F. W. Karasek R. E. Clement J. A. Sweetman Chemistry Department University of Waterloo Waterloo. Ontario N2L 3G1
Canada
Preconcentration for Trace To successfully conduct many analyses for organic compounds present at trace concentrations in gas, liquid, or solid samples, it is necessary to selectively extract these compounds and use a concentration step prior to analysis. Many of the techniques developed for preconcentration are described in specialized books and reviews (I,2); a number appear in works on headspace analysis (3).Because of the diversity of this subject, it was felt that an exhaustive review and treatment of all these techniques would not be manageable in this REPORT. Therefore, our discussion will be confined primarily to methods we have found workable and useful for analysis of trace organic constituents in environmental samples. A complete scheme for trace analysis of organic compounds generally consists of sampling, extraction, preconcentration, prefractionation, and analysis by gas chromatography (GC), and gas chromatography/mass spectrometry (GC/MS).Determination of organics a t parts-per-trillion (ppt) levels can be performed by combining sensitive and selective detection with sample preconcentration. In many instances, limits of detection can be made arbitrarily small by increasing the degree of preconcentration. The preconcentration step is often an integral part of the sampling and extraction procedure. For example, in the standard method for analysis of atmospheric airborne particulate matter, large volumes of air are drawn through a filter to obtain a sample. This represents simultaneous sampling and preconcentration. After extracting the l050A
sorbed organics from the collected particulates, a second preconcentration step is performed before analysis. If interfering components must be removed from the sample matrix, one or more prefractionation steps may be required in the procedure. Preconcentration procedures are especially important for samples derived from the environment. Many toxic or carcinogenic compounds are distributed in the environment from a wide variety of sources. Because of their serious effects a t low levels, it is necessary to achieve the greatest analytical sensitivities possible for these suhstances to properly assess their environmental impact. In some cases, detection limits as low as a few ppt are required. In addition to the polynuclear aromatic hydrocarbons (PAHs), much attention has been focused on many halogenated pollutants such as pesticides, polychlorinated biphenyls (PCBs), trihalomethanes (THMs), and the polychlorinated dibenzofuran8 (PCDFs) and dibenzo-p-dioxins (PCDDs). Such suhstances can he found in air, water, and solid and biological samples. Although the extraction and preconcentration procedures may vary for different sample types, problems of sample losses, contamination, interferences, and reproducibility must be considered for all preconcentration methods. Most preconcentration techniques fall into two classes: solvent extraction followed by solvent reduction, or sorbent trapping with subsequent solvent elution or thermal desorption. There are many variations of these methods, and they are frequently used in combination.
ANALYTICAL CHEMISTRY. VOL. 53, NO. 9, AUGUST 1981
Solvent Extraction and Preconcentratlon Sample preconcentration by solvent reduction is most frequently performed following the extraction of solid and biological samples by liquid partition. Proper choice of the extracting solvent can often be the critical step in the sample preparation procedure. It is not always correct to assume that a solvent that efficiently removes a compound from one sample matrix will recover the same compound from a different sample type. This was recently demonstrated by a study that compared the relative efficiencies of various solvents for extraction of organic compounds from municipal incinerator fly ash (4). Methanol, which had been found to have a very high extraction efficiency for organics on airborne particulate matter ( 5 ) , demonstrated very poor recoveries for organics from fly ash using Soxhlet extraction. Figure 1illustrates the poor recoveries for the tetrachlorinated dihenzo-p- dioxins (TCDDs) from.municipal incinerator fly ash using methanol. The top trace of Figure 1is a GC/MS selected ion monitoring (SIM) analysis of a methanol extract from fly ash using the characteristic 321.9 mlz ion of the TCDD isomers. The bottom trace shows results obtained for the same fly ash sample by following the methanol extraction with a benzene extraction. Both extractions were accomplished using a Soxhlet apparatus for 20 cycles with 200 mL of each solvent. This clearly illustrates that no preconcentration step can give adequate re0003-2700/8l/A351-1050101.00/0
0 I 9 8 1 American Chemical Society
