Technical Guide: Comparing SPR and QCM

Selecting the right lab equipment for your projects is always a challenge. With the number of techniques providing similar data, it can be difficult to determine which is most applicable to your work. In this respect, if your research involves measuring film thickness, swelling, fouling, deposition and/or self-assembly, surface plasmon resonance (SPR) and quartz crystal microbalance (QCM) are two techniques that you should consider.  

SPR and QCM have many features in common; both use sensors made from thin gold films to detect changes in mass, both are label-free and employ a flow system to transport solvent and sample across the sensing field. Despite these similarities, SPR and QCM measure your data in fundamentally different ways. SPR is an optical technique, detecting the change in mass based on the plasmonic principle; essentially a change in the way the sensor interacts with light. QCM is a mechanical technique, measuring the frequency of oscillations for a thin, gold-coated quartz crystal. 

The difference between the mechanical and optical sensing system in the instruments has far-reaching consequences for the type and quality of data each can give you. In this article, a few key differences between QCM and SPR are outlined so that you can choose the best technique for your projects.

Films which absorb solvent:

When working with films that absorb solvent, both dry weight and wet weight measurements are important, and there are some key differences in mechanical and optical measurement techniques to take into account. Since QCM weighs polymer films mechanically, any solvent absorbed into the film is also weighed and in this respect, QCM gives the “wet weight” measurement. Conversely, SPR measures films optically without “weighing” the solvent in the interfacial layers; this is the “dry weight” measurement. The difference between the two techniques can be used to determine the absorption properties of a polymeric film by measuring both wet and dry weight, under the same conditions. 

Deciding whether to focus on obtaining wet weight measurements, dry weight measurements, or both, is an important consideration when selecting an instrument most suitable for your projects. Pairing QCM and SPR can eliminate artifacts in data caused by water, and yield valuable insight into the absorption properties of your films.  

Experiments in a liquid phase environment:

Differences in the phase of the environment can impact the accuracy and precision of your results. For example, the Sauerbrey equation (which is used to obtain data in QCM experiments) is very accurate for vacuum and gas phase calculations; however, in a liquid environment, it becomes susceptible to response changes caused by viscoelastic effects. Although SPR is also prone to similar response changes, they are not as intense, and can largely be corrected for by using a reference channel.

This leads to another important difference; referencing. Referencing is easily done using SPR, but with QCM, having a second sensor on the same chip results in damping of oscillations and disrupts instrumental performance. It is, therefore, necessary to use a second, separate reference sensor for QCM, introducing an additional source of error. Furthermore, since QCM is a mechanical technique, susceptibility to external vibrations or mechanical noise is higher when compared to SPR.

Overall, while both SPR and QCM are susceptible to measurement noise, SPR is better equipped to handle liquid phase measurements, reference out much experimental noise, and handle mechanical noise.

Mass transfer effects & sensor surface nonuniformity: 

Mass transfer and sensor uniformity can be challenging error sources to resolve for SPR and QCM experiments. When comparing QCM and SPR, QCM tends to be more susceptible to these errors. This arises largely from the difference in the sensing area, QCM having a much larger sensing area than SPR. The result of this area difference is that 5-10 times more sample is needed and the sensing area will be less uniform. Additionally because of the larger volumes, and subsequently lower flow rates, mass transport effects are much more pronounced when looking at the binding of a dissolved biomolecule to the activated sensor in QCM. 

For highly sensitive or precious samples, or for ensuring your data is as accurate and repeatable as possible, limiting the sensing area and mass transfer effects can improve your experiments. 

Which technique is right for your project? 

Comparing the data quality of SPR and QCM, SPR is less sensitive to mechanical disruptions and viscoelastic effects and can be easily referenced using the same sensor. SPR’s smaller sensing area minimizes mass transfer effects while improving surface uniformity, providing you with the highest data quality for your research needs. If your polymer research involves measuring film thickness, swelling, fouling, deposition and/or self-assembly, SPR can be a great tool to help you obtain more repeatable and accurate data for your publications.

OpenSPR is a user-friendly and low maintenance benchtop SPR solution that is currently being used by hundreds of researchers. With access to SPR technology on your own lab bench, you can get the high-quality data you need to accelerate your research and publish faster.

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