Detecting molecules at ppb or ppt levels is like finding a needle in a haystack. However, this is exactly the level of sensitivity required in a variety of gas monitoring applications, such as contaminant detection in gases used in semiconductor production, or measuring hazardous air pollutants and emissions in environmental health and safety applications. Traditionally, methods such as gas chromatography (GC) and GC mass spectrometry (GC-MS) have been used for this purpose but, as these techniques cannot meet the throughput requirements for inline monitoring, other options should be considered in those settings. Fourier transform infrared (FTIR) spectroscopy offers a higher speed alternative but, until recently, these instruments could not provide the sensitivity necessary for many applications. Optically enhanced Fourier transform infrared (OE-FTIR) spectroscopy can help overcome these issues, providing real-time actionable results and detection limits in the ppb to ppt range.
Identified by the spectrum
FTIR spectroscopy is a robust analytical method that allows both qualitative and quantitative gas measurements. The backbone of this method is the telltale spectrum – a collection of peaks at specific energies – that each molecule generates when exposed to infrared (IR) radiation. These spectra are unique to each compound and can therefore be used to identify the species that are present in the gas mixture. Additionally, since the intensity of the peaks is proportional to the amount of a specific compound present, it is also possible to determine concentrations.
Fast and dynamic, High sensitivity
One of FTIR’s main advantages is the speed; gas is continuously probed with an IR beam as it flows though the spectrometer, allowing real-time measurements. In addition, FTIR analyzers do not need calibration, and do not require expert knowledge to use. This is a different story from techniques like GC and GC-MS, where a sample must be collected and sent to a remote laboratory to be analyzed by a specialist. Another boon of FTIR technology is its wide dynamic range, allowing it to handle large variations in compositions and concentrations, from high percentages all the way down to ppb.
As mentioned earlier, FTIR spectroscopy did not previously offer the sensitivity required for use in the semiconductor or environmental monitoring applications, which is why the use of this technology is not widespread in these areas. However, there are gas analyzers that have been developed specifically with these applications in mind, using optical enhancement (OE) technology to dramatically improve the analyzer’s signal-to-noise ratio (SNR). This allows the instruments to detect some compounds at concentrations down to 100 ppt, offering all the benefits of FTIR with detection limits that rival GC-MS.
Summary
High-end OE-FTIR gas analyzers are easy to set up and can be fully operational within a day of installation. These systems come fully calibrated, do not require specialized technicians or chemists to operate, and can generate high quality analytical data from the outset. Many come with integrated software that enables automatic data collection, analysis and reporting, with algorithms that use regression analysis – such as the least squares method – to allow for easy identification and quantification of each compound in the sample.
Many applications – such as environmental monitoring or control of gas purity in semiconductor production – rely on ultra-sensitive gas analysis. GC and GC-MS used to be the only candidates for this job, as they were the only solutions that offered sufficient sensitivity. However, recent developments in FTIR technology have put FTIR analyzers on the list of top candidates. These systems can provide equal sensitivity to GC or GC-MS, but offer significant benefits for inline gas monitoring applications, including higher throughput and greater ease of use.
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