given the complexity of natural organic matter ( nom ) in environmental matrices [ 13 ] , it is has been difficult , over years , to determine its characteristic behaviour in water systems . studies in the last decade , however , have shown that nom is made up of fractions that include biopolymers , humic substances , building blocks , low molecular weight neutrals , and low molecular weight acids . this information is of particular importance because of specific chemical moieties or nom fractions have been shown to have different interactions with other components in water , for example . the low molecular weight acids are of particular interest because they influence water chemistry ( ph , ionic strength , and divalent cations ) , dipole - dipole interactions hydrogen bonding , and complexation reactions . undoubtedly , these considerations would make the extraction of these acids challenging . in order to choose the most appropriate extraction method that will be desirably recover most aliphatic and aromatic carboxylic acids from water , this research provided essential preliminary assessment on selected solid phase extraction methods . clearly , the determination of carboxylic acids in water calls for an effective primary extractive stage before chromatographic analysis . some of the extraction approaches reported in literature include precipitation , conventional electro - dialysis , the liquid - liquid extraction and solid phase extraction ( spe ) [ 1012 ] . despite all the advantages of the above mentioned methods for example , the liquid - liquid separation techniques use solvents whose distribution coefficient are low [ 13 , 14 ] . a number of spe sorbents that extract carboxylic acids in water are commercially available [ 15 , 16 ] the liquid chromatography with organic carbon detection ( lc - ocd ) , in this study , provided information on the changes in hydrophobicity / aromaticity of nom after the spe separation . this would in turn confirm the removal of the carboxylic acids from water . in this present work , we aimed to assess the extraction efficiencies of silica , c-18 and strata x resins for recovery of aromatic and aliphatic acids in raw water used for cooling purposes in typical power generating stations in south africa . in addition , the aromatic / hydrophobic characteristics of the water before and after spe extraction were compared . clearly , the determination of carboxylic acids in water calls for an effective primary extractive stage before chromatographic analysis . some of the extraction approaches reported in literature include precipitation , conventional electrodialysis , the liquid - liquid extraction , and solid phase extraction ( spe ) [ 68 ] . despite all the advantages of the above - mentioned methods , they still suffer from shortcomings . for example , the liquid - liquid separation techniques use solvents whose distribution coefficients are low [ 9 , 10 ] . a number of spe sorbents that extract carboxylic acids in water are commercially available [ 11 , 12 ] . few studies , however , report on extraction of carboxylic acids followed by chromatographic determinations . the liquid chromatography with organic carbon detection ( lc - ocd ) , in this study , provided information on the changes in hydrophobicity / aromaticity of nom after the spe separation . this study sought to assess the extraction efficiencies of silica , c-18 , and strata x resins for recovery of aromatic and aliphatic acids in raw water used for cooling purposes at eskom , south africa . in addition , the aromatic / hydrophobic characteristics of the water before and after spe extraction were compared . this is part of a larger study that seeks to characterize nom in cooling water , a step forward in building up databases of significant nom fractions ( including carboxylic acids ) , which govern chemical reactions that control scale formation in pipes . eventually , an effective modelling strategy could be developed to monitor these reactions in water , at different physicochemical conditions with the view of scale minimization [ 4 , 13 ] . the latter was used to assess the hydrophobicity / hydrophilicity levels of nom as a function of these carboxylic acids in industrial cooling water . in the present study , silica based c-18 , silica , and strata x resins were employed in the extraction of aliphatic and aromatic carboxylic acids prior to their gc  gc - tofms determination and lc - ocd assessment . some of these ( acetone , methylene chloride , tetrahydrofuran , and methanol ) were obtained from merck ( darmstadt , germany ) , while others ( n - hexane and n - propanol ) were obtained from sigma aldrich ( germany ) . the strata x and c-18 spe resins were obtained from phenomenex ( torrance , ca 90501 - 1430 , usa ) . the synthesis of these spe sorbents is reported in literature [ 14 , 15 ] . three critical steps were followed in each extraction : ( a ) conditioning of the sorbent ; ( b ) sample loading and isolation of the analytes ; and ( c ) the elution step . the cartridges were conditioned with 5  ml methanol and then followed by 5  ml of deionised water . approximately 1.5-litre raw water samples were separately loaded onto the spe columns at a rate of 10  ml / min using the vacuum manifold apparatus . the analytes were eluted from strata x spe by 5  ml acetone , followed by 5  ml of methylene chloride , and from the silica based cartridges using a mixture of tetrahydrofuran ( thf ) and methanol in a 1  :  1 , v / v ratio . the eluted samples were dried in a controlled lamina flow apparatus at 25c , reconstituted using pure methylene chloride ( gc - grade ) , and thereafter injected into the gc for determination . the determination of carboxylic acids from water after spe extracted was performed using gc  gc gas chromatograph ( pegasus 4d , leco corporation , south africa ) with a time of flight mass spectrometer ( tofms ) . the gc technique employed the rxi 5sil - ms primary column ( 40  m long with internal diameter ( i d ) of 0.25  mm and 0.25  m film thickness ) and the rxi 17sil - ms secondary dimension column ( 4  m long with an i d of 0.25  mm and the film thickness of 0.25  m ) . nitrogen gas , compressed air , and liquid nitrogen were used for quad - jet thermal modulator operation . the injector temperature for samples was set at 280c , at a volume of 1  l on a splitless mode . helium was used as the carrier gas at a flow rate of 1.0  ml / min and a head pressure of 90  kpa . the transfer line temperature was set at 300c and the ion source temperature at 240c . electron impact ionization energy was set at 70  ev with offset of 300  v totaling to the detector voltage of 1,600  v. the first - dimension column temperature was started at 55c and held for 4.5  min , then increased to 280c at a ramping rate of 10c / min , and held for 4.5  min , whereas the secondary column temperature started at 75c held for 4.5  min , then ramped to 310c at a ramping rate of 10c / min , and held for 4.5  min . the data was processed by identifying aliphatic and aromatic carboxylic acids with their respective unique compound masses ( q masses ) on spectral databases and first- and second - dimension retention times . the ms analysis was carried out at mass range of 40500 amu and acquisition rate was 100  spectra / sec . raw water samples filtered through spe cartridges were analysed using lc - ocd ( doc - labor , karlsruhe , germany ) . the raw water samples were obtained from the vaal river which feeds lethabo power station with cooling water . the lc - ocd technique , developed to identify fractions natural organic matter in water , gives quantitative information and qualitative results regarding molecular size distribution of organic matter in water . water samples were injected into a column filled with a chromatographic gel material where large molecules were eluted first followed by the smaller compounds . to achieve quantification of organic carbon , organic nitrogen , and specific uv absorbance at -254 , their responses in the samples at different retention times were measured with an organic carbon detector , organic nitrogen detector , and uv detector , respectively . table 1 shows fractions of the natural organic matter components based on their retention times and peak areas . after inline filtration through 0.45  m , the lc - ocd analysis was carried out following the procedure described by huber et al . , 2011 . results obtained from samples extracted by silica , c-18 , and strata x cartridges and determined by first- and second - dimension chromatographic separation are presented in this section . the unique masses ( q masses ) , retention times , library hit similarities , and peak areas for selected carboxylic acids ( especially aromatic acids ) are shown in table 2 . the chromatograms of the total carboxylic acids and aromatic carboxylic acids are provided separately in the same figures ( figures 1 , 2 , and 3 ) . using deconvolution chromatof software and the structured unique masses ( q masses ) spectra in gc  gc - tofms [ 16 , 21 , 22 ] , the coeluting carboxylic acids were also identified . hitherto , a comparison , in terms of aliphatic and aromatic carboxylic acid extraction efficiency , was evaluated and results are discussed in this section . the hydrophilic or hydrophobic changes occurring in water after removal of carboxylic acids are shown in table 3 and figure 4 . as shown in table 2 , raw water using