Upon excitation at 170-240 nm, diamonds emit strong luminescence in wavelength range of 300-700 nm. The spectral features observed in the photoluminescence excitation (PLE) spectra show two vibrational progressions, A and B, related to nitrogen defects N2 and N4, respectively. We used PLE spectra excited in region 170-240 nm to identify the type of diamond and demonstrate quantitative analysis of the B center as a N4 nitrogen defect in diamonds; the least detectable concentration of the N4 nitrogen defect is about 13 ppb, and the sensitivity of PLE is about 30 times than that practicable with infrared absorption spectra.