Sample Valve For Chromatographic Apparatus

Abstract

Chromatographic apparatus is described wherein the improvement comprises a sample valve including slidably engaged members formed of a hard, wear-resistant alumina lapped to a high degree of flatness. These valve members are adapted to be molded using relatively inexpensive techniques. The engaged surfaces of the valve present cooperating ports and cavities which are adapted, when the valve members are shifted from one position to another, to inject a fixed amount of a sample mixture into a column. The valve surfaces also are formed with grooves carrying a flow of carrier to aid in isolating the injection ports from adjacent sample ports.

Parent Case Text

This application is a continuation-in-part of our copending application Ser. No. 829,576 filed on June 2, 1969. This invention relates to chromatography. More particularly this invention relates to a metering valve for injecting a precise amount of sample mixture into a chromatographic column.

Description

Chromatography is an analytical procedure of separating components in a mixture by passing a sample of the mixture through a column containing a material which retains the components for differing periods of time. Thus, the individual components emerge at different times from the column. A detector placed at the output of the column provides an electrical signal in the form of a series of peaks, each reflecting the concentration of a respective component.

The chromatographic technique is capable of determining the concentration of minute amounts of a component in the mixture. To make fullest use of this capability, the sampling of the mixture must be carried out consistently and accurately. Specifically, the sample valve employed to inject a sample of the mixture into the column must be essentially leakproof, and must be capable of reliably metering a predetermined amount of sample each time it is actuated. Moreover, the valve must be effectively "neutral" with respect to the sample fluid. This is, the valve must not alter passage of the sample through the input conduits (e.g., it must not adsorb the sample), it must not react with the sample, and it must not contaminate the sample with foreign matter, even of very tiny amounts.

Over the past decade or so, a variety of different types of sample valves have been devised, some of which have gone into extensive commercial use. However, notwithstanding design efforts by many people, none of the sample valves has turned out to be truly satisfactory for the intended use. One of the principal problems with available valves is excessive wear with usage, leading ultimately to leaks requiring repair or replacement of the valve. For example, in conventional valves with sealing surfaces formed of stainless steel and Teflon, the valve may not last beyond 300,000 actuations without thereafter causing undue leakage. Moreover, conventional sample valves are inordinately expensive to manufacture. Thus, there has existed for some time an urgent need for a new valve design providing important improvements in longevity and reduced cost.

In a preferred embodiment of the invention to be described hereinbelow in detail, there is provided a sample valve which will operate without significant leakage for a substantially greater number of operating cycles than conventional valves. This new valve moreover is relatively inexpensive to make. The new valve incorporates planar sealing surfaces molded of a specially selected ceramic material, advantageously very hard, dense, and smooth-surfaced alumina which has been found to be unusually well suited for the chromatographic application, meeting all of the stringent requirements discussed above, yet capable of being manufactured at relatively modest cost.

The deterioration of prior art sample valves often arises from scratching effects caused by small hard particles that get in between the valve surfaces. The scratches in the valve surfaces destroy the fluid tightness of the valve which then becomes unable to provide an accurate sample for analysis, and permits leaks into the column which create errors. The valve according to the invention, however, provides an unusually long-lasting useful life free from abrasion effects due to such dirt or other particles.

Accordingly, it is an object of the invention to provide superior chromatographic apparatus including a relatively inexpensive sample valve capable of reliably injecting a sample into a carrier stream and operable over many cycles without deterioration. Other objects, aspects and advantages of the invention will be understood from the following description of a preferred embodiment of the invention considered together with the drawings wherein:

FIG. 1 is a block diagram and perspective view of a sample valve in accordance with the invention as used in connection with a chromatographic instrument;

FIG. 2 is a perspective schematic representation of fluid sampling port interconnections for the valve of FIG. 1;

FIG. 3 is a plan view of a valve slide employed with the valve of FIG. 1;

FIG. 4 is a sectional view of the valve slide taken along line 4--4 in FIG. 3;

FIG. 5 is a plan view of a valve seat employed with the valve of FIG. 1;

FIG. 6 is a sectional view of the valve seat taken along line 6--6 in FIG. 5;

FIG. 7 is a sectional view of the mounted valve slide and seat taken along line 7--7 in FIG. 3; and

FIG. 8 is a plan view of the manifold employed with the valve of FIG. 1.

