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Anal. Chem. 1991, 63, 2012-2016
Development of a Miniaturized Gas Chromatograph-Mass Spectrometer with a Microbore Capillary Column and an Array Detector Mahadeva P. Sinha* J e t Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109 George Gutnikov Department of Chemistry, California State Polytechnic University, Pomona, California 91 768 A miniaturized focal plane mass spectrograph with an array detector k demonstrated to measure multiple maw spectra from narrow GC peakseMedfrom a rhort "bore capiRary cdumn (50 tun Ld., 3 m In length). The GC peaks of dlchlorodifluoromethane, chloromethane, bromomethane, and chloroahane from a mMue of compounds are found to have full width at haifmaximum In the r a w 200-300 ms and are separated by -700 m. The simultaneous measurement of all Ions by the array detector enables mass spectral measurements of these peaks powlblo without affecting the GC resolution and the sendtlvtty of the mass spectrograph. The system IS shown to possess high "thky (e-g., 7.5 X W 4 g for benzene) and a linear dynamic range of >loa. A microbore capRlary column requires an extremely mall canler gas flow rate (-0.05 atm cma mln-' for helium) and drastlcally reduces the weight and power needs of the mass spectrograph. The combination of such a c o h n wtth a miniaturized focal plane mass spectrograph Is thus uniquely witsd for the development of a field-portable, high-performance GC-MS system.
INTRODUCTION The combination of gas chromatography with mass spectrometry (GC-MS) represents one of the most reliable methods for the analysis of complex mixtures of organic compounds ( 1 , 2 ) . However, because of its mass, size, and power requirements, the analytical instrument has not been amenable for on-site, real-time measurements of pollutants. For field measurements in real time, a miniaturized GC-MS system is needed that is fast and possesses high sensitivity and efficiency. The speed of analysis in a GC-MS system is controlled by the GC separation time. Fast separation with high efficiency can be achieved by the use of narrow-bore capillary columns of short length (3-5). However, the application of microbore (5100pm i.d.1 capillary columns to the separation of complex organic mixtures imposes constraints on the sample size, injection of the sample, and the detection and identification of the eluted components (6,7). These may be the reasons why 100 pm i.d. columns have been the smallest ones to be used in commercial gas chromatographs. The low sample capacity requires a sensitive detector. The minute sample volumes necessitate reliability of their injection (8, 9 ) and a minimum dead volume to maintain column efficiency. Also, the extremely narrow peaks that result from the use of microbore columns require low times constants for measurement devices (6). In addition, the column outlet should be maintained under vacuum conditions for increasing the separation speed (10, 11). In order for a mass spectrometer to prove itself as a suitable detector for such narrow GC peaks for a microbore column, the mass spectrometer must acquire data at a fast rate. Typically, 5-10 mass spectra should be measured to define 0003-2700/91/0363-2012$02.50/0
a peak profile (7).Even with short capillary columns having 104 theoretical plates, several peaks may be eluted per second, particularly in the early part of the chromatogram. For such cases,a spectral scan rate of greater than 101s will be required to reconstruct the chromatogram. A quadrupole mass spectrometer, most commonly used with GC, may be scanned at a rate of 0.1 sfdecade but at the expense of sensitivity and reproducibility of the signal (6). In some cases the sensitivity of such a scanning maas spectrometer can be enhanced by the technique of selected-ion monitoring (SIM) (12). SIM, however, provides only partial mass spectral information and, therefore, has limited application for the unambiguous identification of compounds in complex mixtures. A nonscanning maas spectrograph would be uniquely suited for the measurement of narrow GC peaks. The capability of the nonscanning mass spectrograph for measuring the intensities of all masses at the same time confers on it an almost unlimited speed for obtaining mass spectra. The sensitivity of a nonscanning type of mass spectrometer is also inherently greater than that of a scanning type mass spectrometer because the latter measures the signal at a given mass peak only for a short dwell time. In contrast, a nonscanning mass spectrograph measures all mass intensities during the entire time of signal generation from the analyte. This type of measurement is equivalent to single-ion monitoring a t all masses simultaneously. However, the lack of a detector with high gain (such as an electron multiplier) for a nonscanning mass spectrograph has severely restricted its application for obtaining mass spectra from trace amounts of samples. Recently, an array detector for a focal plane mass spectrograph (Mattauch-Herzog type, nonscanning) has been developed in our laboratory (13, 14). The detector, known as an electrooptical ion detector (EOID), possesses the simultaneity of a photoplate (used in focal plane spectrograph) and the high gain of an electron multiplier. The EOID integrates the signal for a wide range of time (20 ms to 30 s), and multiple mass spectral measurementsof transient samples (narrow chromatographic peaks) can be made without sacrificing sensitivity by an appropriate selection of the integration time. The timeresolved measurementthus performed can also be exploited to augment the resolution of the gas chromatograph for closely spaced peaks. The outlet of the GC column can be maintained a t a low pressure by connecting the column to the ion source of the mass spectrometer. Direct coupling of the gas chromatograph to the ion source of the mass spectrometer also eliminates the dead volume stemming from GC-MS interfaces and allows for complete utilization of the sample amount for analysis. Because of the low-volume flow rates, microbore columns are compatible for their direct coupling to the mass spectrometer. A small pump may be adequate to maintain the proper operating vacuum conditions in the mass spectrometer. Our objective is to develop a field-portable, high-performance GC-MS instrument for environmental measurements. 0 1991 American Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 63, ELECTRIC SECTOR7
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Figure 1. Schematic of the microbore capillary column gas chromatograph and the focal plane mass spectrograph assembly. The sample injector is pneumatically actuated and is provided with pilot valves and a digital valve interface for fast sample injection.
