Determination of Vapor Pressures for Organophosphate Esters

Article pubs.acs.org/jced

Determination of Vapor Pressures for Organophosphate Esters Sandra Brommer,† Liisa M. Jantunen,*,‡ Terry F. Bidleman,§ Stuart Harrad,† and Miriam L. Diamond⊥ †

School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham B15 2TT, U.K. Air Quality Processes Research Section, Environment Canada, Egbert, Ontario L0L 1N0, Canada § Department of Chemistry, Umeå University, SE-901 87 Umeå, Sweden ⊥ Department of Earth Sciences, University of Toronto, Toronto, Ontario M5R 0A3, Canada ‡

S Supporting Information *

ABSTRACT: Organophosphate compounds are ubiquitous in the environment and to better understand and predict their environmental transport and fate, well-defined physical-chemical properties are needed. The subcooled liquid-phase vapor pressures at 298.15 K (p298) were determined for 11 chlorinated and nonchlorinated phosphate flame retardants (PFRs) by the capillary gas chromatography retention time method (GC-RT). Values of log (p298/Pa) ranged from −5.22 to −1.32 and enthalpies of vaporization (ΔlgH/kJ·mol−1) ranged from 82.0 to 109. Log (p298/Pa) by GC-RT showed good overall agreement with estimates using the Modified Grain Method (EpiSuite) and with the mean of experimental and in silico literature values, whereas values for the chlorinated PFRs appeared to be overestimated. SPARC modeling seriously underestimated p298, especially for the less volatile compounds. The Junge−Pankow adsorption model at 288.15 K predicted that most of the PFRs would be predominantly in the particulate phase in urban air and distributed between the particulate and gaseous phases in background air.

1. INTRODUCTION Organophosphate flame retardants and plasticizers (PFRs) are used for a variety of industrial purposes, including flame retardants, plasticizers, antifoaming agents and hydraulic fluids.1−3 Recently, the use of PFRs as replacements for polybrominated diphenyl ethers (PBDEs) and other brominated flame retardants has increased.2−6 PFRs have been reported in relatively high concentrations compared to PBDEs in dust from Europe2,7,8 and the U.S.,9,10 as well as in indoor air.11,12 PFRs can undergo transport over long distances and have been detected in arctic air13 and marine surface waters.3,14 Prediction of the transport and fate of PFRs in the environment depends on reliable data for their physicochemical properties. A key property is the subcooled liquid-phase vapor pressure (p). As discussed in detail below, reported values for p of a given PFR compound often differ by an order of magnitude or more. Moreover, many are in silico estimates, which can vary among software packages and even versions of the same software. Gas chromatographic retention time (GC-RT) data have been used in the past to determine p at 298.15 K (p298) and enthalpies of vaporization (ΔlgH) for a variety of substances.15−23 Hinckley et al.17 applied the GC-RT method to determine p298 of seven organophosphate insecticides and three PFRs (TEHP, TBOEP, and p-TMPP) (names and abbreviations are given in Table 1). The resulting p298 fell within the 0.95 confidence interval of some literature values, but overestimated reported values of p298 for others. At that time it was not clear whether the discrepancies were due to systematic overestimation of p298 by the GC-RT method for moderately polar compounds (by early elution from the nonpolar stationary phase)15,17 or simply the inadequacies of © 2014 American Chemical Society

the literature database. The present study was conducted to determine p298 and ΔlgH of 11 PFRs by the GC-RT method and to evaluate the accuracy of the values obtained by comparison to previously reported experimental measurements and in silico estimates. 1.1. The GC-RT Method. The GC-RT method has been widely used to determine vapor pressures of organic compounds.15−22,24 Test compounds are cochromatographed with a retention time reference compound at a series of isothermal temperatures on a short GC column containing a nonpolar stationary phase. The reference compound is chosen to have well-established values of ΔlgHref and pref,T over the temperature range of measurements. A log−log plot of relative retention time (RRT) of test compound (x) and reference compounds (tx/tref)T versus pref,T is made at the different temperatures (T/K) according to eq 1. The ratio of vaporization enthalpies (ΔlgHx/ ΔlgHref) and constant C are evaluated from the slope and intercept: log(tx /tref )T = (1 − Δl g Hx /Δl g Href ) log(pref,T /Pa) − C (1)

