A method to distinguish the four most common biologically relevant underivatized hexoses, d-glucose, d-galactose, d-mannose, and d-fructose, using only mass spectrometry with no prior separation/derivatization step has been developed. Electrospray of a solution containing hexose and a lithium salt generates [Hexose+Li]+. The lithium-cationized hexoses adduct water in a quadrupole ion trap. The rate of this water adduction reaction can be used to distinguish the four hexoses. Additionally, for each hexose, multiple lithiation sites are possible, allowing for multiple structures of [Hexose+Li]+. Electrospray produces at least one structure that reacts with water and at least one that does not. The ratio of unreactive lithium-cationized hexose to total lithium-cationized hexose is unique for the four hexoses studied, providing a second method for distinguishing the isomers. Use of the water adduction reaction rate or the unreactive ratio provides two separate methods for confidently (p ≤ 0.02) distinguishing the most common biologically relevant hexoses using only femtomoles of hexose. Additionally, binary mixtures of glucose and fructose were studied. A calibration curve was created by measuring the reaction rate of various samples with different ratios of fructose and glucose. The calibration curve was used to accurately measure the percentage of fructose in three samples of high fructose corn syrup (<4% error).

A method has been developed that is capable of distinguishing an exhaustive list of underivatized D-pentoses with only a mass spectrometer. Electrospray ionization (ESI) of a solution containing a pentose and a lithium salt yields [Pentose + Li]+. These lithiated pentoses adduct water in a quadrupole ion trap. The reaction rate of water adduction is unique for several of the pentose isomers. Additionally, there are multiple potential gas-phase lithiation sites to form [Pentose + Li]+. A mixture of ions with at least one reactive (water adducting) and at least one unreactive (non-adducting) lithiation site is formed for each pentose. The water adduction reaction rate along with the unreactive fraction of lithiated pentose can be used to completely discriminate all D-pentoses.

An extractive electrospray ionization (EESI) source design is presented to improve the reproducibility of the interactions occurring in EESI. This design uses three concentric capillaries to deliver the solvent, sample and nebulizing gas. Coaxial EESI was found to improve the inter-experiment variation by a factor of four and intra-run relative standard deviation by a factor of 2.4. Unlike most standard EESI designs, the device has the form factor of a standard electrospray emitter and can be used without any further instrument modifications.

Differential ion mobility spectrometry (DIMS) is capable of separating components of complex mixtures prior to mass spectrometric analysis, thereby increasing signal-to-noise and signal-to-background ratios on millisecond timescales. However, adding a DIMS device to the front end of a mass spectrometer can reduce the signal intensity of subsequent mass spectrometric analysis. This is a result, in part, of ions lost due to inefficient transfer of ions from the DIMS device through the aperture leading into the mass spectrometer. This problem of transferring ions can be at least partially corrected by modifying the front end of the inlet capillary leading to the vacuum of the mass spectrometer. The inner diameter of the ion-sampling end of the inlet capillary was enlarged by drilling into the face. This results in a conical flare at the front end of the capillary, while the other end of the capillary remains unmodified. These flared capillaries allow for a greater number of ions from the DIMS device to be sampled relative to the unmodified standard capillary. Four flare dimensions were tested, differing by the angle between the wall of the flare and the outer wall of the inlet capillary. All flared capillaries showed greater signal intensity than the standard capillary with a DIMS device present without reducing the resolving power. It was also observed that the signal intensity increased as the flare angle decreased. The flared capillary with the smallest flare angle showed greater than a fivefold increase in signal intensity compared with the standard capillary.

