Microlayer sampling

Consequently, for an accurate quantification of surfactants in the water column, sampling must be performed by taking surface microlayer samples (at depths between 0 and 3—5 mm), using a surface sampler, and at various greater depths with Ruttner or similar bottles. 

Surface microlayer samples have shown sulphur-gas anomaly patterns that are more closely related to known mineralisation than sulphur-gas patterns from deeper soil samples (Lovell, 1979). At Johnson Camp, Arizona, mineralisation is best expressed by sulphur compounds from the surface microlayer, while sulphur compounds from 0-5 cm reflect the same mineralisation to a lesser extent, and sulphur compounds from 30-40 cm show the least expression of the mineralisation. A comparison of concentrations of COS, CS2, and SO2 degassed from soils collected at depths of 0.5-2 cm and 30-40 cm at the same sites near Casa Grande, Arizona, showed almost identical patterns of sulphur-gas concentrations over a 150 km (58 square miles) area. Average concentrations of COS, CS2 and SO2 were slightly higher in the shallow samples than in the deeper samples (Hinkle, unpublished data, 1981). These data indicate that, at least in arid areas, surficial soil and microlayer samples are superior to deeper augered samples. 

The world s oceans have also been monitored for PCB concentrations. PCB levels reported in sea water from various oceans include 0.04—0.59 ng/L in the north Pacific, 0.02-0.20 ng/L in the north Atlantic, and 0.035-0.069 ng/L in the Antarctic (Giam et al. 1978 Tanabe et al. 1983, 1984). PCB levels were several orders of magnitude higher in sea-surface microlayer samples taken from industrial areas, compared to sites further offshore (Cross et al. 1987). PCB concentrations of 0.3-3 ng/L, have been detected in sea water from the North Sea (Boon and Duinker 1986). 

Qualitatively, the Middle Atlantic Bight samples were very similar to those collected in the California Bight. The predominant components were the same in both locales, suggesting that a limited number of compound classes are dominant in microlayer films, but present in varying proportions. The specific mixtures of surfactants in the microlayers sampled in this study strongly influenced air-sea interfacial quasi-static elasticity. Pre-... 

Carlson DJ, Canty JL, Cullen JJ (1988) Description of and results from a new surface microlayer sampling device. Deep-Sea Res35 205-1212... 

Carlson DJ, Cantey LL, Cullen JJ (1988) Description of and results from a new surface microlayer sampling device. Deep-Sea Res 35 1205-1213 Cini R, Lombardini PP (1978) Damping effect of monolayers on surface wave motion in a liquid. J Colloid Interface Sci 65 387-389 Csanady GT (1990) The role of breaking wavelets in air-sea gas transfer. J Geo-phys Res 95 749-759... 

Fig. 2. Force area curves for untreated microlayer samples collected 1 km offshore from Mission Beach, San Diego (Barger et al., 1974). Reproduced with permission of the authors and publisher.

It is evident that many of the sampling devices described in the previous section in fact collect thin layers of water adjacent to and including the air/ sea interface itself. These type of surface film samples have become known as surface microlayers (Liss, 1975). Although such layers are in fact operationally defined by the devices used for their collection, there is the understanding that something happens to the properties of ordinary, bulk seawater at and near the air/sea interface which is contained in such microlayer samples. In this way, the microlayer becomes a real phenomenon in much the same way that the particulate state, understandable in a common-sense sort of way, is also usually defined by operational methods such as membrane filtration. 

Thus, the presence of a discontinuity in properties near or at the air/sea interface can be inferred from a difference in properties between the bulk seawater and the microlayer sample which contains, the interfacial region. For an organic surfactant adsorbed at or near the sea surface, for example, this will manifest itself by a higher concentration or enrichment in the microlayer sample. There is, nevertheless, the vexed question of how much of a screen microlayer sample (of typical thickness —200 pm) consists of a real concentration anomaly near the interface and how much consists of seawater of bulk composition. Since there have been a number of differing and intractable opinions voiced in this regard, it is timely to examine closely what can be said, in terms of known surface chemistry, about the structure of the microlayer. 

The microlayer surface excess, defined by the simple formula given above, is perhaps the best variable for describing what is present in excess in a microlayer sample compared with the same amount of subsurface water as a result of surface processes. It is, in particular, almost certainly independent of the thickness of the sample obtained with one or other sampling device used. Thus different samplers should give the same results in terms of surface excess when the concentration in the microlayers taken from the same piece... 

Particulate carbohydrates and protein in screen and rotating drum microlayer samples and subsurface seawater, after Daumas et al. (1976)... 

There are, unfortunately, no studies to date of the dissolved protein content of microlayer samples. With the recent development of many sensitive techniques for the analysis of amino-acid mixtures in seawater using liquid chromatography and fluorescence detectors (e.g., Dawson and Pritchard, 1978), it should be relatively simple to analyse for combined amino acids after hydrolysis of the microlayer samples. Analyses of free amino acids in the microlayer seem not to have been performed to date either, but since considerable degradation of surface-adsorbed proteins may take place as a result of UV irradiation, this may be a fruitful area for future research. 

Collection of sea surface microlayer samples with a metallic screen and subsurface samples with Niskin bottles or bucket filtration through fibreglass filter Whatman GF/C extraction at pH 2 of large samples (20 or 1001) with chloroform extraction of the particulate fraction with CH3OH—CsH 1 1 TLC, GLC... 

Open-ocean seawater (filtrate) samples. Chlordane was not detected in microlayer samples. 

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