Optimization of solid-phase microextraction conditions for

(1) Material Safety Data Sheet for Phenol; Genium Publishing Corp.: Schenectady,. NY, 1985. (2) Rowe, W. Evaluation Methods for Environmental Standard...
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Anal. Chem. 1994,66,160-167

Optimization of Solid-Phase Microextraction Conditions for Determination of Phenols Karen D. Buchholz and Janusz Pawllszyn* The Guelph-Waterloo Centre for Graduate Work in Chemistry, and the Waterloo Centre for Ground Water Research, University of Waterloo, Waterloo, Ontario, Canada N2L 3G 1 In solid-phasemicroextraction (SPME), sorbent-coatedsilica fibers are used to extract analytes from aqueous or gaseous samples. After extraction, the fibers are directly transferred to the injector of a gas chromatograph, where the analytes are thermally desorbed and subsequentlyseparated and quanWied. This is a fast and simple analytical technique, which does not require solvents. An SPME method based on poly(acry1ate)coated fibers has been developed for the phenols regulated by US.EPA wastewater methods604 and 625 and OntarioMISA Group 20 regulations. The method is capable of sub parts per billion detection limits of detection, and precision of 5-12 RSD, dependiig on the compound. Low pH levels and saturated salt conditions significantly increase the sensitivity of the method. Acid and salt conditions have been used to normalize the matrix of a sewage sample so that the amount extracted for heavier chlorinated phenols and nitrophenolsis comparable to that for a spikedlaboratorywater sample. Chromatographic problems associated with free phenols have been overcome by simultaneous in situ derivatization and extraction of their acetates. The SPME of phenolics from the headspace over water has also been investigated. The results demonstrate suitability of the SPME approach to analysis of polar compounds. The release of phenol and its derivatives into the environment is of great concern because of their toxicity,' widespread use in industry,2 and role in drinking water poll~tion.~ It is therefore essential to have a rapid, yet sensitive method of analysis for these compounds. Current standard methods of phenol analysis in wastewater, such as US. Environmental Protection Agency (EPA) method 604 Phenols? and the acid-extractable section of EPA method 625,5 are based on liquid-liquid extraction. They require extensive cleanup procedures that are time-intensive and involve expensive and hazardous solvents, which are undesirable for health and disposal reasons. The risk of sample loss increases with each step in the extraction and concentration process, resulting in recoveries which can vary widely from 40 to 89%, along with poor precision.6 ~~~~~

~~

(1) Material Safety Dora Sheet for Phenol; Gcnium Publishing Corp.: Schcncctady, NY, 1985. (2) Rowe, W. Eualuarion Merhods for Environmenral Srandords; CRC Press: Boca Raton, FL, 1983. (3) Frescnius,W., Quentin,K. E.,Schneider, W., Eds.WarerAnalysis: APracrical Gutde ro Physico-Chemical, Chemical and Microbiological WaferExamimrion and Quality Assurance; Springer-Vcrlag: Berlin, Germany, 1988. (4) SEPA Method 604-Phenols. Fed. Regisr. 1984, 49, 153. (5) SEPA Method 62S-Base/Neutrals and Acids. Fed. Regisr. 1984,49,43290. (6) Hall, J. R.; Florance, J. R.: Strother, D. L.; W a s , M. N. EPAMerhodSrudy 14: Method 604-Phenols; Contract No. 68-03-2625, USEPA, 1984.

$60 Ana&t/caIChem/stty, Vol. 00, No. 1, January 1, 1994

Solid-phase microextraction (SPME) is a fast, simple, and sensitivetechnique, which does not require the use of solvents. SPME involves the extraction of organic pollutants from aqueous or gaseous samples into the solid-phase coating of a silica fiber support. The analytes partition between the sample matrix and coating until an equilibrium is reached. By use of a modified syringe, the fibers are then transferred to a gas chromatograph, where the analytes are thermally desorbed in the injector of the instrument and subsequently analyzed. This process is significantly simpler than conventional techniques, thereby reducing the potential for analyte loss during the extraction process. The time required is in the order of minutes and solvents are completely eliminated from the extraction process. The method has also been successfully automated.7 This technique has been previously applied to substituted benzenes in water,' the chlorinated compounds listed in U.S. EPA method 624,8 and selected PCBsa9 The phenol method is the first SPME application for polar compounds. Most of the other SPME methods were based on a poly(dimethy1siloxane) coating, which is relatively nonpolar. It is expected that, for the phenols, it will be necessary to use a more polar phase or derivatization procedure to reduce their polarity and improve the chromatographic properties. Derivatization would also improve their chromatographic properties.

