The reduction and elimination of matrix interferences in graphite furnace atomic absorption spectrometry

0584-8547/87 S3.oof0.00 0 1987krgamon Journals Ltd.

Spectrochimrca Acta, Vol. 42B, No.8,pp.937-949, 1987 Printed inGreatBritain.

INVITED

REVIEW

The

XIAO-QUAN

Research Center for Eco-Environmental

Sciences, Academia Sinica, P.O. Box 934, Beijing, China

(Received 13 April 1987) Abstract-The approaches to reduction or elimination of matrix interferences encountered in graphite furnance atomic absorption spectrometry is reviewed. These techniques include matrix modification, application ofactive gas, and coating tubes with metallic compounds. The research work carried out in the author’s laboratory is emphasized. A more universal matrix modifier, palladium, is proposed for the determination of mercury, lead, tellurium, bismuth, arsenic, thallium and indium in environmental samples.

1.

INTRODUCTION

ELECTROTHERMAL atomic absorption spectrometry is one of the most promising methods for the determination of trace elements in biological and environmental materials due to its inherently high sensitivity and specificity. However, problems arise in analyzing samples composed of complex and variable materials. Potential chemical interference effects that often occur in a pulse-operated electrothermal atomizer frequently cause depression of the atomic signal due to covolatilization of the analyte with the matrix. There are various ways of reducing such interferences involving the use of a constant temperature furnace Cl], graphite platform [2], probe [3], capacitive discharge heating [4] or stabilized temperature platform furnace, STPF, and Zeeman background correction[5] to avoid temporal and spatial nonisothermality of the furnace during heating. Chemical treatment of the sample in the graphite furnace is a simple approach to alleviate interferences encountered, especially in the determination of volatile elements in heavily matrixed samples. In this paper some of the applications of the chemical treatment (matrix modification) in graphite furnace atomic absorption spectrometry will be reviewed with emphasis on the research work conducted in the author’s laboratory.

2.

MATRIX MODIFICATION USING INORGANIC METALLIC SALTS

In the determination of volatile elements such as cadmium, lead, selenium, arsenic, mercury and tellurium, the modification of the matrix by the addition of chemical reagents to the furnace is necessary either to prevent the loss of analyte during charring by increasing the stability of the analyte or to volatilize the matrix constituents prior to atomization of the analyte by increasing the volatility of the matrix. EDIGER [6] showed that the addition of fluoride, sulfate or phosphate made possible the use of a higher-charring temperature for cadmium. A diammonium hydrogen phosphate-nitric acid-electrothermal atomization procedure was suggested for cadmium in human urine to retard the atomization rate of cadmium sufficiently to resolve the atomic absorption signal of the analyte from the nonatomic absorption of the matrix [7]. Magnesium nitrate added to phosphate further stabilized cadmium, lead and tin [5]. The presence of nickel caused a marked shift of the beginning of the vaporization of selenium [6]. The effect of nickel is probably the formation of relatively non-volatile nickel selenide in the furnace. Mercury has been difficult to determine in the graphite furnace because of its extreme volatility. Adding sulfide to the mercury in a nitric acid solution resulted in much greater thermal stability allowing a charring temperature up to 300°C [6]. The atomic absorption of mercury was also enhanced in the SA4X:88-1\

931

938

Invited

review

presence of hydrochloric acid and hydrogen peroxide due to the formation of HgCl,.H,O, adduct of higher stability [S]. KIRKBRIGHT~~ al. [9] evaluated various matrix modifiers for mercury and found that K,Cr,O, and Na,S permitted charring temperature of about 250°C. In our laboratory metal salts including the salts of palladium, platinum, iridium, copper and nickel have been used as matrix modifiers to stabilize mercury, lead, tellurium, bismuth, arsenic and other relatively volatile metals. The experimental results are discussed individually. 2.1. Mercury Mercury is often lost during drying and charring in the graphite furnace, so it is difficult to determine mercury with graphite furnace atomic absorption spectrometry. However, when microgram quantities of gold, platinum or palladium were used as a matrix modifier, mercury was stabilized as a result of the formation of stable amalgams. In the HGA 72 the tolerable charring temperature for mercury in the presence of gold, platinum or palladium can be raised to 250,300 or 5OO”C, respectively, as shown in Fig. 1. The higher charring temperature is a distinct advantage since organic components can be burned off and hence molecular absorption can be reduced. For waste water samples (100 ~1) containing mercury of 0.010-0.080 pg/ml and palladium of 40 pg/ml, average recoveries were 8&l lo’;/,. The detection limit of the method, defined as a quantity of analyte which will produce an absorbance equal to twice the noise was found to be 0.2 ng of mercury if a volume of sample solution of 100 ~1 was taken. The method has been applied for the determination of mercury in waste water, down to the 0.007 pg/ml level [IO]. 2.2. Lead The direct determination of trace amounts of lead in sea water with graphite furnace atomic absorption spectrometry is troublesome since the large amounts of alkali and alkaline earth halide present cause serious loss of lead during the preatomization stage. The interference observed was due to the covolatilization of lead with sodium chloride or the vapor phase formation of chlorides of lead from the magnesium chloride matrix, the lead species being partially lost with the rapidly expanding inert gas phase. In order to minimize these interferences a matrix modification technique using micrograms of palladium or platinum has been proposed for the determination of lead at ppb levels [ 111. In the presence of palladium or platinum the charring temperature of lead can be raised to 12OO”C, a temperature high enough to volatilize the salts present in sea water, as shown in Fig. 2. The interference of sulfate and phosphate ions can be eliminated by the addition of La(N03)3 [12]. For lead concentrations greater than 2 ppb in sea water, a direct injection method was recommended. For lead concentrations lower than ppb levels, the sea water samples were extracted with APDC-CHCl, system. The absorbance reading versus charring

