Oxygen

Oxygen is determined by both positive and negative ion APIMS. The positive ion mode can be used with either corona discharge or ⁶³Ni sources. However, the ⁶³Ni source must be used for negative ion mode.

Positive Ion Mode

In bulk gases such as nitrogen, helium, argon, oxygen is determined by monitoring O₂⁺ primarily formed by the reaction

O₂ + N₄⁺ → O₂⁺ + 2N₂

We collect our signal in ion counting mode so that the standard deviation of the signal is the square root of the number of counts. In analog mode the standard deviation only can be computed be physically measuring it.

We observe about 1000 counts per second in positive ion mode so for a one second integration time 1000 counts are observed. The signal to noise is the square root of the number of counts or 33 in this case. Defining the lower limit of detection as the concentration at which the S/N is 3, the lower limit of detection is 100 pptv. With an integration time of 100 sec the S/N = 10.

However, like most determinations the LLD is determined by fluctuations in the blank. In this case the blank is due mostly to H₂O∙N₂⁺. A convenient way to determine the blank is to insert a very good purifier in line which removes the O₂ leaving only the signal for H₂O∙N₂⁺. The lower limit of detection in counts based on the 95 percentile confidence limit is computed from the relationship

Here σ is the standard deviation of the signal, _ns is the number of signal measurements and n_o is the number of blank measurements. In the above example if n_s = n_o = 1000 and σ = 33 the lower limit of detection is 4 counts or 4/1000 =0.004 ppbv or 4 pptv for an integration time of 1 second. Actually the trend in the absolute sensitivity of the instruments limits the length of time the signal can be studied before instrument drift dominates the signal.

Negative ion Mode

In negative ion mode oxygen is usually the most electroegative ion species present. In this case the available thermalized electrons attach to oxygen

O₂ + e → O₂^-

O₂^- forms complexes with several species including H₂O and NH₃. The oxygen concentration can be computed from the sum of the intensity of the O₂^- peak and all its complexes. In systems where H₂O is the primary impurity other than O₂ the oxygen concentration is proportional to the ratio of the O₂^- peak intensity and the O₂^- ∙H₂O peak intensity. In these systems the oxygen concentration can be determined with precision down to 10 pptv and perhaps lower.

In liquid ammonia oxygen can be determine by following the signals for O₂^-, O₂^- ∙H₂O and O₂^- ∙NH₃. The lower limit of detection is 100 pptv which is higher than for nitrogen because the ammonia is diluted 10/1 with clean nitrogen to prevent damage to the APIMS pumps. The LLD can be restored to that observed in nitrogen if the pumps were replaced with ones than can survive in pure ammonia.