The Schlieren Effect

In 1864, Toepler introduced the term schliere as an inequality of the optical refraction of an otherwise homogeneous mass [75]. The phenomenon is now known as the Schlieren effect [76] and in English-language documents, terms such as streak and stria have also been used. 

In flow analysis with spectrophotometric detection, the Schlieren effect is generally the ultimate limitation on the signal-to-noise ratio and hence on the detection limit. Furthermore, it can lead to systematic measurement deviations, thus degrading accuracy. 

In general, different radiation wavelengths correspond to slightly different indices of refraction. The main consequence of this aberration is that the refraction angle is wavelength dependent [80]. This aspect is very important for minimising the Schlieren effect by dual-wavelength spectrophotometry (see also 4.23.5). 

Refraction of radiation can also occur in fluidic regions with refractive index gradients because optical artefacts analogous to fluidic lenses are established. Regardless of the process (reflection or refraction) and the optical artefact (mirror or lens) involved, the deflection of radiation inside a fluidic environment can result in external distortions, characterised by the Schlieren effect. 

In spectrophotometry, this effect alters the power of the emergent radiation beam (Fig. 4.1), because some radiation is deflected. Consequently, a systematic deviation in the measurement is observed. 

---

The schlieren system of optics is an analytical method that is particularly well suited to following the location of a chemical boundary with time. It is routinely employed in ultracentrifuges and also in electrophoresis experiments, as we see in Chapter 12. Schlieren optics produces an effect that depends on the way the refractive index varies with position, that is, the refractive index gradient rather than on the refractive index itself. Therefore, the schlieren effect is the same at all locations along the axis of sedimentation, except at any place where the refractive index is changing. In such a region, it will produce an optical effect that is proportional to the refractive index gradient. The boundary between two layers is thus per-... 

The Schlieren effect tends to be pronounced. It should be noted that undesirable concentration gradients leading to the appearance of the Schlieren effect (see also 4.2) are dependent on differences in the original composition of the sample and carrier solutions and that the effects are more pronounced in single-line systems. 

The Schlieren effect tends to be minimal because exploitation of... 

Radiation losses are reduced with a well collimated radiation beam, as orthogonality between the incident beam and any interfaces results in less pronounced refraction. Regarding flow analysis, transient liquid/liquid interfaces that are oblique to the incident beam can be established inside the flowing sample and this is related to occurrence of the Schlieren effect [3]. This effect is more pronounced in spectrophotometers with inferior optical components and design. 

Loss of radiation by reflection and refraction inside the sample is mainly due to the Schlieren effect, which can manifest itself as a consequence of the establishment of lenses and mirrors between neighbouring fluid elements with different refractive indices (see also 4.2). These losses are more pronounced in flow analysis [3], as perfect homogeneity of the monitored sample is not attained in practice. Losses are minimised by avoiding the presence of discontinuities along the optical beam, i.e., by efficient mixing of the sample with the carrier/reagent streams. In this regard, exploitation of a chemically inert carrier/wash stream is beneficial [5]. 

Lack of homogeneity of the sample may also be a source of deviation from the Lambert—Beer law in view of the increased possibility of radiation losses at the liquid/liquid interfaces, as well as the presence of gas and/or liquid mini-bubbles and suspended matter (see also Fig. 4.1). In flow analysis, effects due to discontinuities along the monitored sample zone tend to be more severe therefore the Schlieren effect can become more pronounced, the possibility of evolution of gaseous mini-bubbles is... 

The Schlieren effect in flow analysis has often been referred to as a refractive index effect, but both reflection and refraction are involved (Fig. 4.6). Indeed, it is a phenomenon more related to light reflection. 

FIGURE 4.9 Observations of the Schlieren effect in everyday life. Source Courtesy of G.S. Settles and reproduced with permission from "Schlieren and shadowgraph techniques" by G.S. Settles, ISBN 978-3-540-661559, Springer (2001), see Ref. [821. 

The Schlieren effect is also observed in liquid media. A pedagogical demonstration involves the addition of a concentrated sucrose solution to a beaker containing water. Immediately after addition, the aqueous medium becomes turbid (Fig. 4.10, left). Transparency is restored by letting the solution stand for a few minutes (Fig. 4.10, centre). The medium may become turbid again if the solution is gently stirred again... 

FIGURE 4.10 Batchwise observation of the Schlieren effect. Photographs show a beaker of water to which a concentrated sucrose solution is added. From left to right the situation during sucrose addition, after about 5 min standing time, and after gentle shaking. 

In spectrophotometry, the Schlieren effect can alter the analytical results because it influences the propagation of the radiation beams involved (Fig. 4.1). Collimation of the light beam plays an important role in this respect, as an effect analogous to a shadowgraph is observed when a spectrophotometer with poor optical properties is used [83]. 

Regarding the influence of the Schlieren effect on analytical procedures, the effect of differences between the temperature of the ambient environment and the sample on the absorbance was originally reported in relation to stopped-flow procedures [84]. The systematic deviation in absorbance was proportional to the temperature difference and was also dependent on the optical properties of the instrument. The effect also manifested itself when identical solutions at different temperatures were mixed [85] a linear dependence of the measurement on the temperature... 

In flow analysis, the Schlieren effect tends to be more pronounced [28] because a perfectly mixed flowing sample is not practically achievable, and an analyte concentration gradient is always present. Undesirable concentration gradients and/or discontinuities along the monitored sample can give rise to the formation of relatively steady liquid lenses as well as a myriad of randomly distributed transient mirrors. These optical artefacts lead to fluctuations in the emergent radiation that can alter the measured signal. 

The influence of the Schlieren effect on the analytical output depends on the optical performance of the spectrophotometer [83], the number of mirror interfaces and the intensity and direction of the concentration gradients. The latter parameters are closely related to the concentrations of the solutions involved, the mixing conditions and the kinetics of solution inter-mixing and hence the transport number of the related chemical species, as discussed below. 

The Schlieren effect degrades the signal-to-noise ratio, and hence the detection limit, in flow-based spectrophotometric analytical procedures and the accuracy because the related noise is superimposed on the analytical signal. 

FIGURE 4.11 Recorder traces showing the influence of the Schlieren effect caused by a 35 salinity matrix on the signals for different F concentrations under good mixing... 

Anal. Chim. Acta 351 (1997) 265, I.D. McKelvie, D.M.W. Peat, G.P. Matthews, P.J. Worsfold, Elimination of the Schlieren effect in the determination of reactive phosphorus in estuarine waters bp flow-injection analysis, with permission from Elsevier. 

The Schlieren effect has been investigated mainly in relation to UV—Visible spectrophotometry [3,28,92,93], but it is also important in fluorimetry. Analytical procedures involving chemi- or bioluminescence detection are less affected due to the geometry of the flow cell and the integrating nature of the light collection process. The influence of the Schlieren effect on turbidimetric and nephelometric detection is less pronounced because mixing tends to be more efficient due to the presence of solid particles in the flowing sample. 

Little attention has been given to the influence of these transient liquid mirrors and lenses on the amount of stray light reaching the detector. Investigation of this process is strongly recommended, as the Schlieren effect can lead to an increase in detected radiation, hence degrading the linearity of the analytical response curve [13]. 

From these historical reports, it is also possible to conclude that the Schlieren effect in flow analysis is a consequence of the combined effects of two components, namely deflection of the beam of radiation by transient (A) and/or relatively steady (B) optical artefacts [3]. 

TABLE 4.3 Features Related to the Components of the Schlieren Effect... 

---