Anal. Chem. 1997, 69, 4799-4807
Elemental Speciation Analysis by Multicapillary Gas Chromatography with Microwave-Induced Plasma Atomic Spectrometric Detection Isaac Rodriguez Pereiro, Vincent O. Schmitt, and Ryszard Łobin´ski*
CNRS, URA 348, Universite´ Bordeaux I, 351, Crs. de la Liberation, F-33 405 Talence, France
Multicapillary column gas chromatography (MC-GC)/ microwave-induced plasma atomic emission spectrometry (MIP AES) was developed for fast speciation analysis of organotin compounds in the environment. Ethylated butyltin compounds could be separated isothermally within less than 30 s (instead of ∼5-10 min) without sacrificing either the resolution or the sample capacity of conventional capillary GC with oven temperature gradient programming. Careful optimization of the pressure and temperature GC program allowed a comprehensive organotin speciation analysis including phenyltin compounds within less than 2.5 min, increasing the sample throughput 6-fold. Compatibility of MC-GC with an MIP atomic emission detector (MIP-AED) was discussed. MC-GC/ MIP-AES was validated for the analysis of sediment (PACS-1 and BCR 462) and biological (NIES11) certified reference materials. Speciation of organometallic (Pb, Sn, Hg, Mn) compounds released into the environment as a result of anthropogenic activity has been attracting considerable attention in recent years.1-3 In particular, the degradation of edible aquatic resources by tributyltin (Bu3Sn) and, to a lesser extent, by triphenyltin (Ph3Sn), which have been used as biocides in antifouling paints to control the attachment and growth of organisms on the ship hulls, has raised important ecotoxicological and economic concerns.4,5 The strong dependence of the toxicity on the chemical form has stimulated the development of analytical methods able to distinguish between the toxic target species and the relatively harmless products of their environmental transformation in waters, sediments, biota, and foodstuffs.6-8 (1) Batley, G. E., Ed. Trace Element Speciation: Analytical Methods and Problems; CRC Press: Boca Raton, FL, 1987. (2) Krull, I. K., Ed. Trace Metal Analysis and Speciation; Elsevier: Amsterdam, 1991. (3) Vela, N. P.; Olson, L. K.; Caruso, J. A. Anal. Chem. 1993, 65, 585A-597A. (4) Hugget, R. J.; Unger, M. A.; Seligman, P. F.; Valkirs, A. O. Environ. Sci. Technol. 1992, 26, 232-237. (5) Fent, K. Crit. Rev. Toxicol. 1996, 26, 1-49. (6) Harrison, R. M., Rapsomanikis, S., Eds. Environmental Analysis Using Chromatography Interfaced with Atomic Spectroscopy; Horwood: Chichester, 1989. (7) Uden, P., Ed. Element-Specific Chromatographic Detection by Atomic Emission Spectroscopy; ACS Symposium Series 479; American Chemical Society: Washington, DC, 1992. (8) Hill, S. J.; Bloxham, M. J.; Worsfold, P. J. J. Anal. At. Spectrom. 1993, 8, 499-515. S0003-2700(97)00410-1 CCC: $14.00
© 1997 American Chemical Society
The analytical problem consists of bringing the organotin species [BunSn(4-n)+ and PhnSn(4-n)+, n ) 1, 2, 3] present in a sample to a sensitive (absolute detection limit at the picogram level) tin-specific detector in the time-resolved manner in a minimum of time. The commonly accepted technique has been capillary gas chromatography (GC) with spectrometric detection because of the high resolution and sensitivity, the latter due to the efficient (quasi-100%) sample introduction into a detector and virtually no energy losses for the desolvation and vaporization of the mobile phase.9 Atomic absorption spectrometry (AAS),10,11 flame photometric detection (FPD),12 microwave-induced plasma atomic emission spectrometry (MIP-AES),13-15 inductively coupled plasma mass spectrometry (ICPMS),16 and electron ionization MS17,18 have been the most widely used for detection of organotin compounds in GC eluates. The analysis time and the sample throughput have long been controlled by the length of the sample preparation step, which was traditionally complex, multistep, and time-consuming.9 However, a series of recently reported rapid 3-5-min-long microwaveassisted approaches integrating sample decomposition/leaching, extraction of the analytes into a nonpolar solvent, and their derivatization changed this situation.19-21 This is the duration of a