Sampling aerosols for lead isotopes on a global scale

To reliably sample aerosols in a wide range of environments from the polluted to the almost pristine, ultra-clean materials, procedures and laboratory conditions are needed. Also, when multiple samples need to be taken at numerous sites the sampling procedures need to be simple and the equipment inexpensive and easy to operate. A system described here which meets these requirements uses a disposable plastic monitor fitted with an acid-cleaned PTFE 0.45 μm front and a 30–60 μm backing filter connected to a small diaphragm pump. When ion exchange chemistry is required the system gives a Pb blank of 70 pg. Without ion exchange chemistry the blank amounts to 15 pg on a complete filter sectioned into 8-π/4 sectors giving reliable isotopic data on filters containing >400 pg of Pb. The lowest blank on the complete filter, without cutting, is 11±8 pg which permits Pb isotopes from aerosols to be measured on a 120 pg sized sample with acceptable accuracy and precision (0.2% for ²⁰⁶Pb/²⁰⁷Pb and ²⁰⁸Pb/²⁰⁷Pb ratios). The factor limiting the quality of measurements below this level is the uncertainty in the blank contribution and not the sensitivity of the thermal ionisation mass spectrometer used here. The analysis of selected filters from the United Kingdom and the Republic of Ireland are used to illustrate the method. Air containing 0.11 ng m⁻³ of Pb yielded isotopic ratios ²⁰⁶Pb/²⁰⁷Pb=1.083±0.003, ²⁰⁸Pb/²⁰⁷Pb=2.361±0.003 and ²⁰⁶Pb/²⁰⁴Pb=16.9±0.3.