M
ost organic contaminants are present in environmental waters i n trace amounts at t h e pglL level and lower. As a result, 'ery low detection limits are required for directly monitoring drinking water or studying the fate and transport of organics directly in the environment. In addition, many polar organic compounds cannot be analyzed at a trace level in water samples because of the lack of simple methods for their extraction. In general, water samples are too dilute and too complex to be analyzed by gas chromatography (GC) or liquid chromatography (LC) without some preliminary sample preparation: extracting traces of the oreanic of interest fro& the aqueous media, concentrating these traces, and often removing 0 t h components from the matrix that have heen coextracted and may interfere with the chromatographic analysis RecentlY interest has grown in trace enrichment techniques that use solid-phase extraction W E ) as an alternative to the laborious and time-consuming liquid-liquid extractions (LLE). Moreover, recoveries of many Polar analYtes Obtained using LLE are low because of their relatively high partial solubility in water. Automatic devices are now commercially available for the on-line Of to GC Or Lc.Neveras theless* SPE does not straightfoward as LLE to many environmental chemists because of the sorbent choices and berecoveries On the sample volume. We will discuss the similarity between the extraction process with apolar sorbents and classical elution LC. This similarity allows most Of the parameters for to be Obtained from LC data. Some wellknown methods that predict retention in LC can also be applied to SPE and help in the choice of a sor-
bent. We will also present the limits of the common sorbent C18 silica for the preconcentration of polar organic (as related to the wateroctanol partition coefficients of analytes). Finally, the potential of two other reversed-phase sorbents for extracting more polar organic is examined.
~
Description ofthe SPE procedure SPE can be used off-line (i.e., the sample preparation is completely separated from the chromatographic analysis) or online (i,e., it is directly connected to the chromatographic system) (I, 2). methodologies, samples are percolated through a sorbent packed in disposable or enmeshed in an inert matrix of a membrane-based extraction disk. A typical off-line SPE sequence is described in the box (p, 578A), Disposable cartridges contain a sorbent bed that varies from mg to looo mg, The sample volume that can be applied ranges from one to several hundreds of milliliters; the large volumes are for designed with large reservoirs, ~h~ sample can be processed by injecting the contents of a syringe onto the
MAR1E - C L A I R E H E N N I O N VALERIE PICHON Ecole SupBrieure de Physique et de Chimie de Labomtoire de Chimie 75231 05, Fmnce
576A Environ. Sci. Technol., Vol. 28, No. 13, 1994
SPE cartridge or it can he aspirated through the cartridge by vacuum. Various vacuum manifolds allow batches of up to 24 samples to be prepared simultaneously. Automation of the whole off-line SPE procedure is now possible using commercially available sample preparation workstations that perform each step iequentially. Compared to LLE, the off-line SPE technique is faster and saves a substantial amount of solvent. Samples can even be percolated through a sorbent in the field, avoiding the transporting and storing of voluminous samples. These adsorbed analytes generally store well. In addition, automated units reduce the risk of sample contamination. Nevertheless, off-line procedures have the _e inherent disadvantage of losses during the evaporation step and loss of enrichment factor resulting from the injection of an aliquot. On-line coupling of SPE to a GC or LC instrument avoids many of these problems. On-line coupling of SPE to LC is particularly easy to perform in any laboratory (see box, p. 579A). More accurate quantitative results can he expected because there is no sample manipulation between the preconcentration and the analysis steps. The sample volume can be lower than off-line SPE because the entire sample is transferred and analyzed. The SPE sequence runs during the analysis time of the previous sample. Automatic devices are now commercially available. The sorbents, chemistry, and principles are identical in off-line and on-line SPE. The only differences are in the packing size of sorbents-larger in off-line cartridges (40-60 pm) than in on-line precolumns (5-10 p m LC-grade sorbents)-and the amount of sorbent, which may be 1000 mg or more in off-line cartridges. The size of a precolumn cannot be increased because this will lead to hand broadening during on-line elution. The amount of sorbent is less than 20 mg
0013-936X/94/0927-576 A$04.50/0 0 1994 American Chemical Society
in the classical precolumns (1 cm x 0.2 cm i.d.) and, in general, less than 100 mg. Basic principles As the development of a SPE procedure is often an empirical process for those expert only in LLEs, we will give a simple approach to modeling the SPE process, which can help select a convenient sorbent. To a first approximation, SPE can be considered as a simple liquid chromatographic process. The sorbent is the stationary phase, and the mobile phase is the water in the aqueous sample during the extraction step or the organic solvent during the elution step. Organic compounds that do not elute with the water are trapped on the sorbent. High enrichment factors are obtained when analytes are strongly
retained by the sorbent in the presence of water and when there is a low retention with the eluting organic solvent. The choice of the sorbent is guided by the aqueous nature of the sample. Water should not easily elute the target organic compound. Thus bare silica and silica modified with polar groups are not usually good sorbents because water is the strongest eluting mobile phase. Better sorbents are reversed-phase stationary phases for neutral organics and ion-exchangers for ionic compounds. The main sorbents for retaining organic compounds in aqueous samples are reported in Table 1 with the corresponding separation mechanisms, the nature of the elution solvent, and the characteristics of the organic compounds that are preconcentrated.