An initial estimate of the vapor pressure pGC at 298.15 K is then obtained from log(pGC,298 /Pa) = (Δl g Hx /Δl g Href )log(pref,298 /Pa) + C (2) Received: November 7, 2013 Accepted: March 17, 2014 Published: April 4, 2014 1441

dx.doi.org/10.1021/je401026a | J. Chem. Eng. Data 2014, 59, 1441−1447

Journal of Chemical & Engineering Data

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Table 1. Chemicals Used for Vapor Pressure Determinations abbreviation4

CAS no.

purity, %

source

melting point, K41

boiling point, K41

tris(2-chloroethyl)phosphate tris(chloroisopropyl)phosphate tris(1,3-dichloroisopropyl)phosphate tris(2-butoxyethyl)phosphate tris(2-ethylhexyl)phosphate triphenyl phosphate ethylhexyldiphenyl phosphate tris(2-isopropylphenyl)phosphate tri-o-cresyl phosphate tri-m-cresyl phosphate tri-p-cresyl phosphate hexachlorobenzene

TCEP TCIPP TDCIPP TBOEP TEHP TPHP EHDPP TIPPP o-TMPP m-TMPP p-TMPP HCB

115-96-8 13674-84-5 13674-87-8 78-51-3 78-42-2 115-86-6 1241-94-7 64532-95-2 78-30-8 563-04-2 78-32-0 118-74-1

97 a 96 94 97 98 91

238 233 300 203 199 323.5 219 219 26232 298.5 350

603 > 543 509 to 510 at 5 mmHg 494 at 4 mmHg 488 at 4 mmHg 518 at 11 mmHg 648 > 57330 683 683 703

p,p′-dichlorodiphenyltrichloroethane

p,p′-DDT

50-29-3

p,p′- dichlorodiphenyldichloroethylene

p,p′-DDE

72-55-9

p,p′-dichlorodiphenyldichloroethane

p,p′-DDD

72-54-8

o,p′- dichlorodiphenyltrichloroethane

o,p′-DDT

789-02-6

polycyclic aromatic hydrocarbon mix

PAH mix

polychlorinated biphenyls

PCB

Sigma Aldrich Seelze, Germany Sigma Aldrich Seelze, Germany Sigma Aldrich Seelze, Germany Sigma Aldrich Seelze, Germany Sigma Aldrich Seelze, Germany Sigma Aldrich Seelze, Germany Sigma Aldrich Seelze, Germany Sigma Aldrich Seelze, Germany Wellington Guelph ON, Canada Wellington Guelph ON, Canada Wellington Guelph ON, Canada Accustandard New Haven CT, USA Accustandard New Haven CT, USA Accustandard New Haven CT, USA Accustandard New Haven CT, USA Accustandard New Haven CT, USA Accustandard New Haven CT, USA Accustandard New Haven CT, USA

chemical name

>98 97 98

a

According to the Sigma-Aldrich catalogue, this is a mixture of isomers with typical composition: main isomer 66% tris(1-chloro-2-propyl) phosphate; minor components, bis(1-chloro-2-propyl) (2-chloropropyl) phosphate and (1-chloro-2-propyl) bis(2-chloropropyl) phosphate.

