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Anal. Chem. 1088, 58,366-370
Time Gating for the Elimination of Interferences in Electrothermal Vaporization-Inductively Coupled Plasma Atomic Emission Spectrometry M. W. Tikkanen’ Scientific Laboratory Division, Health a n d Environment Department, 700 Camino de Salud NE, Albuquerque, New Mexico 87106
T. M. Niemczyk* Department of Chemistry, University of New Mexico, Albuquerque, New Mexico 87131 The use of an electrothermal vaporizer as a sample Introduction system for an Inductively coupled plasma spectrometer allows for a time sequencing of appearance tlmes of various sample constituents according to the constituent volatilities. This fact can be utliized to eliminate some interferences, as long as the introduction of the analyte and the interferent can be separated In tlme. Time gating the data collection system is shown to be necessary to eliminate certain spectral interferences. The application of these concepts to the ellmlnation of the spectral Interference noted for Ai on As determlnatlons and easliy Ionized elements on Pb, Mn, and Fe determlnatlons is discussed.
Because of its many strengths inductively coupled plasma atomic emission spectrometry (ICP-AES) has become a very popular technique. One of the advantages often claimed for ICP-AES is that it is relatively interference free. Indeed, when compared to most atomic emission sources, the ICP is less susceptible to interferences. On the other hand, some interferences have been well-documented. These have included spectral interferences and effects due to the presence of significant concentrations of an easily ionizable element (EIE) in the plasma. Most spectral interferences occur when a secondary line of an element, or its ion, overlaps the measured line of the analyte element (I,2). Other examples of spectral interferences that have been documented include shifts in the background at the analyte wavelength due to the presence of an electron-ion recombination band or broadening of a spectral line of a concomitant species (3). When a large concentration of an easily ionizable element is introduced into the plasma, both enhancements and suppressions of companion analyte signals have been noted (4-12). In this case it appears that the presence of a large concentration of easily ionizable species causes changes in the transport, vaporization, ionization, excitation, etc., mechanism(s) in the plasma. In some cases, whether an enhancement or a suppression is noted has been shown to be dependent on the region of the plasma viewed. All of the above types of interference are due to the concurrent presence of the analyte and interferent in the plasma. Thus, it will be difficult to ever circumvent these interferences using conventional pneumatic nebulization systems. The electrothermal vaporizer (ETV) is a most useful device when coupled to ICP-AES systems (13-20). Very small sample volumes are used and extremely low absolute detection limits have been achieved. Unlike a pneumatic nebulization system the electrothermal vaporizer produces a pulse of sample material that is swept into the plasma, resulting in a pulse Current address: Applied Research Laboratories, 9545 Wentworth St., Sunland, CA 91040.
of analyte(s) radiation. The “pulse mode” of sample introduction partially accounts for the low detection limits achieved. The mass of analyte material is introduced to the plasma with high efficiency over a rather short time interval. This results in an instantaneous, but high, concentration of analyte element in the plasma. By proper choice of measurement time, or time gating the detection system, the signal can be measured only during the time when the analyte is present in the plasma resulting in a relatively high signalto-noise ratio (16, 18, 20). Electrothermal vaporizers have been widely used in atomic absorption systems with the same properties noted above to produce detection limits superior to flame atomic absorption. Interferences in ETV atomic absorpton spectrometry, which have been extensively documented, are primarily due to the ETV system which must serve as an atomizer, i.e., reduce the analyte to its atomic state. In ETV-ICP-AES the vaporizer serves only as a vaporizer, and atomization and excitation primarily occur in the plasma. Although some interferences associated with the ETV system are to be expected when coupled with an ICP, common interferences such as oxide formation, dimer formation, etc., associated with ETV atomic absorption will be eliminated because the atomization is not necessarily a consequence of the atomizer heating. For this reason ETV systems are more appropriate for use with plasma sources than they are in atomic absorption systems. It is well-known from atomic absorption studies that elements of different volatilities are released from the atomizer surface at different times (assuming similar heating programs). A relatively nonvolatile element will be released from the atomizer surface later in time than one that is relatively volatile (21). The appearance of various analytes from a multielement solution would be expected to follow the relative volatilities of the elements. When a multielement solution is introduced to an ETV-ICP-AES system, the retardation of plasma appearance time with decreasing volatility for the various components is demonstrated (20). Thus, some elements will pass through the plasma before the appearance of less volatile species. If the simultaneous presence of two species is cause for a plasma related interference, the timesequenced appearance of elements of varying volatilities can be used to eliminate some plasma related interferences. In this paper the use of an ETV system to sequence in time the arrival of various components of a sample into an ICP direct reader system will be discussed. Then, by using a multichannel “time-gated detection sequence, we will demonstrate the elimination of some well-documented ICP interferences. EXPERIMENTAL SECTION Instrumentation. The 31-channel, 0.75-m Jarrell-Ash 965 AtomComp direct reading spectrometer, associated computer, electrothermal vaporization system, gas flow control, plotter, and modifications necessary to allow the acquisition of signal-vs.-time profiles have been previously detailed (1420).The preselected
0003-2700/86/0358-0366$01.50/00 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 2, FEBRUARY 1986
Table I. Experimental Operating Conditions and Instrument Parameters
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Direct Reader 0.53 nm/mm in 1st order linear reciprocal dispersion entrance slit height 3 mm 25 fim entrance slit width 50 fim exit slit width 0.036 nm in 1st order working resolution Plasma forward power 1100 w reflected power
