GC-Computer System

GC-Computer System Nowadays, a large number of data-processing-computer-aided instruments for the automatic calculation of various peak parameters, for instance relative retention, composition, peak areas etc., can be conveniently coupled with GC-systems. A commercially available fairly sophisticated computer system of such type are available abundantly that may be capable of undertaking load upto 100 gas-chromatographs with ample data-storage facilities. In fact, the installation such as multi GC-systems in the routine analysis in oil-refineries and bulk pharmaceutical industries, and chemical based industries have tremendously cut-down their operating cost of analysis to a bare minimum. 

Communication between the user and the computer should be as simple and reliable as possible. Several input and output devices may be used which of them is attached to the system is a question of philosophy and cost and depends on the configuration of the gc-computer system. 

With these handicaps in mind, several manufacturers of gc-computer systems have developed nonlinear separation methods designed to be used in the time domain. These methods are based on the fact that a gaussian peak is defined by three or four points which assume a S3unmetrical or asymmetrical shape. 

The software of many gc-computer systems includes the calculation of Kovdts indices a program in Fortran has been published by Gastello et Schomburg 2 b, 84) has described the advantages of automatic... 

In considering the economics of a gc-computer system, the cost of purchase or rent must be set against the savings and advantages expected from using the computer. While it is relatively easy to calculate the sum spent or... 

Data Handling System—Any commercially available GC integrator or GC computer system capable of accurately integrating the area of the methanol peak is satisfactory. 

Once a mass spectrum from an eluting component has been acquired, the next step is to try to identify the component either through the skill of the mass spectroscopist or by resorting to a library search. Most modem GC/MS systems with an attached data station include a large library of spectra from known compounds (e.g., the NIST library). There may be as many as 50,000 to 60,000 stored spectra covering most of the known simple volatile compounds likely to be met in analytical work. Using special search routines under the control of the computer, one can examine... 

Fast chromatography involves the use of narrow-bore columns (typically 0.1-mm i.d.) that will require higher inlet pressures compared with the conventional wide-bore capillary columns. These columns require detectors and computing systems capable of fast data acquisition. The main disadvantage is a much-reduced sample loading capacity. Advances in GC column technology, along with many of the GC-related techniques discussed below, were recently reviewed by Eiceman et... 

Pyrolysis-Gas Chromatography-Mass Spectrometry. In the experiments, about 2 mg of sample was pyrolyzed at 900°C in flowing helium using a Chemical Data System (CDS) Platinum Coil Pyrolysis Probe controlled by a CDS Model 122 Pyroprobe in normal mode. Products were separated on a 12 meter fused capillary column with a cross-linked poly (dimethylsilicone) stationary phase. The GC column was temperature programmed from -50 to 300°C. Individual compounds were identified with a Hewlett Packard (HP) Model 5995C low resolution quadruple GC/MS System. Data acquisition and reduction were performed on the HP 100 E-series computer running revision E RTE-6/VM software. 

An important use for FT-IR spectrometers is in coupling them to gas chromatographs in what is called a coupled or hyphenated technique, i.e. GC-IR (p. 117). This enables the spectra of eluting peaks to be recorded without the need to trap the component or stop the gas flow. Computer enhancement and manipulations of the recorded spectra are additional features of both stand-alone FT-IR spectrometers and GC-IR systems. 

Lao RC, Thomas RS, Bastien P, et al. 1982. Analysis of organic priority and nonpriority pollutants in environmental samples by GC/MS/computer systems. Pergamon Ser Environ Sci 7 107-118. 

Different capillary columns are available for organic acid separation and analysis. In our laboratory, the gas chromatography column in all GC-MS applications is crosslinked 5% phenyl (poly)methyl silicone, 25 m internal diameter 0.20 mm stationary phase film thickness 0.33 pm (Agilent HP-5, DB-5, or equivalent). Several instrument configurations are commercially available, which allow for positive identification of compounds by their mass spectra obtained in the electron impact ionization mode. A commercially available bench-top GC-MS system with autosampler (Agilent 6890/5973, or equivalent) is suitable. Software for data analysis is available and recommended. The use of a computer library of mass spectra for comparison and visualization of the printed spectra is required for definitive identification and interpretation of each patient specimen. 

