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Anal. Chem. 1909, 6 I , 2320-2322
Ionspray Mass Spectrometry/Mass Spectrometry: Quantitation of Tributyltin in a Sediment Reference Material for Trace Metals’ K. W. M. Siu,* G. J. Gardner, and S. S. Berman Division of Chemistry, National Research Council of Canada, Montreal Road, Ottawa, Ontario, Canada K I A OR9
Tributyltln (TBT) concentratlon In a sedlment reference material for trace metals, PACS-l, was determined by uslng Ionspray mass spectrometry/mass spectrometry. TBT was extracted Into isooctane or 1-butanol, dlluted wlth methanol contalnhg 1 mM ammonium acetate, delivered to the lonspray tandem mass spectrometer by uslng flow injection, and quantttated by means of selected reaction monltorlng of the daughter/parent palr of m / r 179/291. The minlmum detectable amount of TBT was about 5 pg of Sn absolute 01 0.2 jtg of Sn/g of sedlment. PACS-1 was found to contain tributyltln at a concentratlon of 1.29 f 0.07 pg of Sn/g of sedlment .
INTRODUCTION Organotin compounds have wide ranging chemical and toxicological properties. They find application in usages as diverse as poly(viny1 chloride) stabilizers, fungicides, pesticides, and marine antifoulants, to name just a few. The most important organotin in the marine environment is tributyltin (TBT), the active ingredient in antifouling paint. TBT is toxic to shellfish at ng of Sn/mL levels. As a result, the use of TBT as an antifoulant is now banned in many countries. Model studies have shown that TBT has a high tendency to adsorb on particulates and thus be available to benthic organisms (1I 2). The two most common techniques for TBT determination are gas chromatography with flame photometric detection (GC-FPD) ( 3 ) and hydride generation-atomic absorption spectrometry (HGAAS) ( 4 , 5 ) . Both methods require fairly extensive sample work up including extraction, cleanup, and derivatization. We have recently shown that ionspray mass spectrometry/mass spectrometry (ISMS/MS) to be an extremely sensitive technique for T B T determination, having a minimum detectable amount in the low picogram range (6). Ionspray (IS) and its related technique, electrospray, are fairly new ion desorption techniques for atmospheric pressure ionization mass spectrometry (APIMS). As well, they are excellent methods for interfacing liquid chromatography (LC) or capillary zone electrophoresis to APIMS (7-9). In IS, the analyte solution-whether it is a flow injection analysis (FIA) carrier solution, a liquid chromatographic eluant, or an electrophoretic effluent-is allowed to flow through, at a rate of 1-500 pL/min, a very narrow-bore capillary tubing (50-200 pm) polarized to a high voltage (2-3 kV). A coaxial flow of nitrogen is installed to faciliate spraying, thus effecting a lower optimum polarizing voltage or a higher maximum operable flow rate than electrospray. Droplets emerging from the capillary tubing are electrically charged. These are pnematically sheared off and/or electrically repelled into the surrounding gas once surface tension is overcome. As a droplet evaporates during its flight to the sampling plate (counterNRCC 30533.
electrode), coulombic repulsion increases due to a decrease in its surface area. Ion evaporation takes place when coulombic repulsion overcomes ion solvation forces and results in ions being ejected into the surrounding gas. The most significant difference between ionspray and other introduction/ionization techniques for mass spectrometry is that, in IS, no ionization process exists; the analyte must be present as ions in solution otherwise it will not be seen. This means that the analyte must have an ionizable functional group or be able to form an ionic adduct in solution. Polar organotin compounds, e.g. butyltin halides, acetates, etc., form pentacoordinated anionic adducts readily in solution (6, 10). These have been detaiIed in a recent study (6). In addition, cationic species may also be formed due to heterolytic cleavage of the tin-(pseudo)halide bond (6). When monitored by IS, tributyltin chloride shows an intense TBT+ ion at m / t 291. This report describes application of IS to the quantitation of tributyltin in a sediment reference material for trace metals, PACS-1.
