GB2101312A - Method for the microanalysis of chemical substances - Google Patents

Abstract

A method for the microanalysis of chemical substances in which a sample is burned in the presence of fluorine, the products of combustion then being analysed for volatile fluorides by means of a mass spectrometer or a IR spectrometer without the need for separation. The method enables the determination concurrently of a greater number of elements than has been practicable using conventional techniques.

Description

SPECIFICATION Method for the microanalysis of chemical substances This invention relates to a method for the elementary microanalysis and ultramicroanalysis of chemical substances, where a sample of the substance is burned in the presence of a combustion gas and then analysed for determination of its constituents.

Elementary microanalysis is the most important method used in the quantitative analysis in organic chemistry.

Using this method, the substance under analysis is oxidized in a stream of oxygen and is then separated and determined using various methods. Analyzers like these, however, can be used for the analysis only of hydrogen, carbon, nitrogen, oxygen indirectly, and sulphur, for the reason that these elements, unlike others, combine with oxygen to form highly volatile oxides or, in the case of nitrogen, are set free as elements.

The present invention aims to provide an improved method of microanalysis permitting the detection and determination of concurrently a greater number of elements than had been practicable heretobefore.

According to this invention we propose that the sample of the substance is burned using fluorine gas and then gas analysed.

The electronegativity of fluorine causes a great many more elements to be displaced from their compounds than with the aid of oxygen.

Combustion with the aid of fluorine gas, therefore, enables a substantially larger variety of elements to be reduced to volatile fluorides and, thus, concurrently to be determined by gas analysis.

Combustion of said description will produce volatile fluorides with totally 31 elements, and volatile substances in element form with 3 elements (Oz, N2 and Kr). Gas analysis will additionally permit the simultaneous analysis of combustion products without requiring resort to separation.

The present invention makes it possible, therefore, to analyse concurrently 34 different elements in a single process operation rather than a mere 3 elements concurrently and 2 elements successively as heretobefore.

It has also been shown that when using fluorine, the substance under analysis is burned completely for a very accurate quantitative analysis with the present method. Whereas combustion using oxygen in a conventional manner will occasionally produce incomplete transformation and, thus, faulty results even when additives and a higher temperature of 9500C is used.

A further benefit provided by the method of the present invention is that with fluorine, combustion can often be achieved at temperatures as low as room temperature, and that at 4000C it is certain to be complete.

For gas analysis, use is preferably made of mass spectrometry. Such as unit can readily be combined with the combustion system to form a unitary construction, so that elements of a substance under analysis can be determined not only concurrently but also very rapidly in a single process operation without the risk of contamination.

In cases involving solely special analysing tasks, the place of a mass spectrometer can be taken by an IR spectrometer.

When combustion is conducted with excess fluorine, the fluorides formed are predominantly accurately defined and mostly of the highest degree of oxidation. Exceptions are noted with chlorine and xenon. In a further aspect of the present invention the use of an additive, more particularly CsF, is recommended for substances containing these elements, so that chlorine and xenon may form a defined degree of oxidation.

In the presence of such elements as Ti, Nb and Ta, it will be advantageous to heat the gas analysis system to 500C. This makes sure the fluorides formed with these elements will be volatile.

In a further aspect of the present invention a small amount of oxygen is added to fluorine when Ru, Rh or Pt is present. This prompts the formation of volatile dioxygenyle salts.

In the periodic system of elements of Fig. 1 the elements framed in fat line will form volatile fluorides and permit of direct determination using the method of the present invention. Shown together with the elements are their fluoridation products.

In a further aspect of the present invention the samples to be analysed are enclosed in thin capsules, preferably of Sn or Ag, and inserted in a metal tube serving as a magazine. The use of a suitable control gate will then permit automatic sample feeding, which when combined with automatic valves controlling the admission and exhaust of gases and combustion products, will enable the rapid, production-type and automatic performance of analyses.

The invention is described more fully in light of the accompanying drawings.

Fig. 2 is a schematic arrangement illustrating a fluorine combustion elementary analyser with a mass spectrometer. Before the analyser is operated, any contamination is removed from it.

For the purpose, the inlet member 23, which has been evacuated to 10-5 Torr, is charged by opening a valve 11 with fluorine at about 1 bar from a portable container 10 holding pure fluorine. The fluorine is again exhausted after the lapse of at least 24 hours using a pump 13 which resists attack by fluorine. This cycie is repeated until the mass spectrometer 12 ceases to indicate the presence of contaminants in the fluorine.

For making an analysis, a nickel tube 14 is filled with 0.1 to 1 milligram of the nonvolatile substance under analysis. It is then threaded on a valve 1 5 and evacuated by means of a pump 13.

In order to prevent the sample from being atomized or carried away during evacuation of the air, a sintered-metal restrictor 1 6 is provided. A thermocouple-operated vacuum meter 20 switches the system over to high-vacuum operation when about 10 mbar are achieved.

