Efficient ‘‘total’’ extraction of perfluorooctanoate frompolytetrafluoroethylene fluoropolymer

Barbara S. Larsen,* Mary A. Kaiser, Miguel A. Botelho, Stanley F. Bachmura and L. William Buxton

Received 15th May 2006, Accepted 17th July 2006

First published as an Advance Article on the web 14th August 2006

DOI: 10.1039/b606801d

To determine the optimum conditions for the complete extraction of perfluorooctanoate (PFO)

from polytetrafluoroethylene fluoropolymers, sample preparation and pressurized solvent

extraction (PSE) conditions were investigated. Solvent extraction temperature, solvent residence

time, relaxation time between extractions, and the effects of heating before PSE showed that

methanol at 150 uC extraction temperature and a 12 min solvent residence time were the most

efficient conditions. Preheating the polymer before extraction at 150 uC for 24 h significantly

enhanced the quantity of PFO removed. Heating above 150 uC resulted in loss of PFO. PFO was

determined by liquid chromatography with tandem mass spectrometry.

Introduction

Ammonium perfluorooctanoate (APFO) is a processing aid

used in the production of many fluoropolymers. Reports have

shown low levels of perfluorooctanoate (PFO) and perfluoro-

octane sulfonate (PFOS) in human blood sera,1–3 with

additional reports showing PFO, PFOS, and other perfluori-

nated compounds in wildlife.4–6 The US Environmental

Protection Agency (US EPA) has identified both commercial7

and consumer use8 of fluoropolymers as possible contributors

to these levels. One specific commercial use is in analytical

instrument systems, due to the chemical inertness and stability

of fluoropolymers. However, background levels of perfluori-

nated materials have been associated with instrument proce-

dures and blanks.9 This complicates low-level detection of

perfluorinated carboxylic acids due to possible contamination

from the analytical systems and procedures used in the

analyses. Since fluorinated materials are found at low levels

in human sera and in wildlife, it is important to know if

exposure is possible from fluoropolymer processing aids

remaining from the polymerization process. Many fluorinated

polymers would need to be examined for this purpose,

therefore optimal analytical conditions must be ascertained.

This study is an extension of a previous study10 designed to

select the most efficient and complete extraction method and

solvent for quantifying total PFO content in polytetrafluoro-

ethylene (PTFE). In that study we showed that methanol was

the best solvent and that pressurized solvent extraction (PSE)

was the most efficient technique. In this study we show the

effect of sample pretreatment and sample ‘relaxation’ on the

efficiency of PFO extractability. Optimization of temperature,

residence time, and relaxation time of the sample for the PSE

with methanol as the solvent was performed on a PTFE

polymer for maximum extraction of PFO.

Experimental

Apparatus

PSE was performed using a pressurized solvent extractor, an

ASE1 series 200 fitted with 11 mL stainless steel cells, stainless

steel frits, cellulose extraction cap filters, and polyetherether-

ketone (PEEK) seals (Dionex, Sunnyvale, CA). 40 mL

I-CHEM glass collection vials were used (Chase Scientific

Glass Inc., Rockwood, TN). Sample evaporation was con-

ducted with Reacti-Therm III and Reacti-Vap III (Pierce

Chemical Co., Rockford, IL) evaporators. The oven of a

model 5890 gas chromatograph (Agilent Technologies,

Wilmington, DE) was used for sample heating. The oven

temperature was verified with a NIST traceable thermometer

(VWR International Inc., Bridgeport, NJ) to be within

¡0.8 uC.

Materials and reagents

Polytetrafluoroethylene fluoropolymer resin was obtained

from a commercial lot (average particle size approximately

500 mm). Analytical grade methanol and standard grade

Ottawa sand was purchased from EMD Chemicals, Inc.