The design and operation of an inexpensive, miniature low-temperature plasma ion source is detailed. The miniature low-temperature plasma ion source is operated in a “flow-through” configuration, wherein the gaseous or aerosolized analyte, caffeine or pyrolyzed ethyl cellulose, in a carrier gas is used as the plasma gas. In this flow-through configuration, the sensitivity for the caffeine standard and the pyrolysis products of ethyl cellulose is maintained or increased and the reproducibility of the ion source is increased. Changes in the relative intensity of ions from the aerosol produced by pyrolysis of ethyl cellulose are observed in the mass spectrum when the low-temperature plasma ion source is used in the flow-through configuration. Experiments suggest this change in relative intensity is likely due to differences in ionization efficiency rather than increased fragmentation of ethyl cellulose pyrolysis products during ionization. Flow-through low-temperature plasma ionization with the miniature ion source is shown to be a promising technique for the ionization of compounds in gases or aerosol particles.

Low-temperature plasma ionization, a technique that causes minimal fragmentation during ionization, is investigated as an ionization technique for mass spectrometric detection of the compounds in ambient organic aerosols in real time. The experiments presented in this paper demonstrate that ions are generated from compounds in the aerosol particles. The utility of this technique for detection of both positive and negative ions from the pyrolysate of multiple natural polymers is presented. Ultimately, low-temperature plasma ionization is shown to be a promising ionization technique for detection of compounds in organic aerosols by mass spectrometry.

Differential ion mobility spectrometry (DIMS) separations are described using similar terminology to liquid chromatography, capillary electrophoresis, and drift tube ion mobility spectrometry. The characterization and comparison of all these separations are typically explained in terms of resolving power, resolution, and/or peak capacity. A major difference between these separations is that DIMS separations are in space whereas the others are separations in time. However, whereas separations in time can, in theory, be extended infinitely, separations in space, such as DIMS separations, are constrained by the physical dimensions of the device. One method to increase resolving power of DIMS separations is to use helium in the DIMS carrier gas. However, ions have a greater mobility in helium which causes more ions to be neutralized due to collisions with the DIMS electrodes or electrode housing, i.e. the space constraints. This neutralization of ions can lead to the loss of an entire peak, or peaks, from a DIMS scan. To take advantage of the benefits of helium use while reducing ion losses, linked scans were developed. During a linked scan the amount of helium present in the DIMS carrier gas is decreased as the compensation field is increased. A comparison of linked scans to compensation field scans with constant helium is presented herein. Resolving powers >7900 are obtained with linked scans. However, this result highlights the limitation of using resolving power as a metric to describe DIMS separations.

Measurement of herbicide residues in environmental matrices is typically performed using liquid or gas chromatography coupled to mass spectrometry, generally with one or more stages of sample processing or purification prior to analysis. Paper spray ionization enables the rapid mass spectrometric analysis of such samples without the use of chromatography or sample cleanup techniques. Samples are applied to a paper strip and dried, after which they may be stored or transported. By applying solvent and a high voltage to the paper strip, the analyte is extracted from the paper and ionized by electrospray from the tip of the paper strip. Qualitative and quantitative measurement of triazine herbicides and the chloroacetanilide herbicide metolachlor are demonstrated using samples spiked into water and crop extracts at part-per-billion concentrations. The linear dynamic range includes the U.S. statutory maxima for atrazine in crops and human health hazard levels in water, as well as E.P.A. levels of concern and regulatory limits for metolachlor in crops and water.

Differential ion mobility spectrometry (DIMS) separates ions based on differences in their mobilities in low and high electric fields. When coupled to mass spectrometric analyses, DIMS has the ability to improve signal-to-background by eliminating isobaric and isomeric compounds for analytes in complex mixtures. DIMS separation power, often measured by resolution and peak capacity, can be improved through increasing the fraction of helium in the nitrogen carrier gas. However, because the mobility of ions is higher in helium, a greater number of ions collide with the DIMS electrodes or housing, yielding losses in signal intensity. To take advantage of the benefits of helium addition on DIMS separations and reduce ion losses, linked scans were developed. In a linked scan the helium content of the carrier gas is reduced as the compensation field is increased. Linked scans were compared with conventional compensation field scans with constant helium content for the protein ubiquitin and a tryptic digest of bovine serum albumin (BSA). Linked scans yield better separation of ubiquitin charge states and enhanced peak capacities for the analysis of BSA compared with compensation field scans with constant helium carrier gas percentages. Linked scans also offer improved signal intensity retention in comparison to compensation field scans with constant helium percentages in the carrier gas.