118 (TIPPP), 284 (HCB), and 235 (p,p′-DDT). Data were collected and processed using Agilent Chemstation. 2.3. Data Treatment. The log (tx/tref)T measurements obtained over the range of isothermal temperature runs were regressed versus log (pref,T/Pa) according to eq 1. Example plots are shown in Supporting Information SI-1. Log (pGC,298/Pa) was calculated from eq 2. To avoid excessively long run times, the more volatile RT reference compound HCB was used for TCEP and TCIPP, and less volatile p,p′-DDT as an RT reference for the other PFRs. Koutek et al.21 found that errors in the GC-RT method were minimized by selecting reference compounds that were closer in volatility to test compounds. The temperature dependence of pHCB,T was

These equations assume that the infinite dilution activity coefficients in the GC stationary phase are the same for test and reference compounds, and that the ratio of vaporization enthalpies is constant over the temperature range of measurements.17,21 In practice, it is necessary to adjust for nonideality by making a log−log calibration plot of p298 vs pGC,298 for a series of low-polarity compounds (usually polycyclic aromatic hydrocarbons and chlorinated hydrocarbons) for which values of p298 have been measured or estimated by methods other than GCRT.15−20 The resulting plot is used to derive p298 from the experimental pGC,298.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Chemical names, “practical” abbreviations following Bergman et al.,4 and sources of the PFRs targeted in this study are listed in Table 1. Structures are shown in Figure 1. 2.2. Experimental Procedures. PFRs, retention time reference compounds hexachlorobenzene (HCB) or p,p′-DDT, and other calibration compounds (see below) were cochromatographed according to previously described techniques.15,17 Approximately 2 ng of PFRs in isooctane was injected in splitless mode on a DB-1 capillary column (1.0 m × 0.25 mm i.d., 0.25 μm film, J&W Scientific, USA) installed in an Agilent 6890N GC5973 mass selective detector (MSD). Injector, source and quadrupole temperatures were 473 K, 423 K, and 503 K. Isothermal runs were made between 333 K and 363 K for TCEP and TCIPP and 383 K and 413 K for other less volatile PFRs. Flow rates varied between (1 to 5) mL·min−1; the faster flow rates were used for the lower temperature runs. As reported previously,16 this flow rate variation does not affect the RRTs. The MSD was operated in the selected ion monitoring mode using ions 249 (TCEP), 277 (TCIPP), 299 (TBOEP), 326 (TPHP), 368 (TMPP isomers), 99 (TEHP), 381 (TDCIPP),

log(pHCB,T /Pa) = 10.90 − 3556/(T /K)

(3)

which was derived from the thermodynamically consistent “final adjusted value” (FAV)25 of log (pHCB,298/Pa) = −1.03, and the slope = −ΔlgHHCB/2.303·R, where ΔlgHHCB/J·mol−1 = 68082 and R/J·mol−1·K−1 = 8.314. This value of 68082 was derived from the FAV internal energy of phase change for solubility in air (Δlg UHCB/J·mol−1 = 65690)25 plus 2391 to convert internal energy to enthalpy.26 FAVs of physicochemical properties have been derived by first combining literature values for solubilities of chemicals in three phases (air, water, and octanol) and three partitioning properties among these phases (octanol/water, octanol/air, air/water), and then adjusting these six properties to achieve thermodynamic consistency while minimizing error.25,26 The ΔlgHHCB is not at 298 K, but an average value over the reported temperature range of HCB vapor pressure measurements (250 to 416 K).25 Equation 3 gives pHCB,T that are about 60% lower than those used as reference values (not from FAVs) in the GC-RT determination of p298 for fluorinated chemicals.20 1442



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Figure 1. Structures of test compounds used for vapor pressure determinations.