0.40 [

:“tV____ 400

800

1200

Temperature

1600

2000

2:OO

, “C

Fig. 1. Effect of matrix modifiers on the charring and atomisation temperatures of mercury [lo]. (0) 20 ng Hg + 400 ng Au; (0) 20 ng + 400 ng Pd; (A) 20 ng + 400 ng Pt; ( x ) 20 ng Hg in aqueous solution.

Invited review

939

OLO “u 0.30 -

6 ; 0.20 . 13 q 0.10 0

-

200

LOO 600

800

Temperature,

1000 1200 1400 “C

Fig. 2. Effect of matrix modifiers on the charring temperature of lead in the presence of NaCl or MgCI,(l1).(V)1.2ngPb;(m)1.2ngPb+20~gNaCl;(~)1.2ngPb+20ygNaCl+2ygPd;(~)l.2ng Pb + 20 pg NaCl + 2 pg Pt; (A) 1.2 ng Pb + 10 pegMgC&; (A) 1.2 ng Pb + 10 pegMgC$ + 2 pegPd; (0) 1.2 ng Pb + 10pegMgCl*+ 2 pg Pt.

temperature is shown in Fig. 3. As can be seen from the figure for lead in APDC-CHCIJ a maximum absorbance appears at 800°C. With an increase of temperature, lead begins to volatilize and absorbance decreases. When palladium is added along with the organic extract of lead, a charring temperature of 1200°C may be tolerated and the sensitivity is increased. This technique was also applied to the determination of lead in urine [13]. The mechanism by which palladium and platinum stabilised lead has been studied. The temperature-time profile for the graphite furnace at the heating rate of 0.1 K/ms and the absorbance-time profile for lead alone and lead in the presence of palladium or platinum are shown in Fig. 4. The appearance temperatures for lead in the platinum or the palladium matrix is shifted to higher temperatures (curve a, 1080K; curve b, 15OOK;curve c, 1480K). This observation is consistent with the formation of a more stable compound or alloy between lead and palladium or platinum, the compound being an intermediate in the formation of lead atoms. 2.3. Tellurium Tellurium is one of the volatile elements and its loss in the graphite furnace increases with increasing charring temperature. EDIGER [6] used nickel to stabilize tellurium. We recommended the addition of micrograms of palladium, platinum or iridium as matrix modifier to prevent such loss [14]. The charring temperature can be raised to 1300°C and the sensitivity is better than that obtained with nickel as shown in Fig. 5. Based on this technique an analytical method for differential determination of tellurium (IV) and tellurium (VI) was proposed. Tellurium in water was preconcentrated by extraction with KI-MIBK and then

0.20 1 0.15 S c $ 0.10 4 Xl =l 0.05 -

0

a

1 0

I

200

LOO Charring

600

800 temperature

1000

1200

lf,Oo

.‘C

Fig. 3. Effect of charring temperature on the absorbance of lead in the APDC-MIBK extracts [13]. (0) 0.8 ng Pb + 321 pg La, (0) 0.8 ng Pb + 321 pg La + 2 pg Pd. (By permission of Polyscience Publication Inc.)

940

Invited review

Time

.s

Fig. 4. Absorbance-time profile of lead in the presence or in the absence of palladium or platinum as a matrix modifier [13]. (a) 1.2 ng Pb in 0.01 M HNOI; (b) 1.2 ng Pb + 4 pg Pt; (c) 1.2 ng Pb + 4 pg Pd. (By permission of Polyscience Publication Inc.)

