Anal. Chem. 1981, 53, 1077-1082
1077
Sampling and Determination of S ,SI S-Tributyl Phosphorotrithioate, Dibutyl Disulfide, and Butyl Mercaptan in Field Air Bruce W. Hermann' and James N. Seiber' Department of Environmental Toxicobgy, Unhrers& of California, D a h , California 956 16
A method was developed for shwtlaneously trapplng airborne S,S,S-trlbutyi phosphorotdthloate (DEF), dl-n-butyl disulfide, and n-butyl mercaptan by accumulatlve sampling through a two-stage, high-volume sampler. The upstream sampllng stage, conslstlng of XAD-4 resln, quantltatlvely trapped DEF and dibutyl dlsutflde; POskiampPng extractlon wlth ethyl ether, cleanup through a micro-Florisll column for dibutyl disulfide, and element-selective gas-liquid chromatographic (GLC) anaiysls led to method recoveries of 81 % for DEF and 107% for dibutyl dlsulflde at splking levels correspondlng to ca. 80 ng/m3. Esthnated detection limlts were 0.1 for DEF and 1.0 ng/m3 for dibutyl disulfide. The downstream sampllng stage, conslsthg of mercuric acetate impregnated sHka gel, trapped butyl mercaptan as a mercaptide salt; mercaptan was regenerated through a postsampllng workup conslstlng of treatment with hydrochlorlc acld and extractlon with oleflnfree pentane. Analysls of the pentane phase by element-seiective GLC led to quantltatlon of butyl mercaptan at splking levels correspondlng to a detectlon llmlt of ca. 1 ng/m3 with an overall recovery of 65%. The methods were tested for sampllng airborne levels of the three chemlcals In the vicinity of cotton defollatlon treatments.
The organophosphorus compounds DEF (S,S,S-tributyl phosphorotrithioate) and Folex (merphos, S,S,S-tributyl phosphorotrithioite) are two of the major defoliants used in the US.as cotton harvest aids. The use of these chemicals has elicited complaints of headache, nausea, and respiratory distress from nearby residents during the defoliation season in Arizona and California ( I , 2), due apparently to the odors associated with the defoliation treatment. The odors are considered to arise from the presence in the air of dibutyl disulfide and butyl mercaptan, both of which are formulation impurities and potential environmental conversion products of the parent defoliants. For the determination of airborne levels of sulfur-containing compounds associated with the use of organophosphorus defoliants, methods were required for sampling and analyzing DEF, dibutyl disulfide, and butyl mercaptan at low levels in agricultural field air. Since merphos forms DEF rather rapidly by oxidation in water (3) and air (4,there was no need for a separate method for merphos assay. We previously reported a method for sampling DEF and other organophosphorus pesticides based on accumulative sampling through XAD-4 macroreticular resin @)-a general purpose adsorbent useful in sampling a variety of airborne pesticides (6) and other trace organics (7).The volatility of butyl mercaptan resulted in poor retention by XAD-4 during high volume sampling, necessitating use of an alternative 'Present address: Shell Chemical Co., P.O.Box 4248,Modesto, CA 95352. 0003-2700/81/0353-1077$01.25/0
sampling medium. Reported methods for sampling volatile SH-containing compounds are based on formation of nonvolatile metallic salts in the sampling chamber (8), using silver (9, IO), lead (81, gold (II), and mercury (12) as the trapping metal or metallic ion. The latter report was particularly appropriate to our situation, since accumulative sampling was quantitative for butyl mercaptan when a glass-fiber filter coated with mercuric chloride was employed. However, the sampling efficiency dropped a t flow rates higher than ca. 1 L/min-a limitation which also applied to direct sampling of volatile Scontaining compounds through Porapak and other macroreticular resins (13). For the determination of butyl mercaptan at atmospheric levels below the odor threshold of ca.1ppb (14pg/m3) (I4),accumulative sampling at flow rates considerably in excess of 1 L/min was desirable for our application. Our aim, then, was to develop a high-volume sampling system capable of simultaneously trapping DEF, dibutyl disulfide, and butyl mercaptan, from which the three compounds could be extracted and isolated at atmospheric levels in the low ng/m3 range. The attainment of a workable method satisfying these objectives is the subject of this report.
