Analyttca Chmuca Acta, 259 (1992) 19-23 Elsevler Science Pubhshers B V , Amsterdam

19

Determination of magnesium and calcium by ion chromatography with post-column reaction

fluorescence detection

Tim Wllhams and Nell W Barnett *

Department of Chemrcal Scrences, Deakm lJnrvers@, Geelong, victona, 3217 (Australu)

(Received 2nd September 1991, revised manuscnpt received 19th November 1991)

Abstract

A method IS presented for the determmatlon of magnesium and calcmm at trace levels The metals are separated from each other and other elutmg species by using an aqueous lactic acid eluent m conjunction wth a low-capacity strong cation-exchange stationary phase Once separated, the magnesium and calcium ions react with qumolm-bol- 5-sulphomc acid to produce fluorescent species [A(ex) = 390 nm, Atern) = 510 nm] The cahbratlon graph 1s linear for at least three orders of magmtude for both magnesium and calcmm with detection hmlts of 0 6 and 40 Fg l-l, respectively No interference was observed from other trace metals present and magnesium and calcmm were successfully determined m a certified reference material (USEPA Trace Metals III)

Keywords Chromatography, Fluonmetry, Ion exchange, Calcium, Magnesium, Waters

The separation and sensitive determmatlon of magnesium and calcmm has been a continuing analytical problem for many years Catlon-ex- change procedures are well established for alka- line-earth metal separations [l] and sensitive de- tection can be performed by post-column reac- tion with a chromogenic reagent such as 4-(2- Pyrldylazo)resorcmol, Arsenazo-I and Arsenazo- III [2] These methods have been useful for the determmatlon of Mg and Ca m potable waters [l] Molecular absorption spectrophotometrlc techniques, however, are of hmlted use for apph- cations that require higher sensltlvltles There 1s much interest m the determination of ultra-trace contammants m high-punty waters and reagents Ultra-sensltlve trace metal determmatlons are also required by mdustnes that produce hlgh-pur- sty materials such as those used m nuclear reactor design and m the production of semiconductors and optical flbres

In an attempt to nnprove sensltivlty, fluores- cent post-column reagents have been used for the detection of Ca and Mg Karcher et al [31 em- ployed qumohn-8-01 as the fluorescent post-col- umn reagent after on-hne hqmd-liquid extractlon of the eluted metal ions using a hquld segmented phase separator The fluorescence of the com- plexes was monitored m the extraction solvent (lsobutyl methyl ketone) They achieved the ana- lyte separation with an Ion-panmg mobile phase, this, combined with the hqmd-hqmd extractlon, resulted m poor chromatographlc peak shapes for Mg They demonstrated the apphcablhty of this approach for nme metal cation speaes with de- tectlon lrmlts ranging from 3 to 90 pg 1-l with Mg at 10 ,ug I-’ (no figures were given for Ca)

In an extensive study by Soroka et al [4], 78 metal species were examined for the fluorescence properties of their chelates with qumohn-8-01-5 sulphomc acid (&HQS) and the usefulness of

0003-2670/92/$05 00 0 1992 - Elsevler Science Pubhshers B V All rights reserved

20 T WILLIAMS AND N W BARNETT

8-HQS m chromatographlc systems as a metal fluorlmetrlc detection reagent was clearly demon- strated However, problems were expenenced with the metal separations as a direct result of mcorporatmg the 8-HQS m the mobile phase With the 8-HQS m situ, Mg (and other M*+ metal Ions present) eluted with the solvent front In an attempt to improve the separation system, a solvent gradient programme was employed How- ever, Mg and Ca were not baseline resolved and the background signal, due to the gradient elu- tron, rose slgmflcantly The use of S-HQS as a post-column reagent was tried but suffered from a poor baseline as a result of the separation system used

