Enhanced ionization of organic salts in secondary-ion mass spectrometry

Ammonium chloride increases the absolute signal intensity for the intact cations of organic ammonium, pyrylium and phosphonium salts when admixed in 10−10³-fold excess. This signal enhancement results in improved signal-to-noise ratios and is achieved without addition of extraneous ions to the spectrum. The ionization efficiency (secondary ions produced per incident primary ion) is similar for neat and ammonium chloride diluted samples. However, the ionization yield (secondary ions produced per sample molecule) is enhanced by the matrix by four orders of magnitude. Sensitivity, calculated here for a desorption ionization method, is 0.5−1 × 10⁻¹⁰ C μg⁻¹, and is less than that of electron ionization and chemical ionization sources. Spectra of submicrogram amounts of organic species in ammonium chloride are stable and can be recorded continuously for many hours, as compared to less than an hour for the neat compounds. Even subnanogram amounts of sample diluted in ammonium chloride give spectra which last a number of minutes. Matrix-diluted samples may be withdrawn from the spectrometer and stored for days before being reanalyzed without incident. With this matrix, the technique is essentially nondestructive. Total ionization yields for matrix-diluted samples are estimated at 0.01-0.1%. Primary-ion fluxes impinging on matrix-diluted samples exceed static SIMS limits without evidence of beam damage.