Ionization Current Detector For Chromatographic Analysis

Abstract

An ionization current detector of improved efficiency includes facilities for directing heated nitrogen gas upward against a test sample, which is disposed on an aligned periphery of a rotary conveyor. The gaseous product resulting when the test sample is volatilized by the hot nitrogen is in turn sucked upward into a partial vacuum formed in an overlying, axially aligned nozzle having a venturi-like construction into which heated hydrogen gas is introduced. The hydrogen gas thereby admixed with the gaseous sample is ignited by the further application of forced air at the output of the nozzle, and the resulting hydrogen flame ionizes the sample. A pair of electrodes positioned above the air inlet measures the ionization current increase in the detector caused by the ionization of the sample.

Description

BACKGROUND OF THE INVENTION

In the present designs of ionization detectors employing a hydrogen burner for chromatographic analysis of gaseous test products, the normal ionization current level in the detector resulting from the hydrogen flame is increased when the gaseous test product is ignited by the flame.

Such gaseous test product is generally introduced into the detector on a moving linear conveyer (typically a wire) on which a test sample is coated. Inside the detector the sample is either directly volatilized by the hydrogen flame itself or, in more complex designs, is volatilized first in a separate pyrometer portion of the detector, and the resulting gaseous product is forced by a gas stream into the hydrogen flame. In either case, the increase in ionization current in the presence of the gaseous product is employed for chromatographic analysis of the product in the normal manner.

These existing schemes have several disadvantages. They tend to have a low detection sensitivity and are susceptible to noise. Moreover, because of their relatively small burner area and their association with a wire or other linear conveyer, only a small portion of the available test sample is actually ignited in the hydrogen flame, resulting in a low efficiency; and while more efficient mechanical conveyers such as rotary centrifugal types are available, they are not compatible with presently known detector designs or with convenient modifications of such designs.

SUMMARY OF THE INVENTION

The present invention contemplates an improved ionization detector that is more sensitive, efficient and noise-free than existing designs and that is compatible with a rotary conveyer. In one embodiment, particularly suitable for liquid chromatography, the detector is provided with a nozzle having an elongated central chamber centered on a longitudinal axis of the detector. The nozzle has a gas port in radial communication with an intermediate portion of the chamber.

The input end of the nozzle is positioned in axially spaced relation to, and concentric with, a heat source including a heated cylinder through which a first gas (such as nitrogen) is directed upward toward the nozzle. The test sample-collecting periphery of a rotary conveyer is disposed at the longitudinal axis of the detector intermediate the heated tube and the input of the nozzle. In this position, the hot nitrogen directed at the conveyer periphery volatilizes the collected test sample.

The resulting gaseous test product is sucked up into a partial vacuum formed in the nozzle when a second heated gas (e.g., hydrogen) is forced through the gas port into the intermediate portion of the nozzle, which is advantageously constricted relative to the input and output ends of the nozzle.

The hydrogen constituent of the resulting admixture of gases in the nozzle is ignited when mixed with air introduced into a diffusor section coupled to the upper end of the nozzle. The resulting hydrogen flame ionizes the gaseous test product, and the increase in ionization current level in the diffusor is sensed by a pair of electrodes therein.

One feature of the invention is the provision of an auxiliary rectifying screen in the diffusor above the air inlet, such screen serving to reduce the noise level and to maintain a uniform hydrogen flame. Where the gaseous product contains phosphorous or a halogen such as chlorine, an additional order of detection sensitivity may be obtained by disposing a layer of an alkaline metal salt on the auxiliary screen.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be further set forth in the following detailed description taken in conjunction with the appended drawing, in which the single FIGURE depicts an improved ionization detector in accordance with the invention for use with a rotary (centrifugal) conveyer.

DETAILED DESCRIPTION

In the drawing, an illustrative ionization detector constructed in accordance with the invention has a longitudinal axis 30 which is intersected by a peripheral region of a rotary conveyer designated as 3. Such conveyer may be arranged in a conventional manner to rotate a column of a raw material sample (not shown) and to collect the sample, after centrifugal action, on the conveyer periphery.

The test sample (not shown) on the conveyer periphery is exposed to an axial stream of a first gas from a source 31 (illustratively nitrogen). Such gas is heated to a temperature illustratively in the range of 200 - 800.degree.C by passing it through the interior of a cylinder 1 which is heated via a surrounding coil 2. The heated gas serves to volatilize the overlying test sample on the conveyer periphery.

Disposed above the periphery of the conveyer 3 and axially aligned with the heated tube 1 is the input end of an elongated nozzle 4 which serves as a fraction collector. The nozzle 4 has an interior passage or chamber including an intermediate portion 32, which is preferably constricted as shown so that the fraction collector defines a Laval nozzle.

A second gas, illustratively hydrogen, is directed into the nozzle 4 via a tube 6. Such tube defines a gas in the nozzle which communicates with the interior of the constricted intermediate portion 32. The hydrogen gas thus introduced into the portion 32 is preheated in a suitable manner to a temperature of about 500.degree.C. Such introduced hydrogen gas is transported upwardly through the nozzle chamber and into a diffusor 7 that communicates with the upper end of the nozzle chamber for ionizing the test sample as described below and for measuring changes in the ambient ionization current caused thereby.

