Contamination, troubleshooting

The book is divided into four parts. Part I, The Fundamentals of GC/MS, includes practical discussions on GC/MS, interpretation of mass spectra, and quantitative GC/MS. Part II, GC Conditions, Derivatization, and Mass Spectral Interpretation of Specific Compound Types, contains chapters for a variety of compounds, such as acids, amines, and common contaminants. Also included are GC conditions, methods for derivatization, and discussions of mass spectral interpretation with examples. Part III, Ions for Determining Unknown Structures, is a correlation of observed masses and neutral losses with suggested structures as an aid to mass spectral interpretation. Part IV, Appendices, contains procedures for derivatization, tips on GC operation, troubleshooting for GC and MS, and other information which are useful to the GC/MS user. Parts I to III also contain references that either provide additional information on a subject or provide information about subjects not covered in this book. 

This section introduces simple polymer reaction chemistry used to produce many commodity polymers. Understanding this simplified approach to the chemistry of polymer production Is Important In troubleshooting many extrusion processes, especially those that are producing unwanted degradation products that contaminate the discharge resin. There are two general types of polymer production processes 1) step or condensation reactions, and 2) addition or vinyl polymerization reactions. An overview of the reaction mechanisms wifi be presented in the next sections. 

Once the contaminant is identified, the troubleshooter must determine how the material entered the feedstock stream. Process controls must be identified and implemented to mitigate the contaminant source. 

Several case studies are presented in the next sections that show some common root causes of contamination in injection-molded parts. In these case studies, the problem is presented in a manner that the troubleshooter would encounter during a trial or information-gathering session. In each case study, the modifications required to fix the process are detailed along with supporting fundamental information. Two of the case studies used (ET) screws to eliminate the defects. ET screws and other high-performance screws will be discussed in Chapter 14. 

Monitoring of contaminants is somewhat like troubleshooting, but each contamination case study also has a scientific aspect, because each pollution case serves as a large-scale tracing experiment. The tracing of pollutants complements the methods presented in section 6.5, which is devoted to the establishment of hydraulic interconnections, the basic premise that underlies most groundwater models. 

Retention can also be influenced by other components of the mobile phase present in small concentrations. The user may not be aware of the presence of these components. In such a case, troubleshooting can be quite difiicult. A common example of this problem is water in normal-phase chromatography. Water is always present in all solvents (see Table 9.3) and can shift retention substantially. However, this is not the only example. For instance, a contamination of methanol with amines that influences the retention of basic analytes in reversed-phase chromatography has been observed. To avoid this situation, the use of HPLC-grade solvents is generally recommended for HPLC applications. 

In modern chemical plants, thousands of measurements are recorded at frequencies that can exceed 1 Hz. Aside from plant operating conditions including pressures, temperatures, flow rates, and stream composition, other recorded variables include product purity, contamination levels (air, water, soil), and even safety compliance. All this information is stored in enormous databases. This historical record may be interrogated to monitor process performance, and control, for troubleshooting, to demonstrate environmental compliance and modeling. Often smoothing techniques are required to help identify trends in the data that may be masked by low signal-to-noise ratios. 

Table 3-12 presents a troubleshooting guide for plastic foam film. One must be aware that causes of problems are not always obvious for example, Fig. 3-35 shows contamination in the die presenting a problem that is almost impossible to resolve by equipment controls. 

Leak detection and leak location using radiotracer techniques are probably the most widespread applications of radiotracers in industrial troubleshooting. Leaks create problems in process plants or in pipelines, spoil the quality of the final product or reduce the capacity of oil and gas pipelines and contamination of surface or ground water and soil could also happen (IAEA, 2009) (IAEA, 2004). 

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