With reference now to FIG. 1, there is shown chromatographic apparatus 10 incorporating a sample valve 12 connected to a source 14 of carrier gas such as helium. The carrier gas is continuously supplied through the valve 12 to a conduit 34 leading to a chromatographic column schematically shown at 16. (In a practical embodiment of the invention, the column 16 typically would consist of a small-diameter tube, wound in compact configuration, but has been shown as an upright element merely for illustrative purposes.)

At the output of the column 16 is a detector 18, such as a thermal-conductivity cell, to provide electrical signals representative of the various components separated by the column. The electrical signals are passed to a utilization device 20 which may include an electronic integrator, memory, display, controller, recorder and alarm, in various combination as needed for utilization of the chromatographic measurement. The gases which emerge from the column are vented at an exhaust port schematically indicated at 22.

The chromatographic instrument operates by injecting in series with the carrier gas stream flowing to the column 16 a preselected amount of a mixture to be analyzed. A supply of the mixture is illustrated at 24. The mixture gas in the embodiment of FIG. 1 flows continuously through a conduit 26 to the valve and thence through another conduit 28 leading to a sample vent.

The valve 12 serves upon actuation to inject a precisely metered quantity of the mixture into the carrier gas stream flowing through conduit 34 to the column.

The valve 12 basically comprises two relatively slidable parts, a valve slide 38 and a valve seat 40. Below the valve seat 40 is a manifold 42 which interconnects the several conduits with ports located in the valve interaction surfaces between the slide and the seat. The valve seat 40 is secured to the manifold 42, as by means of solder glass fusing, and the manifold in turn is supported by a suitable housing (not shown).

The valve slide 38 and valve seat 40 are each formed of a hard, wear-resistant material presenting interaction surfaces lapped to a high degree of flatness and polished to a high degree of smothness. The flatness after lapping is of the order of a quarter wavelength of light, that is, about one to five millionths of an inch. The preferred material is a ceramic comprising at least 80 percent pure alumina, advantageously better than 90 percent pure alumina, the remainder being a suitable binder such as glass silicate material, or the like. Both interacting slide surfaces of the valve should be made of the selected ceramic material. The finest results have been achieved by using alumina which is at least 99.5 percent pure.

There are many different kinds of materials encompassed by the term ceramic, and those particular kinds historically understood to typify basic ceramic material, i.e., materials such as pottery clay or glass, are not appropriate for use in a chromatographic valve. Even more modern ceramics, including a number of categories of alumina, are not suitable for achieving the high performance required. For example, many types of alumina are chromatographically active (i.e., adsorbent), and thus, if used as the interface material in a chromatographic valve, would interfere with the passage of the sample fluid through the valve.

To highlight this adsorbent characteristic of alumina, it may be noted that alumina is well known in the art of chromatography as an excellent packing material for chromatographic columns; see, for example, "Principles of Adsorption Chromatography" by Lloyd R. Snyder, 1968, pages 163-168. Certain classes of alumina also may tend to react with the sample, as noted on page 357 of the above-mentioned text by Snyder. Thus one considering the application to chromatographic valves of ceramics such as alumina would necessarily face serious difficulties with potentially adverse characteristics for such application.

It has been found, however, that such serious problems can be avoided by the use of specific ceramics which perform surprisingly well when used as the interface material in a chromatographic valve as disclosed herein. In more detail, now, the basic ceramic material for this purpose should be a highly-pure sintered crystalline metal oxide, and especially the poly-crystalline form (of which alumina may be considered a principal representative). Such materials have been referred to in the art as "technical" ceramics, or "engineering" ceramics, reflecting the high purity of metal oxide. One accepted standard is 80 percent pure metal oxide or better.

