aerosol particles affect human health and participate in climate interaction , acid precipitation , and visibility reduction on the globe .
the study of organic aerosols and their chemical composition is essential in understanding many processes in the atmosphere , such as solar radiation scattering , cloud condensation as nuclei actions in cloud formation , and participation in oxidation processes [ 1 , 2 ] .
individual monocarboxylic acids and  , -dicarboxylic acids ( dcas ) , as well as n - alkanoic acids , n - alkenoic acids , and aliphatic dcas , have been identified as the major organic compounds in aerosols and coniferous forests [ 3 , 4 ] .
especially aerosol samples from urban , continental background and remote marine sites have been of interest .
it has been shown that a majority of aerosols consist of organic compounds containing hydroxyl groups from alcohols and/or carboxylic acids [ 511 ] .
it has also been proven that the possible sources of carboxylic acids ( ca ) are anthropogenic and biogenic emissions and photochemical reactions , particularly between olefins and ozone [ 1217 ] . in many studies ,
oxalates have been observed as the most abundant organic species in atmospheric aerosols , and therefore , they have traditionally been studied by many research groups [ 13 , 18 ] . in spite of that
, the research interest has also focused on longer chained dcas due to their stability against analytical procedures in sampling and separation .
yang et al . noticed in their physical and chemical studies on c2c9 dcas that succinic acid ( c4 ) exhibits the lowest photooxidation rate in gas , while oxalic acid has the highest rate in a liquid - phase reaction system .
oxalic acid ( c2 ) is the end product of the oxidative decomposition of various organic precursors .
generally , the predominance of oxalic acid ( c2 ) is up to 50% of the total atmospheric dcas , but also malonic ( c3 ) and succinic ( c4 ) acids have been observed in aerosols from distinctly different sites .
[ 1925 ] clegg et al .   have reported that oxalic acid ( c2 ) in the atmosphere is partitioned almost completely into aqueous aerosol droplets , except under conditions of low relative humidity , low aerosol ph , and temperatures greater than + 15c .
this is understandable since the acids are easily ionised and water soluble . at present , it is well known that atmospheric aerosols contain not only salts and nonvolatile compounds but also volatile and semivolatile organic compounds [ 6 , 9 ] .
the work provided thermodynamic data  in particular , liquid - state saturation vapour pressures of three common slightly water - soluble secondary organic aerosol components , namely , malonic ( c3 ) , succinic ( c4 ) , and glutaric ( c5 ) acids .
longer chain dcas ( adipic ( c6 ) , pimelic ( c7 ) , suberic ( c8 ) , and azelaic ( c9 ) acids ) have been determined previously by bilde . to identify the organic compound aerosols , selective and sensitive separation for the sample compounds is needed .
various in - line couplings of separation , identification , and sample preparation devices have been modified for the purpose of concentrating volatile acids [ 69 , 25 ] .
extended sample pretreatment and quite long analysis times dominate the use of chromatographic methods [ 2729 ] .
traditionally , analyses of cas have been conducted by using gas chromatography ( gc ) [ 3032 ] .
our group has earlier published the use of capillary electrophoresis ( ce ) determination of cas in various matrices [ 8 , 28 ] .
but , there are also other research groups who have extensive experience in ce methodologies for dcas in various matrices , for example , in food products [ 3335 ] . due to the many possibilities of separation and detection in ce ,
the technique is favourable for simultaneous analyses of various groups of compounds , like cas and dcas . in addition
, the technique is capable to separate cas from inorganic anions and cations [ 28 , 36 ] .
then , three different chromophores ( 2,3-pyrazinedicarboxylic acid , 2,6-pyridinedicarboxylic acid , and 2,3-pyridinecarboxylic acid ) were tested and the most suitable of them was chosen for the further studies .
it contained 2,6-pyridinedicarboxylic acid as the chromophore for detection and myristyl trimethylammonium bromide as the surfactant to make the separation fast .
the samples were detected in the water - soluble fraction of the aerosol , which was obtained by a commercial microfibre filter .
dcas were identified with reference chemicals , which were spiked into the samples . in this project , we have studied pretreatment , separation , and concentration techniques that can be online coupled as preseparation techniques with capillary electrophoresis .