EXPERIMENTAL SECTION Materials. The modified syringe assembly used in SPME has been described in detail and illustrated previo~sly.~ A I-cm small length of fiber is glued into a 21.5-cm length of 30- or 32-gauge stainless steel tubing with high-temperature epoxy resin. The tubing is inserted into a Hamilton 7005 syringe and glued to the top of the plunger. When a vial is sampled, the plunger is pulled back, drawing the fiber into the needle. Once the needle has pierced the septum, the plunger is pushed down to expose the fiber to the sample. Magnetic stirring was used for the SPME extractions to ensure proper mixing of the sample. All analyses were done with 40-mL EPA vials equipped with 1-in. stir bars. Once an equilibrium is reached between the analyte concentration in the solution and the fiber coating, the plunger is drawn back up and the needle is removed from the septum. The syringe needle is then used to pierce the septum of the GC injector, where the analytes are desorbed off the coating and enter the GC column for separation and analysis. (7) Arthur, C. L.; Killam, L.; Buchholz, K.; Berg, J.; Pawliszyn, J. Anal Chem. 1992, 64, 1968. (8) Arthur, C. L.; Pratt, K.; Motlagh, S.; Pawliszyn, J. J . High Resolur. Chromarogr. 1992, I S , 741. (9) Arthur, C.; Pawliszyn, J. Anal. Chem. 1990,62, 2145. 0003-2700l94l03660160804.5010

0 1993 Amerlcan Chemical Soclety

Initial work was done with poly(dimethylsi1oxane)-coated fibers, with a 100-pmfilm thickness obtained from PolyMicro Technologies, (part no. FHS 110110300). Thepoly(acry1ate)coated fibers with a 95-pm film thickness were also supplied by PolyMicro Technologies (part no. FHA 060072200). The poly(acry1ate) fibers were conditioned under helium at 350 OC for 4-5 h prior to use. Reagents. One-milliliterstandard aliquotsof the EPA 604/ 625 and MISA phenols dissolved in methanol were purchased from Supelco Canada. The EPA 604/625 stock contained 500 pg/mL phenol, 2-chlorophenol, 2-nitrophenol, 2,4dimethylphenol, and 2,4-dichlorophenol; 1500 pg/mL 2,4,6trichlorophenol and 2,4-dinitrophenol; and 2500 pg/mL 4-chloro-3-methylpheno1,4-nitrophenol, 2-methyl-4,6-dinitrophenol, and pentachlorophenol. The MISA stock standard contained 2000 pg/mL each of the MISA Group 20 phenols. The stock was diluted by a factor of 10, using 2-propanol to obtain a working standard which was used to prepare spiked water solutions. The effect of low pH and salt on the SPME technique was examined. A pH 2 buffer was prepared with 25 mL of 0.2 M KCl and 6.5 mL of 0.2 M HC1 in 100 mL of water,1° and saturated salt solutions were prepared with NaCl. In the derivatization experiments, 1.2 g of sodium bicarbonate was added to a 30-mL water sample spiked with the target phenols, followed by 0.05 mL of acetic anhydride. The solutions were mixed until the evolution of carbon dioxide ceased and then analyzed by SPME. Instrumentation. For the gas chromatography-flame ionization (GC-FID) experiments, a Varian Model Vista 6500 GC, equipped with a split-splitless injector, and connected to the data system of a Model 6000 GC was used. The injector was maintained at 200 OC and desorption times of 2.0 and 7.0 min were used for the poly(dimethylsi1oxane) and poly(acrylate) fibers, respectively. The detector was maintained at 275 OC, with a nitrogen make-up gas flow of 23.0 mL/min, hydrogen at 30.0 mL/min, and air at 280 mL/min; range 12 and attenuation 32. A Varian 3400 GC, equipped with a SPI injector, with a Saturn ion trap mass spectrometer was used for the gas chromatography/mass spectrometry (GC/MS) work. The injector, transfer line, and detector temperatures were 200, 280, and 250 OC, respectively. The mass spectrometer was tuned to FC-43 (perfluorotributylamine). The filament emission current was set to give an ionization time of 20 ms. Default values of the segment breaks were used at values of 10-99/ 100-249/250-399/400-650 amu, and the segment tune factors were 80/80/120/85. A mass range of 40-325 amu was scanned, and the detector was turned off for the first 300 s of the run. For all analyses, a 30-m PTE-5 (Supelco) column with a 0.25." i.d. and 0.25-pm film thickness was used. The column flow rate for both GC-FID and GC/MS instruments was 1.0 mL/min, and the temperature program was as follows: 35 OC for 7 min, ramp to 190 OC at 10 OC/min, final hold time at 190 OC for 7.7 min. Headspace Apparatus. The experiments involving the SPME of phenols from headspace used both magnetic mixing (10) Weast, R. C., Ed. CRC Handbook of Chemistry and Physics, 59th 4.CRC ; Press: West Palm Beach, FL, 1979.

and sonication as agitation techniques. For the magnetic stirring, 40-mL EPA vials containing 5 mL of solution with the 1-in. stir bars were used. The water solution containing the spiked phenols was acidified to below pH 1 with concentrated sulfuric acid and saturated with sodium chloride. For the sonication experiments, a Tekmar sonic disrupter with a 3-mm-diameter microtip probe was used. Since the probe base was too large for the EPA vials, a 200-mL Berzelius beaker was used. A rubber stopper wrapped with Teflon tape was used to seal the beaker. Two holes were drilled into the stopper to accommodate the sonicator probe and needle of the SPME syringe assembly.