0.30

3 c

r

020.

:: & In :: 0.10 . A 0

LOO

800 Temper&we

1200

1600

,*C

Fig. 5. Effect of matrix modifiers on the charring temperatures of tellurium [14]. (A) 0.8 ng Te; (A) 0.8 ng Te + 100 ng Ni; (0) 0.8 ng Te + 2 pg Pt; ( x ) 0.8 ng Te + 2 c(g Pd; (0) 0.8 ng Te + 2 pg Ir.

back extracted with ammonium hydroxide solution. This solution containing tellurium was introduced into the graphite furnace with matrix modification using micrograms of palladium, platinum or iridium. Concentration of tellurium (IV) and tellurium(V1) in several water samples were found to be less than 0.02 ppb and 0.03-0.06 ppb, respectively. 2.4. Bismuth There is a very rapid loss of signal above 600°C for bismuth in the graphite furnace. GLADNEY [ 151 reported that, when nickel is added charring up to 1200°C and good sensitivity were achieved. We employed micrograms of palladium as a stabilizing reagent. The tolerated charring temperature for bismuth in aqueous solution is raised to 1200°C as shown in Fig. 6 [ 16). It is worth noting that the charring temperature for bismuth in the MIBK extract may also be raised to lOOO”C,while at this temperature negligible absorbance of bismuth is obtained if no palladium is added. The fact is of importance if solvent extraction is a prerequisite for the determination of bismuth. A method based on graphite furnace atomic absorption spectrometry for the determination of trace levels of bismuth in water, sea water and urine has been developed. Determination of bismuth down to 0.02 ng/ml in sea water and urine can be achieved. The sensitivity of the method of matrix modification in conjunction with solvent extraction is greater by two orders of magnitude than by a direct injection technique. 2.5. Arsenic The stabilizing effect of palladium, zirconium, barium, nickel and molybdenum on arsenic in the graphite furnace has been studied [ 17). The results in Fig. 7 show that molybdenum

941

Invited review

0

200

600

Charring

IL00

1000

temperature

,“C

Fig. 6. Effect of palladium on the charring temperatures of bismuth [16], (0) 0.4 ng Bi in methyl isobutyl ketone, (A) 0.5 ng Bi in an aqueous solution, (0) 0.4 ng Bi in MIBK + 1 pg Pd; (A)0.5 ng Bi in an aqueous solution + 0.8 pg Pd. (By permission of Polyscience Publication Inc.)

I

0

I

I

I

LOO

I

I

800

I

1200

*

I

1600

Temperature,‘C

Fig. 7. Effect of ashing temperature on the absorbance of arsenic (0.8 ng) in the presence and absence of matrix modifiers [17]: (A)alone; (0) + 4.8 pg MO; (V) + 20 pg Ni; (A) + 20 pg Ba, (0) + 20 pg Zr; (V) + 2pg Pd. (By permission of Elsevier.)

and nickel stabilize arsenic up to 1400°C but, with zirconium and barium, loss of arsenic occurs above 800°C though the sensitivity for arsenic is apparently improved. It should be noted that in the presence of microgram amounts of palladium, not only is the maximum tolerable charring temperature for arsenic raised to 13OO”C,but also the sensitivity is 50 % better than that in the presence of nickel. BROOKS et al. [18] reported the arsenic peak height was about four times greater if a tantalum-coated graphite tube was used. However, the peak height obtained with their coating technique is lower than that available with palladium matrix modification. Therefore, palladium has been used as a matrix modifier for the determination of arsenic in soil, coal fly ash and biological samples. 2.6. Thallium Severe interferences are frequently encountered in the determination of thallium in a variety of samples by graphite furnace atomic absorption spectrometry, a separation or preconcentration procedure is indispensable. However, when both palladium and ascorbic acid were used as matrix modifiers the charring temperature for thallium could be increased

942

Invited

review

to 850°C and no interferences were observed from the sample matrices so that direct determination of thallium in river sediment, coal and coal fly ash down to 0.10 pg/g was feasible [ 191. The thallium absorbance was suppressed by the perchloric acid added during the sample decomposition procedure. Ascorbic acid added in combination with the palladium matrix modifier was effective in reducing the interference. The suppression effect of perchloric acid on thallium signal was mainly due to the formation of molecular TlCl and molecular absorption was virtually absent when an adequate amount of ascorbic acid was added as shown in Fig. 8. 2.7. Indium The low sensitivity for indium in graphite furnace was assumed to be due to the vaporization of molecular species of indium during the preatomization stages, and many efforts were made to find suitable matrix modifiers which might prevent such a loss. Among the ions tested, palladium was found to be the best stabilizer. Figure 9 shows that the sensitivity for an aqueous solution was increased 3-fold and the maximum tolerable ashing temperature was increased to 1OOOC. The appearance temperatures for indium in the

0

0.0001 0.001 HCI04

0.01

concentration.

