Washington John W, Henderson W Matthew, Ellington J Jackson, Jenkins Thomas M, Evans John J
United States Environmental Protection Agency, National Exposure Research Laboratory, 960 College Station Road, Athens, GA 30605, USA.
J Chromatogr A. 2008 Feb 15;1181(1-2):21-32. doi: 10.1016/j.chroma.2007.12.042. Epub 2007 Dec 23.
With the objective of detecting and quantitating low concentrations of perfluorinated carboxylic acids (PFCAs), including perfluorooctanoic acid (PFOA), in soils, we compared the analytical suitability of liquid chromatography columns containing three different stationary phases, two different liquid chromatography-tandem mass spectrometry (LC/MS/MS) systems, and eight combinations of sample-extract pretreatments, extractions and cleanups on three test soils. For the columns and systems we tested, we achieved the greatest analytical sensitivity for PFCAs using a column with a C(18) stationary phase in a Waters LC/MS/MS. In this system we achieved an instrument detection limit for PFOA of 270 ag/microL, equating to about 14 fg of PFOA on-column. While an elementary acetonitrile/water extraction of soils recovers PFCAs effectively, natural soil organic matter also dissolved in the extracts commonly imparts significant noise that appears as broad, multi-nodal, asymmetric peaks that coelute with several PFCAs. The intensity and elution profile of this noise is highly variable among soils and it challenges detection of low concentrations of PFCAs by decreasing the signal-to-noise contrast. In an effort to decrease this background noise, we investigated several methods of pretreatment, extraction and cleanup, in a variety of combinations, that used alkaline and unbuffered water, acetonitrile, tetrabutylammonium hydrogen sulfate, methyl-tert-butyl ether, dispersed activated carbon and solid-phase extraction. For the combined objectives of complete recovery and minimization of background noise, we have chosen: (1) alkaline pretreatment; (2) extraction with acetonitrile/water; (3) evaporation to dryness; (4) reconstitution with tetrabutylammonium-hydrogen-sulfate ion-pairing solution; (5) ion-pair extraction to methyl-tert-butyl ether; (6) evaporation to dryness; (7) reconstitution with 60/40 acetonitrile/water (v/v); and (8) analysis by LC/MS/MS. Using this method, we detected in all three of our test soils, endogenous concentrations of all of our PFCA analytes, C(6) through C(10)-the lowest concentrations being roughly 30 pg/g of dry soil for perfluorinated hexanoic and decanoic acids in an agricultural soil.
为了检测和定量土壤中低浓度的全氟羧酸（PFCA），包括全氟辛酸（PFOA），我们比较了三种不同固定相的液相色谱柱、两种不同的液相色谱-串联质谱（LC/MS/MS）系统以及八种样品提取物预处理、萃取和净化组合对三种测试土壤的分析适用性。对于我们测试的色谱柱和系统，在沃特世LC/MS/MS中使用C(18)固定相的色谱柱，我们对PFCA实现了最高的分析灵敏度。在该系统中，我们实现了PFOA的仪器检测限为270 ag/μL，相当于柱上约14 fg的PFOA。虽然用乙腈/水对土壤进行简单萃取能有效回收PFCA，但溶解在提取物中的天然土壤有机质通常也会产生显著的噪声，表现为与几种PFCA共洗脱的宽的、多峰的、不对称峰。这种噪声的强度和洗脱曲线在不同土壤中变化很大，它通过降低信噪比来挑战低浓度PFCA的检测。为了降低这种背景噪声，我们研究了多种预处理、萃取和净化方法的组合，这些方法使用了碱性和非缓冲水、乙腈、硫酸氢四丁铵、甲基叔丁基醚、分散活性炭和固相萃取。为了实现完全回收和最小化背景噪声的综合目标，我们选择了：（1）碱性预处理；（2）用乙腈/水萃取；（3）蒸发至干；（4）用硫酸氢四丁铵离子对溶液复溶；（5）离子对萃取至甲基叔丁基醚；（6）蒸发至干；（7）用60/40乙腈/水（v/v）复溶；（8）通过LC/MS/MS分析。使用这种方法，我们在所有三种测试土壤中检测到了所有PFCA分析物（C(6)至C(10)）的内源浓度——在一种农业土壤中，全氟己酸和全氟癸酸的最低浓度约为30 pg/g干土。
