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Anal. Chem. 1992, 6 4 , 159-165
Optimization of Comprehensive Speciation of Organotin Compounds in Environmental Samples by Capillary Gas Chromatography Helium Microwave-Induced Plasma Emission Spectrometry Ryszard lobifiski, Wilfried M. R. Dirkx, Michiel Ceulemans, and Freddy C. Adams* Department of Chemistry, University of Antwerp (UIA), Universiteitsplein 1, B-2610 Wilrijk, Belgium The potential of the microwave-Induced plasma atomlc emlsslon detector for capillary gas chromatography (GCAED) as a tool for the speciation of organotln compounds in envlronmental samples Is studied. The operatlonal variables are optimized for chromatographlc resolution and detectlon limlts. A comprehensive method for the determination of mono-, dl-, trC, and some tetrarrlkylated organotln compounds In water and sediments by GC-AED is developed. Ionlc organotln compounds are extracted as diethyl dlthlocarbamates Into pentane and, after Its evaporatlon, dissolved in a small volume of octane and derlvatlzed by pentylmagnesium bromide to give the solution of gas chromatographable species. The absolute detection llmlt Is about 0.05 pg (as Sn), and the callbration graph Is linear over three decades. The analytical response Is Independent of the elution temperature of the compound allowlng for Internal standard calibration. The accuracy of the method developed Is conflnned by Independent analysis by GC-AAS. A method to detennlne the empirlc formula of the compounds detected Is proposed.
INTRODUCTION There is a continuously growing interest in the determination of organotin compounds due to the still increasing number and output of their anthropogenic sources ( 1 , 2 ) . The major applications of organotin compounds concentrate in plastics industry, agriculture, and the production of antifouling paints (3-5). Although only 30% of organotins worldwide are consumed as biocides, they constitute almost the total input of organotins in the environment due to their direct introduction. Organotins are subject to degradation processes in the environment leading to the progressive loss of the organic groups down to inorganic tin. The toxic properties of organotins we related to the number and nature of organic groups in an R,SnX,-,, compound (R and X denoting alkyl and anion, respectively), producing the maximum biological activity for R3SnX. Within this class toxicity is strongly dependent on the nature of the organic substituent but very little on the anion. This is the dependence of toxicity on the chemical structure of the compound which makes speciation analysis of paramount importance. A large number of organotin compounds have very similar physicochemical properties. Gas chromatography is preferred to HPLC for the separation due to its larger resolving power, possibility of identification of unknown compounds, and availability of more sensitive but not necessarily selective detectors. However, the organotinspresent in the environment are mostly polar and involatile and need to be derivatized before gas chromatographic separation. Although hydridization or ethylation (on- or off-line) followed by purging have recently gained some popularity (6),most methods still involve extraction of ionic organotin compounds after chelation pentyl- (8),or hexylation (9). followed by Grignard ethyl-
(a,
When analyzing real environmental samples, some difficulties arise. As the extrads are usually rich in various organic compounds, the probability of peaks overlapping between them and organotins is fairly high when an unspecific detector like flame ionization or electron capture detection (FID and ECD) is used. Since the concentration of organotin compounds is usually smaller by more than 1 order of magnitude, any quantitative or even qualitative analysis becomes impossible without cleaning up the extracts and thus possible contamination and/or losses of analytes. Another reason for the need of an extensive clean-up procedure is not to contaminate the injection liner and the column, as the low sensitivity of most detectors makes the injection of concentrated extracts necessary. The most common techniques of detection used in gas chromatography for the speciation of organotin compounds are flame photometry (FPD) ( 7 , l O )and atomic absorption spectrometry (AAS) (8, 11, 12). Absolute detection limits reached are in the range 1-50 pg. Many other more or less sophisticated detection techniques have been proposed (13, 14). Helium atmospheric pressure microwave induced-atomic emission spectrometry (MIPAES) has recently gained popularity as a sensitive and selective detector for gas chromatography. Different laboratory-made systems have been described (15-18), and recently the first commercial one appeared on the market (19-21). The commercial instrument was found capable of detecting many non-metals at the picogram level. No specification, however, is available for environmentally interesting metals such as tin or lead. Some reports indicate a great potential of MIPAED for environmental analysis due to both high elemental sensitivity and selectivity ( I 7,18). Others, however, find tin difficult to determine at the ultratrace level due to formation of refractory oxides and the resulting low sensitivity (16,22). GC-MIPAED of volatile tin hydrides was found to give subnanogram detection limits (23). Very recently, this technique was applied to the analysis of fish and sediment samples (24) with a detection limit of 6 pg. The present paper describes the optimization of GC-AED for comprehensive organotin speciation at the trace level for nine species. The method is applied to the analysis of water and sediment samples, and the results are compared with those obtained by GC-AAS.