GC run that has now become the virtual bottleneck of an analytical procedure. Indeed, the high retention index of Ph3SnEt and the large (over 300 °C) difference between the boiling point of the most volatile (SnEt4) and the least volatile (Ph3SnEt) of (9) Dirkx, W. M. R.; Łobin ˜ski, R.; Adams, F. C. Anal. Chim. Acta 1994, 286, 309-318. (10) Dirkx, W. M. R.; Łobin ˜ski, R.; Adams, F. Anal. Sci. 1993, 9, 273-278. (11) Forsyth, D. S.; Hayward, S. Fresenius’ J. Anal. Chem. 1995, 351, 403-409. (12) Jiang, G. B.; Maxwell, P. S.; Siu, K. W. M.; Luong, V. T.; Berman, S. S. Anal. Chem. 1991, 63, 1506-1509. (13) Scott, B. F.; Chau, Y. K.; Rais-Firouz, A. Appl. Organomet. Chem. 1991, 5, 151-157. (14) Łobin ˜ski, R.; Dirkx, W. M. R.; Ceulemans, M.; Adams, F. C. Anal. Chem. 1992, 64, 159-165. (15) Liu, Y.; Lopez-Avila, V.; Alcarez, M.; and Beckert, W. F. Anal. Chem. 1994, 66, 3788-3796. (16) Kuballa, J.; Wilken, R. D.; Jantzen, E.; Kwan, K. K.; Chau, Y. K. Analyst 1995, 120, 667-673. (17) Sta¨b, J. A.; Brinkman, U. A. T.; Cofino, W. P. Appl. Organomet. Chem. 1994, 8, 577-585. (18) Tolosa, I.; Bayona, J. M.; Albaiges, J.; Alencastro, L. F.; Tarradellas, J. Fresenius’ J. Anal. Chem. 1991, 339, 646-653. (19) Szpunar, J.; Schmitt, V. O.; Donard, O.; Łobin ˜ski, R. Trends Anal. Chem. 1996, 15, 181-187. (20) Rodriguez Pereiro, I.; Schmitt, V. O.; Szpunar, J.; Donard, O.; and Łobin ˜ski, R. Anal. Chem. 1996, 68, 4135-4140. (21) Szpunar, J.; Schmitt, V. O.; Monod, J. L.; Łobin ˜ski, R. J. Anal. At. Spectrom. 1996, 11, 193-199.
Analytical Chemistry, Vol. 69, No. 23, December 1, 1997 4799
Table 1. Optimum Instrumental Conditions for Speciation of Organotin Compounds by MC-GC/MIP-AED GC Parameters injection port split/splitless injection port temperature 250 °C injection mode inverse pulsed split 35 psi (0.55 min) 65 psi injected volume 0.5-2.5 µL split flow 0-250 mL min-1 oven program initial temperature 175 °C initial time 0.55 min rate 100 °C min-1 final temperature 200 °C final time 2 min AED Parameters transfer line temperature cavity block temperature wavelength helium flowa ferrule purge spectrometer purge flow solvent vent H2 pressure O2 pressure a
270 °C 270 °C 303.419 nm 280 mL min-1 45 mL min-1 2 L min-1 0.16 min 20 psi 10 psi
Measured at the cavity vent.
the analytes seldom allow a GC run shorter than 15 min. The additional time necessary to cool the oven from the end temperature (>250 °C) to the starting temperature (∼60 °C) limits further the sample throughput. Fast GC, pioneered by Desty,22 has been the focus of interest for many years.23-25 An increase in efficiency has been achieved by reducing the column inner diameter (down to 10 µm), which allowed the use of shorter columns and, consequently, separations within several seconds instead of several minutes at the expense, however, of injection volume.23-25 The latter is critical in trace environmental analysis. Only recently, a bundle of ∼1000 1-mlong, 40-µm-i.d. wall-coated open-tubular capillaries, denoted as a multicapillary column, was reported to overcome this limitation,26,27 but, to our knowledge, no detailed reports have yet been published in the open literature. The objectives of this paper are to develop a rapid separation of ethylated butyl- and phenyltin compounds on a multicapillary column and to render multicapillary GC (UC-GC) compatible with MIP-AES for speciation of organotin compounds in sediments and biological materials. EXPERIMENTAL SECTION Apparatus. Chromatographic separations were carried out using an HP Model 6890 gas chromatograph (Hewlett-Packard, Wilmington, DE) equipped with a capillary split/splitless injection port with electronic pressure control. Detection was achieved with (22) Desty, D. H. Adv. Chromatogr. 1965, 1, 199-227. (23) Schutjes, C. P. M.; Vermer, E. A.; Rijks, J. A.; Cramers, C. A. J. Chromatogr. 1982, 253, 1-16. (24) Tijssen, R.; van den Hoed, N.; van Kreveld, M. E. Anal. Chem. 1987, 59, 1007-1015. (25) Annino, R. J. High Resolut. Chromatogr. 1996, 19, 285-290. (26) Cooke, W. S.; Wals, J. W.; Wiedemer, R. T. Book of Abstracts of Pittcon ’97, Atlanta, March 16-21, 1997; p P19. (27) Thiem, T. L.; Suto, C. C.; Bunner, J. C. Book of Abstracts of Pittcon ’97, Atlanta, March 16-21, 1997; p 403.
