Secondary ion emission from solutions: time dependence and surface phenomena

Delayed Desorption Improves Protein Analysis by Desorption Electrospray Ionization Mass Spectrometry

Protein analysis by desorption electrospray ionization mass spectrometry (DESI-MS) is limited and often accompanied by a mass-dependent loss in sensitivity as protein molecular weight increases. Previously, incomplete dissolution was identified as a potential contributing factor to this limitation for larger proteins. Here, we developed a unique two-step configuration in which a prewetting solvent is applied to the sample surface proximal to DESI analysis by a wetting quill to increase dissolution time and the detection of larger proteins. After optimizing the system with a mixture of proteins containing cytochrome c, myoglobin, and chymotripsinogen, we demonstrate the ability of delayed desorption to improve the analysis of larger proteins such as bovine serum albumin. Albumin and other serum proteins, including even larger ones, were also detected directly from diluted goat serum. An additional feature of this technique is the ability to deliver multiple solvents with potential synergistic or cooperative effects. For example, when using acetonitrile solutions of formic acid and ammonium bicarbonate as the prewetting and DESI spray solvent, respectively, the intensity of chymotrypsinogen improved dramatically compared to controls but less so for smaller proteins such as myoglobin and cytochrome c. Adduct removal was also observed for all proteins. These early results demonstrate the ability of this two-step technique for the use of multiple additives and increased dissolution times compared to standard DESI-MS experiments.

Deconstructing desorption electrospray ionization: independent optimization of desorption and ionization by spray desorption collection

Spray desorption collection (SDC) and reflective electrospray ionization (RESI) were used to independently study the desorption and ionization processes that together comprise desorption electrospray ionization (DESI). Both processes depend on several instrumental parameters, including the nebulizing gas flow rate, applied potential, and source geometries. Each of these parameters was optimized for desorption, as represented by the results obtained by SDC, and ionization, as represented by the results obtained by RESI. The optimized conditions were then compared to the optimization results for DESI. Our results confirm that optimal conditions for desorption and ionization are different and that in some cases the optimized DESI conditions are a compromise between both sets. The respective results for DESI, RESI, and SDC for each parameter were compared across the methods to draw conclusions about the contribution of each parameter to desorption and ionization separately and then combined within DESI. Our results indicate that desorption efficiency is (1) independent of the applied potential and (2) the impact zone to inlet distance, and that (3) gas pressure settings and (4) sprayer to impact zone distances above optimal for DESI are detrimental to desorption but beneficial for ionization. In addition, possible interpretations for the observed trends are presented.

Physical/chemical separations in the break-up of highly charged droplets from electrosprays

Highly-charged droplets, as formed by an electrospray process, are known to undergo asymmetric fission to form smaller droplets. We have observed a chemical and physical separation phenomenon that occurs in the droplet break-up process and is related to a compound's surface activity in solution. Two experimental approaches demonstrated that the smaller satellite droplets and the progeny droplets generated by the spray formation and asymmetric fission processes to be surfactant-enriched. These smaller droplets were also effectively separated from the larger primary and residual droplets because of their smaller inertia and high surface charge density, and a region attributed to the initially formed smaller satellite droplets was found to be strikingly confined in a narrow periphery region of the electrospray. The phenomenon may have utility for chemical separations and have significant implications for the sensitivity and selectivity of electrospray ionization-mass spectrometry.

Formulation of matrix solutions for use in matrix-assisted laser desorption/ionization of biomolecules

We report a simple method for converting solid matrices into useful matrix solutions for matrix-assisted laser desorption/ionization (MALDI). This method is based on the dissolution of the solid matrix in a liquid support of low volatility such as glycerol. An appropriate solubilizing reagent was added to promote the dissolution of the matrix materials into the liquid support. Selection of the solubilizing reagent is empirically related to an acid-base relationship, i.e., an acidic solid matrix requires a basic organic compound to form a stable matrix solution in the liquid support and vice versa. A tenfold increase in the solubility can be obtained for many solid matrices when appropriate solubilizing reagents are added into the glycerol support. This solubility enhancement is tentatively attributed to the ion-pair formation in a polar nonvolatile liquid support. In addition, the hydrophobicity of the solid matrix seems to play an important role in the efficiency of the resulting matrix solution. By using glycerol as liquid support, a hydrophilic matrix, such as 2,5-dihydroxybenzoic acid (DHB), showed a substantial "peripheral effect," in which good analyte ion signals could only be recorded at the peripheral region of the sample droplet. More hydrophobic matrices, such as alpha-cyano-4-hydroxycinnamic acid (alpha-CCA), exhibit better and more homogeneous responses at different regions of the droplets. The performance of these matrix solutions was evaluated in terms of the durability, reproducibility, sensitivity, high mass capability, and generality. A typical sample droplet can afford more than an hour of repeated sampling with excellent shot-to-shot reproducibility. A low picomole sensitivity was demonstrated using a luteinizing hormone releasing hormone (LHRH) in a Fourier transform ion cyclotron resonance mass spectrometer with a homemade external MALDI ion source. By using a commercial MALDI time-of-flight mass spectrometer, proteins with masses as high as 66,000 Da were successfully analyzed by using these matrix solutions.

