Resolution in the linear time-of-flight mass spectrometer

Water nucleation at extreme supersaturation

We report water cluster formation in the uniform postnozzle flow of a Laval nozzle at low temperatures of 87.0 and 47.5 K and high supersaturations of lnS ∼ 41 and 104, respectively. Cluster size distributions were measured after soft single-photon ionization at 13.8 eV with mass spectrometry. Critical cluster sizes were determined from cluster size distributions recorded as a function of increasing supersaturation, resulting in critical sizes of 6-15 and 1, respectively. Comparison with previous data for propane and toluene reveals a systematic trend in the nucleation behavior, i.e., a change from a steplike increase to a gradual increase of the maximum cluster size with increasing supersaturation. Experimental nucleation rates of 5 · 10¹⁵ cm^-3 s^-1 and 2 · 10¹⁵ cm^-3 s^-1 for lnS ∼ 41 and 104, respectively, were retrieved from cluster size distributions recorded as a function of nucleation time. These lie 2-3 orders of magnitude below the gas kinetic collision limit assuming unit sticking probability, but they agree very well with a recent prediction by a master equation model based on ab initio transition state theory. The experimental observations are consistent with barrierless growth at 47.5 K, but they hint at a more complex nucleation behavior for the measurement at 87.0 K.

An ion trap time-of-flight mass spectrometer with high mass resolution for cold trapped ion experiments

Trapping molecular ions that have been sympathetically cooled with laser-cooled atomic ions is a useful platform for exploring cold ion chemistry. We designed and characterized a new experimental apparatus for probing chemical reaction dynamics between molecular cations and neutral radicals at temperatures below 1 K. The ions are trapped in a linear quadrupole radio-frequency trap and sympathetically cooled by co-trapped, laser-cooled, atomic ions. The ion trap is coupled to a time-of-flight mass spectrometer to readily identify product ion species and to accurately determine trapped ion numbers. We discuss, and present in detail, the design of this ion trap time-of-flight mass spectrometer and the electronics required for driving the trap and mass spectrometer. Furthermore, we measure the performance of this system, which yields mass resolutions of m/Δm ≥ 1100 over a wide mass range, and discuss its relevance for future measurements in chemical reaction kinetics and dynamics.

Derivation of the Statistical Distribution of the Mass Peak Centroids of Mass Spectrometers Employing Analog-to-Digital Converters and Electron Multipliers

The mass peak centroid is a quantity that is at the core of mass spectrometry (MS). However, despite its central status in the field, models of its statistical distribution are often chosen quite arbitrarily and without attempts at establishing a proper theoretical justification for their use. Recent work has demonstrated that for mass spectrometers employing analog-to-digital converters (ADCs) and electron multipliers, the statistical distribution of the mass peak intensity can be described via a relatively simple model derived essentially from first principles. Building on this result, the following article derives the corresponding statistical distribution for the mass peak centroids of such instruments. It is found that for increasing signal strength, the centroid distribution converges to a Gaussian distribution whose mean and variance are determined by physically meaningful parameters and which in turn determine bias and variability of the m/z measurements of the instrument. Through the introduction of the concept of "pulse-peak correlation", the model also elucidates the complicated relationship between the shape of the voltage pulses produced by the preamplifier and the mean and variance of the centroid distribution. The predictions of the model are validated with empirical data and with Monte Carlo simulations.

Prospects for a statistical theory of LC/TOFMS data

The critical importance of employing sound statistical arguments when seeking to draw inferences from inexact measurements is well-established throughout the sciences. Yet fundamental statistical methods such as hypothesis testing can currently be applied to only a small subset of the data analytical problems encountered in LC/MS experiments. The means of inference that are more generally employed are based on a variety of heuristic techniques and a largely qualitative understanding of their behavior. In this article, we attempt to move towards a more formalized approach to the analysis of LC/TOFMS data by establishing some of the core concepts required for a detailed mathematical description of the data. Using arguments that are based on the fundamental workings of the instrument, we derive and validate a probability distribution that approximates that of the empirically obtained data and on the basis of which formal statistical tests can be constructed. Unlike many existing statistical models for MS data, the one presented here aims for rigor rather than generality. Consequently, the model is closely tailored to a particular type of TOF mass spectrometer although the general approach carries over to other instrument designs. Looking ahead, we argue that further improvements in our ability to characterize the data mathematically could enable us to address a wide range of data analytical problems in a statistically rigorous manner.

