Chem. Anal. (Warsaw), 41, 809 (1996)

Determination of Metal Contents in Sewage Sludgesand Ashes of Refinery-Petrochemical Origin

Using Flame At()mic Absorption Spectrometry

by Zofia Kowalewska

Research-Development Center for RefineryIndustry, ChemikOw 5, 09-411 Plock, Poland

Key words: flame atomic absorption spectrometry, petrochemical sludges, petrochemi­

cal ashes, metals

The methods of determination of Na, K, Fe, Mg, Ca, Mn, Zn, Cu, Pb, Ni, ~ Cd and Crin refinery-petrochemical sludges and ashes were evaluated. The sample preparationprocedure depended on the type of sample and included butyl-sulfuric mineralization,dry mineralization in muffle furnace, digestion in HF + HCl04, H3B03, HCI, anddigestion in HF + HCI04, H3B03 followed by fusion with LiBOz. The approvedprocedure was controlled by the analysis of reference material- fly ash CTA-FFA-l andby comparing results obtained by calibration graph and standard addition methods. Inthe case of chromium the results were significantly lower than expected - probably dueto the losses of volatile chromium compounds. In most cases the relative standarddeviation is at the level of a few percent.

Celem badan bylo przygotowanie metod oznaczania Na, K, Fe, Mg, Ca, Mn, Zn, Cu,Pb, Ni, V, Cd, Cr w osadach sciekowych i popiolach pochodzenia rafineryjno-petroche­micznego za pomoc~ plomieniowej ,absorpcyjnej spektrometrii atomowej. Proceduraprzygotowania pr6bek do analizy byla zr6inicowana w zaleinosci od typu pr6bki iobejmowala: mineralizacjC( butylo-siarkow~, mineralizacjC( such~ w piecu muflowym wtemp. 600°C, roztwarzanie w HF + HCI04, H3B03, HCl oraz roztwarzanie w HF +HCI04, H3B03 pol~czone ze stapianiem pozostalosci z LiBOz.Wyniki przeprowadzo­nych badan osad6w sciekowych i popio16w pochodzenia rafineryjno-petrochemicznegooraz materialu referencyjnego - fly ash CTA-FFA-l, a takie por6wnanie wynik6wotrzymanych meto~ dodatku wzorca i wykresu kalibracyjnego wslcazuj~ na dobr~

dokladnosc oznaczen wiC(kszosci pierwiastk6w wedlug przyjC(tej procedury. W przypadkuchromu stwierdzono zaniione wyniki, co najprawdopodobniej wi~ie siC( ze stratamilotnych form tego pierwiastka podezas odparowywania na plycie elektryeznej. WZgl~dneodchylenia standardowe oznaczen metali pozostaj~ zwykle na poziomie kilku procent.

810 z. Kowalewska

The disposal of sludges and ashes, being the result of waste treatment [1,2], is aserious problem in refinery-petrochemical plants processing several million tons ofcrude oil per year. This type of materials may be used for industrial, road-making andagricultural (fertilizers) [2,3] purposes. The content of harmful substances (e.g. heavymetals) maybe a critical factor in the evaluation of the suitability of these materials.The knowledge of the heavy metals concentration is also an important parameter forthe waste treatment quality control.

The main goal of this work was the evaluation of the method of determination ofNa, K, Fe, Mg, Ca, Mn, Zn, Cu, Pb, Ni,~ Cd and Cr in refinery-petrochemical sewagesludges and ashes. According to the literature data [3-5] sewage sludges and ashesof different origin contain heavy metals at the level between J,tg g-l and several %.This level of concentration is high enough to be determined using flame atomicabsorption spectrometry (FAAS). FAAS is a common analytical method, well estab­lished and relatively interference-free. One of its main advantages, especially forsmall laboratories, is low investment and running cost.

FAAS requires solid samples, or at least the analyte containing part of the sample,to be dissolved. Table 1 shows literature data [3,6-18] concerning methods ofpreparation of sludges, wastes and ashes of different origin for the metals determi­nation. The most important components of samples, with respect to the saluplepreparation method, are aluminosilicates and organic materials.

Aluminosilicates are chemically resistant materials. If they are present in thesample methods involving hydrofluoric acid digestion [9,12,18] or fusion withLi2B407 or LiB04 [14,16] are usually applied.

