WO1988008527A1

WO1988008527A1 - Atomic absorption spectroscopy

Info

Publication number: WO1988008527A1
Application number: PCT/AU1988/000122
Authority: WO
Inventors: John William Steiner
Original assignee: The State Of Queensland
Priority date: 1987-04-29
Filing date: 1988-04-29
Publication date: 1988-11-03
Also published as: AU603674B2; AU1707188A; EP0374148A1; GB2228997A; GB8924200D0

Abstract

A process for the chemical determination of an analyte in a specimen using thermally activated atomic absorption spectroscopy where a specimen may be injected directly through sampling port (12) into an atomiser furnace (10) without complicated pretreatment. Constituents of the specimen which may interfere with the process are removed by volatilisation controlled by introduction of reactive gases through gas ports (13, 14) creating reducing, oxidising or neutral environments, and also controlling pH. Volatile substances causing deterioration of the atomiser furnace may also be inhibited or neutralised by introduction of such gases. Suggested gases, by way of example only, are carbon monoxide, ammonia, hydrogen, methane, argon and oxygen.

Description

Title: "ATOMIC ABSORPTION SPECTROSCOPY" BACKGROUND OF THE INVENTION

(1) Field of the Invention

THIS INVENTION relates to thermally activated atomic absorption spectroscopy.

(2) Prior Art

Analysis of complex specimens such as biological material (e.g. plant and animal tissue, whole blood) by atomic absorption spectroscopy (AAS) using electrothermal atomisation (ET) generally requires complex pre-treatment of the specimen to remove interferents. This pre-treatment is usually required to be performed by highly skilled personnel and usually utilises high quality glassware, chemicals and reagents. As a . consequence it is both time consuming and costly.

The analytical process in the analysis by ET/AAS of such samples usually consists of three phases: (i) drying phase; (ii) ashing phase;

(iii) atomisation phase. The procedure is generally as follows:

The digested and pretreated specimen solution is injected in small quantity into the graphite atomiser whereupon the atomiser is heated to a temperature (e.g. 120°C - 250°C) sufficient to evaporate the liquid vehicle or solvent carrying the analyte. The remaining specimen is then ashed by heating the atomiser to 250°C - 600°C. During this phase it is common for a current of inert gas (e.g. argon, nitrogen) to be passed through one of two gas ports into the atomiser and emerge therefrom via a sampling port to act as a purge to remove some undesired volatile contaminants produced by the ashing. It is also known to use a mixture of gases containing an organic gas (e.g. argon/methane) where the organic gas contributes carbon atoms at higher temperatures which act to form a pyrolytic coating of carbon on the surface of the atomiser, thereby offering some protection from oxidising effects of some of the specimen constituents. The introduction of gases in the normal procedure then, is to purge atomised contaminants from the atomiser and to offer some protection for the atomiser carbon from oxidation by specimen constituents and atmospheric oxygen. The specimen may often, after the ashing stage, contain thermally stable compounds, predominantly oxides, which are not purged by the gases due to their stability. Interferents that were not removed from the sample solution, by the pretreatment may then interfere spectrally through non-atomic absorption^• peaks. Further, thermally stable compounds (again predominantly oxides, possibly even including oxides of the analyte) may remain in the atomiser even after analysis, thereby interfering with subsequent specimens and giving rise to - the phenomenon known as memory effect.

Chemical modifiers in the form of reactive solutions are known (e.g. ammonium sulphate). These are added to the specimen before/or during the drying stage in an attempt to chemically remove interferents. Due to the chemical nature of these modifiers they can only be added to the atomiser before/or during the drying phase (i.e. generally less than the boiling point of the solution, usually between 80°C - 150°C).

It is seen then that the prior art processes used for ET/AAS and described above have disadvantages in that:

- complex pretreatment by skilled personnel is required to attempt to "clean" the sample;

- this process is expensive and time consuming; - high quality glassware and reagents are usually required;

- thermally stable compounds still remain during the analysis phase contributing to interference; - compounds oxidising part of the specimen in the atomiser contribute to the breakdown of the atomiser - adding further to the expense by necessitating relatively frequent replacement thereof;

- thermally stable compounds may remain in the atomiser from one specimen to the next causing memory effects;

- use of chemical modifiers may only be implemented prior to or during the drying stage of the process; - - use_. of chemical modifiers commonly requires accurate additions of known chemicals so as not to otherwise interfere with the process;

• - pyrolytic coating by organic gases are only partially effective in prevening oxidation of the atomiser furnace itself.

