Complete text of publication follows. Recently a number of atmospheric-pressure ionization sources have emerged in the field of mass spectrometry, yielding a field collectively referred to as ambient desorption/ionization mass spectrometry (ADI-MS). These sources include desorption electrospray ionization (DESI), direct analysis in real time (DART), desorption atmospheric-pressure chemical ionization (DAPCI), and the low temperature plasma (LTP) probe. Advantages of these ADI-MS sources include direct analysis of solid, liquid, or gaseous samples, high ionization efficiency, and simple mass spectra, consisting of mainly the molecular ion or protonated molecular ion. The ability to ionize samples directly from a surface under atmospheric conditions greatly reduces analysis time by decreasing or eliminating the need for sample pre-treatment. Recently our group has developed an ADI-MS source based on an atmospheric-pressure glow discharge, termed the flowing atmospheric-pressure afterglow (FAPA). In the present study, fundamental parameters and novel applications of the FAPA source will be presented. The FAPA source consists of an atmospheric-pressure glow discharge in a pin-to-plate configuration. A hole in the plate allows excited species from the discharge to either create reagent ions from interactions with atmospheric gases or directly desorb/ionize analytes placed into this afterglow region. By positioning the FAPA in front of a mass spectrometer and introducing a sample into the afterglow region, direct, fast qualitative measurements were made from thin-layer chromatography plates, fruit skins, and animal tissue. Quantitative analysis was performed either by using a motorized stage to scan the sample through the afterglow or by coupling the FAPA with laser ablation (LA). With this method, detection limits on the order of 1 to 100 fmol and sensitivities >10 counts/fmol were achieved. In addition, using the LA-FAPA combination to perform mass spectral imaging will be presented.
New-found interest in the development of ionization sources for mass spectrometry, inspired by the advent of ambient desorption/ionization mass spectrometry, has led to a resurgence in plasma-source development and characterization. Dielectric-barrier discharges, particularly the low-temperature plasma (LTP) probe format, have been at the forefront of this field due to their low power consumption and relatively simple design. However, better fundamental understanding of this desorption/ionization source is needed to improve the analytical capabilities of such a device. Here, we use relatively fast (2.5 ms per spectrum) time-resolved mass spectrometry to characterize the temporal reagent-ion distribution from a low-frequency LTP probe. Different voltage waveforms were found to heavily influence the discharge properties and, consequently, ion production. Ion signals from short discharge pulses, ca. 40 μs, were found to be significantly broadened, ca. 10 ms, prior to extraction into the mass spectrometer. Additionally, higher frequencies of a sine-wave LTP produced the largest flux of reagent ions, which existed for most of the voltage waveforms. Finally, temporal signals for reagent and analyte ions were measured and related to specific ionization processes: proton transfer and charge transfer.
A plasma-based ambient desorption/ionization mass spectrometry (ADI-MS) source was used to perform molecular mass spectral imaging. A small amount of sample material was ablated by focusing 266 nm laser light onto a spot. The resulting aerosol was transferred by a nitrogen stream to the flowing afterglow of a helium atmospheric pressure glow discharge ionization source; the ionized sample material was analyzed by a Leco Unique time-of-flight mass spectrometer. Two-dimensional mass spectral images were generated by scanning the laser beam across a sample surface. The total analysis time for a 6 mm (2) surface, which is limited by the washout of the ablation chamber, was less than 30 min. With this technique, a spatial resolution of approximately 20 microm has been achieved. Additionally, the laser ablation configuration was used to obtain depth information of over 2 mm with a resolution of approximately 40 microm. The combination of laser ablation with the flowing atmospheric pressure afterglow source was used to analyze several sample surfaces for a wide variety of analytes and with high sensitivity (LOD of 5 fmol for caffeine).
Ion optics are crucial for spectrometric methods such as mass spectrometry (MS) and ion mobility spectrometry (IMS). Among the wide selection of ion optics, temporal ion gates are of particular importance for time-of-flight MS (TOF-MS) and drift-tube IMS. Commonly implemented as electrostatic ion gates, these optics offer a rapid, efficient means to block ion beams and form discrete ion packets for subsequent analysis. Unfortunately, these devices rely on pulsed high voltage sources and are not fully transparent, even in their open state, which can lead to ion losses and contamination. Here, a novel atmospheric-pressure ion gate based on a resonant acoustic field structure is described. This effect was accomplished through the formation of a resonant, standing acoustic wave of alternating nodes and antinodes. Alignment of an atmospheric-pressure gaseous ion beam with an antinode, i.e. a region of transient pressure, of the acoustic structure acted as a gate and blocked ions from impinging on ion-selective detectors, such as a mass spectrometer and a Faraday plate. The velocity of the ion stream and acoustic power were found to be critical parameters for gating efficiency. In the presence of an acoustic field (i.e., a closed gate), ion signals decreased by as much as 99.8% with a response time faster than the readout of the ion-measurement devices used here (ca. 75 ms). This work demonstrates the basis for a low-cost, acoustic ion gate, which is optically transparent and easily constructed with low-power, off-the-shelf components, that could potentially be used with MS and IMS instrumentation.
