Gas chromatography/negative ion chemical ionization mass spectrometry (GC/NICI MS) is characterized, elucidating instrument performance for the continuous measurement of peroxyacetyl nitrate (PAN) and its homologues. Chromatograms and linear response at parts per trillion by volume (pptv) levels are examined to evaluate the accuracy of calibration using 4 types of base gases: dry nitrogen, dry synthetic air, purified compressed air, and thermally PAN-decomposed air. The sensitivity to PAN is further checked against humidity, which may be a source of interference in ambient measurements. The experimental results indicate that oxygen, water vapor, and other trace gases in ambient air do not affect the PAN sensitivity. Although significant sensitivity drifts were observed during the course of measurements made at a remote site in Japan, this problem was overcome by performing frequent calibrations with isopropyl nitrate (IPN).