Improvements to a Commercial Constant Pressure High Speed Liquid Chromatograph N. J. Pound,l R. W. Sears, and A. G. Butterfield Research Laboratories, Health Protection Branch, Ottawa, Ontario, KIA OL2, Canada
As a consequence of the current interest in high speed liquid chromatography (HSLC) (1-3), numerous pumping systems are now available which are capable of delivering solvent flows against the high column back pressures encountered in this technique (4-6). The simplicity and low cost of the gas displacement pump make it particularily attractive for HSLC a t low-to-moderate pressures (1001000 psi); however, problems inherent with solvent manipulation and the introduction of samples into a pressurized system through a conventional injection port detract from the desirability of using this type of pump. In this communication, modifications to a Varian Aerograph LC-4000 Liquid Chromatograph are described which overcome many of these inconveniences. Although the standard instrument provides satisfactory performance, particularly for routine analyses, the.addition of a system of valves allows venting of the helium pressure and changing, replenishing, and vacuum degassing of the mobile phase without dismantling the apparatus. This is particularly useful in nonroutine applications when frequent changing of the mobile phase is required. The standard LC-4000 injection port has been replaced by a high pressure "septumless" injection port which, in conjunction with the valves, permits the use of a direct on-column stop-flow injection technique. This eliminates the problems associated with sample injection directly into a pressurized system.
APPARATUS A schematic of the instrument is shown in Figure 1. Connecting Tubes and Fittings. Stainless steel fittings and tubing (0.125-in. 0.d.) and Swagelok connectors (Crawford Fitting Co., Solon, Ohio) are used throughout, except when high pressure is not a factor-e.g., vent and vacuum lines, where Teflon tubing (0.125-in. 0.d.) is employed. Liquid Chromatograph (Figure 1). (A) Helium cylinder, 3000 psi; ( E ) 2-stage high pressure helium regulator, 100-1500 psi; ( E ) high accuracy pressure gauge, &in. dial, 0-1000 psi; ( F ) gas purifier (Matheson of Canada, Ltd., Whitby, Ont.); (C) pressure relief valve, 1000 psi max. (Hoke, Inc., Cresskill, N.J.); (D,P ) 2-way ball valves; (G, M , N ) 3-way ball valves (Whitey Research Tool Co., Emeryville, Calif.); (H)union tee; ( K ) union cross (Swagelok, Crawford Fitting Co., Solon, Ohio); ( R ) 7-micrometer in-line filter (Nupro Co., Cleveland, Ohio), ( J ) 1000-psi constant pressure pump; (S) high pressure injection port, CV detector, 1-mV dual-pen strip-chart recorder (Varian Aerograph, Walnut Creek, Calif.); water bath, (No. 3084, Lab-Line Instruments, Inc., Melrose Park, Ill.).
DESCRIPTION AND OPERATION Helium pressure, precisely controlled by the use of a high pressure regulator.@) in conjunction with a high accuracy pressure gauge ( E ) , forces the mobile phase through the instrument. A pressure relief valve (C) guards against pressures exceeding the 1000-psi maximum of the solvent reservoir (4The . mobile phase is protected from possible impurities in the helium by a gas purifier ( F ) . _ I
8
A HELIUM
"
SO1 VFNT RE:
II WATER BATH
J
II LJ
Figure 1.
RECORDER
Schematic of modified high speed liquid chromatograph
To whom inquiries should be directed. ( 1 ) J . J . Kirkland. E d . , "Modern Practice of Liquid Chromatography," Wiley-Interscience. New York, N . Y . , 1971 (2) N. Hadden et ai.. "Basic Liquid Chromatography," Varian Aerograph, Walnut Creek, Calif.. 1971 (3) J . J. Kirkland,Ana/.Chem., 43 (12), 36A (1971). ( 4 ) R . E. JentoftandT. H . Gouw,Ana/.Chem., 38 (12). 949 (1966). (5) L . R. Snyder, J. Chromatogr. Sci.. 7, 595 (1969). (6) R . A. Henry, "Modern Practice of Liquid Chromatography," J. J, Kirkland, Ed., Wiley-lnterscience, New York. N.Y.. 1971, p 55.
