Preconcentration methods for the analysis of water by x-ray spectrometric techniques

Anatytica

Chimica

Elsevier Scientific

Acta,

143

Publishing

( 1982)

3-3.1

Company,

Amsterdam

-

Printed

in The

Ncthcrlnnds

Review

PRECONCENTRATION METIIODS FOR THE ANALYSIS BY X-RAY SPECTROMETRIC TECHNIQUES

It. VAN

OF WATER

GitIEKEN

Department

(I
of

Chemistry,

2nd July

Unirvrsity

of .-Intwerp

(lJ.I..A.).

B-‘610

Il’ilrijl:

(Ifclgium)

1983)

SUhIhlrZHY

All puhlishcd proccdurcs for multi-clcmcnt prcconccntrrrtion of tr:acc ctlcnic-nls. prior X-ray fluorescence analysis Of w3tw. iIR? reviewed and criticnlly t~vnlu;lted. Ilost prcconcentration methods applied to the: determination of single c,lcments in \v:rler 3rc to

also

listed.

analytical tcchniquo. -X-ray spcctromc?txy (x.r.s.; is a ~vell establishc?d Its underlying spectroscopic principles are well understood, as are its possible pitfalls. The technique offers the advantages of a truly multi-element charxter, acceptable speed and economy, and case of automation. Recent instrumental advances include the upgrading of the classical wavc?lc!ngthdispersiv~ instrument with respect to elcmcntal range, reliability and automation; and about ten years ago, the advent of the non-dispersive or so-called energydispersive detection mode wiLh a semi-conductor detector, and of particleinduced s-ray emission (p.i.s.e.) with a nuclear accelerator as escit;ltion source. Each of the x.r.s. modes has its characteristic advantages, and fervent be applied on a routinrt supporters as well, and each can advantageously Yet, direct s.r.s. has the distinct disbasis for many analytical problems. advantage that its sensitivity is often not sufficient to yield the low level of determination that is desirable in many modern analytical problems. ‘I’hc! combination of s.r.s. with a preconcentration step extends the application research and monirange significantly, e.g., in the fields of ~nvironmcntal toring. Not surprisingly, much effort has already been devoted to designing and studying suitable prcconcentration techniques that are attractive fol X.T.S. of aqueous samples. (process) as Preconccntration, defined by IUPAC [ 11 as “an opc?ration a result of which the ratio of the concentration or the amount of microcomponents (trace constituents) and macrocomponents (matris) increases”, not only improves the analytical detection limit; it also reduces matris effects, and so enhances the accuracy of the results and facilitates calibration. Further, preconcentration allows the sample volume taken to be increased and so improves the representative nature of the results. Iiowcvcr, 0003-267O/S’>/OOOO-0000/~02.75

6.) 1982

Msevier

Scientific

Publishing

Company

preconcentration increases the time required for analyses, complicates the analyses, and may involve risks of contamination and losses of trace elements or of certain species. In principle, any preconcentration and separation method developed for any analytical technique could be used in combination with x.r.s. However, s.r.s. operates best on solid samples, gives optimal sensitivity (especially in the energy-dispersive mode) and accuracy for thin homogeneous targets, and offers sufficient spectral resolution for assessing several elements simultaneously. Thus multi-element preconccntration leading to solid thin targets will be ideal for x.r.s. Conversely, preconcentration techniques developed for x.r.s. can be combined with other techniques, e.g., with neutron activation if the sodium, chlorine, bromine and other elements leading to tedious spectral interferences are not significantly collected, with emission spcctromotry and spark-source mass spectrometry if the resulting sample is a solid conductor without an excess of major ions, and with flame atomic absorption and inductively-coupled plasma emission spectroscopy if the preconcentrated elements can easily be cluied into a small volume. In view of the present developtnents in multi-element analysis, the latter group of techniques will probably become of increasing importance. Direct analysis of whole or filtered water is almost invariably done on a few millilitres placed in a cup with a thin Mylar or polyethylene bottom in a non-evacuated s.r.s. setup. Keported detection limits for energy-dispersive x-ray spectrometry (e.d.x.r.s.) with 30-min counting include, for example, 100 mg 1-l for K, 10 mg 1-I for Ti, 1 mg 1-’ for Fe, 0.3 mg 1-l for Zn and 0.1 mg I-’ for Sr when tube excitation and a secondary target are used [ 21, and 10 mg 1-l or more for these elements when radioactive sources arc used for cstritaiion [ 3, 4 1. The correction for x-ray absorption in the Mylar film has been investigated in detail in this context [ 51. Special cells for operation in vacuum have hcen proposed [ 61 . Typical detection limits in the ppm range are ~:crtainly not satisfactory for many environmental water applications and necessitate a preconcentration step. Several studies have been aimed at x.r.s_ of the particulate matter in water and the environmental or geochemical applications thereof, by means of spectrometry (w.d.x.r.s.) [7-91 or e.d.x.r.s. wavelength-dispersive: s-ray [‘LO-121 and also p.i.x.e. [ 1%-161 _ In general, a simple filtration through a thin membrane, like a 0.45-pm pore-size Millipore or, preferably, a Nuclepore is quite satisfactory for collecting the particulate matter in a membrane, suitable form for measurement. The only problems are then those pertaining to corrections for x-ray absorption and particle size effects. Necessarily much less obvious is the choice of the preconcentration method to be used ior the dissolved ions in water: the literature abounds with alternative procedures. Based on a thorough computer search through the analytical and environmental literature since 1967, as many as possible of water were collected together; articles on preconcentration for x.r.s. more than 170 relevant articles were obtained and studied. Table 1 gives a

TABLE

1

Number of publications s-ray spcctromctry

. _-

.- _

blulti-element

._-

pertaining

lo prcconcc~ntralion

._.- --_

.

dctcrminrrtions: -..-.

..---

Ccncnl

._-. _. -

5

Physical preconcttntrcltion (evaporation, frcczc-dving,. Ion-eschangc wsins Coprecipitntion Extraction Immobilization :iftrr chelation Elcctrodeposition .___~_. _. _ _..-._ -a()f which

_

120

.)

21 nrc on ion.cscllarrgc!

-. iilttirs.

in water

3lld

__

dt*tcBrmin:ltions:

~-

S”

a*la,mcnts

--~_

Singlc.clcmcnt U Cl, Ur and

23 .lSJ 36 G 7 5

of dissolwd

._ .--

---

---

.>

SO,-‘

.---

____ .I3 7 9

I

St> Cr I’. K, I’cs. Xi. ..\s. 110. I32. II)!

--

for

- .-_-.

