ANALYTICA CHIMICA ACTA

Analytica Chimica Acta 305 (1995) 256-264 ELSEVIER

Application of an indirect tensammetric method for the determination of non-ionic surfactants in surface water

Andrzej Szymanski, Bogdan Wyrwas, Zenon Lukaszewski *

Institute of Chemistry, Technical University of Poznan, PL-N-965 Poznan, Poland

Received 17 May 1994

Abstract

Three alternative procedures for the determination of non-ionic surfactants (NS) in surface water have been developed. The simplest one consists of the filtration of water samples, the extraction of NS with ethyl acetate, the evaporation of the solvent and the determination of NS using an indirect tensammetric method (ITM). The detection limit is 15 pg 1-l. The procedure using gas-stripping separation of NS shows a detection limit of 1.5 /lg 1-l. Chlorophyll extracted from water plants

causes a serious interference. Therefore the sample must be filtered before extraction. The extractive procedure shows good precision and recovery of spiked surfactants from river water samples. Comparative studies of the newly developed and the classical BiAS procedures were performed. The ITM procedure shows a detection limit of two orders of magnitude better

than the BiAS procedure. Therefore the required volume of water sample is reduced from 5000 to 100 ml. Due to the sim- pler separation procedure the ITM is substantially less time-consuming, cheaper and requires no sophisticated equipment or well-trained staff. An additional advantage of the ITM compared with the BiAS procedure is the lower sensitivity to the choice of standard surfactant and the broader “spectrum” of NS that can be determined. The results of a half year current

analysis of NS in river water have been included.

Keywords: Bismuth active substances; Indirect tensammetric method ; Non-ionic surfactants; Tensammetry; Waters

1. Introduction

Surface water is a very complex matrix containing both natural compounds (carbohydrates, proteins, lipids, humic substances, etc.) as well as synthetic compounds. Surfactants are the most important syn- thetic substances with respect to the carbon supply. Their flux to surface water is some orders of magni- tude higher than fluxes of pollutants like polychloro- biphenyls (PCB) or polyaromatic hydrocarbons (PAH) [ll. On the other hand, surfactants can in- crease the mobility of different pollutants in an

* Corresponding author.

aquatic environment. Therefore the concentration of surfactants in surface water should be controlled. Different classes of surfactants show different toxic- ity and biodegradability. Non-ionic surfactants (NS) are the second class of synthetic surfactants in terms

of production [2] but their share quickly increases and currently it is almost equal to the amount of leading anionic surfactants produced. On the other hand, NS offer the widest range of utility due to easy manage- ment of properties following comparatively easily management of their structure. Several hundreds of individual NS can be present in surface water. There- fore the total concentration of NS is an important characteristic of surface water, despite of attempts to

0003-2670/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved

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A. Szymanski et al. /Analytica Chimica Acta 305 (1995) 256-264 257

specifically determine certain groups of NS (i.e.,

oxyethylated alkyl phenols [1,3-51 or oxyethylated alcohols [6-q]).

The procedures currently used for the determina- tion of the total concentration of NS are cumber-

some. The BiAS procedure [lO,ll] is recommended in Europe and the CTAS procedure [7], in the USA.

In both procedures multistage separations are applied

to extract NS from the water matrix and to remove all interferents. Therefore these procedures are time-

consuming, expensive, require non-typical equip- ment and well-trained staff. The BiAS procedure contains a step which may be a source of serious er-

ror [12]. Both BiAS and CTAS procedures are much more suitable for the determination of higher concen-

trations of NS, rather characteristic for sewage or sewage treatment plant liquors and effluents than

surface water. Therefore these procedures are hardly suitable for the control of NS in river water because

of a usually much lower concentration range. The majority of the attempts to improve the pro-

cedures for the determination of total concentration of NS applied liquid chromatography (LC) [6-91, pre- suming similarity of the results for the concentration

of oxyethylated alcohols (being the major group of NS) and the total concentration of NS. Derivatisation with phenyl isocyanate to ensure the detection is

considered as a promising one [6,7]. However, the results obtained using the classical procedures and the newly developed LC procedure differ substantially

[s]. Another method using a substantially different principle of measurement is needed for the solution

of the controversy which results are correct and what precisely is measured in a particular procedure.