(Darmstadt, Germany). Methanol was furthered purified by

passing it through a C18 reversed-phase liquid chromatography

column. Water was obtained from a Simplicity water

purification system (.18.2 MV cm) (Millipore, Billerica,

MA). Reagent-grade ammonium acetate was obtained from

Sigma–Aldrich (Milwaukee, WI). House nitrogen obtained

from a Zero Air nitrogen generation system backed up by boil-

off from a liquid nitrogen tank was used for evaporation.

Pentadecafluorooctanoic acid (PFOA) and APFO used for

spiking/recovery was purchased from Oakwood Products, Inc.

(West Columbia, SC). A dual 13C enriched PFOA internal

standard was synthesized in-house. Standards for the seven-

point external calibration curve were prepared by aqueous

dilution of a 1000 ppb (mg L21) standard solution prepared

in methanol to concentrations of 0.5, 1, 5, 10, 25, 50, and

100 ppb.

E I du Pont de Nemours and Company, E228, PO Box 80228,Wilmington, DE, 19880-0228, USA.E-mail: Barbara.S.Larsen@usa.dupont.com; Fax: +1 302 695 1351;Tel: +1 302 695 4876

PAPER www.rsc.org/analyst | The Analyst

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House nitrogen, from time-to-time, was found to contain

quantifiable levels of perfluoroheptanoate, PFO, perfluorono-

nanoate, perfluorodecanoate, perfluoroundecanoate, and

perfluorododecanoate that contributed to background levels

in evaporated samples. Blank water did not show any

quantifiable levels of these anions.

Pressurized solvent extraction procedure

All cells were preconditioned by filling with Ottawa sand, 3 mm

from the top, the top secured, and the cell loaded into the

instrument. The instrument solvent reservoir was filled with

methanol and the lines were rinsed four times prior to use. The

preconditioning cycle parameters were 1600 psi (11.0 kPa),

150 uC, 7 min heating duration, 100% volume flush, and 240 s

purge for 1 cycle. The solvent extract was discarded and a fresh

40 mL collection vial was set in place. Each collection vial was

cleaned before use by soaking twice in water followed by a

methanol soak, and then allowed to air-dry. In each extraction

experiment five preconditioned cells with Ottawa sand were

used, three containing polymer (1.5 g), a recovery check

standard of PFOA and a blank. After extraction each

collection vessel was evaporated to dryness with N2 gas

purging at y45 uC, and PFO determined by LC/MS/MS.10

The thermal stability experiments were performed by

inserting a standard quantity of PFOA or APFO in a

preconditioned cell and heating the cell in the GC oven for

24 hours at 150, 175 or 200 uC. The cells were then extracted

using the PSE. Optimization of the temperature parameter

consisted of three experiments: one at 65 uC, one at 125 uC and

one at 150 uC. The parameters for the extractions were 1600 psi

(11.0 kPa), 12 min heating duration, 100% volume flush, 240 s

purge, and 4 cycles. Using the optimum temperature of 150 uC,

additional residence time experiments were run using duration

settings of 4 min and 30 min.

The sequential extraction of the PTFE polymer (1.5 g) was

performed using the optimum parameter of 150 uC 1600 psi

(11.0 kPa), 12 min heating duration, 100% volume flush, 240 s

purge, and 4 cycles. The extraction procedure was repeated on

each cell a total of six times with no rest time in between. The

relaxation experiment was performed similar to the sequential

experiment with the addition of a 24 h resting time with the

cells maintained at room temperature or 150 uC in between

extractions. The final experiment heated the cells at 150 uC for

24 or 48 hours prior to initial extraction. The resting time at

150 uC for 24 and 48 hours was repeated for six extractions.

Each data point represents an average of four extraction

experiments.

Analytical method

The PFO extract taken after solvent evaporation was

reconstituted by adding 1 mL of methanol to the collection

vial and shaking for 30 min on a wrist-action shaker. The

methanol was then transferred to a 5 mL Class A volumetric

flask and brought to volume with the LC mobile phase A

described below. The dual 13C-enriched standard (final

concentration 50 ppb) was added as an internal standard to

all of the reconstituted samples. The concentration of PFO was

determined using LC coupled with negative ion electrospray

tandem mass spectrometry (LC/MS/MS) (Micromass Quatro

Ultima, Beverly, MA).