Differential ion mobility spectrometry (DIMS) has the ability to separate gas phase ions based on their difference in ion mobility in low and high electric fields. DIMS can be used to separate mixtures of isobaric and isomeric species indistinguishable by mass spectrometry (MS). DIMS can also be used as a filter to improve the signal-to-background of analytes in complex samples. The resolving power of DIMS separations can be improved several ways, including increasing the dispersion field and increasing the amount of helium in the nitrogen carrier gas. It has been previously demonstrated that the addition of helium to the DIMS carrier gas provides improves separations when the dispersion field is the kept constant as helium content is varied. However, helium has a lower breakdown voltage than nitrogen. Therefore, as the percent helium content in the nitrogen carrier gas is increased, the highest dispersion field accessible decreases. This work presents the trade-offs between increasing dispersion fields and using helium in the carrier gas by comparing the separation of a mixture of isobaric peptides. The maximum resolution for a separation of a mixture of three peptides with the same nominal molar mass was achieved by using a high dispersion field (~72 kV/cm) with pure nitrogen as the carrier gas within the DIMS assembly. The conditions used to achieve the maximum resolution also exhibit the lowest ion transmission through the assembly, suggesting that it is necessary to consider the trade-off between sensitivity and resolution when optimizing DIMS conditions for a given application.

Alternative ion activation methods in a quadrupole ion trap mass spectrometer (QITMS) have been studied to determine their utility for iTRAQ. The collisional activation methods, thermally assisted CID (TA-CID) and high amplitude short time excitation (HASTE) CID, allow the low-mass cut-off (LMCO) to be reduced to ∼10% of the parent ion mass-to-charge ratio allowing the iTRAQ reporter ions to be trapped and detected. An alternative to CID for ion activation/dissociation in a QITMS is infrared multiphoton photodissociation (IRMPD), which can be performed at LMCOs of <10% of the parent ion mass-to-charge ratio. Reported here are experiments comparing the efficiency of these methods for relative quantification using iTRAQ. All methods generated the reporter ions but there were differences in overall MS/MS efficiency and the conversion efficiency to the iTRAQ reporter ions.Graphical abstractPlot of conversion efficiency producing iTRAQ reporter ions vs. activation method.Highlights► iTRAQ with a quadrupole ion trap. ► High iTRAQ reporter ion conversion efficiency with IRMPD. ► Methods to decrease the low-mass cut-off in MS/MS experiments using a quadrupole ion trap mass spectrometer.

Thermally-assisted collision-induced dissociation can substantially increase the amount of dissociation in quadrupole ion trap mass spectrometer experiments. The experiments discussed here were performed to assess the effect on dissociation pathways as the bath gas temperature is increased in thermally-assisted collision-induced dissociation (TA-CID) experiments. Double resonance experiments in which a product ion was ejected during collision-induced dissociation of the parent ion provided data to asses competitive versus consecutive dissociation pathways. Consecutive dissociation pathways are indicated when lower mass product ions decrease in intensity when a higher mass product ion is ejected during CID. For the peptide ions studied, those that dissociate to give predominately N-terminal product ions show increased consecutive dissociation with increased temperature during TA-CID. For peptide ions that dissociate via formation of C-terminal product ions, competitive dissociation pathways were more prevalent and increasing the temperature had much less effect. N-terminal product ions consecutively dissociate to smaller N-terminal ions whereas C-terminal product ions dissociate to internal fragments.Product ion formation pathway tree for FLLVPLG obtained by double resonance/CID at a bath gas temperature of 100 °C.