The tensammetric method can be very useful both as a separate method being able to compete with the procedures currently used, as well as a reference

method for answering what is measured in the cur- rently used and newly developed procedures. The in-

direct tensammetric method (ITM) can be especially useful for this purpose. In contrast with the direct tensammetric measurement, in the ITM only one sig-

nal is produced for the NS mixture (the decrease of the tensammetric peak of ethyl acetate) [13]. Con- trary to the behaviour of the NS mixtures in direct tensammetric measurements, the analytical response for mixtures of NS in the ITM is roughly additive [14]. The ITM measurement of NS concentration is

tolerant to the presence of anionic surfactants [15]. All

these advantages show the ITM as the method being able to compete both with the procedures currently used and with the LC procedures being developed.

The aim of this paper was developing, on the ba- sis of ITM, a complete analytical procedure useful for

a routine control of NS in surface water. The ITM

should be combined with a properly working separa-

tion scheme for the reduction of a number of possi-

ble interferents included in surface water and for pre- concentration of NS. The part of the separation

scheme of the BiAS procedure, i.e., the gas stripping of NS from the water sample to the ethyl acetate, was selected as the starting point of this work. The addi-

tional aim of this work was to compare the devel-

oped procedure with the classical BiAS procedure. Both potentiometric and UV spectrophotometric

measurement of the bismuth concentration were ap- plied in the BiAS procedure. For comparison, as a

third procedure, the newly developed BiAS-ITM procedure [ 161 used. The BiAS-ITM is a mixed pro-

cedure, combining the sublation and the precipitation of non-ionic surfactants according to the BiAS pro- cedure with their determination using the ITM.

2. Experimental

2.1. Apparatus and reagents

A Radelkis OH-105 polarograph and EC0 Chemie

General Purpose Electroanalytical System PAUTO- LAB were used with a voltage scan rate of 400 mV min-‘. The applied amplitude of the alternating volt-

age was 2 mV. Controlled-temperature HMDE equipment (Radiometer), having an additional plat- inum wire auxiliary electrode, was used. All poten- tials cited are against the saturated calomel electrode.

The beaker of the measuring cell was replaced by a quartz beaker, to prevent adsorptive loss on the glass surface [17]. The ceramic frit on the end of the salt bridge was protected by the fitting of a polyethylene tube. This ceramic frit exhibits very large adsorptive ability [17], and this protection very effectively re- duces the adsorptive loss of surfactants. The measur- ing cell was carefully cleaned with methanol be- tween the measurements.

Potentiometric titration was performed using an X-Y recorder and peristaltic pump. The measuring

258 A. Szymanski et al. /Analytica Chimica Acta 305 (1995) 256-264

cell was equipped with platinum and SCE electrodes

and a stirrer.

A Carl-Zeiss Jena Specord M40 UV-visible spec-

trophotometer was used. The determination of bis-

muth was performed at 264 nm. The following surfactants were used without addi-

tional purification. Oxyethylated alkyl phenols: Rokafenol N-l (ICSO Blachownia, Poland),

Rokafenol N-3 and Rokafenol N-5 (both Rokita, Poland) and Triton X-100 (Rohm and Haas); oxyeth-

ylated oxo-alcohol having a C10_13 hydrophobic part

of the molecule: Oxetal D104 (Zschimmer und Schwarz); oxyethylated alcohols having a C,,_,, hy-

drophobic part of the molecule: surfactant 18-2 (Technical University of Poznan), Marlipal 1618/10,

Marlipal 1618/18, Marlipal 1618/25 and Marlipal 1618/40 (all Huels). Ethyl acetate (POCh) of gas

chromatographic grade was used. Other reagents were AnalaR grade.

