Ultrasonic Degasser for Use in Liquid Chromatography V. E. Dell'Ova, M. B. Denton, and M. F. Burke' Department of Chemistry, University of Arizona, Tucson, Ariz. 85721
The advent of high pressure pumps and small volume detectors has brought a revival of liquid chromatography (LC). The driving force provided by these pumps is sufficient to allow rapid analysis while using highly efficient narrow bore columns. While much progress has been made in LC column technology, many of the major problems of the detection system remain unsolved. The most commonly used detectors in LC are microcell ultraviolet and refractive index systems. Both of these detectors respond as a function of the concentration of the sample solute in the eluent and therefore benefit from minimizing the cell volume and maximizing the cell path length. Typical values for such systems are 6- to 8.microliter volumes with one-centimeter pathlengths. A problem resulting from this geometry is that dissolved gases, in certain solvent systems, can form bubbles under high pressure gradients which then causes extreme noise problems as they pass through the narrow detector cells. The noise problem arising from dissolved oxygen can become especially critical in LC applications involving the use of aqueous and mixed aqueous-nonaqueous solvent systems. This problem is not unique to LC. Several methods for the removal of dissolved gases from liquids have been used ( I ) . The most common approach to this problem has been to use either heating, or vacuum, or a combination of these to degas the solvent (2, 3). A second approach has been suggested ( 4 ) . That is to place a back pressure upon the detector which forces the gas bubbles to remain dissolved. This has two disadvantages. First, this can cause the detector seals to leak and. second. it is inconvenient for sample collection. The use of heat and/or vacuum for the degassing of solvents has the disadvantage of requiring not only the necessary hardware but an inordinate amount of time, often approaching 6 hours, plus the additional time required for the solvent to return to a reasonable operating temperature. An alternate method of degassing solvents has been investigated and found to be quite useful, especially for use with the more difficult to degas aqueous solvents. This method involves the use of ultrasonic radiation to remove the dissolved gases. The degassing of a liquid by means of ultrasonic radiation has been known for some time. Examples of this include the degassing of molten metals and beer (5). The term ultrasonic is used to describe sound waves with a frequency higher than those detectable by the human ear. In general, this refers to frequencies above 16 kilohertz. When these waves are propagated through a medium, regions of rarefaction and compression are formed. These regions correspond to areas of large negative and positive pressure which couple to cause cavitation. Cavitation is a term used to describe the formation of bubbles or cavities in liquids. If the liquid contains dissolved Author to whom correspondence should be addressed. (1) R. Battinoand D. F. Evans, Ana/. Chem.. 38, 1627 (1966). (2) R. Battino. D. F. Evans, and M . Bogan. A n a / . Chim. A c l a . . 43, 518 (1968). (3) R. Battino. M . Banzhof, M . Bogan, and E. Wilhelm, Ana/. Chem.. 43, 806 (1971). ( 4 ) J. J. Kirkland, E d . , "Modern Practices of Liquid Chromatography," Wiley-Interscience, New York. N.Y., 1971. ( 5 ) B. Brown and J. E . Goodman, "High-Intensity Ultrasonics," D. Van Nostrand Company, Inc., Princeton, N.J., 1965.
gases, the gases exist for the most part as minute gas bubbles which will diffuse from the regions of high compression to the regions of rarefaction. The small bubbles will tend to coalesce into larger bubbles which then rise more readily to the surface of the liquid. This phenomenon can be employed as an extremely efficient means of degassing a liquid. High intensity ultrasonic waves can be produced in a number of ways. The three most common ways are mechanical, magnetostrictive, and piezoelectric generators. The one employed in this study falls in the third category. When a voltage is applied to a piezoelectric material, it undergoes a mechanical deformation which is proportional to the applied voltage. If the transducer is placed in an alternating field, it will be expanded and compressed, thereby producing longitudinal ultrasonic oscillations in the surrounding medium. The electrical frequency is adjusted so as to be at resonance with the fundamental mechanical frequency of the transducers so that the amplitude of the oscillations will be at a maximum. The transducer used in this study is a lead zirconatetitanate, polarized piezoceramic. Ceramic piezoelectric transducers have two major advantages over the conventional quartz piezoelectrics (6). First, ceramics can be easily manufactured. They can be made as large as desired and in almost any shape. Their characteristics can be controlled in many ways by adding different compounds in different proportions. This makes them versatile, cheap, and readily available. Second, they have a large electric energy to acoustic energy conversion efficiency. This makes ceramics suitable for the production of high intensity ultrasonic waves. For a more detailed discussion of ultrasonics see references (5-7).
