Scott D. Hanton

Investigations of matrix-assisted laser desorption/ionization sample preparation by time-of-flight secondary ion mass spectrometry

Matrix-assisted laser desorption/ionization (MALDI) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) analyses are compared to gain insight into some of the details of sample preparation for MALDI analysis of synthetic polymers. ToF-SIMS imaging of MALDI samples shows segregation of the cationization agent from the matrix crystals. The amount of observed segregation can be controlled by the sample preparation technique. Electrospray sample deposition minimizes segregation. Comparing ToF-SIMS and MALDI mass spectra from the same samples confirms that ToF-SIMS is significantly more surface sensitive than MALDI. This comparison shows that segregation of the oligomers of a polymer sample can occur during MALDI sample preparation. Our data indicate that MALDI is not as sensitive to those species dominating the sample surface as to species better incorporated into the matrix crystals. Finally, we show that matrix-enhanced SIMS can be an effective tool to analyze synthetic polymers, although the sample preparation conditions may be different than those optimized for MALDI.

Comparison of Mass Spectrometric Techniques for Generating Molecular Weight Information on a Class of Ethoxylated Oligomers

The results of fast atom bombardment (FAB), time-of-flight secondary ion mass spectrometry (ToF-SIMS), matrix-assisted laser desorption/ionization (MALD/I), electrospray ionization (ESI), and field desorption (FD) analyses of ethoxylated oligomers of 2,4,7,9-tetramethyl-5-decyne-4,7-diol (Surfynol® 104) were compared.Each of these desorption mass spectrometry (MS) techniques can produce spectra of unfragmented cationized oligomers. From the observed ion series we calculate average molecular weight information. We have compared the results of mass spectrometric analyses of a series of ethoxylated Surfynol surfactants. Our data indicate that FAB, ToF-SIMS, MALDI/I, and ESI produce similar results for the lower molecular weight species, but that as the average molecular weight increases FAB and SIMS produce slightly lower results than MALD/I and FD. This could be due to increased fragmentation. ESI produced a result similar to FAB and SIMS for the highest average molecular weight material. Further experiments compare the mass spectral results with gas chromatographic quantitative data. Although gas chromatography is not expected to accurately analyze the higher mass oligomers, we observe significant differences in intensities of the short-chain oligomers (especially the 0- and 1-mers) when compared to the desorption mass spectrometer results. These differences may reflect poor cationization efficiency for very short oligomer chains in the mass spectrometric analyses.

Silver cluster interferences in matrix-assisted laser desorption/ionization (MALDI) mass spectrometry of nonpolar polymers

Potential difficulties associated with background silver salt clusters during matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) of nonpolar polymers are reported. Silver salt cluster ions were observed from m/z 1500 to 7000 when acidic, polar matrices, such as 2,5-dihydroxybenzoic acid (DHB), all-trans-retinoic acid (RTA) or 2-(4-hydroxyphenylazo)benzoic acid (HABA), were used for the analysis of nonpolar polymers. These background signals could be greatly reduced or eliminated by the use of nonpolar matrices such as anthracene or pyrene. Representative examples of these background interferences are demonstrated during the analysis of low molecular weight nonpolar polymers including polybutadiene and polystyrene. Nonpolar polymers analyzed with acidic, polar matrices (e.g., RTA) and silver cationization reagents can yield lower quality mass spectral results when interferences due to silver clusters are present. Replacing the polar matrices with nonpolar matrices or the silver salts with copper salts substantially improved the quality of the analytical results. In addition, it was found that silver contamination cannot be completely removed from standard stainless steel sample plates, although the presence of silver contamination was greatly reduced after thorough cleaning of the sample plate with aluminum oxide grit. Carry-over silver may cationize polymer samples and complicate the interpretation of data obtained using nonpolar matrices in the absence of added cationization reagents.

Updating Evidence for Cationization of Polymers in the Gas Phase during Matrix-Assisted Laser Desorption/Ionization

Matrix-assisted laser desorption/ionization (MALDI) techniques have been developed to determine the chemical structure of a variety of industrial polymers. Despite the enormous popularity and power of MALDI, the details of the cationization mechanisms of the process are currently rather poorly understood. The MALDI cationization of polymer analytes was investigated previously by Hoberg and co-workers.1 They used layered samples to explore the role of gas-phase cationization in MALDI of polymers. This paper seeks to extend the work initiated by Hoberg and co-workers and update the results of the earlier work. The new experiments take advantage of a MALDI instrument with delayed extraction and show that separation in the initial acceleration region of the mass spectrometer is not a key component of cationization. Investigations of tri-layer samples by time-of-flight secondary ion mass spectrometry (ToF-SIMS) also show that there is considerable interaction among the layers. These interactions, specifically the solubility of the salts in the solvents, account for the presence of the bottom layer cation in the mass spectra. The absence of the top layer cation in the mass spectra is due to the inability of the polymer oligomers to penetrate that layer during desorption.