Infrared multiphoton photodissociation (IRMPD) in a quadrupole ion trap is not selective for a parent ion. Product ions are decreased in abundance by continuous sequential dissociation and may be lost below the low mass cut-off. The IRMPD process is made selective by resonantly exciting trapped ions into an axially offset laser path. Product ions form and collisionally relax out of the laser path to accumulate in the center of the trap. The technique, termed selective broadband (SB) IRMPD, limits sequential dissociation to preserve first generation product ion abundance. The abundances of larger product ions are maximized by completely dissociating the parent ion, but continuous sequential dissociation does not form small product ions below the low mass cut-off associated with conventional IRMPD. Smaller product ions are further increased in abundance in another tandem mass spectrum by performing sequential stages of SB-IRMPD, adjusting the trapping rf amplitude to dissociate larger product ions at the same qz range. Thermal assistance is used to perform SB-IRMPD at higher bath gas pressures for increased sensitivity.

A multiplexed tandem mass spectrometry (MS/MS) technique known as iterative accumulation multiplexing (IAM) has been implemented on a hybrid quadrupole Fourier transform ion cyclotron resonance mass spectrometer (Q-FTICR-MS). The IAM experiment resulted in obtaining MS/MS spectra for six analytes in two MS/MS experiments while characteristic resolving power and mass measurement accuracies were maintained. Parent-product ion correlations were graphically represented in a “ratiogram” where each product ion is encoded with a ratio unique to the parent ion from which it was formed. This is the first example of multiplexed MS on a FTICR instrument where the ions are encoded externally to the ICR cell. By performing the encoding external to the ICR cell, one set of ions can be encoded while the previous set of ions is being analyzed in the cell, maximizing the use of the continuous ion current emanating from the electrospray ionization source.

A method of performing collision induced dissociation (CID) on the charge-reduced parent ion as it is formed during electron capture dissociation (ECD), called ECD+CID, is described. In ECD+CID, the charge-reduced parent ion is selectively activated using resonant excitation and collisions with the helium bath gas inside a linear quadrupole ion trap ECD device (ECDLIT). It has been observed that ECD+CID can improve the sequence coverage for β-endorphin over performing ECD alone (i.e., from 72 to 97%). Perhaps just as important, ECD+CID can be used to reduce the extent of multiple electron capture events observed when performing ECD in the ECDLIT. Consequently, the abundance of mass-to-charge ratios corresponding to ECD product ions that contain neutralized protons is decreased, simplifying the interpretation of the product ion spectrum.

A focused laser is used to make infrared multiphoton photodissociation (IRMPD) more efficient in a quadrupole ion trap mass spectrometer. Efficient (up to 100%) dissociation at the standard operating pressure of 1 × 10−3 Torr can be achieved without any supplemental ion activation and with shorter irradiation times. The axial amplitudes of trapped ion clouds are measured using laser tomography. Laser flux on the ion cloud is increased six times by focusing the laser so that the beam waist approximates the ion cloud size. Unmodified peptide ions from 200 Da to 3 kDa are completely dissociated in 2.5–10 ms at a bath gas pressure of 3.3 × 10−4 Torr and in 3–25 ms at 1.0 × 10−3 Torr. Sequential dissociation of product ions is increased by focusing the laser and by operating at an increased bath gas pressure to minimize the size of the ion cloud.A focused IR laser makes IRMPD a practical experiment at normal operating bath gas pressures in a quadrupole ion trap mass spectrometer.Figure optionsDownload full-size imageDownload high-quality image (65 K)Download as PowerPoint slide

Infrared multiphoton dissociation (IRMPD) combined with ion trajectory simulations has been used to obtain probability maps of ion position as a function of different operating parameters in a quadrupole ion trap mass spectrometer. The factors that contribute to the depth of the pseudopotential trapping well are analyzed, and their effects on the efficiency of IRMPD are demonstrated. Ion trajectory simulations are used to substantiate experimental results and demonstrate in greater detail the dynamic nature of the ion population’s positional distribution. In particular, it is shown that the so-called “qz value” used during photodissociation can be of great consequence, as can the frequency of ac trapping voltage applied to the ring electrode. The results reveal that parameters which increase the pseudopotential well have the effect of decreasing the size of the ion cloud and maximizing overlap between the irradiating laser and the ions. Thus, while the common understanding of IRMPD dictates otherwise, IRMPD fragmentation efficiencies really depend on many ion trap operating parameters, much as collision-induced dissociation does.

