Chapter II Sample Handling in Ion Chromatography

33 CHAPTER

I1

SAMPLE HANDLING I N I O N CHROMATOGRAPHY P. R. HADDAD

1. 2. 2.1 2.2 2.3 2.4 3. 3.1 3.2 3.3 3.3.1 3.3.2 3.3.3 3.3.4 4. 4.1 4.2 4.3 4.4 4.5 5. 5.1 5.2 5.3 5.3.1 5.3.2 5.3.3 5.3.4 5.3.5 5.3.6 5.4 6. 6.1 6.2 6.3 7. 8.

Introduction Sample c o l l e c t i o n and d i s s o l u t i o n Sample c o l l e c t i o n E x t r a c t i o n methods Sample d i g e s t i o n Combustion methods Sample cleanup methods Introduction F i 1t r a t ion Chemical m o d i f i c a t i o n o f t h e sample Batch methods u s i n g ion-exchange r e s i n s D i a l y t i c techniques D is pos a b l e c a r t r i d g e columns Chemical r e a c t i o n o f s o l u t e s Contamination e f f e c t s Introduction Contamination from p h y s i c a l h a n d l i n g o f t h e sample Contamination from f i l t r a t i o n d e v i c es and c a r t r i d g e columns Contamination from chromatographic hardware components Contamination o f t h e column Sample h a n d l i n g f o r u l t r a - t r a c e a n a l y s i s Introduction Use o f l a r g e i n j e c t i o n volumes Use o f p r e c o n c e n t r a t i o n columns Hardware c o n s i d e r a t i o n s Choice o f e l u e n t C onc en t r a t o r column c h a r a c t e r i s t i c s A p p l i c a t i o n t o samples o f l o w i o n i c s t r e n g t h A p p l i c a t i o n t o samples o f h i g h i o n i c s t r e n g t h Conclusions Use o f d i a l y t i c p r e c o n c e n t r a t i o n methods M a t r i x e l i m i n a t i o n methods Introduction On-column m a t r i x e l i m i n a t i o n Post-column m a t r i x e l i m i n a t i o n Conc 1us ion Acknowledgement References 1.

INTRODUCTION

The t e r m " I o n Chromatography" was f i r s t i n t r o d u c e d i n 1975 and was o r i g i n a l l y i n t e r p r e t e d t o mean t h e chromatographic a n a l y s i s (by i o n exchange) o f i n o r g a n i c i o n s . Throughout t h e e a r l y y e a r s o f i t s development,

i o n chromatography was concerned c h i e f l y w i t h t h e d e t e r -

m i n a t i o n o f i n o r g a n i c anions because i t p r o v i d e d t h e most convenient

34

means of analysis for these species. However, the technique now embraces a much wider range of solutes and separation methods and some overlap with alternative liquid chromatographic methods currently exists. It is fair to say that ion chromatography can be considered to include any liquid chromatographic procedure used for the determination of ionic and ionisable solutes. Fig. 1 shows the main separation modes applicable to i on chromatography.

ION SUPPRESSION

A

Weak Acids

Fig. 1

Weak Bases

ION INTERACTION

A

Anions

Cations

ION EXCHANGE

ION 1:XCI.USION

rJl Anions

Cations

Weakly Ihsrociaad Anions

OrpllC

Acids

Separation modes for ion chromatography.

Ion suppression involves adjusting the pH o f the mobile phase so that the solutes of interest are non-ionised (or at least only partially ionised). In this form, these solutes can be separated by conventional reversed-phase 1 iquid chromatography and the technique of ion suppression is most successful for weak acids or bases. Ion-interaction methods are applied to the separation of fully ionised solutes by reversed phase HPLC and involve addition to the mobile phase of a relatively hydrophobic ionic species with a charge opposite to that of the solutes of interest. This species is called the "ion-interaction reagent" and serves to convert the non-polar stationary phase into an ion-exchanger suitable for the separation of the desired ionic solutes. Ion-exchange chromatography is a well established technique in which the stationary phase contains ionic functionalities and the solute ions are separated using an eluent which contains a competing ion of the same charge sign as the solute ions. Classical ion-exchangers are not suitable for ion chromatography because their high ion-exchange capacities require the use of high ionic strength eluents, and this creates problems with detection of the eluted ions. For this reason, a new generation of high efficiency, low capacity ion-exchangers has evolved for ion chromatography. It has been common to subdivide ion-exchange methods into those using chemical means to suppress the background conductance of the eluent, and those using electronic means to achieve the same result. Since this classification is

35 somewhat a r b i t r a r y , no d i s t i n c t i o n between t h ese two approaches w i l l be made i n t h i s c ha p t e r . The f i n a l s e p a r a t i o n mode, i o n - e x c l u s i o n chromatography, i n v o l v e s t h e s e p a r a t i o n o f s o l u t e i o n s u s i n g an ion-exchanger w i t h f u n c t i o n a l i t i e s h a v i n g t h e same charge s i g n as t h e s o l u t e s . Donnan e x c l u s i o n , s i z e e x c l u s i o n and hydrophobic i n t e r a c t i o n s a l l p l a y a r o l e i n t h e s e p a r a t i o n o f t h e i n j e c t e d s o l u t e s , and t h e t e c h n i q u e i s most successful

for

partly

i o n i s e d species.

Full

details of

all

o f the

above

s e p a r a t i o n modes can be found elsewhere ( r e f s . 1, 2). There e x i s t s a g r e a t v a r i e t y o f d e t e c t i o n methods f o r i o n chromatography and t h e main approaches

a r e l i s t e d i n F ig.

2.

Conductivity

d e t e c t i o n i s most p o p u l a r because i t i s u n i v e r s a l l y a p p l i c a b l e , s e n s i t i v e and convenient t o use, however a l t e r n a t i v e u n i v e r s a l methods such as i n d i r e c t UV a b s o r p t i o n a r e becoming w i d e l y used.

I n many i o n chromato-

gra phic d e t e c t i o n modes, t h e e l u e n t p r o v i d e s a s i g n i f i c a n t d e t e c t o r s i g n a l and i t i s t h e r e f o r e h e l p f u l t o d i f f e r e n t i a t e between d i r e c t d e t e c t i o n ( i n which t h e d e t e c t o r s i g n a l f o r t h e s o l u t e exceeds t h a t o f the eluent)

and

indirect

detection

( i n which t h e r e v e r s e s i t u a t i o n

a p p l i e s ) . Indeed, most o f t h e d e t e c t i o n methods shown i n F ig. 2 can be employed i n e i t h e r a d i r e c t o r i n d i r e c t mode w i t h an a p p r o p r i a t e c h o i c e o f e l u e n t . A d e t a i l e d d i s c u s s i o n o f t h e t h e o r y and a p p l i c a t i o n s of

ion

chromatographic d e t e c t i o n methods i s a v a i l a b l e elsewhere ( r e f . 3 ) . I n t h e c o n t e x t o f sample h a n d l i n g i n i o n chromatography, a p r i m e c o n s i d e r a t i o n i s t h e d i v e r s i t y of s e p a r a t i o n and d e t e c t i o n methods des c ribed above.

A j u d i c i o u s s e l e c t i o n o f t h e s e p a r a t i o n and d e t e c t i o n

modes, and t h e e l u e n t used, can o f t e n mean t h a t sample p r e p a r a t i o n i s minimal.

For example, ion-exchange p r o v i d e s good s e p a r a t i o n of charged

species, w i t h uncharged o r p a r t i a l l y charged species e l u t i n g as a group a t t h e column v o i d volume. I n c o n t r a s t , t h e r e v e r s e a p p l i e s t o i o n e x c l u s i o n chromatography where f u l l y i o n i s e d species a r e u s u a l l y t o t a l l y excluded and e l u t e a t t h e v o i d volume, uncharged s o l u t e s a r e r e t a i n e d .

whereas p a r t i a l l y charged and

I n o t h e r words, t h e t wo t echniques a r e

b e s t s u i t e d t o d i f f e r e n t sample m a t r i c e s . The same can be s a i d f o r t h e d e t e c t i o n mode used, s i n c e many s o l u t e s can be d e t e c t e d s e l e c t i v e l y and t h i s g r e a t l y reduces t h e r e q u i r e m e n t f o r sample p r e p a r a t i o n . F or example, n i t r a t e and n i t r i t e can be determined i n cured meats u s i n g a s i m p l e aqueous e x t r a c t i o n coupled w i t h ion-exchange

s e p a r a t i o n and d i r e c t UV

a b s o r p t i o n d e t e c t i o n a t l o w wavelength. Under t hese c o n d i t i o n s o t h e r i o n s i n t h e sample, p a r t i c u l a r l y t h e v e r y h i g h l e v e l s o f c h l o r i d e p r e s e n t , a r e n o t d e t e c t e d ( F i g . 3). separation

s h ould

Finally, the

e l u e n t f o r t h e i o n chromatographic

be s e l e c t e d on t h e b a s i s o f e l u o t r o p i c s t r e n g t h , SO-

36

lubility of the sample, pH, compatibility with the detection mode used, and chemical reactivity with the sample.

Fig. 2

Detection modes for ion chromatography.

[OOSA"

L I

I)

I

5

10

Time lmml

la1

Fig. 3

I

15

I

0

[0054"

L 5

10

Time lminl

Ibl

1

15

I

0

10 5 lfme imlnl

I

15

ICI

Separation of nitrite (A) in nitrate (B) in standards (a), bacon sample (b), and spiked bacon sample (c). The sample f o r (c) was spiked with 0.5 ~9 of both nitrite and nitrate in a 25 jd injection. Column: Vydac 302 IC 4.6. Eluent: 11.0 mM methanesulphonic acid at pH 5.0. For chromatogram (a), 10 pl o f a 50 ppm solution of nitrite and nitrate were injected. Reproduced with permission from ref. 4.

It is thus pertinent to begin this discussion of sample handling in ion chromatography by emphasising that the correct choice of separation and detection modes is imperative if laborious sample preparation

37 procedures are t o be avoided. The chromatographer must t h e r e f o r e be f u l l y conversant w i t h the a l t e r n a t i v e s a v a i l a b l e . The sample handling methods discussed i n t h i s chapter should be viewed as a secondary means o f ensuring the success o f t h e i o n chromatographic analysis. 2.

