Kenneth L. Busch

Planar Chromatography Coupled to Mass Spectrometry

Abstract Applications of thin-layer chromatography/mass spectrometry are expanding rapidly due to commercial availability of the devices, and improved understanding of the procedures required to measure good quality mass spectrometric data. Several of the common approaches to TLC/MS coupling are pursued in our laboratory; recent focus has been on techniques of sample preparation and plate treatment that allow direct TLC/MS analysis to be completed on almost any instrument. Specific examples to be covered are the direct derivatization of alkaloid samples on Empore TLC plates for volatilization into an electron/chemical ionization source, development and concentration of thin-film fluorescent dyes for analysis by liquid secondary ion mass spectrometry (LSIMS), and the use of a CCD-based imaging system to explore the integration of optical and mass spectrometric information for the characterization of samples separated by thin layer chromatography.

Use of 2‐Hydroxy‐1‐naphthoic Acid as a Matrix for Matrix‐assisted Laser Desorption/Ionization Mass Spectrometry of Low Molecular Weight Porphyrins and Peptides

Use of the matrix-assisted laser desorption/ionization matrix 2-hydroxy-1-naphthoic acid provides a 2-4-fold enhancement of ion signal for low molecular mass porphyrins over other matrices including α-cyano-4-hydroxycinnamic acid, 2-(4-hydroxyphenylazo)benzoic acid, sinapinic acid and 2,5-dihydroxybenzoic acid. Tetraphenyl-, octaethyl-and tetramethoxyphenylporphyrins give strong signals corresponding to [M + H] + ions and limits of detection between 100 and 500 fmol. Significant fragmentation is observed for the octaethylporphyrins at high laser power and moderate fragmentation is observed for the other classes of porphyrins. The 2-hydroxy-1-naphthoic acid matrix also gives strong signals corresponding to [M + H] + and/or [M + Na] + ions for peptides, including methionine enkephalin, bacitracin, bradykinin, renin substrate and insulin.

Calibration point for liquid secondary ion mass spectrometry tandem mass spectra measured with an EBqQ hybrid mass spectrometer

A protocol for establishing standard instrument conditions for the measurement of product ion tandem mass spectra from parent ions generated by liquid secondary ion mass spectrometry (LSIMS) is presented. The ion at m/z 319 (C + ) from 2-butyltriphenylphosphonium bromide is selected as the parent ion and then is subjected to collision-induced dissociation. The relative abundances of product ions at m/z 263 and 183 are monitored as a function of collision energy while keeping the target gas pressure constant. The collision energy at which these two most abundant product ions are equal is defined as the calibration point at that collision gas pressure. LSIMS tandem mass spectra acquired at the calibration point show good reproducibility for several model compounds. Spectra obtained at the calibration point at two different collision gas pressures, representing multiple and single collision conditions, also show a high degree of similarity. Use of this protocol may aid in the establishment of a tandem mass spectral library.

Analysis of Planar Chromatograms by Fast Atom Bombardment and Laser Desorption Mass Spectrometry

Abstract The applications of fast atom bombardment mass spectrometry coupled with thin-layer chromatography in a study of phospholipids, alkaloids derived from bloodroot rhizomes, and diuretics are summarized. Use of the mass spectrometric data to trace compounds via characteristic fragment ions increases the effective resolution of the chromatographic resolution. Quantitative values derived in TLC/FAB experiments for the diuretic amiloride hydrochloride is linear over two orders of magnitude encompassing the clinically useful range. The use of a laser desorption ionization method and Fourier transform mass analysis for TLC plate detection is also described; matrix and concentration effects are significant in this case, but several successful applications are described. An interface device for the linkage

POSITIVE ION LIQUID SECONDARY ION MASS SPECTRA OF AROMATIC SULFONIUM SALTS

Abstract Organic sulfonium salts exist as pit-charged species in the matrices normally used for liquid secondary ionization mass spectrometry. Positive ion LSIMS mass spectra of the sulfonium salts therefore exhibit the intact cation (C+) as the dominant ion species. Generally, the fragment ions are of 30–40% relative intensity and are formed through simple cleavages and rearrangements. Cluster ions such as CAC+ and C3A2 + are also present in the mass spectra (A is the counter ion of the salt). These cluster ions provided confirmation of the molecular weight. Adduct ions are formed between organic sulfonium salts and matrices used in LSIMS such as diethanolamine and triethanolamine. Beam-induced reactions are identified by comparison of mass spectra obtained with electrospray ionization, and by changes in ion products with changes in solvent matrix.