the studies were performed with anions of homologous series of dicarboxylic acids c2c10 ( oxalic ( c2 ) , malonic ( c3 ) , succinic ( c4 ) , glutaric ( c5 ) , adipic ( c6 ) , pimelic ( c7 ) , suberic ( c8 ) , azelaic ( c9 ) , and sebacic ( c10 ) acids ) .
the novelty of the study is the obtained knowledge about the suitability of sample pretreatment techniques to enrich dcas in environmental aerosol samples .
chemicals for the electrolyte solutions were 2,6-pyridinedicarboxylic acid ( 2,6-pdc ) ( sigma - aldrich , steinheim , germany ) , myristyltrimethylammonium hydroxide ( mtah ) ( waters , milford , usa ) , and methanol ( rathburn , walkerburn , scotland ) . oxalic acid ( c2o4h2 , purity 99% , pka11.27 , pka2 4.27 ) , malonic acid ( c3h4o4 , purity 99% , pka1 = 2.83 , pka2 = 5.69 ) , succinic acid ( c4h6o4 , purity 99% , pka1 = 4.2 , pka2 = 5.6 ) , glutaric acid ( c5h8o4 , purity 99% , pka1 = 4.32 , pka2 = 5.54 ) , adipic acid ( c6h10o4 , purity 99% , pka1 4.43 , pka2 5.41 ) , pimelic acid ( c7h12o4 , purity 99% , pka1 4.46 , pka2 5.58 ) , suberic acid ( c8h14o4 , purity 99% , pka1 4.526 , pka2 5.498 ) , azelaic acid ( c9h16o4 , purity 99% , pka1 4.550 , pka2 5.598 ) , and sebacic acid ( c10h18o4 , purity 99% , pka1 4.720 , pka2 5.450 ) were from sigma - aldrich ( sigma - aldrich finland , helsinki , finland ) .
mandelic acid ( ma , separation standard , is1 ) and trichloroacetic acid ( tca , extraction standard , is2 ) , tri - n - octylphosphine oxide ( topo ) , dissolved in dihexyl ether ( dhe ) , and tris[hydroxymethyl]-aminomethane ( tris , pka = 8.06 ) were from fluka ( buchs , switzerland ) and merck ( darmstadt , germany ) . a 0.1  m solution of sodium hydroxide was prepared from naoh pellets ( akzo nobel , bohus , sweden ) .
distilled water was deionised ( milli - q instrument , 18  m  cm at 25c ) before use .
concentration calibration was performed by diluting the c2c10 stock solutions and making them as dcas standards .
the obtained data was used for calculation of calibration with five concentration levels ( 1  mg / l , 5  mg / l , 10  mg / l , 20  mg / l , 50  mg / l , and 100  mg / l ) for each of the dcas .
the final analyses were done in an optimised electrolyte ( bge ) containing 4  mm 2,6-pda and 0.5  mm mtah at ph 11.0 . before use , the electrolyte solution was filtered through gelman acrodisc cr ptfe syringe filters ( 13 cr , 0.45  m , gelman sciences , ann arbor , mi , usa ) .
the suitability of the chemical and instrumental parameters was cross - checked with standards concentration at 5 
the investigated samples in the study were taken on a roof of a high - rise building ( four floors ) in the kumpula campus area ( 5  km from the centre of helsinki , finland ) .
the samples were immobilised from aerosols onto cellulose membrane ultrafilters ( pore size 0.2  m , pm2.5 ) by using a cascade impactor ( 12 stages ) .
a 24-hour sample collection was used at a 90  m / h flow rate . for extraction one 25  25  mmpiece of the pm2.5 membrane
the filter pieces were diluted into 5  ml of milli - q water during 60 minutes .
the extract was purified with solid - phase extraction ( oasis hlb ) using methanol as the eluent .
the solvent after addition of a few l of 0.1  m naoh was evaporated under a nitrogen atmosphere , followed by dissolving precipitation in either milli - q water ( 200  l ) .
in addition to the cascade impactor collection , aerosols were also sampled on 47  mm whatman qm - a quartz microfibre filters ( whatman international ltd . , maidstone , uk ) .
the sample collection was made at a flow rate of 20.5  ml / min for 48  h. prior to use , the filters were preheated at + 500c for 10  h to remove organic contaminants .