The temperature dependence of pDDT,T was log(pDDT, T /Pa) = 12.38 − 4665/(T /K)

(4)

which was based on five reports for the vapor pressure of solid p,p′-DDT and four experimental values for the entropy of fusion (ΔslSDDT, J·mol−1·K−1), which were used to convert solid- to subcooled liquid-phase vapor pressure.16 The ΔlgHDDT (89320 J· mol−1) is an average over the temperature range for which vapor pressures of p,p′-DDT were reported (273 K to 373 K).16 Two calibration plots were prepared with log (p298/Pa) versus log (pGC,298/Pa), using calibration compounds with established values of p298. One plot (Figure 2, r2 = 0.88) was made with lower

Figure 3. Calibration plot using p,p′-DDT for estimating p298/Pa from pGC,298/Pa (regression parameters are log (p298/Pa) = 1.083 log(pGC,298/Pa) + 0.252, r2 = 0.947).

A second calibration plot (Figure 3, r2 = 0.95) based on p,p′DDT as the reference compound was made for higher molecular weight compounds benz[a]anthracene, benzo[a]pyrene, p,p′DDE, p,p′-DDD, o,p′-DDT, and PCBs-15, -52, -61, -101, -155, -202, and -209. This plot was used for the other PFRs.

Figure 2. Calibration plot using HCB for Estimating p298/Pa from pGC,298/Pa (regression parameters are log (p298/Pa) = 0.718 log (pGC,298/Pa) − 0.178, r2 = 0.880).

log(p298 /Pa) = 1.083 log(pGC,298 /Pa) + 0.252

molecular weight PAH compounds phenanthrene, anthracene, pyrene, fluoranthene, and the chlorinated hydrocarbons hexachlorocyclohexanes (α-, β-, and γ-isomers) and PCB-29, using HCB as the reference compound to determine pGC,298. This plot was used to estimate p298 of TECP and TCIPP from measured pGC,298. log(p298 /Pa) = 0.718log(pGC,298 /Pa) − 0.178

(6)

Calibration compounds and sources of their p298 values are listed in the Supporting Information Table SI-1 and SI-2. FAVs were selected wherever possible; other p298 values were determined by methods other than GC-RT. The temperature dependence of p for the PFRs was expressed by the Clausius−Clapeyron equation: log(p/Pa) = A + B/(T /K)

(5) 1443

(7)



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Table 2. PFR, log (pGC,298/Pa) and log (p298/Pa) with Standard Uncertainties u(p298/Pa), ΔlgH/kJ·mol−1, with Standard Uncertainties u(ΔlgH/kJ·mol−1), and Parameters of eq 7 for PFRs PFR TCEP TCIPP o-TMPP m-TMPP p-TMPP TEHP TIPPP TPHP TBOEP TDCIPP EHDPP

log(pGC,298/Pa) −1.60 −1.79 −4.20 −4.56 −4.80 −4.55 −5.05 −3.53 −4.21 −3.36 −3.97

log (p298/Pa) −1.32 −1.46 −4.29 −4.68 −4.95 −4.68 −5.22 −3.57 −4.31 −3.38 −4.05

u(log p298/Pa) 0.20 0.20 0.20 0.31 0.32 0.31 0.33 0.29 0.30 0.28 0.28

p298/Pa −2

4.8·10 3.5·10−2 5.1·10−5 2.1·10−5 1.1·10−5 2.1·10−5 6.0·10−6 2.7·10−4 4.9·10−5 4.1·10−4 8.9·10−5

ΔlgH/kJ·mol−1a

u(ΔlgH/kJ·mol−1)

B

A

82.0 85.2 99.3 103.7 105.7 106.0 108.5 92.8 102.6 91.3 99.9

10.2 10.6 3.5 3.6 3.7 3.7 3.8 3.2 3.6 3.2 3.5

−4281 −4452 −5185 −5418 −5523 −5538 −5669 −4846 −5359 −4766 −5218

13.04 13.47 13.20 13.62 13.72 14.02 13.96 12.72 13.76 12.63 13.53

The ΔlgH values for the PFRs are averages over the temperature ranges of the GC-RT measurements (333 K to 363 K for TCEP and TCIPP and 383 K to 413 K for other PFRs).