Ambient desorption/ionization mass spectrometry (ADI-MS) is an emerging field that aims to eliminate sample pretreatment and separation steps by directly desorbing and ionizing analytes from a sample surface for analysis by mass spectrometry. Although numerous applications of ADI-MS have been presented, little has been done to characterize problems caused by matrix effects. In the present study, ionization-related matrix effects were investigated for three plasma-based ADI-MS sources: the flowing atmospheric-pressure afterglow (FAPA), direct analysis in real time (DART), and the low-temperature plasma (LTP) probe. Small amounts of vapor-phase matrices were mixed with a continuous stream of gaseous analyte and introduced into each ionization source. A decrease in analyte signal upon introduction of a matrix signaled an ion-suppression event. When the matrix species had a proton affinity the same as or greater than that of the analyte, all three sources suffered analyte signal suppression, even at moderate matrix-to-analyte concentration ratios. In every case, the FAPA was the least susceptible to the ion suppression process. In contrast, when the proton affinity of the matrix species was lower than that of the analyte, no matrix effect was observed with DART, although an effect persisted for both FAPA and LTP. Indeed, matrix-to-analyte mole ratios of 10 were sufficient to entirely suppress analyte ion signals in the LTP. These findings demonstrate that matrix effects in ADI-MS are important for qualitative as well as quantitative analyses.
Ambient mass spectrometry is a powerful approach for rapid, high-throughput, and direct sample analysis. Due to the open-air desorption and ionization processes, random fluctuations of ambient conditions can lead to large variances in mass-spectral signals over time. The mass-spectral data also can be further complicated due to multiple analytes present in the sample, background-ion signals stemming from the desorption/ionization source itself, and other laboratory-specific conditions (e.g., ambient laboratory air, nearby hardware). Thus, background removal and analyte-ion recognition can be quite difficult, particularly in non-targeted analyses. Here, we demonstrate the use of a cross-correlation-based approach to exploit chemical information encoded in the time domain to group/categorize mass-spectral peaks from a single analysis dataset. Ions that originate from ambient (or other) background species were readily flagged and removed from spectra; the result was a decrease in mass-spectral complexity by over 70% due to the removal of these background ions. Meanwhile, analyte ions were differentiated and categorized based on their time-domain profiles. With sufficient mass resolving-power and mass-spectral acquisition rate, isolated mass spectra containing ions from the same species in a sample could be extracted, leading to a reduction in mass-spectral complexity by more than 98% in some cases. The cross-correlation approach was tested with different ionization sources as well as reproducible and irreproducible sample introduction. Software built in-house enabled fully automated data processing, which can be performed within a few seconds. Ultimately, this approach provides an additional dimension of analyte separation in ambient mass-spectrometric analyses with information that is already recorded throughout the analysis.
The advent of ambient desorption/ionization mass spectrometry (ADI-MS) has led to the development of a large number of atmospheric-pressure ionization sources. The largest group of such sources is based on electrical discharges; yet, the desorption and ionization processes that they employ remain largely uncharacterized. Here, the atmospheric-pressure glow discharge (APGD) and afterglow of a helium flowing atmospheric-pressure afterglow (FAPA) ionization source were examined by optical emission spectroscopy. Spatial emission profiles of species created in the APGD and afterglow were recorded under a variety of operating conditions, including discharge current, electrode polarity, and plasma-gas flow rate. From these studies, it was found that an appreciable amount of atmospheric H(2)O vapor, N(2), and O(2) diffuses through the hole in the plate electrode into the discharge to become a major source of reagent ions in ADI-MS analyses. Spatially resolved plasma parameters, such as OH rotational temperature (T(rot)) and electron number density (n(e)), were also measured in the APGD. Maximum values for T(rot) and n(e) were found to be ~1100 K and ~4×10(19) m(-3), respectively, and were both located at the pin cathode. In the afterglow, rotational temperatures from OH and N(2)(+) yielded drastically different values, with OH temperatures matching those obtained from infrared thermography measurements. The higher N(2)(+) temperature is believed to be caused by charge-transfer ionization of N(2) by He(2)(+). These findings are discussed in the context of previously reported ADI-MS analyses with the FAPA source.