Analytes were separated using a model 1100 liquid

chromatograph (Agilent Technologies, Wilmington, DE)

modified with low dead-volume internal tubing. A guard

column, Hypersil C18 2 mm 6 50 mm (Thermo Keystone,

Bellefonte, PA), was installed between the mixer and the

autoinjector. 25 mL of the reconstituted extract was injected

onto a Hypersil ODS 2.1 mm 6 200 mm (Thermo Keystone,

Bellefonte, PA) at a flow rate of 0.3 mL min21 and maintained

at 60 uC. Duplicate injections were made for all samples.

The initial gradient mobile-phase composition was 15%

mobile phase B, where mobile phase A was 2 mM ammonium

acetate–5% methanol and mobile phase B was 100% methanol.

A linear gradient was used from 15%–67% B over sixteen

minutes. The conditions were returned to 15% B for four

additional minutes. Typical elution time for PFOA was

approximately 16.5 min. A typical chromatogram for these

conditions is shown in reference 10. PFO was observed in the

negative ion mode as an anion at 413 amu (CF3(CF2)6 COO2).

The internal standard was observed at 415 amu

(CF3(CF2)513CF2

13COO2).

Selected ion monitoring for the transition of 413 A 369 (loss

of CO2) for the analyte and 415 A 370 (loss of 13CO2) for the

internal standard was used for quantitative analysis. Samples

that fell outside of the calibration range were rediluted and

reanalyzed. A seven-point external calibration curve was

prepared (not including zero) with each calibrant run in

duplicate, bracketing the samples. A typical calibration curve

consisted of all fourteen-calibration points. The acceptance

criterion for the calibration curve requires a correlation

coefficient (R2) ¢ 0.985, with 1/X weighting.

A methanol blank was run after each 100 ppb standard. The

limit of quantitation (LOQ) was defined as 0.5 ppb, the

concentration of the lowest calibration standard. An accep-

tance criterion was set so that the area of the 0.5 ppb standard

must be at least five times greater than any background peak

obtained in the solvent blank. No quantifiable area was

observed in the solvent blank.

Results and discussion

In order to optimize the extraction of PFO from fluoropoly-

mer, it is necessary to understand the thermal stability of

APFO and PFOA at elevated temperatures. Recovery results

from PFOA and APFO spike recovery experiments show that

150 uC is the maximum temperature at which PFO can be

reproducibly recovered (Table 1). The results from 175 uC and

200 uC show recoveries that fall outside the acceptable

recovery range of 70 to 130%. This observation of 150 uC as

Table 1 PFOA/APFO 24 h thermal stability scouting study

StandardConcentration(ppb)

% Recovery150 uC

% Recovery175 uC

% Recovery200 uC

APFO 300 94.5 2 11APFO 30 97.2 0 86PFOA 300 98.5 3 4PFOA 30 78 19 63

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the maximum operating temperature for extraction corre-

sponds with data from recent NMR thermolysis studies11,12

that demonstrated significant decomposition of PFOA and

APFO at elevated temperatures. This observation also agrees

with recent studies of commercial PTFE-coated nonstick fry

pans. Fry pans are thermally treated at elevated temperature

during the manufacturing process. In a surface extraction

study no quantifiable PFO was detected.13 In a different total

extraction study of nonspecified PTFE coating that was

physically removed from the pan surface, a very low-level of

PFO was found.14

Determining efficient extraction parameters for PSE

requires selection of optimal temperature and solvent residence

time. In a previous study10 methanol was shown to be a

preferred solvent for efficient extraction of this fluoropolymer.