EXPERIMENTAL SECTION Apparatus. GC-AED. An HP Model 5890 Series I1 gas chromatograph (Hewlett-Packard, Avondale, PA) equipped with a split/splitless injection port interfaced to an HP Model 5921A atomic emission detector equipped with a turbo makeup gas valve was used. Injections were made by means of an HP Model 7673A automatic sampler. GC-AAS. A laboratory-interfaced GC-AAS system described earlier (425)consisting of a Model 3700 Varian gas chromatograph and a Perkin-Elmer Model 2380 atomic absorption spectrophotometer was used. The system was adapted to the use of a RSL 150 Megabore column. Chromatograms were recorded on
0003-2700/92/0364-0159$03.00/00 1992 American Chemical Society
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ANALYTICAL CHEMISTRY. VOL. 64, NO. 2, JANUARY 15, 1992
Table I . Concentration of Organotia Compounds in the Multicomponent Standard Stock Solutions Used (as Sn) conipd
abbrev for the ionic form
concn, pgimL
Me $nPe MelSnPe, Pr ,SnPe Bu,Sn MeSnPe Bu,SnPe Bu2SnPe. BuSnPe PeSn
TMT DMT TPrT TEBT MMT TBT DBT MBT TEPT
3.51 4.51 5.29 5.25 6.15 4.64 4.99 4.47 3.71
a Spectra-Physics SP 4290 integrator in the peak height mode. Reagents. The plasma gas and carrier gas used for the chromatography were helium, 99.9999%. The reagent gases for the AED were hydrogen, 99.9997%, and oxygen, 99.9995%. All the gases were supplied by 1'Air Liquide, Belgium. Bu3SnC1 (96%), Bu2SnC12(95%), BuSnCl, (95%), Me3SnC1 (99%), Me2SnC12(97%), MeSnC1, (97%), Pr3SnC1 (98%), npentylmagnesium bromide (2.0 mol/L) in diethyl ether, and butylmagnesium chloride (2.0 mol/L) in tetrahydrofuran were obtained from Aldrich Chemical Co. (Milwaukee, WI). Pentylated alkyltin standards were prepared by pentylation of organotin salts as described earlier (8). The concentration of tin in each standard stock solution was determined by flame AAS after the decomposition of the organotin compound with a mixture of sulfuric acid, nitric acid, and hydrogen peroxide as described earlier (8). The purity of standards was controlled by GC-AAS or GC-AED. The Pr,SnPe used as internal standard was found to contain admixtures of PrlSn and Pr2SnPe2. Hence, neither the former species nor Pr2Sn2+could be determined by the proposed technique. The stock solutions were mixed to give a multicomponent standard solution. The concentrations of organotin compounds (as Sn) in the multicomponent standard stock solution together with the abbreviations used throughout (for nonderivatized compounds) are given in Table I. Working standard solutions were prepared by a series of dilutions with octane from the stock solution. A dilution factor not larger than 1:lOO was always applied. Butylated alkyltin standards were prepared similarly but their concentrations were not confirmed independently. The latter standards were used just for tentative identification of peaks found in the chromatograms run for real samples. Citric AcidlPhosphate Buffer Solution (PH5.0). A 10.297-g sample of citric acid monohydrate and 18.156 g of disodium hydrogen phosphate dihydrate (Merck, Darmstadt, Germany) were dissolved in 1 L of water. Sodium Diethyl Dithiocarbamate (NaDDTC) Solution. A 2.25-g sample of NaDDTC (Merck, Darmstadt, Germany) was dissolved in 10 mL of water. The daily prepared aqueous solution was extracted with pentane before use. The solution of DDTC in pentane used in sediment analysis was obtained by shaking the aqueous solution of NaDDTC with 10 mL of pentane and 20 mL of a 0.5 M H2S04solution. ICN Alumina B-Super I (ICN Biomedicals, Costa Mesa, CAI was used in the clean-up step for sediment samples. All other reagents used were of analytical reagent grade. Deionized and further purified