0.1

1.0

M

Fig. 8. Effect of perchloric acid concentration on atomic absorption of thallium and molecular absorption of TIC1 [19]. Atomic absorption: (0) 20 ~1 of 180 ppb Tl + 10 jd of 200 ppm Pd + 20 ~1 of HCIOI, (0) 20 ~1 of 30 ppb Tl in 10 % (w/v) of ascorbic acid + 10 ~1 of 200 ppm Pd + 20 ~1 of HCIOI. Molecular absorption: (A) 20 ~1 of 1000 ppm Tl + 10 ~1 of 200 ppm Pd + 20 ~1 of HCIO,; (A) 20 ~1 of 1000 ppm Tl in 10% (w/v) of ascorbic acid + 10 ~1 of 200 ppm Pd + 20 ~1 of HC104. (By permission of Polyscience Publication Inc.)

0.10 9

O.LO

l-l

”n

0

-

0

600

1200 Temperature

1800

2

2400

, “C

Fig. 9. Effect ofashing and atomisation temperature on the atomic absorbance of indium and the In0 molecular absorption from aqueous solution [20] Atomic absorption: (0) 1 ng In; (0) 1 ng In + 2 pg Pd. Molecular absorption: (A) 1 pg In; (A) 1 pg In + 5 pg Pd. (By permission of Elsevier).

943

Invited review

absence and presence of palladium were found to be 920 and 1340°C respectively. The stabilizing effect of palladium on indium caused the upward shift of the appearance temperature. We assume that the addition of palladium stabilizes indium by forming more thermostable compounds or alloys and decreases the formation of volatile species such as InO, thus enhancing the atomic absorbances. This was proved by the decrease of In0 absorbance in the presence of palladium. The recommended method is applicable to the determination of indium at levels below 1 pg/g in geochemical samples [20]. 2.8. Antimony Some loss of antimony occurred during the preatomization stage when the charring temperature was raised above 1OOOC. To prevent such losses, nickel has been used as a matrix modifier [21]. Comparing the stabilizing effect of nickel, platinum and copper, it was found that copper is the most effective modifier for antimony. In the presence of copper, the critical ashing temperature for antimony in aqueous solution and in chloroform extract can be raised to 1300 and llOO”C, respectively. The sensitivity for antimony was improved by a factor of 1.8 for aqueous solution and 2.2 for the organic solution. The results are shown in Fig. 10. A selective procedure for separating antimony (III) from antimony (V) by extraction with N-benzoyl-N-phenylhydroxylamineCHC1, was described. Antimony in chloroform extract was determined with GFAAS using copper as a stabilizer. The recommended method has been applied satisfactorily to the determination of antimony (III) and antimony (V) in various types of water at sub-ng/ml levels [22]. 2.9. Gallium In the determination of gallium in inorganic materials with graphite furnace atomic absorption spectrometry severe interferences are frequently encountered. In order to search suitable matrix modifiers a variety of metal ions were tested. Among the metals studied, nickel, gave high sensitivity and an allowable charring temperature of 1200°C. The effect of charring and atomization temperature on gallium absorbance was examined, and the results are shown in Fig 11. In the presence of nickel the tolerable charring temperature for gallium could be increased to 1200°C and the sensitivity improved by a factor of 6. The appearance temperatures of gallium in the absence and presence of nickel were 1100” and 1530°C respectively. In addition, the interferences from sample matrices were greatly reduced. The mechanism of the enhancement effect of nickel matrix modification on the determination of gallium is ascribed to the formation of a more thermostable solid solution or alloy [23], thus resulting in reduction of analyte loss in the preatomization stage. In order to verify this assumption the molecular absorption of GaO was measured at 244.5 nm and the results are shown in Fig. 12. When no nickel was added, the molecular absorption of GaO over the range of vaporization temperature from 800 to 13OO”C,reached a maximum at 13OO”C,and then decreased with further increase in temperature. There was a plateau over

200

600

1000

1LCKI 1800 2200

Temperature,

2600

‘C

Fig. 10. Effect of ashing temperatures on the absorbance of antimony (III) in the presence and absence of matrix modifiers [223. (0) 1.0 ng Sb in 0.01% tartaric acid solution; ( X ) 1.0ng Sb + 20 fig Ni; (0) 1.0 ng Sb + 20 pg Cu; (A) 1.0 ng Sb in BPHA-CHCI, extract; (A) 1.0 ng Sb in BPHA-CHCIa extract + 20 pg Cu. (By permission of Pergamon Press.)

