Disc Conveyor Flame Ionization Detectors

Abstract

Effluent to be tested is deposited on a porous alumina disc, the disc being rotated into a heater where the eluent is vaporized leaving a residue (the sample) whereupon the sample is then transported by the conveyor into a dual jet flame ionization detector (FID). After passing through the FID the disc then passes through an oxidizer where any remaining residue from the disc are removed by oxidation whereupon the disc then passes through a cooler, cooling the disc prior to the disc returning to a point where more effluent will be deposited for further analysis.

Description

BACKGROUND OF THE INVENTION

The present invention relates to chromatographic devices and more particularly to a chromatographic device using an alumina disc as a sample carrier. The flame ionization detector has been combined with a porous disc conveyor, on which a wide range of molecular weight samples eluted from a liquid chromatographic column or other source can be deposited and detected. This porous conveyor eliminates the spiking noise usually associated with metal conveyors (wire, belt or chain) produced as a result of sample back diffusion along the metal conveyor.

DESCRIPTION OF THE PRIOR ART

By and large with commercially designed detectors either the total effluent or a portion of it is deposited on a conveyor of some type. The eluent is then flash evaporated in a heater leaving a sample material on the conveyor. One method of detection involves burning the sample material directly in the flame as the conveyor passes through the detector. An alternate and most frequently used concept removes the sample material by pyrolysis in a purged enclosure from which the pyrolysis products are swept into a FID. This technique is widely used in commercial detectors because it is easier to achieve a better signal to noise ratio than is possible with the direct burning technique. Nevertheless sample diffusion on hot metal conveyors which manifest itself in the form of signal spikes is still present with many of the pyrolysis designs. The noise problem is more evident with all designs when broad peaks are observed, as frequently encountered in gradient elution fractionation and gel permeation chromatography. A U.S. Pat. No. 3,316,674 issued to Owens et al. illustrates using a perforate metal support such as a platinum screen for a conveyor. When using a metallic or glass surface, the sample deposited by the applicator creeps as the conveyor enters the flame ionization detector which results in back-mixing and noisy peaks in the readout. Another disadvantage in using a metallic strip as a conveyor is that it distorts due to the heat added during detection of the sample causing the sample to creep even further than when deposited by the applicator on the metal conveyor.

It has been the practice in the past for the applicator to be made from a metal or glass tube preventing a uniform deposit of all of the effluents on the conveyor without back diffusion or wetting along the external surface of the applicator tubing (capillary action). This behavior causes both loss and back-mixing. Sample loss occurs when the carrier solvent vaporizes to deposit the sample material in the form of rings 3 to 5mm above the tip. The extent of this wetting will vary at constant flow rate and is manifested by a slow pulsation along the applicator tube. As a result, a portion or all of the sample material deposited by one pulse can be diluted subsequently by a more extensive pulse which dissolves and carries it down to the applicator tip, while placing a new sample ring at a higher position. After analysis, a ring at maximum wetting level is observed on glass and stainless steel applicators. Therefore, the use of such applicators as shown by the prior art devices causes a substantial variation in the analysis results.

Prior art devices also do not reveal how to ensure against residue build-up on the conveyor as a result of the vaporization process leaving a residue on the conveyor resulting in eventual plugging of the pores of whatever conveyor type is used. This is not a problem with normal samples, however, when samples like pitch are analyzed, a non-detectable carbonaceous residue is left on the conveyor which can result in eventual plugging of the pores causing sample creep, back-mixing and noisy peaks in the readout equipment.

An even further problem with the prior art devices is that cooling of the conveyor is not accomplished other than by normal room air currents. If the disc conveyor is not adequately cooled this results in loss of sensitivity for lower boiling eluents such as pentane, and increased base line instability or noise.

Prior art devices also corporate a single nozzle for the flame ionization detector whereas if dual nozzles are used, uniform coverage is accomplished eliminating sample blowout which is caused when only one flame is used causing further loss of instrument sensitivity and meaningful analysis results.

SUMMARY OF THE INVENTION

The present invention solves the above problems by incorporating a disc conveyor preferably formed of alumina and having a high apparent porosity approximating 40 percent which provides an excellent conveyor for the sample as it is eluted from the applicator. An even further advantage of the alumina disc conveyor is its dimensional stability under the application of the heat in the heater, FID and oxidizer assemblies.