in a Millipore Milli-Q system water was used throughout. The glassware used was cleaned with a common detergent, thoroughly rinsed with tap water, soaked for 12 h in a 10% nitric acid solution, and finally rinsed with deionized water just before use. Procedures, GC-AED and GC-AAS Conditions, The optimum parameters used for gas chromatography and atomic emission detection are listed in Table 11. The gas chromatographic and atomic absorption spectrometer parameters used in the comparative analysis of sediment and water samples are listed in Table 111. Speciation of Organotin in Water (8). A 1.5-L water sample and 600 mL of a citric acid/phosphate buffer solution of pH 5.0 were placed in a separatory funnel, and 3 mL of NaDDTC solution was added. The sample was extracted twice with a 30-mL portion
Table 11. Optimal GC-AED Parameters injection port injection port temp injection vol split ratio column column head pressure oven program initial temp ramp rate final temp
GC Parameters split/splitless 170 "C 1 PL 1:20 HP-1: 25 m X 320 pm 130 kPa of helium
X
0.17 pm
60-80 "C 15-20 "C 230 "C
Interface Parameters transfer line HP-1 column transfer line temperature 250 "C AED Parameters wavelength 303.419 nm helium makeup flow 240 mL/mina scavenger gases H2 pressure 50 psi O2 pressure 20 psi spectrometer purge flow 2 L/min nitrogen solvent vent off time 1.5 min column-detector coupling column-to-cavity cavity temp 250 "C ' Measured at the cavitv vent.
Table 111. Optimal GC-AAS Operating Parameters injection port injection port temp injection vol column argon (carrier gas) flow rate oven program initial temp ramp rat,e final temp GC detector block temp transfer line transfer line temp wavelength light source slit hydrogen flow rate air flow rate MHS-20 furnace
GC Parameters packed column with wide bore on-column liner 170 "C 4 lrL RSL 150: 15 m X 530 um 6 mL/min
X
1.2 um
100 "C 10 "C
245 "C 230 "C Interface Parameters deactivated fused silica 530 pm i.d. 250 "C AAS Parameters 286.4 nm Sn EDL (8 W) 0.7 nm normal 350 mL/min 45 mL/min 900 "C
of pentane by shaking for 2 min each time. The combined extracts were transferred to a 50-mL Erlenmeyer flask and evaporated to dryness under reduced pressure at 25 "C using a rotary evaporator. Then 250 gL of octane spiked with a known amount of Pr,SnPe (as internal standard) and 1 mL of 0.5 M n-pentylmagnesium bromide solution were added followed by a gentle agitation of the mixture for 2 min. Then the mixture was transferred to a specially designed capillary separatory funnel (2%) and shaken for 1min with 10 mL of 0.5 M sulfuric acid to destroy the excess of the Grignard reagent. The aqueous phase was discarded and the octane one rinsed with 10 mL of water. After the aqueous phase was discarded, the octane layer was transferred to a small conical vial containing a small amount of anhydrous Na2S04and analyzed by GC-AAS or after dilution five times with hexane by GC-AED. Speciation of Butyltin Compounds in Sediments (26). A 5-g sediment sample was placed in a 100-mL Pyrex Erlenmeyer flask, and 20 mL of concentrated acetic acid (96%), 10 mL of water,
ANALYTICAL CHEMISTRY, VOL. 64, NO. 2, JANUARY 15, 1992
3 mL of acid DDTC solution in pentane, and 25 mL of hexane were added. The flask was closed with a glass stopper and ultrasonically treated for 30 min. The organic phase was pipetted off and the sediment leached again for 30 min with a fresh portion of hexane by magnetic stirring. After drying over anhydrous Na&304,the combined extracts were processed as described above. Subsequently, the octane layer was removed and analyzed by GC-AED after five times dilution or by GC-AAS after an aluminum oxide clean-up procedure.