4800 Analytical Chemistry, Vol. 69, No. 23, December 1, 1997
Figure 1. Van Deemter (Golay-Giddings) curves obtained for a standard of Bu4Sn in isooctane, 1 µL injected in split. Column temperature, 175 °C. (A, 0) Multicapillary column; (B, 2) HP-1 capillarry column (30 m × 0.32 mm × 0.17 µm). HETP, height of a theoretical plate.
an HP Model G2350A atomic emission detector. Injections were made by means of an HP 6890 series automatic sampler. Data was handled using an HP Model D3398A ChemStation. Analyte species were separated on a multicapillary column which consisted of ∼900 1-m-long, 40-µm-i.d. capillaries coated with 0.2 µm of SE30 (Alltech Associates, Inc., PA). It was connected at both ends to deactivated alumina tubes (∼0.3 m × 0.53-mm i.d.). A piece (0.6 m × 0.32 mm) of deactivated silica tubing (Alltech) served as a transfer line to the detector. Zero-dead-volume feather-light connectors (Alltech) with associated ferrules were used. An HP-1 (30 mm × 0.32 mm × 0.17 µm) capillary column was used for comparison studies. Organotin compounds from biological materials and sediments were extracted in a 50-mL open vessel made of borosilicate glass using a Microdigest Model A301 (2.45 GHz, maximum power 200 W) microwave digester (Prolabo, Fontenay-sous-Bois, France) equipped with a TX32 programmer. Reagents. Analytical grade reagents obtained from Aldrich (Milwaukee, WI) and water deionized and further purified in a Milli-Q system (Millipore, Milford, MA) were used throughout unless otherwise stated. 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. The derivatization reagent was sodium tetraethylborate (NaBEt4) purchased from Strem (Bisscheim, France). The reagent was manipulated under dry nitrogen to prevent its degradation. Fresh 1% (w/v) aqueous solutions were prepared every 8 h. The acetate buffer was prepared by dissolving 1 M sodium acetate in water, followed by adjusting the pH to 5 with concentrated acetic acid. Tetramethylammonium hydroxide (TMAH) was bought from Fluka (St. Quentin-Fallavier, France). The plasma gas and carrier gas used for GC was helium, 99.9999%. The AED reagent gases were hydrogen, 99.996%, and oxygen, 99.995%. All the gases were supplied by AGA (Bassens, France). Standards and Samples. Individual stock solutions (3 mg mL-1 as Sn) of BuSnCl3 (MBT), Bu2SnCl2 (DBT), Bu3SnCl (TBT), PhSnCl3 (MPhT), Ph2SnCl2 (DPhT), Ph3SnCl (TPhT), and Pe3SnCl3 (Aldrich, France) were prepared in methanol. Diluted standards of each compound and mixtures of them were prepared weekly by dissolving the concentrated standards in methanol. Standard
Figure 3. Isothermal separation for a mixture of organotin compounds: 1, BuEt3Sn; 2, Bu2Et2Sn; 3, PhEt3Sn; and 4, Bu3EtSn. Oven temperture, 170 °C; carrier gas flow rate, 130 mL min-1.
