Method for the quantitative elementary analysis of preferably organic substances

Abstract

A method of quantitatively determining the chemical composition of a substance by contacting the latter with a gas which has been excited by exposure to high-frequency discharge to thereby convert the components of the substance into gaseous compounds which may be measured quantitatively by an analytical method.

Parent Case Text

This is a continuation, of application Ser. No. 765,543, filed Oct. 7, 1968, and now abandoned.

Description

The invention relates to a method of quantitatively determining the chemical composition of a sample by means of an analysis apparatus.

The quantitative determination of the chemical composition of organic substances is one of the most important analytical procedures. Combustion analysis procedures are generally utilized for quantitatively determining the amounts of carbon, hydrogen, oxygen, nitrogen, etc. in a sample. In such procedures, carbon dioxide and water are formed and the quantity of these compounds may then be measured. In this method, which is known as Pregl's method, the samples are heated in a stream of oxygen. Gas burners have often been used as a source of heat in the past although at the present time, electric furnaces are generally used. Comparatively high temperatures of about 500.degree. to 600.degree.C are required so that special types of refractory glass are used for the manufacture of the combustion tubes. During the heating, the substances are decomposed and the oxidation products distilled and precipitated on the colder portions of the combustion tube so that the heat source must be moved gradually along the tube. The carbon dioxide compounds and water so formed are determined gravimetrically or in some other way after they have been absorbed by suitable substances.

The details of this known method have been improved over the years, but as every chemist knows, the method still has a great number of disadvantages, the most important of which are the relative slowness of the method especially in the analysis of substances which are difficult to decompose and the relative lack of exactness and sensitivity of the method. Another disadvantage is that the method requires such high temperatures that the analysis of explosive materials and the like is impossible.

In another known method for quantitative chemical analysis, nitrogen is converted into ammonia by means of hydrogen activated by a high-frequency discharge inside a tubular reaction chamber. This method is based upon the fact that activated gases have greater reactivity than ordinary gases. However, the yield from this method is so low that the same is suitable only for laboratory use and it has not been possible to utilize the same for ultimate quantitative analysis.

According to another known method for oxidizing hydrocarbons to determine the sulphur content thereof, first a layer of absorbent material is intorduced into a sampling vessel disposed inside a combustion chamber, then the sample to be analyzed is poured over the absorbent layer. Then the absorbent layer is covered with a separating layer composed of a refractory material, for example and thereafter a weighed quantity of an electrically conductive material, such as hydrogen reduced iron is placed on top of the absorbent layer. The conductive material is then heated to a high temperature for a short period of time by means of an induction coil surrounding the combustion chamber for the purpose of evaporating the sample material which had been absorbed by the absorbent layer. The total heat available is a combination of the heat produced by the induction process and the exothermic heat of reaction generated during the formation of iron oxide. The ceramic separating layer serves to ensure progressive evaporation of the sample only to the extent of its decomposition. The evaporating decomposition products are then brought into contact with the oxygen passing through the reaction chamber outside the sampling vessel whereby the same are oxidized. A second induction coil and another heating arrangement serve to heat the oxygen to the extent necessary to ensure complete combustion. The products of combustion are collected and analyzed in a manner known per se. Application of this method to the analysis of substances other than hydrocarbons is not contemplated. Besides, this method is unsuitable for the analysis of highly sensitive and explosive substances because of the high temperature level required.

It is the object of the invention to improve upon conventional methods of quantitative ultimate analysis so as to render the same suitable for the analysis of any organic and inorganic substance, and particularly of materials which are sensitive to heat or are subject to explosive decomposition.

Another object of the invention is to facilitate the obtaining of more accurate analytical results than has been possible utilizing previously known methods.

Furthermore, it is an aim of the invention to expedite quantitative analysis procedures.

Finally, a further object of the invention is the restriction of the analysis, where necessary to limited areas of the sample to be examined.

