Knight Optical’s Gas Cells
12 Oct 2021
As the name implies, Gas Cells are used to hold a gas or a gas mixture – either for reference measurements or analysis purposes.
Gas Cells are commonly used in Fourier Transform Infrared (FTIR) spectroscopy and laser absorption spectroscopy (LAS). Both of these spectroscopic methods rely on different identifiable absorption peaks in gas molecules. For example, methane (CH4) has two distinctive absorption peaks at 1306cm-1 (7.66µm) and 3019cm-1 (3.31µm).
Here at Knight Optical, we regularly supply custom-made Gas Cells that are specifically manufactured to our customer's exact specifications. By tailoring our Gas Cells to your needs, we can guarantee they are fit for purpose upon receipt.
A Gas Cell's composition comprises two main optical components – the cylinder (tube) and the transmission windows. The tube is often made from a transparent and durable material – such as Pyrex, Borosilicate, Fused Quartz, or Soda Lime Glass. Still, it can be manufactured from other materials, including aluminium. The substrate for the transmission window is more critical and depends on the bandwidth being used. Typically, this is manufactured from materials that transmit into the infrared range, especially near-infrared (NIR), including Sodium Chloride (NaCl), Potassium Bromide (KBr), Calcium Fluoride (CaF2), Barium Fluoride (BaF2), or Zinc Selenide (ZnSe). At Knight Optical, we supply transmission windows separately and already stock a range of suitable windows. What's more, we also stock drilled Cell Windows that are designed for use with demountable cells – often used with liquid samples.
When we provide a complete Gas Cell, the transmission windows will be frit sealed to the cylinder to ensure a hermetic seal. A critical parameter here is the pathlength, where the length of the cell corresponds to the pathlength of the incident light through the gas mixture.
To ensure Knight Optical meets all requirements, we provide fixed short and long varieties, as well as variable, pathlength Gas Cells. The pathlength itself does depend on the gas mixture to be used. If the gas mixture to be analysed has a concentration level at parts per billion (ppb), then a longer pathlength would be preferable, so more molecules are within the light path. If the concentration levels are a percent of the gas mixture, then a short pathlength will be sufficient. Our variable pathlength Gas Cells utilise a folded design, where the incident beam is reflected off mirrors within the Gas Cell to increase its path (rather than travelling straight through).
An inlet is used to fill the cell with gas for analysis. However, this feature can also be an inlet/outlet design for free gas flow into and out of the cell. Customers often specify this in gas monitoring applications, for example, to measure levels of greenhouse gases. Here at Knight Optical, we can supply free flow Gas Cells as custom-made orders.
We pride ourselves on the quality of our products, and with an in-house state-of-the-art Metrology Laboratory, you can be assured that your components are fit for purpose.
We are committed to providing our customers with both high-quality products and excellent service. Contact our technical sales team to discover how Knight Optical’s high-quality Interference Bandpass Filters and superior service can improve your instrumentation and supply chain experience.
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