SAMPLE COLLECTION AND

2.1

SAMPLE COLLECTION

DISSOLUTION

The main concerns when c o l l e c t i n g a sample f o r any a n a l y t i c a l method are t h a t the sample taken i s representative o f t h e m a t e r i a l t o be analysed and t h a t no contamination occurs during the sampling process. S t a t i s t i c a l l y based procedures f o r a c q u i s i t i o n o f a representative sample have been t r e a t e d extensively i n numerous t e x t s and f u r t h e r discussion i s beyond the scope o f t h i s chapter. Contamination i s a very important issue and i s discussed separately i n section 4. Some sampling procedures used f o r i o n chromatography are det a i 1ed be1ow. Gas samples can be conveniently c o l l e c t e d using s o l i d sorbents packed i n t o s u i t a b l e sampling tubes, A known volume o f gas i s drawn through t h e tube and the desired sample components a r e s t r o n g l y adsorbed. One example o f t h i s approach i s the adsorption o f formaldehyde onto charcoal impregnated w i t h an o x i d i s i n g s o l u t i o n o f p r o p r i e t a r y composition ( r e f . 5 ) . The adsorber tube (Fig.

4) comprises a main adsorbent and an a u x i l i a r y o r

back-up adsorbent designed t o detect breakthrough o f formaldehyde from the main adsorbent. Formaldehyde reacts w i t h t h e o x i d i s i n g s o l u t i o n t o produce formate ion, which i s desorbed using d i l u t e hydrogen peroxide s o l u t i o n , p r i o r t o i o n chromatographic analysis. An important aspect of gas analysis i s the procedure used t o provide c a l i b r a t i o n standards. Some standards are a v a i l a b l e commercially, o r a l t e r n a t i v e l y a sample generator can be employed.

Fig. 5 shows one type o f sample generator i n which

standard s o l u t i o n s are i n j e c t e d

i n t o a f l o w i n g stream o f

subsequent evaporation and deposition onto an adsorber tube.

100 mg

50'mg

Impregnated charcoal

Fig. 4

S o l i d sorbent tube f o r sampling o f formaldehyde i n a i r . Reproduced w i t h permission from r e f . 5.

air

for

38

INJECTION PORT

sFbl!?,

11

GLASSWOO’L PLUG

PUMP

& Fig. 5

n

n

L.

^. ..

I

GLASSBEADS

U

WATER BATH NEEDLE VALVE

Sample generator f o r c a l i b r a t i o n o f sorbent tubes. Reproduced w i t h permission from r e f . 5.

D i r e c t absorption o f a sample gas i n t o a l i q u i d f i l m has been reported f o r t r a c e l e v e l s o f ambient n i t r o g e n d i o x i d e ( r e f . 6 ) . Here the absorber s o l u t i o n was guaiacol which was deposited as a methanolic s o l u t i o n i n t o the annular c a v i t y between two c o a x i a l glass tubes, before evaporation o f the methanol t o leave a t h i n f i l m o f guaiacol on the f r o s t e d surface o f the glass.

A f t e r passage of a known volume o f a i r , t h e apparatus was

dismantled and the absorbing s o l u t i o n removed w i t h water and analysed f o r n i t r i t e by i o n chromatography.

Complete absorption o f n i t r o g e n d i o x i d e

was observed and t h e r e was no detectable o x i d a t i o n t o n i t r a t e i o n i n t h e absorbing s o l u t i o n . I n a more simple approach, pyruvic a c i d and methane sulphonic a c i d have been determined i n a i r by passage o f t h e sample through a microimpinger, using e i t h e r d i s t i l l e d water o r d i l u t e potassium hydroxide as the absorbing s o l u t i o n ( r e f . 7 ) . 2.2

EXTRACTION METHODS

The removal o f i o n i c species from s o l i d samples p r i o r t o i o n chromatographic analysis can o f t e n be achieved by aqueous e x t r a c t i o n o f t h e homogenised sample. This process r e l i e s on t h e high s o l u b i l i t y o f i o n i c species i n water. Generally a weighed amount o f t h e d r y sample i s mixed w i t h a known volume o f water, e x t r a c t a n t s o l u t i o n o r eluent, and homogenised i n a blender o r an u l t r a s o n i c c e l l d i s r u p t e r f o r a s p e c i f i e d time. The d i g e s t i s then f i l t e r e d , required

and

injected

onto

the

subjected t o f u r t h e r cleanup where ion

chromatograph.

The

choice

of

e x t r a c t i n g s o l u t i o n i s very dependent on t h e sample m a t r i x and t h e nature of the s o l u t e ions t o be extracted, however water i s t h e prefered ext r a c t a n t whenever possible because a l t e r n a t i v e e x t r a c t a n t s o f t e n i n t r o duce extraneous peaks i n t o t h e chromatogram.

Use o f

eluent as

the

e x t r a c t a n t i s successful o n l y when small i n j e c t i o n columes are t o be used

39 i n t h e f i n a l a n a l y s i s s i n c e t h e presence o f e l u e n t i o n s i n t h e sample prec ludes band compression a t t h e head o f t h e column, w i t h subsequent l o s s of chromatographic e f f i c i e n c y t h r o u g h s o l u t e d i s p e r s i o n . Some samples r e q u i r e e x t r a c t i o n w i t h o r g a n i c s o l v e n t s b e f o r e t h e y a r e s u i t a b l e f o r a n a l y s i s . Fo r example, commercial bromine s o l u t i o n s produced from seawater c o n t a i n h i g h l e v e l s o f c h l o r i d e i o n and t h e a n a l y s i s o f t h e c h l o r i d e can be performed a f t e r d i s s o l u t i o n o f t h e sample i n pot assium bromide s o l u t i o n , f o l l o w e d by e x t r a c t i o n w i t h carbon t e t r a c h l o r i d e ( r e f .