The ceramic material must be chromatographically inert i.e., not chromatographically active, and with respect to alumina this can be achieved by using the so-called alpha alumina, fired at a temperature above about 1,100.degree. C. The purity of the material preferably is higher than 80 percent metal oxide, desirably as high as 99.5 - 99.9 percent pure. The ceramic thus should be very dense, i.e., tightly compacted crystals with very little binder (typically glass) in the intercrystalline spaces. A specific gravity for alumina of above 3.0 grams per cc. is an important characteristic in this respect. As one example, an alumina of 80 percent purity may typically have a specific gravity of about 3.4 grams per cc. Ideally the specific gravity should be even closer to the theoretical maximum value of about 3.98, typical for sapphire. In general the density of the metal oxide used for the valve surfaces should be above about 75 percent of the theoretical maximum and preferably higher.

To eliminate any significant effect of sample adsorption (and simultaneously to assure essentially leakproof operation of the valve), the metal-oxide ceramic material must have an extremely smooth surface, effectively free from pores, i.e., free from cavities of any kind which might tend to temporarily trap particles of the sample or other fluids passing through the valve. The surface finish should be smooth to a value less than about 5 micro-inches (RMS); in some cases, a smoothness of less than one microinch (RMS) will be appropriate. Such smoothness moreover tends to reduce substantially the exposed surface area, and thereby significantly minimizes any tendency of the material to adsorb the fluid sample passing through. To obtain a dense and pure sintered metal oxide, suitable for achieving the required smooth surface finish, entails a quite high sintering temperature, e.g., above about 1,100.degree. C, and preferably between 1,400.degree. and 1,800.degree. C.

There is some uncertainty about the exact mechanism responsible for adsorption in a ceramic such as alumina. The adsorptive effect has been considered possibly to be due at least in part to molecular binding forces, of the type sometimes referred to as Van der Waal forces. However, it has been found that this factor does not appear to be predominant in the present application. If it were predominant, or significant, the adverse effect of such forces would tend to increase as the surface is made more smooth, e.g. similarly to the known characteristic of smooth metal sliding surfaces which tend to stick as a result of the so-called "Jo-block" effect. However, experience with the present valve has indicated that the adsorption effect of the ceramic interface surfaces herein is minimized by providing highly smooth sliding surfaces. Thus, it may be noted that high surface smoothness not only provides improved leak-resistant characteristics, but also advantageously achieves surprisingly superior performance from a chromatographic analysis viewpoint, without detrimental adsorption of sample fluid by the ceramic interface.

The mechanical properties of the valve material also are important. The metal oxide ceramic should for example be very hard, e.g., above about 8.0 on Moh's hardness scale. The described alumina (and, for example, the metal oxide berylia as well) has a hardness of about 9.0 on that scale, providing superior wear and abrasion resistance, as well as the capability of grinding down particles of foreign matter which might enter between the sliding valve surfaces. The ceramic also should have a high impact resistance, preferably above 6.0 inch-pounds on the Charpy test. An alumina of 85 percent purity will have an impact resistance of about 6.3 inch pounds, whereas certain electrical grade ceramics (such as steatite) may have an impact resistance of only 4.5 - 5.0, and clay ceramics may have impact resistances below about 3.5. The tensile strength also should be very high, desirably above 20,000 psi. Alumina of 99.5 percent purity has a tensile strength of about 28,000 - 32,000 psi, whereas the common electricalgrade ceramic steatite has a strength of only about 8,500 psi, and stone ceramic products even as low as 2,500 psi.

The valve slide 38 is reciprocably shiftable along an axis 36 as by means of a suitable driver, such as a solenoid or air cylinder, not shown in the drawing. This valve slide is provided with a spring receiving recess 46 to receive a spring 48 used to urge the slide onto the seat 40. In order to maintain the slide in lateral alignment, the spring 48 acts at an inclined angle to the upper surface 50 of the slide to maintain the side surface of the slide in contact with an abutting cylindrical tube 52. The tube 52 is supported by the valve housing not shown in the drawing, and is made of the same material as the valve slide and seat.