These three reversed-phase sorbents will be compared in this study. The use of ion-exchangers requires some special attention because natural waters contain several milligrams of inorganic ions that very quickly overload the capacity of ion-exchanger sorbent. For this reason, we will not consider ion-exchange sorbents in the present study. Breakthrough. Breakthrough occurs either when solutes are no longer retained by the sorbent or when the capacity of the sorbent has been overloaded. This latter reason is rather unlikely to occur in practical environmental analysis where concentrations are typically of the order of the pg/L level. Thus breakthrough is mostly caused by insufficient retention. Breakthrough volume can be measured by monitoring the UV signal
Environ. Sci. Technol., Vol. 28. No. 13, 1994 577A
I
'
ypical steps in an SPE sequence are as follows: preparation of the sorbent, pplication of the sample, an eventual cleanup, and elution of the concenated analytes. An example sequence ISgiven for a cartridge packed with :18 silica: . The sorbent is activated with 3-5 mL of methanol and conditioned with the ame volume of deionized water (it is important not to allow packing to dry out efore adding the sample). . The aqueous sample is applied and trace organics of i 'apped by the sorbent while the water passes through. . Very often, other components (0)of the samples are either not retained by ie sorbent or trapped with the analytes. A cleanup step can be added. Some f these interferences can be removed by applying 1-2 mL of a washing solution made of water with a small amount of an organic solvent 4. In the last step, the sorbent is dried for several minutes by vacuum suctioi Then the concentrated analytes are eluted with 1-5 mL of organic solvent. 11 is recommended that the s wet the sorbent for 1 mm and that the elution ?meed at a dropwise rate.
I
of a water sample spiked with traces of a solute, S, which has an initial absorbance A,. The spiked sample is percolated through a precolumn. If the compound is retained by the sorbent, the effluent will not contain it and the UV absorbance will be zero. A frontal or breakthrough curve is recorded beginning at a volume, V,, usually defined as 1% of A,, up to a volume, V,, defined as 99% of A,, where the effluent has the same composition as the spiked water sample (Figure la). Under ideal conditions, this curve has a bilogarithmic shape, the inflection point of which is the retention volume, V,, of the analyte. When using the same precolumn i n elution chromatography with water as the mobile phase and with the same flow rate, the injection of 10 or 20 pL of a concentrated solution of the same compound S will generate a peak detected at the same volume V,as represented in Figure Ib. The quantity V, is a key parameter for the preconcentration of the analyte and can be estimated to a 578 A
first approximation from V,. The similarities between SPE and LC indicate that data generated by LC for measuring or estimating V, are useful. The knowledge of the retention behavior of analytes with hydrophobic sorbents should be applied. For example, ionizable compounds are only retained on C18 silica in their neutral form, so that the pH of the water sample is an important parameter, especially for the extraction of weak acids and bases. In practice, one first needs an approximate value of V, in order to select an appropriate sorbent and the amount of the sorbent for off-line preconcentration with, cartridges. More information on the relation between V, and V, can be found in the box. Recoveries. Recovery is defined as the ratio between the amount extracted to the amount percolated and is theoretically 100% only for a sample equal to or lower than V,. The maximum amount preconcentrated is reached at a sample volume of V , (hatched area in Figure
Environ. Sci. Technol., Voi. 28,No. 13, 1994
l a ) and does not correspond to a 100% recovery. Therefore, the recovery in SPE depends on the sample volume a n d on t h e breakthrough volume, related to the amount and the nature of the sorbent. Recoveries obtained with the same sorbent can be compared from one experiment with another only if the amount of sorbent and sample volume are known. It is always possible to obtain a 100% recovery by decreasing the sample volume below the V, value, and a simple calculation indicates whether this volume will allow t h e required detection or not. Limit of ouantification. The absolute detecti'on limit of the chromatographic detection system is usually expressed in pg or ng injected [e.g., 10 ng). Therefore, it is easy to calculate the limit of quantification, CLi,, which corresponds to the maximum amount that can be preconcentrated (hatched area in Figure l a ) or as equal to the product Cli, x V,. If V, is 100 mL, for example, then the concentration limit Cli, is 100 nglL. If a lower limit of concentration is required, the only remedy is to increase V, (or V,) by increasing the amount of sorbent or by selecting another