:; 2 1 e.rc.h

----

classification of this literature on ~)rcconc:cntration for s.r.s. applied to water. It is the purpose of the present review to desc*ribc the pul~lishcci work nearly completely and make this large literature easy to sun’oy. Furthtar, the merits and drawbacks of the various I”“(:o”‘entri~tiorl procedures for s .r.s. are compared. Industrial or various other samples ar(x sometimes dissolvc4 for subsequent s.r.s. measurements: the literature on the trc~atnient of such aqueous solutions. not comparable or Wliltd to environmental waters. will not be considered here. Very few studies have been dedicated to compilrin, 0 espc~rinientally sorncl of the proposed preconcetltration procedures with respect to sensitivity, inter-clement effects and the influenc:ca of humic: subprecision, linearity, _ The results rc?porkcl are ccrtainly not conclusix:c c~nou& stances [ 1 r-.-2O] I about which method is preferable. Of course, it must be !)ornc? in mind that. no method of prcc~oncentratiotl will be a panacea bccauscn thca chol~:e will be influenced by the types of water that have to hc esamincd, the clc~mcnts of interest, the number of samples, the required acc*uracy. precision and WIIsitivity, the available equipment, etc. EV_WOII.-\TIOS

Perhaps the m’ost obvious and straightforward method for prcconcentrating ions from solution is evaporation of the solvent. Yet, in no other method have the inherent problems with respect to s.r.s. been so grossly underestimatid. Evaporation collects quantitatively all non-volatile eler Ictnts irrespective of their speciation in water (which is usually not true for other preconcentration methods). it requires little attention or manpo\ver. and the risks of contxnination are minimal. The obvious drawbacks arc, however, at least as numerous: the evaporation step is time-consuming; samples with high salinity or hardness cannot be preconcentrated efficiently and analysed sensitively; incomplete recovery of the evaporation residue from the> con-

G

raincar can lead to errors unless an intr+mal standard is included; and pelIctizing or some other target preparation is required prior to the x.T.s. step. There are possibly more importrlnt inconveniences. For example, the variatG1it.y of the residue from variable types of water implies variable matris effects unless sophisticated matri.. correction procedures are applied or unless a diluter is used at the expense of lowering the detection limits. Further, the formation of finite crystals, even from very dilute solutions can lead to large and unknown s-ray absorption effects, and the occurrence of fractional crystallization resulting in microscopically inhomogeneous residues may result in very problematic matris corrections. Of course, several ways of removing Lhc water matris physically are available. Straightforward evaporation of a large water sample, pelletizing the! evaporation residue and measuring versus doped calcium carbonate mattis star&u-ds was proposed by Haberet [21] . Mcier et al. [ 221 and Comil and I,cdcnt [ 231 mixed the evaporation residue with an organic binder Lo reduce matrix effects. Freeze-drying of, for example, 80 ml of waste water on 1.2 g of cellulose followed by grinding and pelletizing of the residue for e.d.s.r.s. led Lo 0.1 mg 1-l detection limits for many elements [Z-l]. The variable absorption correction for the residues were then calculated via the Compton-scattered x-ray tube line. Van Dyck [ 251 added to a 250-ml water aliquot a spike of yttrium as an internal standard and, if no substantial residue was espccted, 100 mg of graphite, then freeze-dried, pellctized and mncasured by secondary-target e.d.s.r.s. Calculating the s-ray attenuation cffrac:L for every energy via the coherent and incoherent scatter peaks ratio and the fluorescent peaks, Van Dyck obtained typical detection limits of 5 /lg I-‘, and accuracies around 10% for very variable and complex samples procedure like sewage water and s!udge. The so-called “vapor filtration” [ 261 offers an interesting alternative approach, at least. for similar samples \\?Ltl low salinity. The cellophane bottom of a container is permeable to water vapor but not to water or dissolved material. ‘The bottom surface is osposed to vacuum, the water vapor is pumped away and all the dissolved solids are left behind on or in the membrane. In combination with p.i.x.c., typical detection limits of O.l.-3 pg 1-l have been claimed 1271. In general, the more advantageous detection limits of p.i.x.c. and its prcdominant use in nuclear physics laboratories where chemical manipulations are often unpopular, have resulted frequently in a combination of p.i.x.e. with a simple water pretreatment step consisting of pipetting one drop or at most 1 ml of water onto a thin Formvar or Mylar carrier and evaporating The reported detection limits are then usually around [‘13, 14, 28-311. by partial evaporation at 10 j.Ig 1-1, or lower if the sample is concentrated 50-60°C prior to spotting [ 321. The absorption-enhancing effect in the finite residue crystals, that may plague such an approach, may be reduced by nebulizing the liquid onto the substrate [33, 34 1. A more convincing alternative is the previous addition of homogenizers such as liposomes, i.e., synthetic phospholipid bilayer vesicles 1351. Such vesicles are capable of

entrapping ions and small molecules and interfere with the cwstallization in very small areas. Itobaye et al. [ 351 observed that, in the absenc:e of a homogenizer, 20 pg of salts dissolved in a droplet covering an S-mm tiiamc~tc~r area before drying, may give rise to a crystal with a supc*rficial density of 10 mg cm-‘. Another approach that partially circumvents such prol)lt!ms alld allots larger samples to be evaporated and so bcttcar detection limits to by rc~;lc*llt!d in s.r.s., consists of impregnating a filter paper with the sample solutioli. In combination with w.d.s.r.s., this method was first dcscribcad by I’fciff(ar and Zemany [ 361 and has since been used I)y several authors, e.g.. Feltc~rl et al. [37]. The sensitivity can be improved furthc>r hy applying the ring oven technique, e.g., as recommended by I\ckerm;n~n et al. [ 381 . •Jol~nsoll and Nagel [ 391 proposed spotting within a controllt~d arca which \vas obtained by applying a hydrophobic was ring on the? filtcar paper. Afkr further optimization of the various parameters of such a simple spotting procedure for e.d.s.r.s. of water, the following procedure was propost [ 40] : 1.5 ml of water sample was spotted on a \Vhatnian-‘11 c:cllulosc filter pal)i:l provided with a wa_x ring of 3-cm diameter, and the \vater \V;LS ~?vil~)or~tcd I>y The detection liniits \vcrc posing an unheated air stream from underne;~th. iNId often belo\v .30 /.fg 1 ’ found to be below 100 pg 1 ’ for most elements when the! optimal secondary fluorescence was usctd. I\(:cu~acy ant1 precision better when an interwere usually in the 15--O3 C/orange, but were somewhat nal standard was added to the solution prior to the spotting. The hydrophobic wax ring reduces the spreading of the solution in the filter and any diffcbrential chromatographic effects, and so increases the sensitivity and reduces errors arising from heterogeneous analyte distribution within an csciting boam of non-uniform intensity. Other approaches that give a more homogeneously distributed deposit on a filter paper include the nebulization technique [33, 34, 411 and the multiin 10,-l 5 p 1 tlroi):‘, drop technique [42] in which, e.g., 220 ~1 is depositcti just enough to wet the entire surface. For subsequent p.i.s.c?., Kivits et al. [43] recommended wetting a filter paper fised on a rotating table (1s 000 rpm) with 50 ~1 of a water sample diluted (1:l) with ctlianol i1S a surfer reactant, and air drying. In general, it seems that physical preconcentration methods are quite suitable for simple screening of waters, when cstrcme sensitivity, accuraq and when the trace metal spcciation is and versatility are not required, complex or variable. In fact, physical preconcentration is the only practical possibility when sewage, waste waters or similar samples, are to be analyzfBd.

As in other analytical kchniques, materials with iontschangc properties have cstensively been used for preconcentration in s.r.s., in both the batch and the colun;n mode. The effect of the ion-exchange resin particle size on the

s results obtained by s_r.s. was studied [44], and particle sizes below 50 pm were recommended. Ordinary cation and anion eschangers are of limited use for concentrating tracct ckments from natural water because of their inability to cscludc major eons sclwtive:y. Still, for samples with a limited alkali and alkaline earth ion content, conventionally available cationand anion+?schange resins can be a:>plied. Cesarco ct al. (451 reported on such preconcentration for radioisotope sss. Murata and Noguchi [4S] used a cation cschanger to det.ermine several ions in acidified waste water, by w.d.s.r.s., down to 10 pg 1 ‘, with a 3’? standard deviation at the 1 mg I-’ level. They added 1.5 g of epoxy resin hinder to G g of cation cschnngcr and allowed the mixture to hartlcn into a pellet. Up to 1000 mg 1” Xa’, 200 mg I-’ Mg” and 100 mg 1-l CL” could he tolerated: above this level, the transition metal uptake was strongly depressed. Indeed, as these wsins operate on an ion-association and the abundant alkali and alkaline ions basis, they are not very wlectivc can conipctc? wc?ll wit11 transition metals. In ~3ses \vhere the alkali ions are not of interest, chelating ion-eschange rz!sins arc more promising. The most popular one is undoubtedly Dowes Xl, or Chclcs-100 (HioRad I,aboratories), with iminodiacetate functional groups (Fig. 1). Generally, it offers very high distribution ratios for transition metal ions and for IIg” and I%“, but not for alkali ions, and its chemistry can be accurately predicted by ana.lobT wit11 E:I>T_A. Florkowski et al. [ 4’71 studied t.hc? potential of Clreles-I 00 resin prcconccntrations in the analysis of ground \v;iior rulcl rain \vater. and found that 9000 mg I-’ X1‘ ancl 200 mg I-’ Ca” Y;,__ ‘.._

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Fig. 1. Functional groups of ion cxchnngcrs for prcconcenlration in x.r.s.

