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Note 68: Use of a PC Plug-In UV-Vis Spectrometer To Monitor the Plasma Conditions In GC-CRIMS

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Steven M. Colby1 and Robert S. Kearton2

1Scientific Instrument Services, Inc., Ringoes, NJ 08551

2 Ocean Optics Inc., Dunddin, FL 34698
BobK{at}OceanOptics.com

1998

INTRODUCTION

A low cost PC plug-in UV-Vis spectrometer is used to monitor the status of a microwave plasma for Chemical Reaction Interface Mass Spectrometry (CRIMS) [1]. The CRIMS cavity is placed within a gas chromatograph and connected to the spectrometer via a fiber optic. Examples are shown that illustrate the advantages of optical monitoring of the plasma and versatility of the plug-in system.

Background

The CRIMS method was developed at George Washington University in the early 1980s [1-3]. The technique replaces the need for radioactive tracers by measuring the ratios of stable isotopes. In order to distinguish isotopes of different mass from molecular species such as (M+H)+ etc., the analytes are broken down and mixed with a reactant gas in a microwave cavity. This destroys chemical information about the original molecule but generates the same simple products for all precursors. For example, analytes containing enriched 13C may be detected by reaction with SO2 to produce CO2. The ratio of 12CO2 (44 AMU) and 13CO2 (45 AMU) may then be monitored to selective identify 13C enriched analytes.

CRIMS Chemistry

Abramson [3] and others [4-6] have developed a variety of CRIMS chemistries for distinguishing between the isotopes of specific elements. These elements include: H, C, N, O, P, S, Cl, Se, and Br. The reactant gases employed include NF3, SO2, H2, N2, and HCl of which NF3 and SO2 appear to be the most versatile. Key to the selection of reactant gas is the generation of volatile and stable products whose masses do not overlap with other atomic or molecular species. It is preferable to generate products such as HF and DF which have unique masses in comparison to products such as 1H2H which requires high mass resolution to be distinguished from 1H3+.

Signals resulting from the natural abundance of the tracer isotope can easily be subtracted from the total signal to determine that part resulting solely from enrichment. For example, enriched 13CO2 signal can be found from 13CO2 = m/z 45 - 0.0119 * m/z 44. The factor 0.0119 accounts for both the natural 13C and 17O contributions.

CRIMS Advantages

CRIMS is intended to eliminate the need for radioactive tracers by determining the ratios of stable isotopes. Radioactive tracers are currently used in a variety of industries and have the advantage that other than a small change in mass the tagged species is not chemically modified. Their primary disadvantages are the need to produce and handle radioactive material and the dangers of introducing radioactive isotopes into living organisms. This is a severe limitation in Pharmacology and Biochemistry.

While radiochemical detection methods are normally considered highly sensitive, this is only true for off-line measurements where analysis time is not a limiting factor. In on-line measurements, as those made when analyzing chromatographic effluent, the detection time is limited to the width of the chromatographic peak and the detection limits are dramatically reduced. As a consequence, GC/CRIMS has been shown to produce substantially better signal to noise and/or resolution then on-line radiometric detection (for 14C) [3].

Figure 1

Figure 1. GC-CRIMS Cavity Mounted Inside a Hewlett-Packard Gas Chromatograph

Design of a GC-CRIMS System

A new CRIMS cavity is shown in Figure 1. This device is shown inside a GC oven for GC/CRIMS it could also be used after a desolvation apparatus for HPLC/CRIMS. In either case, a transfer line (fused silica capillary) is used to transport the reaction products to a mass spectrometer. Microwave power is supplied through a gold plated arm. The cavity is tuned via two finely threaded pole pieces located on opposite ends of the cavity. The (1/16" i.d.) reaction tube confines the chromatographic effluent and maintains a pressure sufficiently low to sustain a plasma. The plasma occurs only inside of this tube since the remainder of the cavity is at atmospheric pressure.