Chlorophyll was extracted from Chlorophytum Comosum with methanol. Its concentration was con-

trolled spectrophotometrically. Purified sodium sulphate was used for the prepa-

ration of the aqueous 0.5 M base electrolyte. All so-

lutions were prepared in water triply distilled from quartz. Only freshly distilled water was used.

Determination of NS according to classical BiAS

procedure was performed following [lo] or [18].

2.2. The analytical procedure for the determination of the concentration of non-ionic surfactants in river

water

A river water sample was filtered through paper

filter. The first 200 ml of the filtrate were removed (to prevent adsorptive loss). 10 g of sodium chloride and 0.5 g of sodium hydrogencarbonate were dis- solved in 100 ml of the filtrate in a separation funnel. 10 ml of ethyl acetate were added and the extraction of NS was performed. The extraction was repeated

with the next 10 ml portion of the solvent. Both por- tions of ethyl acetate were evaporated in a small beaker. The residue containing separated NS was dissolved in 1.5 ml of ethyl acetate and transferred into a 25-ml measuring flask. 12.5 ml of 1 M aque- ous sodium sulphate was added and the flask was filled to the mark with water. The mixture containing few excessive ethyl acetate was intensively shaken to ensure homogeneity and the emulsion was trans-

WATER SAMPLE u

FILTRATION

u GAS STRIPPING * OR EXTRACTION

u EVAPORATION

u DISSOLUTION OF THE RESIDUE IN

I.5 ml OF ETHYL ACETATE

U DISSOLUTION OF ETHYL ACETATE SOLUTION OF

SURFACTANT IN BASE ELECTROLYTE

U ITM MEASUREMENT

* - performed from 100 ml in washing flask or from 1000 ml in special apparatus

Fig. 1. Outline for the examined ITM procedures.

ferred into the measuring cell. The emulsion was stirred for 10 min to clear the solution due to evapo-

ration of the excessive ethyl acetate. This excessive ethyl acetate extracts surfactants and therefore it must be removed, which is evidenced by the disappear- ance of turbidity and separated droplets in the sol-

vent. After a quiescent period (30 s) and formation of a new mercury drop the tensammetric curve was recorded in the cathodic direction starting from

- 1.20 V. Measurements were performed without deaeration of the solution. The difference between the

height of the peak of ethyl acetate alone and the peak height in the presence of surfactants was used as ana-

lytical signal. The results were evaluated using a cal- ibration curve of Triton X-100.

The determination of NS in a sewage treatment

plant effluent was performed using the same proce- dure, but after corresponding dilution of the sample with distilled water (1:l for an effluent of the biolog- ical treatment stage or l:lO, for an effluent of the sedimentation stage).

3. Results and discussion

3.1. Model experiments of the determination of

Triton X-100 by ITM in the full procedural cycle

The full procedural cycle includes the separation of non-ionic surfactants from a water sample and the determination of NS by means of the ITM. The out- line of the tested procedures is shown in Fig. 1. Three

Table 1

A. Szymanski et al. / Analytica Chimica Acta 305 (I 995) 256-264 259

Recovery and precision of the determination of Triton X-100 separated from the water phase using different separation schemes (n = 7)

Procedure used Added ( pg)

Triton X-100 DBS

Found ( pg) S.D. R.S.D.

Sublation from 1 1 water sample 4

20

Sublation from 100 ml water sample in washing flask 4

20

Extraction from 100 ml water sample 4

4

20

20

0 4.3 0.49 0.1 I 0 19 0.72 0.04

0 4.2 0.5 I 0.12

0 18.5 0.x3 0.04

0 4.3 0.42 0.10

SO 4.8 0.5h 0.12

0 20 0.77 0.04

50 20 O.YO 0.05

DBS = Dodecylbenzenesulphonate.

separation schemes have been developed during the

gradual evolution of the separation stage. A first at- tempt to combine ITM with a separation procedure was the use of gas-stripping separation of non-ionic surfactants from the water phase to ethyl acetate. This

separation step was adopted from the original ver- sion of BiAS procedure [10,18]. Typically, a l-l vol-

ume apparatus was used and two sublations of sur- factants to 100 ml of ethyl acetate. The ITM mea- surement was performed after evaporation of ethyl

acetate and introduction of the surfactant as a 1.5ml ethyl acetate solution (see 2.2. for details).

