Influence of internal standard charge state on the accuracy of mass measurements in orthogonal acceleration time-of-flight mass spectrometers

Accuracy of mass measurements performed in orthogonal acceleration time-of-flight (oa-TOF) mass spectrometers highly depends on the quality of the signal and the internal calibration. The use of two reference compounds which bracket the targeted unknown, give rise to ions with sufficient signal-to-noise ratio while avoiding detector saturation and produce signals of similar intensity as compared to the target is a common requirement which allow a 5 ppm accuracy on a routine basis. Ion charge state is demonstrated here to be an additional and particularly critical parameter. Using internal references of lower charge state than the target ion systematically yielded overestimated data. Errors measured for quadruply charged molecules were in the range 16-18 ppm when mass calibrants were singly charged ions while accuracy was below 5 ppm when references and target ions were in the same charge state. Magnitude of errors was found to increase with the difference in charge state. This phenomenon arises from the orthogonal acceleration of ions in the TOF analyzer, an interface implemented in all TOF mass spectrometers to accommodate continuous beam ionization sources. Copyright (c) 2007 John Wiley & Sons, Ltd.     

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.     

A simple method for rapid calibration of faraday and ion‐counting detectors on movable multicollector mass spectrometers

Methods are presented for rapid determination of relative efficiencies of Faraday cups in a multicollector array with movable cups and of Daly or electron multiplier detector dead time and gain values. The Faraday calibration approach is based on measuring the same isotopic ratio in two sequences with different collector configurations having one collector in common. Changes in thermal fractionation during the two measurement sequences are monitored using the same two collectors. Since the relative efficiencies are determined by measuring the same ratio in different cups corrected for time‐dependent changes in fractionation, it is unnecessary to use a standard of known composition and calculations are simple, not requiring the solution of multiple equations. Determination of dead time and gain values for a Daly detector are shown based on multidynamic measurements of masses 207, 206, and 208 from Pb standard SRM982 in two sequences consisting of L1‐Daly‐H1 and Daly‐H1‐H2. This provides two equations that can be solved for Daly dead time and gain. This method uses static measurements and is therefore insensitive to signal instability. It also does not require use of a standard of known isotopic composition. The potential of using known cup efficiencies to help determine absolute isotopic abundances is discussed.     

Evaluation of the spectral accuracy of mass spectrometers using compounds containing Cl or Br atoms

Current procedures for the evaluation of spectral accuracy of mass spectrometers are limited by the lack of certified isotopic reference materials and the high uncertainty in the isotopic composition of natural abundance molecules. The calculated uncertainties in the ratio M + 1/M for natural abundance molecules containing any number of C, H, N and/or O atoms are close to 5% relative because of the natural variability of the isotopic composition of carbon. So, we have developed two alternative measurement procedures with much lower theoretical uncertainties for a better evaluation of spectral accuracy in both single and triple quadrupole analysers. The first method is based on the measurement of the M + 2/M, M + 4/M + 2, etc. ratios for halogenated organic compounds containing either Cl or Br. The theoretical uncertainties for these ratios because of natural variability are in the order of 0.3 to 1.0% making them suitable for the evaluation of spectral accuracy with the additional advantage that there is no need to take into account other limitations such as cluster purity or poor mass resolution. This procedure was applied to the evaluation of a single quadrupole GC‐MS instruments using natural abundance PCB and PBDE standards with satisfactory results. The second method can be applied to tandem instruments and takes advantage of the loss of two halogen atoms when PCB and PBDE standards are fragmented by Collision Induced Dissociation. Theoretical SRM transition ratios can be calculated as a pure combinatorial probability with theoretical uncertainties lower than 0.1%. By combining PCBs and PBDEs with different number of halogen atoms, a mass range from 100 to 700 u and abundance ratios from 0.1 to 10 can be evaluated. The use of penta‐chlorinated PCBs and/or penta‐brominated PBDEs is finally recommended for the evaluation of spectral accuracy of mass spectrometers with the EI source. Copyright © 2016 John Wiley & Sons, Ltd.     

