Some figures of merit of a new double focusing inductively coupled plasma mass spectrometer

Abstract

An ICP-MS instrument with a quadrupole filter is hampered by its limited resolution, in particular polyatomic ions are a major source of interference. With a double focusing magnetic sector mass analyser, operated at high resolution, most of these interferences are eliminated. In addition, these mass analysers are characterized by a low instrumental background, yielding superior detection limits at low resolution. In this work a new type of double focusing ICP-MS instrument, the ‘Element’ from Finnigan MAT (Bremen, Germany), was tested. The instrument can be used with low resolution (m/Δm= 300, 10% valley definition) and at high resolution settings (3000 and 7500). At low resolution, a linear response versus mass curve was observed [response of 180 × 10⁶ counts s^–1 per µg ml^–1(ppm) for U and 6 × 10⁶ counts s^–1 for Be]. The sensitivity decreases by roughly a factor of 14 when going from resolution 300 to resolution 3000 and by a factor of 10 from 3000 to 7500. An instrumental detection limit at low resolution of about 8 fg ml^–1(8 ppq) was measured at m/z 240. The detection limit for most elements is not determined by the instrument but by the blank level and laboratory procedures will need to be improved in order to make optimal use of the detection power of the instrument. The stability and the relative abundance of oxide and doubly charged ions are comparable to the values found in quadrupole ICP-MS. Matrix effects were compared for measurements with the ‘Element’ and with a Perkin-Elmer Elan 5000 quadrupole instrument of ¹¹⁵In in the presence of 500 µg ml^–1 of Cs and of 2.5% of ethanol. Though in the ‘Element’ ions are accelerated over 8 kV immediately after the skimmer, matrix effects were found to be similar to those observed for the ‘Elan 5000’. The instrument was used to determine Pt in an environmental material (grass).