The foregoing purposes, aims and objects of the invention are achieved through the use of a method of quantitatively determining the chemical composition of a sample comprising the step of converting the components of the sample into gaseous compounds by contacting the same with a gas selected from the group consisting of hydrogen and oxygen which has been excited by a high-frequency discharge. In accordance with this procedure, the activated gas combines with the sample to form gaseous compounds which may be quantitatively analyzed by conventional techniques.

This method is preferably performed at subatmospheric pressures which permits the analysis to be conducted at low temperatures if this is necessary or desirable in view of the nature of the substances to be analyzed.

Further details and advantages of the method according to the invention will appear from the following description of several preferred embodiments of the invention with reference to the accompanying drawings wherein:

FIG. 1 is a schematic cross-sectional view of an analysis apparatus adapted for the performance of the method of the present invention;

FIG. 2 is a schematic cross-sectional view illustrating another embodiment of an analysis apparatus adapted for use according to the present invention;

FIG. 3 is a schematic cross-sectional view illustrating another variation of an analysis apparatus adapted for the performance of an analysis according to the present invention;

FIGS. 4 and 5 are schematic illustrations of another embodiment of the invention, FIG. 5 showing a partial top view of the apparatus illustrated in FIG. 4 and

FIG. 6 is a schematic illustration of a device similar to the device of FIG. 3 except that a mechanical diaphragm is included for limiting the area of application of the excited gases.

In FIG. 1, the reference numeral 1 designates a tubular container which is closed on its ends by lids 3 and 4 tightly connected to annular flanges 6 of container 1 with gaskets 5 interposed therebetween. A central pipeline 7 for the passage of a reaction gas in the direction of the arrow into the interior 8 of the container 1 extends outwardly from the lid 3.

A central pipeline 9 to which a measuring instrument (not shown) is connected via valve 10 extends from the lid 4.

A pipe 11, to which a partial-pressure measuring instrument 12 or a mass spectrograph (not shown) for the determination and regular observation of the composition of the gas is connected, extends from the side of the container 1. Opposite pipe 11, another pipeline 13 terminates in the container and is connected to a vacuum pump (not shown) via a vacuum valve 14.

The container 1 is surrounded by a high-frequency coil 15 consisting of a plurality of windings which are connected by line ends 16 and 17 to the output of a high-frequency generator 18.

A sample pin 19 containing the sample 20 to be analyzed is disposed in the interior 8 of the container 1. The sample may be any organic or inorganic substance.

For the analysis, the sample 20 is introduced in the sample pan 19 and the container 1 tightly closed by means of lids 3 and 4. Then a partial vacuum is produced in the interior 8 of the container 1 by means of the vacuum pump (not shown) via pipe 13 and valve 14 (which is open during this procedure). After evacuation of container 1, high-frequency coil 15 is energized and at the same time a reaction gas is directed via pipe 7 into the interior 8 of the container 1. Oxygen or hydrogen may be used as reaction gases since these gases are capable of being activated by the high-frequency discharge and feature a particularly high degree of reactivity.

The ultimate analysis generally involves oxidation reactions if elements other than oxygen itself are to be determined. If oxygen itself is to be determined directly, excited hydrogen is generally used as a reactive gas (to form H.sub.2 O which is then determined in a known manner). Excited hydrogen, may also be used for the determination of halogens, sulphur etc. What is said in the following description with respect to excited (or "activated") oxygen is therefore, true for excited hydrogen or for any other excited or activated gas if the use of such other gas should be desirable or necessary under given circumstances.

The reaction between the gas activated by the high-frequency discharge on the one hand and the sample 20 on the other hand develops rapidly and as a result gaseous compounds are rapidly produced. These compounds are then determined quantitatively in accordance with conventional analytical methods. The efficiency of the method according to the invention is ascribable to the fact that at a partial vacuum of approximately 10.sup.-.sup.2 to 10.sup.-.sup.3 torr (1 mm Hg pressure), the free length of the path of the activated and consequently more reactive gas molecules or atoms, is a few centimeters and accordingly the same are allowed to pass substantially unimpeded through the plasma space inside the high-frequency coil 15 to the sample 20 located in the sample pan 19 where the activated molecules or atoms react with the sample. Thus, the frequently objectionable heating of the sample pan 19 is avoided.

