WO1993017321A1

WO1993017321A1 - Electrothermal atomic absorption and preconcentration device

Info

Publication number: WO1993017321A1
Application number: PCT/AU1993/000079
Authority: WO
Inventors: Jaroslav Petr Matousek; Harry Kipton James Powell
Original assignee: Unisearch Ltd
Current assignee: Unisearch Ltd
Priority date: 1992-02-25
Filing date: 1993-02-25
Publication date: 1993-09-02
Anticipated expiration: 1994-08-25

Abstract

A method and an apparatus for the electrothermal atomic absorption spectrophotometric determination of a variety of elements which are affected by severe matrix interferences is disclosed. In one aspect the method relies upon the electrodeposition of the element to be analysed directly into the inner surface of a graphite tube disposed in the furnace assembly. In a second aspect of the method, a metal protective of the inner surface of the graphite tube is either electrodeposited prior to or concurrent with the deposition of the element to be analysed. The apparatus of the invention comprises an atomic absorption spectrometer incorporating a graphite furnace assembly; an automatic sample loader (14, 15) having an electrically conductive capillary tube (13) adapted to hold an aliquot of a sample or other solution; a pyrolytic graphite tube (10) disposed within the furnace assembly of the atomic absorption spectrophotometer so as to permit the drying, if required, ashing and atomisation of a sample contained therein and having an opening (11) adapted to permit the entry of the capillary tube (13) therein; and a source of potential (16) connected to the capillary tube (13) so as to form an anode and to the pyrolytic graphite tube (10) so as to form a cathode.

Description

Electrothermal Atomic Absorption and Preconcentration Device Technical Field

This invention relates to a method and apparatus for the determination by electrothermal atomic absorption spectrophotometry of a variety of elements which are affected by severe matrix interferences, in particular to a method and apparatus whereby matrix interferences are substantially eliminated through the removal of the matrix prior to atomisation of a sample. This invention also relates to a method for the determination by electrothermal atomic absorption spectrophotometry of a variety of elements, in particular to a method whereby a metal is electrodeposited onto the inside surface of a graphite tube, disposed within the furnace assembly, prior to or concurrent with the introduction of a sample for analysis. Background to the Invention

In.the analysis of certain elements that occur in very low concentrations in samples of interest, the principal difficulty in obtaining accurate analytical determinations for these elements is the requirement to preconcentrate the sample to bring the elements into suitable concentration ranges. Frequently this preconcentration step will introduce contamination into a sample both as a result of the addition of chemicals to the sample and through handling, for example from the laboratory environment.

One sample type of considerable interest that is difficult to analyse for these reasons is the determination of trace elements in sea water, particularly trace metals that may be toxic to the environment. Considerable effort has been directed to the development of rapid and reliable methods for the analysis of trace elements in sea water. In this sample type, not only are there significant problems in contamination and loss during sampling and storage of sea water, the high salt content of the matrix and the very low concentrations of the trace metals result in substantial difficulty in the analysis. Accordingly, in the prior art, most methods seek to preconcentrate the metals to a level where satisfactory determination may be made whilst removing the bulk of the matrix encountered in the sample. These methods include solvent extraction, chelating ion exchange, coprecipitation and electrochemical reduction.

Once a concentrate is obtained, it may be analysed using a variety of instrumental techniques, including neutron activation, optical emission spectrometry, mass spectrometry and flame and electrothermal atomic absorption spectrophotometry.

Electrothermal (graphite furnace) atomic absorption spectrophotometry is one instrumental technique which has sufficient sensitivity to permit the measurement of trace elements in sea water without a preconcentration step. However, the matrix introduces significant background absorbance problems since most elements are covolatilised with the bulk of the matrix. Accordingly, the use of direct determination by this technique has achieved only limited success. To circumvent these matrix interferences, chemical modifiers have been used leading to the development of methods for the analysis of several elements. These elements, for example iron, have high atomisation temperatures and must be present in relatively high concentrations. Thus, typically under optimum conditions, the detection, limits for heavy metals in sea water are generally above lμgL~ . For many elements of importance in environmental studies such as lead, copper, cobalt, nickel and chromium, detection limits exceed their natural concentrations. Accordingly, to lower the limits of detection, attempts have been made to eliminate the matrix interferences by electrodepositing the metals in the sample under analysis on carbon rods, mercury drops, high melting point wires of tungsten or iridium and graphite. The deposited metal is then atomised in the furnace of an atomic absorption spectrophoto eter.

