Study on Discharge Behavior of Pencil Lead at Ambient Pressure by Mass Spectrometry

Graphite and clay mixed in pencil lead, with non-standard proportions will be read correctly by computer card reading equipment and identified by non-specialists. In this paper, a rapid and effective method based on atmospheric mass spectrometry utilizes quality evaluation for the pencil lead by the analysis of different degrees of blackness (graphite content) and hardness. At ambient pressure, with about 40% relative humidity, pencil lead was directly used as an ionization device to produce coronal discharge and impact the mass spectrum signals in air at high voltage. Here, experimental conditions were optimized, the relationship between the graphite content in pencil leads (e.g. 4B, 3B, 2B, HB, H and 2H) and their mass spectrum signals were analyzed, and six kinds of pencil leads were discriminated by principal component analysis (PCA). The results showed that the discharge behaviors of the different types of pencil lead have an influence on the signal intensity, which increases with the increasing graphite content in pencil lead. On the basis of the mass spectra raw data, six kinds of pencil lead were correctly identified to more than 99%. The method is available with a good reproducibility and straightforward operation, providing a rapid tool for the quality evaluation for commercially available pencil lead.