi-channel recording advantage is better realized because ions of all masses can be brought into focus simultaneously. The MCA dual-stage ion source compensates dynamically for the mass dependence by incorporating an extraction pulse region followed by an acceleration region that contains a time-dependent waveform correlated with mass. The technique was validated with applications in peptide mixtures and protein digestions. Diagnostic studies for the instrument include m/z range and limits of detection.     

Development of an ion mobility spectrometer for use in an atmospheric pressure ionization ion mobility spectrometer/mass spectrometer instrument for fast screening analysis

An ion mobility spectrometer that can easily be installed as an intermediate component between a commercial triple-quadrupole mass spectrometer and its original atmospheric pressure ionization (API) sources was developed. The curtain gas from the mass spectrometer is also used as the ion mobility spectrometer drift gas. The design of the ion mobility spectrometer allows reasonably fast installation (about 1 h), and thus the ion mobility spectrometer can be considered as an accessory of the mass spectrometer. The ion mobility spectrometer module can also be used as an independently operated device when equipped with a Faraday cup detector. The drift tube of the ion mobility spectrometer module consists of inlet, desolvation, drift, and extraction regions. The desolvation, drift and extraction regions are separated by ion gates. The inlet region has the shape of a stainless steel cup equipped with a small orifice. Ion mobility spectrometer drift gas is introduced through a curtain gas line from an original flange of the mass spectrometer. After passing through the drift tube, the drift gas serves as a curtain gas for the ion-sampling orifice of the ion mobility spectrometer before entering the ion source. Counterflow of the drift gas improves evaporation of the solvent from the electrosprayed sample. Drift gas is pumped away from the ion source through the original exhaust orifice of the ion source. Initial characterization of the ion mobility spectrometer device includes determination of resolving power values for a selected set of test compounds, separation of a simple mixture, and comparison of the sensitivity of the electrospray ionization ion mobility spectrometry/mass spectrometry (ESI-IMS/MS) mode with that of the ESI-MS mode. A resolving power of 80 was measured for 2,6-di-tert-butylpyridine in a 333 V/cm drift field at room temperature and with a 0.2 ms ion gate opening time. The resolving power was shown to be dependent on drift gas flow rate for all studied ion gate opening times. Resolving power improved as the drift gas flow increased, e.g. at a 0.5 ms gate opening time, a resolving power of 31 was obtained with a 0.65 L/min flow rate and 47 with a 1.3 L/min flow rate for tetrabutylammonium iodide. The measured limits of detection with ESI-MS and with ESI-IMS/MS modes were similar, demonstrating that signal losses in the IMS device are minimal when it is operated in a continuous flow mode. Based on these preliminary results, the IMS/MS instrument is anticipated to have potential for fast screening analysis that can be applied, for example, in environmental and drug analysis.     

Accurate mass determination of a metabolite of a potential diagnostic imaging drug candidate by high performance liquid chromatography with time-of-flight mass spectrometry.

A potential drug candidate containing a multiplicity of sulphur atoms, and one unknown in vivo main metabolite, were analysed with an high performance liquid chromatograph time-of-flight mass spectrometer (LC/TOF-MS). Sensitivity and resolution were compared with those obtained with a quadrupole instrument. The LC/TOF-MS provided a more than 200-fold advantage in sensitivity, higher resolving power for the isotopic envelopes of the molecular ions produced by electrospray, and simple procedures for the determination of elemental composition. The exact masses of the candidate and its main metabolite were determined to within 1.5 to 3 ppm, using a single lock mass correction.     

Development of a high-Performance MALDI-TOF mass spectrometer utilizing a spiral ion trajectory

A novel MALDI-TOF mass spectrometer that utilizes a spiral ion trajectory was developed. In this mass spectrometer, the ions sequentially passed through four toroidal electrostatic sectors and revolved along a figure-eight-shaped orbit on a particular projection plane. Each toroidal electrostatic sector had eight stories, and during multiple revolutions, the ion trajectory shifted perpendicular to the projection plane in every cycle, thereby generating a spiral trajectory. The flight path length of one cycle was 2.1 m; therefore, when the ions completed eight cycles, the total flight path length was 17 m. By adopting an ion optical system that had a flight path length five times longer than that in the commonly used reflectron ion optical system, the mass dependence on the mass resolving power was reduced, while improving the mass accuracy of the mass measurements. The basic performance of the system was tested by using standard peptides or the tryptic digest of bovine serum albumin. A mass resolving power of 80,000 (full width at half maximum) was achieved at m/z = 2564 (ACTH18-39). An improved mass accuracy less than 2 ppm was realized over a wide m/z range of 500 to 3000 by correction using one or two internal standard substances.     