For a variety of reasons, none of these methods have proved to be entirely satisfactory. For example, Fairless & Bard Anal. Lett. 5.(7) 433 (1972) disclose a method in which copper was analysed by electrodeposition of the copper into a well in a carbon rod. In this method, a copper containing sample was placed in the well, an electrode immersed therein with electrolytic plating taking place for a 15 second to 2 minute duration. Excess sample was then removed and the copper deposit washed prior to it being subjected to atomic absorption spectrophotometric analysis. The results achieved by the authors led them to conclude that the technique eliminated matrix interferences, but as only about one-tenth of the total copper was deposited "this necessarily limits the technique to samples with higher Cu concentrations where separation of the metal or elimination of matrix effects is of primary importance. There is obviously no improvement in absolute sensitivity" .

In two papers published subsequently to the Fairless & Bard paper, Matousek & Batley Anal. Chem. 4 , 2031 (1977) and Anal. Chem. £52, 1570 (1980) sought to improve upon the Fairless & Bard method by using a method in which the metal was electrodeposited on a pyrolytic graphite tube which was then placed in the furnace assembly of an atomic absorption spectrophotometer and subjected to conventional analysis. As can be seen from Fig. 1 and the accompanying description in the 1980 paper, the^' pyrolytic graphite tube formed a cathode in a cell in which the sample to be analysed was circulated from the cell through the pyrolytic graphite tube and back to the cell. On page 1572 of that paper, the authors state that "During electrolysis, deposition will occur on both the inner and outer faces of the tube. However, provided the same proportion of metal is deposited in this region, it can be assumed that there will be no error involved in measuring by atomisation only the metal on the internal surface. This latter amount must necessarily be greater than that on the external surface, where the efficiency of mass transport is poor due to inefficient stirring" . In that same paper it is disclosed that less than 1% of the available chromium concentration of lμgL~ in the samples analysed was deposited during a six minute electrolysis. Furthermore, it is inherent from the arrangement shown in Fig. 1 that graphite tubes with the electrodeposited metal would have to be removed from the cell and placed in the atomic absorption spectrophotometer for atomisation. Accordingly, it can be concluded that, whilst the method disclosed in the Matousek & Batley papers represents an improvement over the Fairless & Bard method, the fact that a) sample contamination through handling may occur, b) the metal is deposited on both the inner and outer surfaces of the pyrolytic graphite tube, c) a relatively large sample is required for analysis and d) the pyrolytic graphite tube with the deposited metal must be handled and placed in atomic absorption spectrophotometer indicates that this method is of limited value as a routine analytical procedure. This is important in the context of the numbers of samples that would need to be analysed, for example those of an environmental or biochemical nature. In a further publication, Matousek and Grey described a method for the analysis of an element that is capable of being electrodeposited onto a graphite surface from a sample which includes the element in low aqueous concentration and a matrix that interferes in the atomic absorption spectrophotometric determination of that element comprising: a) placing an aliquot of the sample within the inside of a pyrolytic graphite tube disposed in the furnace assembly of an atomic absorption spectrophotometer; b) positioning an anode within the pyrolytic graphite tube so as to be in contact with the sample therein; c) subjecting the sample to electrolysis by applying a potential to the anode and the pyrolytic graphite tube as the cathode so as to electrodeposit the element in the sample onto the inner surface of the pyrolytic graphite tube; d) removing the electrolysed solution; e) washing the electrodeposited element to remove the matrix; and f) subjecting the washed electrodeposited element to electrothermal atomic absorption spectrophotometric analysis.