Calibration Point for Electron Ionization MS / MS spectra measured with multiquadrupole mass spectrometers

A protocol for establishing standard instrument conditions for measurement of product ion MS/MS spectra from parent ions produced by electron ionization is presented. Within this protocol, the ion at m/z 231 (C5F9+) from perfluorokerosene or perfluorotributylamine is selected as the parent ion and subjected to collision-induced dissociation. The relative intensities of product ions at m/z 69, 131, and 181 are monitored as a function of collision energy while keeping the target gas pressure constant within the range of 10−4–10−6 torr (measured), or a beam attenuation of approximately 30-70%. The collision energy at which the ion intensities for product ions at m/z 69 and 181 are equal is defined as the calibration point at that collision gas pressure; the intensity of the ion at m/z 131 is very close to this value as well. Electron ionization MS/MS spectra taken at the calibration point using two different multiquadrupole instruments show good reproducibility for several test compounds. The high degree of similarity may aid in the establishment of a MS/MS spectral library.

Probing the energetics of charge-remote fragmentation in carbocyanine dyes using collision- and surface-induced dissociation mass spectrometry

Carbocyanine dyes used as molecular probes are symmetric ring compounds with long alkyl chains on each ring proximate to a resonance-stabilized charge site. They provide high-quality positive-ion liquid second (LSI) and electrospray ionization mass spectra in which the intact cation is the dominant ion in the mass spectrum. The intact cation of a carbocyanine dye undergoes charge-remote fragmentation (CRF), evident in the LSI mass spectra. CRF is also observed in collision-induced dissociation and surface-induced dissociation of the mass-selected intact cation. Differential CRF processes from the two chains in the molecule are observed with increasing internal energy deposition.

Characterization of supported and unsupported transition metal compounds by liquid secondary ion mass spectrometry

The positive ion liquid secondary ion mass spectra (LSIMS) of metal acetylacetonates (acacs) contain cluster ions that involve multiple metal centers. Several metal acacs form adduct ions with m-nitrobenzyl (m-NBA) alcohol used as the LSIMS support matrix. In these adduct ions, deprotonated m-NBA molecules act as bridges between metal centers. The LSIMS of a series of dirhodium carboxylates show that the electron-withdrawing strength of the carboxylate group controls whether the base peak in the mass spectrum is the molecular ion or an adduct ion formed between the molecule and a matrix molecule of sulfolane. However, no cluster ions are formed for these dirhodium compounds. Similarly, highly charged cationic compounds do not form cluster ions, nor do they form adduct ions with the LSIMS solvent. Instead, the counter ion is retained to maintain a single positive charge in the ion. LSIMS spectra of neutral and charged metallocene compounds are compared. Contrary to the assertions that these compounds cluster extensively in the gas phase, the LSIMS provide no evidence for this process. To support studies of surfaces prepared for catalytic studies, silica- and clay-supported transition metal acacs were characterized by LSIMS. Release of the supported transition metal compounds from the surface occurs during ion exchange.

Fast-atom bombardment mass spectrometry coupled to planar gel electrophoresis

Abstract An interface system between planar gel electrophoresis and mass spectrometry is described that includes a means of rapid electrophoretic gel disruption, concentration of extracted sample in a pre-column, and transfer of the sample molecules to the continuous flow FAB source of a mass spectrometer. Spatial resolution achieved on the surface of the gel is on the order of 2 mm diameter for a spot, and 4 mm2 for a band. Sensitivities to 50 picomoles have been demonstrated for the analysis of small peptides with continuous-flow fast atom bombardment mass spectrometry.

Transmission Characteristics of a Radio-Frequency Quadrupole for Ms/Ms

ABSTRACT Ion transmission through an RF-only quadrupole is affected by both the frequency and voltage applied to the quadrupole. With a change in the collision cell voltage, ion abundances for both precursor and product ions are seen to vary in a modulated fashion. This pattern of ion abundances has been described as quadrupole “nodes”, with changes in ion abundances within a factor of 2-4 from maximum to minimum abundance. These nodes hamper MS/MS spectral reproducibility from instrument to instrument and can camouflage trends observed in energy-resolved MS/MS (ERMS/MS). We find ion abundance nodes to be present for both product and precursor ions, and the collision energies at which the nodes maximize appear to be unique to each mass.