Destruction of organic matrix may be performed using dry (ashing with atmos­pheric oxygen at the temper~ture of 40D-600°C [9,11,12,15-17]) or wet (withoxidizing agents [3,7-10,12,13]) mineralization method. As the dry decompositionat high temperature may cause some analyte losses, some metal-binding substanceslike sulfur or sulfuric acid [15,17] may be added to the sample. The nlost commondecomposing agents used in the wet method are: nitric, sulfuric, perchloric acids andhydrogen peroxide. The application of H2S04 may induce errors connected witheither the precipitation of insoluble analyte sulfates or occlusion of analyte in theprecipitate formed [12,13]. HCI04 should be used very carefully to avoid the risk ofexplosion.

Available equipment is another important factor for choosing decompositionmethod. Rapid development of microwave digestion systems can be observed recently.They shorten the sample preparation procedure [19]. On the other hand microwavedigestion systems are expensive. Therefore, if the~ preparation time is not the keyfactor the conventional decomposition method may be good enough. "The restrictedamount of the sample - usually 250-500 mg organic substance - was in our case also 'an important disadvantage of pr~ssuremicrowave system.

Metals in petrochemical waste by FAAS

Table 1. Preparation of sludge and ash samples for the determination of metals -literature data

811

Sample preparation procedure Source Sample Ref.of energy

H~04 - HzOz - HCI- lIN03; lIN03- H~04'"H:Pz cony. petrochemical sludge 3

Aqua re2:ia cony. sewa2:e slud2:e 6

HN03 - HzOz - HCI (SW846 3050 EPA method); cony. solid waste 7

HCI- lIN03

HN03 - HzOz - HCl (SW 846 3050 EPA method); cony. waste, sediment 8

HN03 (SW 846 3051 EPA method) mw. waste, sediment

HN03; lIN03- HCI; lIN03... HCI04 - HCI; ashing cony. sewage sludge 9in muffle furnace at 550«>C - HCI-HF; ashing inmuffle furnace at 550«>C - HCI -

HN03 - HzOz -HCI; H~04 - HzOz - lIN03 cony. sewage sludge; oil-, 10grease- and wax-containing waste

Ashing in muffle furnace at 400-450«>C - HCI cony. sewage sludge .11

H~04 - lIN03; lIN03 - HZt)2; lIN03 - HCI04 - HF; cony. sewage sludge 12ashing in muffle furnace at 450«>C - HN03 - HCI

H~04 - lIN03; HN03 - HzOz cony. sewage sludge 13

Fusion with LiB02 - HN03 cony. fly ash, silicate 14

Burning with butyl-sulfuric solution - ashing with cony. catalytic cracking feed 15H~04 ..,. ashing in muffle furnace at 525«>C - stockdissolution in KI + HCI solution ...

Ashing in muffle furnace at 525«>C - fusion with cony. petroleum coke 16Li2B407 (or LiB02) - HCI

Burning with butyl-sulfuric solution - ashing with cony. crude oil, its distillates 17H~04 - ashing in muffle furnace at 525«>C - and residues,dissolution in HCI asphaltenes, resins

Aqua regia - HF - H3B03 mw. Sewage sludge, 18fly ash (and others)

cony. - conventional, mw. - microwave.EPA- Environment Protection Agency.

EXPERIMENTAL

Apparatus and reagents

Atomic absorption spectrometer SP9-800, Pye Unicam (Cambridge, UK), equipped with single­element hollow cathode lamps.

Platinum evaporating dishes and crucibles.Muffle furnace, Deutsche Reichsbahn WTPM (Meiningen, Germany).

812 Z. Kowalewska

Analytical grade reagents: sulfuric acid (d = 1.82 g ml-I ), n-butanol, hydrochloric acid (d =1.18 gml-I ), nitric acid (d =1.40 g ml-I ), hydrofluoric acid (d =1.16 g ml-I ), perchloric acid (d =1.60 g ml-I ),boric acid, lithium carbonate, cesium chloride, aluminium nitrate (hydrate).

Lithium metaborate - synthesized from lithium carbonate and boric acid.Single element standard solutions of Na, K, Fe, Mg, Ca, Mn, Zn, Cu, Pb, Ni, ~ Cd and Cr (1000

fA.g ml-I ), Wzorm~t(Warsaw, Poland).Certified reference material: fly ash CTA-FFA-l, Institute of Nuclear Chemistry and Technology

(Warsaw, Poland).Acetylene - "A" purity, dinitrogen oxide-for medical purposes.Four times distilled water from home-made apparatus was used throughout.

