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Solid-phase extraction procedure for the determination of selenium by capillary gas chromatography
129
An&rca ChunacaAcra, 274 (1993) 129-140 Elsevler Science Pubhshers B V , Amsterdam
Solid-phase extraction procedure for the determination of selenium by capillary gas chromatography K. Johansson, U Omemark and A Olm Departmenr of Analytical Chemrsstry,Utuversuy of Uppsala, P 0 Box 531, S-751 21 Uppsala (Sweden) (Rece.wed 6th July 1992)
Ahstraet A solid-phase extractlon procedure to be used m the determmatlon of selenium by gas chromatography wth electron-capture detectlon (GC-ECD) was investigated After denvatlzatlon of Se(N) with 1,2-dlammo-3,5-dlbromobenzene the formed 4,6-d~bromoplazselenol was adsorbed on a porous dwmylbenzene polymer Elutlon was performed wth toluene The proposed solid-phase extraction procedure was compared with a hqmd-hqmd extractlon procedure and the overall effkxency was appromately 97% for sample volumes between 5 and 100 ml Concordant results were obtamed for natural water samples sptked with selemum between the GC-ECD procedure and a hydnde generatIon atonuc absorption spectrometnc procedure Removal of &solved organic carbon m natural waters on XAD-8 and Dowex 1X8 pnor to selenmm determmatlon was also studied Keywords Atomic absorption SpectrometIy, Gas chromatography, Selenium, Sohd-phase extraction, Waters
The low levels of Se(W) present in natural waters [l-3] require analytical procedures havmg good accuracy at the ng 1-l level Gas chromatography urlth electron capture detection (GC-ECD) [4-61 and hydride generation atomic absorption spectrometry (HG-AAS) [1,7] m conlunctlon with appropriate preconcentratlon steps are both smtable techniques for such deterrmnatlons The determmatlon of Se(W) m water by GCECD 1s based on the formatlon of a volatile compound, a pmzselenol, m the reaction between selemous acid and a subsmuted o-phenylenedlamme The plazselenol is normally extracted mto an orgamc solvent before mjectlon mto the gas chromatograph The extraction procedure makes it possible to preconcentrate the sample
Correspondence to K Johansson, Department of AnalytIcal ChemlstIy, Umverslty of Uppsala, PO Box 531, S-751 21 Uppsala (Sweden)
up to 500 tunes [S], leadmg to a detection hmlt below 1 ng l- ’ of Se(W) m the sample In the HG-MS method, Se(W) 1s reduced with sodium tetrahydroborate to volatde hydrogen selerude, which 1s collected at hquld nitrogen temperature The preconcentrated hydrogen selemde 1s vaporized and selemum determmed by AAS The hrmt of detection is 1 to 2 orders of magnitude lower than for systems wrthout preconcentration [9,103 Both methods may be SubJect to interferences from dissolved organic material In the HG-AAS method, Se(W) and/or SeWI) added to ground water [ll], sod extracts [12-141 and extracts of blogemc material [15] have been lost The mterference 1s removed by wet digestion [11,12,15] but this prevents speclatlon Another solution to this problem 1s to remove the mterferent(s) on a sorbent before selemum determmatlon Roden and Tallman [ill demonstrated that the mterferent, probably an orgamc constrtuent, could be
OCKI3-x70/93/$06 00 0 1993 - Elsevler Science Pubhshers B V All nghts reserved
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reversibly bound on the macroretlcular polymeric resin XAD-8, while morgamc Se(IV) and Se(V1) were unretamed As reported by several authors [13-151, this sorbent will not always remove the interference completely In the GC-ECD method with liquid-hquld extractlon, the mam problem connected with the organic material is emulsion formatlon, makmg the phase separation time-consummg if not lmpossible The interferences from organic material can be reduced by extracting the sample with an organic solvent or fdtermg it through a sorbent prior to the derlvatlzatlon and hquld-hquld extraction A sorbent can thus be used as a clean-up step m the GC-ECD method However, It should also be possible to adsorb and collect the plazselenol from the aqueous phase This techmque 1s sometnnes referred to as sohd-phase extraction (SPE) SPE can be used to separate analyte and concomitants by adsorbing the former while the latter are unretamed and 1s a special case of adsorption chromatography In trace analysis, where SPE 1s used to accumulate the analyte, the dlstrlbutlon coefficient should be large for the analyte It IS also important that the adsorptlon capacity of the column 1s adequate, smce the sample may contam other substances, that are also retamed Ideally, the analyte should be adsorbed as a narrow band at the top of the column, because then a short column can be used and the elutlon made m a small volume This paper reports on mvestlgatlons of a SPE method m trace analysis of Se(IV) m water with GC-ECD We also report on determmatlons with GC-ECD and HG-AAS of Se(IV) added to natural water, treated by flltratlon through XAD-8 or Dowex 1X8
EXPERIMENTAL
Apparatus For the gas chromatographlc measurements a Shunadzu GC-14A gas chromatograph equipped with a constant current @j3N1 electron-capture detector was used The column was DB-1701 (15 m x 0 25 mm 1d > with a 0 25-pm film thickness (J&W Saentlflc) The chromatographlc condo-
K_ Johansson et al /Anal.
Chm Acta 274 (1993) 129-140
tlons were carrier gas, helium at 40 cm s-l, split vent, 60 ml mm-l, purge vent, 1 ml nun-l, makeup gas, nitrogen at 45 ml mm-‘, injector temperature, 25O”C, detector temperature, 325”C, and column temperature programme, 100°C held for 2 mm, increased at 8°C mm-l to 175”C, held for 5 mm, Increased at 15°C nun-l to 265”C, held for 5 mm The injections (0 5 111sample + 1 ~1 toluene) were made m the sphtless mode with a Shunadzu AOC-14 automatic Injector, with a change to the split mode after 1 mm The peak areas were evaluated with a Shunadzu C-R5A integrator The HG-AAS apparatus with a cold trap 1s described elsewhere [7] Hydride generation 1s accomphshed by mnung the acidified sample with sodium tetrahydroborate m a flow system The hydnde 1s stripped from the aqueous phase and transported with nitrogen gas Four glass U-tubes m series are used for gas-hquld separation, removal of water vapour and trapping of the hydride Atomic absorption measurements were made with a Perkm-Elmer 3100 spectrometer equipped with the MHS-20 oven and contammg a quartz cuvette at 8OOYZ The hght source, an electrodeless discharge lamp, was operated at 7 6 W The analytical wavelength was 196 0 nm and the spectral band-width 0 7 mn Evaluation of peak areas and peak heights was made with the Model 3100 EDS software supphed by the manufacturer No background correction was applied Determmatlons m the l-10 pg 1-l range were performed with the HG-AAS mstrumentatlon described by Pettersson et al [9] A Phdlps PU 8625 spectrophotometer equipped with a 40-mm flow through cell, a U4XV peristaltic pump (Ahtea AB) and a glass column, bed volume 1 1 ml, (Kabl-Pharmaaa) were used m the study of the XAD-resins The absorbance was measured at 420 nm Total dissolved organic carbon (TO0 m the water samples was determmed on a Shunadzu 500 TOC analyzer by the National Envlromnental Protection Agency in Uppsala The equipment used for SPE is presented in Fig 1 Frve of these devices were connected m parallel to a water aspirator The column (mner diameter 3/8”, volume 4 ml) was made of
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Chm Acta 274 (1993) 129-140
glasssample reservou
polypropylene column pm polyethylene fnts sorbent
her lock needle
Fig 1 Equpment
used for sohd-phase extra&on
polypropylene and contained two 20qm pobethylene fnts The stopcock and adaptor were pu chased from Analyttchem (Harbour City, CAF The sample reservou was made of glass The ion-exchanger tubes were made from Pyrex, length 200 mm, 1 d 8 mm and provided with a cyhndncal funnel, length 70 mm and 1 d 42 mm, at the top Flow was controlled by a clamp around a small piece of PVC tubmg connected to the l-mm drawn out tip These tubes were also used m the recovery expernnents for morgamc selenium on XAD-7 and XADS Chemzcals and reagents All chermcals were of analytical grade from Merck unless othemse stated Demmerahzed, destllled water passed through a M&pore 3 filter cartridge system, Mdh-Q water, was used throughout All glassware was cleaned m 4 M mtnc acid and rinsed with Mdh-Q water 1,2-Dlammo-3,Idrbromobenzene (Br,-PDA) A 5 5 m.M solution of Br,-PDA m 0 5 M perchlonc acid was prepared The solution was punfled by extraction with toluene and should preferably be
131
kept m the dark When 100 ml were extracted Hrlth 5 ml of toluene the Br,-PDA concentration dropped to about 5 2 mM Selenzum(N) A stock standard solution, 1 g 1-l of Se(N), was prepared from an ampoule of selennun dloxlde m dilute nitric acid Its concentration was checked by an amperometrlc titration with thlosulphate after addition of a large excess of iodide [16] Workmg standard solutions were obtained by senal dllutlon of the stock standard solution and contamed 2 ml 1-l of hydrochlonc acid (37%, w/w) Internal standard solutwn Thrs solution was prepared by dlssolvmg lmdane (99%) and aldrm (99%>, both purchased from Applied Science Labs (State College, PA), m toluene The workmg solution contained 2 0 mg 1-l of hndane and 26mgl-‘ofaldrm Lmdane and aldrm are msectlcldes and precautions should be taken to avoid mhalatlon and skm contact 4,6-D&omopuzzselenoi This was synthesized according to the literature [51 The synthesized plazselenol was analysed for selenium after wet digestion according to a variation of Gould’s method 1171 usmg a 10 1 (v/v) murture of concentrated sulphunc acid and fummg rutnc acid by the amperometnc titration method mentioned above The purity was 99 8 f 0 4% A stock standard solution contauung 0 5 g I- ’ of plazselenol was prepared by dlssolvmg plazselenol m toluene Workmg standard solutions were prepared by serial dllutlon with toluene &&urn tetrahydroborate Sodium tetrahydroborate, 0 5% (w/v), was stabilized and purrfled as described elsewhere [7] Hydrochlonc aczd HCl, 37% (w/w), used m the cold trap system was diluted 1 1 and flushed with nitrogen prior to use Resins Sold-phase extractron resm The resin was a porous dlvmylbenzene polymer (PD-lOZPE), 10 pm m diameter, with a narrow particle size dlstnbutlon It was supplied by Dyno Particles (Lrllestrom) as a 10% (v/v) suspension m methanol The resin was not further purified The resm bed was packed as follows, see also Fig 1 The upper