Characterization of large,heterogeneous proteins by electrospray ionization-mass spectrometry

Characterization of heterogeneous proteins as large as 150,000 u was performed by a quadrupole mass spectrometer by using electrospray ionization (ESI). We were able to determine not only the molecular weight, but the detailed heterogeneity for the large glycoproteins as well. The successful application was facilitated by the optimization of the instrument in the high mass-to-charge range up to m/z 4000, where the multiply charged envelopes of the 150,000-u glycoproteins were found. For the analysis of clinically important monoclonal antibodies mass spectral data acquired by this method were consistent with the carbohydrate analysis and were useful in resolving the monosaccharide data into glycoform variations. In the case of the characterization of other large, heterogeneous proteins such as elongation factor 3 and bovine serum albumin, the quadrupole ESI mass spectrometer provided adequate mass resolution and high mass measurement accuracy to discern the modification and degradation of the proteins.     

Coupling of nanoflow liquid chromatography to matrix-assisted laser desorption/ionization mass spectrometry: real-time liquid chromatography run mapping on a MALDI plate

The major obstacle in the use of matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) instruments in the analysis of complex proteome samples is the lack of a direct coupling of a highly resolving separation technique with the mass spectrometer itself. To overcome this drawback, a spotting device for capillary and nanoflow liquid chromatography (LC) with a special liquid deposition principle for lowest volumes was developed. The instrument is able to perform MALDI spotting in real time in order to deposit the LC run on the MALDI plate, and therefore couples the high resolution power of nano-RP-HPLC separation directly with MALDI-MS. This work describes the development and optimization of a method for spotting with online matrix addition, and illustrates its use in the analysis of a complex proteome sample.     

Hybrid mass spectrometers for tandem mass spectrometry

Mass spectrometers that use different types of analyzers for the first and second stages of mass analysis in tandem mass spectrometry (MS/MS) experiments are often referred to as "hybrid" mass spectrometers. The general goal in the design of a hybrid instrument is to combine different performance characteristics offered by various types of analyzers into one mass spectrometer. These performance characteristics may include mass resolving power, the ion kinetic energy for collision-induced dissociation, and speed of analysis. This paper provides a review of the development of hybrid instruments over the last 30 years for analytical applications.     

MICRA: a compact permanent magnet Fourier transform ion cyclotron resonance mass spectrometer

MICRA, a compact Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer is described. The amount of miniaturisation in this device, based on a 1.24 T permanent magnet, remains compatible with genuine FT-ICR performance and analytical power in the mass range 2-1000 m/z, with a mass resolving power of 73,000 at mass 132. A first application of the transportability is the repetitive coupling of MICRA with a large-scale source of IR photons, the free electron laser CLIO.     

Full Scan MS in Comprehensive Qualitative and Quantitative Residue Analysis in Food and Feed Matrices: How Much Resolving Power is Required?

In LC full scan based MS screening methods correct mass assignment is essential. Parameters affecting the accuracy of mass assignment, i.e., analyte concentration, complexity of the matrix, and resolving power, were studied using typical examples from the field of residue and contaminant analysis in food and feed. The evaluation was carried out by analyzing samples of honey and animal feed, spiked with 151 pesticides, veterinary drugs, mycotoxins, and plant toxins at levels ranging from 10 to 250 ng/g. Analyses were performed using a single stage Orbitrap with resolving power settings varying from 10,000 to 100,000 (FWHM). For consistent and reliable mass assignment (<2 ppm) of analytes at low levels in complex matrices, a high resolving power (≥50,000) was found to be required. At lower resolving power settings, the error in the assignment of mass increased due to the coelution of analytes with interferences at the same nominal mass. This negatively affected selectivity and quantitative performance due to the inability to use the required narrow mass-extraction windows. In the case of the less complex honey matrix, a resolving power of 25,000 was generally sufficient to obtain a mass assignment error close to the typical instrument mass accuracy (≤2 ppm) down to low concentration levels of 10 ng/g.     

A Quadrupole mass spectrometer for resolution of low mass isotopes

The qualitative and quantitative identification of low mass isotopes in the mass range 1–6 u poses certain difficulties when attempting to achieve the required resolution with an instrument suitable for deployment within a process environment. Certain adjacent species present in the process sample (HT and D₂) require a resolution greater than 930 to achieve an accurate measurement. We demonstrate here through simulation techniques that this level of performance required is unachievable using commercially available instruments. Using previously reported simulation techniques, this article demonstrates how the required performance for resolving the low mass isotopes can be achieved by a quadrupole mass spectrometer (QMS), which incorporates a quadrupole mass filter (QMF) constructed from hyperbolic electrodes and operated in zone 3 of the Mathieu stability diagram.     

