Analysis of hydrocarbons in aquatic sediments

Journal gf Chromatography, 436 (1988) 503-509 Elsevier Science Publishers B.V., Amsterdam - Printed CHROM.

in The Netherlands

20 145

‘?i Note

Analysis

of hydrocarbons

II. Evaluation of common chlorinated hydrocarbons M. ACEVES,

J. GRIMALT

in aquatic preparative

sediments procedures

for petroleum

and

and J. ALBAIGfiS*

Department of Environmental Chemistry (CID-CSIC).

Jordi Girona 18, 08034 Barcelona [Spain)

and F. BROTO,

L. COMELLAS

and M. GASSIOT

Instituto Quimico de Sarriri, 08017 Barcelona (-Spain) (First received July 28th, 1987; revised manuscript

received October

19th, 1987)

Petroleum and chlorinated hydrocarbons (PHCs and CHCs) are widely distributed in the aquatic environment as a result of numerous human activities, thus becoming pollutants of primary interest. In order to monitor such pollutants, procedures are required for their isolation and characterization from sediment and biota, which have been shown to offer several advantages with respect to water itselPJ. There is an extensive bibliography of analytical methods for PHCs and CHCs in environmental samples. Adsorption column chromatography, using Florisil, silica gel, alumina or their combinations, is by far the most common sample clean-up procedure prior to qualitative and quantitative analysis3. The former Rosen-type separation for saturated and aromatic hydrocarbons4 and that of Holden and Marsden5 for organochlorines have subsequently been refined with the aim of increasing reproducibility, resolution and the linear capacity of the adsorbents6-10. To this end, particularly useful are partially deactivated adsorbents’l-l 5. These methods have recently been comprehensively reviewed’ 6-1 *. However, despite the fact that PHCs and CHCs require similar preparative procedures, little attention has been paid to the simultaneous determination of both classes of pollutants. In a previous paper we focused attention on the extraction step in sediment analysis’ 9, whereas in the present one an evaluation of different clean-up procedures of sediment extracts, including Florisil and silica/alumina column chromatography, is carried out with the aim of designing a common analytical method for PHCs and CHCs. EXPERIMENTAL

Reagents n-Hexane, dichloromethane, diethyl ether and isooctane for residue analysis were obtained from Carlo Erba and distilled from glass apparatus in the laboratory.

0021-9673/88/$03.50

0

1988 Elsevier Science Publishers

B.V.

504

NOTES

Adsorbents Neutral silica gel (Kieselgel 40, 70-230 mesh), alumina (aluminium oxide 90 active, 70-230 mesh) and Florisil (60-100 mesh) were obtained from Merck, article Nos. 10180, 1077 and 12518, respectively. They were cleaned by extraction with dichloromethane in a Soxhlet apparatus for 24 h and, after solvent evaporation, they were heated overnight for activation to respectively 120,350 and 300°C. After cooling in a desiccator they were deactivated by addition of suitable amounts of water (l10%) on a w/w basis. Homogenation was carried out by mechanical shaking for 2 h. The adsorbents were kept in closed containers in a desiccator before use. Following this procedure, no change in activity was observed within the next 5 days, so adsorbents prepared at the beginning of a week can be used without any problem during the week. Test materials Naphthalene, anthracene, benzo[b]fluoranthene, benzo[a]pyrene, perylene, dibenz[a,h]anthracene and coronene were obtained from Fluka. Aroclor 1254, aldrin, heptachlor, pp’DDE, pp’DDT, pp’DDD, ?/HCH, BHCH, dieldrin and endrin were obtained from Analabs. Column chromatography The chromatographic columns were packed with alumina (8 g, top) and silica gel (8 g, bottom) (25 cm x 0.9 cm I.D.) or with Florisil(5 g) (25 cm x 0.5 cm I.D.). The adsorbents were suspended in n-hexane and introduced into the glass columns with gentle lateral tapping in order to avoid retention of air-bubbles. The dead solvent volumes were 20 and 12 ml, respectively, for the alumina/silica and Florisil columns. The solution of PHCs and CHCs was transferred to the column and left to penetrate into the adsorbent. Then, successive mixtures of solvents were added according to the chromatographic conditions to be evaluated. Portions of l-2 ml were collected separately and analyzed by gas chromatography (GC), after vacuum evaporation of the solvent and dissolution in isooctane. Under the conditions described above, up to 150 mg of a mixture of pure hydrocarbons could be separated without overloading the columns. Recoveries were in the range of 80-100% for all the analytes but decreased substantially when mixtures contained less than 5 pg of total hydrocarbons. Instrumental analJ,sis PHCs yere analyzed with a Carlo Erba FTV 4160 gas chromatograph equipped with flame ionization detection (FID) and a splitless injector. A column of 25 m x 0.25 mm I.D. coated with SE-52 was used (surface film thickness 0.15 pm). Hydrogen was the carrier gas (50 cm/s). The temperature was programmed from 60 to 310°C at 6”C/min. The injector and detector temperatures were 300 and 330°C respectively. The injection was in the splitless mode (hot needle technique), the split valve being closed for 35 s. The organochlorinated hydrocarbons were analyzed with a Carlo Erba FTV 4130 Series instrument equipped with a splitless injector and electron-capture detection (ECD). The chromatographic conditions were essentially the same as described above except for the final oven and detector temperatures, which were 290 and 300°C respectively.