An apparatus for performing this method was also disclosed which apparatus comprised an atomic absorption spectrometer incorporating a graphite furnace assembly, an automatic sample loader having a capillary tube adapted to hold an aliquot of a sample or other solution, and an electrode associated with said capillary tube; a pyrolytic graphite tube disposed within the furnace assembly of the atomic absorption spectrophotometer so as to permit the ashing and atomisation of a sample contained therein and having an opening adapted to permit the entry of the capillary tube and anode therein; and a source of potential connected to the electrode so as to form an anode and to the pyrolytic graphite tube so as to form a cathode.

This method and apparatus, however, suffered from the significant disadvantage that the opening in the graphite 5 tube needed to be enlarged to accommodate both the capillary tube and the platinum electrode which also needed to be insulated from the graphite tube at its point of entry. The increased orifice size in turn reduced sensitivity of the analysis and the life time of the 10 graphite tubes.

The present inventors have recognised the deficiencies in these prior art techniques and have sought to provide a method that is highly sensitive, accurate and reproducible, whilst being suited for use as a routine 15 analytical procedure. Disclosure of Invention

Accordingly, in one aspect, this invention consists in a method for the analysis of an element that is capable of being electrodeposited onto a graphite surface from a 20 sample which includes the element in low aqueous concentration and a matrix that interferes in the atomic absorption spectrophotometric determination of that element comprising: a) placing an aliquot of the sample within the inside of 25 a pyrolytic graphite tube disposed in the furnace assembly of an atomic absorption spectrophotometer using an electrically conductive capillary tube which is maintained in electrical contact with the sample therein; 30 b) subjecting the sample to electrolysis by applying a potential to the capillary tube as the anode and the pyrolytic graphite tube as the cathode so as to electrodeposit the element in the sample onto the inner surface of the pyrolytic graphite tube; 3.5 c) removing the electrolysed solution; d) washing the electrodeposited element to remove the matrix; and e) subjecting the washed electrodeposited element to electrothermal atomic absorption spectrophotometric analysis.

In a second aspect, this invention further consists in an automatic sample loader for use in the analysis of an element that is capable of being electrodeposited onto a graphite surface from a sample which includes the element in low aqueous concentration and a matrix that interferes in the atomic absorption spectrophotometric determination of that element, said sample loader having an electrically conductive capillary tube adapted to hold an aliquot of a sample or other solution, the capillary tube being adapted to be positioned automatically within the opening of a pyrolytic graphite tube, such that a sample discharged into the pyrolytic graphite tube from the capillary tube will remain in electrical contact with said capillary tube and wherein the capillary tube is provided with a means to connect it to a source of potential to form it into an anode.

In a third aspect, this invention further consists in an apparatus for the analysis of an element that is capable of being electrodeposited onto a graphite surface from a sample which includes the element in low aqueous concentration and a matrix that interferes in the atomic absorption spectrophotometric determination of that element comprising an atomic absorption spectrometer incorporating a graphite furnace assembly, an automatic sample loader of the said aspect; a pyrolytic graphite tube disposed within the furnace assembly of the atomic absorption spectrophotometer so as to permit the drying, if required, ashing and atomisation of a sample contained therein and having an opening adapted to permit the entry of the capillary tube therein; and a source of potential connected to the capillary tube so as to form an anode and to the pyrolytic graphite tube so as to form a cathode.

Although the aforementioned method was found to be highly sensitive, accurate, and reproducible it suffered from the substantial disadvantage that after repeated firings, the inside surface of the graphite tube in contact with sample was progressively degraded. This degradation did not affect atomisation but it did affect the efficiency of electrodepositing.

In fact in a typical analysis for lead with sodium chloride as the interfering matrix, it was found that after about 70-80 firings the pyrolytic surface was extensively fractured and raised where it had been in contact with sample. It is thought that this may be caused by intercalation compounds formed as a result of the reduction of sodium. Alternatively it is possible that hairline cracks in the pyrolytic coating in the graphite tube permit penetration of sample which under electrolysis generates hydrogen thereby causing the surface to crack.

Since ideally the method should be operated on an automatic basis, when processing large numbers of samples, it is evident that failure of graphite tubes after relatively few samples imposes a severe limitation on the method's utility.

Recognizing this disadvantage, the present inventors sought to provide an improved method whereby the pyrolytic graphite tubes are protected and are thus capable of being used for a substantially larger number of samples.