EXPERIMENTAL Apparatus. A detailed view of the ultrasonic irradiation cell is shown in Figure 1. The body of the ultrasonic degasser is 10.16 cm X 30.48 cm thickwall conical glass pipe (P7370 Corning Glass Works, Corning, N.Y., 14830) with aluminum flanges (Corning P 9325). The lower half of the chamber consists of a phenolic housing which holds the lead zirconate-titanate, polarized piezoceramic (Clevite Corp., Bedford, Ohio 44014). There is a 18.42-cm dia. x 2.54-cm brass plate between the glass pipe and the phenolic board housing. The base plate has a center hole 8.26 crn in diameter. Six fingers made of beryllium-bronze spring stock are used to make electrical contact between the top of the transducer and the brass plate. Contact on the bottom side of the transducer is made by a spring with a banana plug lead soldered to it. The spring is held in the phenolic board housing below the piezoelectric. O-rings are placed between the phenolic board and the bottom of the piezoelectric, the bottom of the brass plate and the top of the piezoelectric, and between the glass pipe and the top of the brass plate. Six holes are tapped through the brass and phenolic board housing, and these are then fitted to the flange and then tightened to form an airtight seal. The top of the apparatus was a 18.42-cm dia. X 0.79-cm brass plate with a three-way valve in the center. An aspirator or a vacuum pump is attached to this valve to evacuate the space above the liquid being degassed. The brass plate has 6 clearance holes and is connected to the flange. An 0 ring is placed between the glass and the plate so as to produce an air-tight seal. The radio frequency power source is composed of an army surplus BC-191 transmitter (Farnsworth and Radio Corporation, (6) R. Goldman, "Ultrasonic Technology," Reinhold Publishing Corporation, New Y o r k , N . Y . 1962. ( 7 ) J. Blitz, "Ultrasonics: Methods and Applications,"Van Nostrand Reinhold Co.. New Y o r k , N . Y . . 1971.
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that up to 7 liters or as little as 100 ml could be degassed if desired. The wavelength used was 350 nm and a slit width of 0.20 mm.
0
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c
Figure 1.
Detailed view of ultrasonic irradiation cell
Fort Wayne, Ind.) and a TU-26R tuning unit operated a t 350 kHz with a 200 watts of input power. The ultrasonic irradiation cell was operated in acyclic mode with 15 sec on and 15 sec off pulse times in order to minimize local heating and facilitate removal of the head gas. The liquid chromatograph used consisted of a 750 psi constant pressure pump (Varian Aerograph, Walnut Creek, Calif., 94598), a 2-mm ID X 60.96-cm stainless steel column packed with porasil C (Waters Associates, Framingham. Mass. 02701) and a specially designed L V detector. The detector consisted of a low volume flow through cell mounted in a variable wavelength spectrophotometer (Gilford Model 2400, Oberlin, Ohio). The flow through cell was constructed from a 2.44 cm X 1.58 cm X 1.00 cm block of Kel-F. A channel 1 mm in diameter from the front face of the block provided an 8-pl cell volume. The flow inlet and outlet are 2.mm o.d. stainless steel tubing which are force-fit through the top, one at either end of the channel. The ends of the channel are sealed with a lens and O-ring on either end. Solvent. Water degassed by boiling for various periods of time and water degassed in the ultrasonic degasser were placed in the pump and a base line run for two hours. Approximately 500 ml were degassed in the ultrasonic cell since this coincided with the volume of the pump. The total cell volume is about 10 liters so
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RESULTS AND DISCUSSION Previous experience in our laboratory has shown that boiling for more than six hours shows no significant improvements. It was therefore decided to compare degassing by boiling for periods of six hours or less to degassing ultrasonically. Visual observation of the ultrasonic cell indicated that no further degassing was occurring after fifteen minutes. The amount of noise was measured in terms of noise spikes per unit time. The normalized ratio of noise for undegassed water, water boiled for one hour, for six hours, and water degassed ultrasonically was 24:17:1:1. Therefore, it can be seen that a fifteen minute ultrasonic degassing is equivalent to boiling water for six hours. There is another important advantage of the ultrasonic method. Liquid chromatograms are not usually run a t 100 "C. When using the boiling technique, the solvent must be cooled. This means that either some method of cooling must be provided or, if the solvent is allowed to cool in the pump by means of ambient air, a two- to three-hour waiting period is required. Also, a non-trivial advantage is that the degassing time for any solvent system can be determined visually, i. e . , when no more bubbles appear after the system is pulsed. Although this technique is equally applicable to any solvent system, water was chosen for this study because it represents a solvent which contains a large quantity of dissolved gases. Salt solutions are even more difficult to degas, and with gradient elution becoming more popular, this problem must be overcome. Reversed-phase chromatography is also being used, and again the degassing problem cannot be overlooked when using this technique. The apparatus used for this experiment was specially designed for the purpose of testing the technique of ultrasonic degassing for use in conjunction with LC. For the average investigator, it may not be practical in terms of cost or time to build such an elaborate instrument. An alternate solution to this problem could be achieved through the modification of an ultrasonic cleaning bath. This should be both easy and inexpensive. Received for review January 11, 1974. Accepted April 1, 1974. The work was partially supported by the National Science Foundation under GP 17332.
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