EXPERIMENTAL SECTION Materials. Unless otherwise specified,reagents and solvents were analytical reagent grade (Mallinckrodt, St. Louis, MO) and used as received. DEF was an analytical standard obtained from Mobay Chemical Co., Kansas City, MO. Butyl mercaptan and dibutyl disulfide were purchased from Aldrich Chemical Co., Milwaukee, WI. Silica gel (Grade 408, Davison Chemical Co., Baltimore, MD) was used as received while Florisil (60-100 mesh, Floridin Co., Hancock, WV) was activated at 130 OC for 48-72 h and used immediately after cooling in a desiccator to room temperature. Pentane (Nanograde, Mallinckrodt) was cleaned prior to use by washing with sulfuric acid, 1% potassium permanganate, and water and stored over calcium chloride until used (15). Standards for gas chromatographywere prepared by diluting 1mg/mL stock solutions with pentane (butyl mercaptan), hexane (dibutyl disulfide), or acetone (DEF). XAD-4 resin (20-50 mesh, Rohm and Haas, Philadelphia, PA) was cleaned prior to use by the following procedure: A Nylon sock (90X 500 mm) was placed in a modified 2200-mL Soxhlet extractor (Corning no. 3885) with the siphon side arm connected to a drain. The sock was filled with methanol-saturated XAD-4 resin. Deionized water was added to the extraction chamber at such a rate that the water continuously drained through the side arm. Addition was continued until the effluent was clear. At this point the water was allowed to siphon completely from the extraction chamber. The washing procedure was repeated with 0.1 N HC1 (ca. 1000 mL) and water (until the pH was 7). Acetone was added to displace the water from the resin (1500 mL or until the eluate was clear). A 2000-mL round-bottom flask containing 1500 mL of acetone was attached and the resin was Soxhlet-extracted for 8 h or 25 cycles. The extraction was repeated in turn with ethyl acetate and ethyl ether (each for 8 h or 25 cycles). The resin was placed into several 3.7 X 60 cm segmented Teflon columns (Savillex Corp., Minnetonka, MN) and dried by passing through a stream of nitrogen (ca. 200 mL/min). This was done for 6-18 h at rmm temperature and 16 h at 40 O C (until the odor 0 1981 American Chemical Society
1078
ANALYTICAL CHEMISTRY, VOL. 53, NO. 7, JUNE 1981
J
/C
I W h l
u1
Figure 1. Diagram of twoeompartment sampling head fied to a high volume air sampler: (A) 100-mesh stainless steel screens; (B) air intake; (C) XAD-4 resin bed; (D) mercuric acetate impregnated silica
gel; (E) fan and motor.
of ether could no longer be detected in the effluent gas). The dry resin was stored in glass jars capped with aluminum foil lined caps. Mercuric acetate impregnated silica gel was prepared as follows: Approximately 400 g of silica gel was added to a 1-L round-bottom flask. Enough methanol was added to cover the silica gel, and the flask was then cooled in an ice bath. Forty grams of mercuric acetate was dissolved in a 400 mL solution of 10% acetic acid in methanol. This solution was added to the silica gel. The methanol was evaporated by using a rotary evaporator until the silica gel was still wet but free-flowing. The mercuric acetate impregnated silica gel was stored in glass jars prior to use. Air Samples. High-volume air samplers (HiVol, Sierra/ MISCO, Berkeley, CA) capable of processing air at flow rates of ca. 1m3/min were fitted with 10 cm diameter aluminum sleeves to hold sampling media in two isolated compartments above the sampler intake (Figure 1). A 100-mesh stainless steel screen supported and kept separate the media in each compartment. The upstream (intake) stage held 100 mL of XAD-4 resin and the downstream stage had 100 mL of Hg(OA~)~-impregnated silica gel. Flow rates were measured with a pressure gauge connected to the sampler with Tygon tubing, using a calibration curve (inches of water vs. m3/min) prepared for each sampler. Readings were made at the beginning, midpoint, and end of each 2-h run. Some field samples were collected with low volume (LoVol) air samplers (5) outfitted with a two-compartment 3.7 X 30 cm Teflon tube adapted to the intake. The upstream (intake) stage held 30 mL of XAD-4 resin and the downstream stage had 30 mL of Hg(OA~)~-impregnated silica gel. Flow rates (ca. 30 L/min) were measured with a flow meter placed at the exit of the sampler pump. XAD-4 Resin Extraction. The XAD-4 resin from the HiVol sampler was placed in a 250-mL Erlenmeyer flask to which was added 120 mL of ethyl ether. The contents were shaken for 2 h on a gyratory table shaker. The ether was decanted through sodium sulfate and Whatman No. 1filter paper into a 300-mL round-bottom flask. Sixty milliliters of fresh ether was added to the resin, shaken for 1 h, and combined with the previous extract. The procedure was then repeated with a third portion, whereupon the entire contents of the Erlenmeyer flask were poured into the funnel to allow complete drainage of the ether. The filter contents were then rinsed with 20 mL of ethyl ether which was combined with the extracts. Resin from the LoVol sampler was extracted similarly, but using glassware and solvent volumes half that for the HiVol sampler. The round-bottom flask was fitted with a three-section Snyder column (Ace Glass, Vineland, NJ) and gently warmed by means of a heating mantle until approximately 5 mL of ether remained. This was placed in a Kontes concentrator tube and column (Kontes, Vineland, NJ) and further concentrated to 1.0 mL, wing a warm (45 "C) water bath. This extract was transferred to a 1-dram vial and stored at -22 OC until analyzed. Silica Gel Extraction. The silica gel was placed in a 200-mL round-bottom flask together with 50 mL of pentane and just