The second separation scheme consists of a gas-

stripping separation performed in a typical laboratory washing flask with a frit. A washing flask is basi- cally a small scale apparatus for gas stripping separa-

tion. This scheme was developed because the pro- curement of an apparatus for gas stripping may be a barrier for small poorly equipped laboratories. The

sample volume is reduced to 100 ml and the ethyl acetate volume to two 10 ml portions. A much better detection limit of ITM in comparison with the proce- dures currently used (almost two orders of magni- tude) makes such reduction of the volume of a sam- ple possible.

A normal extraction of a lOO-ml sample with two

10 ml portions of ethyl acetate has been applied in the third separation scheme.

Triton X-100 was used as a representative non- ionic surfactant in the full cycle experiments. The re- sults of processing 4 pg and 20 pg spikes of Triton X-100 according to three schemes are given in Table 1. Additionally, two series of experiments were per-

formed according to the extraction separation scheme in the presence of an excess of sodium dodecylben-

zenesulphonate (DBS) as a representative anionic surfactant.

All three schemes show satisfactory recoveries and precision on both tested levels of Triton X-100’s

spike. It proves the equivalency of these schemes. Possible differences in the results of the tested

schemes might be caused by kinetic differences, which however, did not appear. The excess of an- ionic DBS does not cause any substantial changes of

the results. The worse but still satisfactory precision of the results for 4 pg spike of Triton X-100 is typi- cal for trace analysis. The smaller the spike, the lower

the precision. The extractive scheme seems to be the best method

for practical analysis because the performance is

simple, only small volumes of water and ethyl ac- etate (evaporated) are needed and no sophisticated equipment is required. The detection limit of 1.5 pg

(defined as 3 times the standard deviation), which corresponds to the concentration of 15 pg 1~ ’ in the water sample, is sufficient for the determination of non-ionic surfactants in river water samples. This value is below the detection limit of the BiAS proce- dure currently used for routine control of non-ionic surfactants in Europe. On the other hand this value can be easily reduced twice by the use of a 200-ml sample, if required. The lowest NS concentration in river water reported in the literature is 8 pg 1~ ’ [19].

The scheme in which gas stripping separation of non-ionic surfactants from a l-l sample is used offers the unique ability to determine concentrations of

260 A. Szymanski et al. /Analytica Chimica Acta 30.5 (1995) 256-264

Ai/..A(

2.0 1 / I A

Fig. 2. Calibration graph of (a) chlorophyll and (b) Triton X-100

(standard). Concentration of ethyl acetate: saturated solution.

non-ionic surfactants as low as 1.5 fig I-‘. Problems

may appear because it is necessary to evaporate 200 ml of ethyl acetate.

3.2. Real samples. Interference of chlorophyll

First attempts to use the developed procedure for the analysis of river water samples gave results that were much higher than expected. The reason was that

chlorophyll was extracted from water plants. There- fore the analytical signal of chlorophyll has been in- vestigated. The results are given in Fig. 2. The cali-

bration graph for Triton X-100 (standard in ITM) has been added for comparison. It is obvious from the

figure that the analytical signal of chlorophyll is much

stronger than that of Triton X-100. To remove this interference the filtration of a sample prior to extrac-

tion or gas stripping separation is necessary. The fil-

tration completely removes the interference of chlorophyll, however, another problem appears: the loss of surfactant during filtration. When 100-200 ml of water sample is required the filtration is per- formed using a paper filter. In this case the first 200

ml of the filtrate should be removed because it has a

decreased concentration due to adsorptive loss. It is even more complicated when a l-l sample is re-

quired. The paper filter gets blocked quickly. There- fore the column filled with gravel was used for filtra-

tion. However, the first 2 1 of water should be re- moved. This volume is necessary to achieve satura-

tion of the surface of the column filling.