944

Invited

review

0 LOO

800

1200

1600

Temperature

2000

2600

, ‘C

Fig. 11. Effect of ashing and atomization temperature on the atomic absorption for gallium in the absence or presence of nickel [24]. (0) 0.8 ng Ga; (0) 0.8 ng Ga + 20 pg Ni. (By permission of American Chemical Society.)

o

0.25

-

0.20

.

zl ?I

z 2 0.15 + P 2 0.10 2 0.05

-

800

1600 Temperature

2400 , ‘C

Fig. 12. Effect of vaporization temperature on the molecular absorption of GaO in the absence or presence of nickel [24]. (0) 1 pg Ga; (0) 1 pg Ga + 50 pg Ni. (By permission of American Chemical Society.)

the temperature range of 160&24OOC. However, very little GaO absorption and very little change in the GaO absorption was observed when the vaporization temperature was varied over the above range if nickel was used as a matrix modifier. These results suggested that the presence of nickel inhibited the formation of gaseous GaO, thus minimizing the loss of atomic gallium as gaseous GaO. Since no serious interferences were encountered and quantitative recoveries were obtained, the nickel matrix modification method has been applied to the determinations of gallium in sediment, coal, coal fly ash and botanical samples [24] avoiding the standard addition method or separation procedures which were frequently used in the literature. 3.

MATRIX MODIFICATION USING ORGANIC COMPOUNDS

Organic acids have been suggested as matrix modifiers [25-271. It has been suggested that the thermal destruction of an aqueous sample solution containing a water-soluble organic material would produce a mixture of carbon and sample which would assist in the efficient formation of atomic vapor. In the presence of ascorbic acid, tartaric acid, citric acid or EDTA, the atomization of volatile elements such as cadmium, lead, zinc and silver would occur at lower temperatures, thus resulting in a more efficient separation of the analyte from involatile sample matrices. The addition of EDTA allowed cadmium to be atomized below 600°C and the atomic absorption was free from intense background absorption [27].The additionof an

Invited review

945

organic acid to the sample solution might also help to reduce chemical interferences. The removal of the depressive effect of chloride may be due to the substitution effect of the reagents, resulting in the formation of a readily volatilized chloride such as HCl or NH_,Cl in the ashing step and the resulting residual salt will be converted to oxides on heating [28]. Ascorbic acid [29] minimized the characteristic interferences of magnesium chloride on the lead absorption signal. It was suggested that pyrolysis of ascorbic acid left a carbonaceous residue which accelerated the reduction of metal oxides during the atomization step [30]. However, other workers [31] postulated that the improvement of the atomic signal was not the result of providing carbon to accelerate the oxide reduction step in the atomization process because oxalic acid which has similar physical properties to ascorbic acid, but could not leave a carbon residue on pyrolysis, also enhanced copper sensitivity. The most easily observed effect of the organic acid was that it acts as a flux, the surface tension of the sample was reduced. The influence may simply be due to the improved thermal contact of matrix and furnace. GUEVREMONT et al. [27] reported that the addition of EDTA promoted reduction and atomization. For cadmium, zinc, lead and silver the addition of EDTA also shifted the atomization to significantly lower temperatures. These features made EDTA an extremely useful reagent in the direct analyses of sea water using graphite furnace atomic absorption spectrometry since the atomic signal was well separated from that of the background absorption. We have found that, in addition to the above mentioned elements, organic compounds including EDTA, citric acid, ascorbic acid and oxalic acid can lower the atomization temperatures for easily volatile elements such as mercury. When selenium was used to stabilize mercury to prevent its loss in the graphite furnace during drying stage, the atomization signal levelled off at a temperature above 900°C at which temperature matrix in the sample began to vaporize causing chemical interferences. However, in the presence of organic compounds the atomization of mercury occurred at 500°C a temperature well below the volatilization of the matrix. Among the compounds studied, citric acid was considered to be the most suitable matrix modifier as it gave very low background absorption and, furthermore, its reducing effect on mercury was not influenced by other concomitants in the sample. The effect of organic compounds on the atomization of mercury is shown in Fig. 13 [32]. Using a solid sampling technique we have determined pg/g levels of mercury in soil. The atomic absorbance of zinc in sea water was severely suppressed because of the presence of magnesium chloride. The recovery of zinc was only l&20 ‘/&The effect may have been due to the gas phase interaction between zinc and magnesium chloride and subsequent formation of ZnCl,. However, with the addition of citric acid, the depressing effect of magnesium chloride was largely removed and 90 ‘A of the added zinc was recovered at an atomization temperature of 1300°C using maximum power heating mode (Fig. 14). This phenomenon may be explained by the volatilization of the chloride or conversion of the zinc chloride to its oxide before the atomization step [33].