Figure 2. Effect of column temperature and He carrier gas flow rate on the peak width for (A) BuEt3Sn, (B) Bu2Et2Sn, and (C) Bu3EtSn.
solutions were stored at 4 °C in the dark. Ethylated derivatives of MBT, DBT, TBT, MPhT, DPhT, and TPhT in hexane were obtained from the Community Bureau of Reference (BCR, Brussels, Belgium). Dilute mixtures of these compounds were prepared weekly in isooctane to give a working concentration of ∼2 µg mL-1 (as Sn). A solution (prepared daily) of ∼25 ng mL-1 (as Sn) for each compound in isooctane was used for the optimization of the separation conditions on the multicapillary column. A PACS-1 reference sediment with certified concentrations of butyltin compounds issued by the National Research Council of Canada (NRCC) was purchased from Promochem (Molsheim, France). A fish tissue reference material (NIES11) with a certified
concentration of TBT and an indicative value of TPhT was a gift from the National Institute for Environmental Studies, Japan. A BCR 462 reference sediment with certified concentrations of DBT and TBT was obtained from the Community Bureau of Reference (BCR, Brussels, Belgium). Procedures. Sample Preparation. Microwave-assisted procedures developed and described in detail elsewhere20,21 were used. For sediments, a sample of 0.1-0.2 g was spiked with 100 µL of the Pe3SnCl solution and leached with 10 mL of acetic acid solution (1 + 1) at a microwave power of 60 W for 2 min. The supernatant solution was transfered to a clean tube by means of a Pasteur pipet, neutralized using 4.9 mL of concentrated aqueous NH3, buffered with 10 mL of buffer, and extracted with 1 mL of NaBEt4 solution into 1 mL of isooctane for 5 min. The organic phase was collected in an autosampler vial and analyzed by MCGC/MIP-AED. For biological materials, a sample of 0.1-0.2 g was placed in an extraction tube and spiked with 100 µL of the Pe3SnCl solution. After the addition of 5 mL of acetic acid, 3 mL of 2% (w/v) NaBEt4 solution, and 1 mL of nonane, the mixture was exposed to microwaves at a power of 40 W for 3 min. After cooling (∼2 min), the supernatant was subject to cleanup on an alumina column and was analyzed by MC-GC/MIP-AED. Alternatively, a sample of 0.15 g of biomaterial was hydrolyzed in a microwave field (50 W, 2 min) with 5 mL of TMAH. The solution was neutralized with 1.3 mL of acetic acid and buffered to pH 5 with 15 mL of the buffer solution. After the addition of 1 mL of isooctane and 1 mL of the NaBEt4 solution, the mixture was shaken for 5 min. The emulsion was broken by submitting the extraction tube to a microwave field for 2 min at 20 W. The supernatant was subject to cleanup on an alumina column, followed by analysis by MC-GC/MIP-AED. Gas Chromatography. Optimum GC and detector operating conditions are summarized in Table 1. RESULTS AND DISCUSSION Features of Multicapillary Columns. It is well known in gas chromatography that efficiency (number of theoretical plates per unit length) increases with the decreasing inner diameter of the capillary.28,29 However, the lower the inner diameter, the lower Analytical Chemistry, Vol. 69, No. 23, December 1, 1997
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Figure 4. Effect of the column temperature on the resolution between butyl- and monophenyltin compounds and on the retention of Ph3EtSn. Carrier gas flow rate, 65 mL min-1.
also the injectable sample volume and, consequently, the higher the minimum detectable concentration of analyte. For these reasons, capillaries with an internal diameter below 0.25 mm are seldom used. A way to overcome the above limitations is to increase the cross section of the carrier flow-rate by assembling a large number (∼1000) of small-diameter capillaries into a bundle. This makes it possible to combine the efficiency of a smalldiameter (∼0.04 mm) capillary with the sample capacity of a conventional capillary column. Figure 1 compares the Van Deemter (Golay-Giddings) curves obtained for Bu4Sn for a multicapillary and for a conventional capillary column with a similar coating (100% poly(dimethylsiloxane)). Whereas a conventional 0.32-mm capillary shows a narrow maximum of efficiency, defined as the minimum value of the height of a theoretical plate (HETP), with a gas velocity range of 20 cm s-1 of He, corresponding to a column flow of 1 mL min-1, the shape of the curve for the multicapillary column shows two important peculiarities. First, the minimum HETP value is half that obtained with the conventional capillary column. Second, this minimum is very broad (80-280 cm s-1 or 60-210 mL min-1), which allows the use of high flow rates to shorten chromatographic separations without sacrificing peak resolution. Consequently, efficient separations can be achieved in a considerably shorter time. The high linear flow rates (up to 3 m s-1) through a multicapillary column set strong requirements in terms of injection time to avoid peak broadening and in terms of the detector’s time constant to assure the acquisition of a representative number of data points for a short transient signal (10 Hz was used to monitor the signal for organotin compounds). The fast injection can be ensured by using a split injector; this causes a loss in sensitivity which should be compensated by the low absolute detection limit of the detector. A microwave-induced plasma atomic emission spectrometer matches the above characteristics in a close to ideal (28) Grant, D. W. Capillary Gas Chromatography; Wiley: Chichester, 1995. (29) Grob, R. L. Modern Practice of Gas Chromatography, 3rd ed.; Wiley: Chichester, 1995.