a

where B = −ΔlgH/2.303·R (as in eq 3 and eq 4). The ΔlgH was calculated from the slope of eq 2 and ΔlgHref for either HCB (TCEP and TCIPP) or p,p′-DDT (other PFRs). These values are summarized in Table 2. The ΔlgH values for the PFRs are averages, defined by the temperature ranges over which the reference compound vapor pressures have been measured (250 K to 414 K for HCB and 273 K to 373 K for p,p′-DDT, see above) and the temperature range of the GC-RT measurements (333 K to 363 K for TCEP and TCIPP and 383 K to 413 K for other PFRs). Since the latter ranges are smaller, they are probably more relevant. 2.4. Uncertainty Estimation in GC-RT Derived Vapor Pressures and Vaporization Enthalpies. RRTs were very reproducible with u(RRT)/|RRT| = 0.002 for three replicates. As discussed previously,16 uncertainties in RRTs tend to be relatively minor and most of the uncertainty in the GC-RT method lies in relating pGC,298 to p298 through the log−log calibration plots. Standard uncertainties of estimates (ue) in these plots were 0.188 (Figure 2) and 0.271 (Figure 3). From these, the standard prediction uncertainties (up) of log (p298/Pa) from log (pGC,298/Pa) were calculated as described previously.16 Relative uncertainties in PFR vaporization enthalpies are determined by the ΔlgHref term in eq 1, through u(ΔlgHref)/ |ΔlgHref|. The GC reference compound used for most PFRs was p,p′-DDT, for which ΔlgHDDT was derived from five literature reports of ΔsgHDDT and an average entropy of fusion.16 The relative uncertainty, u(ΔlgHDDT)/|ΔlgHDDT| was 0.035.16 HCB was used as a GC reference for TCEP and TCIPP. The ΔlgHHCB was derived by the FAV evaluation process, and uncertainties in FAVs were expressed by relative ranking of 1 to 5, with the lower numbers corresponding to the least adjustment of physicochemical properties required to attain thermodynamic consistency (HCB vapor pressure received a ranking of 1).25 The original data and references for log ps,HCB/Pa and ΔsgHHCB/kJ·mol−1 are shown in the top left panel and given in the captions of Figure 2 of ref 25. The ΔsgHHCB/kJ·mol−1 = 77.4 to 101.0, from which we calculated u(ΔsgHHCB)/|ΔsgHHCB| = 0.124. The same relative uncertainty applies to u(ΔlgHHCB)/|ΔlgHHCB|, since a single ΔslSHCB/J·mol−1·K−1 was used to convert solid- to subcooled liquid-phase vapor pressure.25 The u(ΔlgHx)/|ΔlgHx| for PFRs were thus derived by multiplying ΔlgHref/kJ·mol−1 by 0.124 for TCEP and TCIPP and 0.035 for the other compounds (Table 2).