The protection of pyrolytic graphite tubes used in these analyses has now been achieved surprisingly by firstly electrodepositing a protective metal onto the inside surface of the graphite tube and then depositing the analyte from the sample for analysis on the protective metal deposit. The deposition of the analyte from the sample may be the electrodeposition Of the first aspect method or a deposition of an analyte in an aqueous sample followed by evaporation of the sample to dryness.

In addition the protective metal and the analyte from the sample may also be codeposited or a portion of the protective metal electrodeposited followed by a codeposition of protective metal and analyte from the sample.

In the prior art it has been recognized that salts of metals such as palladium may be beneficially used as chemical modifiers in electrothermal atomic absorption spectrophotometry.

In Spectrochimica Acta Rev. 3 (3) 225-274 (1990), the authors state that:

"The aimed effects of chemical modifiers are to alter, and thus be able to tolerate, certain unfavourable features of: (i) the analyte (extremely high or low volatility; the presence of various analyte species in real samples and calibration standards); (ii) the matrix (the presence of interfering concomitants; unmanageable background absorbance;

(iii) the gas phase (e.g. the partial pressure of certain active compounds) . " One way in which such modifiers are used is by deposition onto the inside surface of a graphite tube. Such uses are mentioned on page 234 of this paper. Each of these uses, however, requires deposition of a salt of the metal.

In Spectrochimica Acta 47B (4) 545-551 (1992), the author discloses the use of a graphite tube pretreated by depositing either Pd(N0₃)₂ or Mg(N0₃)₂ onto the inside surface and drying. Such a technique is said to be useful in the analysis of germanium where typically the analysis suffers from premature loss of GeO.

Deposition of metal salts onto the inside surface of graphite tubes has also been used in the technique of hydride generation for the determination of elements such as arsenic, antimony and selenium. In Spectrochimica Acta 44B (3) 339-346 (1989), the authors disclose that a solution of PdCl« in dilute nitric acid was injected into the graphite tube and dried at 110°C for 50 seconds. The generated hydrides were then swept over where the palladium solution had been injected to effect their collection and preconcentration.

From the foregoing discussion it will be evident that whilst deposition of some metal salts onto the inside surface of the graphite tube in electrothermal atomic absorption spectrophotometry has been found to be useful, there has been no recognition of the deposition of metals per se and the possible benefits that they might offer. Accordingly, in a fourth aspect, this invention further consists in a method for the analysis of an element that is capable of being determined using electrothermal atomic absorption spectrophotometry comprising (a) electrodepositing a metal onto the inside of a pyrolytic graphite tube either prior to the introduction of a sample containing the element or concurrently with deposition of the element from the sample into said tube;

(b) either electrodepositing the element from the sample or depositing an aqueous sample and evaporating it to dryness; and

(c) subjecting the sample or the element electrodeposited from the sample and the electrodeposited metal to electrothermal atomic absorption spectrophotometric analysis so as to obtain an analysis for said element.

In a preferred embodiment, a volume of acid, typically about 40μL for a 25^*JL sample is added to the electrodeposited metal and analyte and electrolysed, typically for about 60 seconds. The residual solution is then removed. It has been found that 3% nitric acid is a suitable medium and elements such as lead, cadmium, copper and manganese may be treated in this way. The previously described procedure of drying, ashing and atomisation is then carried out.

In a fifth aspect, this invention still further consists in an improvement to the technique of analysis by hydride generation, the improvement comprising electrodepositing a hydride collecting metal onto the inside surface of a graphite tube prior to introducing the hydride into the tube.

From the foregoing it will be evident that the method of the first aspect of this invention represents a substantial advance over the prior art in that electrodeposition takes place wholly on the inner surface of a pyrolytic graphite tube, substantially all of an element in a relatively small sample may be deposited, the deposited element is washed free of interfering matrix and the pyrolytic graphite tube is retained within the furnace assembly of an atomic absorption spectrophotometer. The use of relatively high voltage during electrodeposition also permits rapid deposition of substantially all of an element in a sample. Furthermore, using the apparatus of this invention, allows the method to be used for rapid, repetitive analysis of many samples.