I
181
I
3.5 1 3.0 2.5 4
x
cn ._ a, 2.0 I Y 1.5
e,
RESULTS AND DISCUSSION Optimization of GC-AED Conditions. GC Conditions. The GC conditions were generally adapted from the already optimized parameters for the GC-AAS technique (8). They allowed for optimum resolution of chromatograms and good shape of the peaks (symmetry factor around 1). A lower oven temperature was found necessary to let the solvent elute prior to the first analytically interesting compound, i.e. (TMT). It is not critical from the point of view of detection but to protect the discharge tube from large amounts of carbon entering it. A much better resolution power of the HP-1 capillary column than the RSL-150 Megabore one allows us to use a faster heating rate i.e. 18-20 OC/min and thus to reduce the time necessary for a GC run. When analyzing real samples, however, it was necessary to reduce the ramp rate to 15 "C/min to obtain a resolution compatible with the large number of organotin compounds present. The temperature of the transfer line had to be kept distinctly above the latest peak elution temperature to avoid condensation and peak broadening. AED Conditions. Wavelength. The instrument allows for measurement of the tin emission signal at two wavelengths: 270.651 and 303.419 nm. The response at both lines being similar, the line at 303.419 nm was chosen for further measurements. Soluent Vent O f f Time. The first peak in the multicomponent standard solution due to T M T lies very close to the octane solvent peak. The solvent must not be allowed to enter the discharge tube at high concentrations, as carbon deposits are formed very quickly in the discharge tube. The chromatogram on the C 193 channel showed that the largest part of the solvent has eluted after 90 s. If more volatile tin species, however, have to be determined, a lower boiling solvent like, e.g., hexane is recommended. The solvent vent off time was thus kept at 1.5 min. Effect of the Makeup Gas Flow. Helium used as a carrier in GC is not sufficient to sustain the plasma discharge and has to be made up before entering the discharge tube. The response of tin (sensitivity expressed as peak area or peak height per mass unit) is known to depend on helium makeup flow rate (16). In order to determine optimum flow rate giving maximum sensitivity, the same amount of the standards was injected at various flow rates. Helium makeup flow rate was measured at the cavity vent with the ferule purge outlet open. This value approximately corresponds to total helium flowing through the torch. Figure 1shows the maximum response at 240 mL/min, which drops off rather rapidly with an increase or decrease of the total flow rate. The optimum flow rate is about 20% higher than in the recommended instrument running conditions for which the tin response is a factor of 2 lower than the optimum. A still higher total helium flow rate results in a decrease in the emission intensity by both cooling the plasma and reducing the residence time of the emitting species. Running an analysis with high flow rates may create the hazard of damaging the spectrophotometer window sitting behind the torch. According to the manufacturer it may happen when the pressure in the cavity exceeds 5 atm. This value corresponds to a flow rate of about 330 mL/min with the solvent vent valve on. Hence, the conditions set in the
a
I
1 .o
0'5
1 200
240
Helium f l o w
280
ml/min.
Flgure 1. Effect of the helium makeup flow rate on the tln response: TPrT 1.0 pg (as Sn).
