Analytrca Chunrca Acta, 264 (1992) 297-301 Elsewer Science Pubbshers B V , Amsterdam

297

8-Quinolinol-Ssulfonic acid as a non-selective post-column reagent for fluorimetric detection of trace

metals in ion chromatography

Tnn Wrlhams and Nell W Barnett

Department of Chemrcal Sccences, Deakrn Vmuersrty, Geelong, victona 3217 (Australd

(Received 7th January 1992, revised manuscript recerved 2nd March 1992)

Abstract

A method IS presented for the determmation of a vanety of trace metals, many of which do not form fluorescent species Hrlth S-qumohnol-5-sulfons acid (8-HQS) The analytes are separated from each other and other elutmg species, m most cases, by usmg aqueous lactic acid eluents m con]unctlon with a low-capacity, strong catlon-exchange, stationary phase After separation the analyte ions react Hrlth Mg-EDTA to displace free magnesium ions which m turn react with 8-HQS to produce the fluorescent species [Atem) = 390 nm, A(,,) = 510 nm] Calibration graphs were linear for at least two orders of magnitude for all analytes with detection hmlts ranging from 3 3 X lo-’ to 14 X 10m6 M

Keywords Pluonmetry, Chromatography, 8-Qumohnol-5-sulphomc acid, Trace metals

There have been relatively few reports [l-5] on the use of S-qumohnol (S-HQ) and Its 5- sulfomc acid (S-HQS) for the fluorlmetrlc detec- tion of trace elements m ion chromatography A crltlcal dlscusslon of earher pubhcatlons can be found m a recent paper from this laboratory [5] The lack of pubhcatlons 1s due to the selective nature of fluorescent reactions and the quenching propertres exhlblted by many cations Thus multi- element determmatlons are dlfflcult to achieve because of the specific nature and different optl- mum detection condltlons reqmred for each mdl- vldual element Also most metals do not form fluorescent complexes with S-HQS [21, and a number of metals are very effective at quenching fluorescent systems Alummum-SHQS IS partlcu- larly susceptible to such quenchmg and has been employed as a post-column reagent for the deter-

Correspondence to Department of Chemical Sciences, Deakm Umverslty, Geelong, Vlctorla 3217 (Austraha)

mmatlon of metal Ions such as lron(III), copper and cobalt [3]

An alternative means of nnprovmg multi-ele- ment capablhty is to develop a post-column reac- tlon system where the elutmg metal cations are mvolved m a reaction that produces a strongly fluorescent species This paper describes the de- velopment of a non-selective fluornnetrlc detec- tlon system based on the displacement by elutmg metal species of Mg 2+ from a Mg-EDTA com- plex, similar to that employed earlier [6] I e

[Mg-EDTA12_ + M”++ [M-EDTA]‘~-“)-

+ Mg2+

The liberated Mg2+ Ions are able to react freely with S-HQS to produce a fluorescent species which IS easily detected Mg2+ ions are particularly useful m their role as the drsplace- ment species smce the rate of dlssoclatlon of the Mg-EDTA complex 1s fast, and the condltlonal

ooO3-2670/92/$05 00 0 1992 - Elsevler Science Publishers B V All rights reserved

298 T Wtiums and N W Bamett /Anal Chm Acta 264 (1992) 297-301

stab&y constant IS smaller than those of the analytes Investigated [71

EXPERIMENTAL

Instrumentatwn The apparatus employed IS shown m Frg 1 A

high pressure inert pump (Waters Action Anal- yser 6OOE, Mllford, MA) was used for the elutlon and separation of the cations, whilst two stainless steel pumps (both Knauer 64, Bad Homburg) were used for the post column reagent pumps All pumps were mamtamed at 10 ml mm-l The standards and samples were located mto a 100 ~1 sample loop and mjected onto the analytical col- umn by means of a 6-port inert swltchmg valve (Rheodyne 9010, Cotatl, CA)

The eluent and Mg-EDTA were mixed with the aid of a PTFE T-piece and the resulting solution was passed mto an open reaction cod (500 x 0 8 mm 1 d) constructed of PTFE The 8-HQS solution was mtroduced via another T- piece, again mto an open delay cod (500 X 0 8 mm 1 d ) Both reactions cods were tightly spl- ralled to limit band broademng The resultant mixture passed mto the fluorescence detector

(mtachl FlOOO) with the excitation wavelength set at 390 nm and the enusslon wavelength at 510 nm The detector was fitted wrth a standard flow cell of 40 ~1 and all connections were aclueved with PTFE tubing