analytes were extracted from the whole filter with 5  ml of milli - q water with ultrasonic agitation for 20 minutes .
they were used for optimisation of sample pretreatment ( ultrasonic agitation , hot water extraction , membrane extraction , and solid - phase extraction ) .
ultrasound extraction was used for the isolation of the dcas retained on the filters during the sampling of urban aerosols in the environment .
the extraction was optimised with real samples , which were spiked with trichloroacetic acid ( tca ) that was used as the extraction standard and the internal standard ( is2 ) . due to the simplicity of the instrument used in the procedure , only the extraction time
finally , the extraction time of 60 minutes was chosen due to the 100% enrichment of the external and internal standards , mandelic acid ( ma , is1 ) and tca ( is2 ) .
hot water extraction ( hwe ) was tested as a comparison method to the ultrasonic agitation .
earlier , it has been shown that hwe is an excellent choice for sample processing of aromatic compounds , pesticides , and bromated retardant compounds .
the extractions were performed with the collected aerosol samples containing the internal standard tca ( is2 ) .
the temperature during extraction was 60c , being low enough to keep water vapour consumption as minimal as possible .
the samples were extracted with a supelco spe instrument ( sigma - aldrich finland , helsinki , finland ) .
the solid - phase materials ( spe ) were oasis hlb ( polymeric reversed - phase polymeric sorbent , waters finland , helsinki , finland ) , strata x ( reversed - phase functionalised polymeric sorbent , phenomenex , copenhagen , denmark ) , and isolute 101 ( strong nonpolar ( hydrophobic ) polystyrene - divinylbenzene copolymer sorbent ) and isolutesax ( quaternary amine functionalised silica based sorbent ) ( ist , hengoed , uk ) .
the amine functionalised sorbent was jtbaker amino ( j.t.baker , deventer , holland ) adsorbent .
the steps in the spe extraction were as follows : conditioning with methanol and water , 1  ml each , sample loading of 5  ml ( carboxylic acid standard mixture , 5  mg / l ) , washing with a methanol - water solution ( 5  :  95 , v / v ) , and eluting with 1  ml of methanol . after completing the elution ,
the eluate was evaporated under nitrogen , and the precipitate was diluted into 50  l of milli - q water .
the spe treatment of the samples was made with three replicates and each of them were analysed with six repetitions with ce - uv .
the membrane extraction device was self - constructed in the laboratory following the instructions published by lthje .
the materials in the support were a porous ptfe membrane ( te 35 membrane filter , schleicher gmbh , dassel , germany ) in between the acceptor and donor devices , with a solution of 10% topo ( tri - n - octylphosphine oxide ) dissolved in dhe ( dihexyl ether ) as an impregnator of the membrane and tris ( tris[hydroxymethyl]-aminomethane ) as an acceptor ( ac ) , and milli - q water producing the flush in the donor side .
the acceptor ( ac ) and donor ( dc ) chemicals were prepared in milli - q water .
the me instrument [ 38 , 39 ] was optimised by using the ac made of tris ( ph 7.5 ) and with a dca standard mixture solution made into acidified water ( 5  ml , ph 2.0 , adjusted with hcl ) . the flow rate in the sample recycling before extraction was 0.15  ml / min for 45 minutes ( giving 1.5 times the original sample volume ) . between the sample pretreatments ,
the membrane system was washed with water and the tris solution for 15 minutes at a flow rate of 0.2  ml / min . in membrane extraction
the whole air sample made to 5  ml was recycled in the me system once , twice , and thrice within 33 , 67 , and 100 minutes , respectively .
internal standard tca ( 25  l of the is2 standard at ph 2 water ) was used in all experiments .
the sample was twice recycled ( flow rate 0.15  ml / min , running time 67 minutes ) before analyses .
the effect of the ph was studied by buffering the sample to ph 7.0 , 8.0 , and 9.0 . between the analyses , samples to measure memory effects
were taken , after washing for 15 minutes at the flow rate of 0.2  ml / min with water - tris solvents ( donor - acceptor ) .
calibration was performed with a mixture containing 25  l of tca ( stock 1000  mg / l ) in a 5  mg / l dcas mixture .