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EXPERIMENTAL SECTION Figure 1 is a schematic of the experimental setup. Its main components include a gas chromatograph, a miniaturized focal plane mass spectrograph, an array detector, and a data system. Gas Chromatograph. The gas chromatograph consisted of a 3.0 m, 50 pm i.d., fused-silica capillary column with a 0.2-pm bonded DB-5 stationary phase, (J.& W. Scientific, Folsom, CA) and was housed in a temperature-programmable oven. Samples were injected with a pneumatically actuated internal sample injector valve (Valco Instruments Co., Houston, TX, Model No. CI 4W.5). To achieve fast sample injection, a digital valve interface (DVI) along with pilot valves was incorporated upstream from the sample injector. Helium was used as the pneumatic gas. The helium pulse output from the DVI is amplified with pilot valves to move the valve actuator faster (15). The time for the loadtu-inject transition of the valve was measured at different helium injection pressure pulses. For the measurement of the transit time, a flag was mounted on the rotating shaft of the injector valve. Two slotted optical limit switches correspondingto the load and inject positions were also mounted on the body of the injector valve. The optical switches sense the motion of the flag that interrupts the light from the emitting diode to the phototransistor and thus generate two voltage pulses for the motion of the flag from load-to-injectposition. The valve transit time, equal to the time interval between the two pulses, was measured on an oscilloscope. Thus samples could be injected reproducibly in about 14 ms. The outlet end of the GC column was introduced through a flange into the ion source of the mass spectrograph. The sections of the column between the oven and the flange, and inside the ion source chamber of the mass spectrograph could be heated independently. However, for the measurements reported here, the injector valve and the column were left at room temperature (22 "C). GC grade helium was used as a carrier gas at a flow velocity of 40 cm s-'. Mass Spectrograph and the Array Detector. A miniaturized focal plane mass spectrograph (Mattauch-Herzog type) was used. It has a 127.0 mm long focal plane and covers a mass
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Real-time, on-site characterization of contaminated environments/sites will be greatly facilitated by a portable GC-MS system. Immediate availability of the analytical results with high accuracy/reliability will save time and cost for locating and determining the level of contamination of sites. It will ensure worker's safety and will be immensely useful in monitoring the effectiveness of remedial steps taken to mitigate the effects of these contaminants. In this paper, a technique based on the use of a short microbore capillary column and a miniaturized focal plane mass spectrograph for the development of a portable gas chromatograph-mass spectrometer is described. Some of the results obtained from the analysis of a mixture of pollutants are also reported.
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Figure 2. Total ion chromatogram obtained from a mixture of compounds consisting of air (1l), dichlorodifluoromethane (16), chloromethane (19),bromomethane (28), chloroethane (30), dichloromethane (59),1,1 ,1-trichioroethane (126), chloroform (162), benzene (188). and trichloroethylene (270). Each component in the mixture has a concentration of 1 ppmv. A sample volume of 0.5 pL was injected, and a signal integration time of 250 ms was used for each frame.