Reagents and standards Analytical-reagent grade chemicals were used

throughout Solutions were prepared with hlgh- quality deionized water (Contmental Water Sys- tems, Sprmgvale) Potassium hydroxide, nitric acid, lactic acid, boric aad, EDTA, trls(hydroxy- methyl)methylamme and potassium sulphate were all obtained from BDH, whilst the 8-HQS was obtained from Lancaster Synthesis Metal stand- ards were obtained by serial dllutlon of 1000 pg ml-’ Spectrosol stock solutions (BDH)

A 1 1 mole ratio of Mg EDTA was obtamed by titration using Enochrome black T as the indicator 0 01 M EDTA (50 00 cm31 was titrated against 0 01 M Mg solution (5 X 10m3 M) at pH 9 0 (NH, solution) and the resultant Mg-EDTA complex was kept m an opaque-glass stoppered flask and was found to be stable for at least 8 weeks The dally workmg solution of Mg-EDTA was made by diluting the stock solution lOO-fold and was buffered to pH 10 0 with boric acid (6 g

mjectlon valve J sample

syrmge -pump :

‘1

pump

u-

a,#&;cal D1 Ll

8-HQS eluent solution

pump =

P

ss_Ts JJJJ fzec;j::nce

reaction reaction COII COII

7 waste

1 I

Mg-EDTA solution

Fig 1 A schematic diagram of the apparatus employed m this study

T Willrams and N W Bamett /AnaL Chm. Acta 264 (1992) 297-301 299

1-l) and ammoma solution The buffermg capac- sty was sufficient to ensure that the resultmg mixture of the eluent and Mg-EDTA solution was pH 10 0

The optmuzed detectlon conditions for Mg determmatlons have been established by Soroka et al [2] A pH of 8 0 gwes the optimum signal m terms of the ugnal-to-background ratlo A pH of 8 0, IS, however, not optimal, given the presence of the Mg-EDTA complex The condItIona sta- blhty constant, at this pH, 1s slgmficantly lowered gwmg rise to a much higher concentration of free Mg2+ and a consequent higher background At a pH of 10 0 the fluorescence signal due to the presence of Mg2+ 1s slightly lowered but the background slgnal 1s slgmficantly decreased, gm- mg a much unproved signal-to-background ratlo A 0 440 g 1-l solution of 8-HQS was buffered with 10 g 1-l Trls to a pH of 10 0 with ammonia solution The resultmg pH of the mtiure of the eluent and the Mg-EDTA solution and the 8- HQS solution was 10 0

Various stationary and mobile phases were used throughout the study ad the separation pa- rameters are described m Table 1

RESULTS AND DISCUSSION

In previous experiments [5] sigmficant im- provement m cahbratlon lmearlty resulted when the reaction co11 was heated Usmg the system described m this paper, no problems were en- countered with hnearlty The effect of heatmg the reaction co11 was again mvestlgated, however,

TABLE 1

I

Ca

I 8

12 min

Fig 2 A chromatogram of 2 1 X 10e6 M Mg*+, 2 5x 10e6 M Ca*+ and 5 6x 10v6 M Sr*+

m an attempt to improve sensltlvlty Upon heat- mg, even only 10°C above room temperature, (ca 38°C) the sensltlvlty for some of the analytes deteriorated quite markedly For this reason all subsequent experiments were camed out at room temperature The detection system created here IS non-selective although still sensltwe Figures 2, 3 and 4 show chromatograms of vanous metal Ions from all areas of the Penodlc Table The negative peaks at the solvent fronts of Figs 2, 3

The mobde and stationary phases reqmred for group metal separations

Metal ions Dlonex stahonaxy phase

Mobde phase

Mg *+, Ca*+, Sr*+ Ba*+ Co*+ Nl*+ Mn*+ Cd*+: Pb*/ Zn*+,‘Fe*+ Lanthamdes 3+

Al3+ In3+

HPICCG2(5cmx46mm~d) HPICCG2(5cmx46mmld) HPICCSS(25cmx46mm~d)

HPICCSSWcmx46mmId)

HPICCG2(5cmx46mmId) HPICCSS(25x46mmId)

LactlcacldO12MpH38 Lack acid 0 16 M pH 4 0 Lactlcac~dOllMpH38

Lack acid gradlent elutton (see Fig 4 for detads) Potassmm sulphate 0 08 M pH 3 0 Lactic acid 0 1 M pH 3 8