EXPERIMENTAL SECTION Instrumentation. The ionspray tandem mass spectrometer has been described in detail elsewhere (11). Briefly, the LC/FIA side consisted of a reciprocating pump (Waters Model 590) capable of delivering from 1 pL/min to 10 mL/min of mobile phase, and a valve injector (Rheodyne Model 7125 or 7520 for injection of 100 or 0.5 pL of samples, respectively). The injector was connected to the ionspray interface via a 1 m X 50 pm i.d. fused-silica tubing. The carrier solution was methanol containing 1 mM ammonium acetate at a flow rate of 5 pL/min. The ionspray probe was built in-house and consisted of two coaxial stainless steel capillary tubes, 33 and 22 gauge (ca. 100 and 400 pm i.d., respectively), polarized to 2.5 kV. Liquid chromatographic effluent/FIA carrier solution was routed to flow in the inner 33 gauge tubing. Nitrogen (40psi, pressure controlled) flowed in the channel between the inner and the outer, 22 gauge tubing, acting as a nebulizer gas. With this setup, a stable ionspray was obtained by using a liquid flow rate as low as 1 pL/min to as high as about 500 pL/min. The MS used was an API mass spectrometer (SCIEX TAGA 6000 prototype). For ionspray, the corona discharge assembly and housing were detached. The ionspray probe tip was positioned about 1-2 cm from the interface plate (the ionspray counterelectrode) and about 1 cm off-axis from the sampling aperture. The interface plate was usually at h450 V. Other MS operating conditions were identical with those used for corona discharge ionization. For tandem MS runs, argon was used as the collision gas at a target gas thickness of about 1.3 X 1014atoms cm-2, which allowed on the average less than one collision event per ion. The laboratory-frame collision energy was 61 eV. Reagents. Tributyltin chloride was purchased from a commercial source (Aldrich). Its purity was confirmed by highperformance liquid chromatography-inductively coupled plasma mass spectrometry (HPLC-ICP/MS). Solutions were made up usually in methanol and refrigerated when not in use. Solvents were “Distilled-in-glass”grade (Caledon). Acids were sub-boiled distilled from reagent grade stocks. All other chemicals were reagent grade or better. The sediment reference material for trace metals was PACS-1 (National Research Council of Canada), which was collected from
0003-2700/89/0361-2320$01.50/0Published 1989 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 20, OCTOBER 15, 1989
CID of m/z 291
I
118,
2321
291
881
+
1
+
I
M/ 1
Ionspray mass s ectrum of tributyltin chloride showing the isotopic distribution of TBTP. Bu = butyl.
Flgure 1.
the Esquimalt Harbour in British Columbia. TBT Extraction. Four grams of PACS-1 plus the appropiate TBT spike was placed in a 50-mL borosilicate glass centrifuge tube, followed by 4 mL of 10 M hydrochloric acid and 8 mL of methanol. The centrifuge tube was placed in an ultrasonic bath and sonicated for 1h. Four milliliters of isooctane was then added. The centrifuge tube contents were shaken vigorously for 3 min and then centrifuged at about 2000 rpm for 10 min. The isooctane phase was removed; 0.5 mL of it was diluted to 25 mL with methanol containing 1mM ammonium acetate (the FIA carrier solution). Alternatively, 4 g of PACS-1 was placed in a 50-mL centrifuge tube, together with 8 mL of 1-butanol. The suspension was sonicated for 1h and centrifuged as previously described. The 1-butanol phase was removed, and 1 mL of it was subsequently diluted to 25 mL with methanol containing 1 mM ammonium acetate. ISMS/MS Analysis. The usual sample size was 0.5 pL. For quantitative analysis, the mass spectrometer was operated under selected reaction monitoring (SRM) of the parent/daughter pair, m / z 291/179. No liquid chromatographic separation was necessary due to the high selectivity in this mode.