Recently reported results (Konn et al. [14]) on the collisional cooling of atmospheric pressure matrix assisted laser desorption ionization (AP-MALDI) and nano-electrospray ionization (nano-ESI) generated ions in a quadrupole ion trap mass spectrometer (QITMS) are inconsistent with measured collisional cooling rates. The work reported here presents a re-examination of those previous results. Collision induced dissociation (CID) has been used to probe various properties of ions contained in a QITMS. It is shown experimentally that when trapping large numbers of ions, an effective dc trapping voltage is induced that varies with changes in the size of the ion cloud. A decrease in the resonant frequency for maximum CID efficiency is observed as the cool time between parent ion isolation and CID is increased. Ion trajectories in a QITMS are simulated to demonstrate how ion density changes over the course of parent ion isolation. The effect of space charge on ion motion is simulated, and Fourier transformations of ion axial motion plus simple calculations corroborate the experimentally observed transient frequency shifts. The relative stability of ions formed by AP-MALDI and nano-ESI is compared under low charge density conditions. These data show that the ions have reached equilibrium internal energy and, thus, that differences in dissociation onsets and “50% fragmentation efficiency points” between the ionization mechanisms are due to the formation of distinct ion conformations as previously shown in reference [28]. The conclusions of Konn et al. [14] are based on invalid experimental procedures as well as inappropriate comparisons of QITMS data to low-pressure FT-ICR data.

Mass spectrometers that use different types of analyzers for the first and second stages of mass analysis in tandem mass spectrometry (MS/MS) experiments are often referred to as “hybrid” mass spectrometers. The general goal in the design of a hybrid instrument is to combine different performance characteristics offered by various types of analyzers into one mass spectrometer. These performance characteristics may include mass resolving power, the ion kinetic energy for collision-induced dissociation, and speed of analysis. This paper provides a review of the development of hybrid instruments over the last 30 years for analytical applications.

Collisional cooling rates of infrared excited ions are measured in a quadrupole ion trap (QIT) mass spectrometer at different combinations of temperature and pressure. Measurements are carried out by monitoring fragmentation efficiency of leucine enkephalin as a function of irradiation time by an infrared laser after a short excitation and incrementally increasing cooling periods. Cooling rates are observed to be directly related to bath gas pressure and inversely related to bath gas temperature. The cooling rate at typical ion trap operating pressure (1 mTorr) and temperature (room T) is faster than can be measured. At elevated temperature and the lowest pressure used for the studies, the rate of collisional cooling becomes negligible compared to the rate of radiative cooling.

Collision induced dissociation (CID) in a quadrupole ion trap mass spectrometer using the conventional 30 ms activation time is compared with high amplitude short time excitation (HASTE) CID using 2 ms and 1 ms activation times. As a result of the shorter activation times, dissociation of the parent ions using the HASTE CID technique requires resonance excitation voltages greater than conventional CID. After activation, the rf trapping voltage is lowered to allow product ions below the low mass cut-off to be trapped. The HASTE CID spectra are notably different from those obtained using conventional CID and can include product ions below the low mass cut-off for the parent ions of interest. The MS/MS efficiencies of HASTE CID are not significantly different when compared with the conventional 30 ms CID. Similar results were obtained with a two-dimensional (linear) ion trap and a three-dimensional ion trap.

