Environ. Sci. Technol. 2008, 42, 9268–9275
Measurements of Oxalic Acid, Oxalates, Malonic Acid, and Malonates in Atmospheric Particulates LIMING YANG AND LIYA E. YU* Division of Environmental Science & Engineering, National University of Singapore, Singapore 119260, Singapore
Received July 1, 2008. Revised manuscript received September 13, 2008. Accepted October 2, 2008.
This study systematically examined effects of analytical approaches on resultant concentrations of oxalic acid, oxalates, malonic acid, and malonates. Results demonstrated that employing separate water extraction and THF extraction is required to properly quantify dicarboxylic acids vs dicarboxylates using IC or GC-MS. Applications of the recommended methods to analyze PM2.5 collected in Singapore showed that concentrations of oxalate ranged from 361.4 to 481.4 ng m-3, which were 10-14.7 times higher than that of oxalic acid. Unlike that of oxalates, malonate concentrations (10.5-23.4 ng m-3) were no more than half of malonic acid concentration (43.8-53.9 ng m-3) in PM2.5. Concentration ratios of oxalate-to-oxalic acid and malonate-to-malonic acid obtained from this work were applied to reported literature data; as a first approximation, in urban environments similar to that in Singapore, quantifiable oxalic acid, oxalates, malonic acid, and malonates in PM2.5 could range from 7.6 to 68.0, 82.2 to 732.8, 6.3 to 150, and 1.3 to 60 ng m-3, respectively. Because photooxidation properties and hygroscopicity of dicarboxylic acids can substantially differ from that of dicarboxylates, more studies are needed to quantify ambient oxalic acid and malonic acid vs oxalates and malonates.
Introduction C2-C9 dicarboxylic acids (DCAs) are important because they can contribute up to 50% of the organic aerosol mass concentration (1). A substantial amount of effort has been devoted to quantify atmospheric DCAs and to specify their effects on hygroscopic properties as well as radiative forcing of atmospheric aerosols. Since 1980 to the present, more than 160 published studies have reported concentrations of oxalic acid (C2 DCA) and malonic acid (C3 DCA) in urban, suburban, remote, and marine atmospheres on three continents (Asia, Africa, and America) (e.g., refs 2-7). Some studies interchangeably equate measured oxalates (malonates) with atmospheric oxalic acid (malonic acid) and vice versa. Surprisingly, the reported concentrations of atmospheric oxalic acid and malonic acid can deviate substantially; more than 125 studies reported oxalic acid and malonic acid as the most abundant DCAs in atmospheric particulates (e.g., refs 5, 8-10), while some found that their concentrations were below detectable levels (11-13). Although sampling sites and seasons could affect the concentration of ambient * Corresponding author phone: +65 6516 6474; fax: +65 6774 4202; e-mail:
[email protected]. 9268
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DCAs, analytical methods could be one of the major factors causing the drastic deviation among reported concentrations. For example, during the winter season in Nanjing, China, Wang et al. (14) reported oxalic acid being most abundant (363-1423 ng m-3) among five identified DCAs in PM2.5, whereas the same research group showed that oxalic acid in PM2.5 at the same location and during the similar season was below detection limit (15). Although primary emissions and secondary ambient reactions at the same location during winter might vary in two different years, such substantial changes in the concentration of oxalic acid suggest that analytical methods, in particular, extraction approach, could be a major responsible factor; before GC-MS injections, the latter study extracted collected particulate samples using organic solvents (dichloromethane and methanol) followed by silylation, unlike the former study, which coupled water extraction with butylation. To verify the hypothesized effects of analytical methods, Table 1 summarizes analytical techniques adopted by 163 studies published during 1980-2007, examining DCAs in snow, ice, and secondary organic aerosols (including chamber studies). DCAs in collected samples were typically extracted using water or organic solvents (e.g., dichloromethane and methanol) followed by quantification using analytical instruments, such as gas chromatography (GC), ion chromatography (IC), or capillary