The flatness and smoothness of the interaction surfaces between the valve slide and seat assures a close and tight fit between them, so as to effect a leakproof seal for the multiplicity of ports and passages. The spring 48 aids in this, and also aids in assuring that any particles which pass into the region between the valve slide and seat will be ground or abraded away by the hard ceramic interaction surfaces.

Referring now to FIGS. 2 and 3, the valve body 40 is formed with four pairs of centrally-located vertical passages 62,72; 86,90 and 62',72'; 86',90' communicating between the manifold 42 and the top face of the valve body. The first two pairs of passages 62,72; 86,90 are operative in injecting a sample into the column when the valve slide 38 is shifted in one direction, and the other two pairs 62',72'; 86',90' are operative in injecting a sample into the column when the slide is shifted in the other direction. That is, the valve is double acting, by injecting a prefixed amount of sample mixture when shifted in either direction. However, a single acting valve is also contemplated by the invention.

Referring now also to FIGS. 5 and 6, the interaction face of the valve slide 38 is formed with a plurality of spaced transverse grooves 70,80; 70',80'; 88,88' adapted to connect together the pairs of valve body passages 62,72; 86,90, etc., in either position of the valve slide. In the position illustrated, groove 70 connects passages 62 and 72, and groove 88 connects passages 86 and 90, while the end groove 80 does not register with any passage. By causing sample mixture to flow up through passage 62, across groove 70 and down passage 72, groove 70 serves in effect as a "metering" section to store a predetermined fixed amount of sample available at any time to be injected into the column. This sample mixture is supplied through conduit 26 and manifold passage 66, and the flow out is through manifold passage 76 to a vent (not shown).

While groove 70 is thus being supplied with a flow of sample mixture, carrier gas simultaneously is flowing up through valve seat passage 86, across the next adjacent groove 88 in valve slide 38, and down through valve seat passage 90 to the conduits leading to the column 16. By shifting the valve slide to the left (FIG. 2), groove 70 with its precisely metered amount of sample mixture is brought into registration with passages 86 and 90. Thus the continued flow of carrier through these passages entrains the sample "slug" and forces it through the column for analysis.

The carrier gas flowing through groove 88 at injection station 54 comes from a manifold passage 82 connecting valve seat passage 86 to the carrier gas conduit leading to supply 14. The output of injection station 54 is directed to the input of the other injection station 56 by a horizontal manifold channel 94. Thus upon shifting slide 38 to the left, the injected slug of sample from metering section groove 70 follows the carrier gas path over to injection station 56, up valve seat passage 86', traverses groove 88' (which replaces groove 70') and flows down through passage 90' and an exit passage 94' out through conduit 34 to the column 16.

With the valve slide 38 in its leftward position, groove 70' is supplied with sample mixture through vertical passages 62' and 72' at sample station 58, venting from outlet passage 62' through manifold passage 78 and sample vent 28. The sample mixture passing up through inlet passage 72' is derived from conduit 26' leading to conduit 26 and thence to mixture supply 24. Thus when the valve slide is shifted back to its original position, this metered quantity of sample moves into registration with passages 86',90' to be entrained by the continuing flow of carrier gas and thereby directed to the column 16 for analysis. The sample mixture connected to passage 72' may be a supply separate from supply 24, to provide redundancy assuring operation if one supply fails.

One of the important benefits of the valve construction described herein is that the operative parts can readily be molded or cast using relatively inexpensive known techniques. Thus, the valve seat and the valve slide can be made in corresponding dies comprising cavities into which may be placed the ceramic material as a powder, to form the green mold. The bottom surfaces of the die have a topography corresponding to the reverse of the desired shape of the interacting valve surfaces, i.e., predominantly flat sections with raised ridges to make the surface grooves and upstanding core members to make the vertical passages.