12 IInn

Zn

Fig 3 A chromatogram of 3 8 x 10v6 M Zn2+, 4 3 X lOa M Ntzf, 4 2x 1W6 M Co2+ and 12X 10v6 M Pb2+

and 4 result from the relatively low pH sample mjectlon “plug” whtch parttaily quenches the background fluorescence signal Callbratlon graphs were constructed for each of the elements with the concentration ranges generally between 1 x 10m7 and 1 x lo-’ M The detector perfor- mance m terms of Imeanty, hmlts of detection and reproduclblhty IS summarLzed m Table 2

In order to evaluate the quantitative perfor- mance and accuracy of the detector, a certified reference material was analysed for Mg and Ca content The certified reference material USEPA Trace Metals III has a rna~esl~ content of 0 10

Fg ml-’ wrth a confidence interval of 0 08-O 16 pg ml-l (significance level 0 05) Using the pres- ent method a mean value of 0095 pg ml-’ (5 replicate mjectlons of the reference material) w&h an R S D of 3 0% was obtamed Simdarly, the calcium concentration was certified at 0 5 pg ml-’ with a confidence mterval of 0 42-O 58 pg ml-’ (~1~~~~~ level 0 05) We obtamed a mean value of 0 53 pg ml-’ (5 replicate injec- trons) with an R S D of 3 2%

T W&urns and N W Bamett /Anal Chim Acta 264 (1992) 297-301

COnGiUSWTlS

A smple aqueous non-selective post-column reagent system based on fluorescence has been demonstrated The use of the displacement sys- tem allows the sensltlve detectlon of a wide range of metals from all areas of the Pen&c Table T’he detectson parameters are extremely good w&h linear calibration graphs and sensltlve detection

Ce

La iin d

c I 12 nil”

1

II 1

Fig 4 A ch~rna~gr~ of various lan~anldes, 2 8~ lo-’ M Lu3+, 3 OX lo-’ M Tm 3+, 3 Ox 10S5 M Er 3+, 3 Ox lo-’ M Ho’+, 3 1X1O-5 M Dy3+, 63x10-s M Tb3+, 66x10-’ M Eu3+, 67~10~~ M Sm3+, 11X1O-4 Pr3+, 71~10~~ M Ce3+ and 72X10V5 M La3+ Elutlon Time O-8 mm, iso- cratic elutton wth 0 25 M lactic acid pH 3 5 Tote 8-18 mm, lmear gradtent from 0 25 M lactic acid pH 3 5 to 0 4 M la&c acid pH 3 5

T willrams and NW Bamett/Anal Chum Acta 264 (1992) 297-301 301

TABLE 2

DetectIon performance for the Mg-EDTA/8-HQS fluores- cence system

Metal Correlation Lmut of detectlon RSD Ion coefficient a (X lo-’ M, (%, 5 rephcate

3 tunes the peak-to- mjectlons of peak base lme noise) 2 X lo- ’ M

standards)

Al3+ 09993 74 31 Ba2+ 09999 14 38 Ca2+ 09991 45 36 Cd’+ 09994 37 37 co2+ 09992 34 36 Fe’+ 09994 45 39 In3+ 0 9991 38 35 La3+ 09996 36 37 Mg2+ 09997 33 31 Mn2+ 09992 46 34 NlZ+ 0 9994 43 33 Pb2+ 09991 36 36 Sr2+ 0 9975 91 42 Zn2+ 09996 38 33

a The correlation coefflclent, r, was calculated by the method of least squares usmg a mean of 3 rephcate mlectlons for each of s1x cahbratlon points

for all those metals tested It IS envisaged that his system will be apphed to the momtormg of trace concentrations of various metals from natural and

waste waters and work IS currently proceeding m this area

The authors would hke to acknowledge the support given by Mr Ken Gawne (The Centre for Chermcal and Blologzal Research, Deakm Um- verslty) for the provlslon of the Dlonex CS5 col- umn used m thrs work

REFERENCES

B Karcher, I Krull, R Schlelcher and S South, Chro- matographla, 34 (1984) 708 K Soroka, R Vlthanage, D Phdhps, B Walker and P Dasgupta, Anal Chem , 59 (1987) 629 P Dasgupta, K. Soroka and R Vlthauage, J Llq Chrom , 10 (1987) 3287 P Jones, L Ebdon and T Wdhams, Analyst, 113 (1988) 641 T Wdhams and NW Barnett, Anal Chun Acta, 259 (1992) 19 P Jones, T Wdhams and L Ebdon, Anal Chun Acta, 237 (1990) 291 D D Pemn, Stab&y Constants of Metal-Ion Complexes Part B Organic hgands, Pergamon, &ford, 1983