.

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*

.._

i.__;,

;.:‘..:

_:..;._;_:

I.:;.,!.. <
.:-

-..

.,.;

and of ion-(??(chan6e/chclating

filters

USC~U~

could be tolerated under their experimental cwndltwns. Clanct ;rnd cwwork(*rs [ 48--501 used 3 Chclcs-100 column in 3 Xlakrolon (*;crtridg(* l .vlllc:t1 ~3s pclletlzed for x.r.s. nicasurcmcnt afkr the collwtion of lrncc mctnls; the> noted Ih3t 3n abund;lnc(B of allk311nc eautll IONS in the wdtor wrnpk n(*c:cs. sitatcs cithcr rec:y(*ling of thtr watt-r sample owr the c*olllrnl~ or prc*vlous dilution. For more versatile I,rc~conc~ntratlons, Lhc 1on-c~s~lw11g:c~sul)S~ratc shoul(l IlOt ShO\Y ilny nffinity for alkdi 3nd alkaline cartl: Ions mulct bc s;c!lt*c:tivc* towrds tr3nsition mc~t;ils. Lcydtan cbt al. [ 51 1 usc~l 100 mg uf l)oly31n1ncpolyurcu rcsln columns prcl):lrcd from trlr;ll~tIryIcncl)cntclmIt1c ;ln(l to111a*nt* diisocyanatc, to prcconcxwtrulc* nickel, copper 3nd zinc qu;intltclt~vc~1. front 4 I of sc3w;iter of n3tur3l pH (In spite of tlw 16 :! I ’ S;I’. 1300 rng I” .\tc” 3nd 500 mg I-’ C:3’* normnlly prcscnt in ?;(!a \\:itcr) and to clc*tc*rmlllr thc*bc* trxc metals by w.d.x.r.s. down to 0.1. 0.3 pg I-’ c~ot1c~t~~~trcil~ofis.Hirrl)a ;111(1 I.icwr [ 321 imnrc~l~~liz~tl l.(2.l,~clro~ypt~~~l~yl.l~u).‘).ntl~,t~llrol or I Iyph;ll~ (see Fig. 1 ) on c*ellulosc I)o:vdcr by dl;lzotl/.cltwn of o.3minopt~c~r~ol~~lllulo~~~ 3nd subsequcant couplin :: wth li-naphthol. 1.3tclr p;~lwrs dcsrwtwd rlw s r.< . dctcrmlnatlon of Fe, %I>, 1% 31x1 c~sp~~.l;rlly Cu and I I III froshwntcr. SC*;IW;I~CI’ and mineral w3Wr lvith this ion ~*schang~~r {52-- 561 wlllcli 11;~s;\ c.;il)ac*ity of 0.5 mmol g-’ 31x1 IS now commc*rc~;~lly ava~lal~le (Rwtlcl de t iwn). ‘t’hc tIctclc.lion limits wcrc’ t~l~l(‘illly lust I)c*low 1 ug I” for t!.d.x.r.s. or wx1.s.r.s.. l)ut sonw \ve;Ik lnflucncc of (:a:’ cewnot tw cxcludcd. (Yonsidcmblc rcwtlrcll I\;Is been dedlc3tc*d to fIndIng sin1l)ll* methods uf ammo t)ilizlng Ion-coll~~cling func.t 10nal groul)s In 3 vc*ry vc*rs;rt iIt* \v;I~. L4*yclc*ll and coworkers 1 ST-601 IKIVC intensively studwd ;uxl used liw s11~1311o11 reaction to immobilize clwlating or c*oordinittlng functional groups onto (:UIItrolled pore g:loss tw;rds or s111ca ccl. ..\ftc*r homogeniz;ttion and I)c~ll~~tl/.ln~. suctl m3tcri3l IS ;I suitable mntrls for s.r.s. clctcrminallorls. ‘l%(*y used .\‘.,j. a~~~i~1octt~yl-~-rrm~rropropyl~r~mc~t1osjtsllanc (Dow.Comlng Z-6030 w;lgcnt) and ) -3minopropyltrlrnrthosysil3nc (Dow.Cornlng S%-203.1 rc’il~!c*lll) 10 immoh~lizc di3n;inc furlctlollal croups. and illso prcparcd ti~rlg. tilr;r>*am:t ~ncl Unohara [Gl ] worked dong, slmiliu llncs and trc3tcd slmllar sllylatctd STOIC*;\ with cithcr monochloronwtatc or s3llc~ylaldchyd~* to produce 3c*ct3tc? or Schlff base functionA groups: they obtainctf cllnchrnc‘nt factors of 500. and x.r.s. detection llmits .uound 10 ~g I-‘. In general, ion eschlmgers will rc*cover hyclr:ttcci Ions. ch3rgcd c:otni)lcsc*s and ions complesed by labile ligrinds. but t!lc recovery dcpcnds on tt1c dw tribution ratio of the ion on the resin, the stability const3nts of the coni. pteses In the solution. the exchange kinetics 3nd the presence of other competing ions. More spcctficully, it is not 31w3ys obvious that Ir3ce lr3nsi.

10 mctais bound to stable complexes and naturally occurring organic material or adsorbed on colloids, will be collected efficiently by ion-exchange resins. In this contest, the use of Chelcx-100 for trace metal preconcentration from natural water has been criticized by Florence and Ratley 1621, who measured very low recoveries from sea water of, e.g., zinc in its natural Lion

spetriation,

although

freshly

added

radioactive

ionic

spikes

were

recovered

fully. This was confirmed by Xbdullah et al. [ 63 ] who found that metals in colloidal form or adsorbed on particulatcs which could be separated from the sample by centrifugation were not retrrincd by Chclex-100, while only the dissolved and electro-reducible species of Cu, %n, Cd and Pb correspond to the fraction taken up by the resin. This problem is not particular to the ion-eschange process, but also plagues most other preconccntration methods. All the prcconcentration procedures based on ion-exchange resins share some drawbacks. First, the wnsitivity is not optimal because the preconcentration factor (ratio of original to final sample weight) is typically around .5000 only (e.g. 100 mg of ion eschangcr per 500 ml of water); moreover, a diluting binder may be necessary for pelleting. Secondly, after column preconcentration, the ion exchanger should be homogenized and pelletized prior to x.r.s. Thirdly, in thick targets of ion-exchanger beads. absorption and particle-size effects can be considerable. The? uw of ion-exchange filters could remedy these problems.