the obtained concentrations for 2.5 , 5 , 10 , 20 , 50 , 100 , 200 , and 400  mg
/ l tca ( optimised as the volume of me permeate ) were 2.991 , 4.975 , 9.901 , 19.61 , 47.62 , 90.91 , 166.67 , and 285.7  mg
was calculated according to the following equation : 
 ( 1)ef = ca  ( final)cs  ( initial ) , 
 where ca ( final ) is the concentration of the enriched sample injected into ce , and cs ( initial ) is the concentration of the dca in the ultrasonicated sample . a hewlett - packard ce system ( agilent technologies , waldbronn , germany )
the compounds were detected with an indirect uv mode using wavelengths of 310  nm as the pilot signal and that of 266  nm as the reference signal .
the wavelengths were optimised using oxalic ( c2 ) and sebacic ( c10 ) acids .
the separation voltage was 24  kv ( reversed polarity ) and the migration window for the carboxylic acids was set to five minutes .
the fused - silica capillaries ( composite metal services , the chase , uk ) had dimensions of 58.5  cm in length ( effective length 50  cm ) , 50  m i.d . , and 375  m o.d .
the new capillaries were conditioned by flushing at high pressure ( 150  kpa ) with 0.1  m naoh , water and the electrolyte solution for 20  min , 15  min , and 20  min , respectively . between analyses ,
a meter lab phm 220 laboratory ph meter ( radiometer , copenhagen , denmark ) was used for the ph measurements .
the combination electrode was calibrated with standard solutions of ph 4.00 , 7.00 , and 10.00 manufactured by merck ( darmstadt , germany ) .
the distilled water was further purified with a milli - q apparatus ( millipore , molsheim france ) .
the studied cda anions of oxalic , malonic , succinic , glutaric , adipic , pimelic , suberic , azelaic , and sebacic acids were separated with capillary electrophoresis ( ce ) with indirect uv detection in an optimised electrolyte solution made of 4  mm 2,6-pda and 0.5  mm mtah at ph 11.0 .
the migration order of the dcas was correlated with their molecular size and the first ionisation constant ( pka1 ) of the acids ( figure 1 ) .
determination of dcas was carried out with ce due to its good separation efficiency in 2,6-pyridinedicarboxylic acid electrolyte .
myristyl trimethylammonium bromide was changed to its hydroxide salt , which was needed to reverse the electroosmosis and to get faster analysis get faster analysis and to enhance sensitivity in uv detection .
due to the development , sebacic acid could be studied at concentration range similar to the other dcas .
tca and ma were the best candidates for the internal standards ( is1 and is2 ) to find out the operational changes of the overall concept and the method in the separation of the c2c10 dcas . in spite of that
, ma was only used for evaluation of the experimental parameters , because it migrated too early and overlapped partly with both monocarboxylic acids ( c1 and c2 ) and inorganic anions ( sulphate and chloride ) , which had migration times ( tm ) between 3.6 and 3.9  min , when real samples were studied .
furthermore , it was sensitive to the ionic strength changes in the sample in the tasks from extraction to separation .
the identification of the dcas in the samples was done by spiking the aerosol extracts with cda standards made to 15  mg / l solutions .
the final method was used to optimise sample pretreatment and concentration of some aerosol samples .
furthermore , they were used to quantify the aerosol samples which were collected with micropore filters and membrane ultrafilters and microfiber filters .
aerosol particles with dcas were extracted from cellulose ultra filters of cascade impactor , which were used for quantification and from whatman qm - a quarz microfibre filters .
microfibre filters were used for method development and choosing the best sample pretreatment technique between utrasonic agitation ( us ) and hot water extraction ( hwe ) .
the performance of us and hwe were compared in extraction of dcas from the filters into purified water .
the result showed that ultrasonic agitation gave better recoveries and it was fast ; the extractions were made with us , which is the most usable due to high recoveries of tca , ma , and dcas obtained in 60-minute treatment .
therefore enrichment prior to ce separation was needed to increase their detectability for indirect uv identification .
thus , we chose solid - phase extraction ( spe ) and membrane enrichment ( me ) for their concentration tests . because only one of the methods was planned to be used , the performance and superiority of them for the aerosol cases were studied .