It also follows that this method is highly sensitive, accurate and reproducible.

The samples to which this method may be applied comprise particularly environmental samples, such as ground waters, sea water and the like, and biological fluids, such as serum. Whilst these latter samples may be aqueous in nature, desirably protein and other interfering organic materials in the matrix should be removed. This is required as proteins, for example, interfere with the electrodeposition procedure-.

Other pretreatments may be automated and readily incorporated into the method of the invention. For example, a sample can be deposited following the electro deposition of the protective metal, reagents added, the residue dried and ashed and then redissolved prior to electrodeposition using the present invention.

The volume of sample to be deposited in the pyrolytic graphite tube will usually be about 5-100μL, typically about 50μL. Preferably the sample would include an electrolyte at a concentration of at least 0.05M as NaCl and a buffer, such as acetate in a concentration of about 0.02M.

It has been found during electrodeposition of a large sample aliquot that a proportion of the sample may be lost through the orifice owing to capillary action. The present inventors have found that one way of controlling this is to incorporate a small amount of surfactant in the sample. An example of a suitable surfactant is Triton X-100 which is incorporated in a concentration of about 0.005% w/v.

Once the sample has been placed in the pyrolytic graphite tube, with the capillary tube placed so as to be in electrical contact with the sample, a potential is then applied to the capillary tube as anode and the graphite tube as cathode. The potential should be constant, typically about 3-6 volts. Alternatively, constant current may be used.

Depending upon whether complete or partial electrodeposition is required, the potential will be applied for about 60-240 seconds. The pyrolytic graphite tube may be coated with pyrolytic graphite or may be totally pyrolytic graphite for improved lifetime.

In the apparatus of this invention, a sample aliquot is taken up into the capillary tube of the automatic sample loader, which then functions to place the sample aliquot into the pyrolytic graphite tube. The capillary tube enters the pyrolytic graphite tube, discharges a sample therein and then remains positioned within the pyrolytic graphite tube to an extent sufficient such that the capillary tube remains in electrical contact with a sample aliquot throughout electrodeposition.

The capillary tube must be electrically conductive and desirably should be unreactive towards a sample. Accordingly, a capillary tube may be formed from materials such as platinum and platinum/iridium alloy.

When electrodeposition has been completed, the electrolysed solution is removed and the deposited element is washed to remove interfering matrix. The apparatus of the invention accomplishes this step through withdrawal of the capillary tube from the pyrolytic graphite tube, collection of a washing medium, discharge of the washing medium into the pyrolytic graphite tube where the element has been electrodeposited, and then withdrawal of the washing fluid in the pyrolytic graphite tube into the capillary tube by suction.

If required, a further sample aliquot may be discharged directly into the pyrolytic graphite tube containing the previously electrodeposited element without any intervening washing. Washing may then occur following the electrodeposition of the second sample aliquot. In either case, it will be evident that the amount of electrodeposited element may be increased as required through the repeated electrodeposition of elements from a plurality of sample aliquots. In effect, this procedure will serve as an efficient means of enhancing concentration, thereby improving the limit of detection.

Typically, a matrix can be removed by washing with lOOμL distilled water. Using this procedure, the background absorption by the matrix will be reduced to approximately 1% of-the original value.

If required, εfiemical modifier may be applied to the electrodeposited element in an amount of about 50μL. It may be either left^~I&wthe pyrolytic graphite tube and dried prior to atomisation, or removed after a preset time.

The electrodeposited element is then ashed and atomised-as

graphite furnace atomic- absorption speetrogfiθtometry.

A wide range ©S-elements may be analysed using this method, including copper, lead, cadmium, zinc, manganese, nickel, chromium, cobalt, silver, gold, mercury, -- palladium_?-platinum, antimony, tin, bismuth, iridium,^" arsenic,^"selenium and gallium.

Whilst the methods of this invention may be performed manually, the use of an automatic sample loader is highly preferred. A suitable apparatus for this purpose is-a GBC PAL 2000 programmable Autoloader modified by using an electrically conductive sampling capillary. This GBC unit may be reprogrammed to perform sample loading, electrolysis, solution removal and several washing and chemical treatment steps as required.