RESULTS AND DISCUSSION As noted earlier (6),T B T compounds respond under both the positive and negative ion detection modes in ionspray mass spectrometry. In the negative ion detection mode, TBT forms adduct anions, whose nature is dependent on matrix composition. This is deemed inconvenient as any analytical methodology based on these adducts would be too matrix dependent. On the contrary, in the positive ion detection mode, polar T B T compounds yield only one tin-containing ion, the TBT+ ion. This is desirable for analytical purposes since the same tin ion is always formed irrespective of counterions, solvents, and matrix composition. No DBT2+ (dibutyltin) and MBT3+(monobutyltin) have been seen. These ions are likely to have too high charge-to-size ratios (their solvation energies are likely too high) to evaporate effectively (11). When chloride ions are present, the singly charged adduct ion (DBT-Cl)+ is observed albeit with sensitivity too low for pursuing. An ionspray mass spectrum of tributyltin chloride is shown in Figure 1,showing the isotopic distribution of the TBT+ ion. A daughter ion spectrum of mlz 291, the most abundant TBT isotope, is shown in Figure 2. The daughter ions, mlz 235, 179, and 123, are apparently formed via loss of one, two, and three butene molecules from the parent ion. Figure 3 shows selected reaction monitoring of the daughter/parent pair of m/z 179/291. The samples were tributyltin chloride standard solutions containing 30, 15, and 50 pg of Sn. Figure 4 shows the same selected reaction monitoring of PACS-1 extracts. By use of the method of standard additions, the T B T concentration in PACS-1 was determined to be 1.29 f 0.07 pg of Sn/g (n = 5). Results obtained by using the two different extraction procedures were indistinguishable.
mi2
Flgure 2.
Collision-induced dissociation (CID) of m l r 291 from T B P .
Bu = butyl. m i z 1791291
h
z
I
i 5 o p g ~
.%
.C
..-e F
d
Time (min) Flgure 3. Flow injection analysis of TBT standard solutions: triplicate injections of 3Q,15, and 50 pg of Sn. SRM of daughterlparent pair, m l z 1791291.
:J
mlz 1791291
PACS
+
2 ug Snig I
%me (min)
Flow injection analysis of PACS-1 extracts: determination by using standard additions method. Flgure 4.
For quantitating T B T in real life samples, e.g. sediment extracts, the use of selected reaction monitoring rather than the less selective single ion monitoring is imperative, particularly when no prior separation is performed. For a sample as complex as a sediment extract, its ionspray mass spectrum is littered with peaks from low to high mass, including m / z 291, whether it contains TBT or not. In the SRM measurement, the non-TBT contribution in the m / z 291 signal was filtered out (any parent ion of mlz 291 that did not yield a daughter ion of m / z 179 was not counted), and the tandem mass spectrometer operated as a T B T specific detector. No interference from DBT and MBT was expected or observed. This tremendously simplified the sample work up since detection as specific as this required minimal processing. As it turned out, no liquid chromatographic separation, off line
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 20, OCTOBER 15, 1989
Table I. TBT D e t e r m i n a t i o n in PACS-ln (pg
of Sn/g of sediment) ISMS/MS
GC-FPD
HPLC-ICP/MS
1.29 f 0.07b (5Y
1.13 f 0.30 (15)
1.18 f 0.15 (7)
'Certified value: 1.27 f 0.22 pg of Sn/g (95% confidence interN u m b e r in parentheses i s the number
val). Standard deviation. of replicate analyses.
