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Laser Ablation-Aerosol Mass Spectrometry-Chemical Ionization Mass Spectrometry for Ambient Surface Imaging Jennifer L. Berry, Douglas A. Day, Tim Elseberg, Brett B Palm, Weiwei Hu, Aroob Abdelhamid, Jason C. Schroder, Uwe Karst, Jose L. Jimenez, and Eleanor C. Browne Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b05255 • Publication Date (Web): 20 Feb 2018 Downloaded from http://pubs.acs.org on February 22, 2018
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Analytical Chemistry
Laser Ablation-Aerosol Mass Spectrometry-Chemical Ionization Mass Spectrometry for Ambient Surface Imaging Jennifer L. Berry,† Douglas A. Day,† Tim Elseberg,‡ Brett B. Palm,† Weiwei Hu,† Aroob Abdelhamid,† Jason C. Schroder,† Uwe Karst, †,‡ Jose L. Jimenez,† Eleanor C. Browne†* †
Department of Chemistry and Biochemistry and Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309, United States ‡ University of Münster, Institute of Inorganic and Analytical Chemistry, Corrensstr. 28/30, 49149 Münster, Germany ABSTRACT: Mass spectrometry imaging is becoming an increasingly common analytical technique due to its ability to provide spatially resolved chemical information. Here, we report a novel imaging approach combining laser ablation with two mass spectrometric techniques, aerosol mass spectrometry and chemical ionization mass spectrometry, separately and in parallel. Both mass spectrometric methods provide the fast response, rapid data acquisition, low detection limits, and highresolution peak separation desirable for imaging complex samples. Additionally, the two techniques provide complementary information with aerosol mass spectrometry providing near universal detection of all aerosol molecules and chemical ionization mass spectrometry with a heated inlet providing molecular-level detail of both gases and aerosols. The two techniques operate with atmospheric pressure interfaces and require no matrix addition for ionization, allowing for samples to be investigated in their native state under ambient pressure conditions. We demonstrate the ability of laser ablation-aerosol mass spectrometry-chemical ionization mass spectrometry (LA-AMS-CIMS) to create 2D images of both standard compounds and complex mixtures. The results suggest that LA-AMS-CIMS, particularly when combined with advanced data analysis methods, could have broad applications in mass spectrometry imaging applications.
In biological systems and pharmacological products, the functions and properties of a compound depend both on its chemical structure and spatial location. Mass spectrometry imaging (MSI) is a powerful tool for investigating these systems since it provides both chemical and spatial information that can be used to create 2D and 3D images.1 Some of the most commonly used MSI methods include matrix-assisted laser desorption/ionization (MALDI) and secondary ion mass spectrometry (SIMS). These techniques typically require the sample to be run under vacuum conditions and/or require sample preparation that may result in spatial dislocation or chemical modification of species of interest.1,2 As a result, there has been an increased interest in techniques that allow the surface to be kept at atmospheric pressure and require little to no sample preparation.1 Laser ablation (LA) at atmospheric pressure coupled with different ionization methods and analytical mass spectrometers has become a key technique for imaging in biomedical research.1,3–6 In this technique, a focused laser beam ablates sample from a surface to create small particles and gaseous molecules that are transferred by a carrier gas flow to the analytical detection instruments.7 LA is commonly used with inductively coupled plasma mass spectrometry (LA-ICP-MS) because of the high spatial resolution, low detection limits, and limited matrix effects.7,8 Despite its many advantages, LA-ICP-MS is limited by the complete loss of molecular information in the ICP conversion of molecules into atomic ions.9
LA coupled with analyzers that offer increased molecularlevel information has opened new opportunities for high spatial and mass resolution imaging of molecular composition.4,10 Techniques such as laser ablation electrospray ionization (LAESI)3 and desorption electrospray ionization (DESI)11,12 are able to analyze samples at atmospheric pressure; however, current versions of these techniques have limited spatial resolution (~200 μm). Laser ablation flowing atmospheric pressure afterglow (LA-FAPA) is able to detect a wider range of species than MALDI, has a high spatial resolution (~20 μm), and can be used for depth profiling.4 LA coupled with atmospheric pressure chemical ionization mass spectrometry (LA-APCI-MS) shows the ability for molecular MSI with high spatial resolution (25 μm).6,10 Despite their many advantages, LA-FAPA4,13 and LA-APCI-MS14 suffer from ion suppression; an effect that is increased at ambient pressures. As many molecular MSI techniques suffer from matrix effects and preferential ionization mechanisms, coupling elemental and molecular techniques provides a more complete picture of chemical composition.15,16 Here, we demonstrate the imaging capability of a laser ablation system coupled with two mass spectrometry techniques common within the field of atmospheric chemistry. These techniques, aerosol mass spectrometry and chemical ionization mass spectrometry, can be used either separately or simultaneously with laser ablation to provide both molecular and bulk chemical information.