Further work by the same group continued the study of S-HQS as a fluorlmetrlc detection reagent for metals separated by ion chromatography [5] A variety of mobile and stationary phases were examined, prmclpally employmg 8-HQS In the eluent Mg and Ca were separated and detected by one of these systems, although under the chro- matographlc conditions chosen Mg may be liable to interference from quenching by certain M*+ metal ions present m real samples The use of 8-HQS as a post-column reagent was briefly men- tioned, although no studies concernmg Ca and Mg detection m this mode were reported Jones et al [6] used a simple post-column reagent sys-

tern for the determmatlon of alummmm which was based on 8-HQS fluorescence, and a slmllar system was employed m thrs study

In this mvestlgatlon, a simple but effective separation system was used that ensured that Mg and Ca are baseline resolved and not subject to interferences from the presence of other metals The utlhzatlon of an almost completely mert sys- tem (all plastic except the post-column reagent pump) ensured that troublesome metal leaching problems experienced by Dasgupta et al [5] were kept to a mlmmum The use of a post-column reaction system means that the 8-HQS plays no part m the separation of the metal species The separations can be carried out using conventional lsocratlc elutlon, which 1s an advantage when trying to extend the range of metals that can be detected m this way

EXPERIMENTAL

Instrumentatwn A schematic diagram of the apparatus 1s shown

m Fig 1 Two liquid chromatographlc pumps were used (Action Analyser 600E from Waters Mdford, MA, and a Model 64 from a Knauer, Bad Homburg) and were set at 0 8 and 12 ml mm-’ for the eluent and post-column reagent,

eluent

analytical column

detector

post-column reagent

Rg 1 Schematic diagram of the mstrumentatlon employed

ION CHROMATOGRAPHY OF MAGNESIUM AND CALCIUM 21

respectwely All connections were made Hrlth PTFE tubing The standards and samples were loaded mto a 100~~1 sample loop and mjected on to the analytical column by means of a six-port mert swltchmg valve (Model 9010, Rheodyne, Co- tatl, CA) The cation-exchange “column” used conslsted of two 50 X 4 6 mm 1 d Dlonex CG2 guard columns connected m series by a very short piece of PTFE tubmg (to mmlmlze band broad- ening) A tightly splralled reactlon co11 was used (1000 mm X 0 8 mm 1 d ) to ensure complete reac- tion prior to fluorescence detection The detector used was a Hitachi FlOOO fluorescence spec- trophotometer havmg a conventronal flow cell (volume 40 ~1) with the excltatlon wavelength set at 390 nm and the emlsslon wavelength set at 510 mn

Reagents Analytlcal-reagent grade chemicals were used

throughout Solutions were prepared with high- purity delomzed water (Continental Water Sys- tems, Sprmgvale) The eluent was lactic aad (0 14 M) adjusted to pH 3 7 wrth potassium hydroxide The post-column reagent was qumohn-8-ol-5- sulphomc acid (0 002 M) and tns(hydrometh- yljmethylamme (Tns, 0 05 M) buffer The reagent solution had a pH of 9 2 which, when mixed with the mobile phase, was lowered to 8 1 Metal standards were prepared by senal dllutlon of 1000 fig ml-’ Spectrosol stock solutions (BDH)

RESULTS AND DISCUSSION

The most intense fluorescence srgnal Gn terms of slgnal-to-noise ratlo) for Ca and Mg occurs between pH 7 5 and 8 5 For this reason, the 8-HQS post-column reagent was buffered at pH 9 2 with Trls (0 05 MI The resultmg mixture of the mobile phase and post-column reagent had an outlet pH of 8 1 Figure 2 shows typical re- sponses for Ca (50 pg 1-l) and Mg (10 pg 1-l) An interference study was carried out to ascer- tam whether there was overlap of other metals on the Mg and Ca signals Using the lactate system, the only catlons hkely to cause Interferences from

