07-09-2013, 03:40 PM
Analytical Instrumentation
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Introduction
Analytical Chemistry deals with methods for determining the chemical composition of samples 0f matter.
A Qualitative method yields information about the identity of atomic or molecular species or the functional groups in the sample.
Quantitative method in contrast, provides numerical information as to the relative amount of one or more of these components.
Analytical methods are often classified as being either classical or instrumental.
Classical methods, sometimes called wet-chemical methods, preceded instrumental methods by a century or more.
In the early years of chemistry, most analyses were carried out by separating the components of interest (the analytes) in a sample by precipitation. Extraction, or distillation
For qualitative analyses, the separated components were then treated with reagents that yielded products that could be recognized by their colors, their boiling or melting points, their solublities in a series of solvents, their odors, their optical activities, or their refractive indexes.
For quantitative analyses, the amount of analyte was determined by gravimetric or by volumetric measurements.
In gravimetric measurements, the mass of the analyte or some compound produced from the analyte was determined.
In volumetric, also called titrimetric, procedures, the volume or mass of a standard reagent required to react completely with the analyte was measured.
Instruments for analysis
An instrument for chemical analysis converts information about the physical or chemical characteristics of the analyte to information that can be manipulated and interpreted by a human.
Thus an analytical instrument can be viewed as a communication device between the system under study and the investigator.
To retrieve the desired information from the analyte, it is necessary to provide a stimulus, which is usually in the form of electromagnetic, electrical, mechanical, or nuclear energy.
The stimulus elicits a response from the system under study whose nature and magnitude are governed by the fundamental laws of chemistry and physics.
The resulting information is contained in the phenomena that result from the interaction of the stimulus with the analyte.
Data Domains
The measurement process is aided by a wide variety of devices that convert information from one form to another.
Before investigating how instruments function, it is important to understand how information can be encoded (represented) by physical and chemical characteristics and particularly by electrical signals, such as current. voltage, and charge. The various modes of encoding information are called data domains.
Analog signals
These quantities are continuous in both amplitude and time as shown by the typical analog signals of Figure 1-4.
Magnitudes of analog quantities can be measured continuously, or they can be sampled at specific points in time dictated by the needs at a particular experiment or instrumental method.
The correlation of two analog signals that result from corresponding measured physical or chemical properties is important in a wide variety of instrumental techniques, such as nuclear magnetic resonance spectroscopy, infrared spectroscopy, and differential thermal analysis.
Digital Information
Data are encoded in the digitaI domain in a two-level scheme.
The information can be represented by the state of a light bulb, a light-emiting diode, a toggle switch, or a logic-level signal etc.
The characteristic that these devices share is that each of them must be in one of only two states.
For example, the signal represented in Figure 1-5c is a train of pulses from a nuclear detector.
The measurement task is to count the pulses during a fixed period of time to obtain a measure of the intensity of radiation.
If the signal crosses the threshold fourteen times, as in the case of the signal in Figure 1-5c, then we may be confident that fourteen nuclear events occurred.
After the events have been counted, the data are encoded in the digital domain by HI-LO signals representing the number 14.
The Electromagnetic Spectrum
As shown in Figure 6-3, the electromagnetic spectrum encompasses an enormous range of wavelengths and frequencies (and thus energies).
In fact, the range is so great that a logarithmic scale is required. Figure 6·3 also depicts qualitatively the major spectral regions.
The divisions are based on the methods used to generate and detect the various kinds of radiation.
Note that the portion of the spectrum visible to the human eye is tiny when compared with other spectral regions.
The spectrochemical methods employing not only visible hut also ultraviolet and infrared radiation are often called optical methods despite the human eye's inability to sense either of the latter two types of radiation.