ion-collecting filters cffer a very elegant way of preconcentration if quantitative recovery can !W accomplished by a single filtration step at natural pH. In fact, ion-collec!.ing filters are more attractive for combination \\-ith s.r.s. than with any othc*r analytical technique. The loaded filter is ideally a thin homogcncous t.ar;:et of low Z number that can be presented dirt:ctly to the x.r.s. machine, without any preparation. Thin targets offer optimal accuracy and sensitivity, because the absorption-enhancement corrections remain small and the background of exciting radiation scatter is limited. Ideally, particulate and dissolved trace metals could thus be detirmined in tapwater, for example, by simply plugging a filter holder with both a common filter and an ion-collecting membrane onto the tap, running a certain volume of water through, and subjecting both loaded filters to .x _r.s. Preconcentration based on paper tape loaded with ion-exchange resin could also be highly automated, as in the “Sample Collection and Preparation with automated e.d.x.r.s., proposed by Carlton and Russ [64). Module” Naturally, much effort has been devoted to preconcentration by ion-collecting fiIters for s.r.s., but, surprisingly, almost all research has been focussed on the ion-collecting membrane types that are least interesting for environmental samples of waters. Grubb and Zcmany [ 651 first proposed the use of ioncxchange membranes for x.r.s. Campbell and coworkers published well-known pioneering work on

11

ion-exchange resin-loaded papers for s.r.s. [SS] and later reviewed their applications [ 67, 681. They used Rec?vt?-_Angel cation-exchange papers SA-2, and found satisfactory rccovcrics for a dozen cations at pII 2 after seven successive filtrations but warned about the effects of abundant alkali and alkaline ions. They also rcportcd good collections of anions on ReeveAngel anion-exchange papers SH-2 with quatcrnary ammonium groups. ‘I’hc! detection limits were just below 1 ~g for w.d.s.r.s. Up to 1977, practically all published work on ion+:ollc*c:ting filters for water research [ 6+80] was devoted to thcl USC of the commctrcially availsblc S/1-2 filters with sulfonic acid functional groups, containing approsimatcl!, 50% Ambcrlitc In-120 resin and 50% cellulose, and having an c~sc:ha~~g!ct capacity of 2 meq g’ ‘. Several of these articles reported only prcliminar> espcrimcnts; others essentially confirmed the original rcBsu1t.s of Caml)t~c~ll ct ai. [ 661, and, depending on the sample volume (typically only 10 .-lo0 ml), the number of passes (G--10), the escitation mode and mcasurcment time and the filtration procedure used, the reported dctcction limits \vcrca typically between 10 and 400 ~g 1-l. in spite of somtl rathctr optimistic preliminary conclusions, most of these authors \vcre well aware of tllct severe restrictions of S:\-2 filters: they have an eschange capacity of on!> 0.25 meq/filtcr and a significant affinity for the alkali and alkalino earth ions. ‘I’hcreforc, the collection of transition mctnl ions is iiot quantitative for lalye sample volumes of natural water in which alkali and particularly alkalinct earth ions are usually abundant: t.he applicable sample volume is thus severely limited, and then the prcconccntration factor is too low to yield cnvironmcntally relevant detection limits. This was confirmc~d in rcccnt intcrcomparison studies [ 17-191. hloreover, passing the sample several times over the filter is inconvenient, and adding acid to adjust the pH to 2 implies a risk of contamination. Clearly S1\-2 filters arc not idckal for direct x.r.s. of water;. Nor did Illrich and IIopkc [‘79] find the SB-2 anion-cschangc filters to be satisfactory for trace metal collection from natural waters, after a complexation step at pII 12 with 0.01-0.02 m&l cyanide or with 0.02 mhl RPPM-S [ 2-(3’-~ulfobcnzoyl)-pyridine-2-pyridylhydrazonc ] _ The authors had cspectcd that such a step would minimize the problems of dctcrmining naturally occurring complexed ions, but at alkaline pIi, large concentrations of calcium and magnesium form a gel that prevents filtration. Kingston and Pclla [Sl] were successful in indirectly using S:\-2 filters for the determination of Ni, Mn, %n, Cu and Pb in sea water, at the 2- 4

pg

l-” level

with

a standard

deviation

of only

0.2 pg I” by c.d.s.r.s.

The

sea water

was first processed for the separation of the trace clcments from Na, K, Ca and Mg, on a Chelex-100 column, and the eluate was evaporated, heatid to sublime ammonium salts, taken up in acid, and only then p:~s~i

through SA-2 filters. Lochmiiller et al. 1821 (Ionic

Chemical

Co.)

that

briefly studied arc characterized

MC-3142 ion+schangc? membranes by a slow metal uptake. Esposurc?

of the membrane to a changing concentration in a stream should therefore lead to a collection that represents an average concentration. Further, P-81 filters (\Vhatman) with phosphate groups and \VA-2 papers (Reeve-Angel) w-ith carbosylic functional groups have been mentioned but not studied in much detail [ 781. It is not likely that any of these materials is vastly superior to S-A-2 fiiters; clearly, more selective filters, with no affinity for alkali or alkaline earth ions, are required. Van Grieken et al. [83] therefore evaluated the commercially available .-\cropor CII filters (Gelman Co.) with Chcles-100 or iminodiacetate groups that \tpre espected to show a much less pronounced affinity for alkali and alkaline earth ions and to collect at natural pH levels. It appeared that filkring 200 ml of water through a pair of Chelcs-100 filters, under a 2-3 bar pressure at pH T-S in not less than 20 min, provided an efficient collection of many trace metals (up to the 0.07 meq capacity per filter pair), with s.r.5. cletcction limits at the 1 pg I-’ level and a precision around 10%. Again, ho\\Pv(tr, the still significant affinity for alkaline earth ions restricted the applicability. \Vith calcium-rich water, for example, only small volumes coulcl be preconcentrated and the detection limits were accordingly worse. Thus the long-term objcctivc: of applying resin-loaded filters for c:ont.inuous on-line collection of ions from drinking water and other near neutral aqueous solutions could not be attained. :\lthough longer filtration times (e.g., 10 h) favor the chelation process relativ e to simple cation cschange and reduce ihe alkaline earth interference, Cheles-100 filters arc not promising either. Ccllu!ose-powder films with immobilized chromotropic acid functional groups (see Fig. 1) allow fast collections at pIi 4- 6.5 with a 0.01 meq capacity, but they suffer even more from Xa* and Ca’* interferences; 5 ppm calcium reduces the transition metal uptake to 40% only [ %I] . tlyphan filters arc prepared by immobilizing 1-(2-hydrosyphenylazo)-2naphthol on short-fibred cellulose powder and pellctizing 0.1 g of the product. which has a 0.05 meq eschange capacity, into thin layers. These are much more suitable: after acetate addition, high rrxovcries of trace metals are possible from 3 I of fresh water and 0.5 M NaCl solutions, at a flo\v rate up to 18 ml min-’ cm-’ at pI1 7, and the detection limits for The influence of alkali e.d.s.r.s. arc typically around 1 pg I-’ [ 52-541. and alkaline earth ions, although small, is not negligible; prcconccntrations i‘rom sea water require thicker filters, smaller sample volumes and lower filtration rates. \Vhen, however, the trace transition metals were first preconcentrated in small Hyphan columns, clutcd by dilute hydrochloric acid and then bound on 100 mg of Hyphen that was prepared as a thin layer for e.d.s.r.s., sea water and 20% NaCl solutions could be analyscd [ 851. Gcndre et al. [86] silylated a silica-gel filter with N-p-aminoethylaminopropyltrimethosysilane and then treated the resulting diamine with carbon ciisulphide to obtain the, dithiocarbamate (Fig. 1) modified filters. Up to 20 pg of transition metals could bc taken up from 50 ml of water at pH of major ions 5-6 after 10 cycles at 10 ml cm ’ min-’ but the influence WIS not studied.