An inductively coupled plasma-time-of-flight mass spectrometer for elemental analysis. Part I: Optimization and characteristics

An inductively coupled plasma-time-of-flight mass spectrometer (ICP-TOFMS) has been constructed and evaluated for elemental analysis. The instrument produces analog spectra similar to those from quadrupole inductively coupled plasma mass spectrometers. The large abundance of Ar ions is deflected away from the microchannel plate detector to reduce detector dead time and space-charge complications. The ICP-TOFMS, operated in a linear (nonreflecting) mode, currently has a resolving power of 500 (full width at half maximum). Present ion optics employed in the instrument require a trade-off between signal-to-noise ratio and resolving power. In addition, mass-dependent kinetic energies in the supersonic beam created in the ICP mass spectrometer interface cause a mass bias in the right-angle TOFMS because the ions must be steered to the detector to compensate for their velocity in the supersonic beam direction. In the current design the sampling duty cycle is only approximately 3%, thereby limiting sensitivity. However, positive potentials applied to the right-angle extraction region can increase sensitivity by a factor of 2–4 by slowing down the ions that enter the extraction zone. The transmission efficiency of the TOFMS is approximately 20% and is limited by divergence of the ion packet in the drift tube.     

Improved fourier-transform ion-cyclotron-resonance mass spectrometry of large biomolecules

Initial results from a Fourier-transform mass spectrometer with a 6.2 Tesla magnet using electrospray ionization show substantial improvements in resolution, mass accuracy, mass range, signal/noise, and tandem mass spectromehy capabilities compared to our earlier 2.8 T instrument that demonstrated the first unit resolution mass spectra of molecules as large as myoglobin (17 kDa). The new instrument exhibits greater than 10⁶ and 10⁵ resolving power for 8.6 and 29 kDa, respectively, proteins. Using an internal standard, the mass measuring error for myoglobin is less than 1 ppm. Nozzle-skimmer dissociation during electrospray of carbonic anhydrase (29 kDa) has yielded 38 fragment ions for which both mass and charge are identifiable; of these, 21 have been assigned to expected oligopeptide fragments.     

Construction and performance of a newly developed X-ray excited optical luminescence spectrometer

A newly developed XEOL spectrometer was constructed having the following features: (1) high energy X-ray tube and its well regulated power supply, (2) minimized leakage of scattered X-rays, (3) sample heating device suppressing host luminescence, and (4) improved resolving power of the optics.The direct determination of rare earth elements in La₂O₃ was explored using this instrument in a performance test. The results showed a great improvement in sensitivity and reproducibility using this method.     

A glow discharge ion source with fourier transform ion cyclotron resonance mass spectrometric detection

A glow discharge (CD) ion source has been coupled to a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer using a four-element electrostatic lens to accelerate and focus ions generated external to the instrument’s high magnetic field into its analyzer cell. Like other CD mass spectrometers, GD-FT-ICR can provide a quantitative measure of bulk analyte concentration with good precision and accuracy. Although detection limits currently attainable are several orders of magnitude higher than the commercially available magnetic sector-based instrument, CD-FT-ICR holds promise for ultrahigh resolving power elemental mass analysis. Several schemes are proposed to lower the detection limits of the technique while still providing high enough resolution to resolve isobaric interferences.     

Design, setup and first results of a miniaturized time-of-flight mass spectrometer with a simple reflector of a new design

A new kind of reflector with only one element has been constructed and used in a miniaturized time-of- flight mass spectrometer. Details from the simulation of the instrument with SIMION and its design are presented and discussed. The instrumental setup is displayed and some first results from measurements are presented. Laser- generated electron ionization (LEI) and resonance-enhanced multi-photon ionization (REMPI) are used for ion production. While both methods yield good mass spectra, the mass resolving power of the instrument is different for both methods. The results of the measurements are compared and discussed with respect to the design. It is found that the resolution of the new instrument is satisfactory for mass spectrometric measurements of small molecules and not limited by the reflector.     

A novel mass spectrometry cluster for high-throughput quantitative proteomics

We have developed and implemented a novel mass spectrometry (MS) platform combining the advantages of high mass accuracy and resolving power of Fourier transform ion cyclotron resonance (FTICR) with the economy and speed of multiple ion traps for tandem mass spectrometry. The instruments are integrated using novel algorithms and software and work in concert as one system. Using chromatographic time compression, a single expensive FTICR mass spectrometer can match the throughput of multiple relatively inexpensive ion trap instruments. Liquid chromatography (LC)-mass spectrometry data from the two types of spectrometers are aligned and combined to hybrid datasets, from which peptides are identified using accurate mass from the FTICR data and tandem mass spectra from the ion trap data. In addition, the high resolving power and dynamic range of a 12 tesla FTICR also allows precise label-free quantitation. Using two ion traps in parallel with one LC allows simultaneous MS/MS experiments and optimal application of collision induced dissociation and electrontransfer dissociation throughout the chromatographic separation for increased proteome coverage, characterization of post-translational modifications and/or simultaneous measurement in positive and negative ionization mode. An FTICR-ion trap cluster can achieve similar performance and sample throughput as multiple hybrid ion trap-FTICR instruments, but at a lower cost. We here describe the first such FTICR-ion trap cluster, its performance and the idea of chromatographic compression.     