electrophoresis (CE) (Table 1); among the analytical instruments, GC and IC were most frequently used. By utilizing water extraction coupled with IC or CE, 78 studies reported that the concentrations of oxalic acid (or oxalates) and malonic acid (or malonates) range from 1.4 to 6920 ng m-3 and 0 to 370 ng m-3, respectively. More than half (41 studies) of the 78 studies addressed data as oxalates and malonates, although these concentrations may actually represent atmospheric concentrations of respective oxalic acid and malonic acid, or DCAs and dicarboxylates (e.g., oxalic acid and oxalates together). When GC was selected as the analytical instrument, results varied significantly depending on extraction and derivatization methods employed; a substantial amount of oxalic acid and malonic acid was often yielded from methods involving propylation or butylation, whereas studies coupling solvent extraction with methylation or silylation found little DCAs; neither C2 nor C3 DCAs was detected after methylation, and only C3 DCAs were measured through silylation (Table 1). While methylated oxalic and malonic acids become more volatile and are easily lost, silylated oxalic and malonic acids (becoming bis(trimethylsilyl esters)) were successfully detected during smog chamber studies (16). DCAs and dicarboxylates (including partially and fully dissociated DCAs) can coexist in airborne particulates; distribution between DCAs and dicarboxylates in ambient particulates varies depending on a few factors (such as water content, pH, and mixing status of particulates). Quantification of DCAs vs dicarboxylates in ambient aerosols is important to properly evaluate their individual effects on the atmosphere because DCAs can substantially differ from dicarboxylates in hygroscopicity and degradation. Peng and Chan (17) showed that sodium succinate (C4 dicarboxylate) exhibits a stronger hygroscopicity than succinic acid. Yang et al. (18) also demonstrated that photodegradation of oxalates can be at least five times faster than that of oxalic acid. Hence, this work systematically quantifies oxalic acid, oxalates, malonic acid, and malonates as standards and in PM2.5 collected in Singapore using four approaches to examine how analytical methods can affect perceived concentrations of atmospheric oxalic acid and malonic acid. Recommendations of analytical 10.1021/es801820z CCC: $40.75
2008 American Chemical Society
Published on Web 11/18/2008
TABLE 1. Summary of Analytical Approaches Employed To Measure Oxalic Acid and Malonic Acid in Particulates during 1980-2007 detection extraction medium
derivatization
analytical instrumenta
water NAb IC, IEC, IC-CD-UV-MS, IC-MS, PILS-IC water NAb CE organic solvents methylation GC-FID, GC-MS, GC-ITMS, GC-irMS organic solvent followed by water propylation water butylation organic solvent silylation
number oxalic acid malonic acid of studies NDc NDc
NDc
67d 13e 35f 4g 38h 16i
a IEC, ion exclusion chromatography; IC-CD-UV-MS: ion chromatography-conductivity detector-ultraviolet-mass spectrometry; GC-FID: gas chromatography-flame ionization detector; CE: capillary electrophoresis; GC-ITMS: gas chromatography-ion trap mass spectrometry; GC-MS: gas chromatography-mass spectrometry; IC-MS: ion chromatography-mass spectrometry; PILS-IC: particle-into-liquid sampler-ion chromatography; GC-irMS: gas chromatography-isotope ratio mass spectrometry. b NA: not applicable c ND: not detected. d References in Supporting Information 1-1. e References in Supporting Information 1-2. f References in Supporting Information 1-3. g References in Supporting Information 1-4. h References in Supporting Information 1-5. i References in Supporting Information 1-6.
FIGURE 1. Schematic of four approaches (a)-(d) for measurements of oxalic acid, malonic acid, oxalates, and malonates. approaches were proposed according to the standard tests and employed to quantify oxalic and malonic acids as well as corresponding dicarboxylates in PM2.5. Concentration ratios of oxalate-to-oxalic acid and malonate-to-malonic acid obtained from atmospheric samples were applied, as a first approximation, to field data reported in literature to engage possible concentrations of C2 and C3 DCAs and corresponding dicarboxylates in urban areas. Findings of this study are intended to demonstrate the importance of characterizing atmospheric DCAs and dicarboxylates in detail.