Disam et al. [ 1371 followed a different and very novel approach: they filtered 0.1-6 I of aqueous samples through a freshly prcp,arcd homogeneous metal sulfide layer (e.g., 450 c(g of zinc sulfide) and sufficiently enriched numerouselements that form insolu!>lc sulfides, to o!)tain ~v.d.s.r.s. detection limits of ahout 0.2 pg I-’ for surface waters. Only Ia’e,“, HPO,‘- and strong chelating agents interfered scrious!y. An ideal complesing tnolcculc for preconcrntrating filters sho111tl fulfi! the following requirements: (1) no affinity for alkali and alkaline earth ions. (2) high stability constants for heavy metal ioIls. (3) formation of a stabllx molecular structure, (4) easy immobilization onto cellulosc~, for csamplc~. Theoretical considerations indicated that 2,2’-ciiaminodictthylamill~~, oftcan called diethylenetrianline (DEN), would satisfy tlresc* rcxluircmcnts very ~vc~ll. A considerable amount of work to optimizca the synthesis of cellulose-I’>I:S filters (see Fig. 1) led to thct following recipe [ SS J . ‘I’ttn dric>d \\‘hatman-41 cellulose filters are pre-swc?!!ed hy soakin, 0 for 30.-60 miri in 200 ml of N,!V-dimcthyIformamic!e (D&IF; distillctci with 10 ml of t)t?nzt?ntf !xr 100 ml is hc~ntt*d to 9O.C of D>IF); thcxn 6 ml of PO<:!, is x!d~d and t!~cx misturtl for at least 15 min. The filters arc’ was!icBc! succcssivc~ly with DAIF. \v;it.cnr. 50% (w/v) NaOII, water, 5% (v/v) actgtic ‘acid and Lvater. The chlorine functional group is then replaced !)p heatin, “ the filters in an escc?ss of piirificd metal u!)takc cn!xicit> DEN at 130°C for ilt least 2 ii. The average transition of the resulting filters is 3.6 ~.~ctqcm-’ or 37 poc!:fi!ter. AIany parameters that might influenw t!ic trace 1:ation u!)ti~kt? t)y the DES filters \vere suhsc~qucntly studied [ 89] . ..\t ~1 filtration rate below 1 .?I ml min-’ cm”’ and at a pII alxxt~ C;, rccoverics of 90 -1 00”~ arc? o!)tainc?d for Pt)-’ a~lc! KO.“. I’or Cr”. k-e.“, Co”, ii’*, %.n”, _-\g’. Cd”. Ku.“. !ig”, of the c:bllcctc*tl sample volumes up to 100 ml cm-‘, there is no clution cations. The possible influence of forrxign sut)stnnccs \vas checkcad t,y adding NO,,. cl-. I-. r-rc:o;, SO, z . H,I’C).; and zlso salts of Na’, K+, Mg”, 0”. srr;, glycine and humic material to numerous multi-rlcment trnlisiiion mt?tn! solutions. \\‘hile the collection of AIn” \vaS dc~prc~sscd hy ScVPrilI agents, the uptake of the other cations \vas only influenced hy X!-1; and amino acids (above 5000 mg l”), which compete with the nitrogen donors in t!lca DI*IS filters, ‘and not by 3.50 g 1-l SaCl or 30 g 1-l CaCl,. :\nions can also !)e IIreconcentrat.4 hy DEN filters [ 901, at least from diluted solutions, at a filtraof at tion rate tIelow 0.7 ml min-’ crnWz and at pIi .!.5, up to a total least 1.5 peq cm-‘). It appcxvcd, however, that the anion colkction meckmism is based purely on electrostatic attraction, and so is not selective: ionic: strengths above 0.01 hI XaCl strongly depress the collection efficiency. ‘I’llis limits the applicability of DEN filters for anion preconcentrations to diluted samples only. The combination of the DEN fitter with conventional secondary-fluorescer e.d.s.r.s. has been ctvaluated [ 911 . _.I perfectly linear relation was noted between the x-ray response and the metal concentration in solution up to the ca. 20 peq capacity of a 10cm’ DKN filter. Practical detection limits were around 0.5 ~g I-’ and often lower. r\ccuracy and precision went IOilCl

I .I

both

around

filters is not trivial

DES

10% for higher is that they and requires

PItECLPIT_.\TION

.\ND

concentration

levels. The major

drawback

arc not commercially some precautions.

available

their

while

of the synthesis

COPHIXIPI’i’ATION

Xlany precipitation procedures have been devised over the years for preconcentration in various analytical techniques. Although they may be less attractive for those techniques that require liquid samples, most can be combined vcv advantageously with s.r.s., particularly if they lead to a homogeneous surface toad on a thin filter. In analytical chemistry, much effort has been devoted to finding specific precipitation reagents. However, for a technique with a high inherent selectivity, like x.r.s., selective prccipitation is usually not necessary and non-specific multi-element reagents are more attractive. The simplicity of the procedure, the completeness of the precipitation and the inherent capacities available will be the most important features. In an effort to measure on infinitely thin precipitate layers only, Smiricz [ 921 first evaporated a 24-1 water sample, dissolved the residue, precipitated and tneasured the sulfides insoluble at pH 2, separated off the iron, and prccipitated and measured the sulfides insoluble at pH 8. This tedious procedure gave detection iimits around 1 pg I-’ . A preliminary investigation of the prcconcentration of a few elements from water as sulfides was also carried out by Ellgren [ 93]_ Alore common in lvater analysis is direct precipitation with an organic reagent, and direct measurements on the filtered precipitate. hlost popular, and undoubtedly also cstremely attractive as reagents for precipitation, arc the dithiocarbamates, in view of the low aqueous solubility of their metal chelates. The pioneering work of Luke [ 94 1 established the conditions for the determination of many trace metal ions by hydroxide precipitation and by coprccipitation with sodium diethyldithiocarbamate (NaDDTC, see technique involves adding DDTC and Fig. 2). Luke’s well-known “coprex” a spike of a suitable metal ion as a coprecipitating agent and as an internal standard, adjusting the pH and filtering off for x.r.s. measurements. \Vatanabe et al. 1951, Florkowski et al. [3] and Holynska and Uisiniek [96] studied the precipitation of several elements, including mercury, in XI ters by NaDDTC. They found that Na’, Ca” and Mg’+ did not interfere, while some interelement effects occurred for the transition metals; the detection limits for w.d.x.r.s. and radioisotope e.d.x.r.s. were 0.05-3 ~_cg1-l and s-10 r.lg I-‘, respectively. Moriyama et al. 197 ] reported that Cr, Mn, Fe, Co, Ni. Cu. Zn, Cd, Hg, Pb, etc.. could be determined in water as carbamates or hydroxides after precipitation with NaDDTC at pIi 9, with detection limits of 0.01-0.3 pg 1-l. For waste water, Sasuga et al. [98] recommended precipitation with DDTC at pH 4.2 and ultrasonic treatment to collect Co, Ni, Cu, %n, Cd and Pb quantitatively and uniformly on a

filter paper, with an 8% precision at the 0.1 ppm levc?l and detection limits of Z-40 and l- 5 ~g I-’ for w.d.x.r.s. and e.d.x.r.s., respectively. ‘I’anoue et al. [99J developed an automated device for DDTC preconcentration and x.r.s. measurement of waste water. Other Japanese articles arc available on this topic (see, e.g., [ 1001 )_ To lower the x.r.s. detection limits, the “copres” prcconccntration method has been combined with a “microdot” filtration procedure, in which the DDTC/hydroxidc precipitate is collected within a filtration area with a diameter of 100 mil, and measured in a separate sarnl,l~?-positioning setup ;vhere the sample is focussed visually relative to a collimator with a SO-mil aperture [ lOl]_ Others evaporated 500 ml of sea water do\vn to 100 ml prior to DDTC precipitation, in order to boost the sensitivity [ 1021 ) or first preconcentrated 10 1 of water on a cationeschangc resin, precipitated with NaDDTC and measured [ 1031. Knochel and Prange [ 1041 found that DDTC can collect many elements quantitatively in a homogeneous precipitate for direct x.r.s. measurements, or for subsequent clution by chloroform onto a special quartz support which is directly suitable for e.d.s.r.s. in the highly sensitive total reflection mode (see below). Leyden et al. [ 191 observed, however, that the recovery with DD’I’C decreases for lower concentrations and is certainly not quantitative at or ammoniumpyrrolidinedithiocarhamatc below 10 pg l-‘. In this respect, (APDC, see Fig. 2) seems generally preferable as a coprecipitating agent.

