Calibration, 296 data system

As mentioned, new types of homemade explosives or alternates to common nitrated explosives have appeared in several terror scenes and pose a threat to public safety (see Frame 12.3). To counteract these threats, instrumentation has to be calibrated, data systems have to be revised, and some modifications in operational parameters may be necessary. Some of the major manufactnrers of explosive detectors have characterized operational conditions for the detection of several alternative explosives withont severe deterioration of performance for the standard explosives. In one example, the temperature of the drift tube was lowered to 169°C, and the desorber was set to 220°C. In addition, the instrument was operated in dual mode (i.e., measurement of the negative and positive mobility spectra almost simultaneously). With these modifications, operational detection limits (ODLs), defined by the authors as the mass of target substance that was required to produce an alarm at a given detection threshold, of 50 to 100 ng were established for TATP, ammonium nitrate, and gunpowder. These detection limits have been improved with the cnrrent generation of explosive detectors, especially for TATP. Under these conditions, the response and detection limit for TNT is degraded by a factor of two, but those for RDX and NG are barely affected the detection limit for PETN was improved. ... 

Documentation of area scanned, top view, side view and all calibration data of ultrasonic instrument and system (comes in a standard 3 page report form)... 

Calibration data transfer from ultrasonic instrument to the system via RS 232... 

Because an increase in resolution causes a decrease in sensitivity, it is best to operate at the lowest resolution commensurate with good results. Some instrument data systems will allow calibration with an external reference material such as perfluorokerosene and then use of a secondary reference material for the internal mass reference. Tetraiodothiophene, vaporized using the solids probe inlet, is recommended as the secondary reference. The accurate masses are 79.9721, 127.9045, 162.9045, 206.8765, 253.8090, 293.7950, 333.7810, 460.6855, and 587.5900. For a higher mass standard, use hexaiodobenzene. Because the mass defect for these internal reference ions are so large, a resolution of 2000 is ample to separate these ions from almost any sample ions encountered in GC/MS. 

GC/MS. GC/MS is used for separation and quantification of the herbicides. Data acquisition is effected with a data system that provides complete instrument control of the mass spectrometer. The instrument is tuned and mass calibrated in the El mode. Typically, four ions are monitored for each analyte (two ions for each herbicide and two ions for the deuterated analog). If there are interferences with the quantification ion, the confirmation ion may be used for quantification purposes. The typical quantification and confirmation ions for the analytes are shown in Table 4. Alternative ions may be used if they provide better data. 

Lacking SOPs on maintenance, calibration, inspection, or testing of data systems or part of systems... 

Instead of the addition of the 1-vector the calibration data may be centered (y — y and x, —3c, respectively). Even if the spectra of the pure species cannot be measured directly then the A-matrix can be estimated indirectly from the spectra provided that all components of the analytical system are known ... 

Equipment must be suitable, maintained and, where appropriate, calibrated. Computer systems used to generate, store and retrieve data should be of appropriate design and capacity, validated and suitably located. If there are computerized systems used to control environmental factors, then these also require the same consideration. Issues relating to computerized systems are covered in Section 9.2.1.8. 

Mass Calibration (time-of-flight) A means of determining m/z values from their times of detection relative to initiation of acquisition of a mass spectrum. Most commonly this is accomplished using a computer-based data system and a calibration file obtained from a mass spectrum of a compound that produces ions whose m/z values are known. 

Beyond simple data storage and instrument control, modern data systems provide extensive data analysis capabilities, including fitted baselines, peak start and stop tic marks, named components, retention times, timed events and baseline subtraction. Further, they provide advanced capabilities, such as multiple calibration techniques, user-customizable information and reports and collation of multiple reports. If a Laboratory Information Management System (LIMS) is available, the chromatographic data system should be able to directly transfer data files and reports to the LIMS without user intervention. The chapter by McDowall provides a terse but thorough description of the... 

Any mass spectrometer requires mass calibration before use. However, the procedures to perform it properly and the number of calibration points needed may largely differ between different types of mass analyzers. Typically, several peaks of well-known m/z values evenly distributed over the mass range of interest are necessary. These are supplied from a well-known mass calibration compound or mass reference compound. Calibration is then performed by recording a mass spectrum of the calibration compound and subsequent correlation of experimental m/z values to the mass reference list. Usually, this conversion of the mass reference list to a calibration is accomplished by the mass spectrometer s data system. Thereby, the mass spectrum is recalibrated by interpolation of the m/z scale between the assigned calibration peaks to obtain the best match. The mass calibration obtained may then be stored in a calibration file and used for future measurements without the presence of a calibration compound. This procedure is termed external mass calibration. 

As shown in the Level column of Figure 10, the various injected volumes can be designated as individual component calibration levels. By doing this, experiments 10-27 can also be used to help validate multilevel, multi-replicate calibration algorithms on a chromatographic data system. 

Experimental Techniques. Chromatography was performed on a Varian model 5060 HPLC equipped with a RI-3 refractive index detector. A Vista Plus Gel Permeation Chromatography (GPC) data system was used consisting of a Vista 401 chromatography data system serially connected to an Apple II microcomputer. The Vista 401 performs data acquisition and allows data storage and automations capability while all SEC data processing is performed on the Apple II by means of user-interactive GPC software for automated, on-line calibration and polymer analysis. 

The size exclusion chromatography for this study was done in the routine manner execept for the inclusion of an online viscosity detector called a Differential Viscometer <3> (Viscotek Corp., Porter, Texas, USAl. This instrument together with an RI concentration detector permits the calculation of intrinsic viscosities across the chromatogram. An IBM PC data system with software is also provided (5). The software acquires data from both detectors, and performs calculations of intrinsic viscosity and molecular weight distributions using the Universal Calibration Method. 

In the absence of a calibrated standard, the radioactivity in an argon-37 sample, supplied by Oak Ridge National Laboratory, has been approximated, using calibration data foar tritium and a pulse height analyzer to observe the peak. In the counter set up, a long line connects the counter to the fill system. The volume of the line is about 310 cm.3,... 

Proton NMR spectra of coal derivatives generally give rise to either broad peaks or complicated multiplets which can be easily divided into band envelopes. For example, the H1 spectrum of a coal-hydrogenation asphaltene (4) consists of three peaks, two of which overlap. A broad peak at lowest field is caused by protons in aromatic and phenolic systems, whereas two higher field peaks are caused by protons bonded to carbons situated o to aromatic rings and those bonded to other nonaromatic carbons, respectively. The ratios of these spectral areas are the same as the ratios of the hydrogens in each of these three hydrogen classes. This accounts for one of the most important characteristics of proton NMR spectra—namely, no calibration data are necessary. 

The output of electronic integrators can be readily fed into a computer system. Here the raw area data can be compared to calibration data, calculations made, and complete analysis printed out. An alternate is to feed the detector output voltage directly into a computer using the proper interface. The capability of the computer can be used to integrate and calculate results. 

Most HPLC instruments include computer data systems which automatically plot peak response versus concentration to generate response factors however, if using an older system, the standard curve can be plotted manually. The calibration curves should be linear with a correlation coefficient of >0.98 and intersect very near the origin. 

This means for improvement concerns the experimental procedures that are used to collect and analyze the calibration samples. In PAC, sample collection can involve either a highly automated sampling system, or a manual sampling process that requires manual sample extraction, preparation, and introduction. Even for an automated data collection system, errors due to fast process dynamics, analyzer sampling system dynamics, non-representative sample extraction, or sample instability can contribute large errors to the calibration data. For manual data collection, there are even more error sources to be considered, such as non-reproducibility of sample preparation and sample introduction to the analyzer. 

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