odorants are troublesome in water samples , because they dramatically influence the esthetic quality and consumers ' acceptability of drinking water [ 1 , 2 ] . these odorants are often associated with the metabolites that are produced in the degradation of cyanobacteria , actinomyces , fungi , and blue - green algae [ 35 ] .
the odorants geosmin ( gsm ) and 2-methylisoborneol ( 2-mib ) are commonly found in lakes and reservoirs [ 6 , 7 ] , which people can smell their odors in water samples even at the concentration of 10  ng  l or less , but it would be difficult to identify and quantify these two trace volatile organic compounds ( vocs ) [ 812 ] . as a result , the low threshold of detection can result in consumer complaints about the odors in recreational waters , aquatic products , and tap water , especially during the outbreak period of algal blooms , even if some other quality indicators of water , such as turbidity , number of algal cells , and suspended matter , are acceptable . in recent years , according to the investigation by chinese academy of sciences [ 14 , 15 ] , the results indicated that freshwater lakes were suffering the odor problems in china , especially in lake taihu , lake chaohu , and lake dianchi . these unpleasant odors occurring in these lakes or
therefore , we had conducted this study in 2012 , aiming to obtain the basic data of common odorants ( gsm and 2-mib ) in environmental waters in jiangsu province . to be more specific ,
the headspace solid - phase microextraction coupled to gas chromatography mass spectrometry ( hs - spme / gc - ms ) was applied to determine the levels of gsm and 2-mib in water samples , and this method can be obtained in our early reports [ 16 , 17 ] . in addition , the water samples were collected in detail as follows : firstly , for water sources of tap water , four kinds of water samples were collected respectively from four cities in jiangsu , including city a ( little lakes as the source ) , city b ( the changjiang river as source ) , city c ( lake taihu as source ) , and city d ( underground water as the source ) ; secondly , for conventional treatment process of tap water , including coagulation and sedimentation , filtration , and disinfection , the water samples were collected respectively from above processes ; thirdly , for different seasons of water samples , we collected the samples in three periods in lake taihu , including the period when a river is at its normal level ( november to december ) , drought period ( february to march ) , and wet season ( july to august ) when the algae bloom .
two common odorants in water , gsm and 2-mib , and internal standard 2-isobutyl-3-methoxypyrazine ( ibmp ) were obtained from sigma - aldrich ( usa ) at a concentration of 100  mg  l in methanol and 1  mg 
l mixed standard solutions of two target compounds in methanol , and all of them were stored in the dark at 4c .
deionized water was prepared on a water purification system ( gradient a10 ) supplied by millipore ( billerica , ma , usa ) .
sodium chloride ( analytical grade , china ) , which was added to the samples before extraction , was conditioned by heating at 450c for 4  h before use .
spme apparatus was purchased from supelco ( usa ) , including fiber 30/50  m dvb / car / pdms ( number 57348-u ) , fiber holder , 60  ml specialized vials for spme , sampling stand , magnetic stirrer , and injection catheter ( number 57356-u ) . after putting nacl and a stir bar in a 60  ml vial
, 40  ml of mixed standard solutions for standard curve or 40  ml environmental water samples were added , and 2  l ibmp ( 1  mg 
the vial was sealed with polytetrafluoroethylene ( ptfe ) septum cap and placed in a water bath .
several minutes after the temperature was achieved in the vial , the outer needle of fiber was used to penetrate the septum , and the fiber was exposed to the headspace for extraction . after 30  min exposure , the fiber was immediately inserted into gc injection port for desorption . according to our early study [ 16 , 17 ] ,
the hs - spme / gc - ms was applied to determine the levels of gsm and 2-mib in water samples , and the optimum conditions for hs - spme were as follows : temperature of extraction and desorption , 65c and 260c , respectively ; time of extraction and desorption , 30  min and 5  min , respectively ; ionic strength , 25% ( w / v ) ; rotation speed , 600  rpm ; solution ph , 5.0 .
a varian 300 gc / ms / ms ( varian inc . , ca , usa ) with ion trap and mass spectrometer was obtained with a varian vf-5 ms capillarity column ( 30  m  0.25  mm  0.5  m ) .
the temperature of the oven started at 40c and was held for 5  min .
then the temperature was 8c  min to achieve 160c ( total time 20  min ) followed by 20c  min to achieve 260c ( total time 25  min ) .
the electron impact ( ei)-ms conditions were as follows : ion - source temperature , 230c ; ms transfer line temperature , 250c ; solvent delay time , 5  min ; ionizing voltage , 70  ev . the mass spectrogram in full scan mode was obtained at the m / z range of 80200  u. based on the ms scan function ( sim mode ) , the process was divided into three main segments .
the method of internal standard was applied to construct calibration curve and determine concentrations of 2-mib and gsm in water .
based on different sources for tap water , four representative cities were chosen to collect water samples , including city a ( little lakes as the source ) , city b ( the changjiang river as source ) , city c ( lake taihu as source ) , and city d ( underground water as the source ) .
five water works were selected from each city , and raw water , output water , and end water ( for consumers ) were obtained respectively from each water work in drought period  february to march  by manual sampling , meaning 15 samples in total in one city .
water samples from the water works were analyzed by the proposed method , which were kept in 350  ml sample vials with ptfe - faced silicone septum and stored at 4c before analysis .
the conventional treatment of tap water includes four processes , as raw water , coagulation and sedimentation , filtration , and disinfection , and end water was provided for consumers by pipe network as shown in figure 1 .