Experimental Section Standard Tests. Before individual tests, all glass apparatuses were heated at 500 °C for more than 2 h to minimize undesired organic contamination. Figure 1 illustrates four approaches ((a)-(d)) measuring standard oxalic acid (Merck), malonic acid (Aldrich), and corresponding dicarboxylates including disodium oxalate (BDH) and disodium malonate (Fluka). The standard solutions were prepared using water or tetrahydrofuran (THF, Merck) (without extraction) to simulate extract solutions that are ready to undergo the following processes (e.g., solvent replacement and derivatization). On the basis of initial solvents utilized, the four approaches are classified into two categories: standards in water (approaches (a) and (b), Figure 1) and standards in organic solvents (approaches (c) and (d), Figure 1). Approach (b) mainly differs from (a) by replacing water in standard solutions with THF followed by silylation and analyses via GC-MS (HP6890 and HP5973, Hewlett-Packard Co.). Similarly, approach (c) differs from (d) by replacing initial THF in standard solutions with ultrapure water before analysis using ion chromatograph (IC, model ICS-2000, Dionex Corporation). To replace solvents (water or THF) in samples, a mild nitrogen flow was
administered to dry standard solutions before a new solvent (THF or water) was introduced to redissolve compounds for further analyses. Procedures of filter extraction, derivatization, and operations of GC-MS (for approaches (b) and (d)) as well as IC (for approaches (a) and (c)) are provided in the following sections of this article. Atmospheric Aerosol Samples. The above-mentioned four approaches were further verified by applying them to measure actual atmospheric samples, which were collected for 24 h daily on May 15, 17, and 18, 2004. During the sampling period, the averaged daily temperature and relative humidity were 29.4 ( 1.5 °C and 70.5 ( 9.2% (based on data supplied by National University of Singapore Geography Weather Station, http://nt2.fas.nus.edu.sg/geog/weather_frame.asp), respectively. Sampling and preparation procedures used in this study were reported previously (19). In brief, atmospheric fine particulates (PM2.5) were collected onto six acidpretreated 47-mm tetrafluoroethylene (TFE)-coated glass fiber filters (Fiberfilm, Pallflex, GelmanSciences) using two Andersen RAAS 2.5-200 samplers and one RAAS 2.5-400 sampler (Andersen Instruments, Inc.). The six filter samples were composite to two sets (three filters per set); each set of filters was loaded with PM2.5 of a total air volume of around 56 m-3 and stored in an organic-free jar in the dark (-25 °C) before extraction using water or organic solvents. Water and Solvent Extraction. Ambient filter samples were extracted using ultrapure water or organic solvents. An organic-free Y-shaped Teflon separator was inserted between filters to maximize the contact between solvents and filters during extraction. Around 35 mL of ultrapure water was introduced followed by a 30-min ultrasonic agitation. All water extracts were filtered through cleaned quartz filters (Whatman QM-A, Whatman International Ltd.) and conVOL. 42, NO. 24, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Quantitative Analyses of Standard Solutions for Individual Oxalic Acid, Malonic Acid, Oxalate, and Malonate through Approaches (a)-(d) Based on Triplicate Testsa Approach
a
b
c
d
solvent of standard solution
water
water
THF
THF
solvent of injected sample (analytical instrument) oxalic acid (0.9 mM; pH ) 3.1 in water) malonic acid (0.8 mM; pH ) 33.3 in water) sodium oxalate (0.9 mM; pH ) 3 8.2 in water) sodium malonate (0.8 mM; pH ) 38.5 in water) characteristics
water
THF
THF
(GC-MS) 93.8 ( 3.4
100.7 ( 3.0
103.7 ( 4.2 99.4 ( 2.5
103.2 ( 2.5
102.1 ( 1.4
NDb
NDb
NDb
96.8 ( 2.5
NDb
NDb
NDb
• less material loss • resolving DCAs and dicarboxylates together
(IC) 101.7 ( 0.1
THF
(IC) 100.4 ( 1.5
(GC-MS) 102.5 ( 3.9
• resolving DCAs only; exclusion of dicarboxylates
a Standard solutions of individual oxalic acid, malonic acid, oxalate, and malonate were quantified through approaches (a)-(d), without involvement of filter extraction. b ND: not detected.