Sample preparation

The samples obtained fronl waste treatment unit were different with respect tostate of aggregation (liquid, liquid with the content of solids, solid) and content ofwater, carbon and petroleum products. The complete digestion was assumed to benecessary to obtain good accuracy of analytical results. The preliminary step ofpreparation included breaking up, homogenizing and drying at 105°C. Then removalof organic matter was necessary. If the petroleum products content was high enoughto be detected with smell, the mineralization procedure was reinforced with theaddition of butyl-sulfuric solution (alcohol addition prevents splashing and atte­nuated burning) [15,17] and then with sulfuric acid.

Butyl-sulfuric digestion procedure

Put 10 g of the sample into the platinum crucible and 10 ml of butyl-sulfuric solution [H2S04 +n-butanol, 1:4(V:V)]. Burn over the Bunsen burner. Add 2 ml of cone. H2S04, heat on the heating platewith additional infrared lamp from the top, until white fumes no longer emerge.

. The coke obtained was then ashed in muffle furnace at 600°C. If the samplescontained only non-petrochemical organic matter the butyl-sulfuric digestion stephad been omitted and the samples had been placed directly in the muffle furnace. Theashing of samples was performed until all carbon was burned out.

The next step of sample preparation was digestion. As the determination of manyanalyzed elements may be influenced by silicon (e.g. Fe, Na, Mn [20,21]) thenecessity of the removal of this element was assumed. Digestion consisted of threesteps: attack withHF + HCI04 (SiF4 removal), evaporation with H3B03 (BF3removal) and dissolution in HCl.

HF + HCIO.., H;J803, HCI dissolution procedure

Weigh 0.2 g of ash, put it into platinum crucible (or take the ash obtained in ashing procedure). Add5 ml cone. HF and 4 ml cone. HCI04• Evaporate on heating plate almost to dryness. Add 5 m14% H3B03•

Evaporate again almost to dryness. The residue dissolve in 4 ml Hel (heat it gently if necessary), transferto 50 ml volumetric flask and fill to the mark with distilled water.

The above described digestion procedure was effective for most of the samples(homogeneous solutions were obtained). However, in some cases small amount ofwhite precipitate remained on the bottom of the flask. For these samples, the residueafter evaporation with H3B03 was additionally fused with lithium metaborate.

Metals in petrochemical waste by FAAS 813

Fusion with lithiwn metaborate

Add 1 g LiB02 to the residue after evaporation with H3B03. Put the crucible into the muffle furnaceand heat up to 900°C until clear, glassy matter is formed. Dissolve and transfer to 50 ml volumetric flaskusing dilute HN03 (1 :20). Fill the flask with diluted HN03 to the mark.

The above described sample preparation scheme is presented in Fig. 1.

Initial sample preparation(dispersing, homogenization,drying)

No LithlUmmetabOnlt8.fUSioIlof the residue from 1118digestion after H.aO.8V~

Yes

Figure 1. Sample preparation procedure scheme

Determination parameters

Determination parameters are shown in' Table 2.The sample and standard solutions contained 0.1% Cs (ionization suppressant) in the case of K,Na,

Ca, Mg, Fe and Crdetermination and 0.1 Cs + 0.5% AI in the case of V determination.

Table 2. Parameters of spectrometric determination

Analytical Bandpass, om Background Flame oxidant Oxidant/C2H2flowlines, om correction rates, I min-1

Na589.6 0.2* No air 4.6/1.1--

K 766.5 0.2* No air' 4.6/1.1

Fe 372.0 0.5 No air 4.6/1.1

Mg202.6 0.5 Yes N20 5.1/4.6

814 Z. Kowalewska

Table 2. (continuation)

Ca 422.7 0.5* No NzO 5.1/4.6

Mn279.5 0.5 Yes air 4.6/1.5

Zn213.9 0.5 Yes air 4.6/1.1

Cu324.8 0.5 No air 4.6/1.1

Pb 217.0 0.5 Yes air 4.6/1.1

Ni 232.0 0.5 Yes air 4.6/1.1

V 318.5 0.2 No NzO 5.1/4.6

Cd 228.8 0.5 Yes air 4.6/1.1

Cr357.9 0.5 No NzO 5.1/4.6

* 90° angle between burner slit and light path.

RESULTS AND DISCUSSION

Analytical results for CfA-FFA-l ash

As no certified reference material for sludges or ashes of refinery-petrochemicalorigin was available the CRM fly ash CTA-FFA-l was used for control of accuracy.This reference material was exp~cted to have the composition similar to that of oursamples. For the comparison pluppses thi~ material was digested in two ways; withHF + HCI04, H3B03, HCI (dissolution was complete) and using similar procedurecombined with fusion with lithium metaborate. The certified (or indicative) valuesfor CTA-FFA-l are shown in Table 3.