SUMMARY OF THE PRESENT INVENTION It has been found by the present inventor that use of gaseous modifiers enables direct analysis of liquid specimens without complicated pretreatment as required in prior art processes. Gaseous modifiers may be added at any temperature (unlike prior art chemical modifiers which, added in a liquid vehicle, could not be added at temperatures above the boiling point of the liquid) . This allows environmental conditions within the atomiser to be quickly and accurately controlled throughout all stages of the process. Hence, even inorganic interferents may be manipulated prior to the atomisation of the analyte. Even where the atomisation temperature of the analyte is less than that of an interferent, chemcial modification is possible in a number of cases to create species with either higher or lower thermal stability than the elemental species, thereby permitting the "cleaning up" of specimens wholly within the atomiser itself prior to actual atomisation of the analyte.

It is an object of the present invention to sustantially reduce and in some cases remove the need for complex pretreatment of samples prior to introduction into the atomiser (apart from simple digestion of solid samples) .

It is a preferred object to provide a gaseous atmosphere within the atomiser during the analysis process that will reduce the rate of deterioration of the furnace. It is. a further preferred object to provide a process whereby memory effects will be substantially reduced.

Other preferred objects will become apparent from the following description. The invention resides broadly in a process for the determination of elemental concentration in a sample using thermally activated atomic absorption spectroscopy including the steps of: introducing an amount of sample in liquid form into the atomiser; changing the temperature of the atomiser in a controlled manner; introducing one or more gases into the atomiser synchronised with the temperature changes, at least one of the gases being a reactive gas; and performing absorption and/or transmission measurements of the resonant radiation for the analyte of interest.

In a preferred form of the invention, the process utilises the gases nitrogen, hydrogen, oxygen. methane, ammonia, carbon monoxide and argon at various stages of analysis suited to the analyte and the specimen type. These gases, or combinations thereof, are introducted at strategic times (and hence temperatures) to effect one or more of the foregoing objects.

BRIEF DESCRIPTION OF THE DRAWINGS In the following description reference is made to the attached illustrations where: FIG. 1 is a sectional side view of a conventional graphite atomiser assembly II, containing the atomiser tube 10 with sample introduction hole 12, optical windows 15 at both ends thereof, conventional gas ports 13 and additional gas ports 14 through which the indicated gases are introduced into the^' atomiser 10. Auxilliary features of the assembly e.g. water casting are not specifically shown as they are not immediately relevant to the present invention;

FIG. 2 is an absoprtion spectrum in respect of silicon in whole blood introduced into an atomiser using conventional inert gas flushing when air is introduced;

FIG. 3 is an absoprtion spectrum for the same blood after the invention process has been utilised.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

So that the invention may be more completely understood the following preferred form using the determination of silicon in whole blood as an example is presented. It should be noted that a very complex specimen (whole blood) containing high concentrations of organic and inorganic interferents has been chosen to demonstrate the effectiveness of the invention process. The application of the process to inorganic interferents is also particularly noted. In the furnace assembly 10 illustrated in FIG. 1 the two conventional gas ports 13 were used separately for the introduction of argon (Ar) and carbon monoxide (CO) respectively. Of the additional ports 14, one was used for oxygen (0₂) (exclusively) and the other for any one or more of the gases methane (CH₄), ammonia (NH₃) or hydrogen (H₂) . Introduction, of the gases was controlled by needle valves (not shown). Other suitable means which in turn may be controlled manually or with various degrees of automation (e.g. computer controlled by an analytical program controlling the AAS instrument) may be used however.

For the determination of silicon in whole blood, the specimen (4 microlitre) was introduced by conventional means (e.g. automated micro syringe) in its untreated state through sampling port 12 into graphite atomiser 10 ^' which was preheated to 60°C. Simultaneously, methane, hydrogen and ammonia were introduced through gas port 14 to create .a highly reducing, highly basic chemical environment in atomiser 10. The atomiser was slowly heated to a temperature of 480°C by application of a suitable current.

During this stage the strongly reducing atmosphere allowed the removal of organic constituents without the formation of extremely stable carbonaceous compounds which commonly tend to form under normal inert conditions.

Gentle suction (not shown) was applied through sampling port 12 to the atomiser 10 while - the atomiser was heating from 320°C to 480°C to encourage drying and the prompt removal of the liquid vehicle and products of this heating phase (ashing by-products) .

When the atomiser 10 reaches 480°C the chemical environment was changed to a strongly oxidising atmosphere for substantially complete removal of any remaining organic constituents. This was continued until the temperature of the atomiser reached 520°C.

At 520°C introduction of ammonia was ceased because of extreme instability of ammonia above this temperature in the presence of oxygen. At 560°C introduction of hydrogen was ceased to avoid explosion with oxygen at the high temperatures. At 660°C introduction of oxygen was ceased to reduce damage to the carbon atomiser furnace.

