Physical interference

3 Physical Interferences. The physical interferences originate in changing physical characteristics of the solutions to be measured (viscosity, surface tension, vapour pressure, temperature). Various solution properties have an effect on the sample intake, nebulization, transport of the sample mist, evaporation of the solvent, vaporization of the analyte, and scatter of the light. 

In practice, very dilute solutions are used in AAS, so that the physical properties of the sample solutions are close to those of the pure solvent (water). Physical properties will be changed if the sample solution contains large amounts of acids, salts, or organic compounds. Concentrated solutions are viscous which may cause problems with the sample intake and nebulization. The absorption signal decreases with increasing viscosity. Viscosity 

Nebulization is clearly dependent on the surface tension. The surface tension of the organic solvents is smaller than that of water and, hence, in organic solutions the droplets of the sample mist are smaller than in aqueous solutions. The vaporization will be improved if the vapour pressure of the organic solvent is greater than that of water. This is why the sensitivity in organic solutions is better than in aqueous solutions. 

Analyte solutions that are deposited onto solid support materials placed into the atomiser are not subject to positional and diffusional errors. 

Originally, this method of sample introduction was devised for specific applications (e.g., Cemik s punched-disc of filter paper for blood lead analysis [6], and the analysis of metalloproteins separated on cellulose acetate membranes [7]), but there is now a commercially available system that uses carbon boats which has been recently applied to the direct analysis of cadmium and lead in blood and urine [ 8]. 

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The method is based on the international standard ISO 4053/IV. A small amount of the radioactive tracer is injected instantaneously into the flare gas flow through e.g. a valve, representing the only physical interference with the process. Radiation detectors are mounted outside the pipe and the variation of tracer concentration with time is recorded as the tracer moves with the gas stream and passes by the detectors. A control, supply and data registration unit including PC is used for on site data treatment... 

For a method to be useful it must provide reliable results. Unfortunately, methods are subject to a variety of chemical and physical interferences that contribute uncertainty to the analysis. When a method is relatively free from chemical interferences, it can be applied to the determination of analytes in a wide variety of sample matrices. Such methods are considered robust. 

We will see that CLS and ILS calibration modelling have limited applicability, especially when dealing with complex situations, such as highly correlated predictors (spectra), presence of chemical or physical interferents (uncontrolled and undesired covariates that affect the measurements), less samples than variables, etc. More recently, methods such as principal components regression (PCR, Section 17.8) and partial least squares regression (PLS, Section 35.7) have been... 

The CLS method hinges on accurately modelling the calibration spectra as a weighted sum of the spectral contributions of the individual analytes. For this to work the concentrations of all the constituents in the calibration set have to be known. The implication is that constituents not of direct interest should be modelled as well and their concentrations should be under control in the calibration experiment. Unexpected constituents, physical interferents, non-linearities of the spectral responses or interaction between the various components all invalidate the simple additive, linear model underlying controlled calibration and classical least squares estimation. 

The large amount of sodium chloride in seawater samples causes nonspecific absorption [366-370], which can only be partially compensated by background correction. In addition the seawater matrix may give rise to chemical as well as physical interferences related to the complex physico-chemical phenomena [371-373] associated with vaporization of metals and of the matrix itself. 

Flow-injection methods are quicker, more precise and use less reagents than other techniques in addition, they are very useful where only a Hmited amount of sample is available. The main advantages of the combination of FIA with DCP/OES are the increase in precision in sample handhng, fewer physical interferences, ahigher throughput of samples and versatihty towards physical and chemical properties of reagents. Some minor disadvantages are the loss of sensitivity compared with continuous nehuHzation, and the fact that only one element can he determined at a time. 

Greenfield ef. ai.l l) observed a reduction of signal intensity that correlates with sample intake effects from the modified solution viscosity and/or surface tension of mineral acids. This, coupled with peristaltic pumping of solutions into the nebulizer, considerably reduces physical interferences. Increased salt concentration also has an effect on solution physical properties. In the experience of these authors, the high levels of salt in the matrix also increases the noise from the nebulizer system. This degradation of nebulizer performance, which is not necessarily accompanied by a proportional reduction in sensitivity, is the cause of the observed deterioration of detection limits in real samples as opposed to ideal solutions. 

Determinations by both techniques can be subject to chemical and/or physical interference effects caused by the sample matrix. However, after fractionation of the sample the species are usually in a less complex matrix, a buffer or electrolyte solution. Consequently, matrix interferences effects are minimised. On the other hand, the species may be diluted in the process and this could be detrimental for the determination of species present at very low concentrations. At the present state of the art GFAAS can be used for the determination of analytes at the 1 pgl level. However, at this level contamination in the reagents and equipment limit the number of species that can be detected with confidence. 

Studies of the formation, chemical composition, and properties of deposits have shown that they consist of partially oxidized organic material, including more or less nitrogen, sulfur, and phosphorus. Compounds of iron, silicon, calcium, and other metals are present in small quantity, together with substantial amounts of lead oxides, sulfates, and halides from combustion of the antiknock fluid. The effects of these deposits are both physical and chemical in nature they may physically interfere with lubrication, heat transfer, gas flow, operation of valves and spark plugs chemically, they may bring about corrosion and oxidation. 