enough concentrated HC1 to cover the silica gel (ca. 60 mL). This was placed in a heating mantle, fitted with a three-section Snyder column and an Allihn water condenser (900 mm, Ace glass) using water that was recycled through an ice bath. The solution was heated (100 V, ca. 55 "C) just enough to maintain a vigorous reflux in the pentane. The reaction proceeded for 1 h, and then the mixture was allowed to cool. The pentane was decanted through anhydrous sodium sulfate into a 100-mL round-bottomflask, along with two additional portions of 10 mL each of pentane which were used to rinse the acidified silica gel. A boiling chip was added and the contents were then concentrated through a three-ball Snyder column to 10 mL. The solution was then further concentrated to 2-3 mL in a Kontes concentrator tube heated to 45 f 1OC in a water bath. The volume of this concentrate was then adjusted to 3 mL, and gas chromatographic analysis was carried out for both butyl mercaptan and dibutyl disulfide. Florisil Cleanup. After DEF in the XAD-4 extract was quantitated by gas chromatography, the remaining ether extract was cleaned on a Florisil column to facilitate analysis of dibutyl disulfide. The column was made by placing a 5-cm bed of Florisil in a disposable Pasteur pipet (Kimble, Toledo, OH) that had been plugged with glass wool. One milliliter of hexane was added to the vial containing 1mL of the resin extract. The solution was placed in a nitrogen evaporator (N-Evap, Organomation Associates, Shrewsbury, MA) and concentrated to approximately 0.3 mL. This was placed at the head of the Florisil column which had first been wetted with 1mL of hexane. The dibutyl disulfide was eluted quantitatively from the column with 4 mL of hexane. Air Sampling Efficiency. The resin air sampling efficiency was tested with the HiVol air sampler. Since the HiVol processes air at a faster rate, it was assumed that if the resin was effective in this sampler, it would be effective in the LoVol air sampler as well. The air sampler was assembled as described earlier, except that a single-stage module using only the XAD-4 resin was used. A piece of glass tubing (20 cm X 1/4 in.) was mounted 20 cm in front of the sampling module at an angle of ca. 45". After the resincharged air sampler was turned on, the air was fortified by slowly injecting 10-100 pL of a standard solution of DEF and/or dibutyl disulfide in acetone into the glass tube with a syringe. This injection was done at a rate which allowed the solvent to evaporate before reaching the middle of the tube. The air temperature and pressure gauge readings were recorded at the beginning and end of the sampling period (three samples ran for 2 h and one for 12 h). A 2-h control sample (no fortification of the air) was also taken. All the efficiency studies were done outdoors at temperatures of 26-30 "C. Once the samples were taken, the resin was extracted and analyzed as described earlier. The glass tube was rinsed with ethyl ether (ca. 25 mL), and the rinsings were collected in a 100-mL round-bottom flask. This solution was concentrated and analyzed in a similar manner as for the resin samples. The amount of sample vaporized was calculated as the difference between the amount added to and that remaining in the glass tube at the end of the sampling period. For DEF 10-30% of the amount fortified remained in the glass tube after sampling. For dibutyl disulfide there was no unvaporized residue remaining in the glass tube after sampling. A similar procedure was used to check for sampling efficiency of Hg(OA~)~-impregnated silica gel toward butyl mercaptan in the HiVol air sampler. Gas Chromatography. DEF was analyzed by using a 75 cm X 6 mm (0.d.) glass column packed with 5% OV 101 on 100-120 mesh Chromosorb W Hp. A Tracor Model 222 gas chromatograph equipped with a Tracor Model 222 flame photometric detector (FPD) in the phosphorus mode was employed with the following conditions: Temperatures were 210 "C (column),250 "C (injector), and 210 "C (detector). Gas flows were 60 mL/min (nitrogen carrier gas), 80 mL/min (air), and 60 mL/min (hydrogen). Butyl mercaptan was analyzed on an F&M Model 400 gas chromatograph with the same composition OV 101 column and a Hall electrolytic conductivity detector (HECD, Tracor, Austin, TX). Temperatures were 35 "C (column), 190 "C (injector), 200 "C (transfer line), and 710 "C (pyrolysisoven), and gas flows were 30 mL/min (nitrogen carrier gas) and 1mL/min (oxygen reaction gas). The conductivity solution (95% ethanol) ran at 0.2 mL/min.
