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1.1. MODERN ANALYTICAL CHEMISTRY
1.1.1. Developments in Modern Chemistry
The field of chemistry is currently facing major changes. As we know, optical, mechanical, and microelectronic technologies have advanced rapidly in recent years. Computer power has increased dramatically as well. All these developments, together with other factors, provide a new opportunity but also challenge to chemists in research and development.
A recent (as of 2003) development in the pharmaceutical industry is the use of combinatorial synthesis to generate a library of many compounds with structural diversity. These compounds are then subjected to high-throughput screening for bioassays. In such a process, tremendous amounts of data on the structure-activity relationship are generated. For analytical measurements, a new, advanced, modern technology called hyphenated instrumentation using two or more devices simultaneously for quantitative measurement has been introduced . Examples of this technique are the high-performance liquid chromatography--diode array detector system (HPLC-DAD), gas chromatography with mass spectrometry (GC-MS), and liquid chromatography coupled with mass spectrometry such as LC-MS and LC-MS-MS. Huge amounts of data are generated from these pieces of equipment. For example, the Hewlett-Packard (HP) HPLC 1100 instrument with a diode array detector (DAD) system (Agilent Technology Inc., CA) produces 1.26 million spectrochromatographic data in a 30-min experimental run with a sampling rate of 5 Hz, and a spectral range of 190-400 nm with a resolution of 1 data item per 2 nm. To mine valuable information from these data, different mathematical techniques have been developed. Up to now, research and development of this kind with the application of statistical and mathematical techniques in chemistry has been confined mainly to analytical studies. Thus, our
Chemometrics: From Basics To Wavelet Transform, edited by Foo-tim Chau, Yi-zeng Liang, Junbin Gao, and Xue-guang Shao. Chemical Analysis Series, Vol. 164.
ISBN 0-471-20242-8. Copyright © 2004 John Wiley & Sons, Inc.
discussion will focus on analytical chemistry but other disciplines of chemistry will also be included if appropriate. The main content of this book provides basic chemometric techniques for processing and interpretation of chemical data as well as chemical applications of advanced techniques, including wavelet transformation (WT) and mathematical techniques for manipulating higher-dimensional data.
1.1.2. Modern Analytical Chemistry
Modern analytical chemistry has long been recognized mainly as a measurement science. In its development, there are two fundamental aspects:
1. From the instrumental and experimental point of view, analytical chemistry makes use of the basic properties such as optics, electricity, magnetism, and acoustic to acquire the data needed.
2. New methodologies developed in mathematical, computer, and biological sciences as well as other fields are also employed to provide in-depth and broad-range analyses.
Previously the main problem confronting analytical scientists was how to obtain data. At that time, measurements were labor-intensive, tedious, time-consuming, and expensive, with low-sensitivity, and manual recording. There were also problems of preparing adequate materials, lack of proper techniques, as well as inefficient equipment and technical support. Workers had to handle many unpleasant routine tasks to get only a few numbers. They also had to attempt to extract as much information as possible about the structure, composition, and other properties of the system under investigation, which was an insurmountable task in many cases. Now, many modern chemical instruments are equipped with advanced optical, mechanical, and electronic components to produce high-sensitivity, high-quality signals, and many of these components are found in computers for controlling different devices, managing system operation, data acquisition, signal processing, data interpretation in the first aspect and reporting analytical results. Thus the workload on analytical measurement mentioned above (item 1 in list) is reduced to minimum compared to the workload typical decades ago.
After an analytical measurement, the data collected are often treated by different signal processing techniques as mentioned earlier. The aim
modern analytical chemistry
is to obtain higher quality or ‘‘true’’ data and to extract maximum amount of meaningful information, although this is not easy to accomplish. For instance, in an HPLC study, two experimental runs were carried out on the same sample mixture. The two chromatograms acquired usually differed from each other to a certain extent because of the variations in instrumentation, experimental conditions, and other factors. To obtain quality results that are free from these disturbances, it is a common practice to carry out data preprocessing first. The techniques involved include denoising, data smoothing, and/or adjustment of baseline, drift, offset, and other properties. Methods such as differentiation may then be applied to determine more accurate retention times of peaks, especially the overlapping peaks that arise from different component mixtures. In this way, some of these components may be identified via their retention times with a higher level of confidence through comparison with those of the standards or known compounds. If the peak heights or peak areas are available, the concentrations of these components can also be determined if the relevant calibration curves are available. Statistical methods can also help in evaluating the results deduced and to calculate the level of confidence or concentrations of the components being identified. All these data obtained are very important in preparing a reliable report for an analytical test. Data treatment and data interpretation on, for instance, the HPLC chromatograms as mentioned above form part of an interdisciplinary area known as chemometrics.