132
75ml glass reservoir was detached, the stopcock closed and the fret at the bottom inserted Smce the particle size of the resin was 10 pm, a glassflbre filter with a pore diameter of about 2 pm was mserted above the fret The resm was added as a methanol slurry until the settled bed was approxnnately 4 mm Then the second fret was inserted The column was condltloned by drammg the methanol followed by 5 ml of 0 25 M perchlonc acid The stopcock was closed when about 2 ml of perchlorlc acid were left above the upper fnt, then the sample reservoir was attached The column was effectively regenerated by passing 5 ml of methanol-pcrchlonc acid (7072%) (98 2) through the column When not used, the column was filled with methanol and stored closed Zen-ex&ange resin Dowex 1X8, (100-200 mesh analytical grade, Serva) was purchased from LabKern (Stockholm) Fmes were removed by decantatlon and the resin (100 g) was placed m a glass filter funnel and washed wth 200 ml of 1 M sodmm hydroxide added m portlons followed by Ml&Q water to neutral ConversIon to the chloride form was achieved by passing 200 ml of 1 M hydrochlortc acid followed by M&-Q water When no detectable chlonde was found m the effluent the resm was dried over mght at 50°C and then condltloned over calcium chloride hexahydrate and its saturated solution A 2-g amount of the condltloned resm (dry weight) was slurrled m a few ml of Mllh-Q water and packed between plugs of quartz wool The resm was washed with 10 ml of 1 M hydrochloric acid followed by M&-Q water to approxnnately neutral effluent Finally 25 ml of 0 025 M hydrochloric acid were passed through the column The solution to be filtered was made 0 025 M with respect to hydrochlonc acid and transferred to a volumetric flask The flask was mounted upside down m the cyhndrlcal funnel of the exchanger tube The flow was adJUSted to about 2 ml mm-’ The effluent was collected and analysed for Se(N) and TOC XAD realm XAD-2 (0 3-l mm anal gr 1, XAD4 (20-50 mesh pract ), XAD-7 (20-50 mesh pract >, XAD-8 (0 3-l mm pract ), XAD-16 (1380 mesh pract ) (Serva), were purchased from Techtum (Umefl) Fines were removed by de-
K Johanmon et al /Anal Chm Acta 274 (1993) 129-140
cantatlon The resm (100 g) was then slurrled m 50 ml of methanol followed by addltlon of 50 ml of water After deaeratlon with mtrogen gas the resin was stored refrigerated A 7 5-ml column was prepared between plugs of quartz wool from the resin m the methanol-water (1 1) mixture After washing of the resin with Mllh-Q water to remove all methanol, 50 ml of 0 01 M hydrochlonc acid were passed through the column The solution to be filtered was made 0 01 M with respect to hydrochloric aad and passed through the column as described above at about 1 ml mm-’ S&d-phase extractionof puzzselenol Standard procedure for the extractrons The Se(N) was derlvatlzed at room temperature or 100°C by adding Br,-PDA to test solutions m 0 25 M perchlorlc acid so that the concentration became 0 1 mM At room temperature the denvatlzatlon reaction was complete after 3 h and at 100°C after 5 mm [6] The SPE columns were condltloned as described above The derlvatlzed test solutions were passed through the columns at a flow-rate of 5 ml mm- ’ followed by 15 ml of 4 M perchlorlc acid The columns were allowed to dram and were then spmned for 2 mm to remove most of the hquld The adsorbed pmzselenol was eluted with 1 ml of toluene (flow-rate of 2 ml mm-‘) into a test-tube with a PTFE-faced screwcap containing 10 ~1 of the internal standard solution The toluene eluate was washed with 15 ml of M&-Q water and dried with anhydrous sodium sulphate prior to inJectIon on the gas chromatograph Effect of sample volume A stock standard solutlon containing 1 pg l- ’ of Se(W) m 0 25 M perchlorlc acid was prepared Standard solutions were prepared by diluting 5-ml portions of the stock standard solution to 25, 50 and 100 ml with 0 25 M perchlorlc acid The procedure from above was then followed As a reference, a 5-ml portion of the stock standard solution was delrvattzed and extracted with 1 ml of toluene + 10 ~1 of the internal standard solution The aqueous phase was discarded and the toluene phase was extracted with 15 ml of 4 M perchlorlc acid followed by 15 ml of Mllh-Q water Finally the
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Chm
Acta 274 (1993) 129-140
toluene phase was dned with anhydrous sodium sulphate prior to mJectlon Effect of sum&~ flow-rate 25-ml portions of a derlvatlzed standard solution contammg 200 ng I-’ of Se(W) were passed through a condltloned SPE column at different flow-rates The condltlons were otherwise the same as described above Eflect of elutzon flow-rate 25-ml portrons of the solution above [200 ng I-’ of derrvatlzed SeWI] were passed through a condltloned SPE cohunn at 5 ml mm-’ The flow-rate of toluene at the elutlon was varied, otherwise the condltlons were the same as above Elutlon profile of plazseienol from the SPE column 20 ml of a derwatlzed solutron contammg 10 CLg1-l of Se(W) were passed through a condltloned SPE column The standard procedure was followed except at the elutlon, which was performed as follows A total volume of 2 9 ml of toluene was passed through the column at a flow-rate of 2 ml mm-’ Fractions of approxlmately 0 5 ml were collected and dduted to 1 ml with toluene The first fraction was further dlluted 1 40 All volumes and dllutlons were controlled by welghmg Elut~on of excess of Br,-PDA 25 ml of denvatrzed standard solution, 160 ng 1-l of Se(W), were passed through a conditioned SPE column at a flow-rate of 5 ml mm- ’ followed by 15 ml of either 0 25, 2, 4, 6 or 8 M of perchlorlc acid Elutlon and analyses were performed accordmg to the standard procedure Pretreatment of natural water samples Natural water samples were collected as surface water from lakes and streams m the vlcmlty of Uppsala After flltratlon through 0 45-pm membrane filter, samples were stored refngerated m polyethylene bottles Prior to the analysis 1 or 2 1 portions of the samples were wlthdrawn, acidified to pH 2, spiked with selemte and deaerated with nitrogen Each sample was then spht mto three equal portlons of which two were subjected to the filtration procedures on XAD-8 and Dowex 1X8, respectively, as described above Flnally, the two effluents and untreated sample were analysed for Se(W) wth GC-ECD and HGAAS and m addltlon analysed for TOC
133
Determuuztwn of Se(N) with GC-ECD Sample preparatwn 25 ml of the pretreated sample and 0 5 ml of perchlorlc acid (70-72%) were added to a conical flask The flask was placed on an ordinary hot-plate and removed as soon as the sample started to boll Immediately 0 5 ml of 5 2 mM Br,-PDA was added and the mouth of the conical flask was covered with a watch-glass The plazselenol formatlon was complete after 5 mm and the sample was cooled to room temperature m a water bath Sold-phase extractron The sample was quantltatlvely transferred to the sample reservotr and the sample was passed through the condltloned column at a flow-rate of 5 ml mm- ’ When most of the sample had passed the column and the hquld level had reached the top of the sorbent, the stopcock was closed The sample reservoir was detached and 15 ml of 4 M perchlorlc acid were added The column was allowed to dram and then spmned for 2 mm to remove most of the hquld One ml of toluene was added to the column A syringe was used to supply the pressure needed for the toluene to pass through the column The effluent was collected m a test tube containing 10 ~1 of the internal standard solution and equipped wth a PTFE-faced screw-cap The toluene phase was washed with 15 ml of M&-Q water (rapid phase separation was obtamed by spmnmg the test-tube) and dried by adding anhydrous sodmm sulphate prior to mJectlon on the gas chromatograph Lzqud-lzqud extractton The extractlon was performed as described m Ref 6 Determmatwn of Se(IV,, wrth HG-AAS To 25 ml of the sample containing < 5 ng of selenium, 5 ml of deaerated 6 M hydrochloric acid were added The hydride trap was munersed mto the hqmd nitrogen and 30 s allowed for lowering of the temperature The penstaltlc pump was turned on When the sample followed by small portions of 1 M hydrochloric acid to rmse the tubmgs had been consumed after 3 mm, the pump was stopped After 10 s the hydnde was released by replacmg the liquid mtrogen with ethanol at - 15°C and the measurmg cycle of the spectrometer was mltlated The cold ethanol was
134
not removed until the whole peak had been monitored
RESULTS AND DISCUSSION
Selectwn of solui-phase extraction sorbent The selection of the SPE sorbent depends on