Dynamic range of mass accuracy in LTQ orbitrap hybrid mass spectrometer

Using a novel orbitrap mass spectrometer, the authors investigate the dynamic range over which accurate masses can be determined (extent of mass accuracy) for short duration experiments typical for LC/MS. A linear ion trap is used to selectively fill an intermediate ion storage device (C-trap) with ions of interest, following which the ensemble of ions is injected into an orbitrap mass analyzer and analyzed using image current detection and fast Fourier transformation. Using this technique, it is possible to generate ion populations with intraspectrum intensity ranges up to 10(4). All measurements (including ion accumulation and image current detection) were performed in less than 1 s at a resolving power of 30,000. It was shown that 5-ppm mass accuracy of the orbitrap mass analyzer is reached with >95% probability at a dynamic range of more than 5000, which is at least an order of magnitude higher than typical values for time-of-flight instruments. Due to the high resolving power of the orbitrap, accurate mass of an ion could be determined when the signal was reliably distinguished from noise (S/Np-p)>2...3).     

First signal on the cryogenic fourier-transform ion cyclotron resonance mass spectrometer

The construction and achievement of the first signal on a cryogenic Fourier-transform ion cyclotron resonance mass spectrometer (FTICR-MS) are reported here, demonstrating proof-of-concept of this new instrument design. Building the FTICR cell into the cold bore of a superconducting magnet provided advantages over conventional warm bore design. At 4.2 K, the vacuum system cryopumps itself, thus removing the requirement for a large bore to achieve the desired pumping speed for maintaining base pressure. Furthermore, because the bore diameter has been reduced, the amount of magnet wire needed to achieve high field and homogeneity was also reduced, greatly decreasing the cost/Tesla of the magnet. The current instrument implements an actively shielded 14-Tesla magnet of vertical design with an external matrix-assisted laser desorption/ionization (MALDI) source. The first signal was obtained by detecting the laser desorbed/ionized (LDI) C(60)(+*) ions, with the magnet at 7 Tesla, unshimmed, and the preamplifier mounted outside of the vacuum chamber at room temperature. A subsequent experiment done with the magnet at 14 Tesla and properly shimmed produced a C(60) spectrum showing approximately 350,000 resolving power at m/z approximately 720. Increased magnetic field strength improves many FTMS performance parameters simultaneously, particularly mass resolving power and accuracy.     

A compact liquid chromatography-mass spectrometry instrument for the quantitation of immunosuppressants

Liquid chromatography tandem mass spectrometry (LC-MS/MS) plays an important role in clinical diagnostics. Although LC-MS/MS is superior in terms of accurately quantifying molecules in complex matrices, instrument footprint, operation and maintenance complexity also hinder its expansion as the analytical technique of choice. In this study, a compact LC-MS instrument was developed, in which an assembled liquid chromatograph was coupled with a miniature ion trap mass spectrometer. The overall instrument has a footprint of 69 cm × 31 cm × 31 cm, and it requires no gas supply as well as minimum maintenance. Furthermore, the use of LC-MS is in accord with conventional clinical diagnostic protocols, and the choice of ion trap offers tandem MS performance. The results showed that the use of LC could improve both mixture analysis capability and detection sensitivity of the miniature mass spectrometer. After optimization, feasibility of this instrument in clinical practice was demonstrated by the quantitation of four widely used immunosuppressants in blood samples. Relatively good linearities were obtained, which spanned the reference ranges of effective therapeutic concentrations of each immunosuppressant. Intra-day and inter-day accuracy and precision of analytical method were also assessed. This work showed that a compact LC-MS instrument could be used in clinical diagnosis, either to replace conventional lab-scale instruments or to be used in POCT applications.     

Evaluation of different combinations of gated trapping,RF-only mode and trap compensation for in-field MALDI Fourier transform mass spectrometry

MALDI, while providing advantages such as the ability to do in-depth and repeated exploration of the sample, challenges the existing performance capabilities of Fourier transform mass spectrometry (FTMS). The challenge arises because MALDI-produced ions have high mass-to-charge ratios and uncertain kinetic-energy distributions. We demonstrate that a combination of a gated trapping event, a RF-only mode pressure focusing event, and an electrically compensated trap provides a compelling advantage in meeting these challenges. Removal of any of the above combination elements significantly degrades the detection performance of substance P from 850 K resolving power at 34.9 kHz and of melittin from 278 K resolving power at 16.5 kHz when using a 3-Tesla magnet-based spectrometer.