Improvement of the Dynamic Range of Data Acquisition System in Single Particle Mass Spectrometry

Atmospheric particles have complex chemical compositions because of the various emission sources and complex chemical reactions in the atmosphere. Chemical species vary greatly in content for a single particle, which poses a number of challenges in single particle analysis. Single particle mass spectrometry is an on-line technology for the size and chemical characterization of airborne particles at single particle level. The laser ionization process usually generates a large number of different ion species with significant differences in signal intensity. The signal intensity vary from several mV to dozens of V, which is greatly beyond the capacity of normal 8-bit dual-channel data acquisition system. The inadequate system dynamic range will result in the saturation and distortion of the signals with high intensity while loss of the signals with low intensity. In order to detect the high and low intensity ions at the same time, a new 8-bit four-channel data acquisition system was developed, in which two channels were designed for the positive ion analysis and the other two for the negative ions. For each ion mode, the original signal from the detector was split into two parts and then connected to two channels, respectively. The two channels were characterized with two different voltage scales, i.e., the higher scale was used to sample of intensive signal and the low one was used for the small signal. Then, the signals collected by the two channels were combined together by using a specific algorithm. In order to further increase the dynamic range, an attenuator was added to attenuate the even bigger signal. Preliminary experimental results showed that the highest intensity of ions detected was up to 20 V and the lowest intensity was below 10 mV when the two voltage scales were set to 5 V and 500 mV and a 6 dB attenuator was equipped. The system dynamic range reached 2 000, more than 40 times of the normal data acquisition system. The bigger dynamic range will benefit (ⅰ) more accurate chemical composition of the single particle, since a lot of small signals can be retrieved in the novel system; (ⅱ) better identification of emission sources due to improved chemical fingerprint; (ⅲ) higher detection sensitivity and particle utilization. Conclusively, the new data acquisition system is of great importance for the more accurate source apportionment of aerosol particles (e.g., PM 2.5) and the study of the quantitative analysis by single particle mass spectrometry.