RESULTS AND DISCUSSION Coating Evaluation. The equilibrium distribution constant between a solid-phase coating and water, K, is used as measure of that coating's affinity for the target a n a l ~ t e : ~ K = CJCaq

(1)

where C, is the analyte concentration in the solid phase and Cas is the concentration in the aqueous phase. Using moles per volume to replace the concentration terms, K can be rewritten as follows: K=

nsvaq

(2) V,(Vaqc"aq - ns) where n, is the moles extracted by the solid phase at equilibrium, Vaq and V,are the volumes of the aqueous sample and solid phase, and Pa,is the initial concentration of the analyte in the sample. A coating with a high affinity for an analyte will have a high K value associated with it since a large amount, n,, will be extracted. This will result in good sensitivity. If a coating does not have a relatively high K value associated with it toward a particular analyte, the sensitivity of the method can be improved by using a thicker coating, which will increase V, and therefore, 4. To determine when equilibrium is reached, a fiber is exposed to a standard water solution spiked with the target analytes for different lengths of time. A fresh solution is used for each time interval. The GC area counts from the thermal desorptionsvs the timethat the fiber wasexposed tothesolution is plotted to obtain an exposure time profile. Poly (dimethylsiloxane)-Coated Fibers. The poly(dimethylsiloxane) fiber had been used successfully for previous SPME applications and was readily available in our laboratory. Figure 1 shows the exposure time profile for the poly(dimethylsiloxane) coating and demonstrates that only 15 min is required for all analytes to equilibrate with this fiber. Unfortunately, as indicated by the K values in Table 1, this coating was not suitable for phenol analysis. 2-Chlorophenol, 4-nitrophenol, and 2,4-dinitrophenol could not be extracted in large enough amounts to be detected by GC-FID. Table 1 lists the GC/MS detection limits for the compounds that could be extracted. With the exception of the nitrophenols, the GC/MS limits are within range of the EPA method 625 guidelines. The GC-FID results, however, were several orders of magnitude larger than the EPA method 604 limits. When the poly(dimethylsi1oxane) fiber was used to extract nonpolar compounds such as substituted benzenes, it was observed that the Kvalues corresponded to the octanol-water AnaMicalChemistry, Vol. 66, No. 1, January 1, 1994

161

F 3

f

I

I a

e

8

a

a

4-ChlOrO-3-methylphuaOl 2,4,6-TriChloroph.nol Pult.chlorop~ol

m

15 55 100

I

PhrnOl 2-NitrophulOl 2,I-Dinuthylphnol

I i

a

2,I-Dichloroph.no1 2-)c.thyl-4,6-dinitrophuaol

I

I i

I

I

I

I

n

10

20

T h e (minutoa)

Flgwe 1. Exposure time profile of poly(dimethylsiloxane)fiber. The CK:area countsfrom thethermaldesorptkns,and therefore the amount extracted, level off In 15 min, indicating that equilibrlum has been reached. T1#. 1. ~ ( & n o t h ~ x ~ ~ ) - W a #rMkrtlm tor co"b( K ) and D.(.otkn~

SPME GCIMS detection limit#

compound phenol 2-chlorophenol 2-nitrophenol 2,Cdimethylphenol 2,4-dichlorophenol 4-chloro-3-methylphenol 2,4,&trichlorophenol 2,4-dinitrophenol 4nitrophenol 2-methyl-4,6-dinitrophenol pentachlorophenol

K

WmL)

1.3 0.2

7.6 0.76 0.75 0.20 0.17 0.53 0.12 3411 1311 156 0.14

4.8

1.3 4.6 2.4 15 0.1 0.1 6.0 370

0.589

3.369 6.116

0.011 0.044 0.011

3 13 12

1.5 7 5

1 4 3

Expressed as percent area of initial desorption peak.

I

T i m (minutam) ,

W(dhnn*Woxm)CV

% carryovee silica rod f i i coating vol surface area blank blank blank thickness (rm) (XloC cmS) (ans) 1 2 3

a

. 1

T&Io~*

partition coefficient (Kow)values, commonly found in the literature." With the phenols, however, there is a dramatic difference. Although the target phenolicswith relatively high Kowvalues tend to have higher K values, the K values are several orders of magnitude lower than the K,values. Octanol is able to extract both polar and nonpolar compounds because it contains a nonpolar saturated hydrocarbon chain as well as a polar hydroxyl end group. Poly(dimethy1siloxane) is relatively nonpolar and can easily extract the nonpolar substituted benzenes. Its behavior is similar to that of hexane in a liquid-liquid extraction. The high affinity of this phase for these compounds is reflected in their Kvalues, which range from 126 for benzene to 654 for o-xylene." For phenols, in particular thq nitrophenols,it is apparent that a moreselective, polar coating is required for their extraction.

Carryover. With the poly(dimethylsi1oxane) coating, carryover was a problem with pentachlorophenol. When the fibers were placed in the GC injector, a large percentage of this compound remained in the coating and was desorbed with subsequentinjections,thereby interferingwith the analysis of following samples. Previous work with this coating showed that it bleeds above 200 OC. Pentachlorophenolhas a boiling point of 310 OC, which indicates that a higher desorption temperature may be required. Boiling point alone, however, is not the only factor. 2-Methyl-4,6dinitrophenol,which boils at 312 OC, does not show any signs of carryover. To discover the cause of the carryover, both the solidphase coating and the silica rod support were considered. Three different combinations of coating and silica core thicknesses of the poly(dimethylsi1oxane) fiber were investigated and are summarized in Table 2. All results are for 2-min desorptions at 200 "C. The results are for three successive blank desorptions of the fiber following its exposure to a 8 pg/mL solution of pentachlorophenol and are expressed in terms of percent initial desorption peak area. By comparing the 15and 100-pm coated fibers, which have the same size of silica rod, it is evident that carryoverincreaseswith coating thickness. Unlike a chromatographic grade coating, the poly(dimethylsiloxane) coating used for our research is industrial grade and probably contains relatively large amounts of uncapped hydroxyl end groups that could strongly bind to the analyte.12 A comparison of the 55- and 100-pm fibers suggests that the silica rod is also a factor. If the coating was the only cause of carryover, the-55-pm fiber should have less than the 100pm one; however, the results are very similar. It is possible that impurities, such as metal ions, present in the silica core or cladding could be retaining the analyte by chemisorption. For example, aluminum and boron can form strong Lewis acid centers on the silica surface which effectively retain aromatic compounds.12 Concentration is another factor. The results for the lO0-pm coating in Table 2 were for a 8 pg/mL solution. When a 0.8 pg/mL solution was sampled, the carryover was 2.5% for the first blank and dropped to 0.1% by the third blank. In Situ Derivatization. Derivatization was investigated as a means of increasing the affinity of the phenols for the poly(dimethylsiloxane) coating. Derivatization with acetic anhydride to form phenol acetates is one of the simplest methods since it does not require the extraction of the phenols into an organic solvent prior to the addition of the derivatizingreagent. This method has been previously reported for chlorophen o l ~ , ~cresols,l5 ~ J ~ and the Ontario, Canada Municipal ~

(11) Arthur, C.L.; Killam, L.; Motlagh, S.; Lin, M.; Potter, D.;Pawlisyn, J. J. Environ. Sci. Techno1 1992, 26, 1979.

162 Analytical Chmlstty, Vol. 66, No. 1, January 1, 1994

(12) Rotzsche, H. Srariomry Phases in Gas Chromatography; Elsevicr Science Publishing Co.: New York, 1991. (13) Janda, V.; Van Langenhove., H. J. Chromatogr. 1989,472, 327.

FREE PHENOLS

-

2N PCP

TOT

Phe

2C

2N 24DM 24DC4C3M246TC 4N

-

PEE

PCP

Flgure2. Extraction enhancement with derlvatizedphenols. Improved extraction with acetates compared to free phenols for the poly(dimethylslbxane)Rber: phenol(PHE), 2chlorophenol(2C), P-nitrophenol (2N), 2,4dimethylphenol(24DM),2,4dichlorophenol(24DC), Cchloro3-methylphenol (4C3M), 2,4,6-trichlorophenoI (246TC), 4-nitrophenol (4N), and pentachlorophenol (PCP).

Industrial Strategy for Abatement (MISA) Group 20 phenols.16 In our experimentsinvolving the EPA 604/625 phenols, 2,4-dinitrophenol, and 2-methyl-4,6-dinitrophenoldid not appear to undergo the derivatization reaction. For the other compounds, however, the reaction was successful with acetate yields ranging from 85 to 100%. The derivatization procedure made the compounds less polar by replacing the hydroxyl group with an acetate group and therefore, created a compound with more affinity toward the poly(dimethylsi1oxane) coating. As shown in the bar graphs in Figure 2, the amount of acetate extracted for most of the target analytes was several times greater than the free form. Also, 2-chlorophenol and 4-nitrophenol, which were not detected in their free forms with GC-FID when this fiber was used, can now be analyzed as their acetates. 2-Nitrophenol was the only compound for which the amount of derivative extracted was significantly less than the free phenol. The amount of free phenol extracted is about 25 times higher than the amount of derivative that is extracted for the same initial free phenol concentration. The in situ derivatization can also be performed directly in the polymeric coating. The derivatization reagent can be absorbed or chemically bound to the fiber, prior to its exposure to the sample. Then, when the analytes partition into the organic phase, they are simultaneously derivatized and retained in the coating. This SPME approach is particularly useful for very polar analytes such as carboxylic acids since these compoundscannot be derivatized directly in the aqueous phase. Besides improving the sensitivity of the method for most of the target phenols, derivatization also had chromatographic advantages. Free phenols can hydrogen bond with GC column stationary phases, which results in peak tailing and interferes with integration.17 Figure 3 shows GC/MS chromatograms of both free and derivatized phenols. The phenol acetates have significantly better peak shape than the free phenols in the lower chromatogram. The improvement is particularly evident for pentachlorophenol. With the free phenols, the ~~

~~~

(14) Lee, H.; Sotkker, Y.; Chau, A. Anal. Chem. 1987, 70, 1003. (15) Coutts, R.; Hargesheimer, E.; Pasutto, F. J. Chromatogr. 1979, 179, 291. (16) Alfieri, A.; Crawford, G.; Ahmad, I. Anal. Chem. 1989, 72, 760. (17) Zyka, J., Ed.Znstrumentation in Analytical Chemistry; Ellis Horwood: New York, 1991; Vol. I.

688

1e:m

ACETATE DERIVATIVES

10:m

l3:a

16 :40

Figure 3. Comparisonof free and derivatlzedphenols.The top GC/MS chromatogram is from an SPME extraction of free phenols using a poly(dimethylsi1oxane)fiber. Abbreviations as in Figure 2. The bottom GC/MS chromatogramshows that the correspondingacetate derhrative peaks could not be detected for 2,4dinitrophenol and 2-methyl-4,6dinitrophenol.

chromatographicseparationof phenol and 2-chlorophenolcan be a problem; however, this is not the case when derivatization is used since there acetate forms elute several minutes apart. Derivatization also improved the carryover profile for pentachlorophenol. The correspondingacetate typically had only 2% carryover on the first blank compared to 12% for the free phenol using a 100-pm fiber, 8 pg/mL pentachlorophenol solution, and desorption conditions of 2 min at 200 OC. Poly(acry1ate)-Coated Fibers. The results for the poly(dimethylsiloxane) fiber suggested that a more polar phase was needed for the analysis of free phenolics. Although the derivatized SPME method was successful for most of the target analytes, the analysis of nitrophenols was still a problem. Derivatization has many advantages, but the method could be simplified if this step was omitted. As a result of these concerns, a poly(acry1ic) acid based derivativewas investigated as a possible coating for phenol analysis. The results for this coating demonstrate that it was able to extract all of the target compounds. With this solid phase, 40 min was required for all analytes to reach equilibrium. The slower equilibration times with this coating compared to the poly(dimethylsi1oxane) is due to the nature of the solid phase. Poly(dimethylsi1oxane) is a polymeric liquid, which the analytes can easily diffuse into and through, whereas the poly(acry1ate) coating is a solid. Table 3 compares theKvalues for theEPA 604/625 phenols with this fiber. In order to determine some sort of relationship AnaIyticalChemistty, Vol. 66, No. 1, January 1, 1994

163

Tablr 3. Coatlng-Wator DktrlbuHon Constank (K), OctanoCWater a d SoluMYly In Water Partillon CMnkknb (&.),

compound pentachlorophenol 2,4,6-trichlorophenol 2,4-dichlorophenol 4-chloro-3-methylphenol 2-chlorophenol 2,4-dimethylphenol

2-methyl-4,6-dinitrophenol 2-nitrophenol 4-nitrophenol 2,4-dinitrophenol phenol

acrylate K value

2 2 1

solub in water21 (mg/L)

170 60 47 16 9.3 9.1 7.3 3.7 2.4 1.7 1.3

5.01 3.69 3.23 3.10 2.15 2.42 2.12 1.78 1.90 1.53 1.46

5 800 4600 3850 28500 4200 100 2100 16000 5600 87000

between the analytes and their K values, the octanol-water partition coefficients or KO, values, and solubility in water were compared. In general, the higher the KO, value, the higher the K value. This is expected since KO, values are often used to indicate a compound’s affinity for an organic layer over water. As a group, the chlorinated compounds have a high affinity for the poly(acry1ate) coating. As the number of chlorines on the phenol molecule increases, so does the compound’s affinity for the coating. Another trend is the relationship between their solubility in water and affinity for the coating. As the solubility in water increases, the distribution constant for the coating-water equilibrium favors the water. After the chlorophenols, 2,4-dimethylphenol shows the highest affinity for the coating, followed by the nitrophenols. With the exception of 2,4-dinitrophenol, the nitrophenols as a group, show a trend between K value and water solubility similar to that of the chlorophenols. Phenol has the lowest K value, which is expected from its high solubility in water and low KO, value. Although this coating was successful in extracting polar compounds, it can extract nonpolar ones equally as well. The fiber only slightly favors the polar compound over its nonpolar analog. For example, benzene has a K value of 0.6 with this fiber, compared to 1.5 for phenol. Similar results were observed for m-xylene and 2,4-dimethylphenol. Although m-xylene could not be separated on the chromatographic column from p-xylene, and an accurate determination of its K value could not be made, the estimated value of 4.1 is in the same order of magnitude as that of 2,4-dimethylphenol (9.1). The poly(acry1ate) coating has an affinity for both polar and nonpolar compounds since its structure consists of a hydrocarbon chain backbone with relatively polar ester side chains. When this coating was used to extract acetate derivatives, however, the amount extracted was essentially the same as for the free phenol. This gives some indication of the different partitioning mechanisms between the two fiber coatings. For the poly(dimethylsi1oxane)fiber, the partitioning was strongly dependent on the polarity and/or sizeof the endgroup, whereas for the poly(acry1ate) coating, these factors had no obvious effect. These results agree with the observations of the similar K values for phenol and benzene and 2,4-dimethylphenol and m-xylene. The poly(acry1ate) coating had another advantage over the poly(dimethylsi1oxane) one in that the carryover was relatively low. A 8 pg/mL solution of pentachlorophenol only 164

AnaIyticalChemistry, Vol. 66, No. I , January 1, 1994

resulted in 1.O% carryover on the first blank, using a 200 OC desorption temperature. Also, if the fibers were conditioned prior to use at 350 “C under helium for 4-5 h, the chromatograms of the fiber blanks were very clean and contained no evidence of any degradation peaks. The polarity of the extracting phase can be increased further by using fibers coated with ion exchange polymers. They are very useful for extracting ionic analytes.’* Scope of the Method. Using the poly(acry1ate) fiber, an SPME method for phenols was developed. Table 4 shows the linear range for both FID and MS detectors. With both detectors, most of the analytes were linear over a 100-fold concentration range, which compares favorably with those cited in the EPA method 604 validation study.6 The lower end of the linear range was usually limited by instrument sensitivity. In most cases, if an analyte peak could be detected, its GC area counts were linear with the concentration range above it. With the MS, the top of the range was defined by the capacity of the ion trap detector, which was overloaded at the higher concentrations. For the FID, the higher end of the linear range was limited by the solubility of pentachlorophenol in water. Detection limits were also determined for FID and MS detectors. For FID, the calculated limits were determined by comparing the GC area counts of the lowest detectable standard concentration to a peak threshold level of 10 000. For example, if a 3 ppb standard gave an area count of 30 000 for a particular analyte, the detection limit was estimated to be 1.O ppb. A threshold of 10 000 was arbitrarily chosen as a conservative measure of the instrument’s noise. The threshold typically varied from 500 to 10 000, depending on the age of the column. For the GC/MS results, detection limits were calculated by comparing the signal-to-noise ratio (S/N) of the lowest detectable concentration to a S/N of 3. For example, if a 10 ppb solution gavea S/N of 6, the detection limit was calculated to be 5 ppb. All procedures were carried out in duplicate. Table 4 lists the FID and GC/MS detection limits, along with the regulatory limits. The FID results are all within 1 order of magnitude of the required EPA 604 limits, with the exception of phenol and 2-nitrophenol. The GC/MS results all exceed the required limits for EPA 625. Theoretically, pentachlorophenol should have the lowest detection limit because it has the highest affinity for the fiber coating, as indicated by its K value. However, because of its poor GCFID and GC/MS response factors, the limit is higher than it should be. The precision of the method was determined by doing 10 consecutive fiber injections. The average GC area counts and corresponding relative standard deviations were determined for each analyte in the mixture and are listed in Table 4. Most of the analytes have a precision of 5% RSD or better, with the exception of the nitrophenols and pentachlorophenol, at 12% RSD. Enhancementwith Acid and Salt. The addition of acid and salt, singularly and in combination, was investigated as a means of enhancing the amount extracted by the fiber. Table 5 shows the factor increase obtained for all conditions using a (18) Otu, E.; Pawliszyn, J. Solid Phase Microextraction of Metal Ions.Microchim. Acta 1993, 112, 41.

Tabk 4. PoIy(acryla10) FWnr Umar Rango, Dot.cuOn unltr,and R.cbkn

SPME linear range (pg/mL) compound

GC-FID

0.2-2.0 0.02-2.0 0.02-2.0 0.02-2.0 0.002-2.0 0.008-8.0 0.05-5.0 0.5-5.0 0.08-8.0 2-methyl-4,6-dinitrophenol 0.8-8.0 pentachlorophenol 0.008-8.0 phenol 2-chlorophenol 2-nitrophenol 2,4-dimethylphenol 2,4-dichlorophenol 4-chloro-3-methylphenol 2,4,6-trichlorophenol 2,4-dinitrophenol 4-nitrophenol

SPME detection limits (rg/L) GC-FID

GC/MS

GC-FID

GC/MS

(%RSD)

0.007-0.7 0.007-0.7 0.007-0.7 0.007-0.7 0.007-0.7 0.007-0.7 0.007-0.7 0.07-0.7 0.007-0.7 0.007-0.7 0.007-0.7

30 0.61

0.80 0.24 0.38 0.02 0.02 0.01 0.08 1.6 0.75 0.44 0.11

0.14 0.31 0.45 0.32 0.39 0.36 0.64 13.0 2.8 16.0 7.4

1.5 3.3 3.6 2.7 2.7 3.0 2.7 42.0 2.4 24.0 3.6

4.2 4.2 5.2 4.8 4.9 4.0 4.5 8.9 9.3 5.6 12

11

2.1 0.64 1.4 0.80 32 7.8 1.7 1.4

factor increase pH2+ pH2+ salt PK* 60 min value2l p H 2 salt salt

0.9 1.2 0.9 1.1 1.1 1.1 1.1 10.1 0.6 2-methyl-4,6-dinitrophenol 4.5 pentachlorophenol 1.4

phenol 2-chlorophenol 2-nitrophenol 2,Cdimethylphenol 2,4.-dichlorophenol 4-chloro-3-methylphenol 2,4,6-trichlorophenol 2,4-dinitrophenol 4-nitrophenol

5.5 1.6 0.1 5.0 0.8 1.7 0.1 0.2 0.1 0.1 0.2

6.0 3.8 4.3 5.6 2.0 1.9 1.5 16.8 3.0 6.3 1.1

precision SPME

GUMS

Tabk 6. EHod ot Acld and Sail

compound

EPA detection limits (rg/L) 604 625

6.0 4.1 4.3 5.6 2.3 2.3 1.9 17.0 3.1 6.5 1.3

9.89 8.48 7.23 10.63 7.85 9.55 7.42 4.09 7.15 4.35 4.74

40-min extraction time, which was also used for control samples of the same concentration at neutral pH and with no salt added. For example, when the extraction is performed under saturated salt conditions, 5.5 times more phenol was extracted compared to the control sample. When the pH was lowered to 2, the amount extracted for most of the analytes remained the same, except for those with pKa (negative log of the acid dissociation constant) values below 7. When the effect of acid on 2-methyl-4,6-dinitrophenol, pentachlorophenol, and 2,4-dinitrophenol is examined, it is evident that the lower the pKa value, the greater the improvement in the amount extracted. At neutral pH, these analytes are still largely in their ionic form. When the pH is lowered, their acid-base equilibriums shift significantly toward the neutral form, which has a greater affinity for the fiber, thereby increasing the amount extracted. The effect of neutral molecules becoming insoluble as the water molecules prefer to solvate the salt ionslg is commonly known as “salting out”. This effect was observed for highpKa compounds such as phenol(pK, 9.89) and 2,4-dimethylphenol (pKa 10.63). At pH 7, these analytes are already in their neutral form, which is readily extracted by the fiber coating. The presence of the salt decreases their solubility in the water and forces more of these analytes into the fiber, thereby shifting the equilibrium of the phenol even further in favor of its neutral form. This results in a significant positive increase in the amount extracted for these compounds, which is 5 times greater than that extracted under the control conditions. (19) Fasenden, R.;Fessenden, J. Organic Luboratory Techniques; Brooks/Cole Publishing: Monterey, CA, 1984.

For analytes which have a considerable portion of their molecules in the ionized form, the saltingout effect is a negative factor. By adding salt, the ionic strength of the solution increases, and more of the ionized phenols are formed at the expense of the neutral molecules. For these analytes, the distribution constant is in favor of the salt water matrix instead of the fiber coating, and the amount extracted is only about one-tenth of that extracted under the control conditions. The positive effects of both acid and salt can be realized when they are used in combination. With pH 2 and saturated salt conditions, the amount extracted for every analyte in the mixture was greater than the control sample at pH 7 and with no salt added. Under these conditions, all phenols are in their neutral form and are salted out of solution and into the fiber coating. In general, the improvement was by a factor of 2-4. 2,4-Dinitrophenol shows the greatest increase with a factor improvement of 17. Under the combined acid and salt conditions, a time of 60 min was required for all analytes to reach equilibrium. This is likely due to slower diffusion through the saturated salt solution compared to pure water. MISA Compounds. Before the SPME phenol method was applied to environmental samples, the range of analytes was expanded to include those covered under the MISA program Group 20 regulations. The MISA method involves liquidliquid extraction with methylene chloride and GC/MS analysis. It covers 20 phenolics, 10 of which are covered in EPA methods 604 and 625, as well as additional cresol and di-, tri-, and tetrachlorophenol isomers. Table 6 summarizes the linear range with GC/MS, and the detection limits, which easily surpass the MISA guidelines for every target analyte. Figure 4 is a GC/MS chromatogram of a laboratory water sample spiked with the MISA phenols. Matrix Normalization. Figure 5 shows a GC/MS chromatogram of a sewage sample obtained from the Wastewater Technology Centre, Burlington, ON, Canada. The sample was found to contain 80 ppb phenol and 0.3 ppb 2,4dichlorophenol. Along with these target phenolics, a range of other species were also extracted with the fiber. To determine the effect of this complex matrix on the SPME process, 20 pg of each of the MISA phenols was spiked into 30 mL of the sewage sample. The amount extracted was compared to that obtained from a laboratory water sample spiked with the same concentration. Table 7 shows the recoveries. Some of the nitrophenols and the heavier chlorinated phenols were poorly extracted compared to the clean Analytical Chemlstry, Vol. 66, No.

I, January I, 1994

185

Table 6. Linear Range and Detectlon Lhnlts for MISA Compounds

Table 7. Analyte Recovery from Sewage Mat*

compound

% recovery4

% recovery acid salt

phenol 2-chlorophenol o-cresol m-cresol p-cresol 2,4-dimethylphenol 2,4-dichlorophenol 2,6-dichlorophenol 4-chloro-3-methylphenol 2,3,5-trichlorophenoI 2,4,6-trichlorophenol 2,4,5-trichlorophenol 2,3,4-trichlorophenol 2,4-dinitrophenol Cnitrophenol tetrachlorophenol isomers

74.2 128 118 95.8 95.8 104 105 21.3 91.5 56.1 21.5 64.5 66.8 2.7 38.4 16.7 0 8.0