ANALYTICAL CHEMISTRY, VOL. 53, NO. 7, JUNE 1981
Table I. Recoveries from XAD-4 fortification no. of compound level, pg trials DE F dibutyl disulfide butyl mercaptan
Extraction 100 10 100 10 100 10
3 3 3 3 3 3
Sampling and ExtractionC 100 7 10 3 dibutyl disulfide 100 3 10 3 butyl mercaptan 100 2 DEF
11111
1070
i
% recov-
ery 83 (1.5) 81 (1.8)
106 (1.3) 107 (3.1) 94 (1.7) 92 (2.3) 86 (3.2) 80 (1.2)
102 (1.2) 100 (1.2) 0
a One standard deviation in parentheses. Fortified to Fortified to air intake of resin just prior to extraction. high-volume sampler; ran for 2 h at 1 m3/min.
Alternately, butyl mercaptan was analyzed on the Tracor 222 instrument with the flame photometric detector in the sulfur mode using the same composition OV 101 column. Temperatures were 36 O C (column), 100 "C (injector),and 190 "C (detector), and gas flows were 20 mL/min (nitrogen), 80 mL/min (air), and 60 mL/min (hydrogen), For confirmatory analysis, a 50 cm X 6 mm (0.d.) glass column containing 60-80 mesh Poropak QS (Waters Associates, Milford MA) at 200 "C was substituted for the OV 101 column using either the HECD or FPD detectors. Dibutyl disulfide was analyzed on the F&M Model 400 instrument with the HECD and the same composition OV 101 column but at a column temperature of 160 "C and nitrogen carrier gas flow of 30 mL/min. Alternatively,disulfide was analyzed on the Tracor 222 instrument equipped with the FPD and same composition OV 101 column, but at a column temperature of 140 O C and nitrogen carrier gas flow of 60 mL/min.
RESULTS AND DISCUSSION Recoveries using XAD-4 resin in the high-volume air sampler are in Table I. The resin proved to be an efficient sampling medium for DEF and dibutyl disulfide. Recoveries for DEF matched closely with the 84 f 6% reported previously for that compound on XAD-4 a t lower (30 L/min) flow rates (5). Less than quantitative recoveries apparently are due to limitations in the postsampling extraction with diethyl ether, since resin spiked directly with DEF through which no air was passed gave 81-83% recovery for the extraction alone (Table I). The high sampling efficiency of XAD-4 resin for DEF is apparently due to the low vapor pressure of DEF, which we determined to be 1.38 X mmHg (30 "C), and the ability of relatively nonpolar DEF to partition to the hydrophobic resin phase. Our lower spiking level for DEF, 10 lg, corresponds to an air concentration of ca. 80 ng/m3 to the highvolume air sampler. A detection limit of ca. 0.1 ng/m3 was estimated by comparing the GLC-FPD peak from the spiked sample with that from an air blank. The dvailability of a number of highly sensitive element-selective GLC detectors for DEF, including the FPD in either phosphorus or sulfur modes and HECD, greatly facilitates low-level analysis of this compound. XAD-4 resin trapped, retained, and released dibutyl disulfide quantitatively in similar fortification experiments (Table I). Although considerably more volatile than DEF, dibutyl disulfide still has a relatively high boiling point (226 "C)and relatively low polarity. Both factors can affect retention on hydrophobic resins (7, 16). The higher overall method recoveries for dibutyl disulfide when compared with DEF apparently are due to a better extraction efficiency of the disulfide from XAD-4 with ethyl ether (Table I).