NOTES

505

RESULTS AND DISCUSSION

Although there is no universally recommended procedure for the separation of hydrocarbons and related compounds in environmental samples, a general preference is the use of silica and/or alumina for PHCs and Florisil for CHCs3. Columns of alumina overlying silica gel in different v/v ratios, e.g., 4: 1, 3: 1, 2: 1 and 1:1, have been favoured and intercalibrated for PHCs in sediments8*20. On the other hand, Florisil has been adopted in a reference method for pesticide residue analysis2’ and thus used in CHC monitoring programmes 22. Therefore, our attention was focused on the behaviour of PHCs and CHCs on both types of adsorption systems. The aim of the study was not the isolation of fractions of individual components, but of well defined classes, suitable for FID and ECD in gas chromatographic analysis. In this context, it is known that the high adsorptive capacity of the above chromatographic materials can result in poor recoveries due to excessive tailing or irreversible adsorption of the components. Uncontrolled variations of activity, particularly with Florisil, result in poor separation reproducibility. The use of partially deactivated adsorbents (by adding a certain amount of water) has been shown to reduce these deleterious effects, and also increase the linear capacity of the adsorbents, but at the expense of some loss in resolution. The optimum ranges of water content for the separation of polycyclic aromatic hydrocarbons (PAHs) and CHCs on silica have been reported to be 46% and 2-3% respectively’ 1,14*23.Alumina and Florisil, being more active adsorbents, would require slightly higher percentages of water for a similar performance, e.g., 5, 6% for pesticides10,23. Taking into account these features, the chromatographic conditions and the mixture of PHCs and CHCs reported in Figs. 1 and 2 were selected. The results shown have been obtained consistently in our laboratory after more than 5 years of routine operation. The mixture does not contain saturated hydrocarbons because they are always eluted in the first column volume of hexane (fraction I in Fig. 1). By comparing the composition of the eluates from the different columns, it is seen that water deactivation increases the migration of the components of the mixture, but in different ways. In the alumina/silica column (Fig. l), the PAHs are far more sensitive than CHCs to water deactivation, to the extent that they are resolved by ring number for columns with 1% of water and are all eluted in fraction III (20% dichloromethane) both in the 3 and 5% water deactivated columns. This behaviour is a likely consequence, on the one hand, of the weak interaction of PAHs with the active sites of alumina which, in turn, is characteristic for the crystalline structure of this adsorbent. Thus, the separation of these hydrocarbons is determined by the selectivity effects resulting from the linear arrangement of the condensed rings24. At higher degrees of deactivation, this weak interaction cannot take place and the aromatic hydrocarbons are less resolved. On the other hand, the series of CHCs are spread over all fractions of increasing polarity. For instance, DDE, DDT and DDD are eluted according to the strength of the dipole moment corresponding to their polychloromethyl group, and dieldrin and endrin are the more strongly retained compounds of the series, most likely due to the interaction of their epoxide group with the acidic sites of the silica. Deactivation with 5% of water has the advantage that polychlorobiphenyls (PCBs), aldrin, heptachlor and DDE are eluted even ahead of the PAHs, in fraction II (10% di-

NOTES

506

____+__--~~-____+____I_____

1

~

Naphthalene Anthracene Benzo(b)fluoranthene Benzo(a)pyrene Perylene lJibenz(a,h)anthracene Coronene

___I_._.__i__-_:_r_-~--‘_I__:

____I____I_~~____t____C____.

?CBs Aldrin Heptachlor pp'DDE pp'DDT pp'DDD YHCH pHCH lieldrin :ndrin

ml

column. Solvent: Fig. 1. Elution of the test mixture of PHCs and CHCs on a (1:l) alumina/silica chloromethane in n-hzxane (proportions indicated at the bottom; integrated elution volume, above).

di-

507

NOTES

20 0

I

“““‘I

40 10

Fig. 2. As Fig. 1, but on a 5% water profile.

60

deactivated

80 20

100 I

(1:l) alumina/silica

ml 50 column,

with a different

elution

chloromethane), where only monocyclic aromatics are isolated from PHCs of environmental origin. At the same time, most of the PAHs of environmental concern, namely those having three to six rings, are collected in a single fraction (III). With 3% water deactivation, all PAHs, PCBs and the DDT family are coeluted in fraction III, whereas the HCH isomers and dieldrin and endrin are eluted, in fractions IV and V, respectively. The composition of the eluting solvent can also have marked effects on the elution of components of different polarities. In order to improve the resolution between PAHs and CHCs in the 5% water deactivated column, we eluted fraction III again with 10% dichloromethane in hexane. The result is shown in Fig. 2. In this case, fractions II + III contain the naphthalene + phenanthrene derivatives plus the PCBs and the DDT family, whereas the more condensed aromatics are eluted in fraction IV with 20% of dichloromethane in hexane. These conditions are particularly useful for a rapid analysis of CHCs. In contrast to alumina/silica, in the Florisil column (Fig. 3) a small increase in deactivation (from 6 to 7% of water) affects primarily the elution pattern of the CHCs, resulting in a significant decrease in the retention volumes of the semi-polar compounds, such as pp’DDT and YHCH. The same effect is also observed for the PAH mixture but at higher water content, e.g., 9% where the two- and three-ring aromatics start to be eluted earlier. However, the chromatographic behaviour of the more strongly retained compounds, i.e., dibenz[a,h]anthracene and coronene, remains unaffected by water deactivation, evidencing the strength of their interactions with Florisil. In this context, chemisorption may be an important retention mechanism for this adsorbent, as has been described for perylene25. With 7% of water deactivation the resolution obtained is similar to that on silica/alumina under the conditions shown in Fig. 2. The hexane fraction (I + II) contains the PCBs and the DDT family together with the two- + three-ring PAHs. From a general standpoint, the most convenient separation is obtained with 6% of water deactivation which

508

NOTES

6

I II Ill lv _________~______;_________~_________ _-------- L--~--l--_____I_______. ________-I_----_____--,--~____. __ ._______I____________~---~_________. ._______C----_____--I-_-~~__--_____ ______-_c----______c______ ________J____________J______~__.

PCBs

-_~____________r_____---'_________ -------_L--_________t_________ly_______-

6

Naphthalene Anthracene Benzo(b)fluoranthene Benzo(a)pyrene Perylene Dibenz(a,h)anthracene Coronene

--_I~IIIT__TI~~_~~_~~~~~~~:__III:___, ._______---+--___-__r__-_____ ________--,___-____+---_____ II____~_m_____~________,__-_____ -------!_-______---_ clmmmmn__,_________

Aldrin Heptachlor pp'DDE pp'DDT pp'DDD jHCH pHCH Dieldrin Endrin

7

7

9

9

Fig. 3. As Fig. I, but on a Florisil column. Solvent: the bottom; integrated elution volume, above).

diethyl

ether in n-hexane

(proportions

indicated

at

509

NOTES

provides the following fractions: PCBs, aldrin, heptachlor and DDE (fraction I), two to three aromatic rings and DDT (fraction II) and the rest of the PAHs and CHCs (fractions III + IV). Although in principle silica/alumina (3 or 5% water deactivated) seems to offer group separations of more practical interest, the results shown here give the choice of a variety of conditions for each need or application, taking advantage of the variety of interactions available for each solute after adequate water deactivation of the absorbents. PAHs are more sensitive to deactivation than CHCs, probably by weaker interaction with the adsorbents. When the extracts, however, contain substantial amounts of neutral lipids, i.e., fatty esters, the use of Florisil is more appropriate because it exhibits greater retention for these components, whereas on silica/alumina they are eluted together with the PAHs. In any case, the main advantage of these procedures is the simplicity and the high reproducibility of the separations, namely the elution volume for each group of components, which permits the handling of large series of samples, as it is usually necessary in monitoring studies. ACKNOWLEDGEMENT

Financial support of this research was provided by Consejo Superior de Investigaciones Cientificas and Comision Asesora de Investigation Cientifica y Tecnica. :

,

I t ,i

i

REFERENCES _ 1 R. A. Baker, Contaminants and Sediments, Vols. 1 and II, Ann Arbor

Sci. Publ., Ann Arbor,

1 , ,2 D. J. H. Phillips, Quantitative Aquatic Biological Indicafors, Applied Science Publ., Barking, 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

MI, 1980. 1980, p.

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