92b 92b 95.9 97.8 97.8 96.5 78.8 83.4 85.0 71.3 66.1 66.1 71.9 111 118 61.4 83.1 32.2

detection limits compound phenol 2-chlorophenol o-cresol m-cresol p-cresol 2,4-dimethylphenol 2,4-dichlorophenol 2,6-dichlorophenol 4-chloro-3-methylphenol 2,3,5-trichlorophenol 2,4,6-trichlorophenol 2,4,5-trichlorophenol 2,3,4-trichlorophenol 2,4-dinitrophenol 4-nitrophenol 2,3,5,6-tetrachlorophenol 2,3,4,5-tetrachlorophenol 2,3,4,6-tetrachlorophenol 2-methyl-4,6-dinitrophenol pentachlorophenol

linear range (pg/L)

SPME

MISA

(dL)

(pg/L)

6.7-670 6.7-670 6.7-670 6.7-670 6.7-670 6.7-670 6.7-670 6.7-670 6.7-670 6.7-670 6.7-670 6.7-670 6.7-670 67-670 6.7-670 6.7-670 6.7-670 6.7-670 67-670 6.7-670

0.80 0.24 0.09 0.11 0.11 0.02 0.02 0.01 0.01 0.02 0.08 0.07 0.04 1.6 0.75 0.01 0.01 0.01 0.44 0.11

2.4 3.7 3.7 3.4 3.5 7.3 1.7 2.0 1.5 1.3 1.3 1.3 0.6

42.0 1.4 1.6 0.4 2.8 24.0 1.3

2-methyl-4,6-dinitrophenol pentachlorophenol

+

a A 100% recovery represents the amount recovered from a laboratorywater sample spiked at the same level asthe sewage sample. Coeluted on GC column.

1

250

200

156

100

.~ 23:20 1o:oo

13:20

16~40

Flgure 4. MISA Group 20 phenols: (1) 2-chlorophenol, (2) phenol, (3) res sol, (4)m+ pcresol,(5) 2,4dimethylphenol, (6) 2,4dichlorophenol, (7) Z,Michlorophenol, (8) 4-chloro-3-methylphenol, (9) 2,3,5-trichlorophenol, (10) 2,4,6-trichlorophenoI, (11) 2,4,5-trichlorophenol, (12) 2,3,4-trichlorophenoI, (13) 2,4dinitrophenol, (14) 4-nitropheno1, (15) tetrachlorophenol isomers, (16) 2-methyl-4,6dinitrophenol, and (17) pentachlorophenol

1O:OO

13:20

16:40

50

20 :00

20:OO

23:20

Figure 5. Sewage sample. Phenol (80 ppb) and 2,4dichlorophenol (0.3 ppb) were extracted from a sample of primary clarifier effluent.

water sample, indicating that the matrix had a significant effect on the extraction. The experiment was repeated using acid and salt to adjust both matrices to the same extent. The sewage and laboratory water were acidified to a pH of 1.7 with several drops of sulfuric acid and saturated with sodium chloride. 2,4-Dinitrophenol and 2-methyl-4,6-dinitrophenol could now be recovered in amounts comparable to the clean 166 AnalyticalChemistry, Vol. 66, No. 1, January 1, 1994

0

Phe

2C

2N

240M 24DC 4C3M 246TC PCP

F'lgure 6. Comparison of headspace and control results.

water matrix, and the recoveries of the chlorinatedcompounds also improved substantially. These results suggest that matrix effects can be overcome to a large extent by normalization to extreme acid and salt conditions. Headspace Analysis. Preliminary investigations to determine the feasibility of extracting phenols from headspace were conducted. If the fiber is exposed directly to samples high in particulate matter, material from the matrix could coat the solid phase and interfere with the extraction. To avoid this, the headspace above the liquid could be sampled. Headspace SPME for BTEX and selected PAHs has proved promising20 and demonstrated that the distribution constant for the extraction,K, can bedefined in terms of the partition coefficient between the coating and gas, K1, and between the gas phase and water, K2:

K = K1K2

(3)

The success of this method dependson the transfer of analytes from the aqueous phase to the headspace. The phenolics in the test mixture have low Henry's law constantvalues, ranging from 1.6 X l o 8 atmm3/mol for 2,4-dinitrophenol to 4.2 X (20) Zhang, Z.; Pawliszyn, J. Anal. Chem. 1993,65, 1843.

l o 5 atm-m3/mol for 2,4-dichlorophenol2~and, therefore, low KZvalues. Most of the target EPA 6041625 phenolics could be forced into the headspace by decreasing their solubility in the aqueous phase through saturation with sodium chloride and acidification to below pH 1 with a few drops of concentrated sulfuric acid. Even with these extreme conditions, however, most analytes needed 1-2 h to reach equilibrium, and pentachlorophenol still had yet to equilibrate after 4 h. These long equilibration times are due to the slow transfer of the phenols from the aqueous layer through the headspace to the fiber.20 Some of the nitrophenols were not detected in the headspace, but for those compounds that were, the amount extracted by the fiber was greater than that of a control sample because of extreme pH conditions (Figure 6). For the control results, the fiber was exposed directly to the aqueous layer, at pH 7 and with no salt added. The exposure time profiles were repeated using sonication as an agitation method instead of magnetic stirring to improve

the mass transport of analytes in the system.Z2 Now only 1 h was required for all analytes to equilibrate, but the amount extracted was only about one-tenth of that with the magnetic stirring. A possible reason for a decrease in the amount extracted is a leak in the system. Further modifications are currently being made to the seal around the extraction vessel and should correct this problem.

(21) Montgomery, J. Groundwater ChemicalsDesk Reference;Lewis Publishers:

Recehred for revlew June 1, 1993. Accepted October 1, 1993..

cllebia,Mi, 1990. (22) Moth.&. S.: Pawliszyn, J. On-line Monitoring of Flowing Samples Using . Solid &e Microextrkion-Gas Chromatography. Anal. CMm.,420, in press.

ACKNOWLEDGMENT The authors acknowledgeDr. James Barker of the Waterloo Centre for Ground Water Research (WCGR) for his advice and encouragement, and Brian MacGillivray of the Wastewater Technology Centre, Burlington, Ontario, for providing the sewage sample. Financial support for this work was been generously provided by the Natural Sciencesand Engineering Council of Canada (NSERC), Supelco Inc., and Varian Associates.

Abstract published in Aduance ACS Abstracts. November 15, 1993.

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