8). Free bromine i s e x t r a c t e d and t h e r e m a i n ing aqueous s o l u t i o n can be analysed d i r e c t l y by i o n chromatography u s i n g c o n d u c t i v i t y d e t e c t i o n . Methanol e x t r a c t i o n o f tetramethylammonium i o n f rom s h e l l f i s h has been r e p o r t e d ( r e f . 9), b u t t h e methanol must b e evaporated and t h e sample r e dissolved i n hydrochloric a c i d before i n j e c t i o n .

2.3.

SAMPLE DIGESTION

When samples ( p a r t i c u l a r l y s o l i d s ) a r e n o t amenable t o simple aqueous e x t r a c t i o n , i t becomes necessary t o d i g e s t t h e sample t o o b t a i n a q u a n t i t a t i v e measure o f t h e i o n i c components. T r a d i t i o n a l l y , sample d i g e s t i o n p r i o r t o a n a l y s i s has been performed u s i n g c o nc ent rat e d a c i d s , used e i t h e r a l o n e o r i n m i x t u r e s . T h i s approach i s g e n e r a l l y i n a p p l i c a b l e t o i o n chromatography because o f t h e l a r g e

excess o f t h e a c i d a n i o n ( s ) i n t r o d u c e d and t h e r e s u l t i n g low pH of t h e sample d i g e s t . The excess a n i o n r e s u l t s i n column o v e r l o a d i n g and t h e appearance o f a m a j o r peak i n t h e f i n a l chromatogram, w h i l s t t h e low pH o f t h e d i g e s t can cause d i s r u p t i o n o f t h e m u l t i p l e e q u i l i b r i a e x i s t i n g between t h e e l u e n t s p e c i e s and t h e column, l e a d i n g t o severe b a s e l i n e perturbations. One s uc c es s f u l

F o r t h e s e reasons,

a c i d d i g e s t i o n has been r a r e l y used.

a p p l i c a t i o n however i s t h e d i s s o l u t i o n o f g e o l o g i c a l

samples i n phos p h o r i c a c i d p r i o r t o t h e d e t e r m i n a t i o n o f f l u o r i n e (as fluoride)

by

i o n chromatography

(ref.

10).

The

fluorosilicic

acid

produced i n t h e d i g e s t i s v o l a t i l i s e d and c o l l e c t e d on a s i m p l e condenser apparatus i n s e r t e d i n t o t h e d i g e s t i o n tube. The condensed f l u o r o s i l i c i c a c i d i s removed w i t h sodium h y d r o x i d e and i s convert ed t o f l u o r i d e i o n which i s t h en determined by i o n chromatography. An a t t r a c t i v e a l t e r n a t i v e t o a c i d d i g e s t i o n o f samples i s t h e use of f u s i o n techniques.

I n t h i s process t h e sample i s mixed w i t h a s u i t a b l e

f l u x m a t e r i a l and i s heated u n t i l t h e f l u x becomes molt en. The m i x t u r e i s then allow ed t o c o o l and t h e f u s i o n cake d i s s o l v e d i n a s u i t a b l e s o l v e n t and t h en analysed. T y p i c a l f l u x m a t e r i a l s i n c l u d e sodium h y d r o x i d e , sodium carbonate and l i t h i u m t e t r a b o r a t e ( r e f . 11). Once again, t h e main

40 problem with this method is the compatibility of the final digest solution with the ion chromatographic eluent, but in this case some of the fluxing materials are identical to eluent components. For example, sodium hydroxide (ref. 12) and sodium carbonate-bicarbonate (ref. 1) are common eluents in ion chromatography. Thus fluoride (ref. 13) and chloride (ref. 14) have been successfully determined in geological materials after fusion with sodium carbonate and injection onto an ion chromatograph using a carbonate-bicarbonate eluent, and boron and fluoride have been determined in glasses after fusion with sodium hydroxide (ref. 15). Fusion methods are generally quite time-consuming because of the necessity to redissolve the fusion cake and in some cases the high pH of the digest presents a problem. Further disadvantages are the limited applicability of the method, possibile interference from the high level of sodium or lithium present, and the loss of nitrate from the sample during fusion, presumably due to the formation of volatile oxides of nitrogen

.

COMBUSTION METHODS Analysis of some non-metallic elements in organic samples can be achieved by total combustion of the sample, conversion of the desired elements into gaseous compounds, collection of these gases in a suitable absorber and finally, i o n chromatographic analysis of the absorber solution. This approach is suited to the determination of halides (to form such products as HF, HCl, HBr and HI) and sulphur and phosphorus (which form SO2 and P205, respectively). The experimental conditions employed for the combustion determine the nature of the final products and in some cases, multiple products are formed for the same element. When this occurs, the composition of the absorber solution should be carefully chosen to convert all forms of an element to a single species suitable for ion chromatographic determination. The simplest apparatus for combustion of organic samples is a Schoeniger flask, as shown in Fig. 6. A Pyrex glass or quartz vessel containing absorber solution and a small amount of sample (about 0.1 g) in a paper cup is filled with oxygen and inverted. The sample is then electrically ignited and the gases produced are trapped in the absorber solution which provides a gas-tight seal at the mouth of the flask. After an appropriate amount of time has elapsed, the absorber solution is removed and analysed. The advantages of this method are that it is inexpensive, rapid and simple, whereas the major disadvantage is that the 2.4

41

oxygen pressure is limited to atmospheric pressure. This in turn limits the size of the sample which can be analysed and ultimately renders the method fairly insensitive.