Table 3. Analytical results for fly ash CTA-FFA-1 using different digestion methods (n = 4 or 5)

Certified value andOur results (95% confidence level)

Element (concentrationconfidence interval digestion with HF +unit)or indicative value

digestion with HF+HCI04,HCI04, H3B03H3B03,HCI and fusion with LiBOz

Na(%) 2.19 ± 0.08 2.23 ± 0.04 2.58 ± 0.19

K(%) 2.20 2.20 ± 0.03 2.01 ± 0.20

Fe(%) 4.89 ± 0.14 5.07 ± 0.11 4.60 ±0.42

Mg(%) 1.55 1.58 ± 0.08 1.57 ±0.04

Ca(%) 2.29 2.08 ±0.04 2.12

Mn (~gg-l) 1066 ± 4.1 1008 ± 18 1079 ±20

Zn (~g g-l) 562± 58 557 ± 17 537 ± 89

Cu (~g g-l) 158 ±9 162±2 144 ± 15

Metals in petrochemical waste byFAAS 815

Table 3 (continuation)

Pb (J.tgg-l) 369 ± 46 366±7 318 ±47

Ni (J.tgg-l) 99.0 ± 5.8 103±8 105 ±4

V(J.tg g-l) 260 ± 10 250 ± 17 265 ±28

Cd (J.tg g-l) 2.8 2.3 ± 0.3 -Cr (J.tgg-l) 156±8 124±8 117±6

,

From the results obtained for the reference material sample it can be concludedthat:

- both alternative of sample preparation procedure lead to accurate results for themajority of analyzed elements;

- in both cases the results obtained for chromium are too low. The most probablesource of the error is the loss of the element in the form ofvolatile compounds duringheating. The possibility of interferences in FAAS measurements was excluded usingstandard addition and successive dilution method - SASD [22];

- precision of the results after HF + HCI04, H3B03, HCI digestion was at thelevel of a few percent (usually <5%);

- additional step - fusion with LiBOz - decreases the precision of determinationof Na, K, Fe, ~ Cu and especially that of Zn and P.b.

Analytical results for refinery-petrochemical samples

Determination of metals in sewage sludges and ashes from refinery-petrochemi­cal plant was made. The average results for several samplings of different sampletypes are presented in Table 4. The comparison ofthe content of investigated elementsin CTA-FFA-l fly ash and petrochemical sludges and ashes is shown in Figs. 2 and3 (the results for sludges were expressed as concentratioIl in ash). In some cases thesludge ashes and reference sample composition differ significantly. In effect, in orderto confirm the accuracy of resu'lts supplementary comparison 'of data obtained usingstandard addition and calibration graph was made. Both series of results are in goodagreement (Table 5). This shows that petrochemical matrix does not influence themetal determination and therefore the calibration graph method may be used in thiscase.

The mineralization stage may influence the accuracy of results for sewag~ sludgesamples. This effect could not be evaluated with CTA-FFA-l determinations. Drymineralization method is considen~d to be the most ri~ky with respect to the loss ofvolatile analytes. TabJe 6 shows re~ults for two samples that were analyzed with andwithout dry mineralization step. No significant difference between those results canbe found confirming that no analyte loss occurs under the digestion parameters.

The precision of determination of metals in the sludge from industrial centrifugeafter mineralization in muffle furnace expressed as realtive standard deviation for alldetenninations remain at the level ofa few percent, ranging from 1.3 (Fe, Cu)to 9.7 (Cr).

Tab

le4.

Ana

lyti

cal

resu

lts

for

met

als

inre

fine

ry-p

etro

chem

ical

sam

ples

.Res

ults

are

aver

aged

for

seve

rals

ampl

ings

for

each

type

ofs

ampl

e

Con

tent

Sam

ple

Met

alco

nten

t,~g

gl9

61

(%fo

ras

hsa

mpl

e,ex

cept

for

Cd

and

Pb)

Sam

ple

type

H2O

,as

hpr

epar

atio

n

%%

proc

edur

eN

aK

Fe

Mg

Mn

Zn

Cu

Cr

VN

iC

dP

b

Flu

idiz

ed-b

ed18

1.7

"a",

furn

ace

feed

then

"b",

180

7022

0020

622

150

203.