Analysis of Organic Compounds sults for quantitative work unless the initial extraction technique gives high, or at least known and reproducible, recoveries of the desired compounds from the initial sample. Prior to the GC/MS analyses (shown in Figure 1)of the benzene and methanol extracts from fly ash, each extract was concentrated by a factor of 2000 by removing solvent using a rotary evaporation apparatus. Figure 2 shows one such unit, although several other types are available commercially. Solvent containing the extracted compounds is rotated in a ilask partially submerged in a water bath. The water bath can be maintained at temperatures lower than the normal boiling point of the solvent hy reducing the internal pressure of the rotary evaporator using aspirator suction. Volatile components will be largely lost in this procedure. However, for many applications the compounds of interest are nonvolatile compared to the extracting solvent. In these applications the rotary evaporation technique is rapid and straightforward, although severe losses of even nonvolatiles can he experienced without careful sample handling. Table I shows the recoveries of components of a standard mixture that was spiked into 200 mL of benzene solvent and then reduced to its original volume (100pL) by rotary evaporation. Losses of these magnitudes are not uncommon when reducing sample sizes to such small volumes. A major problem exists in the physical handling of the sample. Very small volume losses to the glass walls of the recovery flask or to the disposable glass pipets com-
Flgure 1. Selected ion monitoring analysis of tetrachlorcdlbenz+pdii n fly ash; a and b are plolted with the same full-scale values. (a) Methanol extraction. Ib) Benzene reextraction of a monly used for sample transfer may result in significant and nonreproducihle component loss. Table I1 shows recoveries of total organic compounds after spiking 200 mL benzene with 100 pL of a concentrated fly ash extract and then concentrating to the original 100-pL volume of the spiked extract by rotary evaporation. The peaks detected by GC were divided into three categories: early-eluting compounds with GC elution temperatures less than 150 " C , middle-eluting compounds (150 to 230 "0,and late-eluting compounds with GC elution temperatures greater than 230 "C. The large losses of the early, more volatile components dem-
onstrate the difficulties of using rotary evaporation preconcentration for compounds of high or medium volatility. To obtain the original concentrated fly ash extract, an initial rotary evaporation step was performed, which caused losses of many of the volatile compounds prior to the additional losses reported in Table 111. Many recovery studies have been reported in the literature. There is a large variation between some studies in which similar preconcentration procedures were employed, even though very reproducible results were ohmined fur each individual study. Recoveries of PAH compounda, for example, have been reported as being
ANALYTICAL CHEMISTRY, VOL. 53. NO. 9. AUGUST 1981
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I Figure 2. Apparatus used for solvent reduction. Left: rotary evaporator. Right: Kuderna-Danish concentrator with Snyder column from 70 to 95%.When performing recovery studies, several replicates should be performed. In applications involving frequent analyses, recoveries need to be checked periodically, particularly when modifications to the procedure are made or if the analvses are to be performed by someone hexperienced with the procedure. New batches of solvent should always he checked for levels of trace impurities when used in procedures requiring preconcentration of the solvent extract before analysis. Several studies have reported the use of correction factors to allow for the organics lost during extraction and preconcentration. For such applications it is important that the range of recoveries for different replicates he examined, and not just the mean recovery and standard deviation. Although the overall recovery may be acceptable, a single experimental value can easily vary by 30-50%from the expected value, especially for the determination of organics a t pph or lower concentrations. One method of coping with this problem is to spike the sample before extraction and concentration with an isotopically labeled standard that is at a concentration similar to the compound to be determined. For example, "CI-labeled 2,3,7,8tetrachlorodibenzo-p-dioxin (TCDD) has been used in the determination of this compound (6). During the analysis by GC/MSSIM, the labeled TCDD can be determined with the nonlabeled TCDD by monitoring the mlz 321.9 ion (nonlabeled) and the 327.9 ion (labeled TCDD) in a single GC/MS analysis. The use of labeled 1052A
substances is expensive and not practical for analysis of a large number of compounds, but is well-suited for special applications such as the analysis of TCDD.
Solvent Concentration Figure 2 also shows the apparatus that is mmt often used as an alternative to the rotary evaporation method. The Kuderna-Danish evaporative concentrator equipped with a Snyder column also operates by solvent distillation. However, it is a more effective concentrator for compounds of higher volatility. Since the concentrator is operated a t ambient pressure, the rising vapors must build up sufficient pressure to force their way past the stage of the Snyder column. Each stage consists of a narrow opening covered by a loose-fitting glass insert. Since the vapors are slow to work their
way through the different stages of the Snyder column, there is initially a large amount of condensation of these vapors, which returns to the bottom of the Kuderna-Danish apparatus. Besides continually wash& the organics from the sides of the glassware, the returning condensate also contacts the rising vaDors and helps to recondense vola