sorbent that will provide a higher retention in the presence of water for the analyte of interest. Determination and prediction of breakthrough volumes. Recording breakthrough curves is time consuming, and reading V, at 1% of A, is difficult and not always accurate. Moreover, the sample should be spiked at a trace level in order to not overload the sorbent capacity, and the UV signal of the effluent should be monitored at very low absorbances (which may lead to problems with baseline stability or noise). Moreover, many compounds are difficult, and sometimes impossible, to record because they have poor UV properties. A faster method that is easily performed using the on-line setup consists of preconcentrating water samples of increasing volumes, each containing the same amount of analytes, and then measuring the peak areas or heights eluted on-line from a precolumn (4). As the sample volume increases, the analyte concentration decreases. Provided that breakthrough does not occur, the amount preconcentrated remains constant and the peak areas in the on-line chromatograms following elution are constant. When breakthrough occurs, the
the precolumn or sorbent bed in the cartridge. Values of K, are estimated from chromatographic measurements using analytical columns packed with reversed-phase sorbents, such as C18 silicas, which are eluted with mobile phases composed of water-methanol mixtures. The advantage of this method is that experimental data are obtained rapidly by measuring the capacity factor K of the analyte in methanolwater phases. Over a range of methanol concentrations, often between 15% and 90%, there is a linear relationship between the logarithm of the K and the percentage of methanol. This has been observed iample for alkyl silicas, apolar styrenedivinylhenzene copolymers PRP-1 I L simple on-line setup consists of the following: The sorbent is packed in (from Hamilton) or PLRP-S (from mall precolumn, which is placed in the sample loop position of a six-pa Polymer Laboratories), and porous witching valve. graphitic carhon (PGC) (as shown in Figure 3 for phenol) (6). . The conditioning, sample application, and eventual cleanup occur via impie pump in the injection valve in the "load position'' (-). From rapid measurements with three or four mobile phases contain'. The precolumn is coupled to an analytical column by switching the valve to ing different methanol concentrale "injection position" (- - -). The adsorbed compounds are then eluted ditions, k', can he estimated by ectly from the precolumn onto the analytical column using a suitable mobile graphically extrapolating to zero Ihase, which also enables the chromatographic separation of the trapped ompounds. methanol content. In Figure 3, the experimental values of log K , should he similar to predicted values because the relation is linear 'ABLE 1 over the entire range. Werkhovensorbents that can be used for the solid-phase extraction of Goewie et al. (3) found good agreeirganic compounds present at a trace level in aqueous samples, ment between V, values for some .C separation mechanism involved, nature of the eluting solvent, chlorophenols derived from experiind characteristics of the organic com ounds that can be mental breakthrough curves and !xtracted from a sufficient water samp e volume for trace-level values calculated from extrapolated letermination K , values. Differences were on the Separation order of 10-20%. iO&"l mechanism Elution solvent Extractable analyles I However, according to Schoenilkyl bonded silicas Reversed-phase Organic solvents Nonpolar to weakly the relationship bemakers et al. (7), (CB. CIS) (methanol. moderately polar log K and the methanol contween acetonitrile . . .) neutral compounds tent is a better fit with a quadratic polar styreneReversed-phase Or anic solvents Nonpolar and relationship for some compounds. divinylbenzene ðanol, moderately polar Therefore, the extrapolated value is copolymers acetonitrile.. .) neutral compounds only an approximate value, but for ;raphitized carbons n-,a=a*'-ihase Organic solvents Nonpolar to olar neutr-I trace enrichment studies this is ac(methanol, compound: acetonitrile, ceptable. As an example, the log K, THF . . .) I value of cyanuric acid was estige Water (pH adjusted Cationic and anionic mated to 2.5 L 0 . 