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Anal. Chem. 1984, 56,2692-2697
Protection Agency: Research Triangle Park, NC, revised 1981; Section 5. (45) McCullough, W.; Craun, J., US Food and Drug Administration, Kansas Clty, MO, personal communication, 1983. (46) Heikes, D. Bull. Environ. Contam. Toxicol. 1980, 24, 338-343. (47) Lamoureux, G.; Gouot, D.; Davis, D.; Rusness, D. J . Agrlc. Food Chem. 1981, 29, 996-1002.
(48) Heikes, D.; Griffitt, K.;Craun, J. Bull. Envlron. Contam. Toxlcol. 1979, 21, 563-588. (49) Yurawcez, M. "Laboratory Information Bulletin"; US Food and Drug Admlnlstration: Washington, DC, 1976; No. 1710A.
RECEIVED for review April 12,1984. Accepted August 9,1984.
Determination of Ionic Alkyllead Compounds in Water by Gas Chromatography/Atomic Absorption Spectrometry D. Chakraborti,' W. R. A. De Jonghe,2 W. E. Van Mol, R. J. A. Van Cleuvenbergen, and F. C. Adam$* Department of Chemistry, University of Antwerp (U.I.A.),B-2610 Wilrijk, Belgium
Ionic alkylead compounds are extracted from water samples into pentane by complexation with sodlum diethyldlthlocarbamate. Inorganic lead is complexed with EDTA. The organic phase Is evaporated to dryness by vacuum dlstillatlon, after which butylation with n-butyl Grignard reagent is carried out in a microvolume of n-nonane. The analysis is performed by gas chromatography with atomlc absorption spectrometric detection. The effects of various parameters on the anaiytlcai performance are dlscussed. With 500 mL of water, detection limits for individual species are situated at the ng.L-' level.
A thorough understanding of the environmental behavior of organic lead compounds is not yet possible, owing to the lack of precise data on the occurrence of trialkyllead and dialkyllead species (PbR3X and PbR2X2,with R = Me, Et and X = arbitrary anion) (1). Analytical methodology for determining these analyks at ambient concentrations is indeed only poorly developed, and emphasis in environmental studies has been given so far to measurements of the tetraalkyllead (PbR4) compounds. For the latter, the combination of gas chromatography with an element-specificdetection system was proven to be very successful (1-6). This approach has been attempted also in the case of the trialkyllead compounds despite their low vapor pressure, thermal instability, and chemical reactivity, which make a direct quantitative gas chromatographic analysis extremely difficult (7). The use of gas chromatography (GC) for the determination of trialkyllead ions at the trace levels with which they are thought to be present in biological tissues was first described by Hayakawa (8). In this method the trialkyllead compounds were extracted as chlorides into an organic solvent, by saturating the aqueous solution to be analyzed with NaC1, and then they were quantified using GC with electron capture detection. A similar method was described by Chau and Wong (9), though in this case detection was based on atomic absorption spectrometry (AAS). The authors stated that the sensitivity obtainable in this way is much less than that for PbR4 compounds. Robinson et al. (10) also described a GC/AAS based method in which both PbEtsCl and PbEt4 were determined directly in seawater. With a detection limit of 1mg.L-l the sensitivity is, of course, insufficient for any environmental Present address: Department of Chemistry, Jadavpur Universit 32 Calcutta, India. YPresent address: Analytical Laboratory, Metallurgie HobokenOverpelt (M.H.O.),B-2710 Hoboken, Belgium.