Flgure 2. Gas chromatograms of dibutyl disulfide In (A) an XAD4 air blank without Fbrisil cleanup and (B) and XAD4 alr sample splked with dibutyl disulfide with Florid1 deanup. Disulfide in B cwresponds to 2-ng injection on a 5 % OV 101 column (160 "C) wkh HECD detector. Silica gel in 200 mL RB llask
12 N ncl to cover silica gel
-L
50 mL penlane
FII WISnyder column and Ailihn condenser
ExtracI at reliux lor 1 h
penr
Filler lhrough sodium lullate
HCi and silica gel
+lo
mL pentane HCI and silica gel
into RB llask
I-
Fit wISnyder column
I
Heal at 95' with water bath
Evaporate to 3
mL
Flgure 3. Schematic of procedure for extraction and concentration of butyl mercaptan from mercuric acetate impregnated silica gel.
While dibutyl disulfide was readily quantifiable at the 1O-pg (ca. 80 ng/m3) spiking level without cleanup, attempts to measure lower concentrations failed due to the presence of rather large GC peaks eluting in the retention region of the disulfide (Figure 2). Use of Florid column chromatographic cleanup of the DEF-disulfide resin extract after DEF analysis completely eliminated these interferences, allowing for quantitation of disulfide a t levels corresponding to 1 ng/m3 when high-volume sampling was employed. Butyl mercaptan was not trapped by XAD-4 resin under the high-volume sampling flow rates employed for DEF and dibutyl disulfide (Table I). The higher volatility of butyl mercaptan (bp 98 "C) and its relatively greater polarity are apparent causes. We thus focused our efforts on developing an alternate sampling medium for mercaptan which was compatible with high-volume sampling. After an initial evaluation of several uncoated trapping agents (silica gel, XAD-7, and molecular sieve 5A), and several solids (silica gel, XAD-4, glass beads, and glass wool) coated with mercuric salts, development proceeded with mercuric acetate coated on silica gel. The analytical process consisted of trapping butyl mercaptan as a mercuric mercaptide salt, postsampling release of mercaptan to pentane by treatment with hydrochloric acid, concentration of mercaptan in pentane, and determination by GLC (Figure 3).
1080
ANALYTICAL CHEMISTRY, VOL. 53, NO. 7, JUNE
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1981
i;;Iusn J
---
0
1
2
0
1
2
0
1
A
C
0
D
2
TIME (mln)
Flgure 4. Effect of pentane cleanup on gas chromatograms of butyl mercaptan samples: (A) solvent blank using unpurlfled pentane, (B) solvent blank using cleaned pentane, and (C) 2 ng of butyl mercaptan standard. A 5% OV 101 column (35 "C) and HECD detector were used.
The trapping step was patterned after that employed by Okita (12),substituting mercuric acetate on silica gel as the coated matrix in place of mercuric chloride on glass-fiber filters. The use of largeparticle silica gel accommodated high sampling flow rates while providing an extensive surface for interaction of mercaptan with mercuric salt. The partial oxidation of mercuric acetate to orange mercuric oxide which occurred during collection of some air samples did not reduce sampling efficiency. Minor losses of powdered mercuric acetate occurred by migration through a dry sampling bed; a slightly moist sampling bed containing residual acetic acid minimized such losses. Displacement of mercaptan from coated silica gel was accomplished with hydrochloric acid. Acid was added to just thoroughly wet the silica gel to minimize the formation of soluble mercury complexes, which have been shown to lower mercaptan recoveries (8). Limiting the volume of hydrochloric acid in this way also eliminated the need for a phase separation of pentane and aqueous acid, as pentane could be directly decanted from the mixture through sodium sulfate giving a clear solution in dry pentane for further concentration and analysis. The pentane solution was analyzed for both mercaptan and disulfide because of partial conversion to the latter during the procedure. Butyl mercaptan was quantitated as the sum of mercaptan and disulfide in the pentane solution. Pentane was chosen as the partitioning solvent because of its low boiling point (36 "C), minimizing losses of butyl mercaptan during evaporative concentration. However, it was necessary to remove unsaturated compounds present in the commercial solvent prior to its use to prevent formation of substances which interfered in subsequent GLC determination with the HECD (Figure 4). The interfering substances are apparently chlorinated hydrocarbons arising from addition of HC1 to residual olefin in the pentane. Severe losses can occur in two steps of the procedure. During hydrochloric acid treatment of silica gel to release mercaptan, it was necessary to heat the mixture to 55 "C-well above the boiling point of pentane. To maintain pentane reflux and minimize solvent and mercaptan loss, we employed ice water in an Allihn condensor and fresh pentane was added periodically to replenish the small amounts of solvent which escaped even with the precaution of a well-chilled condensor. Similarly, subsequent concentration of the pentane extract was carefully controlled by using a three-ball Snyder column under constant visual observation and heating water kept at 45 f 1 "C by circulation through a constant-temperature
