Abstract
An atmospheric pressure ionization mass spectrometric (API-MS) method was studied for the determination of residual NO in high purity N2 gas. The API-MS is very sensitive to NO, but the presence of O2 interferes with the NO measurement. Nitrogen gas in cylinders as sample gas was mixed with NO standard gas (971 ppm) and/or O2 standard gas (1050 ppm) by a standard gas diluter and the gas was introduced into API-MS. The calibration curves of NO and O2 had linearity in the region of 02 ppm but the slopes changed at every cylinder. Effect of O2 on NO+ peak was additive and proportional to O2 concentration in the range of 00.5 ppm (O2).The increase in NO+ intensity due to O2 was (0.070.13) %/O2, 1 ppm. Determination of NO and O2 was carried out by standard addition method to eliminate the influences of variation of slopes. The interference due to O2 was estimated from the product of the O2 conentration and the ratio of slope "A" to slope "B". The slope "A" is the change in the NO+intensity with O2 concentration. The slope "B" is the change in the NO+ intensity with NO concentration. The analytical values obtained were about 50 ppb for O2, (0.61.3) ppb for interference of O2, and (913) ppb for NO (error due to interference of O2 was subtracted). The analytical value for O2 is considered to be adequate to estimate the interference of O2, still it requires further examination.