for cooling at eskom chiefly contains straight - chain and branched carboxylic acids ( all referred to as aliphatic carboxylic acids ) . the results showed a total of 42 carboxylic acids extracted by silica spe from the vaal river water . of these , there are aliphatic carboxylic acids ( 90.48% ) while aromatic acids were 9.52% of the total ( table 2 and figure 1 ) . these aromatic acids included 1,2-benzenedicarboxylic acid , bis(2-methyl ) propyl ester ; benzoic acid , 3,5-bis(1,1dimethylethyl)-4-hydroxy - ethyl ester ; phthalic acid , di(2-propylpentyl ) ester , and 7-phenylheptanoic acid . comparatively , the number of aromatic carboxylic acids recovered by the silica spe cartridge was lower than those recovered by c-18 ( 25% ) and strata x ( 28% ) cartridges . a possible explanation for the relatively large number of carboxylic acids in this river could be degradation and aggregation . related studies have reported short and medium chain carboxylic acids due to anaerobic fermentation of organic matter [ 16 , 18 ] . clearly , microbial activity and changing physicochemical parameters ( such as ph , ionic strength , and temperature ) influence the distribution of nom fractions . the result of this influence explains the fact that higher molecular weight and aromatic compounds in water degrade and aggregate forming smaller intermediates , which include carboxylic acids of varying molecular weights . they include eicosanoic acid ( 1st t 
 r = 728 ; 2nd t 
 r = 2.800 ) ; hexanedioic acid , bis(2-ethylhexyl ) ester ( 1st t 
 r = 728 ; 2nd t 
 r = 2.950 ) , eicosanoic acid , 2,3-bis(acetyloxy)propyl ester ( 1st t 
 r = 840 ; 2nd t 
 r = 2.650 ) , 9,12,15-octadecatrienoic acid , 2-(acetyloxy)-1-[(acetyloxy)methyl]ethyl ester , ( 1st t 
 r = 840 ; 2nd t 
 r = 2.800 ) ; octanedioic acid ( 1st t 
 r = 1048 ; 2nd t 
 r = 2.800 ) , decanoic acid , 2-propenyl ester ( 1st t 
 r = 1048 ; 2nd t 
 r = 3.070 ) . some other carboxylic acids were eluted at different times in the primary dimension but at the same time in the secondary dimension . these included benzoic acid , 3,5-bis(1,1-dimethylethyl)-4-hydroxy - ethyl ester ( 1st t 
 r = 676 ; 2nd t 
 r = 2.800 ) ; octanedioic acid ( 1st t 
 r = 1048 ; 2nd t 
 r = 2.800 ) and 1,2-benzenedicarboxylic acid , bis(2-methyl ) propyl ester ( 1st t 
 r = 688 ; 2nd t 
 r = 2.810 ) . the total carboxylic acids extracted by the c-18 resin were 20 , 15 of which were aliphatic and 5 aromatic ( table 2 and figure 2 ) . the aromatic acids included 1,2-benzenedicarboxylic acid , bis(2-methylpropyl ) ester ( 1st t 
 r = 688 ; 2nd t 
 r = 2.820 ) ; benzenepropanoic acid , 3,5-bis(1,1-dimethylethyl)-4-hydroxy- , methyl ester ( 1st t 
 r = 1080 ; 2nd t 
 r = 2.660 ) ; phthalic acid , di(oct-3-yl ) ester ( 1st t 
 r = 1136 ; 2nd t 
 r = 2.570 ) ; 1,2-benzenedicarboxylic acid , butyl 2-ethylhexyl ester ( 1st t 
 r = 1152 ; 2nd t 
 r = 2.590 ) ; phthalic acid , butyl undecyl ester ( 1st t 
 r = 1188 ; 2nd t 
 r = 2.520 ) . of these aromatic acids , only 1,2-benzenedicarboxylic acid , bis(2-methylpropyl ) ester aliphatic carboxylic acids were less retained on c-18 ( 75% ) compared to silica spe ( 90.48% ) . this is because c-18 consists of a silica surface functionalized with hydrophobic groups , which favour retention of the carboxylic acids in the reversed - phase [ 23 , 24 ] . additional studies have reported the application of the c-18 solid - phase in extraction of organic acids using the reverse phase and henceforth higher retention of those with higher hydrophobicities . the retention efficiency of c-18 was 25% for the aromatic carboxylic acids ( relatively more hydrophobic ) compared to silica 's 9.52% . the remaining 75% of the carboxylic acids extracted using c-18 were mainly medium and long chain aliphatic acids . except for cis-13-octadecenoic acid ( 1st t 
 r = 712 ; 2nd t 
 r = 2.670 ) and 9,12-octadecadienoic acid ( 1st t 
 r = 716 ; 2nd 20 t 
 r = 2.750 ) that had similar retention times , the time of flight rapid separation and mass spectral signatures did not show other closely eluting or coeluting compounds . this strata x polymeric resin has the capacity to adsorb both polar and nonpolar compounds with more capacity for the former . it consists of copolymers that have aromatic rings in their structures which allow for electron - donor interaction between the sorbent and the  bonds in the analyte . therefore , this sorbent overcomes many limitations of bonded silicas because their hydrophobic surface contains a relatively large number of active aromatic sites that allow these - interactions with aromatic carboxylic compounds . more aromatic carboxylic acids were retained in the strata x than the two other sorbents ( table 2 and figure 3 ) . except for m - hydroxymandelic acid , tris(trimethylsilyl ) , all the other aromatic acids extracted by strata x had > 70% gc - library hit similarity . the retention efficiency of aromatic acids for strata x 28% is higher than the one obtained using c-18 ( 25% ) and silica ( 9.52% ) . table 2 shows a total of 25 carboxylic acids extracted by strata x with 18 aliphatic carboxylic acids and 7 aromatic carboxylic acids . the strata x extracted 72% aliphatic acids of the total carboxylic acids and 28% aromatic acids . these were cis-10-heptadecenoic acid ( 1st t 
