Totally serious analysis of N2O – capillary column selection

A few years ago, Jaap wrote a blog, “Can I analyze NO, N₂O and NO₂ via GC?”, which received a vast response. If you are just starting to dive into the analysis of N₂O, scroll through the blog comments where you can find many valuable insights from our blog readers. What I would like to show now is the selectivity of capillary columns capable of separating N₂O from permanent gases.

While N₂O is considered a greenhouse gas, it has a variety of beneficial uses in multiple industries. We all know it as “laughing gas”, thus the analysis is commonly done in medical and forensic labs. N₂O is soluble in fat and inhibits bacterial growth, which makes it an ideal propellant in cans of whipped cream. It’s also a powerful oxidizer, yet stable at room temperature, so it powers our rockets into space. Analysis of this light gas is often performed in the ppb concentrations in atmospheric samples. So, all the mentioned characteristics make it an easy gas to analyze using gas chromatography. Separation of N₂O from permanent gases at room temperature can be attained using almost any adsorbent column. The large surface area of those materials offers enough retention for the light analytes to separate without using sub-ambient temperatures. Analysis times will be short, and any injection errors (e.g. system dead volume or slow sample transfer on column) will lead to peak broadening, affect resolution between the components, and raised system detection limits.

Rt Q BOND column is often a number one choice for this analysis. CO₂ (3) and N₂O (4) will be separated at 40°C with enough resolution that even overloading a column (observed as peak tailing) will not interfere with peak integration (blue chromatogram overlay).

Column: Rt-Q-BOND 30m x 0.53mm x 20µm (cat.# 19742); Sample: 0.1-0.5% permanent gas mix with nitrous oxide and C2 hydrocarbons (black overlay); Injection: Inj. Vol.: 200µl, split ratio 10:1; Oven Temp: 40°C Isothermal; Carrier Gas: He, constant flow 5.1 ml/min; Detector: TCD; Notes: Blue chromatogram overlay is 200µl injection of 80 vol% carbon dioxide and 20 vol% nitrous oxide. Overlays are not in the same y scale (blue is compressed).

Although GC-µECD[1] or GC-MS[2] detection are generally preferred over the TCD, TCD is an easy to use universal detector that can be used when N₂O concentration is above 50 ppm.  The advantage of GC-µECD is obvious; the ability of N₂O to absorb emitted high energy electrons increases sensitivity for this compound down to ppb detection limits.

Alumina columns will absorb CO₂, and are extremely sensitive to water.  Absorbed CO₂ and water will effect column loading capacity over time.  However, these can be easily removed by conditioning the column at its maximum temperature.

Column: Rt-Alumina BOND/Na₂SO₄ 30m x 0.53mm x 10µm (cat.# 19755); Sample: 0.1-0.5% permanent gas mix with nitrous oxide and C2 hydrocarbons (black overlay); Injection: Inj. Vol.: 100µl, split ratio 10:1; Oven Temp: 40°C Isothermal; Carrier Gas: He, constant flow 4.0 ml/min; Detector: TCD

N₂O performance on Molecular Sieve 5A column was unanticipated.  The small, polar molecules of N₂O fit well in the pores of 5A zeolite and offer excellent retention for nitrous oxide.  Notice that the temperature of the analysis is 200°C (Isothermal).  This is the maximum temperature where carbon monoxide is still separated from methane, and oxygen and nitrogen are baseline resolved under the analysis conditions. And, the unpredictable part, at that temperature N₂O requires almost 6 minutes to elute from the column.

Column: Rt-Msieve 5A 30m x 0.53mm x 50µm (cat.#19723); Sample: 0.1-0.5% permanent gas mix with nitrous oxide; Injection: Inj. Vol.: 200µl, split ratio 10:1; Oven Temp: 200°C Isothermal; Carrier Gas: He, constant flow 4 ml/min; Detector: TCD

ShinCarbon columns are not available as capillary columns, however, I wouldn’t be able to complete the overview without showing this analysis using porous carbon material.  The analysis was performed at 165°C, although starting with a lower oven temperature is an option if oxygen and nitrogen must be separated.  ShinCarbon columns are available with 0.53mm ID and can be used with capillary inlets.  The micro-packed option requires Micropacked Inlet Conversion Kits that quickly convert your capillary inlet to an inlet for micropacked columns.  Or, micropacked columns can also simply be installed using “pigtails”.

Column: ShinCarbon ST 2m x 1.0mm 100/120 mesh (cat.# 19808); Sample: 0.1-0.5% permanent gas mix with nitrous oxide and C2 hydrocarbons; Injection: Inj. Vol.: 200µl, split ratio 10:1, Oven Temp: 165°C Isothermal; Carrier Gas: He, constant flow 15 ml/min; Detector: TCD; Notes: Resolution between CO2 and N₂O is great enough that overloading the column will not affect quantification of nitrous oxide.

In summary, N₂O will separate well from all the permanent gases using any of the above mentioned columns.  The concentrations in most of the samples we are analyzing are very low and require very sensitive detectors and large injection volumes. Next time I will focus on exploring limits of detection using a capillary column and a µECD (required injection volume, and split ratios used).

Literature: [1] https://doi.org/10.1016/j.trac.2013.11.004 [2] https://doi.org/10.1016/j.jchromb.2014.12.034