first , the me method with the reagents and the membrane was modified and further optimized based on the studies published in literature [ 38 , 39 ] .
the optimisation was focused on acceptor buffer concentration and its ph , the flushing and collection timings of donor and acceptor solvents , the solvent flow rate during extraction , and the circulation time of the donor solution .
the results show ( figure 2 ) that ptfe membrane worked selectively by improving the sensitivities of is2 and c2c10 by 726% and between 15 and 25% , respectively .
me with the chosen parameters gave the highest enrichment for c2c4 and medium extractability for c8c10 .
it was observed that acids with low pka1 could be moved to acceptor solution better than the dcas with high pka values ( figure 3 ) .
the longer chain acids did not have high diffusion through the membrane , although the acceptor solution tris[hydroxymethyl]-aminomethane ( tris , pka = 7.5 ) was ionised to aid the extraction of the anions .
it was noticed that the lower ionic strengths in the acceptor solvents were more efficient than the stronger solutions .
the results showed that optimal me concentration for the organic acid anions could be obtained , when the sample was two times recycled at optimal donor solution of tris ( amine ) at ph 7.5 .
however , there were still after the procedure memory effect from the sample at 0.15  ml / min flow rate .
it was four to seven times more efficient for c4c10 dcas than the electrolyte solutions modified to ph 8.0 and 8.5 .
low flow rates should be preferred for repeatable and quantitative separation recoveries in me . as expected , recycling of the sample ( study carried out with one to seven cycles ) improved the recoveries .
however , for the dcas concentrations in one cycle were only improved with 15%25% from that of pure solvent extraction reported earlier . to improve the sensitivities , 67  min recycling time would be needed . however , the time was too long .
our studies resulted in conclusion that the membrane would need more than 2-hour water cleaning before a new aerosol sample would be introduced to prevent contamination .
solid - phase extraction ( spe ) was preferred over membrane extraction ( me ) .
spe was used after solvent extraction of the dcas after extraction from the impact filters .
spe materials ( oasis hlb , strata x , isolute 101 and sax , and j.t.baker nh2 and quaternary amine ) were tested for the offline concentration and purification of the air samples .
the recoveries of the dcas are shown ( figure 4 ) with four spe materials , excluding the baker materials that could not be optimized for the aerosol samples .
oasis hlb and strata x were the best for further looking in terms of low inorganic matrix background and good extraction efficiency followed by intensive response for anions of oxalic ( c2 ) , malonic ( c3 ) , succinic ( c4 ) , glutaric ( c5 ) , adipic ( c6 ) , pimelic ( c7 ) , suberic ( c8 ) , azelaic ( c9 ) , and sebacic ( c10 ) acids .
oasis hlb and strata x enhanced selectivity for nonspecific analytes with nonpolar retention mechanism on the material surface that was either end - capped or non - end - capped polymer . as seen in figure 4 , isolute 101 sorbent was even superior to oasis and strata x materials , when the acids c7c10 were studied , but it was not used for the further studies due to its 50% higher capability to retain inorganic anions , such as cl and so4 in the samples .
the material with octadecyl ( non - end - capped ) functionalised trifunctional silane was activated by ionization of polar functional groups in the process .
the best operating material in terms of enrichment of the homologous carboxylates , repetition , and low matrix effects was oasis hlb .
the spe optimization was made with 5  mg / l standards for 6 times .
the average values of recovery rsd% in the spe analysed with ce were 87 , 9.0 , 4.0 , 5.9 , 10.0 , 6.0 , 5.8 , 8.1 , and 19.7 for c3 to c10 , respectively .
standard deviation on the average was 1 - 2% . because the correlation was linear as a function of concentration and the 5  mg / l concentration ( 6.8  nl into separation ) was low enough to show the working at low concentration range for the aerosol samples , the methods development was accepted with oasis hlb material . with
however , by using of spe methods c2 and c3 could also be quantified , although the spe technique did not improve their amount from that obtained with us .
therefore , the recoveries of dcas were remained on the same level obtained with the us method . however , in spe enrichment the concentrations of c4c10 were remarkably improved .
