Critical instrument design

Practical FTIR solutions have been developed by paying attention to the fundamental design of the instrument. Moving an FTIR instrument out of the benign enviromnent of a laboratory to the more alien environment of either a process line or that of a portable device is not straightforward. A major emphasis on the instrument design in terms of both ruggedness and fundamental reliability of components is critical. Furthermore, issues such as enviromnental contamination, humidity, vibration and temperature are factors... 

Dow s Critical Instruments Program describes identification of critical instruments, and provides guidelines for their design, installation, testing, maintenance, and documentation. 

Identification of critical instruments is the responsibility of the owner of the process and usually is a part of process research and design. In addition to recognizing critical instruments, the operation must be reviewed to make certain that instrumentation is adequate to prevent a serious incident. 

Once a critical instrument loop is identified, a procedure for testing the entire loop must be written. The test procedure will influence the design of the new system, since, if possible, the test should be an actual performance test. For example, if a high temperature should close a valve, the ideal test would consist of raising the temperature to see if the value closes. Efforts should be made to avoid test procedures which require temporary wiring disconnects, valve closures, and so on, which might not be returned properly to operating condition. 

As described above, performance testing is an important consideration in the design of a critical instrument loop. Components of the system must be selected for ease of testing, as well as for their ruggedness and reliability. 

Conditional on satisfactory on-site inspection, assembly, installation, SAT, critical instrument calibration, and design qualification, the computer system is available for the in situ qualification phases. 

Critical instruments assigned a Class 1 include those necessary to avoid a failure which may cause the perils listed above or instruments which fail to inform of upset conditions which may result in perils. Testing of these instrument systems may be mandated by regulatory agencies, in-house technical safety review committees, HAZOP studies, or designated as critical by operations supervisors. All of these shutdown systems and alarms must be prooftested in accordance with a proper schedule. [8]... 

In most ESI interfaces, the position of the spray needle is relatively fixed, while in some systems, the needle position can be optimized. Researchers do not agree on the importance of optimization of the spray needle position. Some instrument designs are more serrsitive to the needle position than others. The needle position becomes more critical at lower flow-rates. 

The dynamic range of a mass spectrometer is defined as the range over which a linear response is observed for an analyte as a function of analyte concentration. It is a critical instrument performance parameter, particularly for quantitative applications, because it defines the concentration range over which analytes can be determined without sample dilution or preconcentration, which effects the accuracy and precision of an analytical method. Dynamic range is limited by physiochemical processes, such as sample preparation and ionization, and instrumental design, such as the type of mass analyzer used and the ion detection scheme. 

The Installation Qualification (IQ) verifies that the equipment or system and/or corresponding utilities are installed in accordance with design specifications, manufacturer recommendations, and cGMPs. In addition, the IQ will confirm that critical instruments are calibrated and that system components are properly identified. Any exceptions will be documented, corrected, and/or justified. 

It is evident that the quality of communication within a project is critical. The work is a team effort. One discipline s output is another discipline s input. The information transfer has to be co-ordinated in time and co-ordinated in content (to ensure that it is appropriate and comprehensive), and there has to be an effective feedback loop to ensure that information is understood and that perceived problems and conflicts are properly handled. Outputs of one discipline often affect the work, completed or still to be done, of other disciplines, in ways not identified or planned as prime input information. (For example, consequences of instrumentation design, see Section 18.5) At management level, a balance constantly has to be struck between the often-conflicting demands of time, cost, quality, and scope for correct compromises, comprehensive information is essential. The larger the project or the more ambitious the time schedule, the greater the need for quick and appropriate communication. 

---