Enormous advances in our understanding of the chemistry underlying life processes have identified new targets for therapeutic agents. The discovery of effective therapeutics to address these targets is often accomplished through parallel synthetic and screening efforts. In almost all cases, what has enabled target identification and allowed parallel approaches to drug discovery to be effective are the development of either new analytical tools or the improvement of currently existing ones. Among these tools, mass spectrometry has evolved to become an irreplaceable technique in the analysis of biologically related molecules. This article will guide researchers in drug discovery through the basic principles of mass spectrometry.

The performance of quadrupole ion traps using argon or air as the buffer gas was evaluated and compared to the standard helium only operation. In all cases a pure buffer gas, not mixtures of gases, was investigated. Experiments were performed on a Bruker Esquire ion trap, a Finnigan LCQ, and a Finnigan ITMS for comparison. The heavier gases were found to have some advantages, particularly in the areas of sensitivity and collision-induced dissociation efficiency; however, there is a significant resolution loss due to dissociation and/or scattering of ions. Additionally, the heavier gases were found to affect ion activation and deactivation during MS/MS, influencing the product ion intensities observed. Finally, the specific quadrupole ion trap design and the ion ejection parameters were found to be crucial in the quality of the spectra obtained in the presence of heavy gases. Operation with static pressures of heavy gases can be beneficial under certain design and operating conditions of the quadrupole ion trap.

An alternative to resonant excitation collision-induced signal enhancement (CISE) is presented. This alternative utilizes boundary activation instead of resonant excitation to effect CISE and is called boundary activated collision induced signal enhancement (BA-CISE). There are three ways to effect BA-CISE to enhance the signal for an MSn+1 experiment. Each technique utilizes the βz = 0 boundary, which ions encounter from high to low mass/charge ratio. BA-CISE is shown to produce an almost 900% increase in the C2 ion of [maltohexaose + Li]+. The use of a heavy collision gas in addition to the helium bath gas generally produced a signal enhancement inferior to the same experiment without the heavy gas.

Dissociation pathways of alkali-cationized peptides have been studied using multiple stages of mass spectrometry (MSx) with a quadrupole ion trap mass spectrometer. Over 100 peptide ions ranging from 2 to 10 residues in length and containing each of the 20 common amino acids have been examined. The formation of the [bn−1 + Na + OH]+ product ion is the predominant dissociation pathway for the majority of the common amino acids. This product corresponds to a sodium-cationized peptide one residue shorter in length than the original peptide. In a few cases, product ions such as [bn−1 + Na − H]+ and those formed by loss, or partial loss, of a sidechain are observed. Both [bn−1 + Na + OH]+ and [bn−1 + Na − H]+ product ions can be selected as parent ions for a subsequent stage of tandem mass spectrometry (MS/MS). It is shown that these dissociation patterns provide opportunities for peptide sequencing by successive dissociation from the C-terminus of alkali-cationized peptides. Up to seven stages of MS/MS have been performed on a series of [b + Na + OH]+ ions to provide sequence information from the C-terminus. This method is analogous to Edman degradation except that the cleavage occurs from the C-terminus instead of the N-terminus, making it more attractive for N-terminal blocked peptides. The isomers leucine and isoleucine cannot be differentiated by this method but the isobars lysine and glutamine can.

This is a review of the gas-phase reactions, in beam type mass spectrometers, that change the charge states of ions. The Cooks research group has been a leader in both the understanding and use of these charge-changing reactions. The charge of ions can be manipulated in beam type instruments via collisions with neutral gas atoms or molecules in the high ion kinetic energy regime. The two major processes, charge inversion and charge stripping due to high energy ion/neutral collisions, are discussed. Charge permutation reactions often result in the formation of unique ion structures that aid in the differentiation of isomers. The products of charge permutation reactions may possess excess internal energy that cause the ions to fragment, and these fragment ions may provide complementary information to that obtained from high energy collision-induced dissociation. The charge of ions also can be changed as a result of a collision with a surface in the low ion kinetic energy regime. Charge exchange and charge inversion processes that occur as a result of low energy ion/surface collisions are presented.