The avoidance of heating is of vital importance, for example, in the analysis of explosive substances. In such case, the reaction may be caused to take place at ambient temperature or even at considerably lower temperatures. FIG. 2 shows an analysis apparatus designed for this purpose which comprises a tubular container 21 which is closed at its ends by means of lids 22 and 23 and gaskets 24. A pipeline 25 mounted on the lid 22 serves for the supply of the reaction gas. A measuring instrument (not shown) can be connected with the interior 28 of the container 21 via a valve 26 and a pipe 27 extending from the lid 23.

Pipes 29 and 30 terminating in the sides of the container as well as the vacuum valve 31 are the equivalents of members 11, 13 and 14 respectively, of the analysis apparatus shown in FIG. 1.

The container 21 is surrounded by two highfrequency coils 32 and 33 which are connected to the high-frequency generator 34. A sample pan 36 containing the sample 35 is disposed in the interior 28 of the container 21 in the area between the coils 32 and 33 and a cooling device 37 is provided beneath the sample pan 36. The coolant passes to the cooling device 37 via a feeder pipe 38 extending through the wall of the container 21. The coolant discharge outlet is designated by reference number 39. Water, alcohol or liquid nitrogen for example, are used as coolants depending on the temperature to which the sample 35 is to be exposed.

A shielding net 40 which extends over the entire cross-sectional area of the container 21 is provided on both sides of the sample pan 36 for shielding the sample 35. The shielding nets 40 prevent the high-frequency fields of the coils 32 and 33 from extending as far as the sample 35 and sample pan 36. On the other hand, the activated gas molecules penetrate the shielding nets 40 so that the reaction is allowed to proceed unimpeded.

It is also possible through the use of the present invention to arrange the high-frequency field for the activation of the reaction gas on the side of the sample. A suitable apparatus for that purpose is shown in FIG. 3. The container 41, designed as an upright cylinder, is surrounded by the high-frequency coil 42 in its upper portion only. The sample pan 45 which contains the sample 44 is located in the bottom portion of the container 41. A cooling device 46 with a feeder pipe 47 and a return pipe 48 for the liquid or gaseous coolant is provided below the sample pan 45. The inlet 49 for the reaction gas terminates laterally above the sample pan 45 in the interior 50 of the container 41. The sample 44 is shielded from the high-frequency field produced by the coil 42 by means of a shielding net 51 provided above the gas inlet 49.

In FIGS. 4 and 5 the container in which the analysis is performed, is designated by reference numeral 52. This container has the shape of an upright cylinder, the upper opening of which is closed vacuum-tight by means of a lid 54 and a gasket 53. A heating plate 55 is mounted on the lid 54. As appears from FIG. 5, four pipelines 56 to 59 extend from the container 52. The pipeline 56 serves for supplying reaction gas taken from a gas container 60 to analysis container 52 via a needle valve 61. A diffusion pump 64 is connected to the opposite line 57 via a vacuum valve 62 and a freezing trap 63 and another pipeline 65 connects diffusion pump 64 with a backing pump (not shown).

A Pirani-type measuring element 66 with a bakeable ionization manometer is connected to a third pipeline 58 of container 52 while the pipeline 59 leads via a needle valve 67 to a bakeable mass filter 68.

The container 52 and the pipelines 56 and 57 are surrounded by a filament winding 69. A high-frequency winding 71 connected to a high-frequency generator 70 surrounds the container 52 and the filament winding 69 thereon.

The bottom portion 72 of the container 52 is provided with an inwardly protruding hollow central extension 73 which supports a table 74 adopted for carrying the sample 75 to be analyzed.

A heating and cooling rod 76 which is used for the heating or cooling of the table 74 depending on the nature of the sample to be analyzed can be introduced into the hollow extension 73. Consequently, rod 76 may be utilized in cooperation with the filament winding 69 to provide specific temperature conditions inside the container 52.

