Detector troubleshooting

The best practice in troubleshooting an interface is first to determine that the problem is in the interface. If upon connecting the GC column to an alternate detector, the problem is no longer evident, then it is likely an interface problem. Problems with capillary columns usually involve column plugging. This problem can be alleviated by breaking off a small section at the front of the column. Because plugging can be caused by a cold spot... 

However, those who have worked with smaller diameter columns have often experienced lower performance and other difficulties. This is primarily due to extra-column effects the bandspreading in the injector, detector and tubing, or the gradient delay volume of the instrument. Troubleshooting guidelines for sorting out the causes of these difficulties are available in Reference f. With proper care, 2-mm columns can be run on a standard modern HPLC instrument with few difficulties. Smaller i.d. columns require special instrumentation. 

Most manuals supplied by chromatograph manufacturers provide instructions for proper maintenance and troubleshooting. When checking the detector, it is advisable to use a sample that will not be adsorbed by the column, such as a hydrocarbon. For electron capture detectors a suitable test substance would be a halo-carbon, rather than a sensitive chlorinated pesticide. 

Set the flow rate of the HPLC system to 1 ml/min across a 5-p.m x 250-mm x 4.6-mm C18 polymeric support reversed-phase column or equivalent at ambient temperature. Set the detector at 520 nm. Inject 50 ul sample into the HPLC system and start a gradient similar to that outlined in Table FI. 3.2. Analyze data as described (see Data Anaylsis in Critical Parameters and Troubleshooting). 

It is imperative that the HPLC instrument, including the detector, is working correctly. The easiest way to check this is by first running a blank. If there is no response, one can move onto injecting the standards. If there is a response to the blank, the column may have been overloaded prior to this run. Refer to a troubleshooting guide for the specific HPLC system. The internet is also an invaluable source for troubleshooting (e.g., see Internet Resources). Keep in mind that the source of the problem may not be the system but may in fact be the column. 

Part II shows you how to make the best use of the common columns and how to keep them up and running. (Chapter 6 on column healing should pay for the book in itself) It discusses the various pieces of HPLC equipment, how they go together to form systems, and how to systematically troubleshoot system problems. We will take a look at the newest innovations and improvements in column technology and how to put these to work in your research. New detectors are emerging to make possible analysis of compounds and quantities that previously were not detectable. 

V. Inouye, H. Kanai, et al., Guide for troubleshooting operator problems in the Hall 700 and 700A electrolytic conductivity detectors, J. Chromatogr. Sci., 22 262-263 (1984). 

Rood, D. 1999b. Detectors. In, A Practical Guide to the Care, Maintenance, and Troubleshooting of Capillary Gas Chromatographic Systems, 3rd edn (D. Rood, ed.), pp. 156-211, Wiley-VCH, Weinheim. 

Since the response of the detector (and the separation) is a function of a flow rate, it is essential that the standard response curve be determined at the same flow rate as the tablet assay. If retention times differ significantly from the runs of the standards, there is a need to troubleshoot the HPLC to determine where the problem resides. Refer to Chapter 3 for a discussion of retention time precision. 

Many detectors track the number of hours the lamp is ignited. Although the lamp life may vary, the detector s meter reading can be a helpful guide for troubleshooting. Lamps can sometimes operate for more than 2000 h. To distinguish a lamp problem from air bubbles, one should stop the mobile phase flow. A lamp problem will persist when the flow is stopped, whereas, if the problem is due... 

Records also help prevent mistakes, such as introducing water into a silica column, or precipitating buffer in the system by adding too much organic solvent. Many analysts occasionally modify their HPLC systems for a variety of reasons. Reliable records are the best way to ensure that a modification does not introduce problems. For problems relating to pumps, detectors, automatic samplers, and data systems, instrument manuals provide suitable troubleshooting guides. 

For the case of radioisotope tracer experiments, nonintrusive methods are used to get the outlet concentration of tracer by utilizing collimated scintillation detectors. Radioisotope tracers have many advantages such as on-line detection, high detection sensibility, and availability in different compatible forms over conventional tracers.This method can also help in troubleshooting and checking the performance of industrial TBR under operational conditions. 

Infrared spectroscopy is widely used in industry. The applications range from quality control and quality assurance of raw materials to customer complaints (troubleshooting) to quantitative analysis and online process monitoring and control. Most of the instrumentation is based on a Fourier transform infrared (FTIR) spectrometer but specialist applications use instruments as diverse as a nondispersive infrared detector through to mid-infrared diode lasers. 

An invaluable tool for troubleshooting and maintenance is the instrument control chart. Anything useful can be control charted, such as detector response factors, deviation from expected retention time, etc., but the analysis results from a check gas is one of the most popular items to chart. The most important information a control chart can show is when the instrument needs maintenance or recalibration. While it may seem like a good idea to recalibrate every day, the act of recalibration has error associated with it, and thus overcalibrating can be as much of a problem as undercalibrating. Statistical tools can... 

While open tubular (OT) columns are the most popular type, both open tubular and packed columns are treated throughout, and their advantages, disadvantages, and applications are contrasted. In addition, special chapters are devoted to each type of column. Chapter 2 introduces the basic instrumentation and Chapter 7 elaborates on detectors. Other chapters cover stationary phases (Chapter 4), qualitative and quantitative analysis (Chapter 8), programmed temperature (Chapter 9), and troubleshooting (Chapter 11). Chapter 10 briefly covers the important special topics of GC-MS, derivatization, chiral analysis, headspace sampling, and solid phase microextraction (SPME) for GC analysis. 

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