6n ;9s. Fig.

PAri

l!8- 1201 L?. Reagents

useful

for

(co)precipiLntiorl

prior Lo s.r.s.

1r1 gcncral, the type of dithioc:ubmete substituent influences the solubility of the chelates and their sensitivity to hydrolysis rather more than the selectivity of the comples formation which favours the transition metals in all cases. Ulrich and Hopke [‘i9], comparing several x.r.s. preconcentration mc!thods, concluded that :ZPDC at pH 4 was the best non-specific precipitating agent, .superior to DD’I’C for %n and Pb; they found adequate recoveries for Fe, Xi, Cu, Zn, Se, Pb, IIg, Cd, Ti, Cr, Th and TI, independent of the all~:~linc ion c*onc:entration level. Elder et al. [ 1051 had earlier precipitated at PI! 2 with a fresh ..WDC solution and obtained quantitative recoveries for 01, IIg and Pb, though not for Fe and Zn; but they noted a strong depression of zinc rc%coveries in natural water. In the analysis of polluted rivers by rac~ioisotopc-inducIcd e.d.s.r.s., hleicr and Unger [ 1061 reported a standard cieviation of ;<:‘Aat the 125 pg I-’ level and of 15% at the 15 pg I-’ level and tlc~tcction limits around 6 pg l-‘. for both DDTC and i!PDC precipitations. Several authors have tried to improve the precipitation characteristics by adding n carrier, as in the “coprcs” technique [ 94 I. In precipitation, the use of cilXTic?IX, or of a coprecipitation technique, is especially important when, in escccdingly dilute solutions, the solubility is not low enough to prevent c:r;rnpletc precipitation, or when the particle size of the precipitate is very small so that it passes through the filters, or when a supersaturated solution can esist. Pradzynski and coworkers [ 107, 1081 used _-IPDC and a Fe>*carrier, and reported e.d.x.r.s. detection limits of 1 pg 1-l for V, Cr, Mn, Cu, Zn,
carrier. for x.r.s.

In a very [ 58,

66,

recent 95,

study,

105,

114,

where seven preconcentration 116,

1381

were compared

procedures for sensitivity,

li

precision, detection limit, linear range and interferences by concomitant ions and salts. I.cyc!en et al. [ 201 recommended the latter l)UDTC procipitation technique as being the? most valuable!. They inwstigatcd the trace c?lcnwnts Cr, Mn, Fe, Co, Ni, Cu. Zn, As, Se, Sh, IIg!, ‘1’1. ..\g, Cd and Pb, and found nearly quantitative recoveries (csccpt for Cr and As) in variety of field-sampled waters; the c.d.s.r.s. detection limits w~rc l-5 pg 1 I for IOOml wakr samples. Various other multi-clement col,rc~cipit;itin!:~~ ;lgellt.S IlilVC IJCC~ 1xoposc.d. each with specific advantages ulld drawbacks wlativc! to the dithiocarham;~tcs. Panayappan et. al. [ 116) reported that. a combination of l)olyvinyll>yrrolidinc and thionalide as prccipitatin g agents for s.r.s. can provide particx~lnrly rapid and quantitative prec~onc~entratiolls, not influenctxl by Ca, Alg and alkali metal ions. .A combination of l,lO-I>henanthrolinc~ and tc~tral~hc~nyll~oroti also appears to c:ollcct most transition metsal ions cluic:kly and cIuantitativi?l~ at pH 5 without interferences from Ca and AIg, and to rtllow e.cl.s.r.s. clet~~c:tion limits of 0.1 ,rg, e.g., in no-nil water samples [ 117 ] _ l-(2-Pyridylazo)-2-nal~t~thol (I’_-\S. sect Fig. 2) is anotllcr clic~laiing ag:ont \vhich forms strong insoluble c:hclates with many transition metal ions: I’_-1N but not in cold water. l’iischcl is very soluble in hot water and Fttianol, introcluced the use of l’,\N for prclc:onr.clitr;ition purpuscts anti I1181 \Vatanahc et al. [95] applied it in combination with \v.d.s.r.s. Coc~rystallizations with PL\N have been found to be very c:onvenicant and cafficicnt ( 1191 _ _.lddition of 20 mg of P.-W to a neutral 2-l wattxr sampler at 707C. and filtration after cooling leads to nearly quantitative rwoveric5 of at least 15 cntiolls. independent of the sample salinity and, up to tlic lOO+eq calxxity of 20 rng of I’.-\N, of the ion cx~nwntration. Enrichmc~nt. factors as high as 10’ can be obtaincxcl routinely; cwunting statistics would thcBn allo~v 6B.cl.s.r.s. detection limits of 0.03 pg 1 ’ but the P;\N blank levels lcacl to realistic: limits c:an be a~hievcd at thct 10 .ug 1 ’ around 0.5 /lg 1-l . _-I precision of 5-107 level. Some disturbance by high contents of humic! material and, e.g.. sulfidca has been noted [ 1201. For waters with ;I wlatively constant comlwsition like seawater, however. this method is quite attractive. _Although there is an estensive literature on coprec:ipitation with hydrated iron osidc, this method has seldom heen uscad in s.r.s. work. Yet, Brunins and coworkers [ 121, 1221 claimed that 2-10 mg 1 ’ I*‘t?’ would quickly collect even non-ionic forms of dissolved transition metals cafficiclntly wirhout interfcrcncc? from Ca, Xfg, etc., while the I:t?,-I<,, s-ray fan bc uwd 3s an internal standard; thcay showed espcrimentally than Zn” and I’b” could he measured by w.d.s.r.s. in polluted river water. In a recent study, it was found that hydrated iron aside can ixldecd scavcngc: Xi”. Cu’)+, %n” and 1%” from is not significantly inseawater adjusted to pII 9 [ 1231. The collection fluenced by the major components. It depends on the concentrations of both the iron carrier and the transition metal, ;\Jrtunakly in a very predictabic way. Indeed, the ratio of the resulting trace metal concentration in the precipitate to the remaining concentration in solution is constant. and il