30
Hydrogen
70
50
flow,
psi
Figure 2. Effect of hydrogen flow rate on the tin response: TPrT 1.0 pg
(as W .
present work are considered safe for a routine basis. Hydrogen Flow Rate. Tin is prone to form refractory oxides in a microwave-induced plasma in the presence of oxygen which tend to accumulate on the wall of the quartz discharge tube. These properties result in a decrease in sensitivity, peak tailing, and subsequent memory effects (27). The presence of oxygen is necessary to prevent carbon deposition at the discharge tube coming from hydrocarbons which are abundant in uncleaned environmentalextracts. The negative effect of oxygen can be compensated for by adding some hydrogen to the plasma gas which acts as a tin scavanger. Furthermore hydrogen seems to support tin excitation (11) probably by forming very volatile hydrides which are easily excited. For these reasons the concentration of hydrogen in the plasma gas was optimized to obtain a maximum tin response. In the instrument used the hydrogen flow is directly proportional to the supply pressure due to a built-in flow restrictor. The hydrogen supply pressure was thus taken as a variable, and repetitive injections of the same standards amount were made at various H2supply pressures (Figure 2). A distinct maximum occurs at 50 psi. No signal was observed when the same amount of standards was injected in the absence of hydrogen in the plasma. A decrease in the response at higher flow rates is due to quenching of the microwave energy and thus finally decreasing the emission intensity of tin. Peak tailing is minimal (symmetry factor around 1)even after 100 runs. However, at this time considerable peak tailing was observed on the carbon and hydrogen channel which was impossible to remove by burning out with an increased stream of oxygen. The exchange of the discharge tube becomes mandatory in these conditions. Confirmation of Elemental Identity. Although the photodiode array coupled to a monochromator provides very
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 2, JANUARY 15, 1992
:
40.30;
-
3000,
Wavelength,
1
I
300
302
run, provided the analytical lines are not separated from each other by more than 40 nm. Otherwise two or more injections are necessary. For tin speciation the acquisition of chromatograms on carbon, hydrogen, and tin channels allows the determination of the elements’ ratios in any compound and thus to calculate the empirical formulas. In this way an additional identity proof for eluted peaks independent of the retention time can be provided. The most sensitive emission lines for carbon (193.031 nm) and hydrogen (656.302 nm) are far separated from each other and from the tin line. Three runs are thus necessary to acquire data indispensable to calculate the empirical formula. Another problem arising is the difference in optimum operational parameters such as flow rates, filters, and reagent gases. In this work no manual correction for gas flows from element to element was made. When the chromatogram was run on the C 193 or H 656 channels, the turbo valve was not activated, which gave the He makeup flow rate about 130 mL/min lower for these elements than for tin. No dilution of the response for carbon and hydrogen was thus observed. Other settings were as recommended by the manufacturer in standard software. The calculations depend on the condition that all the compounds of interest respond similarly in the plasma irrespective of their structure. This is not always true, as shown by Huang and co-workers (29). Indeed, our experiments have confirmed the dependence of the response on the molecular structure. Using a saturated hydrocarbon (tetra- or hexadecane) as a standard to quantify carbon and hydrogen in organotin species fails. A defined organotin species (TPrT) spiked a t exactly known concentration was therefore attempted as the standard for quantifying carbon, hydrogen, and tin on appropriate channels. This was possible due to the absence of signal discrimination with the retention time. Figure 5 shows the chromatograms registered for the same multicomponent standard injection amount on C 193, H 656, and Sn 303 channels. The peak a t 5.368 min from 5.3 pg of T P r T (as Sn) corresponds to the standard. The formula to be determined is denoted as C,H,Sn,. The x , y, and z coefficients were calculated according to the following equations:
nm
301
306
Wavelength, nm
Figure 4. Three-dimensionalsnapshot of (1) TEBT 5.25 pg; (2) MMT, 6.15 pg; (3) TBT 4.62 pg. good selectivity of the instrument for tin, hydrocarbons present a t high concentrations may give rise to unspecific emission on the tin channel. This may happen, e.g., when analyzing uncleaned real environmental sample extracts. The identity of the tin peak may be confirmed by taking an emission spectrum a t the peak apex and comparing it with the tin emission pattern. Figure 3 shows the typical emission spectrum measured at the organotin peak apex in the 240-280-nm range and in the 290-330-nm range after substraction of the carbon and helium background contribution. Tin lines at 242.949, 270.651, 300.914, 303.419, 317.505, and 326.234 nm can be identified (28). Figure 4 shows a smaller part of the emission spectrum but taken point by point when the chromatogram is run. The identity of several peaks at the same time as organotin peaks may be confirmed by identification of the characteristic pair of the Sn peaks a t 300.914 and 303.419 nm (Sn 300 and 303 channels). Determination of Molecular Formulas by GC-AED. Identification Proof. An extremely useful feature of the GC-AED used is the possibility of the determination of empirical formula of the eluted compounds. The AED is equipped with a photodiode array spectrophotometer allowing for the determination of up to four elements within the same
z =
I d S n 303)/mst
where IComp and Zst denote the emission intensity (peak height) for the analyzed compound and the standard, respectively, measured on correspondent channels. The symbols pomp and mst denote the injected amounts (in moles) of the determined compound and the standard, respectively. The coefficients 14,32, and 1correspond to the number of C, H, and Sn atoms, respectively, in a molecule of the standard. The ratio x:y:z obtained in this way was normalized to the closest integer value for the number of carbon atoms. The values for x , y, and z found in this way are given in the empirical formulas shown in Table IV. The determination of the empirical formula by the above method cannot match the subpercent precision of the classic microanalysis. It offers, however, interesting advantages like a possibility of carrying out the measurements on-line and considerably lower amounts of sample necessary for the analysis (picograms instead of milligrams). I t seems to be a valuable confirmative tool for identification of unknown chromatographic signals.
ANALYTICAL CHEMISTRY, VOL. 64, NO. 2, JANUARY 15, 1992
163
Table V. Analytical Figures of Merit for the Calibration Curves: y = A Bx for Organotin Compounds
+
compd
intercept
std dev
slope
B std dev
TMT DMT TEBT MMT TBT DBT MBT TEPT
-1.017 0.740 1.218 1.116 0.153 0.116 -0.095 -1.446
0.510
0.253 0.241 0.222 0.208 0.244 0.229 0.226 0.223
1.4313-3 3.2743-3 4.0383-3 3.7003-3 2.3293-3 2.4743-3 1.6713-3 1.7323-3
A
2
6
4
Time
9
I0
(min.1
1.502 2.156 2.312 1.099 1.255 0.758 0.653
correlation coeff 0.9999 0.9997 0.9995 0.9995 0.9999 0.9998 0.9999 0.9999
5n 303
2
5n
6
4
E
10
I
0
43
E
4 4
1
39 2
3
4
5
6
7
Figure 6 . Chromatogram for 1 pL of tin standards in octane splitless injection: (1) TMT, 0.18 pg; (2) DMT, 0.23 pg; (3) TEBT, 0.26 pg; (4) MMT, 0.31 pg; (5) TBT, 0.23 pg; (6) DBT, 0.25 pg; (7) MBT, 0.22 pg;
303 7
,"
(8) TEPT, 0.19 pg.
2
6
4
Time
E
10
(min.1
Figure 5. Chromatogram for a mixture of tin standards (Table I) diluted 1 5 0 (A, top) on the C 193 channel, (B, middle) on H 656 channel, (C, bottom) on the Sn 303 channel: (1) DMT; (2) tetradecane; (3) TPrT; (4) hexadecane; (5) TEBT; (6) DPrT; (7) MMT; (8) TBT; (9) DBT; (10) MBT; (1 1) TEPT.