4802 Analytical Chemistry, Vol. 69, No. 23, December 1, 1997
way. Absolute detection limits for many elements are at the 0.1pg level, and the response time is fast.30,31 Another advantage is the low dead volume of the detector, which does not distort the sharp (half-width down to 0.3 s) chromatographic peaks. The high elemental selectivity minimizes the background even in the analysis of complex samples. Separation of Organotin Compounds on a Multicapillary Column. Sample preparation for organotin speciation analysis usually produces two series of ethyl derivatives:32 one for butyltin compounds, BuSnEt3 (bp 208 °C), Bu2SnEt2 (bp 245 °C), and Bu3SnEt (bp 265 °C), and one for phenyltin compounds, PhSnEt3 (bp 254 °C), Ph2SnEt2 (bp 285 °C), and Ph3SnEt (bp 363 °C). The boiling point values are approximate; they were obtained by extrapolation as described elsewhere,33 assuming (1) a linear dependence of the chromatographic retention time on the boiling point of an eluting compound and (2) a linear increase in the retention time with the number of carbon atoms in a similar (alkyl or aryl) structure. The ideal method should allow the separation of butyl- and phenyltin compounds in one run, but methods for butyltin compounds only are not uncommon. The objective of the optimization was to reduce as much as posible the duration of the GC run, keeping in all the cases a baseline resolution between the analytical peaks. Two independent approaches, one for butyltins only, and one for comprehensive speciation including phenyltins, were considered. Three separation modes were evaluated: (1) isothermal and isobaric (isocratic), (2) isothermal and pressure (flow) programmed, and (3) pressure (flow) and temperature programmed. Isothermal and Isocratic Separation. This mode is particularly advantageous because it practically eliminates the need for a GC (30) Wylie, P. L.; Quimby, B. D. J. High Resolut. Chromatogr. 1989, 12, 813818. (31) Becker, G.; Colmsjo, A.; Janak, K.; Nilsson, U.; Ostman, C. J. Microcolumn Sep. 1996, 8, 345-351. (32) Ceulemans, M.; Dirkx, W. M. R.; Łobin ˜ski, R.; Adams, F. C. Fresenius’ J. Anal. Chem. 1993, 347, 256-262. (33) Feldmann, J.; Hirner, A. V. Int. J. Environ. Anal. Chem. 1995, 60, 339359.
A
Table 2. Reproducibilty of MC-GC/MIP-AED Analysisa retention time
peak area
peak height
peak width (min)
compd
mean (min)
RSD (%)
mean
RSD (%)
mean
RSD (%)
mean
RSD (%)
MBT DBT TBT MPhT DPhT TPhT
0.193 0.304 0.497 0.409 0.951 2.241
0.3 0.2 0.2 0.2 0.1 0.1
21.4 23.8 25.2 23.8 11.3 16.3
5.5 4.0 4.3 3.8 3.9 2.5
31.5 28.7 21.8 25.6 12.3 6.0
4.7 3.4 3.4 3.5 1.7 2.8
0.011 0.013 0.017 0.015 0.014 0.042
4.9 3.6 2.5 2.1 0.0 1.7
a Results obtained for 10 injections of 1 µL of a mixture of ethylated butyl- and phenyltin compounds (∼20 ng mL-1 each) in isooctane.
Table 3. Sensitivity and Detection Limits in Speciation of Organotin Compounds by MC-GC/MIP-AEDa
B
system response per picogram injected compd
area
height
area
height
MBT DBT TBT MPhT DPhT TPhT
0.9 1.0 1.1 1.0 0.6 0.5
1.3 1.3 1.0 1.1 0.6 0.2
0.27 0.24 0.22 0.24 0.40 0.48
0.16 0.16 0.21 0.19 0.35 1.05
a
Figure 5. (A) Chromatogram for a mixture of ethylated organotin compounds under optimum instrumental conditions (cf. Table 1): 1, BuEt3Sn; 2, Bu2Et2Sn; 3, PhEt3Sn; 4, Bu3EtSn; 5, Ph2Et2Sn; and 6, Ph3EtSn. (B) Contributions to the cavity flow rate during the chromatographic run.
