B. Schueler

A secondary ion time-of-flight mass spectrometer with an ion mirror

Abstract A new time-of-flight mass spectrometer has recently been constructed at the University of Manitoba (Manitoba TOF II). It incorporates a single-stage ion mirror that enables a resolving power m /Δ m FWHM = 10000 to be obtained, an improvement by a factor ∼ 3 over a previous linear instrument (Manitoba TOF I). The signal/background ratio is also considerably improved. Two detectors are used. One is situated behind the mirror to measure neutral fragments from unimolecular decay in the first leg of the flight path. The other measures the undissociated parent ions and the charged daughters in the reflected spectrum. This system enables measurement of correlations between the charged and the neutral daughter, thus yielding information about the structure of the parent ion.

Secondary ion mass spectrometry by time-of-flight

Abstract Some properties of the Manitoba time-of-flight secondary ion mass spectrometer are described and compared with those of other instruments. Topics discussed include properties of the primary ion beam, preparation of small electrosprayed targets, secondary ions from alkali halides (metastable decay and increase in yield after irradiation), data handling and sensitivity.

Automated dry fraction collection for microbore high-performance liquid chromatography-mass spectrometry.

A method is presented for the collection of dry fractions from a microbore high-performance liquid chromatographic column. These fractions are electrosprayed onto a foil strip that is being moved past the spray in steps. These solid deposits are in a form which is compatible with solid-phase secondary-ion mass spectrometry, in particular the time-of-flight instrument that has been developed in our laboratory. Because the type of ionization used in static secondary-ion spectrometry is essentially non-destructive, the non-volatile eluent is available for any other analytical method after mass analysis. The chromatogram of a mixture of peptides was re-constructed from the mass spectra of fractions collected in this way. This chromatogram is shown and its features are examined.

Secondary ion mass spectra of alkali halides

Abstract Mass spectra of positive and negative secondary ions from various alkali halides have been measured in the Manitoba time-of-flight mass spectrometer. The ions were produced by Cs + and K + bombardment at primary ion energies of 3 to 19 keV for the positive spectra, and 11 to 28 keV for the negative spectra. The ions measured were those emitted within a time interval ∼ 20 ns after the primary ion impact. The secondary ion yields are strongly dependent on the sample composition and treatment; prior irradiation may change the yield by an order of magnitude or more. The secondary ion yields also depend strongly on the energy loss of the primary ion, but the ratio of yields of different cluster ions from a given target is almost independent of this parameter. The results appear to be consistent with models in which the clusters are ejected directly from the target, but do not determine whether or not they possess the original surface structure. The results may also be described by a recombination model if the recombination is essentially complete.

A data system for time-of-flight measurements

Abstract Time interval measurements in a time-of-flight mass spectrometer are digitized by a LeCroy 4208 time-to-digital converter, and fed into a CAMAC processor for analysis and histogramming. In a preliminary version of the system a DEC LSI 11/23 carried out these tasks, but in the present system an INCAA CAPRO 68K handles the higher speed tasks, leaving general housekeeping to the LSI 11/23. The new system has largely overcome the limitations of some earlier systems. Data can now be stored in a 120-Kword × 32-bit histogram in 1 ns bins. Resolution M ΔM ( fwhm ) is ∼ 3200 at mass ∼ 720 u for stable ions; it appears to be limited by the axial velocity spread of the ions ejected from the target. Spectra can be accumulated at ∼ 20 000 counts/s. Although the system has been designed for time-of-flight mass spectrometry, it may be useful for other fast timing applications.

Unimolecular decay measurements of secondary ions from organic molecules by time-of-flight mass spectrometry

Abstract Unimolecular dissociations of positive ions ejected by keV Cs + bombardment from para -substituted phenylalanine F-X (X = H, HO, OCH 3 and halides) were studied by time-of-flight mass spectrometry. The first-order rates of dissociation and the corresponding fragmentation pathways were determined with the aid of a movable 45° ion mirror. Variation of ion yields and fragmentation pathways for [M + H] + , [M + Na] + , [M + 2Na - H] + , [M-45] + , and [tropylium] + decompositions were observed for the various substituents. The effect of varying the primary ion energy from 3 to 13 keV was studied for the ion yields and decay constants from phenylalanine (X=H). Although the ion intensities varied significantly, especially at low primary ion energies, the rate constants remained unchanged.

Ion-Neutral Correlations Following Metastable Decay

An important property of linear time-of-flight (TOF) mass spectrometers is their high efficiency for detecting the products of metastable decay [1,2]. Ions that decay in the field-free drift region contribute to the parent ion peak because the centre-of-mass velocity remains constant. However, the normal TOF spectrum provides no information on metastable decay paths or rate constants. Such data can contribute to structural elucidation and understanding of the desorption process.

Time-of-Flight Investigations of Secondary Ion Emission from Metal Halides

The bombardment of metal halides (MX) by keV — primary particles causes the emission of secondary atomic (M+, X-) and cluster [(MX)nM+; (MX)nX-; n = 1,2,...) ions. Once the secondary cluster ions leave the primary interaction region, they can undergo metastable decays due to the vibrational energy stored in the cluster bond during the initial production process. A large fraction of secondary cluster ions from metal halides, as well as molecular ions from organic materials, has indeed been observed to be metastable. We have studied the metastable decays of some CsI clusters and determined the corresponding rate constants.

Coincidence Measurements with the Manitoba Time-of-Flight Mass Spectrometer

Observations of secondary ion multiplicity have been reported for high energy (~MeV/u) bombardment of large organic molecules [1,2] and CsI [2]. The average number of secondary ions detected per primary projectile was >4, and some incident particles ejected more than 32 secondary ions. Significant correlations among ions desorbed from thiamine by 252Cf fission fragments have also been observed [3]. Similar measurements with ~10 keV Cs+ ions have recently been made possible with a new data system [5] installed on the Manitoba time-of-flight mass spectrometer [4].

Time-of-Flight Measurements in Secondary Ion Mass Spectrometry

In recent years a number of mass spectrometers have been constructed for measuring the mass spectra of secondary ions by time-of-flight methods [1]. References The secondary ions may be produced by bombardment of the sample with high energy (MeV) primary ions [2] or with low-energy (keV) ions. Instruments using both low-energy ion bombardment (SIMS) and time-of-flight (TOF) are listed in table 1.