Endpoint detection

Physical methods for endpoint detection have been suggested. Hellsten [226] proposed an instrumental turbidimetric method to determine the endpoint, which does not need indicators. Since chloroform is emulsified by the anionic surfactant, changes in the optical density can be followed by a colorimeter thus detecting the endpoint when the emulsion breaks. Another turbidimetric method based on commercially available automatic titrators has also been proposed [227],... 

Other detection methods are based on optical transmittance [228-231], Alcohol sulfates have been determined by spectrophotometric titration with barium chloride in aqueous acetone at pH 3 and an indicator [232] or by titration with Septonex (carbethoxypentadecyltrimethylammonium bromide) and neutral red as indicator at pH 8.2-8.4 and 540 nm [233]. In a modified two-phase back-titration method, the anionic surfactant solution is treated with hyamine solution, methylene blue, and chloroform and then titrated with standard sodium dodecyl sulfate. The chloroform passing through a porous PTFE membrane is circulated through a spectrometer and the surfactant is analyzed by determining the absorbance at 655 nm [234]. The use of a stirred titration vessel combined with spectrophotometric measurement has also been suggested [235]. Alternative endpoint detections are based on physical methods, such as stalag-mometry [236] and nonfaradaic potentiometry [237]. 

Marquis and Lebel [534] precipitated potassium from seawater or marine sediment pore water using sodium tetraphenylborate, after first removing halogen ions with silver nitrate. Excess tetraphenylborate was then determined by silver nitrate titration using a silver electrode for endpoint detection. The content of potassium in the sample was obtained from the difference between the amount of tetraphenyl boron measured and the amount initially added. 

The determination of charge in galvanostatic electrolysis is particularly simple, since i f(t) Q = it. Again, a suitable protocol for endpoint detection must be defined [39], and product isolation is possible. 

An extensive database has demonstrated that many chemicals that are positive in this test also exhibit mutagenic activity in other tests. There are, however, examples of mutagenic substances, which are not detected by this test reasons for these shortcomings can be ascribed to the specific nature of the endpoint detected, differences in metabolic activation, or differences in bioavailability. On the other hand, factors which enhance the sensitivity of the bacterial reverse mutation test can lead to an overestimation of mutagenic activity. The bacterial reverse mutation test may not be appropriate for the evaluation of certain classes of chemicals for example, highly bactericidal compounds (e.g., certain antibiotics) and those which are thought (or known) to interfere specifically with the mammalian cell replication system (e.g., some topoisomerase inhibitors and some nucleoside analogues). In such cases, mammalian mutation tests may be more appropriate. 

The process variations that have been intrinsic to CMP since the beginning of its use in semiconductor manufacturing have led to endpoint detection (EPD) being viewed as the holy grail of CMP. 

Numerous approaches have been proposed for use in CMP for in situ EPD. They include optical, electrical, and acoustic sensing. Given the benefits of EPD, it is no surprise that many of these methods have been awarded patents. Some of these methods, most notably current sensing, have been developed to become commercially viable products while others remain laboratory curiosities. For a review of in situ endpoint detection methods up to early 1998, see the work of Bibby and Holland [68]. 

Many endpoint detection systems, based on mechanisms, such as those based on reflected optical light [9], spindle motor current [10], pad temperature [11,12], have been used to resolve this problem, with limited success. Some systems may work with blank wafers or wafers with relatively low pattern density (at the STI level, for example), but for the PMD or ILD levels no useful results have been reported. The presence of a pattern at the PMD or ILD levels adds a great deal of complexity to the signals. Currently, use of an endpoint detection system to control the final post-CMP thickness is still a fertile topic for research and development. 

Total sulfate may be determined in a 50 50 water-methanolic formaldehyde solution by titration with standardized 0.1//lead perchlorate. Endpoint detection is effected using a combination lead ion-selective electrode, and the level of sulfate is typically 13.8 wt % [14]. 

Depending on the samples being titrated, electrode contamination may be an issue. The electrodes may become coated when substances such as oils and sugars are titrated. This results in delayed endpoint detection and an overtitration. A dark brown color on the electrodes indicates that they should be cleaned. The coatings can usually be removed by polar organic solvents or by cleaning the platinum physically. 

Visible spectra of Mg2 -Calmagite and free Calmagite at pH 10 in ammonia buffer. [From C. E. Dahm, J. W. Hal. and B. E. Mattioni, "A Laser Pointer-Based Spectrometer for Endpoint Detection of EDIA nrations. J. Chem. Ed. 2004,81. 1787.]... 

The Karl Fischer titration of water uses a buret to deliver reagent or coulometry to generate reagent. In bipotentiometric endpoint detection, the voltage needed to maintain a constant current between two Pt electrodes is measured. The voltage changes abruptly at the equivalence point, when one member of a redox couple is either created or destroyed. 

Some method of signaling is required to indicate when the amount of titrant generated is equivalent to the amount of unknown present, and all of the endpoint detection methods used in volumetric titrimetry are, in principle, applicable to coulometric titrations. A list that covers most of the published coulo-metric titration procedures is given in Table 25.2. It is beyond our scope here to describe any of these in detail because each of these methods is a subject for discussion in its own right. Discussions of the equations for a number of types of titration curves are found in texts by Lingane [15], Butler [16], and Laitinen and Harris [17]. 