Every organic chemical has a mass spectrum, which is a combination of ions with different masses and different intensities (abundances). To identify a compound, its mass spectrum is compared to the mass spectra of standards, analyzed under the same instrument settings, and to the EPA/National Institute for Standards and Technology (NIST) mass spectra library. The EPA/NIST library is stored in the database of the computer that operates the instrument. A comparison to the library spectra is possible only if there is consistency in the compound spectra generated by different GC/MS systems at hundreds of environmental laboratories. To achieve such consistency, the EPA methods for GC/MS analysis include the mass... 

Combined GC-MS-computer systems with repetitive scanning can lead to the identification of GAs as minor components of complex extracts at levels down to 1CT- -1 g. In such cases mass fragmentograms can be constructed in which the distribution of ions of particular m/e values are plotted throughout a GC-MS run. Thus if the presence of a particular GA is suspected, characteristic ions in the mass spectrum of the derivatized GA are plotted. An identification can be made if the ions peak at the same retention time as the GA and have the same relative intensity as in the mass spectrum of the authentic compound (see Figure 6). In this way GAs can be detected which are masked in the GC trace by other compounds of similar retention time (cf. 33). 

Mass spectrometry (MS) is now a well-accepted tool for the identification as well as quantitation of unknown compounds. The combination of MS with powerful separation methods such as gas chromatography (GC) or high-performance liquid chromatography (LC) provides a technique which is widely accepted for the identification of unknown components in complex mixtures from a wide variety of problems such as environmental pollutants, biological fluids, insect pheromones, chemotaxonomy, and synthetic fuels. The importance of such analyses has grown exponentially in the last few years there are now well over a thousand GC/MS instruments in use around the world, most with dedicated computer systems which make possible the collection from each of hundreds of unknown mass spectra per day (1). 

Fig. 8E.1 Scheme showing the basics of AEDA and Charm analysis. Different sequential dilutions of the sample extract are analyzed in the GC-O system. In AEDA the judge simply marks the retention times and the odor descriptions. The FD value is R , where p is the p-th dilution of the extract at which the odor was last detected and R is the dilution rate (3 in the figure). In Charm, he/she presses the space bar of the computer during the odor detection, and the outputs are combined to form the Charm chromatogram... 

Over the last few years, the coupling of a computer or computing facilities to GC-MS systems has become commonplace and indeed essential in... 

The primary role of the computer system monitoring the spectra from a GC-MS run is to assign mass values to each ion peak, subtract background peaks where necessary, correct for intensity bias and then print out or graphically present normalised spectra. These form the basis of an MS analysis. Other important uses of the computer in the manipulation of this data are discussed later. 

Commercial data systems are now available for all the principal GC-MS instruments although a number of independent centres have developed their own systems to suit individual requirements [44]. These range from low cost off-line systems to dedicated mini computers linked to time-shared central computers. Many of these systems are in advance of commercial packages but the different capabilities of their configurations are outside the scope of this article. (For a review of computerised data acquisition and handling see [45]). However it is useful to consider some of the hardware components and requirements of a small on-line computer system (Figure 1.8) for low resolution GC-MS work. 

The GC data system performs the tasks of recording, handling, evaluation, and documentation of the chromatogram. In a modem gas chromatographic system, these can be performed by means of a computer with specialized software. Nowadays, software for calculating the quantitative results and for the method validation is available. 

Each fraction was evaporated to dryness in 10-ml pear-shaped flasks, redissolved in a small amount of methylene chloride, and gas chromatographed. GC conditions were as follows column, 6 ft X 0.125 in. od stainless steel packed with 3% OV-17 on 100-120 mesh Gas-Chrom Q temperature program, 70-330°C at 12°/min, holding at the flnal temperature for up to 20 min and carrier gas flow rate, 28 ml/min. Fractions containing very little material or with GC patterns very much like adjoining fractions were combined. These fractions were then analyzed on the GC-MS computer system to obtain mass spectral data on the individual components. For certain fractions additional information was gained from ultraviolet spectra. 

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