What is the key difference between online and offline mass spectrometers?

The difference lies not in the ‘mass analyser’ itself, but in ‘when and how the sample is introduced’. Offline mass spectrometry involves first collecting the sample, followed by pre-treatment and chromatographic separation before it is fed into the mass spectrometer; the result is a ‘snapshot’ at a specific point in time, and the process often takes hours. Online mass spectrometry, on the other hand, connects the mass spectrometer directly to the reaction or process site, measuring as events occur, yielding a ‘dynamic curve’ that changes continuously over time.

Neo Source INSTRUMENT’s ProC series of online time-of-flight mass spectrometers follows this online approach: utilising photoionisation and soft ionisation, they produce virtually no fragments, thereby enabling real-time analysis of complex gases without the need for chromatographic separation; With a sampling frequency of up to 30,000 Hz, it can obtain a full spectrum within seconds and further provide dynamic time-dependent curves for each component, making it suitable for capturing reaction transients, free radicals and unstable intermediates.

The value of online mode lies in three aspects: firstly, it preserves the time dimension, allowing one to observe ‘what is formed first and what is formed later’; secondly, it is chromatography-free and real-time, making it suitable for process monitoring and process iteration; thirdly, it can detect extremely short-lived unstable species, which is difficult to achieve with offline sampling. The trade-off is that online systems place higher demands on interfaces, atmosphere control and interference resistance, whilst quantification still requires the use of internal standards and calibration.

Taking publicly available case studies from the official website as examples: in methane oxidation coupling (OCM), reactive intermediates such as methyl radicals, peroxymethyl radicals and formaldehyde can be captured in situ; in methanol-to-olefins (MTH), the dynamic changes in products such as formaldehyde (a carbon deposition intermediate) in response to the catalyst can be observed. This type of ‘measure whilst reacting’ information is almost impossible to fully capture through offline sampling; this is precisely where online mass spectrometry is irreplaceable.

In summary: offline mass spectrometry answers the question ‘what is in the sample?’, whilst online mass spectrometry also answers ‘how does it change over time?’.