3. RESULTS AND DISCUSSION 3.1. Vapor Pressures and Enthalpies of Vaporization. Table 2 summarizes log (pGC,298/Pa), log (p298/Pa), ΔlgH/kJ· mol−1, and parameters of eq 7 for the 11 targeted PFRs, based on GC-RT measurements. Values of log (p298/Pa) for the 11 PFR cover 4 orders of magnitude, from −5.22 for TIPPP to −1.32 for TCEP. Values of up for the GC-RT determinations of log (p298/Pa) ranged from 0.20 for TCEP and TCIPP to 0.28 to 0.33 for the heavier PFRs. Calibration plots contain 7−12 points (Figures 2 and 3) so the corresponding prediction windows at 95% confidence for log (p298/Pa) were ± 0.47 for TCEP and TCIPP, and ± 0.62 to ± 0.73 for heavier compounds. The ΔlgH/kJ·mol−1 from GC-RT measurements ranged from 82 to 109 with most > 90, which is similar to the range of ΔlgH values reported previously for tri- to hexabrominated PBDEs,19 PAHs with four or more rings,17,22 PCBs with four or more chlorines,17,23 and some chlorinated pesticides.17 3.2. Comparison of GC-RT Measurements to Other Values. A survey was performed to retrieve published measurements or in silico estimates of p298 for the target compounds. Values of p298 were also predicted using SPARC version 5.1 (SPARC performs automatic reasoning in chemistry)27 and MPBPWIN in the Estimation Program Interface (EPI) Suite,28 version 4.11. Comparison of the acquired results with with our GC-RT determinations and estimates using SPARC and EpiSuite are listed in Supporting Information Table SI-3. Literature values of p at temperatures other than 298.15 K were adjusted by using the Clausius−Clapeyron parameters of eq 7 determined here (Table 2), or by extrapolation of temperature-dependent data in the original report (Supporting Information Table SI-3). This comparison shows that reported p298 values often vary by more than an order of magnitude for an individual PFR. We compared our GC-RT determinations of log (p298/Pa), designated log (p298/Pa)RT for the comparisons, with (1) SPARC estimates, (2) EpiSuite estimates, and (3) mean of log (p298/ Pa)lit, based on literature reports of experimental measurements and in silico estimates other than GC-RT or SPARC. The log (p298/Pa)RT values of Hinckley et al.17 for TEHP, TBOEP, and p-TMPP were not included because these were also obtained via GC-RT and the intent was to evaluate GC-RT versus other methods. Plots of log (p298/Pa)RT versus log (p298/Pa)SPARC, log (p298/ Pa)EpiSuite, and mean log (p298/Pa)lit are shown in Figures 4 to 6. Log (p298/Pa)RT was closely correlated to log (p298/Pa)SPARC (r2 = 0.94) (Figure 4), but with a strong overestimate from the 1444



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that the GC-RT method overestimates p298 for polar compounds. Figures 5 and 6 suggest little or no overall bias for PFRs which are relatively polar, though individual compounds may be problematic. The greatest deviations from the 1:1 line in Figure 5 are for TDCIPP and o-TMPP, which are higher and lower by about 1 log unit, respectively. Figure 6 suggests a high bias for the chlorinated PFRs TCEP, TCIPP, and TDCIPP (above the 1:1 line by 0.91, 0.97, and 1.6 log units). However, the log (p298/Pa)EpiSuite and mean log (p298/Pa)lit values have their own uncertainties. EpiSuite predictions are based on the modified Grain method which extrapolates the vapor pressure from the boiling point at atmospheric pressure. Accurate boiling points for these compounds are often difficult to obtain and hence can be uncertain.29−33 Standard uncertainties (u) of log (p298/Pa)lit values range from 0.35 to 0.99 log units (mean u = 0.71 log unit). 3.3. Implications for Mobility of PFRs. The distribution of semivolatile compounds between the particulate and gaseous phases in air is critical to understanding their long-range transport potential34 and exposure through inhalation.35 The Junge−Pankow (J-P) equation is commonly used to estimate the particle fraction (Φ) in ambient air from p and the surface area available for adsorption, θ = cm2 aerosol/cm3 air.36,37

Figure 4. Log (p298/Pa)RT versus log (p298/Pa)SPARC (gray line is 1:1 relationship; dashed line is the regression line; regression parameters are log (p298/Pa)RT = 0.619 log (p298/Pa)SPARC − 0.930, r2 = 0.944).

Φ = cθ /(p + cθ )

(8)

where c/Pa·cm is often assumed to be 17.2. Estimates of Φ for the PFRs investigated here were made for urban air and clean continental background air, using the θ-values of 1.1·10−5 and 4.2·10−7 cm2 aerosol/cm3 air, respectively, typical of these regimes.37 The average temperature of the earth’s surface (288.15 K) was assumed. Other parameter values are listed in Table 2. Figure 7 shows the modeled particulate−gas distributions in urban and background air. In urban air, TCEP and TCIPP are 37

Figure 5. Log (p298/Pa)RT versus log (p298/Pa)EpiSuite (gray line is 1:1 relationship; dashed line is the regression line; regression parameters are log (p298/Pa)RT = 0.959 log (p298/Pa)EpiSuite − 0.114, r2 = 0.769).