The operation of the apparatus illustrated in FIGS. 4 and 5 corresponds to that of the previously described embodiments of the invention.

It is also possible occasionally to use the high-frequency energy produced for the heating of a catalyst disposed in a sample pan or in the form of a net provided inside the container. This is possible, for example, in conjunction with an apparatus as shown in FIG. 1. Thus, small quantities of a substance can be evaporated, the vapor being entrained immediately by the stream of activated gas.

Excited oxygen is easily capable of oxidizing carbon layers. If, however, a portion of a substance to be analyzed is deposited on the walls of the apparatus during the evaporation or decomposition of the same, such deposits may be positively oxidized by the activated oxygen. This is not the case with conventional combustion apparatus.

It is, of course, easy to shift the high-frequency coil over the reaction tube, but in most cases it will not be necessary to do so. By the reduced pressure which is required by the high-frequency discharge unnecessary amounts of gas are avoided. Moreover, it is possible to immediately determine the residual gases remaining in the reaction vessel. This can be done for example, with modern efficient devices for determining partial pressures, such as the omegatron, the high-frequency mass filter and the like. The high-frequency coil over the reaction zone can also be used to determine the gases spectroscopically by emission or adsorption. In special cases an efficient mass spectrograph may be connected directly to the apparatus. If desired, the gas outside the reaction zone may be concentrated by collecting pumps and may be then determined in any known manner, such as by gas chromatography, heat conductivity, absorption and the like.

By means of the method according to the invention it is even possible to conduct the decomposition layerwise from the surface of the substance to be analyzed. Such layerwise decomposition may be conducted at room temperature or at low temperatures so that the degree of homogeneity in the structure of substances can be determined. The composition of the decomposed layers of a film of synthetic plastics for example, can thus be registered continuously.

It is frequently desirable, for example in connection with the ultimate analysis of substances of biological origin or of substances of a non-uniform internal structure, such as synthetic resins, to obtain an ultimate analysis not of the whole substance or of the whole preparation, but only of certain limited areas thereof, the said limitation referring both to the depth (determination of the composition of surface layers) and to the lateral extension. These particular analyses can also be performed by the method according to the invention following the principle of ashing or incineration by means of activated gases. For such purposes, provision may be made for limiting the application of the activated gases, by appropriate masking, in such a manner that only certain zones of the sample are contacted by the activated gas and the depth-effect of the reaction of the gas with the substance to be analyzed may be controlled by appropriately varying the duration of the reaction.

The area of application of an already excited gas may be limited by using an appropriately designed mechanical diaphragm as illustrated in FIG. 6. The apparatus of FIG. 6 is generally the same as the apparatus of FIG. 3 except that gas inlet 49 is disposed above net 51 and diaphragm means 77 presenting an aperture 78 is provided above sample 44. Thus, only that area of sample 44 which is disposed beneath aperture 78 is exposed to contact with the excited gases. Moreover, the area of contact may be limited by restricting the influence of the excited radiation upon the gas to be excited, for example, by means of electromagnetic or electrostatic fields in accordance with the known laws of interaction between electromagnetic fields and charged particles and/or electromagnetic radiation.

As a result of these operative characteristics the range of application of the analyzing method according to the invention is broadened so as to also cover locally restricted analyses of samples, the composition of which is subject to local fluctuations. Conventional methods are unsuitable for that purpose.

Claims

I claim:

1. A method for quantitatively determining the chemical composition of a sample substance comprising the steps of:

selecting a gas from the group consisting of hydrogen and oxygen;

directing said gas to sample areas defined by means of mechanical diaphragms;

activating said gas by means of an electrical discharge at a sub-atmospheric pressure;

converting the portions of the sample at said areas into gaseous compounds by combining the same with the activated gas; and

thereafter quantitatively determining said chemical composition by analyzing said gaseous compounds.

2. A method as set forth in claim 1, wherein said sample is screened off against the effects of the electric fields activating the gas.