3.3. Real samples. Precision, recovery and examples of application.

River water samples from the Warta River in Poz-

nan were used as an example for testing the preci- sion and recovery of the determination of non-ionic

surfactants in a real water sample. The results are presented in Table 2. The non-spiked river water sample exhibits the concentration level typical for

slightly polluted river water (27-38 pg I-‘). The precision of determination is typical for trace level concentrations and is satisfactory. The recovery of

different spikes of Triton X-100 or Oxetal D 104 is

Table 2

Precision of the determination of “native” river surfactants (Warta River, Poznan; n = 7) and recovery of Triton X-100 and Oxetal D104

spikes added to the sample

Sample Added ( pg) Triton X-100 or Oxetal D104 Found( pg) SD. R.S.D.

River water 0 3.8 0.42 0.11

0 2.7 *

0 3.5 a

River water * spiked with Triton X-100 3.0 6.0

7.0 9.8

10 13.1

20 21.8

50 59

100 104 River water * spiked with Oxetal D104 7.0 9.9

10 13.3

The determination was performed using extractions of a 100-ml sample.

a 3.6 pg when determined using the classical BiAS procedure and a 5-1 river water sample.

A. Szymanski et al. /Analytica Chimica Acta 30.5 (199.5) 256-264 261

Table 3

Examples of the use of the ITM procedure for the determination of concentration of non-ionic surfactants in different surface water samples

Date Determined ( fig I-‘)

ITM MBAS

Warta River, Poman

4-2-1993 34

9-3-1993 25

25-3-1993 28

8-4-1993 23

20-5-1993 29

8-6-1993 32

5-7-1993 27

23-7- 1993 24

13-8- 1993 35

26-10-1993 39

Warta Rber, Czestochowa

21-4-1993 204 a

24-g-1993 222

Prosna Ril’er, Kalisz

21-4-1993 89

Vistula Rir,er, Cracaw

21-4-1993 135 Nida Rii’er, Pinczow

22-8- 1993 79 Effluent of the sewage treatment plant, Czestochowa

24.8- I993 550 EJjfluent of the sewage treatment plant, Poznan-Kozieglowy (only sedimentation treatment)

3-3-1994 2280 h

390

320

230

250

270 320

210 190

220 380

The results for the anionic surfactants determined by the MBAS procedure have been added.

a 255 /.~g l- ‘, determined by the classical BiAS method, and 214 pg I-’ by the BiAS-ITM procedure.

h 2605 ~g I-‘, determined by the classical BiAS method, and 2640 pg I-‘, by the BIAS-ITM procedure.

very good. It is necessary to emphasize that Oxetal D 104 represents this group of non-ionic surfactants

which are not determined by the BiAS procedure be- cause of a too short oxyethylene chain. One sample was determined simultaneously by the classical BiAS

procedure. The agreement between the results of BiAS and the proposed method is satisfactory.

The concentration of non-ionic surfactants in the Warta River was periodically measured during half a

year for further testing the utility of the proposed procedure. Long examination times increase the probability to discover possible unexpected interfer- ences. The results are shown in Table 3 together with

the results for anionic surfactants as determined by the MBAS procedure [20]. The results show a typical

pattern of periodical concentration fluctuations for both classes of surfactants. No unexpectedly differ- ent results were observed. Any significantly different result might be the symptom of undiscovered inter- ferences or losses, but it no prove for this was found.

Anionic surfactants show a characteristic level for moderate pollution. Their level is roughly 10 times higher than the concentration level for non-ionic sur-

factants. It is obvious from these results that this ex- cess of anionic surfactants does not remarkably dis- turb the determination of non-ionic surfactants.