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Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Over the past decade, Aerodyne Aerosol Mass Spectrometers (AMS)17 and Aerosol Chemical Speciation Monitors (ACSM)18,19 have become widely used in atmospheric aerosol research to quantify and characterize the mass concentration and chemical composition of non-refractory submicron aerosols in the field and laboratory. Typical data products include mass spectra time series of chemical species concentrations including sulfate, nitrate, ammonium, chloride and organic aerosol components. Due to the combination of the near unity collection of particles using an aerodynamic lens and efficient pumping away of gases (factor of ~107), very low detection limits are achievable (1-1000 ng m-3 for 1-min averages, depending on the species and instrument version).19 Since the sampling is continuous and rapid, they can be operated to collect data as rapidly as 10 Hz.20 While some versions of the AMS and all versions of the ACSM produce unit mass resolution spectra, high-resolution versions21 have the capability to resolve ion peaks at the same nominal m/z, making possible greatly improved separation of inorganic and organic species and quantification of elemental ratios of organic aerosol (OA; e.g., O/C, H/C, N/C).22,23 Due to the process of flash vaporization of particles prior to detection of the evolved gases, a substantial degree of thermal decomposition can occur for some molecules23 in the AMS and ACSM (hereafter just AMS). Additionally, the ionization method (70 eV electron ionization) results in substantial fragmentation prior to ion detection. For this reason, detection of molecular ion peaks is rare. Consequently, statistical methods such as positive matrix factorization (PMF)24,25 or fragment ion tracer26–29 methods have proven very useful, and been widely applied, for extracting meaningful information from AMS OA data. The main advantage of the use of electron ionization is that the entirety of the non-refractory submicron aerosol mass is quantified and characterized. Many of these capabilities (e.g., fast response, rapid data acquisition, low detection limits, high-resolution ion peak separation, submicron particle range, sensitivity to all molecules), in addition to highly-developed data processing software packages, make the AMS an excellent candidate for use as an analytical tool by coupling with laser ablation analysis. While the AMS is useful for bulk mass and elemental composition, chemical ionization mass spectrometry (CIMS) provides selective detection of parent compounds with limited fragmentation.30–32 Recently, high resolution CIMS has been shown to provide the sensitive, selective, and fast detection of trace species that is necessary for analyzing complex mixtures such as those characteristic of the ambient atmosphere.33,34 A key feature of these CIMS techniques is that reagent ions are generated before introduction to analyte gas phase species. Separation of reagent ion generation from analyte ion generation allows for controlled, reproducible ionization conditions even when sampling within spatially or temporally variable matrices. Specific reagent ion species are generated by exposing a trace concentration of reagent ion precursor in high purity nitrogen to a radioactive, discharge, or x-ray source. The reagent ions are then mixed with the sample gas in a flow tube or drift tube where ion-molecule reactions occur. Typically, the only matrix effect of relevance for the measurement of gas-phase species is sensitivity to gas-phase water vapor concentration that alters ion clustering.31 This effect can readily