centrated down to around 1.5 mL using a TurboVap II workstation (Zymark Co.). The concentrated extract was then transferred to a cleaned 2-mL vial. Solvent extraction was conducted by following a previously reported procedure (19). In brief, before extraction, d50-tetracosane (Aldrich) and pimelic acid (Aldrich) were first spiked as internal standards evenly onto filter samples, including blank filters. This was to track recoveries of respective nonpolar and polar organic compounds in collected fine particulates. Filter samples were then extracted successively using three types of solvents, in the order of THF, dichloromethane (Merck), and hexane (Merck). Each solvent ultrasonication lasted for 10 min. All solvent extracts were filtered through cleaned quartz filters and concentrated down to around 1.5 mL using a TurboVap II workstation (Zymark Co.). The extract was then transferred into a cleaned 2-mL vial and further concentrated down to around 0.15 mL using a microconcentrator (Pierce, Inc.). All solvent extracts were stored in the dark (-25 °C) before chemical analyses. Blank filters were extracted following the same procedures of water or THF extraction to identify and to correct interference of background contaminants. IC and GC-MS Analyses. A Dionex IC system consisting of an eluent generator (EG with continuously regenerated trap columns (CR-TC)), conductivity detector, and anion selfregenerating suppressor (ASRS Ultra II-4 mm) was employed. Oxalates and malonates were determined using an AS-11HC (4 mm) analytical column with an AG-11-HC guard column under an eluent flow rate of 1.5 mL min-1. The eluent consisting of KOH and pure water was operated in a gradient program starting with 1 mM KOH during 0-1 min, followed by increasing the KOH concentration at a rate of 1 mM/min up to 25 mM, and then increasing KOH concentration from 25 to 40 mM during 25-29.9 min before the KOH concentration stayed at 1 mM for one more minute. Four repeated tests of authentic oxalic and malonic acid (spiked onto filters followed by water extraction and IC analysis) yielded respective recoveries of 96.6 ( 4.0 and 97.4 ( 3.0%. For analyses using GC-MS, 50 µL of sample in THF was derivatized using 20 µL of BSTFA under room temperature. The derivatized samples (1-2 µL) were analyzed via GC-MS 20-30 min after the addition of N,O-bis (trimethylsilyl)trifluoracetamide (BSTFA, Sigma). Injected compounds were separated through a 30-m HP-5 MS column (5% phenyl polysiloxane capillary column, 0.25 mm × 0.25 µm, Agilent) following a temperature profile, which stayed at an initial temperature of 60 °C for 3 min before it increased to 280 °C at a rate of 8 °C min-1 and was held at 280 °C for 3 min. One 9270
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microliter of 1-phenyldodecane (1-PD, Aldrich) was coinjected with derivatized extracts to monitor injection loss and instrument performance. On the basis of triplicate analyses of standards spiked onto filters, approaches using GC-MS resulted in a relative standard deviation of 8.4 and 6.6% for oxalic and malonic acids, respectively. Detection sensitivity of IC vs GC-MS of standard oxalic and malonic acids was evaluated by comparing the limit of detection (LOD) of individual compounds. For this study, IC provided a detection sensitivity of oxalic and malonic acids at least 9 times (or up to 55 times) better than that of GC-MS (Supporting Information).
Results and Discussion Standard Tests. Table 2 displays fractions of individual standard solutions detected through the four approaches. All methods quantified more than 93% of C2 and C3 DCAs in the individual standard solutions. When water was used to prepare initial standard solutions, approach (b) yielded less oxalic acid compared to approach (a) (Table 2) because approach (b) involves more steps of solvent replacement (including a drying process) and derivatization, in addition to using GC-MS with lower detection sensitivity than IC. These additional steps could cause more loss of oxalic acid, which is three times more volatile than C3 DCA. On the other hand, when THF was employed to prepare standard solutions, approaches (c) and (d) showed comparable recoveries (Table 2). This could be mainly due to higher detection sensitivity of IC, compensating material loss during solvent replacement of THF with water (approach (c)). Note that although solvent replacement was involved in both approaches (b) and (c), drying THF at 40 °C (approach (c)) could result in less material loss than drying water at 90 °C (approach (b)), and therefore a larger amount of C2 DCA (Table 2). For standard oxalate and malonate, only approach (a) successfully resolved both anions with a recovery of more than 96% (Table 2). None of dicarboxylates was quantified via the other three approaches, which could be due to poor solubility of both oxalate and malonate in THF. This qualifies THF to extract only the free form of oxalic acid and malonic acid. Our laboratory tests showed that, at 22 °C, solubility of oxalate and malonate in THF is less than 0.5 µg/100 mL (or