This green form is fired at a high temperature in a furnace to produce the hard ceramic parts. The flat interacting surfaces then are lapped to a fine degree of smoothness and flatness. Continued operation of the valve tends to improve the seal between the two interacting surfaces, as a result of the self-lapping action of planar surfaces. No lubricant is required between these surfaces, nor are special wedge shapes needed for effective sealing.

In operation, the extremely hard alumina ceramic is not scored by dirt, metal chips or other foreign material, as are the surfaces of conventional chromatographic valves. Wear tests have indicated actual improvement in the sealing surfaces. Such tests have shown that at least about 2,000,000 are possible without developing the usual leakage problems encountered in prior art chromatographic valves. It is probable that even greater numbers of valve actuations would be entirely practical if desired. The available evidence suggests that valves in accordance with this aspect of the invention will have a life which may be upwards of ten times that of prior art valves.

The corrosion resistance of alumina is superior to stainless steel used in some prior chromatographic valves, thus minimizing a problem encountered in certain industrial applications. Such ceramic also withstands higher temperatures than prior valves which included valve surfaces of plastic such as Teflon. Because of this high temperature capability, valves in accordance with the present invention advantageously may, if the ports become clogged with foreign substances, be cleaned simply by baking at an elevated temperature, e.g. high enough to oxidize gum, varnish and tars to a powder which may readily be removed.

A valve of this design may be quite compact, e.g., having an axial length of about 1 inch and width of about three-fourths of an inch. The stroke in a typical unit might be about 0.14 inches. Preferably, resilient bumper stops are provided to cushion the ceramic material from excessive shock at the end of each stroke.

If liquid sample mixtures are being analyzed, capillary action may tend to draw the sample material from the sample ports into the carrier passages leading to the column. To avoid such undesired contamination, the valve advantageously may be provided with what might be termed fluid shields around the sample injection stations 54 and 56. These fluid shields are composed of interconnected gutters through which carrier fluid from source 14 flows continuously to drain so as to intercept any leaking sample fluid and wash it away with the carrier. This carrier may be obtained from a source having a higher pressure than source 14, but in any event after passage through the shield is discarded through vents and not used in the columns.

These gutters are formed in the interaction surfaces of the valve slide 38 and valve seat 40 which cooperate to form enclosed channels between the adjacent surfaces. These channels provide a closed rectangular shield around each of the two sample injection stations 54 and 56. The shields are operated in series, i.e., carrier first flows through one rectangular shield, then passes through an interconnecting channel 114 in the manifold 42 to the other shield, and from there to carrier vent passage 98 and vent conduit 32.

Specifically in reference to the fluid shields, carrier gas at either atmospheric or elevated pressure is first supplied from a flow controlled source (not shown) through shield passage 100-102 in the manifold and valve seat respectively to a corner inlet port 104 in the valve seat. The carrier passes from the port 104 through two separate paths to an opposite corner outlet port 106. One path comprises a first gutter 108 which connects to a corner recess 110, both being formed in the valve seat interaction surface; and a second gutter 112 which is formed transversely in the valve slide 38 and provides a carrier passage between the corner recess 110 and the corner outlet port 106. The second path includes transverse gutter 112' formed in the valve slide to connect to a corner recess 110' formed in the valve seat 40, and a second gutter 108' completing the path from the corner recess 110' to the outlet port 106.

From the corner outlet port 106, the carrier flows down through interconnecting passage 113 and across channel 114 in the manifold to corner inlet port 104' in the rectangular shield around the other injection station 56. The arrangement of this shield is similar to the first shield and thus will not be described in detail.

From the view presented by FIG. 2, it will be appreciated that the injection stations 54 and 56 are enclosed by fluid shields for both positions of the valve slide 38. This result is obtained by providing an additional transverse gutter 112' in the valve slide which is so spaced from gutter 112' that upon valve actuation into the alternate position the gutter 112" is placed where 112' previously was located (at station 54) and 112' is placed where gutter 112 was. In this manner gutter 112 is actually shared between the two fluid shields.