/

h\

Absorption Liquid

Fig. 6

Schoeniger combustion flask.

Larger samples (up to 1 g) can be accommodated in a bomb combustion apparatus such as that shown schematically in Fig. 7. Here high pressures of oxygen (e.g. 40 atm) are used to facilitate complete combustion of the sample. Because of the high pressure generated within the bomb, obvious safety considerations apply and analysis is relatively lengthy because o f the time taken to achieve complete absorption of combustion products in the absorber solution. Absorption may be monitored by a pressure gauge or using a collapsible expansion bag which inflates during combustion and deflates during absorption. As stated earlier, the absorber solution must be carefully selected to ensure that each element is present in a single form suitable for ion chromatographic analysis. Elements such as fluorine and chlorine are converted quantitatively to hydrogen fluoride and hydrogen chloride, respectively, and so can be absorbed with water or dilute sodium hydroxide. Hydrogen halides are also produced from bromine and iodine, but other more oxidised products such as HBr03 and HI03 are also formed. For these species, a reducing agent such as hydrazine sulphate should be added to the absorber solution so that only bromide and iodide are present in the final solution. For sulphur and phosphorus, it is desirable that they be quantitated as sulphate and phosphate, respectively, and therefore the absorber solution should contain an oxidant such as dilute hydrogen peroxide. Some samples, particularly those of a geological origin, may be combusted using furnace techniques. In this method, the sample i s mixed with a suitable combustion accelerator (such as a mixture of iron and

42

copper, o r i r o n , t i n and vanadium pentoxide) and heated i n a ceramic c r u c i b l e i n an i n d u c t i o n furnace w h i l s t oxygen i s passed over t h e sample. The combustion products are c o l l e c t e d i n a s u i t a b l e absorbing s o l u t i o n . Furnace combustion i s very r a p i d due t o t h e h i g h temperature used and l a r g e numbers o f samples can be handled w i t h ease. I n a d d i t i o n , r e s u l t s are very precise and c a l i b r a t i o n does n o t r e q u i r e the use o f a l a r g e number of geochemical standard materials. Fig. 8 shows a furnace combustion apparatus.

Oxygy

charging valve

Pressure relief valve

W steel container with inert liner

Ignition wire

Samplecup

\ Fig. 7

/

High pressure combustion bomb.

Table I l i s t s some a p p l i c a t i o n s o f i o n chromatographic analysis of samples prepared by combustion techniques. 3.

SAMPLE CLEANUP METHODS

3.1

INTRODUCTION

When the sample has been dissolved, i t i s o f t e n necessary t h a t some m o d i f i c a t i o n o f the sample d i g e s t be performed before an i n j e c t i o n can be made onto t h e i o n chromatograph. f i l t r a t i o n step,

T h i s m o d i f i c a t i o n may i n v o l v e a simple

o r i t may be more extensive and i n v o l v e s e l e c t i v e

removal of t h e analyte from t h e sample o r removal o f i n t e r f e r i n g m a t r i x components.

Alternatively,

i t may be necessary t o change the chemical

form of t h e analyte t o improve i t s separation o r d e t e c t i o n i n t h e f i n a l analysis.

43

Fig. 8

Furnace combustion apparatus. Reproduced w i t h r e f . 19

TABLE I

permission

from

A p p l i c a t i o n s o f combustion methods

Matrix

Species determined

Plant materials

c1,

s

Reference

16

B i ol og ic a l sampl es

F , C1, S

Fuels, o i l s and c o a l

S

17 17, 23

Foods

I , F, C1, S F, C1, S F, C1, Br, I , S, P

18, 21 19, 20 22

Geological samples Organic reagents

These sample cleanup procedures o f t e n t a k e t h e m a j o r i t y o f t h e t o t a l a n a l y s i s time and c o n t r i b u t e s i g n i f i c a n t l y t o t h e f i n a l c o s t o f t h e a n a l y s i s , b o t h i n terms of l a b o u r and t h e consumption o f m a t e r i a l s .

In

a d d i t i o n , m a n i p u l a t i o n o f t h e sample can o f t e n i n t r o d u c e a major source of

imprecision

which

can

greatly

chromatographic process i t s e l f .

outweigh

any

variables

in

the

Often, t h e degree o f success achieved i n

t h e sample cleanup step determines t h e u l t i m a t e success o f t h e a n a l y s i s . Sample cleanup can be performed o f f - l i n e ,

p r i o r t o t h e chromatographic

44

analysis, or can be incorporated as an on-line process linked with the chromatographic hardware. The goals of cleanup are to achieve: (ref. 1) reduction of the overall loading of sample on the column in order to prevent peak distortion and loss of chromatographic efficiency, (ref. 2) removal of matrix interferences, (ref. 3) concentration or dilution of the analyte, and (ref. 4) preparation of the sample in the solution most appropriate to the analysis. With the exception of sample preconcentration which is discussed in section 5, the achievement of these goals is discussed below.