applications. Estes et al. (11)reported the determination of PbMe3C1and PbEt3Cl in tap water using fused silica capillary column gas chromatography with microwave excited helium plasma detection (GC/MWPD). This method again involves an initial preconcentration, based on "salting-out" the PbR3+ ions into an organic phase, and a subsequent vacuum reduction to further concentrate the compounds; it is timeconsuming and, moreover, provides semiquantitative results only. The methods based on gas chromatographic separation of PbMe3Cl and PbEtaCl without preliminary preconcentration are clearly of a rather limited utility; moreover they suffer from the inability to incorporate the dialkyllead species in the analysis. These drawbacks led to the development of chemical derivatization techniques. Derivatization modifies part of the molecule to render it more volatile without, however, changing the original structure of the alkyl groups bonded to the lead atom, thus preserving the authenticity of the original compound. Estes et al. (12) reported the n-butyl Grignard derivatization of trialkyllead ions into the corresponding n-butyltrialkylleads, after the extraction of the compounds as chlorides from spiked tap water and industrial plant effluent. Precolumn trap enrichment of the derivatized trialkylbutyllead compounds enabled determinations at the pgL-l level. Final measurements in this case were done by GC/ MWPD using fused silica capillary columns. Almost at the same time, Chau et al. (13) developed a method to include the dialkyllead species. A complexometric extraction with sodium diethyldithiocarbamate was applied, and then the butylated trialkyllead and dialkyllead compounds were simultaneously determined by GC/AAS; a detection limit of about 0.1 pg-L-' was obtained. Forsyth and Marshall (14) isolated the analytes with dithizone and carried out their speciation, this time after phenylation, using fused silica capillary column GC with electron capture detection. The present paper reports in detail on an improved version of the NaDDC extraction and n-butyl Grignard derivatization method of Chau et al. (13), combined with specific detection through a simple and readily demountable GC/AAS arrangement. By use of a specific enrichment step, superior detection limits in comparison with any previously reported could be obtained. The method has been applied to the analysis of rain water samples to demonstrate its applicability in environmental surveys.
EXPERIMENTAL SECTION Apparatus. A Perkin-Elmer 503 atomic absorption spectrometer equipped with a deuterium background corrector was
0003-2700/S4/0356-2692$01.50/0 0 1984 American Chemical Society
ANALYTICAL CHEMISTRY. VOL. 56,
NO. 14, DECEMBER 1984 2893
Table 1. Optimal DC/AAS Operating Parameters for the Determination oP PbR&u and P b w u , Compoundsa column packing material dimensions I
column program injector detector interface gas flow rates argon (carrier g d
hydrogen acetylene air light source wavelength slit recorder range speed
10 % OV-101on Chromosorb W (80-100 mesh) 1.5 m X 6 mm 0.d. X 2 mm i.d.
/ \
50-250 O C at 10 'Cmin-' 150 o c 140 'C 140 'C
40 mLmin-'
110 mLmin-' Lmin-' 25 L m X ' Pb hollow cathode lamp, 10 d 283.3 nm 5
1 mm 2 mV 0.5 cmmin-'
'For PbR, compounds, optimal operating parameters are similar.
used as the detector. The separation of the butylated triauryuead and dialkyllead compounds was achieved by using a Varian 3700 gas chromatograph fitted with a glass column (1.5 m X 2 mm i.dJ packed with 10% OV-101 on Chromosorb W (80-100 mesh). Samples were injected directly onto the column. The output from the built-in peak read device of the AAS was displayed on a fast-response chart recorder (Hitachi Perkin-Elmer 56). The interface between the gas chromatograph and the atomic absorption spectrometer was achieved using nickel tubing (0.5mm i.d.) connected via a stninlewsteel low hold-up union; it WBB heated by applying a variable transformator-controlled voltage between both tube ends, which were well insulated from the rest of the instrument. The temperature was monitored with a thermocouple. On the GC-side the interface was connected to the base of the h e ionization detector of the GC, thus allowing in a simple way the introduction of hydrogen at the beginning of the interface. On the AAS-side the interface was connected to a silica cell normally used with a Perkin-Elmer mercury/hydride system (MHS-IO), which was mounted on top of a three-slot burner for heating in an air-acetylene flame. The optimized operating parameters are summarized in Table I. Reagents. The alkyllead compounds used were supplied by the Associated Oetel Co., South Wirral, England. They included a mixture of the five tetradkylleads (PbMe,, PbMe&, PbMeEk, PbMeEt,, and PbEt,) in diisopropyl ether, and trimethyllead chloride, trietbyllead chloride, dimethyllead dichloride, and diethyllead dichloride in powder form. The latter were used to prepare individual standard stock solutions in water, containing each 10.0 pgmL-' Ph. When stored in a refrigerator, these solutions were stable for at least 4 weeks without noticeable deterioration. Working standards, obtained by further dilution, were prepared immediately before use. Butylated akyllead standards in nonane were prepared by processing aqueous working standards of the corresponding alkyllead salts in a way similar to that described in the procedure below. The standards were subsequently quantitized by AAS after treatment with iodine monodoride according to a procedure denmibed by H a n d and Slater (15). nButylmagnesium chloride in tetrahydrofuran (1.9M)was obtained from Alfa Ventron (Federal Republic of Germany). AU other reagents used were of analytical reagent grade. Procedure. After filtration on a Type RA Millipore filter of 1.2 pm pore size, to 500 mL of the water sample to be analyzed, respectively, 4 g of citric acid and 1g of ethylenediaminetetraacetic acid (EDTA) are added after which concentrated ammonium hydroxide is added to obtain pH 9. The solution is transferred to a 1000-mL separating funnel. Next, 2 mL of aqueous 0.25 M sodium diethyldithiocarh-te (NaDDC) and 10 mL of pentane