The non-ionic surfactant concentration for several other river water samples and two samples of an ef- fluent of a sewage treatment plant were also mea-

sured using the proposed ITM procedure. The results are shown in Table 3. They show that the proposed method works within a wide concentration range.

Each of these water samples represents a different background, i.e., different content of natural sub- stances and anthropogenic pollution. Despite this the

method works in all these cases showing no unex- pected analytical signals. In the case of one river wa- ter sample and one sample of effluent of the sewage treatment plant the results were checked using the classical BiAS and the BiAS-ITM procedures. The

262

Table 4

A. Szymanski et al. /Analytica Chimica Acta 305 (1995) 256-264

Recovery and precision of determination of Triton X-100 spike using the classical BiAS procedure (the bismuth concentration was deter-

mined by UV spectrophotometry)

Number of measurements Added ( pg)

6a 200

;: 500 200

lb 1000

Found average ( pp)

130

590 93

867

SD. (pg) R.S.D.

35.5 0.27

a Model investigations.

b Added to river water sample (containing 25 pg “native” ethoxylates).

results of all three measurements show general agreement which supports the equivalency of all these

procedures in terms of analytical results.

3.4. Comparison of the proposed and the classical BiAS procedure

The precision and recovery of the lTM and classi-

cal BiAS procedures were compared. The results for ITM have already been shown in Table 1 while those for the classical BiAS procedure are shown in Table

4. Much higher Triton X-100 spikes were used in the case of the classical BiAS procedure than that for ITM to perform the experiments within the optimum

range for this procedure. Even in the model investi-

gations the classical BiAS method shows substan- tially poorer precision and incomplete recovery. The recovery of 200 pg Triton X-100 spike from the real river water is even worse. Much better results for Triton X-100 spikes are recoveries of 500 and 1000

,ug. The poor results are the consequence of the loss of the precipitate during the washing of the precipi- tate with glacial acetic acid in the classical BiAS procedure [12]. This loss is roughly constant and in-

dependent of the amount of the precipitated surfac- tant. The lower the amount of surfactant that is pre- cipitated, the higher the percentage of the loss. In contrast to the poor recovery and precision in the classical BiAS procedure, the results for the ITM procedure seem to be quite satisfactory.

The complexity of a mixture of non-ionic surfac- tants separated from surface water can be another source of substantial errors of determination. Each component of the mixture exhibits its own slope of the calibration curve. However, only one surfactant is chosen as the standard (usually Triton X-100 or a

similar surfactant). The difference between the slope of a calibration curve of a particular surfactant and the

calibration curve of the standard creates an error which depends on the choice of standard. Certainly, the results concerning one surfactant validated using the calibration curve of a different surfactant must be

erroneous. Probably these errors for particular surfac- tants partially compensate one another. However, the

content of the non-ionic surfactant mixture separated from the surface water is unknown and the real value of the error of the choice of standard is impossible to

predict. Obviously, the lower the difference of slope of the calibration curves of particular mixture com-

ponents, the lower is the error of the choice of the standard. Studies of the errors due to the choice of

standard were also the subject of comparing the clas- sical BiAS procedure with the proposed ITM proce- dure. The results concerning several oxyethylated al- cohols and oxyethylated alkyl phenols are shown in Table 5. Additionally, the results obtained using the

third procedure (BiAS-ITM [16]) have been added. First of all it is clear that the ITM procedure is able

to produce the analytical signals for ethoxylates hav- ing short oxyethylene chains while BiAS and BiAS- ITM procedures (operating with the precipitate) are

not able to do this or they produce very weak sig- nals. In the classical BiAS procedure the analytical

signal strongly depends on the length of the oxyethy- lene chain while in both procedures applying ITM this dependence is much weaker. Therefore the classical BiAS procedure is much more sensitive to the choice of the standard than the other two procedures. The measure of this kind of error can be the percentage of recovery of a 500 pg spike of different surfac- tants (R) (if validated using Triton X-100’s calibra- tion curve>. This value is given in Table 5. In the