The reason for locating part of each fluid shield in the slide and the other part in the valve seat is to disconnect the moving gutter passages from each other, thereby to prevent "short circuiting" the sample ports directly to the injection ports during movement of the valve slide. The gutters in the valve slide disconnect completely from the valve seat gutters during transit so as to prevent injecting fluid into (or removing fluid from) the injection passages.

In some applications, it may be desirable to enhance the sealing between the valve manifold 42 and seat 40 by a similar shielding technique. Thus, as shown in FIG. 8, a fluid shield is formed in surface 43 by etching a gutter 120 around the several ports. The gutter 120 is fed with carrier at channel 122 communicating with passageway 82 and the carrier in the gutter 120 is removed at channel 124 leading to carrier vent conduit 98.

Although a specific preferred embodiment of the invention has been described hereinabove in detail, it is desired to emphasize that such detailed showing is for the principal purpose of illustrating the invention, and is not to be considered as limiting the scope thereof except as required by the prior art, it being understood that numerous modifications of the structure and techniques will be apparent to those skilled in this art. For example, although the sample mixture metering section in the disclosed embodiment consists of a groove in the valve slide, it will be apparent that for larger samples it may be desirable to form vertical passages through the valve slide to make connection to relatively large volume metering tubes supported on the slide. It will also be apparent that the use of fluid shields is not limited to liquid applications, nor to the mixture metering valve only, but may be beneficial to various types of column switching and backflush valves as used in chromatographic instruments. For some applications, the valve seat and the manifold may be provided with additional passages to furnish a continuous flow of carrier gas through groove 88' in order to assure that no contamination accumulates in that groove. Furthermore, rotary instead of linear valve slide action may be employed with ports and passages and fluid shields correspondingly rotatably aligned. Other changes within the scope of the invention will be obvious.

Claims

We claim:

1. In chromatographic apparatus of the type comprising a separation column to which is connected a supply of carrier fluid to extablish a continuous flow thereof through the column, the apparatus further including a sample valve operable to inject into said flowing carrier fluid a precisely metered amount of a sample mixture to be separated into its constituent components by the column for detection and determination of the concentration of at least one component, said sample valve being of the type having first and second members with interaction surfaces slidably engaged for shifting movement between first and second positions, said members being formed at said interaction surfaces with first passage means cooperable in said first position to connect said carrier fluid supply to said column to establish said continuous flow of carrier thereto, said members being formed at said interaction surfaces with second passage means cooperable in said first position to connect a supply of sample mixture to a metering section to place therein a predetermined amount of sample mixture, with at least a part of said passage means being cooperable in said second position to entrain the sample mixture from said metering section into the flowing carrier fluid to be conducted through said column for analysis;

the improvement in said apparatus wherein both said first and second members, including said interaction surfaces thereof, are made of a technical ceramic which is chromatographically inactive and which comprises metal oxide of at least 80 percent purity sintered to a dense mass at a sintering temperature of at least about 1,100.degree. C and formed with said interaction surfaces having a smoothness effectively eliminating any adsorbent effect of the surfaces on the fluid samples.

2. Apparatus as claimed in claim 1, wherein said interaction surfaces are lapped to a smoothness of less than about 15 micro-inches (RMS).

3. Apparatus as claimed in claim 2, wherein said surfaces are finished to a smoothness less than about 5 micro-inches (RMS).

4. Apparatus as claimed in claim 1 wherein said metal oxide has a density of at least 75 percent of its theoretical maximum density.

5. Apparatus as claimed in claim 4, wherein said metal oxide has a specific gravity of at least about 3.0 grams per cubic centimeter.

6. Apparatus as claimed in claim 5, wherein said metal oxide has a purity of greater than 95 percent and a specific gravity greater than 3.5 grmas per cubic centimeter.

7. Apparatus as claimed in claim 6, wherein said metal oxide is alumina.

8. Apparatus as claimed in claim 7, wherein said alumina has a purity of about 99 percent.

9. Apparatus as claimed in claim 1, wherein said surfaces are flat to within one wavelength of light.

10. Apparatus as claimed in claim 1, wherein said interaction surfaces are free from lubricant.