818

065

0.60

6.1

then

"c';

Slu

dge

from

433.

2c'a

",em

erge

ncy

rese

rvoi

rth

en"b

",14

014

049

0036

034

310

267.

326

011

01.

18.

8th

en"c

"

Indu

stri

al78

12.3

"b",

620

550

1070

095

012

049

085

2598

032

02.

023

cent

rifu

gesl

udge

then

"c"

Ash

from

flui

dize

d"b

",b

edfu

mac

eth

en"c

"

-10

0or "b

",th

en"c

"1.

600.

6611

.60.

860.

980.

480.

070

0.02

90.

800.

3113

130

then

"d"

"a"

-bu

tyl-

sulf

uri

cm

iner

aliz

atio

n;"b

"-

dry

min

eral

izat

ion;

"c"

-di

gest

ion

HF

+H

CI0

4,H

3B0

3,H

CI;

"d"

-fu

sion

wit

hL

iBO

z•

00~ Q

\ N ~ ~ f ~

Metals in petrochemical waste by FAAS

2.5

•

811

2

1.5

1

0.5

•

•

o -+----+-----+---+----.1---+----+------+-------1N. K Mg v

metal

Nt 2ft

Figure 2. Concentration range for Na, K, Mg, \T, Ni, Zn and Fe in sewage sludge ash; • concentrationin fly ash CTA-FFA-l

0.2

~

~..: #.U::i .r;;u u •~

'a 0.1

ca Iti 0

0

J§ ~

2

D 0• 0•

Mn Cu Cr Pb Cd

metal

Figure 3. Concentration range for Mn, Cu, Cr, Pb and Cd in sewage sludge ash; • concentration in flyash CTA-FFA-l

Comparison of analytical results (Figs. 2 and 3) with literature data demonstratesthat the concentrations of most metals in refinery-petrochemiCal samples are withinthe typical range for the samples of different origin. The only exception are Ni andV present in crude oil at relatively high level [17,23]. The concenh:ation of these

818 z. Kowalewska

metals in investigated ashes is several times higher than the values quoted in theliterature. Therefore, the high, specific content of Ni and V may be used to indicatethe refinery-petrochemical origin of sludges or ashes.

Table S. Comparison of the results obtained by standard addition and calibration graph methods

Element Sample Result (expressed as concentration in ash) Standard addition(concentration unit) and calibration·graph

calibration graph standard addition slopes ratio

Na(%) 1 0.420 0.431 1.0

K(%) 1 0.460 0.445 1.1

Fe(%) 1 18.5 19.4 0.92 16.5 17.8 0.9

Mg(%) 1 1.27 1.24 1.02 1.06 1.08 1.0

Mn(%) 1 0.096 0.093 1.02 0.107 0.103 1.0

Zn(%) 1 0.800 0.810 1.02 0.465 0.448 1.0

Ni(%) 1 0.380 0.322 1.12 0.340 0.320 1.1

V(%) 1 0.770 0.737 1.02 1.01 1.01 1.0

Cu (JA.g g-l) 1 385 390 0.92 690 694 1.0

Cr(JA.gg-l) 1 281 277 1.12 161 149 1.1

Cd (JA.g g-l) 1 35 32 0.9

Pb (JA.gg-l) 1 210 203 1.02 415 394 1.0

Conclusions

Presented experiments proved the suitability of the proposed method of analysisof sewage sludges and ashes from refinery-petrochemical plant. The results obtainedfor Na, K, Fe, Mg, Ca, Zn, Cu, Pb, Ni, ~ and Cd are accurate and satisfactory precise(RSD at the level of a few percent).

In the case of chromium significantly low results were Qbserved. As the digestionprocess was complete coprecipitation or occlusion losses were not possible. Inter­ferences in the FAAS determination were also not observed. The most likely sourceof errors is the volatilization of chromium compounds during the heating of thesample.

Tab

le6.

Eva

luat

ion

ofa

naly

telo

sses

duri

ngth

em

iner

aliz

atio

nin

muf

fle

furn

ace

at60

0°C

(met

alco

nten

tsex

pres

sed

asco

ncen

trat

ion

inas

h)

Sam

ple

Sam

ple

stat

eaf

ter

Na

KF

eM

gM

nZ

nC

uV

Ni

Cr

Cd

Pb

prec

ondi

tion

ing

%%

%%

%%

%%

%I-

lgg-

1I-

lgg-

1I-l

gg-

l

1.C

entr

ifug

edr

ym

ass

afte

r0.