The CRIMS plasma is turned on immediately after the solvent slug has passed. Problems may occur at this point if: 1) the plasma fails to ignite or 2) the mixture of gasses within the reaction chamber is incorrect. In the past, the plasma status has been monitored by opening the GC oven and viewing the plasma directly with the eye. The plasma color was indicative of the gas mixture. This has the disadvantages that the GC oven temperature is perturbed and the color evaluation is subjective.

UV-Vis Spectrometer

A solution to the above problems is found through the use of a low cost optical spectrometer. Scientific Instrument Services, Inc. has incorporated a PC100 miniature fiber optic spectrometer from Ocean Optics, Inc. into the CRIMS system. The PC1000 is a low-cost 1024-element linear CCD-array fiber optic spectrometer (200-1000 nm wavelength range) mounted on a half-length, 500 kHz ISA-bus analog-to-digital converter card. The PC1000 conveniently installs into an ISA-bus slot in a PC and couples to fiber optics via a SMA 905 connector. In addition, multiple spectrometer channels can be easily connected to a PC1000 to expand wavelength range, perform multiple tasks or make reference measurements. The PC1000 comes with OOIBaseÔ operating software for WindowsÒ and SpectraScopeÔ operating software for DOS.

Figure 2

Figure 2. The PC1000 UV-Vis spectrometer From Ocean Optics

A critical element in the selection of a spectrometer from Ocean Optics was the OEM support provided. OEM's can create virtually any spectrometer configuration using the OEM board, one of three standard interface platforms (desktop, PC Plug-in and portable), 14 different grating choices, light sources and spectrophotometric accessories. This flexibility allows the OEM to optimize resolution, wavelength range and sensitivity within the desired spectral range. To provide optimal flexibility, Ocean Optics also offers customized spectroscopic components. For software integration, a series of *.dll files are available. These allow the integration of spectrometer software into a developers control system.

In the CRIMS system, light is collected after passing through the 3/32 inch wall of the ceramic reaction tube and directed to the spectrometer using a 2 meter fiber optic. The control software includes a user-friendly display for real-time observation and manipulation of the data. In the future, we expect the CRIMS control software to automatically detect the report the status of the plasma using the spectral data. The OEM support has made the incorporation of the spectrometer into CRIMS control software very simple but powerful.

Results

Figure 3 shows the data obtained using the OOIBase software. Figure 3A shows the OOIBase screen and a spectrum observed with just He carrier gas flowing through the cavity. Figures 3B and 3C, respectively, show the spectra recorded with too much SO2 reactant gas and with an organic contaminant. Figure 3D illustrates the data received under optimal plasma conditions. The intensities of the peaks observed are clearly indicative of plasma conditions. We can also detect the presence of excess gases, sample overloading, or air leaks. Use of the spectrometer has greatly simplified plasma observations and enhanced our ability to explain the interpretation of plasma conditions to users. In the future, output of the spectrometer software will communicate directly with the CRIMS electronics in order to automatically check plasma status and repeat ignition attempts when needed.

Figure 3a

Figure 3a

Figuare 3b

Figure 3b

Figuare 3c

Figure 3c

Figure 3d

Figure 3d

Figure 3. Spectra observed under different CRIMS plasma conditions. Light is detected after passing through a 3/32 ceramic wall. A) He carrier gas, B) Too much SO2, C) Organic contamination, and D) Proper plasma conditions.

Conclusions

The PC1000 has proven to be a valuable addition to the CRIMS system. The support provided for OEM integration made the incorporation of the Ocean Optics product relatively simple. These factors have contributed to the development of significant improvements in the CRIMS product.

References

1. Abramson F. P.; Markey, S. P. Proc ASMS Conf. 1982, 30, 866-867.

2. Markey S. P.; Abramson F. P. Anal. Chem. 1982, 54, 2375-6.

3. Abramson F. P. Mass Spectrom. Rev. 1994, 13, 341-356.

4. Heppner, R. A. Anal. Chem.1983, 2170-2174.

5. Morre¢ , J. T.; Moini, M. Biol. Mass Spectrom. 1992, 21, 693-9.

6. Li, G.; Moini, M.; Rerez, F.; Ibarra, F. E.; Sandoval, D. Proc. ASMS Conf. 1994, 42, 293-4.