Organic ion imaging beyond the limit of static secondary ion mass spectrometry

Secondary ion mass spectra and images were obtained from spikes of choline chloride, acetylcholine chloride, and methylphenylpyridinium iodide deposited onto specimens of porcine brain tissue. Samples were subsequently subjected to a dose of 10-keV Cs(+) sufficient to suppress secondary ion emission characteristic of the targeted analytes. Following ablation of the samples by massive glycerol clusters generated by electrohydrodynamic emission, secondary ion mass spectra and images could be obtained that reflected the identity and location of the spiked analytes. The absolute intensity of secondary ion emission that followed ablation was found to be between 30 and 100% of the intensity obtained prior to exposure to the high dose of Cs'. Not all chemical noise is removed by ablation, however, so that the signal-to-noise ratios after ablation correspond to between 10 and 85% of their values observed under conditions of low primary ion dose.

Massive cluster ablation as preparation for organic secondary ion imaging

A focused Cs+ beam was used to obtain secondary ion mass spectra and images from samples of stearic acid on gold before and after ablation by a beam of massive cluster ions. Ablation appears to have two effects on secondary ion emission. First, the number and intensity of peaks reflecting contamination are substantially reduced. Secondly, the absolute intensity of secondary ion current characteristic of the analyte increases. These features simplify mass spectra and improve contrast in the images obtained.

N-Alkylnicotinium halides: A class of cationic matrix additives for enhancing the sensitivity in negative ion Fast-Atom bombardment mass spectrometry of polyanionic analytes

The addition of some surfactants to the fast-atom bombardment (FAB) matrix previously has been demonstrated to enhance analyte signals in fast-atom bombardment mass spectrometry. In particular, cationic surfactants appear to enhance the negative ion FAB detectability of analytes that exist as anionic species in the matrix solution. It has been proposed that the charged surfactant concentrates the oppositely charged analyte near the surface, which results in larger signals for the analyte. Cationic surfactants that contain a fixed positive charge and an additional basic site were prepared with different hydrophobic moieties and were evaluated for their effectiveness as FAB matrix additives. The compound N-octylnico-tinium bromide (ONBr) is shown to improve greatly the analyte-related signals in negative ion fast-atom bombardment mass spectrometry for a variety of polyanionic analytes, relative to other surfactants (e.g., cetylpyridinium salts). This surfactant not only enhances detectability, but also simplifies the pseudomolecular ion region of the resulting spectra by reducing or eliminating metal cation adduct peaks. The simple mechanism of enhancement via surface activity is evaluated, and alternative mechanisms are considered. It is clearly shown that ONBr, as a FAB matrix additive, will allow mass spectrometry to be used for the analysis of anionic compounds that normally exhibit very low responses.

Fast-atom bombardment-induced condensation of glycerol with ammonium surfactants II: Time dependence of mass spectra and tandem mass spectra

Studies of fast-atom bombardment (FAB)-induced condensation between trimethyltetradecylammonium cations and glycerol have been extended to consider spectral time dependence. To enhance reproducibility of time dependence, a modified FAB target was used. FAB mass spectrometry of deuterium-labeled surfactants and FAB/collision-induced dissociation (CID) of nonlabeled material demonstrate that products of condensation at the surfactant "head group" predominate early in the analysis, while tail adducts become prominent later. This time dependence correlates with the expected surface activity of the products. It is incompatible with gas-phase reaction, but consistent with reaction in the condensed phase. Subtle variations in the surface activities of various condensation products (derived from changes in the number of hydroxyls from the reactive glycerol radical or in the position of attack along the surfactant chain) are reflected in the time dependence of FAB and CID spectra. CID spectra of deuterium-labeled cations provide evidence for intramolecular hydrogen transfer from the surfactant tail to the head within a surfactant radical. This transfer shows no significant kinetic isotope effect.

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 cm(2) 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.

Role of ion-ion recombination for alkali chloride cluster formation in liquid secondary ion mass spectrometry

Liquid secondary ionization mass spectra of solutions of alkali chlorides in glycerol were studied as a function of salt concentration. The experimental abundances of glycerol ions and of Cs(+)(CsCl) n cluster ions were successfully reproduced by assuming that most of the randomly distributed ions pair up with counterions shortly after impact. Further, it is considered that clustering (or proton transfer) reactions occur mainly between an ion that survives the pairing process and ion pairs (or basic analytes) in the immediate vicinity; however, some mixing undoubtedly occurs in the later stages of the desorption process. At the density of the original matrix, the range of proton transfer is calculated to be 5-15 Å, and that of clustering approximately 25% shorter. These reaction distances are inversely correlated with the internal energy of the ejected ions. In general, liquid secondary ionization mass spectra of alkali chloride solutions can be seen to result from competitive ion-ion recombination reactions in the decaying matrix. Finally, from the abundances of cluster ions containing [glycerol - H](-) ions, it is estimated that approximately 1% of the glycerol molecules in the ejected volume are ionized in the collision cascade.