 r = 984 ; 2nd t 
 r = 2.520 ) and e-9-tetradecenoic acid ( 1st t 
 r = 984 ; 2nd t 
 r = 2.540 ) ( figure 3 ) . first , the lc - ocd analyses were performed to identify the major fractions of nom in raw water . second , results showed that the concentration of nom fractions decreased after filtration through the three spe methods , that is , silica , c-18 , and strata x ( table 2 ) . the decrease was more pronounced in c-18 filtered water and followed the order c-18>silica > strata x. the relative degrees of aromaticity against molecularities of nom in the four water samples are presented in the humic substances diagram ( hs diagram ) . from the results , the hydrophobicity degree of the samples depended on figure 4 , which is the humic substances diagram ( hs diagram ) , shows the relationship between molecularity and aromaticity of organic compounds in the water samples . the concentrations of the hydrophobic component followed the order strata x > c-18>silica > raw water and the hydrophilic fraction followed the order raw water > silica > strata x > c-18 . from figure 4 , more aromatic compounds were extracted by c-18 ( h on the hs diagram ) followed by strata x ( f on the hs diagram ) . however the molecular weights of nom components remaining in the water were higher in water extracted by strata x. this agrees with data obtained using 2-dimensional gc . hydrophobicity of samples was also assessed using the specific uv absorbance at the wavelength of 254 ( suva ) . the various suva values and the composition of the samples are tabulated in table 3 . natural organic matter in water is reported to possess a hydrophobic characteristic , when the measured suva values are > 4 [ 26 , 27 ] . accordingly , the aromaticity of the water samples followed the order f > g > h and the molecularity values followed the order g > f > h . effective characterization of raw water samples was achieved by spe prior to gc  gc - tofms determination . the use of strata - x cartridge resulted in the best recovery of aromatic carboxylic acids from raw water . this was relatively higher recovery and could be attributed to the fact that strata x is a polymeric resin with aromatic rings in its hydrophobic structure . the total number of carboxylic acids extracted by silica spe was 42 , with aromatic acids comprising 9.52% of the total and the rest being aromatic . accurate determination of the various carboxylic acids was achieved by the first- and second - dimension gas chromatography . the lc - ocd results confirmed that the aromaticity degrees and molecular weights nom changed after extraction of carboxylic acids . largely , the hydrophobicity degree of the samples depended on the type and number of carboxylic acids that were removed by the spe cartridges .
the extraction and determination of aliphatic and aromatic carboxylic acids as well as their influence on the aromaticity and molecularity relationship of natural organic matter ( nom ) in water are reported in this study . 
 three solid phase extraction ( spe ) sorbents were used and their extraction efficiencies evaluated after chromatographic determinations ( using gas chromatography with a time of flight mass spectrometer ( gc  gc - tofms ) and liquid chromatography with organic carbon detector ( lc - ocd ) ) . more than 42 carboxylic acids were identified in raw water from the vaal river , which feeds the lethabo power generation station , south africa , with cooling water . the aromatic carboxylic acid efficiency ( 28% ) 
 was achieved by using strata x spe while the highest aliphatic carboxylic acid efficiency ( 92.08% ) was achieved by silica spe . 
 the hydrophobic nature of nom in water depends on the nature of organic compounds in water , whether aromatic or aliphatic . 
 the lc - ocd was used to assess the hydrophobicity levels of nom as a function of these carboxylic acids in cooling water . 
 the lc - ocd results showed that the aromatic nature of nom in spe filtered water followed the order silica > strata x > c-18 . from the results , the hydrophobicity degree of the samples depended on the type and number of carboxylic acids that were removed by the spe cartridges .