339

0.23

47.

480.

841

0.13

10.

449

0.10

50.

960.

221

255

198

246

cake

ofo

iled

heat

ing

at10

5°C

slud

ge

ash

afte

rro

astin

gat

0.40

30;

234

8.06

0.80

60.

126

0.46

30.

103

0.93

0.26

624

219

422

260

0°C

ratio

:as

hre

sult

/dry

1.19

1.00

1.08

0.96

0.96

1.03

0.98

0.97

1.20

1.07

0.98

0.90

mas

sre

sult

2.C

entr

ifug

edr

ym

ass

afte

r0.

534

0.48

17.

900.

833

0.13

50.

333

0.07

31.

150.

331

388

140

172

cake

ofs

urpl

ushe

atin

gat

105°

Csl

udge

ash

afte

rro

asti

ng0.

566

0.50

28.

120.

887

0.13

60.

350

0.07

41.

150.

357

340

150

193

at60

0°C

,

ratio

:as

hre

sult

/dry

1.06

1.04

1.03

1.06

1.01

1.05

1.01

1.00

1.08

0.88

1.07

1.12

mas

sre

sult

~ ~ f;j- S· ~ ~ ~ ~ §. - ~ r" ~ ~ ~ ~ 00 ..... \Cl

820 Z. Kowalewska

REFERENCES

1. Kawalczewska I., Notatki Plockie, 1-2, 142 (1990).2. Proceedings of Symposium Petrochemicals - Criteria and Methods of Pollution Control, KalWice,

13-15 April 1994 (in Polish).3. Kalavska D and Kalavsky S., Chem. Anal. (Warsaw), 38,231 (1993).4. Kucowski J., Laudyn D. and Przekwas M., Energy Production andEnvironmentProtection, Warszawa

1994, pp. 296,300 (in Polish).5. Reinmann D.O., in: Wasfelncineration and theEnvironment (Hester R.E. and Harrison RM., Eds.), The

Royal Society of Chemistry, Great Britain 1994, p.49.6. Blankle M., Henson A and Thompson KC.,Anal. Spectrosc. Libr., 5, 321 (1991).7. Kimbrough D.E. and Wakakuwa J., in: Waste Testing and QualityAssurance: Third Volume (Friedman

D., Ed.), ASTM SlP 1075, Baltimore 1992, p.231.8. Binstock D.A, Grohse P.M., Gaskill AJr. and Sellers C., in: Waste Testing andQualityAssurance: Third

Volume (Friedman D., Ed.), ASTM STP 1075, Baltimore 1992, p.245.9. Ritter Ch.J., Bergman StC., Cothern Ch.R and Zamierowski E.E.,At. Absorp. Newslet., 4, 70 (1978).

10. Katz S.A and Jenniss St.W., Regulatory Compliance Monitoring by AtomicAbsorption Spectroscopy,Verlag Chemie International, Deerfield Beach USA 1983.

11. Morton S1. and Roberts J.R., Unicam -AtomicAbsorption Spectroscopy Methods Manual, Unicilm Ltd,Cambridge 1993.

12. Carrondo M.J.I, Lester J.N. and Perry R., Talanta, 26,929 (1979).13.Carrondo M.J.T., Lester J.N. and Perry R..,A~alyst, 104, 831 (1979).14. Boar P.L and Ingram LK,Analyst, 95, 124 (1970).15. Hoggan D., Marquart C.E. and Battles W.R, Anal. Chem., 36,1955 (1964).16.1994 Annual Book of ASTM Standards, D 5056-90.17. Sikora Z., Kowalewska Z., Wojdala A and Kutylo U., Internal report, OBR PR, Plock 1989.18. Bettinelli M., Baroni U. and Pastorelli N.,Anal. Chim.Acta, 225,159 (1989).19. Matusiewicz H. and Sturgeon R.E., Prog. Anal. Spectrosc., 12, 21 (1989).20. Pinta M.,AtomicAbsorption Spec(romt;try. Aplicatio.v in ChemicalAnalysis, PWN, Warszawa 1977 (in

Polish).21. Whiteside P.J. and Milner B.A, AtomicAbsorption Data Book, Pye Unicam Ltd, Cambridge 1984.22. Pszonicki Land Skwara W., Talanta, 36,1265 (1985).23. Yen IF., The Role ofTrace Metals in Petroleum, Ann Arbor Science Publishers, Ann Arbor, MI 1975.

ReceivedApril 1995AcceptedMarch 1996