the apphcatlon The two, perhaps, most commonly used types of sorbents today are bondedphase silica and macroretlcular polymeric resms The bonded-phase sorbents based on silica partlcles have a limited workmg pH range, approxlmately between 2 and 7 5 Above pH 7 5, the slhca substrate IS susceptible to dlssolutlon m aqueous solutions Below pH 2 the sllyl ether linkage IS labile and the functional groups on the surface will begin to cleave, changmg the sorptlve properties m a non-reproducible fashion The polymeric resins, on the other hand, are almost unaffected by extremes of pH The formation of the plazselenol occurs m a reaction between undlssoclated selemous acid and the monoprotonated dlamme, and the denvatlzatlon should therefore be carried out at a pH where these species predommate When Se(IV) 1s derlvatlzed with Br,-PDA, the reaction is carried out m 0 25 M perchlorlc acid [6] This reaction medium makes polymeric resins best suited for the SPE The resin used, PD-lOZPE, 1s a porous dlvmylbenzene polymer It has a small particle aze, 10 pm, which gives good mass transfer properties, and a narrow size dlstrlbutlon so that no problems with backpressure were experienced No other sorbent was studled m detail Equipment used for the sobd-phase extractions
The prazselenol does not adsorb only on the resin (dlvmylbenzene polymer) used, but also on other common polymeric mater& Different materials were cut mto pieces and added to conical flasks together with an aqueous solution of derlvatlzed Se(IV) The flasks were shaken for 3 h and the solution analysed The results are presented m Table 1 The plazselenol was adsorbed on all the Investigated materials except PTFE and glass This makes the choice of material m
K Johansson et al /Anal
Chtm Acta 274 (1993) 129-140
TABLE 1 Adsorption of pmzselenol on different materials [Solutions of derwatlzed Se(W) were shaken together with different materials cut mto small pieces The determmatton of Se(W) was made after 3 h] Material
Recovery (%I
PVC Polyethylene Polypropylene Skone PTFE Glass
4 40 70 4 99 96
the extraction equipment important The equlpment 1s presented m Fig 1 The sample reservoir IS made of glass to reduce the risk of loss of plazselenol due to adsorption on the walls The column IS made of polypropylene However, when toluene was added to elute the adsorbed plazselenol from the sorbent, the wall of the column was also rmsed with the eluent Sohd-phase extraction of puzselenol The detection limit of a procedure
for trace determmatlon m water with a preconcentratlon step will depend on the enrichment factor Smce Se(IV) 1s often present at concentration below 100 ng I-’ 111natural waters [l-3], considerable preconcentratlon might be necessary from a large sample volume When SPE 1s used m trace analySIS it 1s important that the rate of the sorption process is also fast at low analyte concentrations If the sorption 1s slow, there should be a trend towards lower recoveries when the concentration of Se(IV) IS decreased Table 2 contains results on the SPE of 5 ng of Se@% derlvatlzed at different temperatures and m different sample volumes The results are presented as the ratlo, R, between the peak areas of the pmzselenol and the internal standard The data m Table 2 do not mdlcate a trend m recovery with sample volume In a prewous study [61 it was shown that the time for the derlvatlzatlon reaction could be reduced from 3 h to 5 mm by ralsmg the temperature to 100°C No adverse effects on the chromatograms were observed from the increase m derlvatlzatlon temperature To verify this obser-
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135
Glum Acta 274 (1993) 129-140
TABLE 2
TABLE 4
Effect of sample volume on &d-phase extraction of p~azselenol [5 ng of Se(N) were added to 5,2.5,50 and 100 ml of 0 25 M perchlonc acid and dertvatlzed at 22°C or 100°C by addmg Br,-PDA to a concentration of 0 1 mM The reference consisted of 5 ng of Se(W) denvatlzed m 5 ml of 0 25 M perchlonc acid and extracted with 1 ml of toluene R IS the ratio between the areas of the pmzselenol and the aldnn
Effect of flow-rate on the desorptlon of plazselenol [25 ml of a solution wntammg 200 ng I-’ of dematlzed Se(IV) were passed through an SPE column The flow-rate at the desorptlon of pmzselenol mth toluene was vaned R IS the ratio between the areas of the pmzselenol and the hndane
peaks1
5 25 50 100 Ref
[Se(IV)l
R
(ng l-9
22°C
100°C
1000 200 100 50 1000
1055,104l 1054,1036 0 982,1055 1048,1058 1098,1057
1081,1048 1075,1064 1098,1046 1010,1012 1081,1090
vatlon for SPE, the derlvatlzatlon was run at the two temperatures m parallel No adverse effects were observed on the chromatograms or the performance of the SPE column The values of R obtained with derlvatlzatlon at 22“C, R = 1041 f 0025, and lWC, R = 1054 f 0032 (mean f S D., it = 8), are not slgnlficantly different These values are, however, lower than for the reference, R = 1082 f 0 018 Since the recoveu of derwatrzed Se(W) at liquid-hquld extractlon was earher found to be nearly 100% [61, the difference m the R values indicates incomplete recovery of Se(IV1 v&h the SPE procedure Further, the mfluence of sample flow-rate on the recovery of derlvatlzed Se(W) was mvestlgated The results are presented m Table 3 No influence of flow-rate on the recovery could be TABLE 3 Effect of flow-rate on the adsorption of pmzselenol [25 ml of a solution containing 200 ng 1-l of denvatlzed Se(IV) were passed through an SPE column at dtierent flow-rates R IS the ratio between the areas of the pmzselenol and the aldrm peaks] Flow-rate (ml mm-‘)
R
18 42 50 61 75
1026,1089 1041,1058 1053,1063 1061,1075 1063,1075
?=&I Flow-rate (ml mm-‘)
R
03 1 13 3
1082 1051 1086 1066
established A flow-rate of 5 ml mm-l was used in most experunents The adsorbed prazselenol from the SPE column was eluted wrth toluene Toluene was successfully used m the liquid-liquid extraction of plazselenol [6] and was therefore chosen m the SPE No other organic solvents were mvestlgated The flow-rate of toluene does not effect the recuvery m the mvestlgated range, Table 4 The elutlon profile exhibits some tadmg, but when 2 9 ml had passed the column, approximately 98 5% of the plazselenol had been eluted m the first 0 6 ml of toluene, Table 5 One ml of toluene was added to the columns 111all expernnents and smce the void volume of the column was approxlmately 0 15 ml, 0 85 ml passed through the column TABLE 5 Elutton profile of desorbed pmzselenol [20 ml of a solution contammg 10 pg 1-l of denvatlzed se(IV) were passed through an SPE column The column was eluted ~th toluene and fractions of about 0 5 ml were collected and analysed R IS the ratio between the areas of the plazselenol and hndane peaks] Total eluate
R
058 1 13 158 199 244 2 93
49071a 0 330 0 154 0 114 0094 0 036
’ The first fraction was dduted 1 40 before mJe&on on the gas chromatograph The calculated area ratio IS compensated for the ddutlon
136
The reason for the use of two internal standards, hndane and aldrm, was due to the fact that m some expernnents a compound wtth a retentlon close to hndane appeared, wbch made the evaluation difficult The use of aldrm as internal standard, instead of hndane, solved the problem Aldrm IS not back-extracted by 15 ml of 4 M perchlonc acid followed by 15 ml of M&-Q water or 5 ml of 0 25 M perchlonc acid Later, the ongm of the interfering compound was fully mvestlgated and traced to contammated perchlonc acid Clean-up procedure zn the SPE method With hquld-liquid extra&on it has been reported that the plazselenol reaction yields byproducts, which together wth excess of reagent, result m chromatographlc peaks that may overlap the plazselenol peak [5,8,18] However, no mterference was observed when a capillary GC column was used [6] Pmzselenol and hndane were well separated from the byproducts and the reagent However, a clean-up step couId be meluded m order to mmmuze broad peaks from the reagent that appeared after the plazselenol peak [6] In the clean-up step, the toluene phase was washed once with 6 M perchlorlc acid The same reagent peaks appeared m the SPE method By treatmg the SPE cohnnn mth perchlonc acid before elutlon of the adsorbed plazselenol, these peaks were dnnmlshed The effect of the concentration of perchlorlc acid on both the hgand peaks and the plazselenol peak was investigated by addmg 0 25, 2, 4, 6 or 8 M perchlorlc acid to the column, Table 6 and Fig 2 A higher concentration of perchlorlc aad yielded a cleaner chromatogram but when the concentration exceeded 6 M, losses of plazselenol were observed This might be due to protonatlon of the plazselenol [191, changing the sorptlve properties Since the difference m the appearance of the chromatograms from columns treated with 4 or 6 M perchlorlc acid was small, 4 M perchlonc acid was used Calzbratzon of the SPE method In the hqmd-hqmd extraction procedure It was possible to use standard solutions of synthe-
K Johansson et al /Anal Chm Acta 274 (1993) 129-140 TABLE 6 Clean-up step unth perchlonc aad 125 ml of a solution contammg 160 ng I-’ of denvatlzed Se(W) were passed through an SPE cohnnn The column was washed mth 15 ml of a solution of percblonc acid before eMon R IS the ratio between the areas of the plazselenol and the lmdane peaks] HClO,
R
(MI 025 2 4 6 8
1011,1019 1005,1017 1046,1014 0 950,1066 0 740,O 676
