I recently received a question relating to the saturation of GC detectors. The GC user was unsure of the causes of saturation, what effect saturation might have on analytical results, how to know when it is happening, and how to avoid it.
Each style of GC detector will detect compounds based on some physical or chemical characteristic of the sample. In flame ionization detectors (FID), organic compounds are burned in a H2/air flame and form ions that are collected on a polarized electrode (the “collector”). Other detectors respond in different ways, but the response always ends up as an amplified electrical signal (which then is usually digitized and stored).
Figure 1: When an FID is operating correctly, sample components are quantitatively combusted in the flame, creating positive ions and electrons. The positive ions are attracted to the negativelybiased collector while the (negatively charged) electrons are repelled toward the jet.
Every detector is designed for a specific intended use paradigm that involves all setpoint variables and physical design characteristics. Take the FID, for example. The primary source of signal is the efficient combustion of column effluent in a hydrogen flame, as illustrated in figure 1. Column effluent is directed to the flame and mixed with H2 as it passes through a small orifice (the “jet”). In order to yield the theoretically 6-7 orders of magnitude response that are possible with an FID, all the physical aspects of the detector must work together. So, the dimensions of the detector, the orifice size, dimensions and position of the jet, the temperature profile, the flow rates of gases, electrical and electronics design, etc. - all these parameters are designed and specified for a “typical” use paradigm. Saturation occurs when some of the actual parameters of use fall outside the normal operating range.
A detector becomes “saturated” if its increase in signal as a function of increasing amount of analyte is less than expected. Figures 2 and 3 illustrate the roll off in response at the high end of a calibration curve as the detector saturates. Each detector will have its own root cause(s) for saturation depending on the underlying chemical and physical phenomena relating to its response. In the case of an FID, saturation usually results from the flame becoming “starved”; it is no longer able to completely combust the large amount of analyte passing through it. This lack of complete combustion can be influenced by many variables such as limited fuel (H2) or oxidant (air), not enough pre-heat, inefficient mixing, etc. An extreme indication of poor mixing and insufficient combustion is when the flame self-extinguishes when the solvent peak elutes. In many cases, increasing air and/or fuel flows and increasing detector temperature can yield another factor of 5-10 of proportional detector response at the high end of the calibration curve; however these changes would at the same time make LOD worse (increasing noise).
Figure 2: Typical FID calibration curve in log(response) vs. log(amount) showing detection limit and the onset of saturation at high concentrations.
Poor mixing of column effluent with the H2 fuel prior to entering the flame can happen if the jet dimensions are not well matched to the column flow (a bigger orifice is needed for higher column flows, especially if using packed columns). In this case, a change in jet orifice size can help. Using too big of a jet orifice when unnecessary, on the other hand, degrades LOD if used with lower flow columns.
ECDs saturate because of a combination of quenching and charging effects. By diluting the effluent (adding more makeup gas), the saturation point can be raised (again with worse LOD).
NPDs saturate because the plasma at the surface of the bead is stripped of essential Rb ions (it takes time for more to diffuse to the surface from the bulk of the bead).
A TCD might saturate because the concentration of analyte passing through it exceeds the boundaries for which the electronics were designed. And so on … each detector with its own characteristic manifestations of overload.
Figure 3: Alternate view of calibration curve in Response Factor format.
In addition to saturation from what happens within the detector, one can also get saturation of the detector electronics. The amplifiers are designed for a given range of signal. If the signal exceeds that, the signal is limited (“clamped”) to the board’s maximum output. Some designs require that the user set the “range” of the board. For example, the board might be designed for 4 orders of magnitude response, but by setting the range you are able to position the range to low, medium, or high signal levels depending on your needs. If you set it to the most sensitive setting (low signal levels) and a big peak comes through, it signal will “flat top” at the maximum value of the electronics.
You can easily tell if the saturation is electronic vs. physical/chemical processes in the detector. With electronic saturation, the top of the peak is flat (the numerical value of the signal is constant across the top of the peak) with a very abrupt transition to the flat top at the edges. With chemical/physical saturation, the top of the peak is distorted and noisy and has round more gradual transitions at the edges.
The bottom line is that if you need to analyze high concentrations of components, you have to put less sample into the GC (inject less, split more, dilute the sample) and/or adjust the detector parameters to accommodate the higher load (more flame gases, higher temperature, dilute with more makeup gas, change the jet size, etc.). If you do not, the concentration of the saturated analyte will be under reported and the detector may become contaminated, affecting results for other analytes and requiring maintenance or repair.
This blog article series is produced in collaboration with Dr Matthew S. Klee, internationally recognized for contributions to the theory and practice of gas chromatography. His experience in chemical, pharmaceutical and instrument companies spans over 30 years. During this time, Dr Klee’s work has focused on elucidation and practical demonstration of the many processes involved with GC analysis, with the ultimate goal of improving the ease of use of GC systems, ruggedness of methods and overall quality of results. If you have any questions about this article send them to firstname.lastname@example.org