Letter to the Editor Concerning the Viewpoint; “Recognizing the

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Letter to the Editor Concerning the Viewpoint; “Recognizing the Limitations of Performance Reference Compound (PRC)-Calibration Technique in Passive Water Sampling”

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iu et al.1 criticize the use of performance reference compounds (PRCs) for the in situ calibration of passive sampling devices (PSDs), but insufficiently distinguish between absorption and adsorption based PSDs (typically used for “nonpolar” and “polar” compounds, respectively). The problems with the use of PRCs with adsorption samplers have been discussed earlier,2,3 and the authors provide little new insight. Thus for adsorption PSDs we agree, but for absorption PSDs the authors confuse rather than clarify. They list five “shortfalls” of the PRC approach. First, That Isotopically Labeled Counterparts May Not Always Be Available/Affordable and That Alternatives May Be Different Enough As to Affect Quantitation. Best results are obtained by nonlinear least-squares regression (NLS) of PRC retention data that cover the full dissipation range (0−100%, including intermediate values).4 Whether or not PRCs are labeled counterparts of target analytes is irrelevant. A range of deuterated analogues are widely available at reasonable cost. The accuracy of the partition coefficients (Ksw), for PRCs and target compounds, is the real challenge here (presently 0.2−0.5 log units).5 Second, That the Dissipation Rate of a PRC May Not Be Identical to the Uptake of the Target Analyte. This is not a requirement for the PRC approach in absorption PSDs. See previous comment. Third, That It Will Be Difficult to Find PRCs Suitable for Compounds Which Are Strongly Bound to Sorbent Phases. All attempts to use PRCs for in situ calibration of adsorption PSDs are futile as long as we have incomplete mechanistic and quantitative understanding of these samplers, and thus attention should be focused on this first.2,3 Even if the dissipation rate of desisopropyl-d5 responds to changing exposure conditions, it is still not known what this means for the sampling rates (Rs) of other analytes in adsorption PSDs. In contrast, we do have a quantitative understanding of absorption PSDs and can therefore, use PRCs to calibrate these samplers. Fourth, That Differences between Membrane Control and Boundary Layer Control Limit the Applicable Scope of PRC Calibration. The use of a single PRC is inappropriate (see above). Further, membrane control is only an issue for exceptionally short exposure times or exceptionally large sampling rates. In most cases, compounds under membrane control quickly reach equilibrium concentrations (which are independent of uptake kinetics). Finally, the Rs model used in the NLS approach with multiple PRCs can include membrane control.4 Fifth, That the Physical Significance of PRC-Calibrated Analyte Concentrations Is Not Clear. The distinction between “time-weighted average” and “time-integrated” concentrations is artificial. Both are approximate for any time >0. This is fundamental to the use of PSDs as such, and is unrelated to whether or not PRCs are used. Furthermore, the authors © 2013 American Chemical Society

appear to misunderstand the concept of Rs (mass transfer coefficient multiplied by surface area) which is independent of the degree of equilibrium attained.6 We reiterate that the PRC approach remains the best way to determine exposure and compound specific sampling rates for absorption samplers such as SPMDs, LDPE, and silicone rubbers.

Christopher Harman*,† Kees Booij‡ †

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Norwegian Institute for Water Research (NIVA), Gaustadalléen 21, NO-0349, Oslo, Norway ‡ Royal Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Texel, The Netherlands

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Liu, H. H.; Wong, C. S.; Zeng, E. Y. Recognizing the limitations of performance reference compound (PRC)-calibration technique in passive water sampling. Environ. Sci. Technol. 2013, 47 (18), 10104− 10105. (2) Harman, C.; Allan, I. J.; Bauerlein, P. S. The challenge of exposure correction for polar passive samplersThe PRC and the POCIS. Environ. Sci. Technol. 2011, 45 (21), 9120−9121. (3) Harman, C.; Allan, I. J.; Vermeirssen, E. L. M. Calibration and use of the polar organic chemical integrative sampler − A critical review. Environ. Toxicol. Chem. 2012, 31 (12), 2724−2738. (4) Booij, K.; Smedes, F. An improved method for estimating in situ sampling rates of nonpolar passive samplers. Environ. Sci. Technol. 2010, 44 (17), 3798−6794. (5) Lohmann, R.; Booij, K.; Smedes, F.; Vrana, B. Use of passive sampling devices for monitoring and compliance checking of POP concentrations in water. Environ. Sci. Pollut. Res. 2012, 19, 1885−1895. (6) Booij, K., Vrana, B., Huckins, J. N. Theory, modelling and calibration of passive samplers used in water monitoring. In Passive Sampling Techniques in Environmental Monitoring, Comprehensive Analytical Chemistry, 48; Greenwood, R., Mills, G. A., Vrana, B., Eds.; Elsevier: Amsterdam, 2007; pp 141−169.

Received: November 19, 2013 Accepted: November 27, 2013 Published: December 6, 2013 3

dx.doi.org/10.1021/es405153c | Environ. Sci. Technol. 2014, 48, 3−3