sized plazselenol for cahbratlon purpose, since there was good agreement between cahbratlon graphs obtained from these solutions and denvatlzed standard solutions of Se(IV) [6] In the proposed SPE method the recovery of Se(IV) IS approximately %-98% mth the liquid-hquld extractlon method as reference, Fig 3 Consequently, the cahbratlon should be camed out with standards of Se(IV) treated the same way as the sample It should, however, also be possible to use standard solutions of synthesized pmzselenol for cahbratlon purpose if, m addition, one or two Se(IV) standards are analysed wth the same procedure as the samples The standards are evaluated Hnth the cahbration graph obtamed from the synthesrzed plazselenol and the recovery calculated The results from the samples are then corrected for the recovery Chozce of X4D reszn for removal of dzssolved organzc carbon The uptake of orgamc matter was followed by pumping the fresh water sample through a column of the resm and measurmg the absorbance at 420 nm This experiment was sumlar to that described by A&en et al [20] for XAD-1, 2, 4, 7 and 8 Accordmg to the authors, fulv~ aads are a major part of the coloured orgamc compounds present m natural waters They found that resms based on styrene dwmylbenzene copolymers (XAD-1, 2, 4 and 16) were less sultable than those based on acrylic ester polymers (XAD-7 and 8) for adsorptlon of fulvlc acids This was confirmed m our study The degree of adsorption
K Johansson et al /Anal
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Chm Acra 274 (1993) 129-140
and surface area on adsorption Only small dlfferences were noted between XAD-7 and XAD-8 The latter resin is known not to retain inorganic
Increased m the order XAD4, 16,2,7 and 8 The posltlon of XAD-16 IS m agreement with the dlscusslon m [20] on the mfluence of pore size
b
a
Lmdane
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Fu 2 Chromatograms of toluene eluates 25 ml of a derwatlzed standard soluUon, 160 ng I-’ of Se (IV), were passed through an SPE column followed by cleamng wtth 15 ml of (a) 0 25 M, (b) 2 M, (c) 4 M, (d) 6 M or (e) 8 M of perchlorlc acid In (a) the complete chromatogram IS shown and m the others only the analyttcally relevant parts
138
K Johansson et al /Anal Chm Acta 274 (1993) 129-140 TABLE 8
R
l5
Adsorption of coloured orgamcs and total dissolved orgamc matenal from water samples on Dowex 1 x8 and XAD-8 [Coloured orgamcs were determined by spectrophotometrlc measurements at 420 nm Sample volume 330 ml (670 ml for sample V)]
I
l-
Sample 05 -
(I) (II) (III) (IV)
4
0
4
2
6
wa
Fig 3 Cahbratlon graphs R ISthe ratio between the areas of the pmzselenol and the aldrm peaks Results from (01 synthesized pmzselenol standards and (0) 25 ml of denvatlzed selenmm(IV) solutions passed through an SPE column
selenium [11,13] This was found to be the case also for XAD-7 at pg 1-l and ng 1-l levels (Table 7) XAD-8 was used m the subsequent filtration expernnents since It 1s reported [20] not to suffer from bleeding as XAD-7 does Blank levels of TOC were low, < 0 5 mg I-’ and 0 8 mg 1-l for XAD-8 and Dowex 1X8, respectively, usmg a 500~ml solution Results from absorption measurements and TOC analysis of column effluents are presented m Table 8 The percentage of total organic carbon removed hardly exceeds 50%, whereas coloured materral 1s adsorbed more effectively A study of the behavlour of morgamc Se(W) and Se(W) on Dowex 1X8 IS m progress m our laboratory Under the conditions used m this work, 1e , addltron of hydrochloric acid to
Lafssjon SavJa&n Ramsen Trehornmgen (V) Flottsund
TOC
XAD-8
Dowex 1X8
(mgl-l)
Coloured (%)
TOC (o/o)
Coloured (%o)
TOC (%)
26 1 174 13 3
89 85 86
61 52 11
69 93 96
52 45 47
15 1 15 6
83 84
16 49
94 92
46 61
0 025 M, Se(W) passes through the column while Se(VI) 1s retained Our results Indicate that the ion-exchange resin 1s as effective as XAD-8 for removal of coloured orgamcs and other dissolved orgamc compounds It can thus be an attractive alternative to XAD-7 and 8 since Se(IV) and (VI) can be separated at the same tune as organic matter 1s removed Detemunutwn of Se(N) m spiked water The five fresh water samples were not acidified on storage Speclatlon changes and adsorption losses are therefore hkely to have occurred [21] The decrease m Se(N) concentration for sample I after three months of storage was approximately 30% (Tables 9 and 10) Prehmmary measurements mdlcated that the concentrations of Se(IV) were Just a few nanograms per litre
TABLE 7 Recovery of morgamc selenium (average and standard devlatlon) m 0 01 M hydrochlonc acid afier filtration through XAD-7 and XAD-8 (250 ml of a solution Hrlth morgamc selenmm were passed through a 75 ml bed of XAD The effluent was collected as 25 ml fractions and analysed for selenium usmg HG-AAS) XAD-7
XAD-8
Se(W) [Sel/(Lcg 1-l) Recovery (%) Fractions analysed
200 94 f 4 9
SeWI) 0040 96k5 5
200 101 f 3 9
SeW) 0060 106*10 5
200 lOOf 9
SeWI) 0040 104*3 7
200 102*2 8
0080 102+6 7
K Johansson et al /And
Chum Acta 274 (1993) 129-140
139
TABLE 9 Determmatlon of Se(W) m water samples wth GC-ECD [Average and standard devlatlon (ng I-‘) from trlphcate analysis SPE = Soltd-phase extraction, Extr = hqmd-hqmd extra&on] Sample
(I) LafssJon (II) SavJakn (III) Ramsen (IV) Trehommgen (V) Flottsund
Added Se(IV) (ng 1-l)
No pretreatment
XAD-8
SPE
Extr
SPE
Extr
SPE
Extr
80 a 60 40 20 0
54 f2 57 fl 37 f2 18 &2 26kO3
54 *3 59 *2 41 *1 19 *1 33kO6
54* 1 58 f 2 39 f 3 19f2 28
54 *2 56 k2 41 *1 21 *1 42+_01
53 *2 57 f4 42 k3 18 *1 31*07
57 *1 55 fl 42 fl 200*02 38+02
Dowex 1X8
a Se(W) added 3 months poor to analysis
The samples were therefore spiked vvlth Se(IV) to concentration levels, where differences between methods and treatments could be better detected Analysis were carried out vvlthm 24 h after splkmg Determmatlons with the GC-ECD method and the HG-AAS method were performed by different persons GC-ECD The water samples were analysed by the SPE and the liquid-liquid extractlon methods and the results are presented m Table 9 There 1s no srgmficant difference (p = 0 05) m the Se(W) concentrations or m the precision between filtered or unfiltered samples Selemum(IV) concentrations obtained vvlth the two extraction methods mdlcate a somewhat higher mean value for the hqurd-liquid extraction procedure but this difference 1s not sign&ant The major effect of the filtration of the water through XAD-8 or Dowex 1X8 was the faster and more distinct phase separation m the liquid-llqmd extraction procedure In the solid-phase extractions the same
columns were used for samples that had been treated m the same way, e g , all unfiltered samples passed through the same SPE columns The regeneration of the columns appeared to work effectively No accumulation of adsorbed coloured organic mate& was observed on either column and no deterroratlon 111performance was noted after 15 loadings Consequently, there appears to be no need to prefilter the samples through an adsorbing resm 111the SPE method iYG-AAS Standard additions of Se(W) were made to the unsplked waters and the slopes of the standard additions graphs were compared with the sensltlvlty determmed from standards No differences were found and subsequently the cahbratlon graph was used m the evaluation of the Se(W) concentrations The results are given m Table 10 &g&cant higher results (p = 0 05) were noted for subsamples filtered through Dowex 1X8 despite the fact that this was not the general observation Evaluations from peak height
TABLE 10 Determmatlon of Se(W) m water samples urrlthHG-AAS [Average and standard devlatlon (ng 1-l) from trlphcate analysis ND = Not detected1 Sample
(I) (II) (III) (IV) (V)
LafssJon
SavJain Ramsen Trehommgen Flottsund
Added Se(W) (ng 1-l)
80 a 60 40 20 0
* SeGV) added 3 months poor to analysis
No pretreatment
XAD-8
Peak area
Peak height
Peak area
Peak height
Peak area
Peak height
59*1 62 f 4 43 f 2 23*3 ND
55 *2 61 k4 43 *1 208*04 1 *1
59 f 5 58 f 6 44rtl 21 f 2 ND
56 &2 58 k2 44 *1 211*05 2 *1
58+2 65 f 4 47 f 3 23i4 31t2
57 f3 66 *2 47 fl 217*04 45*03
Dowex 1X8
140
or peak area gave the same results, mdlcatmg the absence of volatde compounds causing spectral interferences [7] Analysts of variance was performed on the results m Tables 9 and 10 There 1s a small but slgmficant difference (p = 0 05) between the results obtained wth the two methods The over-all mean was 37 4 ng 1-l with the HG-AAS method compared to 34 8 for the GC-ECD method It 1s noticeable that the analysis by HG-AAS generally gave a Se(W) concentration higher than the spiked level of the samples while the results obtamed with the GC-ECD method were somewhat lower The reason for this difference 1s not known to the authors conclusions Solid-phase extraction usmg a polymeric sorbent can replace the hquld-liquid extraction of the plazselenol m the determmatlon of Se(W) m natural waters with GC-ECD The SPE 1s less tune-consummg than the hqmd-liquid extractlon and makes it possible to run several samples simultaneously Problems with emulsion formation and/or preclpltatlon of organic substances m the boundary layer between the two phases that sometimes occur m liquid-hqmd extraction, are not present m SPE Removal of dissolved organic material 1s most effective on XAD-7 or XAD-8 which m addition do not retam morgamc selemum The use of Dowex 1X8 allows a separation of morgamc Se(N) and Se(W) and it adsorbs dissolved organlcs as effectively as the XAD resins Concordant result were obtamed between GC-ECD and HGAAS for fresh water samples spiked with Se(W) at the ng 1-l level
K Johansson et al /Anal Chtm Acta 274 (1993) 129-140
Thanks are due to Dynochrom Sweden Inc for supplymg the solid-phase extraction resm Thus work has partly been supported by the Swedish Environmental ProtectIon Board