e, LOO 800

1

1200

Temperature,

1600

2000

‘C

Fig. 13. Effect of organic compounds on the atomization temperature of mercury [32]. (A) 5 ng Hg + 0.1 mg EDTA; (0) 5 ng Hg + 0.1 mg citric acid; (A) 5 ng Hg + 0.1 mg oxalic acid; (0) no organic compounds added.

946

Invited review 0.20 s 5 13 5 0.10 .n Q

0 200 800 1200 1600 2000 Temperature.

“C

Fig. 14. Effectiveness ofcitric acid in reducing the interference from MgCl, with the determination of Zn 1331 (0) 0.4 pg Zn + 0.10 mg MgCi,; (0) 0.4 pg Zn + 0.10 mg MgClz + 0.50 mg citric acid.

4. USE OFANACTIVE GAS The introduction of an active gas such as hydrogen during a portion of the furnace heating cycle will reduce the interferences in the atomizer. The role of hydrogen in eliminating the interference from chlorine in the graphite furnace atomic absorption spectrometry method for lead in steel has been studied experimentally [34]. It was found that chlorine was removed as a result of the reaction between iron (IQchloride and hydrogen. Hydrogen chloride formed by removal of a chlorine atom by hydrogen has a large dissociation energy and is more stable at high temperatures. In the determination of selenium the background from NaCl and CaO was halved by adding hydrogen to the argon purge gas [35]. The use of a hydrogen flame [36] in conjunction with the heated carbon rod reduced interference considerably. Both spectral and chemical interferences caused by up to lOOO-foldexcess of interferent diminished when an argon/hydrogen flame burned simultaneously with samples vaporization from the rod. This was attributed to the reducing environment provided by the hydrogen diffusion flame. A low flow rate of hydrogen to argon purge gas resulted [37] in a higher peak absorbance for rhodium and ruthenium and the peak temperatures of these metals shifted to a lower temperature. For aluminum, barium and vanadium the appearance temperature as well as the optimum atomization temperature were much lower in an argon-hydrogen mixture than in pure argon [38]. Hydrogen lowered the atomi~tion temperature of germanium and improved the sensitivity for germanium significantly in a metal atomizer [39]. Obviously, the addition of hydrogen to the protective atmosphere helped to reduce the analyte oxide and the reduction conditions within the atomizer were improved. In the determination of lead in environmental and biological samples we found that atomization for lead occurred at 950°C in argon-hydrogen though a higher temperature of 1100°C should be maintained in pure argon gas. The shift of the atomization temperature to the lower temperature region has the advantage of allowing a better temporal resolution of the atomic absorption from the matrix volatilization. Figure 15 shows that the suppression effect on lead by complex matrices is reduced when argon-hydrogen is used and a two fold enhancement in peak absorbance is achieved for lead in soil [40]. The depression of lead atomic absorption in real sample analysis was most likely caused by chloride present in the sample solution or perchloric acid used in the sample decomposition procedure. The loss of lead in the presence of chloride and perchlorate was probably due to the formation of lead monochloride by atoms stemming from the decomposition of chloride or perchlorate in the atomizer. Lead escaped in molecular form without being atomized, so causing loss of atomic signal. A study on the effect of chloride and perchlorate on lead indicated that the purge of argon/hydrogen allowed the presence of IO-fold excess of sodium perchlorate and &fold excess of magnesium chloride. The suppression of interference when hydrogen was added may be attributed to the binding of free chlorine to form HCl which has a relatively higher dissociation constant (Do = 102 k Calmol-‘) than PbCl (D, = 71 K cal molt ’ f.

947

Invited review 0.25 ,-

I

0

a

4 Time,

12

5

Fig. 15. Absorption signals for lead and background in soil [40]: (a) lead in Ar; (b) lead in Ar-H,; (c) background in Ar; (d) background in Ar-H,. (By permission of Royal Society of Chemistry.)