the five water works were selected from city b ( shorter distance to laboratory than other three ) , and one sample was obtained from each process , that is to say , 4 samples in total in one water work . for different seasons of water samples , we collected the samples in three periods in lake taihu , including the period when a river is at its normal level ( november to december ) , drought period ( february to march ) , and wet season ( july to august ) when the algae bloom .
in addition , lake taihu was used as sampling point , because the algal overgrowth appears every summer in this lake , and the consumers complain that the uncomfortable odors exist in their drinking water during the outbreak period of algal blooms .
the five water works were selected from city c , and raw water , output water , and the end water were obtained respectively from each water work , 15 samples in total for one period .
the analysis of variance ( anova ) was applied to the present work , aiming to obtain the significant differences when comparing average values in the two or more groups .
more specifically , two - way anova was applied to compare the gsm and 2-mib levels in four cities ( tables 1 and 2 ) , while one - way anova was applied in water samples from different processes of tap water treatment and different seasons ( tables 3 and 4 ) .
according to tables 1 and 2 , the significant differences of concentration can not be observed for both gsm and 2-mib among the four cities ( p > 0.05 ) , excepting the 2-mib levels of raw water ( p < 0.01 ) .
more specifically , the 2-mib level of raw water in city d was lower than that in three other cities ( p < 0.01 ) , and the most potential reason could be that the soil or rocks work as natural barrier or filter , keeping the larger particles permeating into raw water . as a result , the level of 2-mib may be lower in city d. however , there are no significant differences for the levels of gsm among four cities ; in other words , the lower 2-mib levels of raw water in city d never necessarily equal the lower levels of gsm in city d similarly . it can be explained that the levels of two odorants were associated with multiple factors , including the species of algae , acidic condition of environment , rainfalls , and light density .
for example , growth and production of 2-mib was characterized in the cyanobacterium phormidium sp .  
another example is that the actinomycetes can cause the odorants , but the total colony counts did not necessarily indicate the direct role of actinomycetes in odor problems in environmental water , because their activities would be restricted by other environmental factors .
consequently , the lower 2-mib occurred in city d , but gsm did not , even though they shared the same water source .
in addition , the decreased levels of gsm or 2-mib were observed among the raw water , output water , and end water ( p < 0.05 ) , as implied in table 4 . because both of gsm and 2-mib were volatile organic chemicals ( vocs ) , the concentrations would decrease in the water transferred from water works to destination , and similar results are obtained by early reports in china .
meanwhile , the concentration of total residual chlorine in pipe network would necessarily reduce the levels of gsm and 2-mib [ 22 , 23 ] .
but , however , an experimental error ( 1070% ) may be observed due to the presence of free residual chlorine , and this error can decline after dechlorination ( less than 10% ) [ 22 , 24 ] . consequently , the average values of two odorants in table 3 may increase from filtration to chlorination . based on table 3
, it is indicated that the concentration of gsm and 2-mib decreased significantly as treatment process moved forward ( p < 0.0001 ) .
the reasons can be as follows : firstly , after the processes of coagulation , sedimentation , and filtration , larger particles had been removed , such as algae , silt , and other plankton , which can produce the musty and earthy odorant in tap water , and the same result was implied in early study .
the last process , chlorination , can kill or remove main cyanobacteria and actinomyces by oxidation [ 26 , 27 ] . and also some reports indicated that the presence of chlorine would substantially reduce the observed gsm and 2-mib concentrations , and its impact was larger for lower organic compound concentrations , under higher residual chlorine conditions .
the water samples of different seasons were collected from city c , the lake taihu of which was used as source water and blue - green algae blooming in summer in this lake . according to table 4 , the significant differences were observed for the levels of gsm and 2-mib among the three periods ( p < 0.01 ) ; in other words , the gsm and 2-mib levels of raw water were higher in period of wet water than corresponding levels in other two periods .
the most important reason was the algae blooming and reservoir 's eutrophication in summer , resulting in higher concentrations of odorants , while the algae or cyanobacteria were dead or inactive in other periods .
moreover , the concentrations were found to correlate with corresponding air and water temperatures , and the average water temperature was 12 , 16 , and 27c for normal level , drought , and wet period , respectively .
the concentrations of odorants followed a trend of higher levels during the warm seasons [ 2830 ] .
consequently , the levels in wet season ( in summer ) were higher than those in other two periods . also , the results implied that levels of odorants were approximately equal between the periods of normal level and drought .
the levels of gsm and 2-mib in water samples were less than 10  ng  l , the odor threshold concentration ( otc ) [ 12 , 31 ] . and the conventional treatment process of tap water plays a significant role in removing common odorants in running water for these source waters .
in particular the largest decrease in concentrations occurs after sedimentation suggesting that removal of gsm and mib in intact algae cells is important .
 this study was conducted to obtain the basic data of two common odorants  geosmin and 2-methylisoborneol ( gsm and 2-mib)in environmental water . 
 more specifically , the headspace solid - phase microextraction coupled to gas chromatography mass spectrometry ( hs - spme / gc - ms ) was applied to determine the levels of gsm and 2-mib in water samples , and the samples were collected depending on water sources , conventional treatment processes , and seasons . 
 the significant difference was shown for the 2-mib levels of source water ( p < 0.05 ) , 
 the concentrations of gsm and 2-mib decreased significantly as treatment process of tap water moved forward ( p < 0.0001 ) , and the significant differences for the levels of gsm and 2-mib were observed among three sampling periods ( p < 0.01 ) . 
 the levels of gsm and 2-mib in all water samples were lower than 10  ng  l1 , the odor threshold concentration ( otc ) , and the conventional treatment process plays a significant role in removing odorants in tap water . 