Also at 660°C argon was introduced to purge the atomiser of oxygen. At 700°C, argon was turned off and ammonia (which was no longer dangerous in the absence of oxygen) , carbon monoxide and hydrogen were again introduced to revert to a highly reducing atmosphere to encourage the decomposition of otherwise thermally stable compounds (e.g. sodium oxide, sodium chloride, iron oxides) which characteristically form at that stage of the heating (ashing) procedure. The carbon monoxide was introduced to remove sodium oxide, sodium chloride and iron oxides as well as to "soak up" any oxygen produced and in so doing to protect (at least partially) the atomiser carbon and to prevent the formation of further stable oxides.

The temperature was then raised and before reaching 1460°C introduction of carbon monoxide was ceased and argon was again introduced. The temperature was then raised to the atomisation temperature for silicon of 2800°C. This sequential synchronised procedure is illustrated in Table 1 hereunder. TABLE 1 In situ gaseous treatment of whole blood sample during the determination of silicon. Gas mixtures and temperature of introduction.

+ means gas on - means gas off

As a further example as to the performance of the invention process, the results obtained following the process as described in the foregoing are contrasted with these obtained by injection of blood into the atomiser and using the conventional purging means with inert gas and air. FIG. 2 illustrates the spectrum obtained using the conventional means. Note the two large non-atomic absorption peaks.

FIG. 3 contains a spectrum for the same specimen obtained using the invention process. Note the large reduction in the non-atomic peaks together with a very large increase in sensitivity for the analyte, silicon.

The foregoing description was presented by way of example only and many modifications may be made to that process.

For example, it is obvious that the process is applicable to other elemental analytes in most samples. It will also be apparent that heating rates; critical temperatures for drying, ashing and atomising; sequence of introduction of the gases; synchronisation of the gases with temperature changes may all be varied to suit particular types of samples, analytes or other analytical peculiarities. Obviously other gases may be used to alter the chemical environment in the atomiser thereby effecting e.g. pH, reduction/oxidation potential or other function (e.g. the "soaking up" function as performed by carbon monoxide in respect of oxygen, or destabilizing otherwise thermally stable species). When the determination is being carried out for an analyte (e.g. Cd) with a lower atomisation temperature than the atomisation temperature of an interferent (e.g. Fe), it is possible, by the introduction of the gases, to produce a thermally stable compound of the analyte and to then raise the temperature in the atomiser to the volatilisation temperature of the interferent. When the interferent has been volatilised, the environment in the atomiser can be changed, by the introduction of gases (and possibly a reduction in temperature in the atomiser) to destabilize the thermally stable analyte compound to enable determination of the analyte.

Various other changes and modifications may be made to the embodiments described without departing from the scope of the invention defined in the appended claims.

Claims

1. A process for a determination of elemental concentrations in a specimen using thermally activated atomic absorption spectroscopy including: introducing an amount of sample in liquid form into the atomiser; changing the temperature of the atomiser- in a controlled manner; introducing one or more gases into the atomiser synchronised with the temperature changes; at least one of the gases being a reactive gas; and performing absorption and/or transmission measurements of the resonant radiation for the analyte of interest. 2. A process as claimed in Claim 1 wherein: the reactive gases are introduced after the ashing phase and prior to or during atomisation. 3o A process as claimed in Claim 1 or Claim 2 wherein: the reactive gases create an oxidising environment within the atomiser.

4. A process as claimed in Claim 1 or Claim 2 wherein: the^" reactive gases create ^"a reducing environment and after the pH within the atomiser.

5. A process as claimed in any of the preceding claims wherein: the reactive gases alter the pH within the atomiser. 6. A claim as claimed in Claim 1 or Claim 2 wherein: inorganic interferents in the specimen are removed through atomisation by inhibiting the formation of stable species of interferents. 7. A process as claimed in any preceding claim wherein: carbon monoxide is introduced to remove oxygen and other constituents from the atomiser environment. 8. A process as claimed in any preceding claim wherein: suction is used during the ashing phase to remove ashing by-products.

9. A process as claimed in Claim 1 wherein: the gases are introduced to destabilize inteferents within the atomiser.

10. A process as claimed in Claim 1 wherein: the gases are introduced to induce the formation of thermally stable analyte species. 11. A process as claimed in Claim 10 wherein: after the formation of the stable analyte species, the temperature in the atomiser is raised to the volatilisation temperature of an interferent in the sample, and then after the volatilisation of the interferent, the environment in the atomiser is changed by the introduction of other gases to destabilize the stable analyte species to enable determination of the analyte.

12. A process as claimed in Claim 1 wherein: an atomiser assembly containing more than two gas ports is used.

13. A process as claimed in Claim 1 wherein: an atomiser generally of the configuration illustrated in FIG. 1 of the drawings is used. 14. An atomiser assembly with more than two gas ports operated in accordance with the process of any one of Claims 1 to 11.

15. An atomiser assembly generally of the configuration as illustrated in FIG. 1 of the drawings.