Flgura 1. Separatlng tunnel
(capillary not to scale)
are added and the mixture is shaken for 2 min. After phase separation, the lower (aqueous) layer is run off in a beaker while the pentane phase is collected in a 50-mL Erlenmeyer flask. In the original funnel, the aqueous solution is extracted a second time with a fresh 10-mL portion of pentane. The combined pentane extra- are then carefully evaporated under vacuum at 20 "C until dry. Thereupon 1 mL of 1.9 M n-butylmagnesium chloride Grignard reagent in tetrahydrofuran is added and the mixture is gently swirled around the recipient for 1 min. Then, 250 pL of nonane and 10 mL of 1 N H,SO, are added. The mixture is shaken for 1 min and then transferred to a separating funnel @@re 1) designed to improve phase separation. The water phase ia allowed to run off, exactly until the nonane phase starts to enter the capillary. After the nonane phase is rinsed with a fresh 10-mL portion of distilled water, the aqueous layer is again discarded, thereby not allowing the nonane phase t o enter the capillary. The nonane layer is sampled from above by means of a 100-pLEppendorfpipet and transferred to a conical micro test tube. After addition of a small amount of anhydrous NaSO, the solution is ready for analysis by GCJAAS,using injections of up to 20 pL.
RESULTS AND DISCUSSION GC/AAS Determination of n -Butylated Alkyllead Compounds. Preliminary testa revealed that the graphite furnace AAS-detection system developed earlier in our l a b oratory for the GC determination of tetraalkylleads (16) was not appropriate in the case of n-butylated trialkyllead and dialkyllead compounds, as they were found to condense inside the inner gas flow entrances of the furnace. In view of the excellent results obtained by Ebdon et al. (I7)with the ceramic tube suspended above an airaeetylene flame for the detection of PbMe, and PbEt4, i t was decided to adopt this approach for the measurement of the P b h B u and Pb&Bu2 species. In this way and also by ensuring an appropriate heating of the interface, condensation could be virtually avoided. The elegant feature of the approach of Ebdon et al. introducing the
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
MHS-10 SILICA ABSORPTION TUBE
I 0
EL RFACE TUBE
Flgure 2. Arrangement 2 cm above burner.
20
40
60
BO
100
carrier gas flow rate
120 140 ( m i min’ll
Flgure 4. Effect of carrier gas flow rate on the sensitivity (average and standard deviation for the four tri- and dialkyllead species studied).
of quartz tube in flame. The tube is positioned 3
m
m
+ u n
0
20
40
60 BO 100 hydrogen flow rate
120
140
160
(ml min” I
Flgure 3. Effect of hydrogen flow rate on the sensitivity (average and standard deviation for the four tri- and dialkyllead species studied).
GC effluent in the atom cell by means of its impingement within the flame is, however, not feasible with burners requiring high gas flow rates such as that of the Perkin-Elmer 503 AAS. In fact, depending on the actual method of introduction used, the analytes were either removed too quickly from the region where light absorption takes place, or else did not enter the atom cell to a sufficient extent. These problems could be solved through the use of a commercially available open silica cell (16.5 cm length, 1.0 cm diameter) normally employed with the Perkin-Elmer MHS-10 mercurylhydride system. As shown in Figure 2, a direct contact of the GC effluent with the flame is thus avoided. An investigation of the optimal working parameters, based on the univariate procedure, was performed by repeated injection of a standard test solution containing p~~p.L-l concentrations of the butylated dialkyl- and trialkyllead compounds, PbMe3Bu, PbEt,Bu, PbMezBuz, and PbEtzBuz in nonane. Figure 3 demonstrates that the introduction of hydrogen gas at the beginning of the interface is extremely important for obtaining high sensitivity. In the absence of hydrogen virtually no atomization seems to occur. This phenomenon was also observed with PbR4 compounds, and in fact for all species studied the dependence of the peak height on the hydrogen flow rate appeared to be similar. The beneficial effect of hydrogen on the AAS sensitivity, which has been described also for other atomization cells (2,17-19),is probably the result of the higher temperature developed in the cell due to the burning of the hydrogen gas. A flow rate of 110 ml-min-l was chosen as the optimal value. Figure 4 shows that the carrier gas flow rate is a rather critical parameter, with a pronounced optimum at about 40 ml-min-l. The decrease in sensitivity at higher flow rates must be attributed to the reduced residence time of the atomic
0
4
8 ia I
12
-
16 timeimini
t
0
4
8 j
b)
1 2 time(mtn 1
Flgure 5. Typical GCIAAS chromatograms for standard solutions of tri- and dialkyilead compounds (a)and tetraalkyllead compounds (b).