The design of single particle laser mass spectrometers

This review explores some of the design choices made with single particle mass spectrometers. Different instruments have used various configurations of inlets, particle sizing techniques, ionization lasers, mass spectrometers, and other components. Systematic bias against non-spherical particles probably exceeds a factor of 2 for all instruments. An ionization laser tradeoff is the relatively poor beam quality and reliability of an excimer laser versus the longer wavelengths and slower response time of an Nd-YAG laser. Single particle instruments can make special demands on the speed and dynamic range of the mass spectrometers. This review explains some of the choices made for instruments that were developed for different types of measurements in the atmosphere. Some practical design notes are also given from the author's experience with each section of the instrument.

Factors contributing to peak broadening and mass accuracy in the characterization of intact spores using matrix-assisted laser desorption/ionization coupled with time-of-flight mass spectrometry

Factors contributing to peak broadening, accuracy and precision in mass assignment in the matrix-assisted laser desorption/ionization characterization of a lipopeptide desorbed from intact Bacillus spores were investigated. These spores were studied as an example of a thick, topologically irregular sample, which present a more difficult target than a pure peptide or protein. The type of matrix, matrix:sample ratio, laser fluence, and localized repetitive laser irradiation were all found to affect the full-width at half maximum of the biomarker. Both in-source and post-source phenomena were shown to contribute. Sample thickness had less effect. Precision and accuracy of mass assignment were also affected by matrix:sample ratio and laser fluence. In general, this sample was responsive to the same experimental variables as pure peptides, and the use of an internal standard produced significant improvements in precision and accuracy.

Improved calibration of time-of-flight mass spectra by simplex optimization of electrostatic ion calculations

A novel time-of-flight mass calibration method has been developed. In contrast to conventional methods, where the relationship between ion flight time and mass is an arbitrary polynomial equation, this method is based on the physics of ion motion. Parameters needed to describe the physics are numerically optimized using a simplex algorithm. Once these parameters are established, unknown masses can be determined from their times-of-flight. This calibration method gives intrinsically well-behaved results, since nonlinearities (due to extraction delay, desorption velocity, etc.) are properly taken into account in the time-of-flight calculation. The simplex method is compared to curve fitting for the analysis of time-of-flight data, and some significant advantages are demonstrated. Salient features of the method include greatly improved mass extrapolation accuracy, no loss of interpolated calibration accuracy, the ability to obtain an accurate calibration with a minimal number of calibrants, and the ability to extract unknown parameters such as desorption velocities.

Ion isolation in a continuous zero angle reflecting time-of-flight mass spectrometer

A continuous zero angle reflecting time-of-flight mass spectrometer capable of tandem mass spectrometry measurements with high resolution and high sensitivity has been developed. The instrument design features two pulsed-ion mirrors in a coaxial geometry. Ions can be reflected back and forth with the mirrors, which increases the net flight length and permits kinetic energy focusing for enhanced resolution. The instrument also contains an electrostatic particle guide which increases ion transmission efficiency and can be used in a bipolar pulsed mode to isolate ions of interest for structural study.

Laser photodissociation of insulin ions generated by matrix-assisted laser desorption

A novel method for studying the ionization step of matrix-assisted laser desorption/ionization (MALDI) is demonstrated. A 193-nm pulse from an ArF excimer laser is used to photodissociate a portion of a plume of insulin ions generated by MALDI. Laser photodissociation (LPD) creates a "hole", i.e., a negative spike in the insulin peak in the time-of-flight (TOF) mass spectrum. The position of the hole in the mass spectrum provides useful measurements of the characteristics (position, time, and velocity) of insulin ions shortly after their creation. Although the performance of the method can be further improved, the data obtained could be used to refine our current understanding of MALDI and to improve the resolution of MALDI-TOFMS.