Table IV. Empirical Formulas Calculated for the Standard Organotin Compounds
compd DMT TEBT MMT TBT DBT MBT TEPT
empiric formula calcd theor
C12H2,Sn C16H36Sn
CizHza.6Sno.9a
C16H36Sn
C19H42Sn
C16H36.9Snma C17H38.1Sn0.97 CiaH4z.3Sni.oa Cd39.aSni.m
C20H44Sn
C20H44,0Sn1.09
C17H3aSn CiaH4oSn
C16H35.6Sn0.91
Determination of Organotin Compounds in Standard Solutions. Calibration Graphs. For calibration, a series of standards were injected at six concentration levels to give 0.1-100 pg (as Sn) on the column. Good linearity was observed over three decades. The analytical figures of merit calculated for particular compounds are shown in Table V. The responses defined as emission intensity per tin mass unit are almost identical for all the analyzed species, indicating no signal discrimination with the increase in boiling point of the eluted species. One level internal standard calibration leads to reliable results for each compound. This virtually eliminates the need for a multistandard calibration curve.
Precision. Replicate injections ( n = 5) have been made to evaluate the precision of the method. It was similar for all the compounds and reached a few percent at the 0.5 pg and higher levels when the peak height mode was used. With the peak area mode the precision at subpicogram levels was far worse and equalled 10-20%. At lower concentrations peak height mode gives more reproducible values than the peak area mode, probably due to imperfections in automatic base line definition. Detection Limits. To evaluate the lowest detectable concentration of tin by the technique described, splitless injections were made. The chromatographic performance could be improved by using cool programmable injector (Gerstel, Germany) with a normal silanized quartz insert. The temperature was raised from 50 to 230 "C at 12 OC/min. The purge time of 45 s was used. Figure 6 shows the chromatogram for a dilute sample run in the splitless mode. The absolute detection limit calculated as 3 times the standard deviation of the noise was 0.05 pg in the peak height mode corresponding to a concentration of 50 pg/mL in the injected extract. When the peak area mode of the measurement was applied, the detection limit was not as low and equaled about 0.14 pg. The lowest quantifiable amount defined as 10 times standard deviation of the noise was 0.16 pg in the peak height and 0.5 pg in the peak area mode, respectively. Table VI compares the detection limits (most of them at the 30 level) obtained by various hyphenated techniques for tin speciation. It can be seen that the detection limit obtained in this work is the lowest obtained so far and is twice as low as this of MIPMS (35). This is much lower than the value obtained by Scott and co-workers (24) with a similar instrument due to higher makeup helium flow, peak height mode instead of peak area used, and some other minor differences influencing the system performance. Moreover full chromatograms of several organotin species have rarely been shown and the authors usually concentrated on the selection of a few particular compounds. The run for a comprehensive chromatogram takes about 15 min and is considerably shorter than in the most compre-
ANALYTICAL CHEMISTRY, VOL. 64, NO. 2, JANUARY 15, 1992
164
Table VI. Comparison of Detection Limits Reported for Various Hyphenated Techniques for the Speciation of Tin
technique
detn limit/pg 31-85 3 50 0.65-1.6 7.3-17.2 11-45 50 6.1 8-18 15
GC-ECD GC-FPD GC-FPD-DCP HG-FPD" GC-AAS HG-GC-QFAAS GC-ND-AFS GC-MIPAES GC-MIPAES GC-MIPAES GC-MIPAES GC-MIPAES GC-MS GC-MIPMS (standard torch) (Ta injector torch) GC-ionspray MS-MS HPLC-ICPMS HPLC-LEI'
" HG
ref 30
I 10
31 11
12 32 16 27 33 24 this work 34 35
6
0.05 200
1-4 0.09-0.35 5 20-40
I
- .
I
,
1
,
. I
,
:0
58j
36
37 38
60
= hydride generation.
GC-NDAFS = gas chromatography-nondispersive atomic fluorescence spectrometry. HPLCLEI = high-pressure liquid chromatography-laser-enhanced ionizntinn. c
193
, 6
4 Time
-
.