oven (a simple insulated Al tube is sufficient) and the need for temperature programming electronics. Moreover, the sample throughput increases considerably because there is no postrun cooling necessary, which is often as long as the run itself. The high efficiency of the multicapillary column makes it possible to work at lower temperatures, sparing the coating. Optimization was carried out using a two-parameter (column flow and column temparature) total factorial experiment with the objective to minimize the retention time of Ph3EtSn while preserving the baseline resolution of the early-eluting butyltin peaks. Column flow was changed in the range 65-200 mL min-1, whereas column temperature was varied from 140 to 200 °C. The maximum oven temperature is limited by the resistance of the ferrules between the alumina tube and the multicapillary column rather than by the stability of the coating. If the separation of butyltin compounds only is considered, a relative large number of combinations of the column flow rate and the column temperature allow baseline resolution of the analyte compounds within times from 0.4 to 1.5 min. Figure 2 shows that a peak width below 0.020 min (a typical value for a conventional 0.32-mm capillary) can be obtained for all ethylated butyltin compounds at temperatures exceeding 170 °C, even at relatively low flows (65 mL min-1). The peak width can be decreased (and thus the response in the peak height mode
detection limits (S/N 3) (ng/mL as Sn)
One microliter injected.
increased) by increasing the flow rate. The system optimizes at 175 °C with the column flow rate of 130 mL min-1, leading to the separation of all the butyltins within a time envelope of 15 s (Figure 3). The retention time of the last-eluting Bu3SnEt is ∼25 s, compared to 5-8 min in procedures which use 0.32-mm capillaries and oven temperature gradient programming.34 Under the elution conditions optimized for the separation of butyltin compounds, ethylated di- and triphenyltin, if present in a sample, are retained strongly on the column. They elute only after 1.3 and 5.7 min, respectively, as distorted broad peaks. A trade-off between the resolution for the early-eluting compounds and the retention time of triphenyltin is thus necessary (Figure 4). The most critical is the separation between Ph3SnEt and Bu3SnEt. At low flow rates (65 mL min-1), the highest possible temperature which still allows baseline resolution of these compounds is 190 °C. Under these conditions, Ph3SnEt elutes as a broad peak at almost 3 min. Hence, the only possibility to elute Ph3SnEt below 3 min in a good shape while maintaining the resolution of butyltin species appears to be to increase the pressure (flow rate) after Bu3SnEt passed, leading to the isothermal and pressure (flow rate)-programmed separation mode described below. Isothermal and Pressure (Flow Rate)-Programmed Separation. In the ideal case, the separation should be run at the conditions described in Figure 3, and as soon as Bu3SnEt has passed, the flow rate should be increased to the maximum value (∼210 mL min-1) within the mimimum of the Van Deemter curve (cf. Figure 1). The reality is marked, however, by the following major restrictions: (1) the maximum pressure increase rate allowed by (34) Michel, P.; Averty, B., Appl. Organomet. Chem. 1991, 5, 393-397.
Analytical Chemistry, Vol. 69, No. 23, December 1, 1997
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Figure 6. Effects of the injected volume and of the split flow on the peak height. Data exemplified for Bu3EtSn. Table 4. Results Obtained for Organotin Compounds in Certified Materials by MC-GC/MIP-AED concn found in PACS -1 (µg/g as Sn)
a
concn found in BCR CRM 462 (µg/g as ion)
compd
certified
found
certified
found
MBT DBT TBT TphT
0.28 ( 0.17 1.16 ( 0.18 1.27 ( 0.22
1.18 ( 0.12 1.00 ( 0.05 1.27 ( 0.04
(13-244)a 128 ( 16 70 ( 14
135 ( 9 106 ( 9 54 ( 2
concn found in NIES-11 (µg/g as chloride) certified
found
1.3 ( 0.1 6.3b
1.19 ( 0.02 6.23 ( 0.70
Reported values in the literature. b Reference value.