1S

the !ogarithms of these ratios are equal to 4.1 i 0.1 for MnZ’, 5.1 ? 0.1 for Co’+ , 4.9 ” 0.1 for Ni’+, 5.7 .! 0.1 for Cu”, 6.5 A 0.1 for %n?-+, 4.5 2 0.1 for Cd” atid >5 for Pb” and rare earth ions, at pH 9. This observation allows a convenient compromise to be achieved between a minimal amount of iron, hcttlce high enrichment factor and low detection limit on the one hand, and a lligh collection yield, hence optimal precision and accuracy on the other hilnd, and to make corrections for inadequate coprecipitation. In routine practice in this laboratory, the use of 10 mg 1” Fe in 200-ml samples of pH 9 is preferred; collection yields are above 90% at 10 ~g 1.’ metal concentmtions (esccpt for hlnz” and Cd’+), the e.d.s.r.s. precision is around 5%. and cictctction limits are around 0.4 pg I- ’ for a 3000-s counting time. The environmental applicability of this approach is still under investigation. Other multi-element coprccipitation agents that have been considered for s.r.s. are 8-quinolinol (osine) [ 1241 (which is less attractive because of incomplete transition metal collections from polluted waters and because it also precipitates Big”, which can lead to variable enrichment factors), alizarin blue, phenylfluorone, cupfcrron, fluorides, tellurium. etc. [ 1251. Dithiols are promising as more selective prccipitants. For example, \i’atanabe and Ueda [ 126 J studied the use of 6-aniline-1,3,5-triazinc-2,4tlithiol either alone or in combination with benzyldimcthyltetradccylamtnonium chloride for enrichments of Cu, Cd and Pb; they determined these three elements in hot-spring waters at levels from several to several ten pg 1-l t;y w.ci.s.r.s. In general, although the literature abounds with precipitation enrichment procedures, many of which are truly attractive for combination with x-r-s., :I common failure of most publications is the lack of thorough and systematic checking of the performance of these procedures for waters containing abundant a!kalinc earth and alkali ions, examination of complications caused by humic material, and the effects of variable speciation of the trace metals. Although acidification to a low pH as required in some procedures (e.g., APDC precipitation) might overcome some of these problems, at the cost of a higher c:ontamination risk, and although U.V. irradiation prior to preconcentration might often be helpful [ 1271, serious checking of the procedures, prior to their routine application in environmental problems, should be regarded as mandatory. LIQUIELIQUID

IXSTRACI’ION

Solvent extraction is undoubtedly the most popular routine preconcentration method, e.g., in atomic absorption spectrometry. Many monoeaphs and reviews on this technique are available, and an excellent general survey by B’richmann [ 1281 refers to much of the work. Extraction procedures are usually simple and rapid, and they may be automated fairly easily. Yet they are not commonly applied in x-r-s., mainly because subsequent evaporation is necessary and because the preconccntration coefficients are usually quite low.

19

added to 250-ml water samples 5 ml of pII 5 buffer, and 5 ml solution, extracted the transition metals after equilibration for 5 min into three s-ml portions of chloroform, and carefully evaporated these on a filter paper carrier. The capacity was ;$OO ~cq of divalcnt transition metals. Similarly, Iwasaki et al. [ 1301 estractcd microgram quantities of various metals with DDTC from solutions buffered at pI1 5-6 into chloroform or with cupferron into chloroform from hydrochloric acid solutions. and dropped the extract on a filter paper. Kuroha and Shibuya [ 131 ] extracted with DDTC from p1.i 5-9 medium into carbon tetrachioride, added polystyrene and an internal standard to the estract. and dried it under i.r. radiation on a h?ylar foil to obkn a very thin film for w.d.s.r.s. ‘The> achieved detection limits down to 0.03-l ~g for 1 ;_j clctments. with a 3-S:; coefficient of v;uiation at the 20-pg Icvcl. Florkowski et al. [ 31 rcportod a few espcriments, using extraction with 8-quinolinol into chloroform or \vith trioctylatnine into xylenc for radioisotopcescitc~d s.r.s. m(~asuremcnts on polluted waters at the l-5 mg 1-l level. An ingenious way of target preparation was proposed by %la&yar and Lobanov [ 1321 who extracted into molten S-quinolinol, and subsequently dried, remelted, cooled, ground and pcllctized the organic phase. Such a method partially avoids the problems inherent in separating a small volume of organic phase from the aqueous sample and in adhesion of the organic: solvent to the wall of the vessel. The procedure is claimed to be useful for trace metal determinations in fresh water, but the enrichment factors arc typically 1+50 only. Similarly, Kawase et al. [ 1331 estracted the .-\P!X chelates of Ni, Cu, Zn, Cd, 1% and Hi from hot solutions into disks, sild measured by c.d.x.r.s. In general, again, very little attention has been paid to specific problems that might be encountered in extraction prcconcentration for natural waters. In a recent intercomparison study of preconcentration methods, disa[)pointing results were often found for APDC cstractions from natural waters [ 171 . Make [ 1291 of a 2% APDC

CHELATION

AND SUBSEQUENT

SORPTION

I~I~IOUII.I%:\‘I’ION

While liquid-liquid extraction involves handling large volumes of cstract and time-consuming evaporations or another target preparation step, rcversctdphase techniques with organic solvents adsorbed on the surface of a small particle support seem more interesting. Such processes correspond to repeated extraction, and quantitative yields can therefore be espccted even with relatively low distribution constants. In their original approach, Knapp et al. [ 1341 formed the transition metals chelates with NaDD’I’C and subsequently adsorbed the chelates onto Chromosorb \V-DhlCS columns. They observed complete sorption of Fe, Co, Ni, Cu. Zn, Cd, Hg and I’b at pII :1 or 5. The carbamates were eluted with 2 ml of chloroform onto a filter paper in a measurement. For loo-ml samples, special tcflon container for w.d.s.r.s. the detection limits were O.l- 1.1 pg.

Such reversed-phase techniques with subsequent elution can be combined advantageously with e.d.s.r.s. by using a totally reflecting sample holder. In this technique, because of the small angle of incidence of a few minutes of arc, the primary incident rays almost cannot penetrate the sample support_ Thus the usual scatter4 irradiation caused by the sample support is almost

completely eliminated and determinations of lo-” g become possible. In water analysis, Kniichcl and Prange [ 104, 1351 formed the transition metal chclatcs with DDTC, adsorbed the chelates on a Chromosorb column and clutcd with ch!oroforrn as a thin film onto a totaNy reflecting quartz glass support. The yield for many elements was nearly quantitative, rcproducibilitics of 3. 6% were achieved in spite of the deposit not being homogeneous, and the detection limits calculated from peak-to-background ratios were typically 0.01 pg 1-l; but even with special precautions, the blank contributions increased the detection limits to 0.02-0.3 ~g I-‘. Yet, Fe, Ni, Cu, Zn, IIg and 1% could he determined conveniently in seawater in this way. Knoth and Schwxnke [ 1361 similarly formed the transition metals-APIA: chclat,es ilfter pipetting a 100~j.fl samplct volume onto a siliconizcd quartz glass support, rinsing off the unreactctd material and measuring by total reflection ct.d.s.r.s.; they obtained quantitative collections of numerous elements, even from 3% SaCI solutions, and clctection limits at the 20 pg level (i.e., 0.2 pg 1-l). Another promising procedure consists in chclatiorl followed by immobilization on activated carbon, the most traditional of all adsorbents. Activated charcoal is known to be a good adsorber for organic and colloidal material, and SO probably also for the species of trace metals that arc bound to naturally occurring organic: and colloidal matter. Free ions are not quantitatively adsorbed onto activated carbon, but addition of a chelating agent will conv.xt them to an adsorlxlble form. The addition of a chelating agent and the su bsequcnt adsorption of activated carbon should thus constitute a powcrful technique for collecting both originally free and colloidal and organic trace metal species. Jackwcrth et al. [ 1371 were probably the first to esploit this idea for preconcentrations in atomic absorption spectroscopy. It has been concluded [ 1381 that. 8-quinolinol is a particularly suitable multiclement chelating agent, because the chelates eshibit high stability constants at nearly natural pl4 levels for many transition metal ions, but not for alkali and alkaline earth ions; further, the S-quinolinolates can be adsorbed onto activated c‘arbon very efficiently and straightforwardly. ‘l’he optimal preconcentration procedure depends somewhat on the type of water under test but consists roughly of adding 10 mg of 8-quinolinol per liter of water sample at pII 8 (either adding solid 8-quinolinol and heating the sample to 6O”C, or adding a 10% 8yuinolinol solution in acetone), adding 100 mg of activated carbon after nn equilibration for 30 min. and filtering off the Quantitative recoveries with enrichment factors near 10 000 suspension. were demonstrated for about 20 ions frotn various media independent of the alkali ‘and alkaline earth content. When the filters loaded with 8quinolinol/