REFERENCES 1 G A Cutter, Anal Chum Acta, 98 (1978) 59 2 H Uchlda, Y Shlmolshl and K. T&I, Enwron Scl Techno1 , 14 (1980) 541 3 D Tanzer and KG Heumann, Anal Chem, 63 (1991) 1984 S Nakashuna and K T&I, Talanta, 15 (1968) 1475 Y Shlmonhl, J Chromatogr , 136 (1977) 85 K Johansson and 8, Olm, J Chromatogr ,589 (1992) 105 U Omemark, J Pettersson and k Olin, Talanta, 39 (1992) 1089 8 Y Shlmolshl and K T&l, Anal Chun Acta, 100 (1978) 65 9 J Pettersson, L Hansson and A Olin, Talanta, 33 (1986) 249 10 V E Negrettl de Bratter, P Bratter and A Tom& J Trace Elem Electrolytes Health DIS ,4 (1990) 41 11 D R Roden and DE Tallman, Anal Chem, 54 (1982) 307 12 S M Workman and P N Soltanpour, Sol1 Sa Sot Am J , 44 (1980) 1331 13 J L Flo and R Fu~n, Sod SCI Sot Am J , 54 (1990) 363 14 MM Abrams and R G Burau, Commun Sod Sa Plant Anal, 20 (1989) 221 15 G A Cutter, Anal Chem , 57 (1985) 2951 16 T A Bengtsson, unpubhshed results 17 E S Gould, Anal Chem ,23 (1951) 1502 18 T StlvJe and G Phthpposslan, Trav Chum Ahment Hyg , 69 (1978) 74 19 J N&e, M Hanocq and L Moue, Mlcrochun Acta, 1 (1980) 41 20 G R A&en, EM Thurman and RL Malcolm, Anal Chem , 51 (1979) 1799 21 GA Cutter, Report EPRI EA-4641, Volume 1, June 1986
An&rca ChunacaAcra, 274 (1993) 129-140 Elsevler Science Pubhshers B V , Amsterdam
Solid-phase extraction procedure for the determination of selenium by capillary gas chromatography K. Johansson, U Omemark and A Olm Departmenr of Analytical Chemrsstry,Utuversuy of Uppsala, P 0 Box 531, S-751 21 Uppsala (Sweden) (Rece.wed 6th July 1992)
Ahstraet A solid-phase extractlon procedure to be used m the determmatlon of selenium by gas chromatography wth electron-capture detectlon (GC-ECD) was investigated After denvatlzatlon of Se(N) with 1,2-dlammo-3,5-dlbromobenzene the formed 4,6-d~bromoplazselenol was adsorbed on a porous dwmylbenzene polymer Elutlon was performed wth toluene The proposed solid-phase extraction procedure was compared with a hqmd-hqmd extractlon procedure and the overall effkxency was appromately 97% for sample volumes between 5 and 100 ml Concordant results were obtamed for natural water samples sptked with selemum between the GC-ECD procedure and a hydnde generatIon atonuc absorption spectrometnc procedure Removal of &solved organic carbon m natural waters on XAD-8 and Dowex 1X8 pnor to selenmm determmatlon was also studied Keywords Atomic absorption SpectrometIy, Gas chromatography, Selenium, Sohd-phase extraction, Waters
The low levels of Se(W) present in natural waters [l-3] require analytical procedures havmg good accuracy at the ng 1-l level Gas chromatography urlth electron capture detection (GC-ECD) [4-61 and hydride generation atomic absorption spectrometry (HG-AAS) [1,7] m conlunctlon with appropriate preconcentratlon steps are both smtable techniques for such deterrmnatlons The determmatlon of Se(W) m water by GCECD 1s based on the formatlon of a volatile compound, a pmzselenol, m the reaction between selemous acid and a subsmuted o-phenylenedlamme The plazselenol is normally extracted mto an orgamc solvent before mjectlon mto the gas chromatograph The extraction procedure makes it possible to preconcentrate the sample
Correspondence to K Johansson, Department of AnalytIcal ChemlstIy, Umverslty of Uppsala, PO Box 531, S-751 21 Uppsala (Sweden)
up to 500 tunes [S], leadmg to a detection hmlt below 1 ng l- ’ of Se(W) m the sample In the HG-MS method, Se(W) 1s reduced with sodium tetrahydroborate to volatde hydrogen selerude, which 1s collected at hquld nitrogen temperature The preconcentrated hydrogen selemde 1s vaporized and selemum determmed by AAS The hrmt of detection is 1 to 2 orders of magnitude lower than for systems wrthout preconcentration [9,103 Both methods may be SubJect to interferences from dissolved organic material In the HG-AAS method, Se(W) and/or SeWI) added to ground water [ll], sod extracts [12-141 and extracts of blogemc material [15] have been lost The mterference 1s removed by wet digestion [11,12,15] but this prevents speclatlon Another solution to this problem 1s to remove the mterferent(s) on a sorbent before selemum determmatlon Roden and Tallman [ill demonstrated that the mterferent, probably an orgamc constrtuent, could be
OCKI3-x70/93/$06 00 0 1993 - Elsevler Science Pubhshers B V All nghts reserved
130
reversibly bound on the macroretlcular polymeric resin XAD-8, while morgamc Se(IV) and Se(V1) were unretamed As reported by several authors [13-151, this sorbent will not always remove the interference completely In the GC-ECD method with liquid-hquld extractlon, the mam problem connected with the organic material is emulsion formatlon, makmg the phase separation time-consummg if not lmpossible The interferences from organic material can be reduced by extracting the sample with an organic solvent or fdtermg it through a sorbent prior to the derlvatlzatlon and hquld-hquld extraction A sorbent can thus be used as a clean-up step m the GC-ECD method However, It should also be possible to adsorb and collect the plazselenol from the aqueous phase This techmque 1s sometnnes referred to as sohd-phase extraction (SPE) SPE can be used to separate analyte and concomitants by adsorbing the former while the latter are unretamed and 1s a special case of adsorption chromatography In trace analysis, where SPE 1s used to accumulate the analyte, the dlstrlbutlon coefficient should be large for the analyte It IS also important that the adsorptlon capacity of the column 1s adequate, smce the sample may contam other substances, that are also retamed Ideally, the analyte should be adsorbed as a narrow band at the top of the column, because then a short column can be used and the elutlon made m a small volume This paper reports on mvestlgatlons of a SPE method m trace analysis of Se(IV) m water with GC-ECD We also report on determmatlons with GC-ECD and HG-AAS of Se(IV) added to natural water, treated by flltratlon through XAD-8 or Dowex 1X8
EXPERIMENTAL
Apparatus For the gas chromatographlc measurements a Shunadzu GC-14A gas chromatograph equipped with a constant current @j3N1 electron-capture detector was used The column was DB-1701 (15 m x 0 25 mm 1d > with a 0 25-pm film thickness (J&W Saentlflc) The chromatographlc condo-
K_ Johansson et al /Anal.
Chm Acta 274 (1993) 129-140
tlons were carrier gas, helium at 40 cm s-l, split vent, 60 ml mm-l, purge vent, 1 ml nun-l, makeup gas, nitrogen at 45 ml mm-‘, injector temperature, 25O”C, detector temperature, 325”C, and column temperature programme, 100°C held for 2 mm, increased at 8°C mm-l to 175”C, held for 5 mm, Increased at 15°C nun-l to 265”C, held for 5 mm The injections (0 5 111sample + 1 ~1 toluene) were made m the sphtless mode with a Shunadzu AOC-14 automatic Injector, with a change to the split mode after 1 mm The peak areas were evaluated with a Shunadzu C-R5A integrator The HG-AAS apparatus with a cold trap 1s described elsewhere [7] Hydride generation 1s accomphshed by mnung the acidified sample with sodium tetrahydroborate m a flow system The hydnde 1s stripped from the aqueous phase and transported with nitrogen gas Four glass U-tubes m series are used for gas-hquld separation, removal of water vapour and trapping of the hydride Atomic absorption measurements were made with a Perkm-Elmer 3100 spectrometer equipped with the MHS-20 oven and contammg a quartz cuvette at 8OOYZ The hght source, an electrodeless discharge lamp, was operated at 7 6 W The analytical wavelength was 196 0 nm and the spectral band-width 0 7 mn Evaluation of peak areas and peak heights was made with the Model 3100 EDS software supphed by the manufacturer No background correction was applied Determmatlons m the l-10 pg 1-l range were performed with the HG-AAS mstrumentatlon described by Pettersson et al [9] A Phdlps PU 8625 spectrophotometer equipped with a 40-mm flow through cell, a U4XV peristaltic pump (Ahtea AB) and a glass column, bed volume 1 1 ml, (Kabl-Pharmaaa) were used m the study of the XAD-resins The absorbance was measured at 420 nm Total dissolved organic carbon (TO0 m the water samples was determmed on a Shunadzu 500 TOC analyzer by the National Envlromnental Protection Agency in Uppsala The equipment used for SPE is presented in Fig 1 Frve of these devices were connected m parallel to a water aspirator The column (mner diameter 3/8”, volume 4 ml) was made of
K Johansson et al /Anal
Chm Acta 274 (1993) 129-140
glasssample reservou
polypropylene column pm polyethylene fnts sorbent
her lock needle
Fig 1 Equpment
used for sohd-phase extra&on
polypropylene and contained two 20qm pobethylene fnts The stopcock and adaptor were pu chased from Analyttchem (Harbour City, CAF The sample reservou was made of glass The ion-exchanger tubes were made from Pyrex, length 200 mm, 1 d 8 mm and provided with a cyhndncal funnel, length 70 mm and 1 d 42 mm, at the top Flow was controlled by a clamp around a small piece of PVC tubmg connected to the l-mm drawn out tip These tubes were also used m the recovery expernnents for morgamc selenium on XAD-7 and XADS Chemzcals and reagents All chermcals were of analytical grade from Merck unless othemse stated Demmerahzed, destllled water passed through a M&pore 3 filter cartridge system, Mdh-Q water, was used throughout All glassware was cleaned m 4 M mtnc acid and rinsed with Mdh-Q water 1,2-Dlammo-3,Idrbromobenzene (Br,-PDA) A 5 5 m.M solution of Br,-PDA m 0 5 M perchlonc acid was prepared The solution was punfled by extraction with toluene and should preferably be
131