In the method of the fourth aspect of the invention a number of advantages arise. These include:- (a) protection of the graphite surface; (b) provision of a more reproducible surface for the plating of analyte;

(c) reduction of the amount of modifier required compared with conventional chemical modification;

(d) provision of a more intimate contact between analyte and furnace to give a more reproducible volatilization; (e) ability to select protective metal according to its volatility to give different atomisation temperatures for variable resolution of atomic and molecular peaks. In the improved hydride technique of the invention, in addition to the foregoing, it provides the possibility of utilizing metals such as palladium which are capable of absorbing up to 900 times their volume of hydrogen.

For the use of the method of the fourth aspect of the invention, the following general protocols may be followed:- (a) electrodeposition of a metal onto the graphite surface followed by electrodeposition of the analyte from the sample; (b) electrodeposition of a metal onto the graphite surface followed by the deposition of the aqueous sample and subsequent drying;

(c) co-electrodeposition of both metal and analyte; and

(d) electrodeposition of a metal, followed by co-electrodeposition of the analyte and the metal. Using the apparatus of the second and third aspects and depending upon the nature of the sample to be analyzed, wash cycles may be used between or following particular deposits as required. A variety of metals may be deposited including palladium, platinum, ruthenium, rhodium, iridium, osmium, gold and the like. The metal to be deposited will be selected on the basis of its volatility and in some cases its functionality as a modifier. Furthermore, to minimize furnace contamination the selected metal will desirably be one which is not frequently analyzed.

Typically the metals will be in aqueous solution in a concentration in the range of the order of lOppm-lOOppm. A volume of this solution in the range of about 25-75μL may be deposited in the graphite tube. Using the apparatus of the second and third aspects, electrodeposition of the metal occurs in about 60 seconds using 5.2V with the current in the range of 15-30mA. It has been found that when palladium is the electrodeposited metal and lead is to be analyzed in the sample in a sodium chloride matrix, a graphite tube may be fired at least 170 times with no microscopically visible signs of tube deterioration. This is in marked contrast to the use of the first aspect method where after 70-80 firings, the pyrolytic coating was extensively fractured and raised.

It has also been found that unlike the first aspect method, the use of a modifier such as ammonium nitrate is not required nor is it necessary to use a surfactant such as Triton.

Furthermore, the concentration of metal required is substantially less than modifier metal salt solutions used in the prior art. Typically only 0.5-5% as much metal is required. In addition to the applications of the inventive method discussed above, it is possible to utilize electrodeposition of a metal as a protective agent to prevent the formation of carbides of refractory elements such as vanadium, chromium, molybdenum and the like. It is also possible that semi-metals and non-metals may be determined by direct formation of a hydride in the graphite tube. In this procedure, following electrodeposition of a metal, the sample is electrolyzed in situ to generate nascent hydrogen at the cathode which then reacts with analyte to form hydride. It is also possible to generate nascent oxygen at the anode which is then used to oxidize organic matter using a TiO„ coated electrode.

Brief Description of Drawings Figure 1 is a schematic representation of an Autoloader of the invention;

Figures 2, 3 and 4 each graphically represents the output of an electrothermal atomic absorption spectrophotometric analysis of lead in aqueous samples. Modes for Carrying out the Invention

In the accompanying Fig. 1 there are shown schematically the essential features of the modified Autoloader 15. Thus , this apparatus comprises a pyrolytic graphite tube 10, shown in cross-section, an opening 11 therein with the platinum/iridium capillary sampling tube 13 disposed therein. The platinum/iridium capillary tube 13 and the pyrolytic graphite tube 10 are connected to a constant voltage or current supply 16 with the platinum/iridium capillary tube as anode and the pyrolytic graphite tube as cathode

A teflon spacer 12 is disposed in the opening 11 with the capillary tube 13 passing therethrough. The spacer serves to both seal the opening and insulate the capillary tube 13 from the graphite tube 10. At an upper end, capillary tube 13 is connected to plastic tube 17 which in turn is in fluid communication with a solution.

An arm 14 permits the capillary sample tube 13 to be moved into opening 11 of the pyrolytic graphite tube and away to withdraw a sample, wash solution or the like.

