By Ruth Lindley and Jeff Gunnell
With a range of technologies on the market for gas analysis, it can be a challenge to know which is best suited to your particular measurement requirement. This blog gives you a guide to choosing between two popular measurement techniques: gas chromatographs (GCs) and quantum cascade laser (QCL) analyzers, as with the recent acquisition of Cascade Technologies, Emerson can now offer QCL technology alongside its existing range of GCs. Both techniques offer excellent sensitivity and accuracy, but depending on application and measurement needs, one or the other will be preferred.
The purchase price of QCL and GC analyzers are similar and both offer multicomponent capability. The main way to choose between the two techniques is by the analysis required for the particular application. GCs are an excellent general purpose analyzer. They can measure liquid samples as well as gases and a wide range of molecules – both large and small. They can separate out complex mixtures and often measure the concentrations of isomers. In principle, dozens of components can be measured. QCL can typically measure up to 12 components per analyzer, but they all must be small gaseous molecules such as CO, ammonia, or hydrocarbons up to C4s. As such, for liquids and larger gaseous molecules, or for very large numbers of components in a stream, a GC is the correct choice.
Sometimes the speed of analysis is important in an application, and in that case, QCL has a clear advantage. In a QCL, the sample flows through a measurement cell where laser beams continuously analyze the the gas. The response time depends on how long it takes to flush the cell, typically <10 seconds to get to 90% of a step change, so the output is effectively continuous and real time. GCs on the other hand work on the principle of injection followed by analysis. Cycle times for a GC vary
from 1 minute to over 15 minutes, depending on the application, and thus the concentration data is periodic rather than continuous. For applications where fast, continuous measurements are required, QCL is therefore the preferred technique.In terms of sensitivity and dynamic range, QCL can offer better performance than GC. QCLs can measure down to low ppb concentrations for some compounds and offer a dynamic range from ppb through to percent concentrations in one analyzer by using multiple pathlengths or spectral lines of varying strengths. GCs are often used for measuring the entire composition of a sample and can measure down to ppm concentrations while also measuring the majority component up to 100%. However, to reach ppb measurements along with high percent level measurements in a GC usually requires separate injection and column trains, making for a more complex and expensive analyzer. So if high sensitivity is needed or there’s a wide dynamic range (for example for online purity measurement with the ability to follow process upsets), then QCL is the better option. If every component in the sample (including the background) needs to be measured, a GC may be more suited.
On cost of ownership, QCL is usually better than GC. GCs require a carrier gas, typically hydrogen, helium, or nitrogen. These are not needed by QCL analyzers, meaning that the cost of ownership, due to the use and management of consumables, is higher for GCs. QCLs inherently have a very stable calibration and they can often carry out validation and calibration checks in real time, using the process gases which are being measured. This is due to the way that spectra are obtained and analyzed, and consequently, checks using injected test gases may only be required every 12 months for QCLs. For GCs, it is more usual to carry out validation checks every few weeks. So if low cost of ownership is essential, then QCL is favorable. If the required measurements can be made by QCL this is likely to be the preferred choice due to lower cost of ownership and reduced maintenance requirements.
The table below gives a summary of the types of applications and measurements commonly encountered for gas analysis and the suitability of the GC and QCL technologies to each.
QCL and GC analyzers are both excellent options for industrial gas analysis and can be utilised across a range of applications and measurement points. The differences in the detection methods can make one or the other better suited to a particular application, and the table above is a guide to selecting the best fit to your application.
The Emerson sales team will be able to further assist in determining the best analytical method for your needs. Please click HERE for find out more information on QCL analyzers, and HERE for more information on gas chromatographs.