Sensitivity and mass accuracy for proteins analyzed directly from polyacrylamide gels: implications for proteome mapping

Matrix-assisted laser desorption ionization (MALDI) mass spectra have been obtained directly from thin-layer isoelectric focusing (IEF) gels with as little as 700 femtomoles of alpha- and beta-chain bovine hemoglobin and bovine carbonic anhydrase, and 2 picomoles of bovine trypsinogen, soybean trypsin inhibitor, and bovine serum albumin all loaded onto a single lane. By soaking the gel in a matrix solution, matrix was deposited over the entire gel surface, allowing MALDI scanning down complete lanes of the one-dimensional gel. As long as matrix crystals were deposited finely on the surface of the gel, time-lag focusing techniques were capable of ameliorating some of the mass accuracy limitations inherent in desorbing from uneven insulator surfaces with external calibration. Eleven measurements on the 5 kDa alpha-subunit proteins of lentil lectin measured over the course of 1 h and referenced to a single calibration yielded a standard deviation of 0.025%. Colloidal gold staining was found to be compatible with desorption directly from IEF and sodium dodecyl sulfate (SDS)-polyacrylamide gels. This direct approach simplifies the interface between gel electrophoresis and mass spectrometry dramatically, making the process more amenable to automation.

Low-power resonant laser ablation of copper

We emphasize two points: (l) the properties and mechanisms of very low-fluence ablation of copper surfaces and (2) the sensitivity and selectivity of resonant laser ablation (RLA). We present results for ablation of bulk copper and copper thin films; spot-size effects; the effects of surface-sample preparation and beam polarization; and an accurate measurement of material removal rates, typically ≤ 10(-3) Å at 35 mJ/cm(2). Velocity distributions were Maxwellian, with peak velocities ≈ 1-2 × 10(5) cm/s. In addition, we discuss the production of diffractionlike surface features, and the probable participation of nonthermal desorption mechanisms. RLA is shown to be a sensitive and useful diagnostic for studies of low-fluence laser-material interactions.

Time-of-flight mass spectrometry: State-of the-art in chemical analysis and molecular science

Time-of-flight instruments form a well-established group of mass spectrometers, with its popularity still increasing. This article gives a survey of the technical basics and important instrumental developments in this field. Special notice is taken of factors that limit the mass resolution and to techniques to overcome them. The possibilities to perform tandem MS experiments in flight-time mass analyzers and their combination with ion-trapping devices are discussed. Finally, some examples of the modern applications of these instruments in molecular science and instrumental analysis are reviewed. © 1997 John Wiley & Sons, Inc.

Time-of-flight mass spectrometry: an increasing role in the life sciences

Although available commercially since the mid-1950s, there has been a renewed and increasing interest in time-of-flight mass spectrometers during the last decade or more. Improvements have been made in mass resolution; and high-speed data acquisition systems have been developed which enable the recording of all ions in each time-of-flight cycle. Most importantly, these instruments have been coupled with several new ionization techniques, which are capable of desorbing the relatively large and intractable biopolymers whose structures are of interest to molecular biologists, biochemists and biophysicists. Primarily these are techniques which employ pulsed lasers, fission fragments and pulsed ion beams, for which a 'non-scanning' and/or high-transmission analyzer provides considerable analytical advantage. In this report we review some basic principles of the time-of-flight mass analyzer, highlighting efforts to improve dynamic focusing for instruments forming ions in the gas phase and static focusing for desorption instruments, and the progression from time-slice to time-array detection. We also review some of the accomplishments of instruments employing the time-of-flight analyzer, including: molecular weight determinations for peptides and small proteins; the analysis of tryptic digests, crude extracts and whole cells; the structural analysis of glycolipids, phospholipids and lipopolysaccharides; and the determination of covalent and metal-linked peptide dimers. We conclude with some recent developments in combining the time-of-flight analyzer with liquid chromatography using the continuous flow probe technique.

Focal Points in Mass Spectrometry

Mass spectrometry has advanced with the renaissance of time-of-flight mass analysis, the use of ion traps as analyzers and reactors, the application of tandem mass spectrometers to problems in ionic reaction mechanisms and chemical analysis, and the development of new desorption ionization techniques. These developments have allowed determination of the molecular weight distributions for polymers through the 10,000-dalton range, as well as the molecular weight and partial sequence of biopolymers of similar size. Surfaces can be characterized by use of the mass, energy, and angle distributions of particles ejected by sputtering or by laser-induced desorption. Mass spectrometry has yielded new information on the kinetics of catalytic surface reactions and on the reactivity of metal clusters.