A second significant advantage of the present invention is through the use of an applicator in which a metal tip having a polytetrafluoroethylene (trademark for Teflon) covered outer surface. By so doing, the present invention has eliminated both sample loss and back-mixing problems as described hereinabove.

A third advantage of the present invention is in the use of a dual nozzle flame ionization detector resulting in uniform heating of the disc conveyor yielding higher response and eliminating sample blowout.

A further advantage of the present invention is through the incorporation of an oxidizer downstream of the flame ionization detector. By use of an oxidizer, the conveyor is burned clean of any residues left after passing through the detection device preventing residue build-up.

An even further advantage of the present invention is incorporation of a cooler which surrounds the edge of the disc conveyor. Use of the cooler extends the range of the detector to lower boiling point eluents such as pentane greatly increasing sensitivity and base line stability.

DESCRIPTION OF THE DRAWINGS

The present invention will be more easily understood from the following detailed description of a preferred embodiment when taken in conjunction with the attached drawings in which:

FIG. 1 is a pictorial view of the apparatus constructed according to this invention;

FIG. 2 is an exploded view illustrating the flame ionization detector and oxidizer according to the present invention;

FIG. 3 is a cross-sectional view of the disc conveyor; and

FIG. 4 illustrates the stable base line and descriptive peaks obtained when using the apparatus of the present invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

Referring to FIG. 1, there is shown the apparatus constructed according to the invention and particularly adopted for quantitative measurements of eluents from a liquid chromatograph. More particularly, there is shown a control panel 30 in which a preferred embodiment of the present invention is mounted comprising essentially the disc conveyor 29, the applicator 27, the eluent heater and exhaust assembly 25, the flame ionization detector and oxidizer assembly 50, and the disc cooler assembly 31.

An ideal applicator uniformly deposits all of the effluent on the conveyor 29 without back diffusion or wetting along the external surface of the applicator wall. Stainless steel and glass tubing have a high level of external wetting causing sample loss and back-mixing. To eliminate sample loss and back-mixing the applicator 27 is made from a teflon covered stainless steel syringe needle resulting in excellent eluent deposition and width control for deposition of the sample on the conveyor 29.

The disc conveyor 29 is preferably made of alumina because of its excellent dimensional stability in the presence of heat as well as its uniform porosity distribution. The use of an alumina disc as the conveyor solves many of the problems inherent in prior art devices using metallic as well as glass as conveyors, namely, dimensional stability and sample containment after being deposited on the conveyor. The general shape of the disc conveyor 29 is shown in FIG. 3 and is shown as having a central bore 58 therethrough with a flat lower surface. The outer flat surface 51 of the conveyor 29 carries the eluted sample as deposited by the applicator 27. Optional holes 52 may also be provided in the disc conveyor 29 further lowering the thermal inertia of the conveyor 29. The disc conveyor is mounted on a shaft 48 and retained on the shaft 48 by a keeper 49. The shaft 48 forms part of a variable speed reversible D.C. motor 49 mounted on the back of control panel 30. The incorporation of a reversible variable speed motor allows a stable base line to be established by reversing direction of rotation of the conveyor 29 through the oxidizer burning any residue left on conveyor 29. In operation, as the disc conveyor rotates clockwise at constant speed an eluent from a chromatographic column is deposited on the conveyor 29 by the applicator 27. The position of the applicator 27 relative to the conveyor 29 is controlled by two micrometer type adjustments 28 and 28a, one being adapted to position the applicator 27 over the conveyor and the other serving to control the distance between the applicator and the surface 51 of the conveyor 29.

Once the sample has been deposited on the conveyor 29 it next passes into the eluent heater and exhaust assembly 25 where the eluent is flash evaporated leaving the sample on the conveyor 29. The heater and exhaust assembly covers approximately 25 percent of the circumference of the disc conveyor 29 and is supported by an exhaust duct 60 which is in turn connected to the heater exhaust blower 10. The blower 10 pulls room air over the conveyor 29 and through duct 60 giving a definite exhaust pattern eliminating hydrocarbons from diffusing into the detector. The heating element 56 is a screw-plug type element with power regulation furnished by a silicon-controlled rectifier (SCR) with the temperature being controlled up to 250.degree. C.