the spe eluent volume was 5000  l and that of the effluent less . to obtain quantitative results
it was evaporated totally under nitrogen and the residue was dissolved into 50  l ( minimum volume of sample in our optimised ce method ) of milli - q water to enrich the acids .
the results were calculated from three samples ( 50  l ) treated with every oasis extraction .
the efficiencies with spe clean - up were mainly between 26 and 134% for the dcas .
depending on the concentration of acid anions in the aerosol samples , in ce analysis inorganic compounds were not always separated from the first two migrating dcas ( c2 and c3 ) , which is why the bge needed further optimisation ( figure 5 ) .
the calibration curves and other validation parameters for the studied compounds under the final conditions are presented in table 1 .
after profound optimisation of the capillary electrophoresis method , sensitivities of the dcas were improved with 50% for the dcas compared to our earlier results .
the reason was the using of another salt of the surfactant in the separation method : instead of mtab , low sensitive mtah was used .
in addition , due to sample processing , inorganic sulphate and chloride of the aerosol in aqueous media could be removed from the final samples by extracting with spe .
our results agreed that the amounts of dcas are very small in the outdoor air samples .
as can be seen from figures 5(a ) and 5(b ) the performance of the capillary electrophoresis was good .
furthermore , a great improvement in the sensitivity could be obtained via the sample preparation procedure that maximizes the dcas signals ( ~100 times ) . in spite of the improvements , only some of the dcas were determined from the aerosol samples . because of the indirect uv detection the dcas in the air samples were identified with spiking of their mixture at 5  mg / l concentration .
the advantage of the sample clean - up and enrichment was the exhaustion of the inorganic species from the sample to obtain electropherogram baseline without great variation .
the ce data showed that the main compounds in the samples were malonic ( c3 ) and glutaric ( c5 ) acids , which were found in the water - soluble fraction of the aerosol samples .
however , also minor amounts of some other cdas could be quantified , namely , c2 , c6c8 , and c10 ( table 2 ) .
the concentrations were calculated to be 510  ng / m ( table 3 ) , when dcas were detected from the isolated and spe ( oasis ) treated aerosol samples and the procedure to enrich the samples was used .
the detection limits ( signal - to - noise ratio 3 ) were at pg level , the highest malonic acid ( c3 ) and the lowest suberic acid ( c8 ) ( table 1 ) .
a simple and repeatable isolation - separation - concentration combination was developed for the determination of homologous dicarboxylic acids from aerosol samples .
overall , the methodology provides efficiency and sensitivity to the separation and provides fast analysis times with repeatability in high alkaline solutions and economical analysis of dcas from aerosol particles .
furthermore , after the concentration of the samples the method is relatively sensitive , when the dca group specific ce method is used .
the pretreatment methods used are supposed to be online coupled with online ce instruments in the future .
when the technology is working , the reproducibility of the separation is supposed to be good enough for monitoring the environmental aerosols better and with lower costs than now .
 the research was performed to study the simultaneous detection of a homologous series of  , -dicarboxylic acids ( c2c10 ) , oxalic , malonic , succinic , glutaric , adipic , pimelic , suberic , azelaic , and sebacic acids , with capillary electrophoresis using indirect uv detection . 
 good separation efficiency in 2,6-pyridinedicarboxylic acid as background electrolyte modified with myristyl trimethyl ammonium bromide was obtained . 
 the dicarboxylic acids were ionised and separated within five minutes . for the study , 
 authentic samples were collected onto dry cellulose membrane filters of a cascade impactor ( 12 stages ) from outdoor spring aerosols in an urban area . 
 hot water and ultrasonication extraction methods were used to isolate the acids from membrane filters . due to the low concentrations of acids in the aerosols , 
 the extracts were concentrated with solid - phase extraction ( spe ) before determination . 
 the enrichment of the carboxylic acids was between 86 and 134% with sample pretreatment followed by 100-time increase by preparation of the sample to 50  l . 
 inaccuracy was optimised for all the sample processing steps . 
 the aerosols contained dicarboxylic acids c2c10 . 
 then , mostly they contained c2 , c5 , and c10 . 
 only one sample contained succinic acid . in the study , 
 the concentrations of the acids in aerosols were lower than 10  ng / m3 . 
1. Introduction
2. Experimental
3. Results and Discussion
4. Conclusions