High-sensitivity LCMS requires high-purity nitrogen carrier gas

To help instrument manufacturer Shimadzu select the right source of nitrogen for its liquid chromatography-mass spectrometry (LC-MS) analysis, BOC conducted a study to analyse nitrogen interference from cylinder, liquid bulk vessel and generator nitrogen sources. This was the first study of the effects of nitrogen source purity on advanced LC-MS equipment.

LC-MS requires nitrogen for drying, auxiliary and nebulising gas for analysis. Depending on equipment model and customer requirements, labs may employ nitrogen with varying purity levels from liquid bulk vessels, cylinders or in-house gas generators. As LC-MS equipment becomes increasingly sensitive with each new model, such as Shimadzu’s LCMS-8060, it is essential to select the best nitrogen source to ensure optimal results.

“As sensitivity increases with each new LC-MS model, any impurity we discover in the background — such as water, solvents or polymers — will have an impact on our results,” said Niron Van, business development manager, Shimadzu Australasia. “When using our in-house nitrogen generator with LC-MS 8060, we were seeing signal suppression — which reduces the sensitivity of results.

“BOC suggested we run these trials with their ultrahigh-purity (UHP), high-purity (HP) and liquid nitrogen products to eliminate the signal noise. Taking into account cost and equipment sensitivity levels, we found that liquid nitrogen from a bulk vessel was the best fit for our LC-MS 8060, as opposed to our in-house generator. We’ve since made the switch.” 

LC-MS is a powerful technique used to detect specific substances with very high sensitivity and selectivity. It is based on the analysis of ions that move through a series of vacuum stages until they reach the quadrupole, which separates the ions. The ions then travel to the detector where they are recorded as signals. When nitrogen with high levels of impurities is used as the carrier gas, it interferes with signal accuracy and reduces the sensitivity of the LC-MS process.

Hydrocarbon (methane), moisture and oil substances were the impurities of particular concern for Shimadzu. After isolating other potential causes of interference, Shimadzu partnered with BOC to examine other nitrogen gas sources to test for ion suppression, baseline disturbance and damaged o-rings in the calibrant delivery system (CDS).

Using standard water and methanol samples, BOC and Shimadzu compared nitrogen purity based on total positive ions counts (higher positive ion counts = higher total impurity) in four different nitrogen sources: UHP and HP from cylinders; liquid nitrogen from a bulk vessel; and Shimadzu’s on-site generator.

Overall, the highest nitrogen purity came from the UHP nitrogen cylinder. Across all tests, the in-house generator had the highest levels of nitrogen impurity. Therefore, high-sensitivity equipment like the LCMS-8060 should use higher grades of nitrogen (UHP or HP from a cylinder, or liquid nitrogen from a bulk vessel) for more precise results.

As a result of the study, Shimadzu switched its carrier gas source for the LCMS-8060 from its in-house generator to liquid nitrogen from a bulk vessel, which has allowed the company to maximise the capabilities and sensitivity levels of the LCMS-8060.

“The switch to liquid nitrogen has shown a significant improvement and we’re quite happy with the results,” said Van. “The background is much cleaner, with an improved signal-to-noise ratio.

“This study really opened our eyes as to how much nitrogen source purity can interfere with our signals. We realised we have to increase our source purity as we develop newer equipment with higher sensitivity. In the future, we’ll need to switch to ultrahigh-purity nitrogen sources, and now we know where to go.”

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