‘2 1

activated carbon were examined by c.d.x.r.s. [ 1391, the responses were linear up to 1 mg I-’ metal concentrations, and the reproducibility was 5%. In synthetic or natural waters containing up to 10 mg 1-l of humic sut)Shices, quantitative recoveries can be maintained [ 124 j , even for the trivalent ions that are strongly bound to humic matter, if sufficient activated carbon is used, i.e., if some x.r.s. sensitivity is sacrificed. The results obtained on natural waters via the 8-quinolinol chelation/activated carbon adsorption procedure are somewhat higher than those obtained by other preconccntration procedures, because collection of the naturally occurring organic: and colloidal trace metal species is more complete. ..\lthough this procedure is not as simple as some other prcconcentrntion techniques. and requires some previous knowledge of the major ion composition of the water sample! (at least if optimal sensitivities are to be reached), it is considered very v;~luzlblc in every case where organic: or colloidal material effects could intctrfcre with the recovery of the total transition metal concentration, and its merits \vcrc established quite well in intercomparison studies [17-l 9; . Recently, Johansson and Aksclsson [ 1401 used osine or APDC chelation and activated carbon adsorption in combination with p.i.s.e. analysis to obtain typical detection limits in the range 0.02-2 pg 1-I for brackish and distilled water, while Fou [ 1411 qualitatively detcctcd trace elements in household tap water by p.i.s.e. after a, probably incomplete, direct collection in an activated charcoal filter.

Preconcentrations by clcctrodeposition imply a very diffcrcnt approach and somewhat more sophisticated equipment. Cathodic deposition has been from simple soluapplied in combination with s.r.s. for preconcentrations tions. Vassos et al. [142] determined 2-40 ~g of Cr. Co, Cu, Hg, Ni or %n by constantcurrent etc?ctroc~c~positioll on 1 cm diameter graphite-rod elec:trodes, in 15 ml of solution containing low concentrations of supporting electrolyte to simulate natural waters. ~Iarsliall and Page [ 143 1 determinc?d Cd, Ni and Zn by e.d.s.r.s. of deposits prepared by flow electrolysis on graphite cloth electrodes, but warned that the mccl~anisms of tile elcctrodeposition of alloys and metal mistures must be studied before this approach can be generally applied in the analysis of water samples. \Vtindt

et al.

[ 1441

proposed

a procedure

for transition

elements

h:sd

on electrodeposition of the anionic organo-i!oml,lc~scs from mised organit: aqueous media in a high potential electric field, and tested it on comples synthetic samples simulating river water. They first collcc:tcd all cations at pH 2 on Dower 50-D’ cation-exchangct resin, and then c1ut.A the anionic cyanoand oxo-complexes of transition metals with a potassium cyanide solution at pII II (while Ca” and Mg” remained on the resin). On addition of about a 0.8 mole fraction of 2-propanol (to increase the solute solvating power while the dielectric constant was kept low), the complexes of Co, Ni, Cu, Zn and Cd were deposited in a high potential field (300-l 500 V) onto anode. Good collection yields and detection limits around an aluminium

m F

i

c

_....

..--.I -*._._-l_L._-___--___._______

mrricr

if Fe cone, <#IO

ppb),

Add to 1 I water: 50 ml tI,SO,,

Reduc!ion by thourcnf I&SO, refluxing, adsorption of SCon activated carbon (for total dissolved SC) 1 g thiourcn nnd 100 mg activated carbon, rcflux for 15 min, filCcr; c.cf.x,r.s.

Add 10 1 11silmpk?of j$l 2 sotne 3 g ascorbic acid antI 100 mg iIcliViIkti citrbon, slir for 15 min, filler; c.tl .x.r.s,

stir for 30 min, filler; radioisotopcexcilcct c,d.x.r.s.

.__-

111-

River, lake, ground tmtf drinking water, SCilwater, rain

River, Inkc, ground Wd drinking WittCr, scn wntcr, rain

tX!il~CtIt COtlCS. iIlltl tyl)C,

atIt

166

1G5

IL!f.

Procedure Oi)~in~i~c(j with respccl to 167 all reduclion reaction parameters, No infhtcncc of sample salinity, Fe” and humic material. Lincnr range O.O&-1Oppb for ScO,“- and ScO,‘-. s = 25% at 0.1 ppb, 11% for 0.1-l ppb, 5% for l- 25 jtpb, 3% for 100 ppb Se DL = 0.06 ppt Se (3000 s counting) Procedure applied in ~nvironmen~i screening [ 168 J, in the range 0.06-2 ppb SC

rcnction time. No influcncc of various Sillls, Itumic miltcriid rlrld Lincnr range 0.05-50 ppb ScO,‘-. s = 6% at 0.5 ppb. DL = 0.05 ppl) ScO,“” (3000 s cnunting) Procedure applied in c~~viro~~tne~~l~l screening [ 1681, in the range 0.05-2 pph SC(W)

pfl,

I’roccdure optitnizcd with respect lo

Linc;1r Km@ 4-l 00 pj,h DL = 0.G ppb (2000 s counting) s = 4.1%;11 10 pph

~t.ll(li~tl l)~lcrt~~itt~li~i)chnmcterisbics”

~l~virnllti~~itt~tl waler

._

Add to 500 ml water of plI 4 100 mi: APDC (anti 200 rl: Fel’

_.--_

‘Type Of Wilier

.“_

I’roccdurc

--__..

Rctjuclion will1ascorbic acid, adsorption of SCon activated cnrbon (sclcctivc for ScO,‘-)

_____m.----.--

l’ABLI2 2 (cotttinucti) --_-

g

27

30

1 pg I-’ were claimed. It is clear that the various reagent additions imply serious contamination risks, and that the environmental applicability of this approach requires further demonstration. P~~~CONCE~‘rRr\TIONS

OF SINGIX

EIXXIENTS

In view of its inherent selectivity, x.r.s. is naturally most appropriate for multi-element determinations. Yet it can, of course, be used as a detection method after a highly selective prcconcentration for determinations of single elements. Many such examples are available in the literature for various fieids of application. The literature pertaining to applications of x.r.s. in water analysis for it single element or only two elements simultaneously is summarized in Table 2. The detail in the summary reflects to some extent the degree of detail in tht: original publication. Of course, many x.r.s. procedures for various matrices often also pass through a dissolution step and an aqueous phase. In Table 2, however, only those methods are included that deal directly with natural waters, or were developed for the purpose of water analyses, or pertain to concentration levels that are realistic in some waters. It appears, not unexpectedly, that most of these single-element procedures have been proposed for uranium assays. Also, several references are available for the halides, for sulphur, selenium and chromium species, and for a few other ions. ‘The characteristics of these procedures, as included in Table 2, allow a relative evaluation in each particular case. CONCLIXION It must be emphasized that this literature review did not reveal any panacea for preconwntration prior to x.r.s. in water analysis. Indeed, no method is simultaneously extremely sensitive, applicable over an infinite concentration range, free of interference and matrix effects, capable of determining all elements in their different oxidation states and chemical speciations, and wry simple, fast and economic. Moreover, a method of choice cannot be sclectcd in a straightforward way, because too much depends on the type and variability of samples, on the number and concentration of elements to be assayed, and on the relative importance that one attributes to speed, quality and cost of the analysis. hlainly because of the confusion, size and complexity of the literature, many rescnrch laboratories are now seen to embark on developing a preconcentration procedure of their own rather than applying or adapting a published method. It is hoped that the present wide review will be of some help to theanalytical chemist who has to make an intelligent selection of a preconcentration method when faced with application of s-ray spectrometry to water samples.

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Hes./L)ev.,

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