or on line, was needed after sediment extraction. That was convenient as some reversed-phase LC materials had been observed to cause degradation of T B T (12), bypassing LC avoided this potential problem. The isooctane extraction isolated T B T as the chloride by virtue of the presence of a high concentration of hydrochloric acid. The T B T form extracted into 1-butanol is unknown. This mattered little as T B T compounds are polar and expected to dissociate into TBT+ and a counterion in a polar solvent such as methanol containing 1mM ammonium acetate. The dilution of the PACS-1 extract (in isooctane and l-butanol) with the flow injection carrier solution served several purposes: (1)dilution placed T B T in a suitable matrix that promoted T B T ionization, (2) it provided a suitable medium where stable ionspray could take place, and (3) T B T concentration in PACS-1 is high, dilution lowered the concentration to a level convenient for analysis. Compared to that of standard solutions of TBT, the response of TBT spikes to PACS-1 was suppressed by a factor of about 2. This was attributed to matrix effects as signals of PACS-1 samples spiked with equal amounts of T B T before and after the extraction were indistinguishable, showing that the discrepancy was not due to nonquantitative TBT recovery. The recoveries for the isooctane and 1-butanol extractions were determined as 95 f 4% and 97 f 6%, respectively. The rationale for using two rather different solvents was that they might extract different portions of the sediment matrix, and therefore one might exhibit less matrix suppression on the TBT signal than the other one. As it happened, the suppressions for both extractants were comparable. The nature of this interference is unknown; adduct formation, ion pairing, different solvation, etc., could all affect the concentration of TBT+ in the gas phase, and hence its ionspray signal. The T B T determination using ionspray tandem mass spectrometry was rapid. The most time-consuming portion was sample extraction/work up, which took about 1.5 h. The analysis itself was extremely fast; 10 injections could be made in a time as short as 12 min (Figure 4). The accuracy of this
method is demonstrated by comparing the TBT concentration in PACS-1 as determined by this method and by two other techniques, GC-FPD (13) and HPLC-ICP/MS (14) (Table I). Agreement among the three methods as well as with the certified value is good. The minimum detectable amount of T B T in sediment, as defined as the signal equivalent to two times the standard deviation of the blank, was about 0.2 pg of Sn/g of sediment, using the present sample work up. The relatively high relative detection limit was due to the 25-fold dilution made on the sediment extracts; most of the blank signal originated not from TBT but from electronic noise. The absolute detection limit was about 5 pg of Sn when matrix interference was absent, which is superior to that of Hg-AAS (20-50 pg of Sn (5))and GC-FPD (30 pg of Sn (13,15)).The ionspray relative detection limit is comparably inferior (0.6 pg of Sn/g for HG-AAS ( 4 ) and 30 ng of Sn/g for GC-FPD (13,16),a consequence of dilution and small (0.5 pL) sample size.
ACKNOWLEDGMENT We thank P. S. Maxwell and J. W. McLaren of this laboratory for GC-FPD and HPLC-ICP/MS analyses of PACS-1. Registry No. T r i b u t y l t i n chloride, 1461-22-9. LITERATURE CITED Donald, 0. F. X.; Weber, J. H. Environ. Sci. Technol. 1985. 79, 1104-1 110. Randall, L.; Weber, J. H. Sci. TotalEnviron. 1988, 5 7 , 191-203. Maguire, R. J. Environ. Sci. Technol. 1984, 78, 291-294. Randall, L.; Han, J. S.; Weber, J. H. Envlron. Technol. Len. 1988, 7 , 571-576. Donard. 0.F. X.; Rapsomanikis, S.; Weber, J. H. Anal. Chem. 1988, 5 8 , 772-777. Siu, K. W. M.; Gardner, G. J.; Berman, S. S. Rapid Commun. Mass Spectrom. 1988, 2 , 201-204. Whitehwse, C. M.; Dreyer, R. N.; Yamashita. M.: Fenn, J. B. Anal. Chem. 1985, 5 7 , 675-679. Bruins, A. P.; Covey. T. R.; Henion, J. D. Anal. Chem. 1987, 59, 2642-2646. Smith, R. D.; Oiiares, J. A.; Nguyen, N. T.; Udseth, H. R. Anal. Chem. 1988, 6 0 , 436-441. Neumann, W. P. The Ofganic Chemistry of Tin; John Wiiey 8 Sons: New York, 1970; p 14. Siu, K. W. M.; Gardner, G. J.; Berman. S. S. Org. Mass. Spectrom., in press. Siu, K. W. M.; McLaren, J. W.; Maxwell, P. S.; Gardner, G. J.; Berman, S. S. Analytical Chemistry of Butynins; Oceans 88, Baltimore, MD, Nov 1988. Siu, K. W. M.; Maxwell, P. S.; Berman, S. S. J . Chromatogr. 1080, 475, 373-379. McLaren, J. W.; Siu, K. W. M.; Lam, J. W.; Maxwell, P. S.;Palepu, A,; Berman, S. S., submitted for publication in Anal. Chem. Aue, W. A.; Hastings, C. R. J . Chromatogr. 1073, 8 7 , 232-235. Hattori, Y.; Kobayashi, A.; Takemoto, S.; Takami, K.; Kuge, Y.; Sugimae, A.; Nakamoto, M. J . Chromatogr. 1084, 315, 341-349.
RECEIVED for review April 3, 1989. Accepted July 19, 1989.