Y
: : m
i
*
I
r 0
'r' -' 0
5
TIME (mln)
i 0
5
5
Figure 5. Gas chromatograms of DEF in X A D 4 field air samples collected 50 m downwind of cotton defollation treatment: (A) before DEF treatment (background) attenuation = 4 X lo-'; (B) during treatment, attenuation = 128 X lo-'; (C)24 h posttreatment, attenuation = 64 X lo-'; and (D) 72 h posttreatment, attenuation = 8 X lo-'. A 5 % OV 101 column (210 "C) and FPD detector were used. reservoir. Use of a boiling chip in the pentane and thorough drying of the pentane prior to concentration were important factors allowing for a low water bath temperature during pentane evaporation. With these precautions quantitative recovery of mercaptan was achieved at 0.1 and 1 pg fortification levels during the pentane concentration. Recoveries of mercaptan, measured as the s u m of mercaptan and disulfide in the pentane extract, were 65-73% through the entire method when mercaptan was spiked to the mercuric acetate impregnated silica gel (no air) and 50-57% through the entire procedure including trapping from air (Table 11). The lowest air spiking level, 1pg, corresponds to a detection limit of ca. 10 ng/m3 for a 2-h HiVol air sample. The best GLC conditions for mercaptan analysis consisted of a 5% OV 101 column with flame photometric detection. It was important to maintain a near ambient column temperature (35 "C)and low carrier gas flow (20 A/&) to move the mercaptan peak well past the solvent tail and thus avoid quenching of mercaptan response in the detector by residual solvent (17). The OV 101 column was more efficient than a Poropak QS column, although the latter proved useful for confirmation of mercaptan particularly when used in conjunction with the HECD detector. With both detection systems, the detection limit was approximately 0.1 ng of mercaptan. Airborne DEF, dibutyl disulfide, and butyl mercaptan were detected in some samples taken near DEF- and merphostreated cotton fields. Figures 5-8 are chromatograms from high-volume samples taken 50 m outside of a field just before, during, and 24-72 h after a commercial treatment with 2.2
ANALYTICAL CHEMISTRY, VOL. 53, NO. 7, JUNE 1981 ~~
1081
~
Table 11. Recoveries of Butyl Mercaptan from Mercuric Acetate Impregnated Silica Gel fortification no. of % level, pg trials recovery Extraction 1 0.1
73 65.7
2 2
Sampling and ExtractionC 3 50 (4.0) 1 3 57 (10.0) Fortified to a One standard deviation in parentheses. Fortified to air intake silica gel just prior to extraction. of high-volume sampler; ran for 2 h at 1 m3/min. 10
C
-
e
0
2
1
3
I
"
1
2
I
)
'
0
1
2
0
L. 1
2
TIME (rnln)
Flgwe 6. Gas chromatograms of dibutyl disulfide in XAD-4 field air samples collected 50 m downwind of cotton defoliation treatment: (A) before DEF treatment (background), (B) during treatment, and (C) 24 h posttreatment. A 5% OV 101 column (160 OC) and HECD detector were used.
A
!$
2
1 TIME
2
3
0
1
2
3
(rnln)
Figure 8. Gas chromatograms of dibutyl disulflde formed from butyl mercaptan In mercurk acetate impregnated silica gel Reid ak samples collected 50 m downwind from cotton defoliation treatment: (A) before DEF treatment (background), attenuatlon = 1; (B) during treatment, attenuation = 16; and (C) 24 h posttreatment, attenuation = 4. A 5% OV 101 column (150
0
U
0
OC) and
HECD detector were used.
while corresponding values for the 24 h posttreatment samples were 450,0.52, and 3.5 ng/m3. Residues of DEF (24 ng/m3) were detected at 72 h posttreatment at the same sampling site. No noticeable interferences were visible at these concentration levels. A GLC peak corresponding to DEF in the pretreatment sample (Figure 5) represents ca. 10 ng/m3 of DEF from spraying of the defoliant at other locations in the general vicinity of the study field. There was no clear pattern of airborne residue concentrations of the three chemicals collected at eight sampling sites in the proximate vicinity of the study field for up to 4 days posttreatment (1). This may be due in part to variations in wind speed and direction during the 4-day period (18). Maximum residues were as follows: DEF, 1243 ng/m3, 350 m southwest of field, during treatment; dibutyl disulfide, 2.5 ng/m3, 350 m north of field, 24 h posttreatment; and butyl mercaptan, 167 ng/m3, 100 m north of field, 24 h posttreatment. The majority of the samples contained much lower air residues than these maximum values. Analyses conducted similarly at a second cotton field treated with merphos gave somewhat comparable resulta, except that the maximum levels of DEF (6080ng/m3, 10 m north of field, during treatment) and butyl mercaptan (1576 ng/m3, 10 m south of field, 2 h posttreatment) were greater than those recorded near the DEF-treated field.
