Turbid samples

It is generally assumed the fluorescence and Fourier transform mid-infrared (FT-IR) spectroscopies do not suffer from the above-mentioned inconveniences and may be applied to turbid samples. Front-face (fluorescence) and attenuated total reflection (FT-IR) techniques may provide information on the structure of adsorbed proteins. 

In practice, very few applications of FEWS sensors can be found outside laboratory applications and demonstration systems. In the near-IR, suitable fibres are readily available but usually there is no real necessity to use them. Possible transmission pathlengths are sufficiently large to allow using standard transmission probes, while turbid samples can be measured using transflection or diffuse reflection probes. In the mid-IR, high intrinsic losses, difficulties in fibres handling and limited chemical and mechanical stability limit the applicability of optical fibres as sensor elements. 

Quantitative determination and monitoring of many species in solution over a wide range of concentrations (10 7 - 1 M) relative precision 0.1-5%. Titrations are especially useful for coloured or turbid samples or for mixtures. 

In cases where water is turbid, samples are generally filtered through glass fiber Alters (GEE) prior to percolation through sorbents. This step recovers waterborne particulates and microorganisms with average diameters >0.7 pm, which are analyzed separately. However, chemicals associated with colloid-sized particulates and DOC are not removed by GFFs. 

We first review the essentials of the phase distribution of the electric fields at the focus of a high numerical aperture lens in Section II. After discussing the phase properties of the emitted signal, in Section HI we zoom in on how the information carried by the emitted held can be detected with phase-sensitive detection methods. Interferometric CARS imaging is presented as a useful technique for background suppression and signal enhancement. In Section IV, the principles of spatial interferometry in coherent microscopy are laid out and applications are discussed. The influence of phase distortions in turbid samples on phase-sensitive nonlinear microscopy is considered in Section V. Finally, in Section VI, we conclude this chapter with a brief discussion on the utility of phase-sensitive approaches to coherent microscopy. 

Aq solns of K azidodithiocarbonate are quite stable at 10° or lower, as is the dry salt itself. At somewhat higher temps the aq solns gradually become turbid. Samples of the dry salt stored in a desiccator at RT gradually turn yel. Solns of the K azido salt, when treated with various oxidizing agents or when subjected to electrolysis, yield the more expl wh ppt, azidocarbondisulfide, (SCSN3)2. An important catalytic effect is exerted by the K azido salt in the reaction between aq KN3 and I2 in the presence of CSa... 

Turbidity may lead to errors in absorbance as particulates may scatter light. If turbid samples must be used, the amount of turbidity needs to be consistent in all dilutions of the unknown sample and in any standards used for calibration. If this is not possible, the sample should be clarified by filtration or centrifugation. 

Problems that may arise with spectrophotometric assays may be due to turbid samples, inadequate mixing, or poor design of controls/blanks. Turbid samples should be clarified by centrifugation (a simple benchtop centrifuge at -5000 x g should be adequate) before reading the absorbance of the clear supernatant. 

The sample should be sent as rapidly as possible to the laboratory, and when the analysis is not to be carried out immediately the bottles are stored horizontally and in a cool, but not excessively cold, place. Turbid samples are either left for some time and then decanted or filtered prior to analysis, the residue being examined separately if necessary. The determination of sulphur dioxide should be made before filtration and as soon as the bottle is opened. 

In numerical simulations and experiments with tissue phantoms, we found that with CR the RMSEP is lower than methods without prior information, such as PLS, and is less affected by analyte covariations. We further demonstrated that CR is more robust than HLA when there are inaccuracies in the applied constraint, as often occurs in complex or turbid samples such as biological tissue.27... 

Similar photosolubility effects have been observed for azo-modified poly(L-omithine) [Scheme 5, VIII (n = 3) 139 and poly(L-a, 3-diaminopropanoic acid) [Scheme 5, VIII (n = 1), 142 monitoring transmittance at 650 nm as a function of irradiation time. The initially turbid samples in HFP/water became dear upon irradiation at 360 nm as a consequence of the trans—>ds isomerization. On new irradiation at 460 nm, the clear solutions became turbid once more as a consequence of the reverse cis/trans isomerization of the azo chromophores. 

High-turbidity samples can clog filters reducing the effective pore size. To avoid this, glass fibre pre-filters should be used and clogged filters changed regularly. 

It is essential that the filter is not a cause of contamination and, if applicable, filters should carefully be washed before use. If possible a portion of sample should be flushed through the filter before sample collection. In general, samples for most organic determinands should NOT be filtered highly turbid samples and all samples collected for dissolved metals SHOULD be filtered. 

Rather than extracting water with solvent, the water sample is poured through a column or filter containing an absorbent resin. The organics will preferentially adsorb to the resin, which is subsequently desorbed with solvent. This technique has been used for PAHs, pesticides, and PCBs and has been well characterized for drinking water. Laboratories should take proper steps to evaluate the efficiency of this technique for effluent samples or turbid samples and may refer to EPA method 3535A or to guideline documents from SPE suppliers (e.g., Supelco bulletin 910). 

Use HPLC or analytical grade buffers, freshly prepared, filtered and degassed before use. Filter or centrifuge the sample to remove particulate matter. Turbid samples should not be injected onto column. 

Compared to conventional microscopy, confocal microscopy delivers superior optical sectioning by using a pinhole in a plane conjugate with the focal (xy) plane. It allows a crisp 3D image to be built from a stack of 2D images even for somewhat turbid samples, but each 2D image is acquired by scanning, which imposes limits on acquisition speed. The technique has been described in detail before [82]. 

The method is typically performed by an instrument that oxidizes the sample to CO over a catalyst, as described in Standard Method 5310 (Standard Methods, 1998). The amount of CO produced is measured by an infrared analyzer. Calibrations are produced by injecting known materials into the analyzer. The accuracy for solutions is typically 1-2%. The accuracy for turbid samples with suspended matter is 5-10%. 

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