In Fig. 2, the method of analysis used was the method of the first aspect, i.e., there was no electrodeposition of a protective metal. In this analysis, the sample solution consisted of 0.02ppm Pb in 0.5MNaCl and 25μL was electrodeposited. The sample contained 0.5ng Pb. In Fig. 3, the method of analysis used was conventional electrothermal atomic absorption spectrophotometric and the sample consisted of an aqueous solution of 5μL of 0.lppm Pb. Thus the sample contained 0.5ng Pb. In Fig. 4, the method of analysis used is one of the protocols of the present invention whereby 25μL of lOppm Pd(N0₃)₂ was firstly electrodeposited followed by electrodeposition of 25μL of 0.02ppm Pb in 0.5MNaCl. The sample contained 0.5ng lead.

A comparison of the graphs shown in the figures indicates that the inventive method gave an output with the peak being shifted towards a higher appearance temperature. This resulted in a more accurate, reproducible analysis as the peaks were further displaced from the sodium background.

Using the inventive method described above and deposition times of 60 seconds, RSD's of the order of 1.5-3% have been obtained.. The present inventors have found that by optimization of the method of this invention, analyses may be done with RSD's as low as 0.2-0.3%.

Whilst this invention has been described with reference to certain preferred embodiments and features, it will be-appreciated by those skilled in the art that numerous variations and modifications may be made to this invention without departing from the spirit or scope thereof.

Claims

1. A method for the analysis of an element that is capable of being electrodeposited onto a graphite surface from a sample which includes the element in low aqueous concentration and a matrix that interferes in the atomic absorption spectrophotometric determination of that element comprising: a) placing an aliquot of the sample within the inside of a pyrolytic graphite tube disposed in the furnace assembly of an atomic absorption spectrophotometer using an electrically conductive capillary tube which is maintained in electrical contact with the sample therein; b) subjecting the sample to electrolysis by applying a potential to the capillary tube as the anode and the pyrolytic graphite tube as the cathode so as to electrodeposit the element in the sample onto the inner surface of the pyrolytic graphite tube; c) removing the electrolysed solution; d) washing the electrodeposited element to remove the matrix; and e) subjecting the washed electrodeposited element to electrothermal atomic absorption spectrophotometric analysis.

2. A method as in claim 1 wherein the aliquot of sample is about 5-100μL.

3. A method as in claim 2 wherein the aliquot of sample is about 50μL.

4. A method as in any one of claims 1 to 3 wherein the sample includes an electrolyte.

5. A method as in claim 4 wherein the electrolyte is in a concentration of at least 0.05M

6. A method as in any one of claims 1 to 5 wherein the sample includes a buffer.

7. A method as in claim 6 wherein the buffer is in a concentration of about 0.02M.

8. A method as in any one of claims 1 to 7 wherein the sample includes a surfactant.

9. A method as in claim 8 wherein the surfactant is in a concentration of about 0.005% w/v.

10. A method as in any one of claims 1 to 9 wherein the potential is about 3-6 volts.

11. A method as in any one of claims 1 to 10 wherein the potential is applied for about 60-240 seconds.

12. A method as in any one of claims 1 to 11 wherein the electrodeposited element is washed with about 100μL of distilled water.

13. A method as in any one of claims 1 to 12 wherein a chemical modifier is applied to the electrodeposited element.

14. A method as in claim 13 wherein about 50μL of chemical modifier is applied.

15. A method as in any one of claims 1 to 14 wherein the element to be determined is selected from the group consisting of copper, lead, cadmium, zinc, manganese, nickel, chromium, cobalt, silver, gold, mercury, palladium, platinum, antimony, tin, bismuth, iridium, arsenic, selenium and gallium.

16. An automatic sample loader for use in the analysis of an element that is capable of being electrodeposited onto a graphite surface from a sample which includes the element in low aqueous concentration and a matrix that interferes in the atomic absorption spectrophotometric determination of that element, said sample loader having an electrically conductive capillary tube adapted to hold an aliquot of a sample or other solution, the capillary tube being adapted to be positioned automatically within the opening of a pyrolytic graphite tube, such that a sample discharged into the pyrolytic graphite tube from the capillary tube will remain in electrical contact with said capillary tube and wherein the capillary tube is provided with a means to connect it to a source of potential to form it into an anode.

