Since process chromatographs can independently analyze the quantities of multiple components in a chemical sample, these instruments are inherently multi-variable. A single analog output signal (e.g. 4-20 mA) would only be able to transmit information about the concentration of any one component (any one peak) in the chromatogram. This is perfectly adequate if only one component concentration is worth knowing about in the process, but some form of multi-channel digital (or multiple analog outputs) transmission is necessary to make full use of a chromatograph’s ability.
All modern chromatographs are “smart” instruments, containing one or more digital computers executing the calculations necessary to derive precise measurements from chromatogram data. The computational power of modern chromatographs may be used to further analyze the process sample, beyond simple determinations of concentration or quantity. Examples of more abstract analyses include the approximate octane value of gasoline (based on the relative concentrations of several components), or the heating value of natural gas (based on the relative concentrations of methane, ethane, propane, butane, carbon dioxide, helium, etc. in a sample of natural gas).
It should be noted that although gas chromatography (GC) is far more prevalent in online industrial process analysis than liquid chromatography (LC), this does not mean the GC technique is limited to the analysis of process fluids existing in the gas phase alone. Gas chromatographs are often used to analyze the composition of liquid process samples, by first boiling that liquid sample within the analyzer so it may be analyzed in gaseous form. This means many of the components within the GC must be operated at temperatures exceeding the boiling point of the lowest-boiling point substance in the sample. While this poses certain technical challenges, it is nevertheless common practice in many industries.
The following photograph shows a gas chromatograph (GC) Precisely fulfilling this purpose – the determination of heating value for natural gas:
This particular GC is used by a natural gas distribution company as part of its pricing system. The heating value of the natural gas is used as data to calculate the selling price of the natural gas (dollars per standard cubic foot), so the customers pay only for the actual benefit of the gas (i.e. its ability to function as a fuel) and not just volumetric or mass quantity. No chromatograph can directly measure the heating value of natural gas, but the analytical process of chromatography can determine the relative concentrations of compounds within the natural gas. A computer, taking those concentration measurements and multiplying each one by the respective heating value of each compound, derives the gross heating value of the natural gas.
Although the column cannot be seen in this photograph of the GC, several high-pressure steel “bottles” may be seen in the background holding carrier gas used to wash the natural gas sample through the column.
A typical gas chromatograph column appears in the next photograph. It is nothing more than a stainless-steel tube packed with an inert, porous filling material:
This particular GC column is 28 feet long, with an outside diameter of only 1/8 inch (the tube’s inside diameter is even less than that). Column geometry and packing material vary greatly with the application. The many choices intrinsic to column design are best left to specialists in the field of chromatography, not the average technician or even the average process engineer.
Instrumentation Basics 