the instrument software is 15 psi min-1 (corresponding to a flow of ∼55 mL min-1), which means almost 3 min is required to reach the flow rate of 210 mL min-1, and (2) the sensitivity of MIPAED for Sn being critically dependent on the plasma gas flow rate, it is necessary to maintain the sum of the column flow rate and the makeup gas constant during the chromatographic run in order to maintain a uniform detector response. The first of the above constraints can be overcome by taking advantage of the pulsed split injection mode. Normally, this injection mode is used to increase rapidly the column pressure at the beginning of a GC run in order to introduce the narrowest possible sample plug on the column and to decrease it after a few seconds to allow the chromatographic separation of the analytes. Here, this mode was used in the opposite way. First, for 0.55 min until TBT has eluted of the column, a low (35 psi) column pressure is maintained, to be followed by a high (65 psi) pressure setting. In this way, the inverse pulsed split injection mode makes it possible to increase the column pressure from 35 to 65 psi, and thus to add ∼110 mL min-1 to the column flow rate, within 5 s. 4804 Analytical Chemistry, Vol. 69, No. 23, December 1, 1997
Regarding the second constraint, the most sensitive detection of Sn is obtained with high makeup gas flows; flow rates of 230250 mL min-1 measured at the cavity vent have been reported to be optimal in the literature.14,15 The cavity flow is the result of the column flow, the normal makeup flow, and the extra makeup flow (a fixed amount of 110 mL min-1 added by a makeup valve). In order to maintain the same optimal cavity flow during the whole GC run, an increase in the column flow rate has to be compensated by a decrease in the cavity helium makeup flow; otherwise, a considerable loss of sensitivity for DPhT and TPhT is observed. The practical problem is that the cavity makup flow is not controlled electronically, whereas its manual adjustment during a chromatographic run is highly irreproducible. An attractive way is to compensate for an increased column flow is to use the extra makeup flow valve of which the opening and closing can be controlled by the software. The consequence of the above is the need to carry out the separation of butyltin species with a much lower column flow (65 mL min-1) than would result from Figure 3, which makes the separation of BuSnEt3, Bu2SnEt2, PhSnEt3, and Bu3SnEt longer
Figure 7. Effects of the injected volume and of the split flow on the peak width for (A) BuEt3Sn and Bu2Et2Sn (data exemplified for BuEt3Sn) and (B) higher boiling compounds (data exemplified for Bu3EtSn).
(0.55 instead of 0.4 min). Also, a loss of sensitivity in the peak height mode needs to be taken into account because of the larger peak width (cf. Figure 3). At 0.55 min, the makeup valve is closed (loss of 110 mL min-1 at the cavity vent), and within 5 s the column flow rate is increased from 65 to 170 mL min-1 (in terms of pressure from 35 to 65 psi), resetting the optimum detection conditions. The retention times for DPhT and TPhT are 1.3 and 4.8 min, respectively.
Note that the instrument hardware does not allow for keeping the cavity vent flow constant. If one of the three flow rate values (column flow, normal makeup, or extra makeup flow) changes, the resulting change in the flow measured at the cavity vent entails a change in sensitivity. Pressure (Flow Rate)- and Temperature-Programmed Separation. Separations on a multicapillary column being fast, the possibility of refining them by oven temperature programming is restricted. Analytical Chemistry, Vol. 69, No. 23, December 1, 1997
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In the discussed case, however, the run can be made considerable shorter if, at 0.55 min, the oven temperature is suddenly increased to 200 °C, which is the maximum allowable column temperature. Figure 5 shows a chromatogram for a mixture of ethylated organotin compounds obtained under the optimum conditions. All the compounds are eluted in less than 2.5 min, the Ph3SnEt peak being much broader than the others. The use of this GC program, which assures a rapid elution of relatively nonvolatile compounds, is also recommended for the analysis of “dirty” sample extracts in order to prevent the contamination of the column, even if Ph3Sn is not of concern. Flow rates of up to 300 mL min-1 can be used to clean the column after the last analyte compound has eluted. Optimization of Injection Conditions. Short retention times of organotin compounds on a multicapillary colum require a fast injection technique to avoid peak broadening. This can be assured by the use of an automatic injector (manual injection is much slower and produces wider peaks25) and by working in the split mode. The parameters to be optimized include the volume injected and split flow. They affect the sensitivity (peak height) and peak width. Figure 6 shows the effect of the split ratio on sensitivity and peak width for the studied compounds. The pattern remains the same for all the compounds. The peak height increases linearly (R2 > 0.995) with the injection volume up to 2.5 µL. Higher injected volumes result in the saturation