Physical interferences are generally considered to be effects associated with such properties as change in viscosity, and surface tension can cause significant inaccuracies, especially in samples that may contain high dissolved solids, or acid concentrations, or both. If these types of interferences are operative, they must be reduced by dilution of the sample or utilization of standard addition techniques, or both. 

The XRF analysis accuracy depends on physical properties of the soil, such as particle size, moisture content, homogeneity, and surface conditions. Drying, grinding, and sieving soil prior to analysis reduce the effects of these physical interferences. 

Physical interferences in FLAA analysis are of the same nature as the ones in ICP-AES analysis high dissolved solids contents in samples change the sample viscosity and alter the aspiration rate. 

PEBBLEs are water-soluble nanoparticles based on biologically inert matrices of cross-linked polymers, typically poly(acrylamide), poly(decylmethacrylate), silica, or organically modified silicates (ORMOSILs), which encapsulate a fluorescent chemo-sensor and, often, a reference dye. These matrices have been used to make sensors for pH, metal ions, as well as for some nonionic species. The small size of the PEBBLE sensors (from 20 to 600 nm) enables their noninvasive insertion into a living cell, minimizing physical interference. The semipermeable and transparent nature of the matrix allows the analyte to interact with the indicator dye that reports the interaction via a change in the emitted fluorescence. Moreover, when compared to naked chemosensors, nanoparticles can protect the indicator from chemical interferences and minimize its toxicity. Another important feature of PEBBLEs, particularly valuable in intracellular sensing applications, is that the polymer matrix creates a separate... 

Figure 5.7. Diagram showing role of N- and C-propeptides in collagen self-assembly. The procollagen molecule is represented by a straight line with bent (N-propeptide) and circular (C-propeptide) regions. Initial linear and lateral aggregation is promoted by the presence of both the N- and C-propeptides. In the presence of both propeptides lateral assembly is limited and the fibrils are narrow. Removal of the N-propeptide results in lateral assembly of narrow fibrils removal of the C-propeptide results in additional lateral growth of fibrils. As indicated in the diagram, the presence of the N- and C-propeptides physically interferes with fibril formation.

Interferences are common in chemical analysis. In general, interferences are classified as physical, chemical, or spectral. Physical interferences are caused by the effects from physical properties of the sample on the physical process involved in the analytical... 

In flame spectrometry, physical interferences are related to transport of determinant from sample solution to the flame. The pneumatic nebulizer functions not only as a spray generator, but also as a pump.1,2 Anything which influences the pumping rate will influence the size of the absorbance signal obtained. The pumping, or aspiration, rate is most sensitive to changes in viscosity of the sample solutions. 

Physical Interferences Chemical Interferences The Literature Interpolation Problem Ionization Interferences Spectral Interferences Spectral Interferences in Flame AAS Background Correction in AAS Spectral Interferences in Flame AFS Spectral Interferences in Flame AES Conclusions about Interferences... 

While chemical and physical interference possibilities can be largely removed by making the standard solution and the sample solution essentially similar, unreproducible temperature settings frequently lead to differences which cannot be corrected. Though there has been no lack of effort to achieve high temperature constancy by the construction of special feedback systems, the lifetime of these systems is very limited when the very high... 

Adequate workspaces should be provided between different equipment to avoid restricted movement of the employee and there should not he any physical interference that can cause error or accident. 

Chemical interference is practically non existent as a result of the high temperature of the plasma. On the other hand, physical interference may be observed. This stems from variations in the sample atomisation speed which is usually due to changes in nebulisation efficiency caused by differences in the physical properties of the solutions. Such effects may be caused by differences in viscosity or vapour tension between the sample solutions and the standards due, for example, to differences in acidity or total salt content. The technique most commonly used to correct this physical interference is the use of internal standards. In this technique a reference element is added at an identical concentration level to all the solutions under analysis, standards, blank and samples. For each element, the ratio of simultaneous measurements of the lines of the element and the internal standard is then determined in order to compensate for any deviation in the response of the plasma. If the internal standard behaves in the same way as the element to be determined, this method can be used to improve the reliability of the result by a factor of 2 to 5. It can also, however, introduce significant errors because not all elements behave in the same way. It is thus necessary to take care when using it. Alternatives to the internal standard method include incorporating the matrix into the standards and the blank, sample dilution, and the standard addition method. 

Physical interferences are due to the effects of the sample solution on aerosol formation within the spray chamber. The formation of an aerosol is dependent upon the surface tension, density and viscosity of the sample solution. This type of interference can be controlled by the matrix matching of sample and standard solutions, i.e. add the same sample components to the standard solution, but without the metal of interest. If this is not possible, it is then necessary to use the method of standard additions (Box 27.3). 

Certain amino acids tend to disrupt the a-helix. Among these are proline (the N-atoms is part of the rigid ring and no rotation of the N-C bond can occur) and amino acid with charged or bulk R groups that either electrostatically or physically interferes with helix formation. 

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