2 using this in order that organic compounds method, which gave a V, value becompounds are (pH of the water in their neutral sample adjusted) tween 30 and 70 mL for the precollorm) umn used in Figure 2 (assuming a porosity of 0.75 for the porous carbon). The experimental V,, was amount extracted decreases and the unknown sample analysis. measured as 50 + 5 mL in Figure 2. elution peak area decreases. CorreAnother method approximates Another aspect of the variations sponding recoveries can he calcu- the breakthrough volume using V,, observed in Figure 3 is that the lated by dividing peak areas oh- which is related to chromatographic breakthrough volume decreases tained after breakthrough by those data and precolumn characteristics when some methanol is added. For obtained before. This is shown in by the equation V, = VJ1 + K,), phenol, using a cartridge packed Figure 2. An advantage of this where V, is the void volume of the with 100 mg of PRP-1, the hreakmethod is that the V, values of sev- precolumn or the cartridge and K, through volume should he 30 mL eral compounds can he estimated is the capacity factor of the solute with water, 20 mL with water consimultaneously by preconcentraeluted by water. Vo can he calcu- taining 5% methanol, and 1 3 mL tion and on-line LC analysis under lated from the porosity of the sor- with 10% methanol. A small perthe real experimental conditions of bent and the geometric volume of centage of organic solvent can be
P
I
Environ. Sci. Technol., Vol. 28, No. 13, 1994
579A
-
Solidphase extraction
-Detector
added to the raw sample without decreasing the recovery only if the retention is very high and if the sample volume percolated is less than the V, value calculated from the corresponding log K for this organic solvent concentration. Choice of the sorbent depending on the polarity of analytes The frontier between polar and nonpolar solutes is undefined. A parameter that characterizes the hydrophobicity and reflects the polarity of a compound is its water-octano1 partition coefficient, Po,? This parameter plays an important role in correlating phenomena of physicochemical, biological, and environmental interest. Many log Po,, values are available in the literature, and calculation methods have been reported (8-10). We characterize solutes with log Po,, values above 3 as apolar, between 1 and 3 as moderately polar, and below 1as polar. When log Po,, is less than zero, the compounds are more solu580 A
ble in water than in organic solvents. n-Alkyl silicas. Most of the offline environmental applications use SPE cartridges packed with n-alkyl silicas ( 1 1 ) . Because C18 silicas usually provide higher retention than C8 silica, C18 silicas should be used for extracting polar organics. Available cartridges packed with C18 silicas have different characteristics and it is well known in LC that retention differs from one to another. This is because retention depends on the number of C18 chains bonded at the surface of the silica. Nevertheless, except for those with a low carbon content, K , is comparable within a 20% variation for most C18 silicas, and that is acceptable. Braumann (12) has gathered many log K , values that have been obtained with different C18 silicas using methanol-water mobile phases. A linear relation was found between the average log K, values and log Pact for closely related com-
Environ. Sci. Technoi.. Voi. 28, NO. 13, 1994
pounds and even for compounds having different polarity and chemical properties. For example, 60 compounds covering a wide range of structures from polar aniline (log Pact = 0.91) to the very hydrophobic p,p'-DDT (log Pact = 6.2) are related by log K, = 0.988 (+0.051)log Po,,+ 0.020 (+0.060). All the relations published in this work and other studies have similar coefficients, showing that values of log K, and of log Pact are very close. Thus, with C18 silicas, k', can be approximated without any additional measurements. Chlorophenol derivatives are used as an example. Depending on the number of substituents, this series ranges from apolar (log Par, = 5) to rather polar compounds (log POLt = 1.5). In Table 2, values of log K,, obtained by extrapolating from changes in K with methanol content, are in good agreement with literature values (12).The values of V, in Table 2 for precolumns are calculated from V, and K, values. V, is the product of the geometric volume of the precolumn and the porosity of C18 silica. An average porosity value is found between 0.65 and 0.7 for most available C18 silicas. V, values calculated for off-line cartridges are based on 100 mg of sorbent with an average density of 0.6 g/mL and a V, calculated as 0.12 mL per 100 mg of sorbent. Values of V, were also experimentally determined on precolumns for all the compounds according to the method described above in Figure 2. Good agreement is observed between the calculated V, values and experimental V, values obtained with the precolumn. One can therefore rapidly obtain V , from log Po,, and determine whether, depending on the concentration limit required, C18 silica is suitable for the extraction. The data reveal a significant difference between apolar and moderately polar analytes: V, values per 100 mg of sorbent are seven liters for pentachlorophenol, 350 mL for 3,5dichlorophenol (log Po,, = 3.561, 16 mL for 2-chlorophenol, and < 5 mL for phenol. To extract moderately polar analytes from a sufficient volume requires an increase in the amount of C18 silica. But even with 1000 mg of sorbent, the sample volume is about 40 mL for phenol. The limitations of C18 silica are clear for some moderately polar organics, especially for on-line methods where the amount of sorbent cannot be increased.
