Power-modulated inductively coupled plasma spectrometry

Details Power-modulated inductively coupled plasma spectrometry Power-modulated inductively coupled plasma spectrometry

2000-05-31

2001-06-05

Berman, Jack (Department: 2881)

Radiant energy

Ionic separation or analysis

With sample supply means

C250S281000

Reexamination Certificate

active

06242735

ABSTRACT:

FIELD OF THE INVENTION
The present invention is related to apparatuses and techniques for ionizing or exciting samples to analyze the samples spectrometrically, and more particularly to apparatuses and methods for ionizing or exciting the samples with inductively coupled plasma and analyzing the ions and exited atoms with mass spectrometers.
BACKGROUND
The inductively coupled plasma (ICP) has been a powerful tool for analytical chemistry since it was introduced as an atomic emission source for optical spectrometry in the 1960's. In the ICP, radio frequency electrical energy is continuously coupled via a spiral load coil into an inert gas flow stream at atmospheric pressure. In a typical design, argon flows through a plasma torch made of three concentric quartz tubes (see, e.g., U.S. Pat. No. 3,958,883 (Turner)). The central gas flow, usually referred to the carrier gas flow because it is used to carry the sample to be ionized or excited, is usually 1 to 1.5 liters/minute, depending upon the characteristics of the sample. An intermediate gas flow of about 1 liter/minute, termed the auxiliary flow, is needed for confining the hot carrier gas and cooling. The outermost flow, termed the coolant gas, both sustains the plasma and protects the glass from melting from the high temperature. The coolant flow is generally 15 liters/minute with typical plasma torch designs.
The electromagnetic field induced by the radio frequency energy, which is typically at 27 MHZ or 40 MHZ, sustains a plasma in the gas. The plasma contains free electrons, ions, and excited atoms and molecules. A chemical sample, usually in a form of aerosol droplets, is introduced into the carrier gas steam through the central quartz tube into the plasma. The aerosol sample is vaporized and decomposed to atoms and small molecules, due to the high temperature of the plasma. Some of the atoms and molecules of the sample are further excited and ionized by the free electrons.
The ICP is a source suitable for coupling with an optical emission spectrometer (OES) since it produces a large number of excited and ionized atoms and molecules from a sample introduced into it (see, e.g., Wendt, R. H. and Fassel, V. A.,
Anal. Chem.,
1965, 37, 920-922). In an optical spectrometer, the emitted light is usually sent to a wavelength dispersive grating and detected by a photodetector array. The output of the detector array is then electronically integrated for a certain time period. The ICP has also been coupled with a mass spectrometer (MS) (see, e.g., Houk R. S. et al., “Mass spectrometry of inductively coupled plasmas,”
Anal. Chem.,
1980, 52, 2283-2289; and U.S. Pat. No. 4,760,253 (Hutton)). For mass spectroscopic detection, quadrupole mass filters are often employed. The chemical composition of the sample is determined by scanning the quadrupole filter within the mass range of interest. The description of the apparatuses and operation of ICP and spectrometers in Wendt, R. H. and Fassel, V. A., Houk R. S. et al., U.S. Pat. Nos. 3,958,883, 4,760,253, supra, and U.S. Pat. No. 4,818,916 (Morrisroe), infra, are incorporated by reference herein.
One of the major drawbacks of using the ICP for chemical analysis is the high cost of instrument manufacture and operation. To obtain useful radiation or to generate ions for routine chemical analysis, the ICP source is usually operated at high power, in the range of 800 to 1600 Watts, depending upon the sample introduced, which results in a very high temperature within to the plasma. Generation of such a high-energy radio frequency waveform normally requires a specially designed electronic circuit with high power output, such as the one described in U.S. Pat. No. 4,818,916 (Morrisroe). When the ICP source or the spectrometer (SP) in an ICP-SP system is not functioning normally, repair is often difficult and expensive. What is needed is an ICP-SP analytical apparatus with low energy consumption and that costs less to manufacture, operate and maintain.
SUMMARY
In the present invention, through modulating the operation of the ICP in an ICP-SP apparatus, in which an ICP is coupled to a spectrometer, a technique of reducing the power consumption of ICP spectrometers is provided, thereby reducing the need for cooling, as well as reducing the heat damage to the apparatus.
In one aspect of the present invention, an analytical apparatus contains an inductively coupled plasma generator (ICPG) and a spectrometer. The ICPG generates a plasma for forming ionic and excited molecular species from a sample introduced into the plasma. The spectrometer associated with the ICPG analyzes the ionic and excited molecular species formed in the ICPG. The spectrometer has an analysis mode wherein the ionic and excited molecular species are identified according to physical characteristics, e.g., mass-to-charge ratios or optical emission characteristics, to provide data on the species and further has a washout mode wherein the spectrometer flushes out interfering ions and molecules and provides no significant data on the sample. The controller modulates the ICPG to operate in power cycles, at each cycle the ICPG operates in an analysis period and a stand-by period. In the analysis period the ICPG operates at one or more analysis power levels to generate plasma to form the ionic and excited molecular species from the sample to correspond to the analysis mode of the spectrometer. In the stand-by period the ICPG stands by at one or more stand-by power levels lower than the one or more analysis power levels to correspond to the washout mode of the spectrometer.
This technique reduces the power consumption, hence the cost for both manufacturing and maintaining an inductively coupled plasma associated with a spectrometer. To reduce the power consumption of an inductively coupled plasma spectrometer, the full plasma power (the power needed to generate useful radiation or ion abundance for chemical analysis) is provided only when a sample is being introduced into the ICP and the spectrometer is recording a signal that reflects the characteristics of the sample. Such a decrease in the power dissipation would reduce or eliminate the need for water cooling, and would also reduce the need for forced-air cooling. A less complex and therefore less expensive radio frequency amplifier with a lower average output power can be used to provide power to this modulated plasma. Additionally, the flow rate of cooling gas through the plasma torch can be reduced when the plasma is at low power.


REFERENCES:
patent: 5480809 (1996-01-01), Salin et al.
patent: 6166379 (2000-12-01), Montaser et al.

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