Forced air is introduced via a tube 12 into the lower end of the diffusor 7 to be mixed with the heated hydrogen gas, resulting in a highly efficient hydrogen flame having a much larger burning area (e.g., in the order of 10 mm) than burners typically used in prior art detectors.

The hydrogen flame, which may be rectified and made more uniform by the provision of an auxiliary screen 10 disposed in the diffusor 7 above the air inlet tube 12, results in the generation of ions of hydrogen. The resulting ambient ionic current in the diffusor 7 (in the absence of the test sample) is picked up by a pair of electrodes 8,9 and passed through an amplifier 33 to a suitable ionization current recorder 34, which may be calibrated to a desired reference current level.

The flow of hydrogen through the inlet tube 6 and into the constricted intermediate portion 32 of the nozzle chamber establishes a partial vacuum therein by Venturi-type action to draw the gaseous test sample from the conveyor 3 through the nozzle chamber to be admixed with the introduced hydrogen. At this point, the test sample constituent of the mixture is ignited by the hydrogen flame. Because of the relatively large burner area discussed above, a large part of the test sample is ionized by the flame, resulting in increased efficiency.

The increased ionization current resulting from the ionization of the gaseous test sample by the flame is registered in the recorder 34. Such increase, of course, has a well-known significance in the chromatographic analysis of the test sample and will not be discussed further here.

An enhancement of detector performance can be obtained if the nozzle 4 is preheated to a temperature in the range of 200 - 800.degree.C. Such heating can be accomplished, e.g., by means of an auxiliary heating coil 5 disposed around the nozzle 4.

Additionally, it has been found that when the gaseous test sample contains a halogen or phosporous, an order of magnitude of improvement in sensitivity can be obtained when a layer 11 of an alkaline metal salt is disposed on the screen 10. Such layer can be especially advantageous in another respect, since in appropriate circumstances it can permit ionization of such gaseous products to take place even in the absence of a hydrogen flame. Such thermo-ionization phenomenon will of course be enhanced by the pre-heating of the nozzle 4 via the coil 5.

Another advantageous characteristic of the described scheme is that a high degree of noise suppression has been observed when the auxiliary screen is used.

The electrodes 8 and 9 are shown as separate elements within the diffusor 7. One of such electrodes may alternatively be constituted by a portion of the detector wall (e.g., the wall of diffusor 7 or nozzle 4, if conductive) or by an internal component of the detector (e.g., the screen 10 or the supply tube 6, if conductive).

In the foregoing, the invention has been described in connection with a preferred arrangement thereof. Many variations and modifications will now occur to those skilled in the art. For example, such arrangements, while especially advantageous for material analysis, can also be used for other purposes such as the control of long-term separation processes. It is accordingly desired that the scope of the appended claims not be limited to the specific disclosure herein contained.

Claims

What is claimed is:

1. In an ionization apparatus for the chromatographic analysis of a test sample:

an elongated nozzle including a central chamber having input and output ends and an intermediate portion, and a gas port disposed at the nozzle periphery in radial communication with the intermediate portion of the chamber;

means disposed in axially spaced relation to the input end of the nozzle for directing a first heated gas axially toward the input end of the nozzle chamber;

conveyer means for transporting a test sample to a position axially aligned with and intermediate the directing means and the input end of the nozzle chamber whereby the heated first gas from the directing means volatilizes the test sample;

means for passing a heated second gas through the nozzle gas port and into the intermediate portion of the nozzle chamber to form a partial vacuum in the nozzle and to thereby draw the volatilized test sample into the nozzle chamber for admixture with the second gas; and

means associated with the output end of the nozzle chamber for ionizing the volatilized test sample passing through the nozzle chamber.

2. Apparatus as defined in claim 1, in which the apparatus further comprises a pair of electrodes for sensing the ambient level of ionized current.

3. Apparatus as defined in claim 1, in which the intermediate portion of the nozzle chamber is constricted with respect to the input and output ends.

4. Apparatus as defined in claim 1, in which the conveyer means is a rotary centrifugal mechanism whose peripheral portion is aligned with and intermediate the directing means and the input end of the nozzle chamber.

5. Apparatus as defined in claim 1, in which the gas directing means includes an axially aligned heated cylinder through which the first gas is passed.

6. Apparatus as defined in claim 1, in which the apparatus further comprises means for heating the nozzle.

7. Apparatus as defined in claim 1, in which the ionizing means comprises means for introducing forced air adjacent the output end of the nozzle chamber to ignite the heated second gas to form a flame, said flame serving to ignite and ionize the volatilized test sample.

8. Apparatus as defined in claim 7, in which the ionizing means further comprises an auxiliary rectifying screen disposed above the forced air introducing means.

9. Apparatus as defined in claim 8, in which the volatilized test sample contains a material selected from the group consisting of phosporous and the halogens, and in which the apparatus further comprises an alkaline metal salt disposed on the auxiliary screen.