5. COATINGTUBES

WITH METALLIC COMPOUNDS

The refractory carbides formed on the surface of the graphite tube such as carbides of zirconium, tungsten, molybdenum, lanthanum and tantalum significantly improved the analytical environment for many elements [41,42]. It was postulated that the thermally stable carbides precluded subsequent carbide formation by the sample and enhanced the atomization efficiency and resulting signal. The improvement in detection limit was demonstrated for beryllium, chromium, manganese and aluminum [41]. We used a zirconium coated graphite tube for the determination of germanium [43]. It has been reported in the literature that the low sensitivity obtained with the standard graphite tube was due to the formation of GeO(g) during atomization [44]. This, however, has not been experimentally confirmed. We have found that the emission line of Pt 265.94 nm could be used for the measurement of the molecular absorption of GeO(g) at 265.94 nm. The temperature-absorption profiles of GeO(g) sampled from an uncoated tube and zirconium coated tube are shown in Fig. 16. When a zirconium coated tube was used, the formation of GeO(g) was greatly suppressed and sensitivity for the determination of germanium increased Sfold. The mechanism of atom formation is not clear, we suggested that the increase in sensitivity may be attributed to the reduction of GeO(g) by the active surface of ZrC. A method for the determination of germinium in coal tly ash has been proposed. Zirconium coated graphite tubes can be used for the determination of selenium in soil [45]. It has been established that extremely complex chemical interferences were encountered in the determination of selenium in soil by graphite furnace atomic absorption spectrometry and very few methods for this determination using electrothermal atomization have been

0: Cd0

000

’ 1200

1600

Temperature

2000

2LOO

2600

, “C

Fig. 16. Molecular absorption of GeO (g) [43]. (0) 5 fig Ge in Zrcoated graphite tube; (e) 5 pg Ge in uncoated graphite tube.

Invited review

948

reported. A rapid and reliable method has been developed in our laboratory. The soil sample was decomposed with acid in the presence of sodium molybdate and ascorbic acid. Selenium (IV) formed in the acid digest was extracted with 1,2-diamino-4-nitrobenzene into chloroform. Selenium in the organic phase was determined by graphite furnace atomic absorption spectrometry using a zirconium-coated graphite tube. The sensitivity was improved by a factor of 1.6 compared with the uncoated tube. 6. CONCLUSION

Over the past decade considerable progress has been made in the understanding, at a fundamental level, of electrothermal atomization in the design and application of the graphite furnace. New directions for future study will probably be concentrated on the investigation of atomization mechanism. No doubt, this relates to the problems of hightemperature technology and metallurgy and to some fundamental aspects of physical chemistry, in addition to the needs of analytical chemistry [46]. Even though a uniform temperature program for all elements and absolute analysis may be a remote prospect, there are other aspects of electrothermal atomization that warrant improvement. Interestingly, the introduction of the platform technique, matrix modification, tube coating technique, high heating rate and Zeeman effect background correction provided the ability to cope with a large excess of matrix. As far as application of graphite furnace is concerned, a wider study of the above individual techniques and in combination with each other [S] will continue to be the subjects of future investigation since it is inherently simple and effective in reducing and eliminating interferences. Obviously, any true achievements in this direction will be applied in trace element analyses by a huge army of analytical chemists. Acknowledgement-The authors greatly thank Dr WALTER SLAVINfor his encouragement and revision of the manuscript for publication. REFERENCES [l] [2] [3] [4] [5] [6] [7] [8] [9] [lo] [ll] [12] [13] [14] Cl51 [16] 1173 [18] [19] [20]

[21] [22]

[23] [24] [25] [26] [27] [28] [29]

[30]