17. An apparatus for the analysis of an element that is capable of being electrodeposited onto a graphite surface from a sample which includes the element in low aqueous concentration and a i.f_.trix that interferes in the atomic absorption spectrophotometric determination of that element comprising an atomic absorption spectrometer incorporating a graphite furnace assembly, an automatic sample loader as claimed in claim 16; a pyrolytic graphite tube disposed within the furnace assembly of the atomic absorption spectrophotometer so as to permit the drying, if required, ashing and atomisation of a sample contained therein and having an opening adapted to permit the entry of the capillary tube therein; and a source of potential connected to the capillary tube so as to form an anode and to the pyrolytic graphite tube so as to form a cathode.

18. An apparatus as in claim 17 or a sample loader as in claim 16, wherein the capillary tube is formed from platinum or platinum/iridium alloy.

19. A method for the analysis of an element that is capable of being determined using electrothermal atomic absorption spectrophotometry comprising

(a) electrodepositing a metal onto the inside of a pyrolytic graphite tube either prior to the introduction of a sample containing the element or concurrently with deposition of the element from the sample into said tube;

(b) either electrodepositing the element from the sample or depositing an aqueous sample and evaporating it to dryness; and (c) subjecting the sample or the element electrodeposited from the sample and the electrodeposited metal to electrothermal atomic absorption spectrophotometric analysis so as to obtain an analysis for said element.

20. A method as in claim 19 wherein the electrodeposition of step (a) is performed prior to the electrodeposition of step (b) .

21. A method as in claim 19 wherein the electrodeposition of step (a) is performed concurrently with the electrodeposition of step (b) .

22. A method as in claim 19 wherein the metal is electrodeposited in step (a) and further metal is electrodeposited with the element in step (b) .

23. A method as in any one of claims 19 to 22 wherein the element in the sample is electrodeposited.

24. A method as in any one of claims 19 to 23, wherein the electrodeposited metal is selected from the group consisting of palladium, platinum, ruthenium, rhodium, iridium, osmium and gold.

25. A method as in any one of claims 19 to 24 wherein the metal to be electrodeposited is an aqueous solution in a concentration of about 10-lOOppm.

26. A method as in claim 25 wherein about 25-75μL of solution is electrodeposited.

27. A method as in any one of claims 19 to 26 wherein the metal is electrodeposited at a potential of about 3-6 volts with the current in the range of about 15-30mA.

28. A method as in any one of claims 19 to 27 wherein a

^* volume of acid is added to the electrodeposited metal and analyte and electrolysed with residual solution being removed following completion of the electrolysis.

29. A method as claimed in claim 28 wherein about 40μL of acid is used for each 25μL of sample.

30. A method as in claim 28 or claim 2^'9 wherein electrolysis is conducted for about 60 seconds.

31. A method as in any one of claims 28 to 30 wherein the acid is about 3% nitric acid solution.

32. A method as in any one of claims 28 to 31 wherein the element to be analysed in the sample is selected from the group consisting of lead, copper, cadmium and manganese.

33. A method as in any one of claims 19 to 32 wherein electrodeposition of the sample is conducted using a method as claimed in any one of claims 1 to 15.

34. A method as in any one of claims 19 to 32 when conducted using an apparatus as claimed in claim 16 or an autosampler as claimed in claim 17.

35. An improvement to the technique of analysis by hydride generation, the improvement comprising electrodepositing a hydride collecting metal onto the inside surface of a graphite tube prior to introducing the hydride into the tube.

36. A method as in claim 35 wherein the hydride collecting metal is palladium.

37. A method as in claim 35 or claim 36 wherein the electrodeposition is conducted using a method as claimed in any one of claims 1 to 15 or 19 to 33.

38. A method as in claim 35 or claim 36 wherein following electrodeposition of the hydride collecting metal, a sample is placed in proximity thereto and electrolysed to generate nascent hydrogen to thereby form hydride.