20 16 12 6 4 1 I 0 1 I

7

Mltl

Fig 2 Responses for 50 pg 1-l Ca-10 pg 1-l Mg aqueous standard

overlap are Mn2+ and Cd*+ None of the M*+ metal ions that were tested (Fe*+, Cu*+, Co*+, Nl*+ Zn*+, Mn*+ and Cd*+) were found to mteiere even at relatively high levels (50 pg ml-l M*+ co-mJected mth 0 1 pg ml-’ Mg and 0 5 pg ml-’ Ca> The detection hrmts, defined as three tnnes the peak-to-peak baseline noae, for Mg and Ca were 0 6 and 40 0 p I-‘, respectively The relative standard devlatlons (R S D s) were calculated from five mjectlons of an aqueous standard contammg 200 kg 1-l of both Mg and Ca and were found to be 3 2% and 3 4%, respec- tively The lmear cahbratlon ranges for Mg and Ca were found to be l-1000 and 50-2500 pg l-‘, respectwely The regression coefficient, r, calcu- lated by the method of least squares and using a mean value of five replicate mJectlons for each of the SIX cahbratlon points, was found to be 0 9997 for the Mg cahbratlon and 0 9987 for the Ca cahbratlon

(b)

I Ca AJL

18 18 12 6 12 6

a -0

Mltl Mlfl

Fig 3 Chromatograms obtamed from the mlectlon the USEPA certified reference matenal Trace Metals III used for the quantltatlon of (a) magnesmm and (b) calcmm, using different detector sensltlwtres

These analytical figures of merit represent a slgmficant improvement over previous studies [3- 51 The increased detection power coupled with the relatively simple chromatography, compared with the earlier work, has facilitated the use of ion chromatography with fluorescent post-column reaction detection for the determmatlon of low levels of Ca and Mg m waters and other matrices

In order to evaluate the quantltatlve perform- ance and accuracy of the system, a certified sam-

T WILLIAMS AND NW BARNETT

ple was analysed The certtfled material USEPA Trace Metals III has an Mg content of 0 10 pg ml-’ with a confidence mterval of 0 08-O 16 pg ml-’ (significance level 0 05) Usmg this method a mean value of 0 096 pg ml-’ (five trials) with an R S.D of 4 6% was obtamed Slmlarly, the calcmm concentration was certified at 0 5 kg ml-’ with a confidence Interval of 0 42-O 58 pg ml-’ (srgmficance level 0 05) Usmg this method, the mean value (three trials) was 0 52 hg ml-’ with an R S D of 3 2% Two of the chro- matograms obtamed for the analysis of Mg and Ca, from the USEPA Trace Metals III certified reference material, are shown m Fig 3

co?lcluswns A snnple, aqueous system for the spectrofluo-

rlmetrlc determmatlon of Mg and Ca has been demonstrated The system overcomes problems experienced with more complex ion-palrmg sol- vents and the ion chromatography ensures the removal of potential mterferences Given the de- gree of separation and sensltlvlty of detection of the two analytes, the techmque lends Itself well to the momtormg of traces of Mg and Ca m a variety of matrices where sensltlve detection hm- its are required

One of the real advantages of chromato- graphic systems 1s their potential for rapid, se- quential, multi-element analysis It has been demonstrated m this work that the sensitive de- termination of selected metals 1s possible Most metals, however, do not form fluorescent com- plexes with 8-HQS and hence cannot be detected usmg the present system Using some of the m- formation gamed from this study, we are cur- rently developmg a fluorescence-based detection system that 1s able to extend greatly the range of metals that can sensltwely be determined

The authors thank Dr R Greenwood-Smith (ICI Scientific Instruments, Australia) for the loan of various pieces of mstrumentatlon and Dr D Wise1 (Board of Works Water Sciences Laborato- nes, Australia) for the provlslon of the USEPA certified reference matenal

ION CHROMATOGRAPHY OF MAGNFSIUM AND CALCIUM 23

REFERENCES

1 M Argue110 and J Fritz, Anal Chem, 49 (1977) 1595 2 J Fritz and J Story, Anal Chem, 46 (1974) 825 3 B Karcher, I Krull, R Schlelcher and S Smith, Chro-

matographla, 34 (1987) 708

4 K Soroka, R Vlthanage, D Phdhps, B Walker and K Dasgupta, Anal Chem , 59 (1987) 629

5 P Dasgupta, K. Soroka and R Mthanage, J bq Chro- matogr , 10 (1987) 3287

6 P Jones, L Ebdon and T Wdhams, Analyst, 113 (1988) 641