Figure 7. Distributions of PFRs between the particulate/gas phases in urban and background air at 288.15 K, modeled with the Junge−Pankow adsorption equation:36,37 dark gray circle, background; light gray circle, urban. Figure 6. Log (p298/Pa)RT versus mean log (p298/Pa)lit (gray line is 1:1 relationship; dashed line is the regression line; regression parameters are log (p298/Pa)RT = 0.971 log (p298/Pa)lit + 0.0162, r2 = 0.680).

almost entirely in the gaseous phase, while 60% or more of the other PFRs are associated with particles. Percentages in the particle phase expected in background air were 10% or less for TCEP, TCIPP, TDCIPP, and TPHP, and 20 to 80% for the other PFRs. In contrast, field studies have shown that PFRs are predominantly in the particulate phase, including TCEP and TCIPP in urban air38 and air over the North Sea.13,39 This suggests that these polar compounds may be more strongly sorbed to particles than predicted by the J-P model, and/or to glass fiber filters used for air sampling. For indoor air sampling SPE cartridges are commonly used,3 which do not distinguish between the two phases. Denuder sampling might be a better approach to determining the particulate/gas distribution for PFRs, as has been demonstrated for fluorinated compounds.40

1:1 line at lower vapor pressures. The average deviation of log (p298/Pa)RT from the 1:1 line was 0.84 log units. The correlations of log (p298/Pa)RT versus log (p298/Pa)EpiSuite (Figure 5) and mean log (p298/Pa)lit (Figure 6) had r2 = 0.77 and 0.69 and only slight overestimate, with average deviations from 1:1 of 0.05 and 0.20 log units. The good correspondence between log (p298/Pa)RT and log (p298/Pa)EpiSuite or mean log (p298/Pa)lit suggests that the bias in Figure 4 is due to underestimation by SPARC, especially for the low vapor pressure PFRs. Hinckley et al.17 expressed concern 1445



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4. CONCLUSIONS Vapor pressures and enthalpies of vaporization have been experimentally determined by the GC-RT method for 11 environmentally relevant PFRs. In general, log (p298/Pa)RT agreed well with log (p298/Pa)EpiSuite and with mean log (p298/Pa)lit. A bias toward higher values by GC-RT, as might be expected from chromatography of polar compounds on a nonpolar stationary phase, was not seen overall, although this is suggested for the chlorinated compounds TCEP, TCIPP, and TDCIPP. The SPARC model underestimated p by over 1 log unit especially for the less volatile compounds. Application of the Junge−Pankow (J-P) adsorption model using p288 indicated that most of the PFRs except TCEP and TCIPP are expected to be particulate phase in urban air and distributed between the particulate and gaseous phases in background air. Higher percentages of particulate-phase PFRs have been measured in field studies, which suggests stronger sorption of PFRs to particles or filters used for sampling. Our measurements provide a consistent set of vapor pressure data that can be used for environmental modeling, risk assessment, and chemical management. Additional experimental measurements by methods other than GC-RT are necessary to provide a more robust database for comparison to estimates made using GC-RT, and to in silico estimates which can vary by an order of magnitude or more.

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ASSOCIATED CONTENT

S Supporting Information *

Additional tables and figures as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] Funding

This research has received funding from the European Union Seventh Framework Program (FP7/2007-2013) under Grant Agreement No. 295138 (INTERFLAME project). S.B. was funded via a studentship from the University of Birmingham, with additional financial support from the Food and Environment Research Agency (FERA). T.F.B. is supported by the EcoChange Program, Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (Formas). Notes

The authors declare no competing financial interest.

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Determination of Vapor Pressures for Organophosphate Esters

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