Tetsumasa Ito

Evaluation of an inductively coupled air-argon plasma as an ion source for mass spectrometry

An inductively coupled air–argon plasma (air–Ar ICP) has been developed using a modified torch and a 40.68 MHz generator (maximum rf power 4 kW), and evaluated as an ion source for mass spectrometry (MS). The outer and aerosol carrier Ar flows were completely replaced with air; however, 1.5 l min -1 of Ar was used as the intermediate gas to maintain a stable plasma discharge at low rf power. The optimized sampling depth and the aerosol carrier air flow rate for maximum analyte signals were found to be 10 mm above the load coil and 0.9 l min -1 , respectively. The analyte signal increases with rf power, but 2 kW was sufficient for the stable discharge and the acceptable analytical sensitivity. In the mass spectra obtained under the optimized conditions, N + , O + and NO + were clearly observed, but the signal for Ar + was weak, which is similar to that for an N 2 ICP. The ion signals for N 2 + and O 2 + were relatively large, compared with N 2 and O 2 ICPs operated with Ar added to the outer gas. The analytical sensitivity of the proposed air–Ar ICP is superior to an Ar ICP using the same equipment for elements with low first ionization potentials (IP) of <6.5 eV, but inferior for elements with high first IPs (>6.5 eV). The secondary discharge increases the average kinetic energy of the analyte ions, the distribution of which is wider in the air–Ar ICP than in the Ar ICP. The ratios of monoxide ion to singly charged ion remain almost constant along the plasma axis. Space charge effects from co-existing elements are also discusssed.

Comparative study of 27.12 and 40.68 MHz inductively coupled argon plasmas for mass spectrometry on the basis of analytical characteristic distributions

Analytical characteristics of 27.12 and 40.68 MHz inductively coupled argon plasmas (Ar-ICP) are compared as an ion source for mass spectrometry on the basis of distribution measurements. Analyte ion signals at 27.12 MHz are found to be more sensitive than at 40.68 MHz, which is based on the count ratio of analyte ion-to-plasma support Ar+. The r.f. power does not affect the ion counts at 27.12 MHz, but the signals are increased with r.f. at 40.68 MHz. The ratio of monoxide-to-singly charged ions is higher at 27.12 MHz for Y+ and Pb+, but the ratio of doubly-to-singly charged ions depends on the elements. The secondary discharge, more strongly observed at 40.68 MHz, might reduce monoxide ion formation. The proportion of the analyte ion (calculated from the ratio of analyte-to-total ion signal in the plasma) is highest in a 27.12 MHz ICP operated at 1.0 KW. Less interference and a wide range of ion kinetic energy distribution in the analytes are observed at 40.68 MHz. The average of the detection limits at 27.12 MHz is slightly lower than at 40.68 MHz.

Inductively coupled nitrogen plasma mass spectrometry assisted by adding argon to the outer gas

An inductively coupled nitrogen plasma (N2 ICP) has been developed using a 40.68 MHz generator with a maximum rf power of 4 kW, and evaluated as an ion source for mass spectrometry (MS). The intermediate and central Ar flows are completely replaced with N2; however, a certain amount of Ar should be added to the outer flow to maintain a stable plasma discharge. The sampling depth for the maximum analyte signals was found to be closer to the work coil, and the signal increases with the N2 carrier flow rate up to 2.0 l min–1. In the mass spectra obtained under the optimized conditions, N+, O+ and NO+ are strongly observed, but the signal of Ar+ is weak. The analytical sensitivity of the proposed N2 ICP is almost equivalent to that of an Ar ICP for elements with low ionization potentials, but inferior for elements with high ionization potentials. The secondary discharge increases the average kinetic energy of the analyte ions, and its distribution in the N2 ICP is wider than in the Ar ICP. A wide energy distribution also seems to lead to higher ratios of doubly charged and monoxide ions to singly charged ions. Space charge effects and ionization interference from co-existing elements are also discussed.