kept m the dark When 100 ml were extracted Hrlth 5 ml of toluene the Br,-PDA concentration dropped to about 5 2 mM Selenzum(N) A stock standard solution, 1 g 1-l of Se(N), was prepared from an ampoule of selennun dloxlde m dilute nitric acid Its concentration was checked by an amperometrlc titration with thlosulphate after addition of a large excess of iodide [16] Workmg standard solutions were obtained by senal dllutlon of the stock standard solution and contamed 2 ml 1-l of hydrochlonc acid (37%, w/w) Internal standard solutwn Thrs solution was prepared by dlssolvmg lmdane (99%) and aldrm (99%>, both purchased from Applied Science Labs (State College, PA), m toluene The workmg solution contained 2 0 mg 1-l of hndane and 26mgl-‘ofaldrm Lmdane and aldrm are msectlcldes and precautions should be taken to avoid mhalatlon and skm contact 4,6-D&omopuzzselenoi This was synthesized according to the literature [51 The synthesized plazselenol was analysed for selenium after wet digestion according to a variation of Gould’s method 1171 usmg a 10 1 (v/v) murture of concentrated sulphunc acid and fummg rutnc acid by the amperometnc titration method mentioned above The purity was 99 8 f 0 4% A stock standard solution contauung 0 5 g I- ’ of plazselenol was prepared by dlssolvmg plazselenol m toluene Workmg standard solutions were prepared by serial dllutlon with toluene &&urn tetrahydroborate Sodium tetrahydroborate, 0 5% (w/v), was stabilized and purrfled as described elsewhere [7] Hydrochlonc aczd HCl, 37% (w/w), used m the cold trap system was diluted 1 1 and flushed with nitrogen prior to use Resins Sold-phase extractron resm The resin was a porous dlvmylbenzene polymer (PD-lOZPE), 10 pm m diameter, with a narrow particle size dlstnbutlon It was supplied by Dyno Particles (Lrllestrom) as a 10% (v/v) suspension m methanol The resin was not further purified The resm bed was packed as follows, see also Fig 1 The upper
132
75ml glass reservoir was detached, the stopcock closed and the fret at the bottom inserted Smce the particle size of the resin was 10 pm, a glassflbre filter with a pore diameter of about 2 pm was mserted above the fret The resm was added as a methanol slurry until the settled bed was approxnnately 4 mm Then the second fret was inserted The column was condltloned by drammg the methanol followed by 5 ml of 0 25 M perchlonc acid The stopcock was closed when about 2 ml of perchlorlc acid were left above the upper fnt, then the sample reservoir was attached The column was effectively regenerated by passing 5 ml of methanol-pcrchlonc acid (7072%) (98 2) through the column When not used, the column was filled with methanol and stored closed Zen-ex&ange resin Dowex 1X8, (100-200 mesh analytical grade, Serva) was purchased from LabKern (Stockholm) Fmes were removed by decantatlon and the resin (100 g) was placed m a glass filter funnel and washed wth 200 ml of 1 M sodmm hydroxide added m portlons followed by Ml&Q water to neutral ConversIon to the chloride form was achieved by passing 200 ml of 1 M hydrochlortc acid followed by M&-Q water When no detectable chlonde was found m the effluent the resm was dried over mght at 50°C and then condltloned over calcium chloride hexahydrate and its saturated solution A 2-g amount of the condltloned resm (dry weight) was slurrled m a few ml of Mllh-Q water and packed between plugs of quartz wool The resm was washed with 10 ml of 1 M hydrochloric acid followed by M&-Q water to approxnnately neutral effluent Finally 25 ml of 0 025 M hydrochloric acid were passed through the column The solution to be filtered was made 0 025 M with respect to hydrochlonc acid and transferred to a volumetric flask The flask was mounted upside down m the cyhndrlcal funnel of the exchanger tube The flow was adJUSted to about 2 ml mm-’ The effluent was collected and analysed for Se(N) and TOC XAD realm XAD-2 (0 3-l mm anal gr 1, XAD4 (20-50 mesh pract ), XAD-7 (20-50 mesh pract >, XAD-8 (0 3-l mm pract ), XAD-16 (1380 mesh pract ) (Serva), were purchased from Techtum (Umefl) Fines were removed by de-
K Johanmon et al /Anal Chm Acta 274 (1993) 129-140
cantatlon The resm (100 g) was then slurrled m 50 ml of methanol followed by addltlon of 50 ml of water After deaeratlon with mtrogen gas the resin was stored refrigerated A 7 5-ml column was prepared between plugs of quartz wool from the resin m the methanol-water (1 1) mixture After washing of the resin with Mllh-Q water to remove all methanol, 50 ml of 0 01 M hydrochlonc acid were passed through the column The solution to be filtered was made 0 01 M with respect to hydrochloric aad and passed through the column as described above at about 1 ml mm-’ S&d-phase extractionof puzzselenol Standard procedure for the extractrons The Se(N) was derlvatlzed at room temperature or 100°C by adding Br,-PDA to test solutions m 0 25 M perchlorlc acid so that the concentration became 0 1 mM At room temperature the denvatlzatlon reaction was complete after 3 h and at 100°C after 5 mm [6] The SPE columns were condltloned as described above The derlvatlzed test solutions were passed through the columns at a flow-rate of 5 ml mm- ’ followed by 15 ml of 4 M perchlorlc acid The columns were allowed to dram and were then spmned for 2 mm to remove most of the hquld The adsorbed pmzselenol was eluted with 1 ml of toluene (flow-rate of 2 ml mm-‘) into a test-tube with a PTFE-faced screwcap containing 10 ~1 of the internal standard solution The toluene eluate was washed with 15 ml of M&-Q water and dried with anhydrous sodium sulphate prior to inJectIon on the gas chromatograph Effect of sample volume A stock standard solutlon containing 1 pg l- ’ of Se(W) m 0 25 M perchlorlc acid was prepared Standard solutions were prepared by diluting 5-ml portions of the stock standard solution to 25, 50 and 100 ml with 0 25 M perchlorlc acid The procedure from above was then followed As a reference, a 5-ml portion of the stock standard solution was delrvattzed and extracted with 1 ml of toluene + 10 ~1 of the internal standard solution The aqueous phase was discarded and the toluene phase was extracted with 15 ml of 4 M perchlorlc acid followed by 15 ml of Mllh-Q water Finally the
K Johansson et al /Anal
Chm
Acta 274 (1993) 129-140
toluene phase was dned with anhydrous sodium sulphate prior to mJectlon Effect of sum&~ flow-rate 25-ml portions of a derlvatlzed standard solution contammg 200 ng I-’ of Se(W) were passed through a condltloned SPE column at different flow-rates The condltlons were otherwise the same as described above Eflect of elutzon flow-rate 25-ml portrons of the solution above [200 ng I-’ of derrvatlzed SeWI] were passed through a condltloned SPE cohunn at 5 ml mm-’ The flow-rate of toluene at the elutlon was varied, otherwise the condltlons were the same as above Elutlon profile of plazseienol from the SPE column 20 ml of a derwatlzed solutron contammg 10 CLg1-l of Se(W) were passed through a condltloned SPE column The standard procedure was followed except at the elutlon, which was performed as follows A total volume of 2 9 ml of toluene was passed through the column at a flow-rate of 2 ml mm-’ Fractions of approxlmately 0 5 ml were collected and dduted to 1 ml with toluene The first fraction was further dlluted 1 40 All volumes and dllutlons were controlled by welghmg Elut~on of excess of Br,-PDA 25 ml of denvatrzed standard solution, 160 ng 1-l of Se(W), were passed through a conditioned SPE column at a flow-rate of 5 ml mm- ’ followed by 15 ml of either 0 25, 2, 4, 6 or 8 M of perchlorlc acid Elutlon and analyses were performed accordmg to the standard procedure Pretreatment of natural water samples Natural water samples were collected as surface water from lakes and streams m the vlcmlty of Uppsala After flltratlon through 0 45-pm membrane filter, samples were stored refngerated m polyethylene bottles Prior to the analysis 1 or 2 1 portions of the samples were wlthdrawn, acidified to pH 2, spiked with selemte and deaerated with nitrogen Each sample was then spht mto three equal portlons of which two were subjected to the filtration procedures on XAD-8 and Dowex 1X8, respectively, as described above Flnally, the two effluents and untreated sample were analysed for Se(W) wth GC-ECD and HGAAS and m addltlon analysed for TOC
133
Determuuztwn of Se(N) with GC-ECD Sample preparatwn 25 ml of the pretreated sample and 0 5 ml of perchlorlc acid (70-72%) were added to a conical flask The flask was placed on an ordinary hot-plate and removed as soon as the sample started to boll Immediately 0 5 ml of 5 2 mM Br,-PDA was added and the mouth of the conical flask was covered with a watch-glass The plazselenol formatlon was complete after 5 mm and the sample was cooled to room temperature m a water bath Sold-phase extractron The sample was quantltatlvely transferred to the sample reservotr and the sample was passed through the condltloned column at a flow-rate of 5 ml mm- ’ When most of the sample had passed the column and the hquld level had reached the top of the sorbent, the stopcock was closed The sample reservoir was detached and 15 ml of 4 M perchlorlc acid were added The column was allowed to dram and then spmned for 2 mm to remove most of the hquld One ml of toluene was added to the column A syringe was used to supply the pressure needed for the toluene to pass through the column The effluent was collected m a test tube containing 10 ~1 of the internal standard solution and equipped wth a PTFE-faced screw-cap The toluene phase was washed with 15 ml of M&-Q water (rapid phase separation was obtamed by spmnmg the test-tube) and dried by adding anhydrous sodmm sulphate prior to mJectlon on the gas chromatograph Lzqud-lzqud extractton The extractlon was performed as described m Ref 6 Determmatwn of Se(IV,, wrth HG-AAS To 25 ml of the sample containing < 5 ng of selenium, 5 ml of deaerated 6 M hydrochloric acid were added The hydride trap was munersed mto the hqmd nitrogen and 30 s allowed for lowering of the temperature The penstaltlc pump was turned on When the sample followed by small portions of 1 M hydrochloric acid to rmse the tubmgs had been consumed after 3 mm, the pump was stopped After 10 s the hydnde was released by replacmg the liquid mtrogen with ethanol at - 15°C and the measurmg cycle of the spectrometer was mltlated The cold ethanol was
134
not removed until the whole peak had been monitored