be accounted for with calibration and controlled/eliminated under laboratory conditions. For the LA-CIMS technique, matrix effects resulting from the use of laser ablation and thermal vaporization of ablated aerosol may exist and have not been thoroughly investigated at this point. In CIMS, the selectivity of detection depends on the ionmolecule reaction mechanism. Thus, there has been significant interest in developing and characterizing the use of novel reagent ions for detection of specific classes of compounds.31,35,36 A common technique uses hydronium ions to detect volatile organic compounds with proton affinities higher than water.37 More selective ionization chemistries include acetate,36 iodide,38 and ethanol39 which detect acidic, polarizable, and basic compounds, respectively. Traditionally CIMS measures trace gases in complex mixtures.31,35,36,40 More recently, it has been used for measurements of aerosol composition.33,41 Coupling CIMS with Time of Flight mass spectrometry (ToF) allows for the simultaneous detection of a wide range of compounds with a second to sub-second time resolution.42 Although quantitative measurements require calibration of individual compounds, linear responses are typical when the reagent ion remains in large excess and the ions are stable on the time-scale of reaction. The soft and selective nature of the ionization coupled with fast time resolution suggests that CIMS is a promising analytical technique for providing quantitative molecular information with high spatial resolution. In this study, we coupled laser ablation with aerosol mass spectrometry and chemical ionization mass spectrometry to demonstrate the ability of these methods to characterize the chemical composition of the ablation-generated aerosols and gases in real time and with high spatial resolution. Several single-component organic molecule samples were tested, covering a wide variety of molecular structures, elemental composition, and functional groups. Twodimensional mapping was demonstrated for standard compounds and multi-component mixtures. EXPERIMENTAL METHODS Experimental Setup. The experimental setup is shown in Figure S-1 and consisted of an online continuous flowthrough configuration. Flow rates were chosen to rapidly flush the LA cell (residence time ~ 150 ms), rapidly transfer the particles and gases to the detection instruments without large losses to tubing walls while maintaining high temporal resolution, and provide enough flow for the nominal sampling needs of each instrument. Dry, ultra-high purity, nitrogen gas (evaporated from a liquid nitrogen Dewar) was used for the carrier flow in the LA system and also as makeup and dilution flows. Stainless steel or copper tubing was used for all flows, except for nitrogen supply lines after the particle thermal denuder prior to CIMS gas and aerosol detection where FEP and PFA Teflon were used. Sample Preparation. Dried droplets of compounds (Table 1 and S-1) on TLC plates were analyzed with LA-AMS-CIMS to determine the ability of each instrument to detect various classes of compounds. Droplets were prepared as 0.1 mol/L solutions or suspensions in water in the absence of any additives that would assist ablation and/or ionization.
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Analytical Chemistry Approximately 0.3 mL of each solution was spotted onto TLC slides. Sampling was done by a single line scan across the droplet for initial detection and multiple line scans to create images. A Tryptophan Tablet (L-Tryptophan 1000 mg, The Vitamin Shoppe, New Jersey, USA) and Neuralgin Tablet (Dr. R. Pfleger Chemische Fabrik GmbH, Bamberg, Germany) were sliced in half for line scans on a flat surface. All standard compounds were >98-99% purity. Laser Ablation. Spatially resolved sampling was performed by a LSX-213 G2+ laser ablation system with a frequency quintupled, Q-switched Nd:YAG laser from Teledyne CETAC, Omaha, USA. The laser was operated with 20 Hz repetition rate and 5 ns pulse duration. Source pulse energy, laser spot size, and scan rate respectively were 0.4 mJ, 50 μm, and 50 µm/s for the Neuralgin tablet and 2,6difluorobenzoic acid droplet and 0.2 mJ, 25 μm, and 12 µm/s for the Tryptophan tablet. The scans consisted of adjacent but non-overlapping lines. Small particles and gaseous compounds ablated from the sample materials were the flushed from the LA cell to the mass spectrometers. Aerosol Mass Spectrometry. An Aerodyne Time-of-Flight High-Resolution Aerosol Mass Spectrometer (HR-ToF-AMS hereafter just AMS) was used to characterize the chemical composition of the aerosols generated. Details of this instrument are described in a previous paper21 as well as some basic features and applications in the introduction of this paper. A brief description