A
able. However, these sorbents are much less popular than C18, and not many data exist for predicting
(mainly XAD-2). These resins can generate impurities that are subsequently difficult to eliminate. Resins have to be purified before use in a Soxhlet apparatus. Several purification methods have been described. Recently, cartridges packed with small beads (80-160 pm) of a new resin, a nonionic highly crosslinked styrene-divinylbenzene copolymer that has been previously purified, have been commercialized. Carbon-based sorbents. Two carbon-based sorbents are available in prepacked disposable cartridges. One is Supelco's graphitic carbon black, GCB, which is not sufficiently pressure resistant to be used in LC; therefore no data can be generated by this method. It has been successfully employed for the extraction of many moderately polar pollutants. A comparison of recoveries for GCB and C18 silica cartridges indicates higher breakthrough volumes for the graphitic carbon (16,17). In addition to good mechanical strength, an appropriate carbon packing in LC should have a homogeneous surface with a minimum of functional groups at its surface to ensure symmetrical peaks (i.e., graphitized carbons are only convenient), a specific area in the range of 50-500 m'lg, a mean pore size no less than 10 nm and, to ensure rapid mass transfer of solutes in and out of the particles, no micropores. These constraints explain why the first useful carbon material for LC was Shandon's Hypercarb, which appeared only at the end of the
K,s.
1980s.
Because of the aromatic rings in the matrix network, electron-donor interactions occur between the aromatic rings or n bonds of the solute. Retention behavior thus is sensitive to changes in the solute electron density caused by the electrondonating or -withdrawing ability of the solute's substituents. These sorbents are therefore more hydrophobic than C18 silica. As shown in Table 3, we measure solute retention values 20-40 times higher with these copolymers than with C18 silica (2, 6). This is especially interesting using on-line preconcentration. Many moderately polar pesticides and other organic pollutants can be analyzed on-line using 100-150 mL of water samples with detection levels below the pg/L (3,6, 13-15). Environmental off-line trace enrichments are often described with Amberlite XAD-type copolymers
As reported in Figure 3, this second sorbent shows a reversed-phase behavior in the sense that the retention decreases as methanol content in the mobile phase increases. Because of its highly ordered crystalline surface and the large graphitic sheets of hexagonally arranged carbon atoms in its structure, it is a more retentive reversed-phase sorbent than C18 silica (18).Recent LC studies show the affinity of PGC toward very polar, water-soluble analytes such as polyhydroxybenzenes (6, IS]. The capacity factor in water of the very polar 1.3.5-trihydroxybenzene or phloroglucinol is about 1000 with PGC,versus 3 with PRP-1 and no retention by C18 silica. Table 3 compares log K, values obtained with C18 silica, PRP-1, and PGC for many moderately polar monosubstituted benzene derivatives and for polar compounds such
ueiaiiea uescripiion 01ine neiarionsntp Detween v, ana
v,
'he sorption of an analyte by SPE can be described by a frontal analysis iodei. Figure 1 shows that the derivative of a breakthrough curve is a Gausian peak (3).When defining the breakthrough volume, V,, under the condions of Figure 1 (1% of AJ, the retention volume, V,. is related to V, by: V, = V, - 2 a" (' rhere av is the standard deviation depending on the axial dispersion of the nalyte along the bed of particles in the precolumn or cartridge. The breakirough volume is therefore controlled by retention and by kinetic parameters. The V, term can be easily calculated from the capacity factor in water, k'w, nd from the void volume, V,, of the precoiumn or cartridge by:
+ k',J
V, = V, (1
F
'he uvterm can be calculated as follows if the number of theoretical plates, 1, of the precoiumn or of the cartridge is know
a,
=L(1 + k'J
(:
fi
or N > 3, N can be determined by the recordin irough curve, assuming these values were the N = V, ( V, - a")
e experimental breakfor other components: (4)
(0,)'
is possible to directly measure N with precolumns because the on-line setup an allow the recording of both the breakthrough curves and the elution peaks ~ydirect injection onto precolumns (4). Because precolumns are packed with tationary phases similar to those used in LC, an estimation of N also can be lerived from the average number of plates in C18 silica columns. As V, can le calculated (see text), prediction V, can be made usin
mns. It is more difficultto measure the efficiency of an SPE cartridge; therefore N
;asto be estimated. SPE cartridges are very differentfrom an LC column and le average particle size is much larger, so a rather poor efficiency is exwted. Miller and Poole (5)have recently studied the kinetic and retention lroperties of an SPE cartridge packed with 500 mg of C18 silica; they mea,ured an average of 20 theoretical plates for a flow rate of 5 mlimin.