Subsequent to being flash evaporated in the heater and exhaust assembly 25 the sample to be analyzed next passes through the flame ionization detector (FID) and oxidizer assembly 50. The assembly 50 as positioned relative to the disc conveyor 29 by a second set of micrometer adjustments 13 and 14 providing both horizontal and vertical adjustment capability.

The FID and oxidizer assembly 50 is made up of a housing 70 which preferably has two chambers 42 and 43. The FID chamber 42 is provided with bores 72 and 73 for receiving bushings 40 having an eccentric bore 75 therethrough. Prior to installing bushings 40 into bores 72, 73 an FID nozzle 22 is inserted through bore 75 of bushing 40 and locked in place with set screw 41. Subsequently, the bushing 40 with nozzle 22 is installed in bores 72, 73 and positioned approximately 2-10mm from the edge of the conveyor 29 prior to locking the bushing 40 in place by a second set screw 36 installed in housing 70.

A collector 37 is also positioned within the FID chamber 42. The collector 37 consists of semi-circular rings approximately 1.8 cm in diameter spaced 3mm apart and positioned approximately 2mm ahead the FID nozzles 22. The collector 37 is positioned in the FID chamber 42 using a bushing 38 through which an insulator 39 passes. A set screw 37 installed in housing 70 contacting bushing 38 allows the final adjustment of collector 37 relative to the FID nozzles 22 and conveyor 29. A view port 23 is provided in the housing 70 to aid in positioning the detector relative to the conveyor 29.

As hereinbefore described the FID nozzles 22 are grounded to the housing 70. The collector 37 is insulated from the housing 70 by insulator 39. The collector lead wire 85 is connected in series with a 300 volt battery (not shown) which in turn is connected in series with an electrometer thus maintaining a positive collector with respect to the FID nozzles 22. Both the collector 37 and FID nozzles 22 are fabricated from platinum resulting in very low thermonic noise emission (approximately 10.sup.-.sup.16 amp/cm.sup.2 at 1,200.degree. C).

The oxidizer chamber 43 is immediately above the FID chamber 42 and separated from it by a partitioning element 90. Two oxidizer nozzles 24 are inserted through perforations 91 in the housing 70 and each is locked in place by a set screw 36 installed in housing 70. The oxidizer chamber 43 is provided with two vent holes 92 allowing venting of excess heat from the chamber 43. By providing an oxidizer chamber a "clean" conveyor is assured by the burning of any residues left on the conveyor after passing through an FID chamber making the apparatus more versatile and reliable.

An end plate 71 (FIG. 2) encloses the housing 70 opposite the conveyor 29. The end plate 71 is provided with an upper cavity 102 with a shoulder 104 against which a sintered stainless steel disc 106 abuts. A lower cavity 103 with a shoulder 105 and sintered stainless steel disc 107 are disposed in the lower portion of end plate 71. A plurality of screws 111 mate end plate 71 with the housing 70. Air is supplied to the upper cavity 102 by an air inlet fitting 33 threadably inserted in a tapped hole 100. Likewise, air is supplied to the lower cavity 103 through an air inlet fitting 21 threaded in a tapped hole 101. By supplying pressurized air to cavities 102 and 103 covered with sintered stainless steel disc, air is supplied at a low velocity such that laminar flow exists insuring a steady flame in both the FID and oxidizer chamber. By providing this laminar flow the present invention overcomes a further disadvantage of prior art apparatus (point source type air inlet).

Referring again to FIG. 1, a plurality of splitter valves having one inlet and two outlets are shown. These valves are utilized to facilitate detector (FID) and oxidizer split ratio for hydrogen and air. Teflon tubes interconnect the various fittings for the supply of air and hydrogen in appropriate volumes. Tube 115 connects fitting 17 to fitting 33 thereby supplying air to the oxidizer chamber. Tube 116 connects 18 to 21 supplying air to the FID chamber. Tube 117 likewise interconnects fitting 20 to one of the two FID nozzles 22 to supply hydrogen to the FID chamber. Similarly, tube 118 interconnects 20a to the second nozzle 22. Finally, tubes 119 and 120 interconnect fittings 19, 19a with oxidizer nozzles 24 supplying hydrogen to the oxidizer chamber 43.

