Anal. Chem. 1981, 53, 171-174
compounds at the parts-per-billion levels relevant to atmospheric and other environmental applications.
ACKNOWLEDGMENT Mr. Itsvan Ary of ERT participated in the early phase of the method development.
LITERATURE CITED (1) Fracchia, Mario F.; Schuette, F. J.; Mueiler, Peter K. Enviion. S d . Techno/. 1967, 7 , 915-922. (2) "Procedures for Determining Exhaust Carbonyl as 2,CDinitro~hen~lhydrazones", Bartlesville Petroleum Research Center, Bureau of Mines, Final Report for Project CRC-APRAC No. CAPE-1 1-68, Revised Aprll 1969; available as Natlonai Technical Information Service Report
(3) (4) (5) (6)
171
No. PB-200-883; National Technical Information Service: Sprlngflekl, VA, 1971. Papa, L. J.; Turner, L. P. J. Chromatogr. Sci. 1972, 70, 744--747. Papa, L. J.; Turner, L. P. J. Chromatogr. Sci. 1972, 70, 747-750. Seiim, Sami J. Chromatogr. 1977, 736,271-277. Grosjean, Daniel; Fung, Kochy; Atkinson, Roger Paper No. 80.50-4, 73rd Annual Meeting of the Alr Pollution Control Assoclation, Montreal, Canada. June 23-27. 1980: Air Pollution Control Assoclation: Plttsburgh, PA.
RECEIVED for review July 15,1980. Accepted October 23,1980. This work was supported by Environmental Researclh & Research and funds part Of Project NO. M-0171-001.
Liquid Chromatograph/Mass Spectrometer Interface with Continuous Sample Preconcentration Richard G. Christensen," Harry S. Hertz, Stanley Melselman, and Edward Whlte V Center for Analytical Chemistry, National Bureau of Standards, Washington, D.C. 20234
A liquid chromatographhnass spectrometer system whlch performs enrichment of the sample In the effluent of a conventional llquid chromatograph prior to Its being introduced into a dlfferentlaiiy pumped quadrupole mass spectrometer is descrlbed. The effluent from the liquid chromatograph is concentrated by evaporation of most of the solvent. Solvent evaporation is accomplished by allowing the effluent to flow down an electrically heated wire with the current to the wire controlled by a feedback loop from a volume-sensing photocell. The concentrated effluent flows through a very small needle valve which regulates the flow into and thereby the pressure inside the mass spectrometer. The valve Is constructed such that llquld Is sprayed into the Ion source of the mass spectrometer. The appllcatlon of the system to polynuclear aromatlc hydrocarbon characterization and to quantltatlon of phenolic compounds In alternate fuels is shown.
The great potential of combined liquid chromatographyand mass spectrometry (LC/MS) as an analytical tool has fostered research by a considerable number of laboratories. In addition to the collection, evaporation, and transfer of LC fractions to the mass spectrometer, a number of means have been used to combine LC and MS. Recent reviews by Arpino and Guiochon ( I ) , Zerilli (Z),and McFadden (a review which contains examples of applications of LC/MS contributed by other authors) (3) describe the operating principles, construction, and performance of these devices. Two methods, direct liquid injection (DLI) and evaporation onto a moving wire or belt, are now offered commercially. Although the DLI technique is intrinsically simple to implement, the moving helt interface has the advantages of concentrating the solute, thereby introducing a greater proportion of the sample into the ion source of the mass spectrometer and of permitting both electron impact and chemical ionization modes of operation. A system which could preconcentrate the liquid stream and introduce the concentrate by DLI would combine some of the advantages of both or-
dinary DLI and the moving belt technique. The interface described in thispaper was designed and built with this aim in mind. The interface device concentrates a liquid stream by allowing it to flow down a resistance-heated stationary wire. The residual liquid is drawn into the mass spectrometer through a capillary tube with a needle valve at the ion source end. The liquid sprays from the needle valve into the ion source of a conventional, differentially pumped, quadrupole mass spectrometer.
