R. T. Short

Peer Reviewed: Organic SIMS of Biologic Tissue

Instrumental developments over the past 20 years now make it possible to identify and map some biologically important molecules by SIMS.

Identification and Mapping of Phosphocholine in Animal Tissue by Static Secondary Ion Mass Spectrometry and Tandem Mass Spectrometry

Secondary ion mass spectra and images were obtained from animal tissue samples using less than 10(13) primary ions/cm2. The mass spectra showed abundant peaks at m/z 184 and m/z 86. Tandem mass spectrometry (MS/MS) was used to identify the source of these ions as phosphocholine. Secondary ion images obtained using MS/MS were used to show that m/z 86 is an abundant gas-phase fragment ion derived from m/z 184. These results are discussed in terms of the physiology of the samples investigated.

A Wide-angle Secondary Ion Probe for Organic Ion Imaging

A secondary ion source has been developed for an organic ion microprobe capable of imaging samples up to 2 em in diameter. The source uses a focused 5 keY Cs+ ion beam which is rastered across the sample surface, and secondary ions from each point on the sample are collected and formed into a low energy beam to be analyzed by a quadrupole mass filter. Dynamic emittance matching is employed to deflect ions from off-axis points on the sample back onto the mass analyzer axis. Rastering and dynamic emittance matching are rapidly controlled by assembly language programs using an IBM/AT (80286) type computer. A low energy ion monitor was used to tune and evaluate the secondary ion source by providing a magnified cross-sectional image of the ion beam at the source exit aperture. A well-focused and centered secondary ion beam was obtained from each point on the sample, indicating that large-scale dynamic emittance matching with high collection efficiency is possible. Mass resolved images of grids and glycerol samples are shown to demonstrate the performance of the integrated secondary ion source mass analyzer and control system.

Use of region II of the a/q stability diagram for fast scanning of a linear quadrupole mass spectrometer

For evaluation as a detector for high-speed gas chromatography, a linear quadrupole mass spectrometer was operated in rf only mode by using Region II (a = 0; 7.514 < q < 7.580) of the Mathieu a/q stability diagram. The available power supply and the diameter of the quadrupole rods of the mass spectrometer placed an upper mass limit of ∼ m/z 93. Scan rates of 1000 scans/s were obtained with mass spectral peaks resolved over an 80-u range The m/z 91 and 92 ions produced from the electron ionization of toluene are resolved with an R1/2 of 135. A potential difference between the source and the quadruple mass filter of up to 1000 V was used to accelerate ions into the quadrupole. Broadening of mass-to-charge ratio peaks results from the time constant of the signal amplification rather than the small number of rf cycles the ions experience. The expected loss of sensitivity relative to Region I is observed, and the problem of mass aliasing is discussed.

Secondary ion emission and images from a biologic matrix

Abstract Secondary ion mass spectra and images were obtained from organic compounds deposited on gold and 30–50 μm thick biologic tissue substrates. Analyte solutions were prepared from acetylcholine chloride, choline chloride, and methylphenylpyridinium (MPP + ) iodide. Tandem mass spectrometry (MS/MS) was used to distinguish secondary ions characteristic of the analyte from secondary ions characteristic of the tissue itself. Effects of primary ion damage appear similar regardless of substrate. Samples of choline chloride deposited on a gold substrate are exceptional; secondary ion emission from such samples appeared unaffected by primary ion dose. Emission of acetylcholine secondary ions was found to decay with a rate independent of primary ion dose, but dependent on the substrate. These results show that the distribution of organic compounds can be mapped from biologic tissue under conditions of static SIMS, but matrix effects and chemical noise must be considered.

Improved energy compensation for time-of-flight mass spectrometry

A model for improved energy compensation in time-of-flight (TOF) mass spectrometry has been developed and tested. This model includes effects of both the acceleration and drift region on mass resolution for surface desorption TOP mass spectrometers that employ ion mirrors to improve mass resolution. Appropriate placement of an additional stage onto the conventional one- and two-stage mirrors provides compensation for flight time spreads, caused by initial ion kinetic energy distributions, in both regions. Experimental results that validate the model calculations are presented for a modified commercial two-stage ion mirror. For example, m/†m for Na+ was improved from ∼ 100 to ∼ 200 using only a 200-eV drift energy and a 58-cm drift path.

Charge compensation for imaging large insulating samples by using secondary ion tandem mass spectrometry

A charge compensation technique has been developed for secondary ion mass spectrometry and imaging of insulating samples as large as 1 cm2 using a triple quadrupole-based microprobe. The microprobe secondary ion extraction field is synchronized with a periodic primary Cs+ beam to allow a sheetlike beam of 5-eV electrons to pass over the sample surface when the extraction field is zeroed. Electrons are attracted to, and neutralize, any points on the sample that have accumulated positive charge. Positive secondary ion images from Teflon®, a well-known insulator, illustrate the effectiveness of charge compensation. Locating and identifying analytes on dry filter paper by using tandem mass spectrometry are also demonstrated.

Monitor for displaying real-time images of low energy ion beams

Analytical Chemistry Division. Oak Ridge National Laboratory. Oak Ridge, Tennessee, USA A monitor that provides real-time images of low energy (< 50 eV) ion beams has been designed, constructed, and tested. The cross-sectional image of the beam at the entrance aperture of the monitor is magnified by a factor of 6.5 and displayed on a CRT, following current amplification by using a dual microchannel plate assembly. The monitor provides unambiguous information regarding the cross section of any low energy ion beam. Anplication in the design and testing of quadrupole-based mass spectrometers is emphasized,