Process Gas Chromatographs. Tony Waters

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СКАЧАТЬ molecules emerge from the column and enter the detector, the detector outputs a signal proportional to the rate of arrival of those molecules.

The number of component molecules arriving at any instant starts at zero, rises to a maximum, and then falls to zero, creating the characteristic chromatogram peak. The ideal peak shape is symmetrical per Figure 1.7, but many imperfections in the instrument conspire to distort that perfection. Figure 1.8 shows some peaks on a real chromatogram.

      Real peaks are not perfect in shape. Note the gradual increase in peak width with elapsed time (left to right) and the asymmetric peak for acetylene.

Figure 1.8 A Real Chromatogram.

      Source: Yokogawa Electric Corporation. Reproduced with permission.

      The output signal of the detector is proportional to the instantaneous number of component molecules it is seeing. Therefore, the area under a chromatogram peak is a true measure of the total number of component molecules passing through the detector. As a compromise, we shall see the occasional use of peak height as a surrogate for peak area as it may be easier to measure.

To relate the measurement of peak area or peak height to the concentration of the analytes, the analyzer uses a calibration factor. In an early PGC, the calibration factor was simply the mechanical setting of an attenuator; usually a variable resistor. Modern instruments get the calibration factors from a calibration procedure that analyzes one or more standard samples containing a known concentration of each analyte. During calibration, the analyzer calculates and stores a calibration factor for each analyte based upon its peak area or height and its known concentration. Then, during process analysis, the analyzer measures the area or height of each analyte peak and multiplies that value by its stored calibration factor to deduce the analyte concentration.

      Chromatographers are so used to seeing peaks on the chromatogram that they even use the word “peak” to describe the molecules of a component as it travels through the column system. In colloquial chromatography, each cluster of molecules in the column is a peak even if those molecules never reach the detector and never appear as a real peak on the chromatogram. This shorthand terminology makes it easier to describe the movement of sample molecules in the column: it's far easier to say:

       “the column separates the propane peak from the ethane peak”

      than:

       “the column separates the cluster of propane molecules from the cluster of ethane molecules”

By the way, we drew most chromatograms in this book with rather wide peaks so you can easily see their shape. Real chromatogram peaks are often much narrower than these, and it can be difficult to see small variations in their topography. You might need to expand the time scale on a computer screen or run at a higher chart speed on a recorder to measure the peak width or see the exact trajectory of the peak.

      The chromatogram readout is a vital design and troubleshooting tool; so much so that it's difficult to overstate its usefulness. Discounting electrical failures that are easy to fix, all faults are visible on a chromatogram, either directly or by comparing the current chromatogram with a previous one.

       All chromatographic faults are visible on the chromatogram. So, to become an expert troubleshooter, you must learn to read the chromatogram!

      Some faults are directly observable, such as a timing error or a missing peak. But a chromatogram holds a lot more information than that; the position and shape of the peaks tell us what they are, and whether the columns are working at full efficiency.

      As an example of chromatogram reading skill, review the chromatograms in Figure 1.7. The peaks are nicely separate from each other, and it looks like a good analysis. But an expert observer would know that the column is not working well. By adjusting a few settings, an expert could improve the performance of the column and do the analysis in half the time!

      The expert observer would also notice that the author has misidentified the last peak; it can't be isobutane on any column. It's probably n‐butane instead. Expert users see information in the chromatogram that eludes novice users. That's why they are experts.

      Keep on reading! The next three chapters will show you how to discern patterns in the shape and position of chromatogram peaks. From those patterns, you will make the same deductions about Figure 1.7 as our expert observer did.

      A chromatogram contains all the information needed to optimize column performance or to diagnose chromatographic problems. If you aspire to be an expert PGC applications engineer or troubleshooter, learn to read the chromatogram. Waters (2017) has summarized these diagnostic skills in a compact format.

      Knowledge Gained

       Chromatography is a general method of separation and includes many different practical techniques.

       Chromatography uses a fluid mobile phase passing over a liquid or solid stationary phase.

       In gas chromatography, the mobile phase is gas; in liquid chromatography, the mobile phase is liquid.

       Chromatographic analyzers separate the desired analytes and then measure them one by one.

       Other analyzers attempt to measure the analyte molecules in the presence of other molecules.

       Gas chromatographs inject a tiny volume of sample into the flowing carrier gas.

       The sample must be a gas or a volatile liquid that quickly vaporizes and enters the column as a vapor.

       In the laboratory, sample injection is by glass syringe, either manual or by an autosampler.

       Process gas chromatographs (PGC) use an automatic injection valve to inject the sample.

       The carrier gas carries the sample into the column where it contacts the stationary phase.

       The stationary phase may be a solid adsorbent or an immobilized non‐volatile liquid.

       Contact with the stationary phase delays some peaks more than others, so separation occurs.

       Special routing valves may direct the peaks into different columns to finish the desired separation.

       PGCs can now use either packed columns or capillary (open‐tubular) columns.

       The separation process takes time; typically one to ten minutes, sometimes longer.

       The carrier gas elutes peaks from the column into a chosen detector for measurement.

       The detector responds to a property of analyte molecules that differs from carrier gas molecules.

       The СКАЧАТЬ