range of 25-500 amu. The mass spectrometer and the detector were designed and built at the Jet Propulsion Laboratory (14). Alnico V-B was used for the fabrication of the magnetic sector. A field strength of 10 kG was maintained in the gap between the pole pieces of the permanent magnet. The details of the EOID have been described previously (13, 14). Briefly, its comprises a two-dimensional microchannel electron multiplier array (Galileo Electro-opticsCorp., Sturbridge, MA), a phosphor-coated fiber-opticwindow, fiber-optic couplers, and photodiodearrays (1024s series obtained from EG&G Reticon Corp., Sunnyvale, CA). An important difference between the present EOID design and the one reported earlier lies in the use of a single continuous electron multiplier array (127 mm X 127 mm, 10 pm channel diameter). The microchannel array (MCA) eliminates the junction discontinuities present in the previous version of EOID, which used several 25.4 mm long MCAs for covering the focal plane. In the EOID, the ions exiting the focal plane impinge on the MCA and initiate an electron-cascading process along the channel length. The electrons coming out at the other end of the channels are accelerated to the phosphorcoated (P-31) fiber-opticwindow (6 pm diameter and 1.0 numerical aperture), producing images of the ions. The ion images are then transmitted to the photodiode array through the fiber-optic couplers. The purpose of the couplers is to minimize the discontinuity in the measurement of ion images. A photodiode array (PDA) has a total length of 38.1 mm, with a 25.4 mm long active region in the middle and has a center-to-center distance of 25 pm between two adjacent diodes. The PDAs were cemented to the couplers and were interdigitated along the focal plane. This arrangement leaves a discontinuityequal to the width of only two diode pixels at each of the junctions. The photodiodes can integrate photon signals from ion images over the desired time. The MS-EOID system has unit mass resolution at mass 500 amu. A related and small version of EOID has also been implemented by other workers (16,17)in different mass spectrograph geometries. The ratio of the highest to the lowest simultaneously detected mass in their measurements was small (- 1.08). Data Acquisition. The signal stored in the photodiodes for the predetermined integration time is called a frame. Each frame constitutes a complete mass spectrum. The signal accumulated in a frame is read serially by an on-line microprocessor-based data system (Masscomp 500) at a rate of -220 kHz under a direct memory access routine. Each diode integrates the signal continuously except for a period of 4 ps while the signal stored in
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Figure 3. Effect of signal integration time on the resolution of GC peaks. Integration times of 100, 250, and 500 ms were used for a frame in (a), (b), and (c), respectively. The peaks corresponding to dlchlorodiflwomethana and chloromethane, and bromomethane and chloroethane are not resolved with 500-ms integration time.
it is read by the computer. The diode is reset upon reading and immediately resumes signal integration. Thus, there is essentially no loss of signal from an analyte by making multiple spectral measurements of short integration times (e.g., 25 ms). The minimum integration time is determined by the readout time of the photodiodes. This allows for the complete mass spectral measurements of GC effluents at a high frequency without any attendant loss of sensitivity. A background spectrum is acquired before the analysis and is stored in the array processor of the data system. Thii spectrum is then substrated from each of the frames on the fly before recording it on the computer disk. No correction for the nonuniformity of gains in different channels of the EOID was made in the present measurement (18). Materials. Kimax brand glass bulbs of known volumes were used for the preparation of analytical samples. These bulbs were fitted with stainless steel valves through metal-to-gh Kovar seals for pumping and rubber septums for the introduction and withdrawal of samples. A stock mixture of known concentrations (lo00 ppmv in air) of some priority pollutants was prepared by introducing the appropriate amount of each compound into an evacuated glass bulb (1.0 L). Air was admitted into the bulb to make a total pressure of 1 atm. A fast and uniform mixing of
the components was achieved by the air rushing into the glass bulb and also by ita subsequent heating (70-80 "C). Proper volume of the stock mixture was introduced into another evacuated container to prepare the analytical sample of desired concentration. Mixing was ensured as described above. Freshly prepared mixtures including the stock mixture were used in all measurements. The internal volume (0.5 pL) of the sample valve was filled with the gas and vapor mixture and was injected onto the GC column. The sample volume of the valve was flushed with the mixture prior to sample injection to minimize the adsorption effects.
RESULTS AND DISCUSSION Figure 2 depicts the mass chromatogram of a mixture of the compounds, each at a concentration of 1ppmv in air,and chromatographed at room temperature. In obtaining the maas chromatogram, the ion intensities of all masses were integratad for a period of 250 ms and recorded consecutively after every integration. Each record (frame) contains a complete mass spectrum of the compound(s) present in the ion source during its integration period. The intensities of all masses in a frame
ANALYTICAL CHEMISTRY, VOL. 63,NO. 18,SEPTEMBER 15, 1991
were summed and plotted against the corresponding frame number to yield the mass chromatogram. The chromatogram shows that the early eluted peaks are closely spaced, with peak-to-peak separations between 2 and 3, and 4 and 5, being