An enclosure 16 having a hingably mounted cover completely encloses the FID and oxidizer assembly and the fittings hereinbefore described. The enclosure is continuously purged with nitrogen supplied through fitting 15 mounted in the control panel 30. By enclosing the FID and oxidizer, background noise produced from room air currents is eliminated further increasing instrument sensitivity and reliability.

A further advantage of the present invention is through the incorporation of a disc cooler assembly 31 which greatly extends the range of the instrument to lower boiling point eluents such as pentane. The assembly 31 is mounted on the control panel 30 having an opening 124 in communication with a duct 125. A cooler exhaust blower 34 draws large volumes of room air over the conveyor 29 exhausting room air is indicated by the arrows 35. Alternately, if the room air temperature is higher than desired a fitting 130 is provided in the disc cooler assembly 31 whereby nitrogen can be supplied aiding in cooling of the conveyor 29. The disc cooler eliminates residual heat in the conveyor and prevents vaporizing of the eluent upon contact when deposited by the applicator 27 on conveyor 29. Premature vaporizing will partially or completely carry the sample material away with the vaporized eluent destroying any meaningful results of the analysis. The use of a cooler further increases sensitivity and base line stability of the instrument.

Claims

I claim:

1. A chromatographic device comprising:

a circular disc conveyor mounted for rotation about a central axis;

means for rotating said conveyor at substantially constant speed about said central axis;

an applicator, said applicator being positioned adjacent the path of said disc to deposit eluents from a liquid chromatographic column near the outer edge of said conveyor;

a heater, said heater being disposed adjacent the path of the outer edge of said conveyor, spaced from said applicator in the direction of rotation of said conveyor and adapted to flash evaporate eluent carried by said conveyor;

a flame ionization detector, said detector being disposed adjacent the path of the outer edge of said conveyor to direct its flame onto said conveyor;

an oxidizer, said oxidizer being disposed adjacent the path of the outer edge of said conveyor and adapted to burn impurities carried by said conveyor after said conveyor passes through said flame ionization detector; and

a cooler, said cooler being disposed adjacent the path of the outer edge of said conveyor adapted to cool said conveyor subsequent to said conveyor passing through said oxidizer.

2. The apparatus of claim 1 wherein said circular disc conveyor is of alumina.

3. The apparatus of claim 1 wherein said applicator is a Teflon covered stainless steel syringe needle.

4. The apparatus of claim 1 further including an enclosure substantially enclosing said flame ionization detector and oxidizer, said enclosure being continuously purged with nitrogen.

5. The apparatus of claim 1 further including an exhaust and cooler blower for exhausting from said heater and said cooler, said blowers being in fluid communication with said heater and said cooler.

6. A chromatographic device comprising:

a circular disc, said disc being rotatably mounted on its central axis;

a drive means coupled to said disc for rotating said disc at a substantially constant speed;

an applicator, said applicator being mounted to deposit the eluents from a liquid chromatographic column on the flat surface of the disc adjacent the outer periphery thereof;

a heater, said heater having a passageway with a U-shaped cross-section and mounted with the legs of said passageway extending along the surfaces of said disc adjacent the periphery thereof;

a flame ionization detector, said flame ionization detector having a passageway with a U-shaped cross-section and mounted with the legs extending along the surfaces of said disc adjacent the outer periphery thereof;

an oxidizer, said oxidizer having passageway with a U-shaped cross-section and mounted with the legs extending along the surfaces of the disc adjacent the periphery thereof; and

a cooler, said cooler being disposed to cool both sides of said disc adjacent the outer periphery thereof.

7. The chromatographic device of claim 6 wherein the passageway in said heater has sufficient length to cover substantially twenty-five percent of the periphery of said disc.

8. The chromatographic device of claim 6 wherein said flame ionization detector utilizes a pair of nozzles to direct a flame onto both sides of said disc.