, B
---
10
I
(mi".)
4
Figure 7. Chromatogram of a sediment sample on the C 193 channel. hensive study so far made by Muller (7). Furthermore the detection limit obtained in this work is about 2 orders of magnitude lower. Analysis of Environmental Samples. The procedures for the speciation of tin in water and sediments have been recently developed in our laboratory (8,26). They involve the extraction of methyltin and butyltin compounds into pentane as diethyl dithiocarbamate complexes a t p H 5. The organic phase is then evaporated to dryness under reduced pressure, after which derivatization with n-pentyl (Pe) Grignard reagent is carried out in a microvolume of n-octane to form pentylated alkyltin compounds R,SnPe,,,, (R = methyl, propyl, or butyl). The quantitation is subsequently performed by GC-AED or GC-AAS. The recoveries of spikes reached 95-1"%0 for most of the species of interest. High selectivity of the AED detector for tin speciation becomes really important when extracts from environmental samples are analyzed. Figure 7 shows the chromatograms obtained for the sediment extract run on the carbon channel. Only the peak from the most concentrated internal standard may be tentatively identified on the basis of the retention time but without any proof of its identity delivered. A nonspecific detector cannot provide with any reliable data unless an extensive clean-up procedure is applied. Figure 8 shows chromatograms run on the Sn channel for a typical sediment and an analyzed water sample (1:20 split mode). A standard chromatogram run under identical conditions is also shown to illustrate the peak identifications. It can be seen that not all of the peaks can be identified. The
:0
6
Figure 8. Speciation of tin in environmental samples: (A, top) chromatogram of a mixture of standards (Table I) diluted 1:lO; (B, middle) chromatogram of a sediment sample; (C, bottom) chromatogram of a water sample. Key: (1) unidentified; (2) unidentified; (3) DMT; (4) TEPrT; (5)TPrT; (6) TEBT; (7) DPrT; (8) MMT; (9) TBT; (10) DBT; (11) MBT; (12) unidentified; (13)TEPT; (14) unidentified. Table VII. Results of the Determination of Organotin Compounds in Water and Sediment Samples by GC-AED and GC-AAS
sample
compd
GC-AAS
GC-AED
river water
TBT DBT MBT TBT DBT
33.1 f 2.0 ng/L 18.2 f 0.6 ng/L 14.1 f 1.5 ng/L 13.3 f 0.9 ng/g 32.0 f 1.8 ng/g
29.8 f 1.1ng/L 21.0 f 0.5 ng/L 19.8 f 0.7 ng/L 10.1 f 0.6 ng/g 33.2 f 1.5 ng/g
sediment
GC-AED chromatograms when compared to GC-AAS ones show more tin peaks due to much higher sensitivity. Some of them cannot be identified because of the lack of standards, and no data on empiric formulas are available because of the virtual absence of signals on the carbon and hydrogen channels. Synthesis of a series of new standards is required together with systematic work on their identification by GC-MS and GC-AED. Results for the analysis of one river water sample (Scheldt River estuary) and a sediment sample are shown in Table VII.
CONCLUSIONS This paper demonstrates the potential of the gas chromatograph atomic emission detector for comprehensive organotin
ANALYTICAL CHEMISTRY, VOL. 64, NO. 2, JANUARY 15, 1992
speciation in environmental matrices. Further work in this field is hampered by the lack of tin standards, as due to its extreme high sensitivity this instrumentation enables quantification of many more compounds than techniques known so far. It may give rise to a study on the environmental impact of many compounds for which to date no reliable data could be obtained because of extremely low concentrations. The development of chemical methods allowing for quantitative extraction and leaching of other tin species from environmental samples especially from sediments is of paramount importance.
ACKNOWLEDGMENT This work was financially supported by EC “EUROCORE” (STEP-CT90-0064-C/DSCN/) and IGBP “Global Change” (GC/OS/OOS) programs.
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RECEIVED for review July 8,1991. Accepted October 21,1991.