Valve D vents the pressure in the line between the helium tank and valve G. Valves G, M , N,and P provide convenient control of helium and carrier flow, facilitate vacuum degassing and mobile phase removal or replenishing procedures, and permit the use of a "septumless" injection port. pressurization (position 1) and venting (position 3) of the solvent reservoir. When the valve is in A N A L Y T I C A L C H E M I S T R Y , V O L . 45,
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position 2 (turned W), the solvent reservoir is isolated from both the helium and vent lines. Valve N permits purging of the line from the solvent reservoir (position 3) and allows the use of a stop-flow injection technique by controlling the flow of the mobile phase to the column (positions 1 and 2). Valves M and P are not essential, since vacuum and solvent inlet lines can be attached directly to unions H and K ; however, these valves facilitate replenishing and degassing of the mobile phase. Residual mobile phase is removed from the reservoir by closing valve N (position 2), venting valve G (position 3) and drawing the solvent into the vacuum trap (7‘)(valve M , position 3). The reservoir is refilled by closing valve G (position 2) and drawing the new mobile phase from an external container into the reservoir through the filling tube (valve M , position 1; valve P, position 1). Most mobile phases are readily degassed by closing valve P (position 2) and applying low vacuum (ca. 100 m m ) to the reservoir for approximately 30 seconds (valve M , position 1). The solvent line from reservoir J to valve N is purged by opening valve G (position 1) and slowly opening valve N to position 3 to expel a few milliliters of solvent into a waste container.
Table I. Solvent Delivery System: Modes of Operation Position of valves (Figure 1 ) Mode
a
Vent reservoir Empty reservoir Fill reservoir Degas solvent Purge Inject
G 3Q 3 2 2 1 1
M 2
Run
1
N
P
1 1 3Q
2 2 2 2 3Q
2 2 1 2 2
2
2
2
1
2 2
3
Open valve slowly.
To introduce a sample directly onto the column, the mobile phase flow is stopped by closing valve N (position 2), the sample is injected through the high pressure “septumless” injection port with a conventional microliter syringe, and the carrier flow re-established by reopening valve N (position 1). These functions are summarized in Table I. Received for review November 30, 1972. Accepted January 24, 1973.
Improved Double Detection Gas Chromatograph-Mass Spectrometer Interface for the Analysis of Complex Organic Mixtures Fabrizio Bruner, Paolo Ciccioli, and Silvana Zelli Laboratorio lnquinarnento Atrnosferico de/ C.N.R.,c / o lstituto di Chirnica Analitica, Citta Universitaria, 00785 Rorna, Italy
Coupling of gas chromatogrphy and mass spectrometry has been a well established technique for several years, and most of the mass spectrometers commercially available are sold with a standard coupling device to be employed for the analysis of the effluent from a chromatograph. However, the two instruments, once coupled, hardly give their best performance. In particular, the gas chromatographic columns do not work in optimum analytical conditions, mainly where linear gas velocity is concerned. Furthermore, “dead volumes,” due to the gas lines connecting the two instruments, very often affect the chromatographic efficiency. This is particularly true when the mass spectrometer is not originally constructed with the aim of coupling it with a gas chromatograph. In the past few years, several apparatus became available that partially overcome the defects cited above. But in the majority of cases, the instrument is a more highly sophisticated detector for gas chromatography than an instrument for organic mass spectrometry. This is so because sensitivity and resolution often give unsatisfactory results in practical work. Usually, the detection of the effluent from the chromatographic column is made by means of the Total Ion Monitor (TIM) which, to eliminate the high background coming from ionized helium atoms, has to be operated with an ionizing energy of the electron beam around 20 eV. When a spectrum has to be taken, the electron energy is suddenly increased to the usual 70-eV value, and this, provocates the base line going off scale during all the scanning time. In the case of complex mixtures, when many spectra 1002
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should be taken during the chromatogram, the original trace is completely lost and one has usually to make two chromatograms, the first being used as reference. This is quite time consuming in cases of very complex mixtures. Moreover it may be difficult to repeat the chromatogram twice with exactly the same analytical conditions, especially if temperature programming is used. From the above considerations we thought that double detection FID and TIM might be of great help in routine work, more so than using it occasionally to test the efficiency of the connecting lines. This paper reports the results obtained by modifying the mass spectrometer presently available in our laboratory, an AEI MS12, for better GC-MS operation.
EXPERIMENTAL Preliminary Experiments. The mass spectrometer was originally equipped with a stainless steel capillary tube (0.25-mm i.d., 1.50 m long) with one end soldered to the entrance of a standard Biemann-Watson separator ( I ) , while the other end was connected to the outlet of the chromatographic column through a rubber septum. The line connecting the separator with the source was a glass tube (2-mm i.d., 30 cm long), sealed to t h e proper source reentrance. With this device, poor chromatograms were obtained compared with those carried out with the same column but using the conventional FID. Without changing anything in the lines connecting the GC with the mass spectrometer, a double detection system was set up to look a t the differences between the two chromatograms in the same conditions. The chromatogram shown in Fig(1) J. T. Watson and K. Biernann, Anal. Chem., 36, 1135, (1964).