cloud in the light path. The sharp decrease for lower flow rates, on the other hand, is also understandable, as these give rise to excessively long GC elution times and a higher probability of thermal decomposition. The latter effect results in severe band broadening and, consequently, in reduced sensitivity. Also in this case, the behavior of the PbR4 species was found to be similar with the species studied. The flow rates of acetylene and air used throughout this work are the same as those recommended by Perkin-Elmer for the three slot burner; they were found compatible with the GC/AAS operating mode. With the optimized parameters (Table I), GC/AAS chromatograms as represented in the typical example in Figure 5 are obtained. The column temperature program adopted proved to give the best separation between the PbMezBuzand PbEtsBu peaks within an acceptable analysis time of about 20 min. However, care should be taken in the interpretation of the peaks due to PbMezBuzand PbEtsBu, especially when only one of them is present. They are indeed separated by 24 s only. As a matter of illustration a chromatogram of the five PbR4 compounds, registered under the same conditions, is also shown. Their retention times are sufficiently different from those of the butylated alkyllead species, so that interference caused by peak overlap can be excluded. The analytical characteristics for the different alkyllead species are summarized in Table 11. Calibration curves are linear from the limit of detection, defined as three times the base line noise, up to a t least 50 ng of Pb. Absolute detection limits range between 0.05 ng of Pb for the PbMeaBu species and 0.1
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984 * 2695
Table 11. Analytical Characteristics of the GCIAAS Determination of PbRaBu, PbRzBuz,and PbR4 Compounds analyte
retention time, min
sensitivity," mmng-'
detection limit,bng of Pb
PbMe,Bu PbMezBuz PbEtaBu PbEtzBuz
6.2 10.5 10.9 13.2
96 61 54
0.05
PbMe, PbMeaEt PbMezEtz PbMeEt, PbEtl
2.3 3.9 5.6 7.3 8.9
120 99 92 76 73
0.08
0.09 0.10
51
0.04
0.05 0.05
L 6 8 10 12 14pH Figure 6. Effect of pH on the determination of PbMe,' (0),PbEt,'
0.07 0.07
" A t a recorder range of 2 mV, 25 mm corresponds to 0.020 absorbance units. bAmount of lead per species required to give a signal of 5 mm (about 3 times the base line noise).
ng for PbEtzBuz. Thermal degradation in the GC column is probably responsible for the lower sensitivity for the analytes with the longest retention times. Hence, it is not surprising that the PbR, compounds generally have lower detection limits (Table 11). Extraction, Dithiocarbamate is well-known to form lead(I1) complexes which can be extracted by organic solvents (20). Chau et al. (13)demonstrated that this also applies to the ionic alkyllead(1V) species, PbR3+ and PbRz2+. Sodium diethyldithiocarbamate proved particularly adequate. In the present extraction procedure, this chelating agent was used in combination with EDTA, so as to prevent coextraction of inorganic lead. If Pb2+is not properly removed, the recoveries of the alkyllead compounds are reduced, as follows from Table 111. With high concentrations of inorganic lead a significant portion of the lead dithiocarbamate, which is insoluble in the small amount of pentane used, may precipitate. In addition, there are indications that a less efficient Grignard derivatization takes place. Moreover, removal of inorganic lead has the additional benefit that no "cleanup" of the GC column is necessary. Under the extremely sensitive measuring conditions adopted, the relatively large concentrations of inorganic lead in practical analytical situations would indeed lead to an intense peak near the end of the chromatogram, requiring a long elution time before returning to the normal base line. From Figure 6 it is clear that for the extraction of the organolead compounds only an extremely limited pH range is suitable. Above pH 9 the recovery of PbMe3+drastically diminishes as a result of the formation of hydroxy complexes (21),whereas below pH 9 a sharp decrease for PbMe?' occurs. The extraction conditions for PbEt3+and PbEtzz+appear to be far less critical. Because the masking capacity of EDTA is low in acidic medium, the extraction efficiency was not investigated further at pH values below 6. I t is interesting to note that Chau et al. (13)apparently did not encounter any problems resulting from the presence of inorganic lead, ob-
(A),
PbMe," (O), and PbEt;' (X) from distilled water. The level of spiking of the organolead species is given in Table 111; 400 KgsL-' inorganic lead was also added. viously because the inorganic to organic lead ratio is less unfavorable. Enrichment and Derivatization. It was found that rotary evaporation of pentane could be continued until dryness without loss of any of the analytes. Evaporation should, however, be stopped as soon as the solvent is completely removed, as prolonged evaporation was found to give rise to decreased recoveries. The subsequent Grignard derivatization then proceeds very smoothly. Care should be taken to remove any traces of tetrahydrofuran from the nonane phase, so as to maintain a constant volume of the final nonane solution. Rinsing of the 1 N HzS04treated extract with distilled water proved to be satisfactory for this purpose. In order to be able to separate the microvolume of nonane from the water layer, a separating funnel provided with a capillary is mandatory. A small amount of anhydrous Na2S04may be added to the final nonane phase to remove any traces of water. Precision, Recovery, and Interferences. The reproducibility of the entire procedure can be estimated at ca. 10% RSD at concentration levels considerably in excess of the detection limit. Table IV provides data on recovery and precision a t concentration levels around 200-300 ngL-l. Repeated experiments with standard solutions proved that recoveries are in excess of 90% for all the compounds under study. Inorganic lead does not interfere with the determination of the tri- and dialkyllead compounds at concentration levels of up to 1 mgL-l in the original solution. Tetraalkyllead compounds do not give rise to interferences in the measurement of the ionic alkyllead compounds. Interference due to other organometallic or organic compounds present in the sample (i.e., Ge, Sn, Se, ...) was not investigated. Due to the specific measurement procedure, such interferences in the chromatograms can be considered as unlikely. Possibly, there is some recovery of organic compounds throughout the entire extraction procedure, but the
Table 111. Recovery ( % ) of Ionic Alkyllead Compunds from Distilled Water in the Presence of Inorganic Lead, without and with Addition of EDTA (1 g)" % recovery for amounts of inorganic lead added 0 rg
analyte
concn, ng-L-'
PbMeaCl PbMezClZ PbEtaCl PbEtzC1,
326 274 302
without EDTA
without EDTA
with EDTA
95 f 3 95 f 2 98 f 4 102 6 99 f 3 93 f 3 95 f 6 98 f 3 99 f 1 98 f 3 99 f 5 107 f 8 'Average f standard deviation for three replicate determinations. cleanup the column (values given are only indicative). 180
102 f 8 105 f 1 104 f 5 102 f 4
10 rg
with EDTA
*
50 pg without with EDTA EDTA
without EDTA
with EDTA
80 f 2 77 f 5 80 f 2 87 3
59 f 6 56 f 7 56 f 4 64 f 5
102 f 3 98 f 2 94 f 5
97 f 1 96 f 3 98 f 2 104 f 5
100 fig
500 pg withoutb with EDTA EDTA 68 31
101 f 2
99 f 4 96 f 2 110 f 8 50 108 f 7 Only one injection performed, in view of the long time required to
*
53
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
Table IV. Recovery and Precision of the Determination of Ionic Alkyllead Compounds in Distilled Water by GC/AAS (10 Replicates) component
concn, ngL-'
Me3Pbt MezPb2+ Et3Pbt EtpPb"
180 326 274 302
recovery mean, % std dev, % 105
9.5 7.8 6.7 7.8
90
102 108
Table V. Results for Duplicate Analysis of Rainwater
component PbMe3+ PbMezEtt PbMeEtz+ PbEt3+ PbMe2+ PbMeEt2+ PbEt?+ inorg Pb (~ug.L-')~
concentration, ngL-' campus downtown" 17 5
