Digital chromatograph

Both gas chromatographic instruments were connected with a PDP 11/45 computer via an analog-to-digital converter. The peak areas were calculated from the digitalized chromatographic data by means of software developed at Delft University of Technology. 

The former has been the more common of the two primarily because of its cost advantage. The latter has the advantage that it has more versatility (it can handle the applications of two independent detector channels as well) and does function as a backup electrometer. In the newer digital chromatographs, the signals from two independent detector electrometer systems are easily and accurately subtracted by the processor. 

S. Chesler and S. P. Cram, Effect of peak sensing and random noise on the precision and accuracy of statistical moment analyses from digital chromatographic data. Anal. Chem. 43 1922 (1971). 

Apparatus. A gas chromatograph equipped with a flame-ionisation detector and data-handling system. The use of a digital integrator is particularly convenient for quantitative determinations, but other methods of measuring peak area may be used (Section 9.4). 

Figure 3.12 Comparison of the chromatographic peak shapes obtained from (a) analogue and (b,c) digital detectors, and the effect on peak shape of the number of data points defining the signal.

The schematic diagram of the experimental setup is shown in Fig. 2 and the experimental conditions are shown in Table 2. Each gas was controlled its flow rate by a mass flow controller and supplied to the module at a pressure sli tly higher than the atmospheric pressure. Absorbent solution was suppUed to the module by a circulation pump. A small amount of absorbent solution, which did not permeate the membrane, overflowed and then it was introduced to the upper part of the permeate side. Permeation and returning liquid fell down to the reservoir and it was recycled to the feed side. The dry gas through condenser was discharged from the vacuum pump, and its flow rate was measured by a digital soap-film flow meter. The gas composition was determined by a gas chromatograph (Yanaco, GC-2800, column Porapak Q for CO2 and (N2+O2) analysis, and molecular sieve 5A for N2 and O2 analysis). The performance of the module was calculated by the same procedure reported in our previous paper [1]. 

Section I of this book includes chapters on the principles and practice of PLC. After this introductory Chapter 1, Chapter 2 provides information on efforts undertaken to date in order to establish the theoretical foundations of PLC. With growing availability and popularity of modem computer-aided densitometers, separation results can be obtained in digital form as a series of concentration profiles that can be relatively easily assessed and processed. From these, relevant conclusions can be drawn in exactly the same manner as in automated column chromatographic techniques. Efforts undertaken to build a theoretical foundation of PLC largely consist of adaptation of known strategies (with their validity confirmed in preparative column liquid chromatography) to the working conditions of PLC systems. 

McConnell, M., Canales, M., and Lawler, G., A validation protocol for analog-to-digital interfaces in chromatographic data systems, LC-GC, 9, 486, 1991. 

Let us dwell on Figure 6.4 for a moment. The standards and sample solutions are introduced to the instrument in a variety of ways. In the case of a pH meter and other electroanalytical instruments, the tips of one or two probes are immersed in the solution. In the case of an automatic digital Abbe refractometer (Chapter 15), a small quantity of the solution is placed on a prism at the bottom of a sample well inside the instrument. In an ordinary spectrophotometer (Chapters 7 and 8), the solution is held in a round (like a test tube) or square container called a cuvette, which fits in a holder inside the instrument. In an atomic absorption spectrophotometer (Chapter 9), or in instruments utilizing an autosampler, the solution is sucked or aspirated into the instrument from an external container. In a chromatograph (Chapters 12 and 13), the solution is injected into the instrument with the use of a small-volume syringe. Once inside, or otherwise in contact with the instrument, the instrument is designed to act on the solution. We now address the processes that occur inside the instrument in order to produce the electrical signal that is seen at the readout. 

The development of digital control computers and of chromatographic composition analyzers has resulted in a large number of control systems that have discontinuous, intermittent components. The nature of operation of both of these devices is such that their input and output signals are discrete. 

The chromatograph Is Interfaced with a Digital LSl-11 microprocessor. Calibration, though requiring observations of chromatograms for monomeric and polymeric standards, can be efficiently achieved. Research over several years shows that styragel/micro-styragel columns are very stable under continuous utilization. 

Figure 6.15. Block diagram of digital oven temperature control system (Hewlett-Packard Co.-5700 series chromatographs).

Figure 6.16. Block diagram of processor based oven temperature control system. The processor and most of the digital parts are shared with other temperature zones and with other functions of the gas chromatograph. (Hewlett-Packard Co.—5830/40 series chromatographs ). ...

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