Imaging the morphology of solvent-free prepared MALDI samples

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry is an important technique to characterize many different materials, including synthetic polymers. MALDI mass spectral data can be used to determine the polymer average molecular weights, repeat units, and end groups. The development of solvent-free sample preparation methods has enabled MALDI to analyze insoluble materials and, interestingly, can provide higher-quality mass spectral data. Although the utility of solvent-free sample preparation for MALDI has been demonstrated, the reasons for its success are only now being discovered. In this study, we use microscopy tools to image samples prepared using solvent-free methods to examine the morphology of these samples. The samples are prepared using a simple vortex method. Our results show that the average particle size of typical MALDI matrices is reduced from their original tens to hundreds of micrometers to hundreds of nanometers. This size reduction of the matrix occurs in one minute using the vortex method. We also observe remarkably smooth and homogeneous sample morphologies for the laser to interrogate, especially considering the relatively crude methods used to prepare our samples.

Determining the time needed for the vortex method for preparing solvent-free MALDI samples of low molecular mass polymers.

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry is an important technique to characterize many different materials, including synthetic polymers. MALDI mass spectral data is used to determine the polymer average molecular weights, repeat units, and end groups. The development of the vortex method of solvent-free sample preparation showed that remarkably short mixing times could prepare samples that yielded high quality MALDI mass spectra. In this paper, we use microscopy images and MALDI mass spectra to evaluate the mixing time required by the vortex method to produce mass spectra for low molecular mass polymer samples. Our results show that mixing times of as little as 10 s can generate homogeneous thin films that produce high quality mass spectra with S/N ∼ 100. In addition, ultrashort mixing times of only 2 s still produce samples with mostly smooth morphology and mass spectra with S/N ∼ 10.

Book Review: ToF-SIMS: Surface Analysis by Mass SpectrometryToF-SIMS: Surface Analysis by Mass Spectrometry Edited by VickermanJohn C.BriggsDavidIM Publications and SurfaceSpectra (2001, Chichester and Manchester, UK) www.impub.co.uk/surface/

In their recent book, ToF-SIMS: Surface Analysis by Mass Spectrometry, John Vickerman and David Briggs have delivered a well-organized and in-depth presentation of developments in this growing field. The book is an excellent resource for both the experienced practitioner and the new student. The overall quality of the book is high and it covers a tremendous breadth of important topics. Anyone with an interest in ToF-SIMS will find something valuable in the book. Overall the chapters are well-written and wellresearched reports examining critical topics in this important field of study. Time-of-flight secondary ion mass spectrometry (ToFSIMS) is a growing field of study valued for its remarkable surface sensitivity. ToF-SIMS uses a focused primary ion beam to bombard a sample. The energy transfer from the primary ions to the surface of the sample generates secondary ions that are characteristic of the atomic and molecular structure of the top few layers of the target. ToF-SIMS experiments can produce high-resolution mass spectra, sensitive detection limits and chemically sensitive images. For these reasons, ToF-SIMS has become a very important analytical technique to characterize the surface of a wide variety of materials, ranging from coatings and polymers to biomolecules and drug delivery systems. The book is composed of 28 chapters organized in sections pertaining to the different stages of the analytical work: Prologue, Instrumentation, Fundamentals, Optimisation Methods, Data Interpretation and Analytical Applications. The chapter authors are recognized in the field and have contributed to the growth of ToF-SIMS as a key surface analytical tool. Most of the chapters are relatively easy to read and have an average length of about 25 pages. The book also contains a useful index that helps locate topics covered by more than one chapter. The Prologue section contains chapters by Vickerman and Benninghoven. Both chapters provide brief overviews of ToF-SIMS that outline the experiments, give a brief discussion of the collision cascade energy transfer model and show some interesting examples. Benninghoven’s contribution to the prologue is particularly interesting. He provides a personal and very interesting history of ToF-SIMS as the technique matured in his labs. The historical perspective provides an effective tutorial on the details of the static SIMS experiment. The Instrumentation section contains three chapters covering the basics of the tools needed to perform ToF-SIMS experiments. The chapter by Scheuler effectively covers timeof-flight mass spectrometry, especially the relatively unique TRIFT instrument (Physical Electronics, Eden Prairie, MN) with which many ToF-SIMS users are familiar. The chapter by Hill on primary ion sources is a good introduction to this key piece of hardware for SIMS experiments. This is a useful chapter for novice users that provides the basic information about liquid metal ion guns (LMIG), surface ionization sources such as Cesium and gas sources such as Oxygen. However, it lacks the depth necessary to be highly valued by experienced users. The third chapter in this section addresses sample preparation and handling by Reich. This is a wellwritten, succinct and very useful chapter that covers many of the sample handling issues that surface scientists face. Reich provides tips and ideas from his own experiences on topics like sending and storing samples, educating clients, preparing unusual samples and the value of cryogenic sample holders. This chapter is easy to read and contains many sketches and drawings that help to communicate the ideas. The Fundamentals section contains four chapters covering the basics of ion generation in SIMS. Ion generation is the Book Review: ToF-SIMS: Surface Analysis by Mass Spectrometry

Variable attenuator for high‐power lasers

A novel variable attenuator is described for use with pulsed, high‐power ultraviolet lasers. The attenuator is comprised of a number of fine metal meshes. The attenuation is accurately and easily changed by varying the number of meshes in the laser beam path.