of the column and, consequently, in a nonlinear response especially for more volatile compounds. Maximum sensitivity is obtained with the split flow close to zero, which means, in fact, a splitless injection. The zero split flow has a consequence of a slight increase in the peak width for the most volatile compounds, MBT and DBT, whereas for the rest of them the effect is negligible (Figure 7). Analytical Figures of Merit. Under the optimum conditions, the reproducibility of retention time reaches 0.1-0.3% (Table 2). Peak width measured at the half intensity is below 1 s, with the exception of the triphenyltin peak, for which the peak is 3 times broader. Measurements in the peak height mode are slightly more precise that those in the peak area mode, with a relative standard deviation of 3-4%. Table 3 summarizes detection characteristics of multicapillary GC/MIP-AED in speciation analysis for organotin compounds. The response factor (emission intensity per mass unit injected) in the peak height mode is strongly dependent on the volatility of the analyte compound and varies by a factor of 6 between the firsteluting BuSnEt3 and the last-eluting PhSnEt3. This is due to the isothermal elution mode. In the peak area mode, there is a difference between the response factor for the first four compounds and that for diphenyland triphenyltin. This is probably due to the different detection conditions as a result of a change (despite the nominal compensation) in the cavity flow rate. Noise is stable throughout the run, and its standard deviation is about 0.07 eu in peak height and 0.08 eu in peak area mode. Detection limits calculated as 3 times the standard deviation of the noise vary from 0.16 pg for BuSnEt3 to 1.05 pg for Ph3SnEt in the peak height mode (Table 3). The run takes 2.5 min, with another 2.5 min necessary for the system to be ready to accept the next sample. The sample throughput reaches thus 12 h-1, compared to 2-3 h-1 in the analysis by the hitherto available methods. 4806 Analytical Chemistry, Vol. 69, No. 23, December 1, 1997
Figure 8. Chromatograms obtained for analysis of certified reference materials by MC-GC/MIP-AED. (A) NIES-11: 1, BuEt3Sn; 2, Bu2Et2Sn; 3, PhEt3Sn; 4, Bu3EtSn; 5, Pe3EtSn; 6, Ph2Et2Sn; and 7, Ph3EtSn. (B) BCR 462: 1, BuEt3Sn; 2, Bu2Et2Sn; 3, PhEt3Sn; 4, Bu3EtSn; and 5, Pe3EtSn. (C) PACS-1: 1, BuEt3Sn; 2, Bu2Et2Sn; 3, Bu3EtSn; and 4, Pe3EtSn.
Validation of the Analysis of Sediments and Biological Materials. The method developed was validated by the analysis of certified reference materials. Chromatograms obtained for NIES-11, BCR-462, and PACS-1 CRMs are shown in Figure 8AC, respectively. Tripropyltin, usually used in capillary GC analysis, was replaced by tripentyltin, which is better resolved from the butyltin compounds. Quantification was done by the method of standard additions. One microliter of extract was injected using a split flow of 45 mL/min, except for BCR-462 (no split flow). NIES11 extracts were submitted to a cleanup step to avoid contamination of the column with fats and proteins. No degradation in column performance was noticed under optimized conditions during the study. Table 4 shows good agreement between the values obtained and the certified ones for the concentrations determined. The MBT concentration found is 3 times higher than the certified value. It is, however, close to the values found by Ceulemans and Adams35 and Chau et al.36 and those found earlier using the microwave-assisted extraction procedure applied in this study. It should be pointed out that it is unlikely that any MBT may be generated by degradation of TBT and/or DBT in a microwave field; the values obtained for the two latter compounds match perfectly the certified values. The lower values of the MBT concentration in sediment CRMs reported elsewhere in the (35) Ceulemans, M.; Adams, F. C. Anal. Chim. Acta 1995, 317, 161-170. (36) Chau, Y. K.; Yang, F.; Brown, M. Anal. Chim. Acta 1995, 304, 85-89.
literature are apparently due to the problems with quantitative extraction of this highly polar compound from a sediment matrix. CONCLUSIONS This paper shows that the use of a multicapillary column can speed up the gas chromatographic separations about 10 times without loss in efficiency. Carrying out the separations in the isothermal mode instead of in the temperature programming mode makes it possible to avoid the use of a cumbersome GC oven and to reduce the dimensions of the separation unit. In particular, the coupling of multicapillary GC with sensitive element-selective detection offers an attractive tool for speciation analysis of anthropogenic organometallic pollutants in complex environmental samples and offers an attractive possibility to be incorporated in a dedicated field speciation analyzer.
ACKNOWLEDGMENT The study was financially supported by the EC (Contract SMT4-CT96-2044 Automated Speciation Analyser) and the French Government (Ministe`re de l’Economie - D.G.C.C.R.F.). I.R.P. acknowledges a postdoctoral fellowship of the Spanish government.
Received for review April 16, 1997. Accepted September 18, 1997.X AC970410E X
Abstract published in Advance ACS Abstracts, November 1, 1997.
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