RESULTS AND DISCUSSION
Selectwn of solui-phase extraction sorbent The selection of the SPE sorbent depends on
the apphcatlon The two, perhaps, most commonly used types of sorbents today are bondedphase silica and macroretlcular polymeric resms The bonded-phase sorbents based on silica partlcles have a limited workmg pH range, approxlmately between 2 and 7 5 Above pH 7 5, the slhca substrate IS susceptible to dlssolutlon m aqueous solutions Below pH 2 the sllyl ether linkage IS labile and the functional groups on the surface will begin to cleave, changmg the sorptlve properties m a non-reproducible fashion The polymeric resins, on the other hand, are almost unaffected by extremes of pH The formation of the plazselenol occurs m a reaction between undlssoclated selemous acid and the monoprotonated dlamme, and the denvatlzatlon should therefore be carried out at a pH where these species predommate When Se(IV) 1s derlvatlzed with Br,-PDA, the reaction is carried out m 0 25 M perchlorlc acid [6] This reaction medium makes polymeric resins best suited for the SPE The resin used, PD-lOZPE, 1s a porous dlvmylbenzene polymer It has a small particle aze, 10 pm, which gives good mass transfer properties, and a narrow size dlstrlbutlon so that no problems with backpressure were experienced No other sorbent was studled m detail Equipment used for the sobd-phase extractions
The prazselenol does not adsorb only on the resin (dlvmylbenzene polymer) used, but also on other common polymeric mater& Different materials were cut mto pieces and added to conical flasks together with an aqueous solution of derlvatlzed Se(IV) The flasks were shaken for 3 h and the solution analysed The results are presented m Table 1 The plazselenol was adsorbed on all the Investigated materials except PTFE and glass This makes the choice of material m
K Johansson et al /Anal
Chtm Acta 274 (1993) 129-140
TABLE 1 Adsorption of pmzselenol on different materials [Solutions of derwatlzed Se(W) were shaken together with different materials cut mto small pieces The determmatton of Se(W) was made after 3 h] Material
Recovery (%I
PVC Polyethylene Polypropylene Skone PTFE Glass
4 40 70 4 99 96
the extraction equipment important The equlpment 1s presented m Fig 1 The sample reservoir IS made of glass to reduce the risk of loss of plazselenol due to adsorption on the walls The column IS made of polypropylene However, when toluene was added to elute the adsorbed plazselenol from the sorbent, the wall of the column was also rmsed with the eluent Sohd-phase extraction of puzselenol The detection limit of a procedure
for trace determmatlon m water with a preconcentratlon step will depend on the enrichment factor Smce Se(IV) 1s often present at concentration below 100 ng I-’ 111natural waters [l-3], considerable preconcentratlon might be necessary from a large sample volume When SPE 1s used m trace analySIS it 1s important that the rate of the sorption process is also fast at low analyte concentrations If the sorption 1s slow, there should be a trend towards lower recoveries when the concentration of Se(IV) IS decreased Table 2 contains results on the SPE of 5 ng of Se@% derlvatlzed at different temperatures and m different sample volumes The results are presented as the ratlo, R, between the peak areas of the pmzselenol and the internal standard The data m Table 2 do not mdlcate a trend m recovery with sample volume In a prewous study [61 it was shown that the time for the derlvatlzatlon reaction could be reduced from 3 h to 5 mm by ralsmg the temperature to 100°C No adverse effects on the chromatograms were observed from the increase m derlvatlzatlon temperature To verify this obser-
R Johansson et al /Anal
135
Glum Acta 274 (1993) 129-140
TABLE 2
TABLE 4
Effect of sample volume on &d-phase extraction of p~azselenol [5 ng of Se(N) were added to 5,2.5,50 and 100 ml of 0 25 M perchlonc acid and dertvatlzed at 22°C or 100°C by addmg Br,-PDA to a concentration of 0 1 mM The reference consisted of 5 ng of Se(W) denvatlzed m 5 ml of 0 25 M perchlonc acid and extracted with 1 ml of toluene R IS the ratio between the areas of the pmzselenol and the aldnn
Effect of flow-rate on the desorptlon of plazselenol [25 ml of a solution wntammg 200 ng I-’ of dematlzed Se(IV) were passed through an SPE column The flow-rate at the desorptlon of pmzselenol mth toluene was vaned R IS the ratio between the areas of the pmzselenol and the hndane
peaks1
5 25 50 100 Ref
[Se(IV)l
R
(ng l-9
22°C
100°C
1000 200 100 50 1000
1055,104l 1054,1036 0 982,1055 1048,1058 1098,1057
1081,1048 1075,1064 1098,1046 1010,1012 1081,1090
vatlon for SPE, the derlvatlzatlon was run at the two temperatures m parallel No adverse effects were observed on the chromatograms or the performance of the SPE column The values of R obtained with derlvatlzatlon at 22“C, R = 1041 f 0025, and lWC, R = 1054 f 0032 (mean f S D., it = 8), are not slgnlficantly different These values are, however, lower than for the reference, R = 1082 f 0 018 Since the recoveu of derwatrzed Se(W) at liquid-hquld extractlon was earher found to be nearly 100% [61, the difference m the R values indicates incomplete recovery of Se(IV1 v&h the SPE procedure Further, the mfluence of sample flow-rate on the recovery of derlvatlzed Se(W) was mvestlgated The results are presented m Table 3 No influence of flow-rate on the recovery could be TABLE 3 Effect of flow-rate on the adsorption of pmzselenol [25 ml of a solution containing 200 ng 1-l of denvatlzed Se(IV) were passed through an SPE column at dtierent flow-rates R IS the ratio between the areas of the pmzselenol and the aldrm peaks] Flow-rate (ml mm-‘)
R
18 42 50 61 75
1026,1089 1041,1058 1053,1063 1061,1075 1063,1075
?=&I Flow-rate (ml mm-‘)
R
03 1 13 3
1082 1051 1086 1066
established A flow-rate of 5 ml mm-l was used in most experunents The adsorbed prazselenol from the SPE column was eluted wrth toluene Toluene was successfully used m the liquid-liquid extraction of plazselenol [6] and was therefore chosen m the SPE No other organic solvents were mvestlgated The flow-rate of toluene does not effect the recuvery m the mvestlgated range, Table 4 The elutlon profile exhibits some tadmg, but when 2 9 ml had passed the column, approximately 98 5% of the plazselenol had been eluted m the first 0 6 ml of toluene, Table 5 One ml of toluene was added to the columns 111all expernnents and smce the void volume of the column was approxlmately 0 15 ml, 0 85 ml passed through the column TABLE 5 Elutton profile of desorbed pmzselenol [20 ml of a solution contammg 10 pg 1-l of denvatlzed se(IV) were passed through an SPE column The column was eluted ~th toluene and fractions of about 0 5 ml were collected and analysed R IS the ratio between the areas of the plazselenol and hndane peaks] Total eluate
R
058 1 13 158 199 244 2 93
49071a 0 330 0 154 0 114 0094 0 036
’ The first fraction was dduted 1 40 before mJe&on on the gas chromatograph The calculated area ratio IS compensated for the ddutlon
136
The reason for the use of two internal standards, hndane and aldrm, was due to the fact that m some expernnents a compound wtth a retentlon close to hndane appeared, wbch made the evaluation difficult The use of aldrm as internal standard, instead of hndane, solved the problem Aldrm IS not back-extracted by 15 ml of 4 M perchlonc acid followed by 15 ml of M&-Q water or 5 ml of 0 25 M perchlonc acid Later, the ongm of the interfering compound was fully mvestlgated and traced to contammated perchlonc acid Clean-up procedure zn the SPE method With hquld-liquid extra&on it has been reported that the plazselenol reaction yields byproducts, which together wth excess of reagent, result m chromatographlc peaks that may overlap the plazselenol peak [5,8,18] However, no mterference was observed when a capillary GC column was used [6] Pmzselenol and hndane were well separated from the byproducts and the reagent However, a clean-up step couId be meluded m order to mmmuze broad peaks from the reagent that appeared after the plazselenol peak [6] In the clean-up step, the toluene phase was washed once with 6 M perchlorlc acid The same reagent peaks appeared m the SPE method By treatmg the SPE cohnnn mth perchlonc acid before elutlon of the adsorbed plazselenol, these peaks were dnnmlshed The effect of the concentration of perchlorlc acid on both the hgand peaks and the plazselenol peak was investigated by addmg 0 25, 2, 4, 6 or 8 M perchlorlc acid to the column, Table 6 and Fig 2 A higher concentration of perchlorlc aad yielded a cleaner chromatogram but when the concentration exceeded 6 M, losses of plazselenol were observed This might be due to protonatlon of the plazselenol [191, changing the sorptlve properties Since the difference m the appearance of the chromatograms from columns treated with 4 or 6 M perchlorlc acid was small, 4 M perchlonc acid was used Calzbratzon of the SPE method In the hqmd-hqmd extraction procedure It was possible to use standard solutions of synthe-
K Johansson et al /Anal Chm Acta 274 (1993) 129-140 TABLE 6 Clean-up step unth perchlonc aad 125 ml of a solution contammg 160 ng I-’ of denvatlzed Se(W) were passed through an SPE cohnnn The column was washed mth 15 ml of a solution of percblonc acid before eMon R IS the ratio between the areas of the plazselenol and the lmdane peaks] HClO,
R
(MI 025 2 4 6 8
1011,1019 1005,1017 1046,1014 0 950,1066 0 740,O 676