The gas-phase ion/ion reactions of iron ions with oppositely charged peptide and protein ions were studied in a quadrupole ion trap. Both Fe+ and FeCO2− were investigated as possible reactant ions for gas-phase cleavage of peptide and protein ions. Several types of reaction products were observed. Charge exchange lowered the charge states of the proteins. Attachment resulted in a complex of the protein ion and the iron ion. In some cases bonds were broken in the protein ions, but it is unclear whether this is due to an insertion of the iron ion into a bond or due to the energetic reaction of oppositely charged species. Some preference was observed for bond cleavage near sulfur. Two disulfide bonds were broken in one case, and bonds adjacent to a cysteine residue were broken in another.

MS/MS has been used to sequence peptides and small proteins for a number of years. This method allows one to isolate the peptide of interest, which makes it possible to analyze impure samples and unseparated mixtures, such as protein digests. Collision-induced dissociation (CID) of the selected peptide ion generates the product ions that provide sequence information. However, often the MS/MS spectrum does not provide adequate information for complete sequence determination. The quadrupole ion trap has the capability to do multiple stages of mass spectrometry, MSn, which can increase the information available to determine the peptide sequence. A regular and predictable dissociation pattern for peptides further simplifies this analysis. By forming predominantly one type of ion, ambiguity is removed as to whether the ion is N- or C-terminal. This pattern can also be advantageous in that ion intensity remains concentrated for the next stage of MS/MS. In this work, a method to take advantage of the MSn capabilities of the quadrupole ion trap by controlling the dissociation pathways is explored. Dissociation is altered by acetylating the N-terminus of the peptide. MSn of a variety of acetylated peptides is used to determine the effects of the identity of the C-terminal residue and the length of the peptide on the dissociation pathways observed.

Tandem mass spectrometry provides information on the dissociation pathways of gas-phase ions by providing a link between product ions and parent ions. However, there exists a distinct possibility that a parent ion does not dissociate directly to the observed product ion, but that the reaction proceeds through unobserved reaction intermediates. This work describes the discovery and kinetic analysis of an unobserved reaction intermediate with a quadrupole ion trap. [a4 − NH3] ions formed from [YGβFL + H] ions dissociate to [(F∗YG − NH3) − CO] ions. It is expected, however, from previous results, that [F∗YG − NH3] ions should form prior to [(F∗YG − NH3) − CO] ions. Double-resonance experiments are used to demonstrate the existence of intermediate [F∗YG − NH3] ions. Various kinetic analyses are then performed using traditional collision-induced dissociation kinetics and double-resonance experiments. The phenomenological rates of formation and decay of peptide rearrangement ion dissociation products are determined by curve fitting decay and formation data generated with the kinetics experiments. The data generated predict an observable level of the intermediate in a time frame accessible but previously not monitored. By examining early product-ion formation, the intermediate ions, [F∗YG − NH3]+, are observed.

Measurement of herbicide residues in environmental matrices is typically performed using liquid or gas chromatography coupled to mass spectrometry, generally with one or more stages of sample processing or purification prior to analysis. Paper spray ionization enables the rapid mass spectrometric analysis of such samples without the use of chromatography or sample cleanup techniques. Samples are applied to a paper strip and dried, after which they may be stored or transported. By applying solvent and a high voltage to the paper strip, the analyte is extracted from the paper and ionized by electrospray from the tip of the paper strip. Qualitative and quantitative measurement of triazine herbicides and the chloroacetanilide herbicide metolachlor are demonstrated using samples spiked into water and crop extracts at part-per-billion concentrations. The linear dynamic range includes the U.S. statutory maxima for atrazine in crops and human health hazard levels in water, as well as E.P.A. levels of concern and regulatory limits for metolachlor in crops and water.