It is also important to know the log Pact for pollutants because predictions can be made without any experiments. Although these data are needed for the commercialization of a new product, in many countries they remain confidential. For some compounds, large differences for this constant exist in the literature depending on whether log Pact were measured by the shake flask method, estimated from chromatographic measurements, or calculated by some other means. There is now an effort to collect reliable data. However, when no data are available, it is always easy to estimate the hydrophobicity of a compound by measuring the retention of the compound in methanolwater with a C18 analytical column and by characterizing the compound with its log K, value. Apolar copolymers. Porous apolar styrene-divinylbenzene polymers are available for LC and can be packed in precolumns. Prepacked PLRP-S precolumns are also avail-
Environ. Sci. Technol.. Vol. 28. No. 13, 1994 581 A
1
TABLE 2
Approximate retention volumes V,, for (a) SPE precolumn (1 cm Y 0.2 cm i.d.) and (c) SPE cartridges packed with 100 mg of C18 iilica predicted from the water-octanol partition coefficient (log and the extrapolated retention in water (log Ww); (b) ?xperimentalmeasurements of V, with the 1 cm x 0.2 cm i.d. irecolumn Retention volume. mL
'entachlorophenol !.3,4,6~Tefrachlorophenol !.4,5~Trichlorophenol 1.5-Dichlorophenol !,4- Dichlorophenol I-Chlorophenol !-Chlorophenol 'heno1
5 4.1
4.8
1400
4.0
200 70
3.56
582 A Environ. Sci. Technol.. Vol. 28,No. 13, 1994
,200 ,200 175k25
7350
60i15 40k10 6-i3
370 16C
1064
32
i
as di- and trisubstituted benzene derivatives. A comparison of moderately polar compounds indicates that retention for PGC is often higher than that obtained for C18 silica, but always lower than that obtained with PRP-1. These surprising results were for di- and trisubstituted benzenes having a log Po,, below one. These compounds are either slightly retained or not retained by silica; log k',s have not been reported. With PRP-1, retention decreases with the polarity of compounds, and the log k', values obtained for derivatives having two polar substituents are always lower than those measured for each corresponding monosubstituted henzene. The opposite behavior is observed with PGC. Because of a different retention mechanism, PGC could be used for trace enrichment of very polar, water-soluble compounds as shown in Table 3. Applications of PGC to the trace-level determination of 2-chloro-4-aminophenol, chloroanilines, aminopheuols, and cyanuric acid have been reported (20). Many pollutants are degraded into hydroxy derivatives, and Table 3 shows that addition of a hydroxy group increases the retention as compared with the parent molecule. It is often pointed out that desorption of analytes from carbon sorbents is more difficult than desorption of C18 silicas. It was shown that the surface of GCB contained some chemical heterogeneities (16) that may be responsible for the difficult desorption. A comparison of C18 silica and Hypercarh LC retention data measured with pure methanol or acetonitrile mobile phases shows that most organic compounds are not retained or just slightly retained on C18 silica whereas some compounds can he well retained on the graphitic carbon. T h u s large volumes of methanol or acetonitrile are sometimes necessary for the elution of analytes from carbon materials. Tetrahydrofuran is a better eluting solvent for many compounds. A better understanding of the behavior of analytes on carbon phases is required for the optimization of the elution solvent. Because the retention mechanism is different, log k', cannot be predicted from the water-octanol constant. Although log k', can be calculated using t h e electronic distribution in the solute molecule, it is not as easy as for C18 silicas or apolar copolymers. From a qualita-
tive point, we find that high retentions are obtained for flat molecules containing several polar groups with delocalized electronic charges
1
via IT bonds and lone pairs of electrons. However, a rapid and easy means is to inject the polar compounds of interest onto an available analytical column of PGC and to estimate log k', values.