LITERATURE CITED
W
- -
0
1
0
1
L 0 1
TIME (rnln)
Flguro 7. Gas chromatograms of butyl mercaptan in mercuric acetate Impregnated silica gel field air samples collected 50 m downwind of cotton defoliation treatment: (A) before M F treatment (background), (B) during treatment, and (C) 24 h posttreatment. A 5 % OV 101 column (35 "C) end FPD detector were used.
kg/ha of DEF. Some differences in detector noise and background for samples in Figures 5 and 8 are due to differences in instrument attenuation settings which produced analyte signals of a convenient size for measurement. Samples collected during spraying gave 1189,2.21 and 14.5 ng/m3 for DEF, dibutyl disulfide, and butyl mercaptan, respectively,
(1) Hermann, B. W. "Atmospheric ResMues of the Defoliants DEF, Foiex and their Sulfur containing Byproducts near Treated Cotton Fields", W.D. Thesis, University of California, Davis, CA, 1980. (2) Oshlma, R. J.; Mischke, T. M.; Gallavan, R. E. "Drlft Studies from the Aerial Application of DEF and Foiex in Fresno and Mer& Counties, Califwnia 1979"; Report to the California Department of Food and Agriculture, Sacramento, CA, March 1980. (3) Teasiey. J. E. Envlron. Scl. Techno/. 1067, 1, 411. (4) Woodrow, J. E.; Mast, T.; Seiber, J. N.; Crosby, D. G.:Moilanen, K. W. 173rd National Meeting of the American Chemical Society, New O r b ans, LA, March 1977; American Chemlcal Soclety: Washington, DC, 1977; PEST 03. (5) Woodrow, J. E.; Selber, J. N. Anal. Chem. 1078, 50, 1229. (8) Seiber. J. N.; Ferreira, G. A.; Hermann, B.; Woodrow, J. E. ACS Symp. Ser. 1080, No. 136, 177-208. (7) Sydor, R.; Pietrzyk, D. J. Anal. Chem. 1078. 50, 1842. (8) Golovnya, R. V.; Arsenyev, Y. N.; Svetlova, N. I. J. Chrometog. 1072, 69, 79. (9) Natusch, D. F.; Klonls, H. 8.; Axelrod, H. D.;Teck, R. J.; Lodge, J. P., Jr. Anal. Chem. 1072, 44, 2067. (10) Natusch, D. F. S.; Sewell, J. R.; Tanner, R. L. Anal. Chem. 1074, 46, 410. (11) Braman, R. S.; Ammons, J. M.; Bricker, J. L. Anal. Chem. 1078. 50, 848. (12) Oklta. T. Atmos. Envlron. 1070, 4 , 93. (13) Black, M. S.; He-rbst, R. P.; Hitchcock, D. R. Anal. Chem. 1078, 50, 848. (14) Verschueren, K. "Handbook of Environmental Data on Organlc Compounds"; Van Nostrand and RelnhoM: New York, 1977; p 152. (15) Vogel, H. J. "Practical Organlc Chemistry"; Longman: London, 1977. (16) Simon, C. 0.; BMleman, T. F. Anal. Chem. 1070, 51, 1110.
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(17) Sugiyama, T.; Suzuki, Y.; Takeuchi, T. J . Chromatogr. 1973, 80, 61. (18) Seiber, J. N.; Woodrow. J. E. Arch. Environm. Contamin. T ~ x i c d . ioai, io, 133.
RECEIVED for review December 29,1980. Accepted March 27, 1981. This paper was presented at the 178th National Meeting
of the American Chemical Society, Paper No. 10 in the Division ofpesticide Chemistry, washington, DC, Sept 10,1979, The authors are grateful for support of this work by NIEHS Training Grant ES 07059, by the Western Regional Pesticide Impact Assessment Program, and by a grant-in-aid from Mobay Chemical Co.