EXPERIMENTAL SECTION Concentrator Wire. The construction of the concentrator wire is shown in Figure 1. The concentrator wire consists of three segments, the active lengths of which are 15, 7.5, and 15 om, respectively. The segments are connected by silver-soldering in butt joints which are then filed to present a short, symmetrical taper between segments. The concentrator wire is held taut and straight by a spring at its lower end through which the lower electrical connection is made. The wire passes across a gap in the DLI probe and the residual stream forms a drop in the gap. When the drop becomes too large, it flows down the wire or along the outer surfacer3 of the DLI probe and is lost. A light emitting diode and photocell are fitted at the gap and sense the size of the drop. Feedback from the photocell controls the current through the wire to hold the drop size constant. The circuit for the photocell sensor and power supply to the wire is shown in Figure 2. An envelope of glass is provided to exclude dust and air currents from the stream on the wire, and a TFE manifold is set into the top of the glass envelope to direct a stream of filtered air or inert gas downward through the tube to carry away solvent vapors. 'I'his cap has passages to conduct the liquid chromatographic effluent to an opening concentric with the concentrator wire. DLI Probe. The construction of the DLI probe is shown in Figure 3. The tip of the probe is formed into a needle valve in order to assure that the pressure drop occurs at the ion source and to control the rate of flow of liquid into the vacuum. The stem of the valve is a 0.10-mm tungsten wire ground to a pencil-point at the tip. The probe itself is formed from a hard-drawn nickel tube with 1mm 0.d. and 0.12 mm i.d. The valve seat at the end of the probe is formed by peening the opening of the tube until it is almost entirely closed (-0.01 mm i.d.). When the seat and stem pieces
This article not subject to U.S. Copyright. Published 1981 by me Amerlcan Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 2, FEBRUARY 1981 b electrical cmneetion
0 8 mm sttomlcrr
0 6mm ~onilonton
Flgure 1. Liquid chromatographic solvent concentrator wire for enrichment of the LC effluent (not arawn to scale). of the valve are assembled and the tungsten wire is rotated, a circular orifice of -0.025 mm diameter is formed in the seat. The position of the wive stem is controlled over a range of0.2 mm with a sprineloaded lever operating at a mechanical advantage of about 201. The lever is moved by a knurled nut on a screw. Fine adjustment is required because the quality of the spray is very sensitive to the position of the valve stem. Mass Spectrometer. The mass spectrometer used is a variation of the Krtranurlear Laboratories SpectrEL, equipped for differential pumping, and fitted with a conventional electron impact ion source. The ion source electronics are those for an atmospheric pressure ionization instrument. The quadrupole volume is pumped with a 280 LIS diffusion pump protected hy a hutterfly valve. and the ion source is pumped with a -150L, s turbomolecular pump. This cornhination allow the vacuum system to be opened rapidly for repairs and modifications. For the developmental work, the instrument was operated with a strip chart recorder for monitoring single ions and an oscillographic recorder fur scanning spectra.
! L.E.D. ; Photo- Dorlington
The mass spectrometer housing is fitted with an additional flange oppmite to the 1)l.l tube inlet. This entry port has been utilized to make measurements of the ion sourre pressure by inserting a tuhe connected to a diaphragmtype pressure gauge. This flange has also heen equipped with a viewing port tn observe the spray from the DLl tube into the ion source. Liquid Chromatography. The liquid chromatograph used was a conventional commercial instrument equipped with an ultraviolet absorbance detector operating at 254 n m and a loop type injector. Ortadecycykilane,aminmilane,and aminwyanosilane hieh.nerformance liauid chromaturraDhic - . columns were used in evbation studies. . Solvents used were Spectro- or HPLC-grade. With the exception of an interference by the antioxidant in tetrahydrofuran, no difficulties were encountered with impurities in the solvents. (The possibility of such interferences must be kept in mind when a solvent stream is evaporated to 1%of its initial flow.) The quantitation of phenol and o-creaol in the shale oil standard reference material (SRM 1580) was performed on a polar aminocyano high-performance liquid chromatographiccolumn with a correspondingpellicular guard column. The mobile phase was n-hexane with 2.5% isopropyl alcohol, and a flow rate of 1.0 mL/min was used. Shale oil WBB injected directly onto the column through a 1.5-pLloop. Quantitation of phenol was by external standard relative to solutions of known phenol concentration prepared in the mobile phase. Cresol quantitationwas by standard addition, adding several known amounts of 0-cresol to aliquots of the shale oil sample. RESULTS AND DISCUSSION Interface Design. Several design considerations were important to the proper functioning of the total LC/MS interface. For the concentrator wire, it is desirable to have a large diameter at the top where the liquid flow is greatest, so that the liquid will be evenly supported by the wire. The diameter needs to be small at the bottom, so the residual flow will keep the wire wetted and no dry spots will develop. It was also desirable to have the resistance of the wire larger at the top (if the wire is to he heated by passing a current through it), so that the heat input will be greatest where the flow rate is large. Low heat input is desired at the bottom where the liquid flow is least so that the evaporation rate is not too sensitive to changes in heating current. These sets of requirements could be met only with a wire that was not of Current Ronqe Select
L.G.....
input 115 V AC
Flgu18 2. Circun diagram for me photocell sensor and power supply controlling ihe heat input lo the concentrator wire.
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I
IOmm OD D
by nickel-plating
Concentrotor wire Figure 3. Details of the direct liquid injection (DLI) probe for transfer of the wncentrated eluate into the mass spectrometer.
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Table I. Flows to Concentrator max solvent n-pentane 2,2,4-trimethylpentane methanol 50%MeOH/H.O
AHv
rate. Jlke m L / A x io:
