Application of SDE to the Analysis of Aqueous Samples

Water was one of the first environmental samples selected to evaluate the feasibility of the SDE

Figure 1 A typical SDE modern design.

technique for the determination of less volatile organic contaminants levels. Table 1 summarizes relevant data concerning some reported methods for the analysis of this matrix.

Quantitative recoveries of spiked organoch-lorinated pesticides, OCPs (globally, in the range 90-106 ppb), and PCBs (globally, in the range 70-104 ppb) in aqueous samples have been reported using the SDE technique. The reported methods allowed the simultaneous extraction and concentration of the analytes in 1-1.3 h in a relatively small amount of a non-polar solvent (1-15 mL). Usually, no additional treatment of the sample or the organic extract was required. The SDE technique was favourably compared with other widely used extraction procedures, such as LLE or solid-phase extraction (SPE) by Ramos etal. in 1995, e.g. similar recoveries have been published for the analysis of PCBs in water at the ppb (ngmL-1) level by using SDE, LLE or SPE. However, the higher repeatability of the SDE procedure (relative standard deviations, RSD, lower than 10%) and the small amount of organic solvent involved, as well as the short sample preparation times, makes SDE a valuable alternative technique for such an extraction, especially when a large number of analyses have to be carried out.

Nevertheless, some limitations of SDE have also been reported for less volatile pollutants in water samples. Nash et al. in 1984 studied different parameters affecting the efficiency of the steam distillation process. They concluded that this technique is probably limited to compounds with a vapour pressure of about 1 kPa at 100°C. Their results also showed that the performance of SDE depends on the concentration investigated and that recoveries tend to increase with the spiking level.

Similar tendencies have been observed by Ramos et al. in 1995 when using the SDE technique for the extraction of water spiked with the 2,3,7,8-sub-stituted-CDD/Fs at different levels of concentration (0.25-2ngmL"\ 0.025-0.2ngmL"1 and 0.00250.02 ngmL~i). The recoveries obtained for the lower and higher boiling point congeners (tetra- and octa-CDD/Fs, respectively) are consistently lower than those found for the rest of the investigated congeners: respectively 40-76% and 73-137% at the highest level of concentration investigated, 39-60% and 62-92% at the intermediate, and 37-55% and 25-72% at the lowest spiking level. These results also show that the SDE recoveries for a given compound decrease with the concentration level when using n-pentane as the extraction solvent. The simple substitution of n-pentane for a solvent more selective for the PCDD/Fs (dichloromethane) increases recoveries from 25-73% to 71-139% for tetra- to hepta-CDD/Fs at the 0.0025-0.02 ngmL^ level. However, no additional improvement is obtained for the octa-CDD/F recoveries (38-56%). In spite of the low recoveries obtained for OCDD/F, the proposed SDE procedure compares favourably with results previously published by using LLE or SPE in terms of repeatability, analysis time and solvent consumption.

Good recoveries (in the range 84-100%) have been reported by Meissner et al. for the analysis of surfactants such as fatty alcohol sulfates and alkyl polyglycosides in water (Table 1). SDE of the fatty alcohols yielded by hydrolysis and subsequent LLE of the original compounds is an attractive technique for the effective clean-up and concentration of these complex mixtures of compounds at the trace level. On the other hand, the application of SDE to the extraction of fatty alcohol ethoxylates with more than three ethoxy units in the molecule cannot be accomplished due to their high solubility in water.

Table 1 SDE methods for the analysis of less volatile organic pollutants in aqueous samples

Compound

Spiking level (ngmL

Solvent (mL)

Extraction time (h)

Cc. factora (water: solvent, v/v)

(%)

(%)

Ref.

OCP

0.004-0.016

Isooctane/toluene

1

167: 1

NRc

90-104

?

Hemmerling

(15)

etal (1991)

Arochlor

0.016

Isooctane/toluene

1

167: 1

NR

98-100

?

1016, 1242,

(15)

1248, 1254

OCP

0.4-4.0

n-pentane (1)

1.3

50 : 1

NR

97-106

?

Godefroot

et al. (1982)

Arochlor 1260

10

n-pentane (1)

1.3

50 : 1

NR

81 -104"

?

Toxic PCBs

0.01-1.0

n-pentane (2)

1

50 : 1

Concentration

70-115

<10

Ramos et al.

(1995)

PCDD/Fs

0.025-2.0

n-pentane (2)

1

50 : 1

Concentration

49-139

< 10

PCDD/Fs

0.0025-0.02

Dichloromethane

1

50 : 1

Change of

49-139

< 10

(2)

solvent

Fatty alcohol

500 nM

Ethyl acetate (2)

3e

100: 1

Derivatization

87-100

5.6-7.0

Meissner

sulfates

et al. (1999)

Alkyl poly-

2 |M

Ethyl acetate (2)

4e

25:2

Derivatization

84

?

glycosides

Concentration factor. bPost-SDE treatment required. cNR, not required.

dRecoveries for some selected peaks.

eThe SDE was conducted after hydrolysis with 4M H2SO4 plus liquid-liquid extraction with diethyl ether of the hydrolysate and concentration.

Concentration factor. bPost-SDE treatment required. cNR, not required.

dRecoveries for some selected peaks.

eThe SDE was conducted after hydrolysis with 4M H2SO4 plus liquid-liquid extraction with diethyl ether of the hydrolysate and concentration.

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