I21 Hennion. M-C.: Scribe, P. Environ-
Conclusion A good knowledge of the LC behavior of analytes with the sorbents used for SPE has shown that approximate values for the extraction parameters can be rapidly obtained from the characteristics of the solute. This, in turn, allows a better sorbent choice for extracting polar compounds. The potential of PGC as a new extraction sorbent of some water-soluble analytes will certainly increase the number of pollutants and degradation products that now can be monitored in the aqueous media.
I41 Subra, P. et al.
References 111
mental Analysis: Techniques, Application and Quolity Assurance; Barcelo, D., Ed.; Elsevier: Amsterdam, 1993: Vol. 13.p p . 23-77. 131 Werkhoven-Gaewie, C. E.; Brinkman, U. A,; Frei, R. W.; Anal. Chem. 1981, 53,207240.
I51 Miller. K. G.; Poole, C. F. I. High Res-
olut. Chromatogr. 1994, 77, 125-34. 1. Chromatogr. 1993, 642, 211-14. I71 Schoenmakers, P. J.: Billet, H. A,; de Galan, L. 1, Chromatogr. 1979, 785, I61 Hennian, M-C.: Coquart, V.
179-95. I81 Rekker, R. F. The Hydrophobic Fmg-
mental Consfont. Pharmaca-Chemistry Library: Elsevier: Amsterdam, 1977; Vol. 1. (91 Noble, A. 1. Chromatogr. 1993. 642,
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?-,a .. I101 Bodor, N.: Huang, M-J. 1. Pharm. 1992,81, 272-81. (111 Font, G. et al. I. Chrornatagr. 1993, 642, 135-61. (121 Braurnann, T . 1. Chromotogr. 1986, 37.7., 191-225. 1131 Liska, 1. et al. Intern. 1. Anal. Chem. 1992,47,267-91. I141 Slobodnik, J. et al. 1. Chromatogr. 1993, 642,359-70. I
~~~
Nielen, M.W.F.; Frei, R. W.; Brinkman, U. A. Selective Sample Handling and Detection in High-Performance Liquid Chromatography, Frei, R. W.; Zeich, K., Eds.: Elsevier: Amsterdam, 1989; Vol. 39 A, pp. 5-78.
1. Chromatogr. 1988,
456,121-41.
~~
Pichon, V . : Hennion, M-C. I . Chrornatogr. 1994, 665. 269-81. I161 Di Corcia, A,; Marchelti, M. A n d . Chem. 1991,63.580-85. I171 Di Corcia, A.; Marchelti, M. Environ. Sci. Technol. 1992, 26.66-74. 1181 Bassler, B.; Hartwick, R. A. I. Chrarnatogr. Sci. 1989, 27,162-65. 1191 Coquart, V.; Hennion, M-C. 1. Chrornatogr. 1992, 600. 195-201. I201 Guenu, S.; Hennion, M-C. 1. Chromatogr. 1994, 665. 243-51. (15)
I
Anihne Disubstituted
Morie-ClOire Hennion [ I ] is a professor 4- Amlnobenmic 3-AminObenmffi
1.4-Dihydmxybenzene 1A-Diamnobenzene
Polysubstrtuted 3,5-Dihydroxybenzoic acid Tnmesic acid (l$,ti-tnbenzo Fyromeilitic acid (1,2,4,5-tetrabenzoic) Not defermined
at the Ecole Sup4rieure de Physique et de Chimie Industrielles de Paris and received a Ph.D. in analytical chemistry from the University ofparis. Her current research interests include liquid chromatographpy and the development of new analytical methods f o r environmental monitoring.
Vd6rie Pinchon frl is research assistant and graduate student in the Department of Analytical Chemistry of the Ecole Sup4rieure de Physique et de Chimie Industrielles de Paris. Her research focuses on selective multiresidue preconcentration using immunoaffinity sorbents far studying pesticide confaminotion in notum1 waters. Environ. Sci. Technol.. Vol. 28, No. 13, 1994 583 A