L. B. S. C.

HACEMAN,A. MURABAKand R. WOODRIFF,Appt. Spectrosc. 33,226 (1979). V. L’vov, L. A. PELlEVAaIId A. I. SHARNOP~LSKH, Zh. Frikt. Spektrosk. 27, 395 (1977). K. GIRI, D. LI~L~oHNand J. M. OTTAWAY,Andyst 107, 1095 (1982). L. CHAKRABARTI, H. A. HAMED,C. C. WAN, W. C. LI, P. C. BERTELS,D. C. GREGoIREand S. LEE, Ad. Chem. 52, 167 (1980). W. SLAVIN,G. R. CARNRICK,D. C. MANNING and E. PRUSZKOWSKA, At. Spectrosc. 4,69 (1983). R. D. EDIGER, At. Absorpt. Newslett. 14, 127 (1975). K. G. BRODIEand J. P. MATOUSEK,Anal. Chim. Acta 69, 200 (1974). J. F. ALDERMEND. A. HICKMAN,Anal. Chem. 49, 336 (1977). G. F. KIRKBRIGHT,SHANXIAO-QUAN and R. D. SNOOK,At Spectrosc. 1, 85 (1980). SHANXIAO-QuANand NIZHE-MING, Acta Chim. Sin. 37, 261 (1979). CA 92:220474x(1980). SHANXIAO-QUANandNIZHE-MING, Huonjing Kexue 1, 24 (1980). CA 93: 120147V (1980). K. C. THOMPSON,K. WAGsrAFFand K. C. WHEATSTON&Analyst 102, 310 (1977). SHANXIAO-QUANTUM NI ZHE-MING, Can. J. Spectrosc. 27, 75 (1982). SHANXIAO-QUANaIIdNIZHE-MING, Acta Sci. Circum. 1, 74 (1981). CA 95:225335Z (1981). E. S. GLADNEY,At. Absorpt. News&t. 16, 114 (1977). JIN LONG-ZHUand NI ZHE-MING,Can. J. Sqectrosc. 24, 219 (1981). SHANXIAO-QUAN,NI ZHE-MING andZHANGLI, And. Chim. Acta 151, 179 (1983). R. R. BROOK$ D. E. RYANand H. F. ZHANG,At. Spectrosc. 2, 161 (1981). SHANXIAO-QUAN,YUAN ZHI-NENGatId NI ZHE-MING,Can. J. Spectrosc. 31, 35 (1986). SHANXIAO-QUAN,NI ZHE-MINGand YUAN ZHI-NENG,Anal. Chim. Acta 171, 269 (1985). R. M. HAMMER,D. L. LECHAKaIId P. GREENBERG,At. Absorpt. Newdeft. 15, 122 (1976). SUN HAN-WEN,SHANXIAO-QUAN and NI ZHE-MING,Talanta 29, 589 (1982). K. WADE~~~ A. J. BANISTER,Comprehensive inorganic Chemistry, Ed. J. C. BAILER, JR, H. J. EMELEUS, R. NYHOLM, A. F. TROTMAN-DICKENSON, Vol. 1, p. 1074. Pergamon Press, New York (1973). SHANXIAO-QUAN,YUAN ZHI-NENGand NI ZHE-MING,Anal. Chem. 57, 857 (1985). J. G. T. REGANand J. WARREN,Analyst 101, 220 (1976). C. W. FULLER, At. Absorpt. Newslett. 16, 106 (1977). R. GUEVREMONT,R. E. STURGEONandS. S. BERMAN,And. China. Acta 115, 163 (1980). K. MATSUSAKI, T. YosHINoand Y. YAMAMOTO,Anaf. Chim. Acra 113,247 (1980). J. G. T. REGANand J. WARREN,At. Absorpt. Newsiett. 17, 89 (1978). M. TOMINAGAandY. L’ME~AKI,And. Chim. Acta 139, 279 (1982).

Invited review [31]

D. J. HYDES, Anal. Chem. 52, 959 (1980).

[32]

NI ZHE-MING and YANG PENG-YUAN, Huanjing

[33]

YANG PENG-YUANand NIZHE-MING,

[34]

W.

[35]

R.

[36] [37] [38]

M. D. AMOS, P. A. BENNETT,K. G. BRODIE,P. W. Y. LUNG and J. P. MATOUSEK,Anal.

[39]

[40] [41] [42] [43] [44]

[45] [46]

949

Huaxue 1,83 (1982). Huanjing Kexue 2,423 (1981). FRECH~~~ A. CEDERGREN,Anal. Chim. Acta 82,93 (1976). D. BEATYand M. M. COOKSEY, At. Absorpt. Newslett, 17, 53 (1978).

Chem. 43,211 (1971). Spectrochim. Acta 39B, 473 (1984). V. SYCHRA, D. KOLIHOVA, 0. YYSKoCKOVAand R. HLAVAT, Anal. Chim. Acta 105,263 (1979). K. OHTAa!Id M. SUZUKI, Anal. Chim. Acta 104,293 (1979). NI ZHE-MING, HAN HENG-BIN and LE XIAO-CHUN, J. Anal. At. Spectrom. 1, 131 (1986). J. H. RUNNELS, R. MERRFIELDand H. B. FISHER, Anal. Chem. 47, 1258 (1975). V. J. ZATKA, Anal. Chem. 50, 538 (1978). GAO Y ING-QIand NI ZHE-MING, Acta Chim. Sin. 40, 1022 (1982). D. J. JOHNSON,T. S. WEsTand R. M. DAGNALL, Anal. Chim. Acta 67, 79 (1973). SHAN XIAO-QUAN, JIN LONG-ZHUand Nr ZHE-MING, At. Spectrosc. 3, 42 (1982). B. V. L’VOV, Spectrochim. Acta 39B, 149 (1984). M. SUZUKI and K. OHTA,