3.2 FILTRATION As with a1 1 other liquid chromatographic methods, ion chromatography requires that the sample be free from particulate matter to prevent fouling of capillary tubing, column end frits and other hardware components. Many samples, such as water samples, are obtained in a fairly clean form which might appear to require no further treatment prior to injection. Despite appearances, all samples must be filtered through a membrane filter of porosity 0.45 lpl or less. Failure to perform this simple step will invariably decrease the column lifetime. Fortunately, sample filtration is very straightforward if disposable filter units are employed. Careful attention must be paid t o sample contamination (see section 4 . 3 ) , particularly by nitrate ion released from the filter membrane. Ultrafiltration devices wherein the sample is forced under pressure through a membrane, can also be applied to difficult samples, for example, the removal of free calcium and magnesium ions from protein material i n biological samples such as serum, milk and egg white (ref. 24). 3.3 CHEMICAL MODIFICATION OF THE SAMPLE 3.3.1 BATCH METHODS USING ION-EXCHANGE RESINS Perhaps the most common chemical modification o f the sample performed in ion chromatography is adjustment of pH of strongly acidic or alkaline samples. Injection of such samples without pH adjustment usually produces an unacceptable chromatogram because of baseline disturbances. Some ion chromatographic detection modes are particularly prone to the formation of system peaks which appear as spurious peaks in the chromatogram. These peaks appear as a direct result of the fact that with some detection methods, such as indirect UV absorption (ref. 3) (or "indirect photometric chromatography"), (ref. 25), a characteristic of the eluent (such

as absorbance) i s being monitored by the detector. Any change i n t h e form of t h e eluent can t h e r e f o r e lead t o a baseline disturbance o r system peak. For example, an eluent comprising potassium hydrogenphthalate a t pH 4 i s commonly employed w i t h i n d i r e c t UV d e t e c t i o n f o r t h e determination of anions. This eluent i s favoured because i t i s an e f f e c t i v e b u f f e r and provides e x c e l l e n t separation o f many inorganic anions.

A t pH 4,

the

phthalate i s present as a m i x t u r e o f p h t h a l i c a c i d and hydrogenphthalate ions, both o f which e x h i b i t d i f f e r e n t UV absorption behaviour. It f o l l o w s therefore t h a t i n j e c t i o n o f a s t r o n g l y a c i d i c o r a l k a l i n e sample w i l l temporarily a l t e r the r a t i o e x i s t i n g between t h e two eluent species and w i l l u l t i m a t e l y produce a system peak ( r e f . 26). The same can be s a i d f o r other detection modes. I t i s u s u a l l y n o t possible t o a d j u s t t h e sample pH by simple a d d i t i o n

of acid o r base because o f contamination o f t h e sample by t h e a c i d anion o r base c a t i o n , since these species may be o f i n t e r e s t i n t h e sample. In such cases i t i s o f t e n p o s s i b l e t o use an ion-exchange r e s i n i n t h e hydrogen form can be added t o an a l k a l i n e sample i n order t o lower t h e pH. The usual procedure i s t o s t i r a known weight o f r e s i n (e.g. a known volume o f sample (e.g.

1 g) w i t h

5 m l ) and t o monitor t h e pH of t h e

s o l u t i o n , n o t i n g t h e time required f o r t h e sample t o reach the desired pH (which i s u s u a l l y t h a t o f t h e e l u e n t t o be used). When t h i s r e a c t i o n time i s determined, t h e process i s repeated w i t h a second sample a l i q u o t b u t w i t h the pH electrode removed. This prevents contamination o f t h e sample by c h l o r i d e from t h e electrode f i l l i n g s o l u t i o n . Whilst t h i s approach i s simple and r e l a t i v e l y e f f e c t i v e , i t s u f f e r s from a number o f drawbacks. F i r s t t h e sample volume r e q u i r e d i s l a r g e and the r e a c t i o n time must be adjusted whenever t h e composition o f t h e sample changes. Second, the r e s i n used must be cleaned thoroughly t o prevent contamination o f t h e sample by ions leached from the r e s i n m a t e r i a l . Third, the sample volume may change due t o uptake o r release of solvent from the resin. F i n a l l y , some l o s s of sample components may occur due t o adsorption 3.3.2

on the resin.

D i a l y t i c techniques

O i a l y t i c techniques i n which selected sample components a r e transfered across a membrane may be subdivided i n t o passive d i a l y s i s and a c t i v e ( o r Donnan) d i a l y s i s procedures. Passive d i a l y s i s involves d i f f u s i o n o f p a r t i c l e s of a s p e c i f i e d molecular weight range through a n e u t r a l membrane.

