Online Analysis and Offline Analysis

Mass spectrometry can be broadly categorised into two types based on the method of sample introduction: on-line and off-line. The difference between the two lies not merely in ‘how quickly measurements are taken’, but in whether certain inherently fleeting pieces of information can be captured.

Offline analysis is the traditional approach: samples are first collected and preserved, then taken to the laboratory for sample preparation, injection and measurement. Its advantages are flexibility and reproducibility—as the samples are stored, they can be re-analysed at any time, and different methods can be employed. The disadvantages are also evident: the time and handling involved between collection and measurement mean that the state of the sample may have changed (through volatilisation, oxidation or degradation); furthermore, this method is inherently incapable of capturing rapidly occurring processes, and can only provide a ‘snapshot at a specific point in time’.

Online analysis, on the other hand, links sample generation with analysis in real time, eliminating the need for manual transfer. As the reaction proceeds, the products are continuously monitored by mass spectrometry. Neo Source INSTRUMENT’s ProC-1 online time-of-flight mass spectrometer, ProC-1C in-situ catalytic/pyrolysis mass spectrometry analysis system, and ProC-1S flue gas multi-component dynamic analysis system all fall under the category of online analysis: The time-dependent changes in multicomponent composition during processes such as catalytic reactions, pyrolysis and combustion flue gas are recorded in real time. Thanks to rapid sampling at the second level (sampling frequency of 30,000 Hz), the online method offers very high temporal resolution, enabling the observation of transient intermediates (such as free radicals) and the kinetic processes of reactions—precisely those phenomena that ‘snapshot’ offline analysis cannot capture.

For spatial analyses such as mass spectrometry imaging, there is a third related concept—in situ, non-destructive sampling: data is collected point by point directly on tissue sections without altering the sample’s spatial position, and the section remains intact after imaging. It shares the same philosophy as online analysis: minimising disturbance and acquiring information in situ; however, whilst online analysis focuses on ‘continuity in the temporal dimension’, imaging focuses on ‘fidelity in the spatial dimension’. New Source’s MSI DPI and MSI LDPI are compatible with Agilent, AB SCIEX and Thermo mass spectrometers, enabling the upgrade of existing mass spectrometers into imaging platforms; the ProC series of online mass spectrometers achieves a mass resolution of 3,500 and a sensitivity of 10 ppb.