sized plazselenol for cahbratlon purpose, since there was good agreement between cahbratlon graphs obtained from these solutions and denvatlzed standard solutions of Se(IV) [6] In the proposed SPE method the recovery of Se(IV) IS approximately %-98% mth the liquid-hquld extractlon method as reference, Fig 3 Consequently, the cahbratlon should be camed out with standards of Se(IV) treated the same way as the sample It should, however, also be possible to use standard solutions of synthesized pmzselenol for cahbratlon purpose if, m addition, one or two Se(IV) standards are analysed wth the same procedure as the samples The standards are evaluated Hnth the cahbration graph obtamed from the synthesrzed plazselenol and the recovery calculated The results from the samples are then corrected for the recovery Chozce of X4D reszn for removal of dzssolved organzc carbon The uptake of orgamc matter was followed by pumping the fresh water sample through a column of the resm and measurmg the absorbance at 420 nm This experiment was sumlar to that described by A&en et al [20] for XAD-1, 2, 4, 7 and 8 Accordmg to the authors, fulv~ aads are a major part of the coloured orgamc compounds present m natural waters They found that resms based on styrene dwmylbenzene copolymers (XAD-1, 2, 4 and 16) were less sultable than those based on acrylic ester polymers (XAD-7 and 8) for adsorptlon of fulvlc acids This was confirmed m our study The degree of adsorption
K Johansson et al /Anal
137
Chm Acra 274 (1993) 129-140
and surface area on adsorption Only small dlfferences were noted between XAD-7 and XAD-8 The latter resin is known not to retain inorganic
Increased m the order XAD4, 16,2,7 and 8 The posltlon of XAD-16 IS m agreement with the dlscusslon m [20] on the mfluence of pore size
b
a
Lmdane
Lmdane PIAZ
0
10
’
I
I
13
14
15
PIAZ
1
26
16
I
I
I
I
13
14
15
16
26
mm e
C
Lmdane
Lmdam
Lindane
s:li.5. .-1. PIAZ
PIAZ
I
J
r
I
26
I
I
I
I
13
14
15
16
mm
I 13
I-
I
I
14
15
mm
I __ 16
26
u
r
I
I
I
I
13
14
15
16
26
mm
Fu 2 Chromatograms of toluene eluates 25 ml of a derwatlzed standard soluUon, 160 ng I-’ of Se (IV), were passed through an SPE column followed by cleamng wtth 15 ml of (a) 0 25 M, (b) 2 M, (c) 4 M, (d) 6 M or (e) 8 M of perchlorlc acid In (a) the complete chromatogram IS shown and m the others only the analyttcally relevant parts
138
K Johansson et al /Anal Chm Acta 274 (1993) 129-140 TABLE 8
R
l5
Adsorption of coloured orgamcs and total dissolved orgamc matenal from water samples on Dowex 1 x8 and XAD-8 [Coloured orgamcs were determined by spectrophotometrlc measurements at 420 nm Sample volume 330 ml (670 ml for sample V)]
I
l-
Sample 05 -
(I) (II) (III) (IV)
4
0
4
2
6
wa
Fig 3 Cahbratlon graphs R ISthe ratio between the areas of the pmzselenol and the aldrm peaks Results from (01 synthesized pmzselenol standards and (0) 25 ml of denvatlzed selenmm(IV) solutions passed through an SPE column
selenium [11,13] This was found to be the case also for XAD-7 at pg 1-l and ng 1-l levels (Table 7) XAD-8 was used m the subsequent filtration expernnents since It 1s reported [20] not to suffer from bleeding as XAD-7 does Blank levels of TOC were low, < 0 5 mg I-’ and 0 8 mg 1-l for XAD-8 and Dowex 1X8, respectively, usmg a 500~ml solution Results from absorption measurements and TOC analysis of column effluents are presented m Table 8 The percentage of total organic carbon removed hardly exceeds 50%, whereas coloured materral 1s adsorbed more effectively A study of the behavlour of morgamc Se(W) and Se(W) on Dowex 1X8 IS m progress m our laboratory Under the conditions used m this work, 1e , addltron of hydrochloric acid to
Lafssjon SavJa&n Ramsen Trehornmgen (V) Flottsund
TOC
XAD-8
Dowex 1X8
(mgl-l)
Coloured (%)
TOC (o/o)
Coloured (%o)
TOC (%)
26 1 174 13 3
89 85 86
61 52 11
69 93 96
52 45 47
15 1 15 6
83 84
16 49
94 92
46 61
0 025 M, Se(W) passes through the column while Se(VI) 1s retained Our results Indicate that the ion-exchange resin 1s as effective as XAD-8 for removal of coloured orgamcs and other dissolved orgamc compounds It can thus be an attractive alternative to XAD-7 and 8 since Se(IV) and (VI) can be separated at the same tune as organic matter 1s removed Detemunutwn of Se(N) m spiked water The five fresh water samples were not acidified on storage Speclatlon changes and adsorption losses are therefore hkely to have occurred [21] The decrease m Se(N) concentration for sample I after three months of storage was approximately 30% (Tables 9 and 10) Prehmmary measurements mdlcated that the concentrations of Se(IV) were Just a few nanograms per litre
TABLE 7 Recovery of morgamc selenium (average and standard devlatlon) m 0 01 M hydrochlonc acid afier filtration through XAD-7 and XAD-8 (250 ml of a solution Hrlth morgamc selenmm were passed through a 75 ml bed of XAD The effluent was collected as 25 ml fractions and analysed for selenium usmg HG-AAS) XAD-7
XAD-8
Se(W) [Sel/(Lcg 1-l) Recovery (%) Fractions analysed
200 94 f 4 9
SeWI) 0040 96k5 5
200 101 f 3 9
SeW) 0060 106*10 5
200 lOOf 9
SeWI) 0040 104*3 7
200 102*2 8
0080 102+6 7
K Johansson et al /And
Chum Acta 274 (1993) 129-140
139
TABLE 9 Determmatlon of Se(W) m water samples wth GC-ECD [Average and standard devlatlon (ng I-‘) from trlphcate analysis SPE = Soltd-phase extraction, Extr = hqmd-hqmd extra&on] Sample
(I) LafssJon (II) SavJakn (III) Ramsen (IV) Trehommgen (V) Flottsund
Added Se(IV) (ng 1-l)
No pretreatment
XAD-8
SPE
Extr
SPE
Extr
SPE
Extr
80 a 60 40 20 0
54 f2 57 fl 37 f2 18 &2 26kO3
54 *3 59 *2 41 *1 19 *1 33kO6
54* 1 58 f 2 39 f 3 19f2 28
54 *2 56 k2 41 *1 21 *1 42+_01
53 *2 57 f4 42 k3 18 *1 31*07
57 *1 55 fl 42 fl 200*02 38+02
Dowex 1X8
a Se(W) added 3 months poor to analysis
The samples were therefore spiked vvlth Se(IV) to concentration levels, where differences between methods and treatments could be better detected Analysis were carried out vvlthm 24 h after splkmg Determmatlons with the GC-ECD method and the HG-AAS method were performed by different persons GC-ECD The water samples were analysed by the SPE and the liquid-liquid extractlon methods and the results are presented m Table 9 There 1s no srgmficant difference (p = 0 05) m the Se(W) concentrations or m the precision between filtered or unfiltered samples Selemum(IV) concentrations obtained vvlth the two extraction methods mdlcate a somewhat higher mean value for the hqurd-liquid extraction procedure but this difference 1s not sign&ant The major effect of the filtration of the water through XAD-8 or Dowex 1X8 was the faster and more distinct phase separation m the liquid-llqmd extraction procedure In the solid-phase extractions the same
columns were used for samples that had been treated m the same way, e g , all unfiltered samples passed through the same SPE columns The regeneration of the columns appeared to work effectively No accumulation of adsorbed coloured organic mate& was observed on either column and no deterroratlon 111performance was noted after 15 loadings Consequently, there appears to be no need to prefilter the samples through an adsorbing resm 111the SPE method iYG-AAS Standard additions of Se(W) were made to the unsplked waters and the slopes of the standard additions graphs were compared with the sensltlvlty determmed from standards No differences were found and subsequently the cahbratlon graph was used m the evaluation of the Se(W) concentrations The results are given m Table 10 &g&cant higher results (p = 0 05) were noted for subsamples filtered through Dowex 1X8 despite the fact that this was not the general observation Evaluations from peak height
TABLE 10 Determmatlon of Se(W) m water samples urrlthHG-AAS [Average and standard devlatlon (ng 1-l) from trlphcate analysis ND = Not detected1 Sample
(I) (II) (III) (IV) (V)
LafssJon
SavJain Ramsen Trehommgen Flottsund
Added Se(W) (ng 1-l)
80 a 60 40 20 0
* SeGV) added 3 months poor to analysis
No pretreatment
XAD-8
Peak area
Peak height
Peak area
Peak height
Peak area
Peak height
59*1 62 f 4 43 f 2 23*3 ND
55 *2 61 k4 43 *1 208*04 1 *1
59 f 5 58 f 6 44rtl 21 f 2 ND
56 &2 58 k2 44 *1 211*05 2 *1
58+2 65 f 4 47 f 3 23i4 31t2
57 f3 66 *2 47 fl 217*04 45*03
Dowex 1X8
140
or peak area gave the same results, mdlcatmg the absence of volatde compounds causing spectral interferences [7] Analysts of variance was performed on the results m Tables 9 and 10 There 1s a small but slgmficant difference (p = 0 05) between the results obtained wth the two methods The over-all mean was 37 4 ng 1-l with the HG-AAS method compared to 34 8 for the GC-ECD method It 1s noticeable that the analysis by HG-AAS generally gave a Se(W) concentration higher than the spiked level of the samples while the results obtamed with the GC-ECD method were somewhat lower The reason for this difference 1s not known to the authors conclusions Solid-phase extraction usmg a polymeric sorbent can replace the hquld-liquid extraction of the plazselenol m the determmatlon of Se(W) m natural waters with GC-ECD The SPE 1s less tune-consummg than the hqmd-liquid extractlon and makes it possible to run several samples simultaneously Problems with emulsion formation and/or preclpltatlon of organic substances m the boundary layer between the two phases that sometimes occur m liquid-hqmd extraction, are not present m SPE Removal of dissolved organic material 1s most effective on XAD-7 or XAD-8 which m addition do not retam morgamc selemum The use of Dowex 1X8 allows a separation of morgamc Se(N) and Se(W) and it adsorbs dissolved organlcs as effectively as the XAD resins Concordant result were obtamed between GC-ECD and HGAAS for fresh water samples spiked with Se(W) at the ng 1-l level
K Johansson et al /Anal Chtm Acta 274 (1993) 129-140
Thanks are due to Dynochrom Sweden Inc for supplymg the solid-phase extraction resm Thus work has partly been supported by the Swedish Environmental ProtectIon Board
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