On the other hand, a c t i v e o r Donnan d i a l y s i s i s t h e

46

transfer of ions of a specified charge sign through an ion-exchange membrane. Both approaches have been applied to the cleanup of samples for ion chromatography. Passive dialysis is a very slow process, requires appreciable volumes of sample (e.g. 5 ml) and normally results in severe dilution of the sample. These factors have mitigated against its widespread use. Nordmeyer and Hansen (ref. 27) have described an automated device for the rapid dialysis of very small samples (e.g. 40 ml) which enables direct injection of the dialysate onto an ion chromatograph. This device is shown schematically in Fig. 9, from which it can be seen that the sample is introduced into the annular cavity formed between a hollow dialysis fibre and an external concentrically mounted small diameter PTFE tube. The eluent is contained inside the fibre and flow is stopped whilst solute components from the sample dialyse into the interior of the hollow fibre. Because o f the small volumes involved, dialysis time is very short (typically less than 1 min), and the sample is then injected directly onto the ion chromatograph. When applied to the removal of free calcium from human serum, linear calibration curves were obtained and peak heights showed a relative standard deviation of less than 5% over a two-week period. The process of Donnan dialysis can be illustrated by reference to a dialysis system comprising 0.1 M NaCl (solution 1) separated from 0.001 M KC1 (solution 2) by a cation-exchange membrane. This experimental arrangement is shown in Fig 10. Cations can diffuse rapidly through the membrane, according to the following equilibrium: Na

+

+

+

K m-Na+m

+ K+

..........(1)

where the subscript m refers to the membrane phase The equilibrium constant for this exchange is given by:

.........(2) where the brackets denote the activity of the species. Since the equilibrium must exist at both surfaces of the membrane, then:

47

......... . ( 3 1 where t h e s u b s c r i p t s 1 and 2 r e f e r t o t h e two s o l u t i o n s on e i t h e r s i d e of t h e membrane. There can be no c o n c e n t r a t i o n g r a d i e n t s f o r t h e same i o n across t h e membrane, t h e r e f o r e (Na

+

=

+

..........(4)

(Na J2

and

Eqn (3) can be s i m p l i f i e d t o g i v e eqn ( 6 ) , o r eqn ( 7 ) i f t h e a c t i v i t y c o e f f i c i e n t i s assumed t o be u n i t y .

........ ..(7)

I n t h e system under c o n s i d e r a t i o n , t h e r e i s a s t r o n g tendency f o r t h e sodium i o n s t o d i f f u s e from t h e h i g h c o n c e n t r a t i o n zone ( s o l u t i o n 1) t o the

low c o n c e n t r a t i o n

zone

(solution

2).

As

this

process

occurs,

corresponding t r a n s f e r o f potassium i o n s from s o l u t i o n 2 t o s o l u t i o n 1 proceeds i n o r d e r t o preserve e l e c t r o n e u t r a l i t y . Thus d i f f u s i o n o f 1% of t h e sodium i n t o s o l u t i o n 2 i s accompanied by t r a n s f e r o f 99% o f t h e potassium i n t o s o l u t i o n 1. I f t h e volume o f s o l u t i o n 1 i s l e s s than t h a t o f s o l u t i o n 2, then t h e c o n c e n t r a t i o n of

potassium i n s o l u t i o n 1 i s

g r e a t e r than t h a t o r i g i n a l l y p r e s e n t i n s o l u t i o n 2. I n t h i s way, sample preconcentration can be accomplished. chemical e q u i l i b r i u m ,

E v e n t u a l l y t h e system w i l l a t t a i n

b u t t h i s s t a t e i s achieved o n l y s l o w l y because

t r a n s f e r o f c h l o r i d e across t h e membrane i s hindered. I n t h e s h o r t t e r m t h e r e f o r e , sample m o d i f i c a t i o n occurs.

Sample in

L

To

column

e Eluent

e

II

4

Sample out

Hollow dialysis fibre

Fig. 9

Schematic representation of a passive d i a l y s i s - i n j e c t i o n device. Adapted with permission from r e f . 27.

Solution 1

Solution 2

0.1 M NaCl

0.05 M NaCl 0.0005 M KCI

Fig. 10

I

0.05 M NaCl

0.0005 M KCI

Equilibrium (days)

Schematic representation o f Donnan d i a l y s i s .

49 I n terms o f i o n chromatographic sample cleanup, Donnan d i a l y s i s can be used t o remove a selected species from a sample, o r a l t e r n a t i v e l y t o s e l e c t i v e l y add an i o n t o a sample. An example o f t h e f i r s t a l t e r n a t i v e i s the removal o f metal cyano complexes from a p l a t i n g bath by Donnan d i a l y s i s i n t o a sodium c h l o r i d e r e c e i v e r s o l u t i o n ( r e f . 28). The second a l t e r n a t i v e can be i l l u s t r a t e d by t h e d i a l y s i s o f sodium hydroxide s o l u t i o n using sulphuric a c i d as t h e r e c e i v e r s o l u t i o n . Here hydrogen ions from t h e sulphuric a c i d s o l u t i o n d i f f u s e i n t o t h e sodium hydroxide through a cation-exchange membrane. The pH o f t h e sample i s t h e r e f o r e lowered, w h i l s t t h e anion content i s t h e o r e t i c a l l y unaltered, a l l o w i n g subsequent determination o f these anions by i o n chromatography. The second o f

the

above

alternatives

suffers

from

a

practical

1 i m i t a t i o n which s e r i o u s l y d e t r a c t s from i t s r o u t i n e use. This l i m i t a t i o n t h a t t h e cation-exchange

is

membrane i s n o t e n t i r e l y

impervious t o

sulphate ions from the r e c e i v e r s o l u t i o n , which means t h a t the sample u l t i m a t e l y becomes contaminated w i t h sulphate

during d i a l y s i s .