Table 2 shows the effects of solvent temperature using a

12 minute solvent residence time. Optimization results for

residence time extractions are shown in Table 3. Together these

experiments indicate that the optimal extraction parameters

are 150 uC with a 12 min solvent residence time. These

conditions were used for all ensuing extractions.

The purpose of this study is to determine the total mass of

PFO in a sample of PTFE. In order to accomplish this goal, a

series of sequential extractions was performed. A limiting

value of 5% of the initial mass extracted was set to determine

the end point or last extraction. The results for the sequential

extraction series are displayed graphically in Fig. 1 as the mass

of PFO extracted from PTFE with each subsequent extraction.

The extraction curve for sequential extractions shows an

exponential removal of PFO under these conditions. Note that

two blank samples and two PEEK seal samples demonstrated

that there was no measurable PFO background in the

extraction system, indicating that the low level measured in

the final extraction was most likely coming from the polymer.

In order to determine if relaxation or thermal treatment of

the polymer might help to expedite PFO removal, a series of

relaxation and thermal treatment experiments were performed.

These experiments were designed to demonstrate if relaxation

or thermal treatment of the polymer would help PFO deep

inside the particle migrate towards the surface, making it more

readily available for extraction. Summaries of the room

temperature and 150 uC relaxation/extraction experiments

are displayed in Fig. 2. A comparison of the sequential

extraction (Fig. 1) with the room temperature, 24 h relaxation

experiment (shown in Fig. 2) indicates that the rest period

alone does not aid the extraction. A comparison of the room

temperature relaxation with the 24 h relaxation at 150 uCexperiment shows that the 150 uC relaxation period gives a

much greater mass on subsequent extractions than at

room temperature. This experiment indicates that the 24 h

relaxation period at 150 uC makes the PFO in this polymer

more accessible for extraction. In order to determine if

thermal treatment or relaxation with thermal treatment was

required to optimize PFO extraction, the polymer was

preheated for 24 h at 150 uC (Fig. 3). These results indicate

that thermal treatment alone greatly increased the quantity of

PFO extracted. Preheating for 48 h rather than 24 h showed no

significant enhancement in the mass extracted.

Table 2 Temperature optimization extraction study of PFO fromPTFE polymer

Temperature/uCConc. PFO inPTFE (ppb)a

Mass PFOextracted/ng

Check standard% recoveryb

65 32.5 ¡ 1.6 48.8 ¡ 2.2 98 ¡ 4.8125 67.2 ¡ 4.0 101 ¡ 6.4 107 ¡ 5.9150 119 ¡ 7.6 179 ¡ 12 118 ¡ 3.7a Average of three sample weights. b Average of five check standardsamples.

Table 3 Residence time optimization extraction study of PFO fromPTFE polymer

Residence time/minConc. PFO inPTFE (ppb)a

Mass PFOextracted/ng

Check standard% recovery

4 71.5 ¡ 5.9 107 ¡ 9.6 99.512 119 ¡ 7.6 179 ¡ 12 11830 105 ¡ 6.1 158 ¡ 9.3 125a Average of three sample weights.

Fig. 1 Sequential extraction of PFO from PTFE polymer.

Fig. 2 Comparison of PFO extraction of PTFE polymer with 24 h

relaxation time at room temperature and 150 uC.

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Conclusions

An accurate, reproducible optimized PSE method to determine

quantitatively PFO in PTFE polymer was demonstrated.

Optimum conditions used methanol, a 150 uC extraction

temperature, and a 12 min solvent residence time. Preheating

the polymer to 150 uC for 24 h prior to PSE greatly enhanced

the quantity of PFO extracted. Heating above 150 uC in this

experiment resulted in the loss of PFO. These optimized

conditions will be used for subsequent determination of total

PFO from similar fluoropolymers.

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Fig. 3 Comparison of PFO extraction from thermally treated PTFE

polymer.

1108 | Analyst, 2006, 131, 1105–1108 This journal is � The Royal Society of Chemistry 2006

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