Ion optics in mass spectrometers

Ion-optical relations are outlined that are relevant to mass spectrometers. Some of the relations are mathematically derived and others are only discussed and rationalized. Above all, however, attempts were made to elucidate basic relations and to provide equations that can readily be used for quantitative calculations of real mass spectrometers. Copyright © 1999 John Wiley & Sons, Ltd.     

Advances in microfabricated mass spectrometers

Mass filters for miniature mass spectrometers are now being constructed using three-dimensional microfabrication techniques. Crossed-field, travelling-wave, time-of-flight, quadrupole and cylindrical ion-trap filters have all been demonstrated, with steadily increasing mass range and mass resolution. This paper reviews the range of available devices and the state of the art.     

Sector field mass spectrometers in ICP-MS

A new generation of sector field mass spectrometers, with improved analytical figures of merit at even lower prices, is commercially available, giving a strong impetus to the development of inductively coupled plasma mass spectrometry (ICP-MS) sector field instrument applications in the analytical community. It is the aim of this paper to give an overview of these instruments, to introduce some basic concepts, to discuss their peculiarities and performance, and to present some selected examples of analytical applications to demonstrate the `state of the art'.     

Multi-turn time-of-flight mass spectrometers with electrostatic sectors

The mass resolution of a time-of-flight (TOF) mass spectrometer is directly proportional to its total flight pathlength. Multi-turn or multi-passage ion optical geometries are necessary to obtain fight distances of sufficient length within reasonable size limitations. We have investigated ion optics for a multi-turn TOF mass spectrometer with electrostatic sectors. The concept of 'perfect' focusing conditions is introduced. Furthermore, a new type of multi-turn TOF mass spectrometer, the MULTUM Linear plus, was developed. It consists of four cylindrical electric sectors and 28 electric quadrupole lenses. It has a vacuum chamber 60 x 70 x 20 cm in size. Mass resolution is demonstrated to increase according to the number of ion cycles. A mass resolution of 350 000 (m/z = 28, FWHM) was achieved after 501.5 cycles. The MULTUM Linear plus analyzer is not simple, however; 28 electric quadrupole lenses are used. In order to reduce the number of ion optical parts, an improved multi-turn TOF mass spectrometer, the MULTUM II, consisting of only four toroidal electric sectors, was also developed. The possibility of tandem mass spectrometric applications using multi-turn TOF mass spectrometers is also discussed.     

Pocket sensitivity calibration of multicollector mass spectrometers

Summary A method is described for the calibration of multicollector mass spectrometers. The instruments are used for nuclear application and equipped with 9 fixed collector pockets. To obtain all required information two solutions have been prepared one for uranium containing U-233, U-235, U-236 and U-238 with an approximate composition of 2111 and the other for plutonium containing Pu-239, Pu-240, Pu-242 and Pu-244 with an approximate composition of 3132. The spectrum was shifted by one and two mass units on both sides of the normal position and the ion intensities collected. These data have been used for the calculation of the relative sensitivity of the pockets involved.

---

Kalibrierung von Multikollektor-Massenspektrometern

     

MS/MS: Cloned mass spectrometers

Coupling mass spectrometers in tandem (MS/MS) can greatly increase the specificity of MS analysis without significantly decreasing its unusual sensitivity and speed, particularly for trace levels of preselected compounds in complex organic mixtures. MS/MS also gives more detailed structural information for larger organic molecules in submicrogram quantities.     

Hybrid mass spectrometers: Versatile research instruments

Hybrid mass spectrometers combine the advantages of different types of mass analyzers to produce highly versatile scientific tools. Improvement on the traditional MS-MS scans is achieved enabling the extraction of additional information. In addition, new MS-MS scan modes have been discovered which display great promise. A new method of obtaining structural information by MS, surface-induced dissociation, is also implemented using the hybrid mass spectrometer.     

Analyzer scan modes for hybrid mass spectrometers

The scan modes that can be used to obtain daughter ion, parent ion, and constant neutral-loss spectra on hybrid mass spectrometers are derived for instruments with a quadrupole combined with a magnetic and/or electric sector. All combinations of sector/quadrupole and quadrupole/sector instruments are discussed. The advantages or lack thereof of the various scans are presented with respect to operation, resolution and potential artifacts.