When the requisite negative pressure has been achieved, the high-vacuum phase is automatically terminated by the closure of a valve 21, and through valve 22, an amount of fluorine is condensed into the combustion tube 14, which is cooled using liquid N2. The selected amount of fluorine, which is approximately 1 to 2 mbar 1, is dosed in a measuring cross 23. After the fluorine has been filled into the combustion tube 14, the valve 22 is again closed.

In the combustion tube 14 the substance is then fluorinated, which in many cases will already occur at room temperature. In order to ensure complete combustion the combustion tube 14 is heated to about 4000C to 4500C for a duration of 5 minutes. Thereafter the volatile fluorides generated by the combustion are ducted into the mass spectrometer 12, after opening the valves 1 5, 22 and 25, for two successive measurements.

An IR spectrometer can be provided in lieu of the mass spectrometer. A computer will finally determine the percent composition of the substance under analysis from the calibration input and the two mass spectra recorded.

Calibration is advantageously achieved by calibrating substances of accurately known compositions.

The analysing process can be made automatic by replacing the combustion unit 26 with a combustion tube 14' having variable regulating valves 15',21 and 22'. A schematic arrangement of such a unit 26' will be found in Fig. 3.

The combustion tube is provided with a crosslinking member 30 flange-bolted to which are three valves 15',21 and 22' under the control of automatic program devices omitted on the drawing.

For automatic analysis, the combustion tube 14'made of highest-purity nickel-is continuously heated to 4000 to 5000C in its lower one-third. The upper end of the combustion tube 14' is air-cooled. The samples under analysis are enclosed separately in thin metal capsules 31 by conventional process, such as cold welding.

The capsule material can be Sn or Ag.

After the metal capsules 31 have been inserted into a metal tube 32 provided on the combustion tube 14', and after this tube has been evacuated, the system is ready for the automatic analysis of the various samples contained in the capsules.

The programmed automatic device will successively control the valves 15',21 and 22' such that no more than one valve at a time will be open. First, valve 21' is opened to establish communication with the pump 13 through the line 33 to evacuate the combustion tube 14'.

Thereafter the valve 15' is operated to admit the fluorine needed for combustion.

Following combustion, which lasts several minutes, the combustion products are finally routed to the mass spectrometer. This is the function served by the manifold valve 21'. As valves, use can be made of metal bellows valves.

After a number of analyses the combustion tube is cleaned to remove residual metal fluoride ashes (SnF4, AgF2, etc.).

Experimental Example No. 1 Triphenylborane B(C5H5)3 was experimentally analysed. 12.10 milligram (50.09 yMol) B(CeH5)3 were charged at 50 Toorl (2755,uMol) with a moderate excess amount of fluorine at minus 1 960C in a sapphire tube (volume: 60 milliliters).

The greater portion was burned when heated to room temperature. A modest amount of black residue remained behind and was then completely eliminated by briefly heating it to 4000 C. Gas analysis by means of mass spectrometry showed combustion products BF3, CF4 and HF to arise at a molecular volume ratio of 1:18.05:14.98.

Experimental Example No. 2 In a second experiment 11.35 milligram (59.58 ,aMol) C7H7ClO2S were burned with 32.4 Torrl (1.77,uMol) fluorine at room temperature and then briefly heated with a stream of hot air.

Combustion was complete with no residue left.

In order to reduce the resultant small amounts of higher fluorine alkanes to CF4, the combustion tube 10 is heated to 4000C for 10 minutes. IR and mass spectrometer gas analysis evidenced the formation of 417 ,uMol CF4, 416 ,uMol HF, 59.5,uMol CIF3,59.5,aMol 02 and 59.5,uMol SF.

Claims (8)

Claims

1. A method for the elementary microanalysis and ultra microanalysis of chemical substances, wherein a sample of the substance is burned in the presence of fluorine and is analysed for determination of its constituents.

2. A method according to claim 1, wherein the gas analysis is conducted by means of a mass spectrometer.

3. A method according to claim 1, wherein the gas analysis is conducted by means of an IR spectrometer.

4. A method according to any of the preceding claims, wherein for substances containing chlorine or xenon, combustion is conducted with an additive, more particularly CsF.

5. A method according to any one of claims 1 to 3, wherein for substances containing Ti, Nb and/or Ta, the gas analysis system is heated to about 400 to 600C.

6. A method according to any one of claims 1 to 3, wherein for substances containing Ru, Rh and/or Pt, combustion is conducted using fluorine and oxygen.

7. A method according to any one of the preceding claims, wherein the samples are enclosed in capsules and inserted into a combustion tube using a magazine-type metal tube for automatic admission of the samples.

8. A method for the elementary microanalysis and ultramicroanalysis of chemical substances, substantially as herein before described.