Vacuum Ultraviolet Photochemistry in Thin Resist Films Paul W. Bohn and James W. Taylor" Department of Chemistty, Unlversity of Wisconsin- Madison, Madison, Wisconsin 53706
Henry Guckel Department of Electrical and Computer Engineering. University of Wisconsin- Madison, Madison, Wlsconsin 53706
A method to measure quantitative absorptlon spectra of thin film samples In the vacuum ultravlolet (VUV) is described. Results between 17.5 and 32.0 nm indicate poiy(methy1 methacrylate) (PMMA) Is a stronger absorber than either poiy(butene-1 sulfone) (PBS) or Kodak 747 (K747) resists. Fourier transform Infrared (FTIR) difference spectrometry Is used to elucidate the VUV photochemical reaction pathways of these three resists. Mechanisms whereby vapor deveiopment of positlve resists may occur are considered, and the failure to obtaln 100% vapor development for PBS is explained.
Thin films of organic polymers have come under increasing scrutiny primarily because of their use as photoresist materials in integrated circuit fabrication. These materials are the medium into which the form of circuitry is written lithographically. Visible radiation has until recently been the source of choice for writing information into the resist. However, the demand for smaller line widths and greater device throughput capability has spurred research into new lithographic methods employing ions, electrons, or X-rays as the exposing radiation (1). X-ray lithography has been developed as a complementary technique to the electron beam approach to avoid some of its inherent difficulties (2). These include the high cost of an electron beam scanning system, the long range of backscattered electrons which can cause exposure to deviate from the desired sample geometry, and the requirement of sequential processing imposed by both vector and raster scanning methods. The use of X-rays to expose resists makes possible high throughput processing because once a mask is available, several wafers may be run at once. In addition, soft X-rays avoid the diffraction limited resolution inherent in visible lithographysand can produce circuit line widths on the same order as electron beam techniques while concurrently reducing backscattering problems (3, 4). Thus, X-ray lithography possesses the same potential for application to high density integrated circuit manufacture as the electron beam technology while having greater throughput possibilities. Assembly of an X-ray lithographic system requires an X-ray source, mask, and suitable photoresist. Choices of any of the three components depend on the nature of the other two and, in particular, on an understanding of their optical properties. For the photoresist this entails characterization of the ab0003-2700/81/0353-1082$01.25/0
sorption properties in the soft X-ray region as well as elucidation of the molecular high-energy photochemical processes occurring upon irradiation. Some data are available for mask and substrate absorber materials in the spectral regions of interest (5, 6), but most polymer molecules have not been investigated below 100 nm (7).Because the first step leading to the ultimate exposure of the resist is absorption of a photon of ionizing radiation (81, the quantitative determination of the linear absorption coefficient in the appropriate spectral region is critical. Qualitative absorption spectra have been studied for three common X-ray resists (91, but quantitative data are needed to compare various resists at comparable wavelength intervals. In addition, although the specific chemistry has been studied in some cases (IO,11),details of the exposure process at short wavelengths are unknown for many photoresists. These details are especially important for positive photoresists such as poly(butene-1 sulfone) (PBS) which have the capability to self-develop (12-14), i.e., not require solvent extraction after exposure. Because the photochemical reactions are wavelength dependent, a continuum source of soft X-ray radiation is needed to explore the molecular consequences of radiation. For these studies an electron storage ring producing synchrotron radiation coupled to a grazing incidence monochromator provided spectrometric capabilities over the wavelength range of 17.5-80 nm. These experiments at relatively low photon energies provide the basis for use of synchrotron radiation to study high-energy photochemical processes and lend themselves naturally to extension to higher energies.
EXPERIMENTAL SECTION Photoresists. Poly(methy1methacrylate) was obtained as a secondary standard (M,= 60000, M,, 33 200) from the Aldrich Chemical Co., Milwaukee, WI and prepared for use as 2.56% (w/v) and 10.22% (w/v) solutions in filtered (1km) methyl isobutyl ketone. Poly(butene-1sulfone) was synthesized by the method of Brown and ODonnell (15) and prepared for use as a 4.74% (w/v) solution in filtered (1pm) 1:l methyl ethyl ketone:cyclohexanone. Kodak 747 Micro Negative Resist (60 cst) was a gift from the Eastman Kodak Co., Rochester, NY. It was prepared for use by dilution with ritered (1hm) xylene mixture to a solution of 2:1 (v:v) K747:xylene. All photoresists were stored in the dark and used under safelight illumination only. Thin Film Samples. Samples for absorption measurements were prepared by modifying the previously described process (9). Two 11-mil wafers (Siltec Lot F-66354, n type (P), (loo), double polished, p 2-25 cm) of single-crystalsilicon were degreased, cleaned, and oxidized to a depth of 100 nm in an 02/HC1 ambient =I
0 1981 American Chemical Society