follows. Aerosols are focused using an aerodynamic lens into a vacuum chamber with differential pumping. Particles impact a heated metal vaporizer (600 °C) where they are flash vaporized. The evaporated gases are ionized by electron ionization (70 eV) and detected using time-of-flight mass spectrometry. In order to separate gas backgrounds from particle signal, a chopper is alternated between blocking (“closed”) and transmitting (“open”) the particle beam every few seconds, and the two signals are subtracted (referred to as MS mode). The technique quantitatively measures nonrefractory material and is either blind or relatively insensitive to non-refractory material (e.g., black carbon, sea salts, mineral dust). AMS data was recorded as 5 s averages which consisted of a cycle of 3 s “open” and 2 s “closed”. The native 5-s data time resolution data was used for all the image plots shown here. In a few initial tests the higher spectral resolution mode, W-mode (~5000 (m/z)/(Δm/z)) was used; however, due to the modest concentrations of aerosols sampled (10-100 µg m3) and need for high time resolution, instead the lower resolution mode, V-mode (~2000 (m/z)/(Δm/z)), was used due to the higher S/N obtained with V-mode. Mass spectra were collected over an m/z range of 11-419. Periodically, 1 minute of data was collected in the Particle Time-of-Flight mode (PToF mode) which produces chemically-resolved size distributions. All AMS data were analyzed with standard ToF-AMS analysis software packages (Squirrel version>1.57I and PIKA version>1.16I) within Igor Pro (Wavemetrics, Lake Oswego, OR).43 All data were processed to yield unit mass resolution mass spectra and m/z time series. Additionally, for some experiments, select ions were fit using highresolution peak fitting to isolate specific ion time series. One important difference in the configuration of the AMS
used in this study is that rather than the standard vaporizer used in most AMS instruments, a newly designed and characterized “capture vaporizer” (CV) was installed. The design of the CV is optimized to eliminate the bounce of particles off of the vaporizer, a phenomenon that otherwise limits the accuracy of AMS quantification due to variable particle collection efficiency. While improved collection efficiency is not an important feature for this study, it is important to note that the spectra collected using the CV tends to lead to increased fragmentation of some molecules compared to the standard vaporizer.44,45 Chemical Ionization Mass Spectrometry. A highresolution time-of-flight chemical ionization mass spectrometer (HR-ToF-CIMS, hereafter CIMS; Aerodyne Research, Inc. and Tofwerk AG) with a heated inlet (300 °C)41,46,47 to volatilize particles was used to provide insight into the ablated material. The heated inlet was 30 cm of ½ inch O.D. and 0.444 inch I.D. 316 stainless steel with a 9 second residence time. Although reactions can occur on the heated stainless steel surface, no evidence for significant effects was observed as the observed molecular ion peaks and fragment peaks were consistent with previous laser ablation studies. The heated inlet was employed to increase signal and to reduce clogging of the mass spectrometer. The high signal observed by the CIMS and the relatively low concentration of particles (~10 µg/m3) suggests that the heated inlet may not be necessary for future studies. The gaseous and volatilized components from the ablation system were sampled at ~0.8 SLPM into the reduced pressure ion-molecule reaction chamber (IMR, 200 mbar). Reagent ions were created by bubbling 20 sccm N2 through pure ethanol and diluted with additional N2 so that 1.2 SLPM ethanol/N2 gas mixture was flowed through a Po210 ionizer (NRD, model 2021). The reagent ions were then introduced into the IMR orthogonal to the sample flow. Resulting ions are subsampled through two RF-only quadrupoles that focus the ions and break up weakly bound ion-neutral clusters. Ions subsequently pass through a lens stack and enter the orthogonal extraction region of the ToF. Mass spectra were collected over a range of 18-962 amu and averaged and saved at 1 second. This instrument has both high resolving power (~8000 (m/z)/(Δm/z)) and high mass accuracy (