Table S

A. Szymanski et al. /Analytica Chimica Acta 305 (1995) 256-264 263

Comparison of the results of the determination of 500 pg spike of different non-ionic surfactants by BiAS, BiAS-ITM [16] and ITM proce-

dures using Triton X-100 as the standard (the bismuth concentration in the BiAS procedure was determined using potentiometric titration)

Surfactant EO Determined ( pg)

BiAS

Surfactant 18-2 2 150

Marlipal 1618/10 10 581

Marlipal 1618/18 18 601

Marlipal 1618/25 25 706

Marlipal 1618/40 40 736

Rokafenol N-l 1 0

Rokafenol N-3 3 92

Rokafenol N-5 5 347

Triton X-100 10 500

R BIAS-ITM

30 115

116 435

120 492

141 483

147 533

0 17

18 345

69 410

100 532

R ITM

23 380

87 443

98 500

97 490

107 548

3.5 272

69 442

82 490

106 500

R

76

x7

100

98

1 IO

54

88

98

100

EO = Number of oxyethylene subunits.

R = Recovery (o/o). _ _

classical BiAS procedure, even in the cases of NS

having 5-30 oxyethylene subunits (which determina- tion by the classical BiAS procedure is recom- mended) the R value strongly deviates from the ideal 100%. In contrast, the ITM procedure shows excel- lent R values for those NS having 5-30 oxyethylene subunits (EO) and even acceptable R values for NS

having l-4 EO. The BiAS-ITM procedure also shows

considerable advantage over the classical BiAS pro- cedure with respect to the R values.

Different characteristics of the ITM and the clas- sical BiAS procedures are shown in Table 6. There is

no doubt that the developed ITM procedure shows advantages over the BiAS procedure in almost all

comparable aspects. Except for the advantages dis- cussed above, the ITM procedure is considerably simpler, cheaper and less time-consuming. On the

other hand, the classical BiAS method, and the newly developed ITM and BiAS-ITM procedures show good agreement when comparing the results for real water and sewage processing liquor samples. These

three independent procedures show good agreement, using different separation procedures and different

final measurements. This, of course, cannot be acci- dental. This fact answers questions which were raised during the classical BiAS procedure and LC mea- surements [21,22]. It seems that LC results do not re- flect the total NS concentration in the real samples.

Table 6

Comparison of the ITM and the classical BiAS procedures

ITM BiAS

Detection limit ( pg)

Required volume of river water (ml)

“Spectrum” of determined non-ionic surfactants

(No. of oxyethylene subunits)

R, ‘I (%:)

R,“(%) Tolerance to anionic surfactants Interferences

The most time-consuming step

1.5 100

100 5000

l-30 5-30

87-100

54-88

Good

Chlorophyll

69-141

O-30

Ion-exchange treatment recommended

Evaporation of 20 ml of ethyl acetate Evaporation of 800 ml of ethyl acetate

a R = Percentage of recovery of spike evaluated using the calibration curve of Triton X-100 (the choice of standard error) (see Table 5). R,,

for NS having 5-30 oxyethylene subunits; R,, for NS having l-4 oxyethylene subunits.

264 A. Szymanski et al. /Analytica Chimica Acta 305 (1995) 256-264

Acknowledgements

This work was supported by the Committee of

Scientific Research (grant No. 4 4008 92 03).

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[19] W.K. Fischer, Tenside Deterg., 17 (1980) 250.

[20] Polish Standard PN-85-C-04550/02, Determination of an-

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[21] D. Brown, H. De Henau, J.T. Garrigan, P. Gerike, M. Holt,

M. Keck, E. Kunkel, E. Matthijs, J. Waters and R.J. Watkin-

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[22] D. Brown, H. De Henau, J.T. Garrigan, P. Gerike, M. Hoh,

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side Surf. Deterg., 24 (1987) 14.