Different experimental approaches are possible with the same endpoint detection method. For example, the titration curve can be plotted and the endpoint determined graphically. First and second derivative curves can be plotted or the derivatives obtained electronically. Another approach is to titrate to a predetermined endpoint signal. This technique is very useful with coulometric titrations, and many examples, especially those involving potentiometric endpoint detection, are found in the literature. The most widely applicable way... 

Table 25.2 Selected Endpoint Detection Methods for Coulometric Titrations...

The uses of constant-current coulometry for the determination of drugs in biological fluids are few, basically due to sensitivity restriction. Monforte and Purdy [46] have reported an assay for two allylic barbituric acid derivatives, sodium seconal and sodium sandoptal, with electrogenerated bromine as the titrant and biamperometry for endpoint detection. Quantitative bromination required an excess of bromine hence back titration with standard arsenite was performed. The assay required the formation of a protein-free filtrate of serum with tungstic acid, extraction into chloroform, and sample cleanup by back extraction, followed by coulometric titration with electrogenerated bromine. The protein precipitation step resulted in losses of compound due to coprecipitation. The recoveries of sodium seconal and sodium sandoptal carried through the serum assay were approximately 81 and 88%, respectively. Samples in the concentration range 7.5-50 pg/mL serum were analyzed by this procedure. 

Optical sensors for ions use indicators, which exist in two different colors, depending on whether the analyte is bound to them. The use of colored indicators is one of the oldest principles of analytical chemistry, used extensively both in direct analytical spectroscopy and in so-called visual titrations. In their sensing application, the indicator is confined to the surface of the optical sensor or immobilized in the selective layer. In that sense, the oldest and most widespread optical sensor is a pH indicator paper, the litmus paper, which is commonly used for the rapid and convenient semiquantitative estimate of pH of solutions or for endpoint detection in acidobasic titrations. Its hi-tech counterpart is a pH optrode (the name of which is intentionally reminiscent of the pH electrode), which essentially does the same thing (Wolfbeis, 2004). The operation principles and limitations of ion optical sensors are common for all ions. 

The aim of an indigo sensor is to keep the leuco-indigo concentration in the solution at a constant value. In the past, different methods were developed for detection of the indigo and sodium dithionite concentration, but up to now with limited success. The sodium dithionite concentration can be determined by volumetric titration with iodine2 22 or with K3[Fe(CN)6]23. The endpoint detection of these titrations can be done visually22,24"25 or... 

Christian, G. D. A Sensitive Amperometric Endpoint Detection System for Microcoulometric Titrations. Microchem. J. 9, 484 (1965). 

In the iodimetric titration procedure, the combustion gases are bubbled through a diluent solution containing pyridine, methanol, and water. This solution is titrated with a titrant containing iodine in a pyridine, methanol, and water solution. In automated systems, the titrant is delivered automatically from a calibrated burette syringe and the endpoint detected amperometrically. The method is empirical, and standard reference materials with sulfur percentages in the range of the samples to be analyzed should be used to calibrate the instrument before use. Alternative formulations for the diluent and titrant may be used in this method to the extent that they can be demonstrated to yield equivalent results. 

However, real-time detection requires access to a special real-time PCR cycler, which is able to detect the increase/decrease of added fluorescence labels during DNA amplification. Although these machines are more and more common for quantitative DNA analysis, their availability in clinical laboratories is still limited. Therefore, the following subsections also include a detailed overview of the classical approaches to quantitative (I)PCR amplificate, analysis which exchanges less demanding PCR equipment for additional hands-on time. The sensitivity of real-time or end-point IPCR detection is quite similar. A comparison of the influence of different endpoint detection methods to the overall sensitivity of IPCR is given in Fig. 5. 

Eig. 5. Several endpoint detection methods were compared for the detection of immuno-polymerase chain reaction (IPCR) amplificate from a direct IPCR (Fig. 3A) of mouse-IgG. Although all IPCR/DNA-detection combinations were able to improve the detection limit of a comparable enzyme-linked immunosorbent assays (ELISA) of approximately 10 amol IgG in a 30-fL sample volume, several differences were observed in actual detection limit, and the linearity of the concentration/signal ratio dependent on the DNA quantification was applied. Best results were obtained for PCR-ELISA (see also Fig. 6) in combination with fluorescence- or chemiluminescence-generating substrates (b, c). With photometric substrates (d) or gel electrophoresis and subsequent spot densitometry (a), a 10-fold decrease in sensitivity was observed. In addition to the more sigmoid curve in gel electrophoresis, an enhanced overall error of 20% compared to 13% in PCR-ELISA was observed for two independent assays. The simple addition of a double-strand sensitive intercalation marker to the PCR-amplificate and measurement in a fluorescence spectrometer further decreased sensitivity (e) and appears therefore to be unsuited for IPCR amplificate quantification. (Figure modified according to references 37 and 65.)... 

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