This

problem could be minimised by increasing t h e p e r m s e l e c t i v i t y o f t h e membrane ( i e . i t s a b i l i t y t o permit t h e t r a n s f e r o f ions o f o n l y one charge sign) however t h i s requires c a r e f u l c o n t r o l over t h e manufacturing process. A more a t t r a c t i v e procedure has been reported by Cox and Tanaka ( r e f . 29) wherein a s l u r r y o f ion-exchange r e s i n i n the hydrogen form i s used t o replace t h e sulphuric a c i d employed as t h e r e c e i v e r s o l u t i o n i n the above example. This method i s c a l l e d "dual ion-exchange'' and s i n c e the counter anion i n the r e c e i v e r s o l u t i o n i s the r e s i n bead i t s e l f , t r a n s f e r across t h e membrane i s eliminated f o r physical

reasons.

It

should be noted t h a t the ion-exchange membrane may a l s o be used i n t h e form o f tube i n s e r t e d i n t o t h e r e s i n s l u r r y ( r e f . 30).

Table I 1 shows

some a p p l i c a t i o n s o f Donnan d i a l y s i s sample cleanup i n i o n chromatography. 3.3.3

DISPOSABLE CARTRIDGE COLUMNS

One o f the most v e r s a t i l e and convenient means a v a i l a b l e f o r sample cleanup i s the use of commercially a v a i l a b l e disposable c a r t r i d g e columns. These devices o f f e r r a p i d sample treatment and can u s u a l l y be employed i n tandem w i t h disposable f i l t e r s so t h a t f i l t r a t i o n and sample cleanup can be performed i n a s i n g l e operation. Table I 1 1 l i s t s some of the common s t a t i o n a r y phases a v a i l a b l e as c a r t r i d g e column packings from a range o f manufacturers.

50 TABLE I1

Applications o f donnan d i a l y s i s cleanup

Sample

Membrane

Reference

Metal cyano

anex

28

catex

30

Anions in Na2C03

catex

30

S042- i n NaCl

anex

30

C1- i n p o l y e l e c t r o l y t e s

anex

31

complexes i n a p l a t i n g bath C1-, NO3

- , SO42-

i n conc. NaOH

e.g..

polyacrylic acid

Table I11

Typical s t a t i o n a r y phases f o r cleanup c a r t r i d g e columns 1. 2.

S i 1i c a

3. 4.

Alumina ( a c i d i c , basic and n e u t r a l )

5.

Cation-exchange (H' o r metal form)

6.

Polymer (e.9.

7.

Activated carbon

C18

Ani on-exchange styrene divinylbenzene and

p o l y v i n y l p y r r o l id i ne) 8.

Chelating agents

9.

Amino

Cartridge columns can be employed i n one o f two ways. The f i r s t method i s s e l e c t i v e removal of t h e s o l u t e ions from t h e sample m a t r i x and t h e solvent used t o e l u t e the sample through t h e c a r t r i d g e should provide chromatographic conditions g i v i n g very strong r e t e n t i o n o f t h e s o l u t e

ions. That i s , t h e capacity f a c t o r s f o r these solutes should be very large. The a l t e r n a t i v e operational mode f o r

c a r t r i d g e columns i s t o

s e l e c t i v e l y r e t a i n m a t r i x components under conditions where the s o l u t e

51 i o n s a r e unretained. That i s , t h e i r c a p a c i t y f a c t o r s approach zero. I t i s g e n e r a l l y i n a d v i s a b l e t o use a c a r t r i d g e column t o achieve chromatographic s e p a r a t i o n o f s o l u t e s which have c a p a c i t y f a c t o r s i n t e r m e d i a t e between t h e abovementioned extremes. The reasons f o r t h i s a r e t h a t experimental f a c t o r s a r e very v a r i a b l e (e.g.. column e f f i c i e n c y , f l o w - r a t e , and packing r e p r o d u c i b i l i t y ) and i n most cases t h e passage o f s o l u t e s along t h e column cannot be v i s u a l l y monitored. Thus even i f a chromatographic separation i s optimised on a p a r t i c u l a r s t a t i o n a r y phase, i t i s probable t h a t t h e separation would be i r r e p r o d u c i b l e due t o changes i n t h e experimental c o n d i t i o n s used. Keeping i n mind t h a t we wish t h e s o l u t e t o be e i t h e r w e l l r e t a i n e d o r not

retained

at

all,

then

several

possibilities

emerge

from

the

s t a t i o n a r y phases l i s t e d i n Table 111. S t a t i o n a r y phases which show some ion-exchange a b i l i t y (such as s i l i c a , alumina, anion and c a t i o n exchangers, and amino phases), and s t a t i o n a r y phases which show c h e l a t i o n a b i l i t y should be s u i t a b l e f o r t h e s e l e c t i v e r e t e n t i o n o f i o n i c s o l u t e s from a m a t r i x composed l a r g e l y o f n e u t r a l ,

organic

species.

Alter-

n a t i v e l y , hydrophobic s t a t i o n a r y phases such as o c t a d e x y l s i l a n e and t h e polymeric phases should be useful

f o r t h e removal o f n e u t r a l o r g a n i c

components w h i l e showing l i t t l e r e t e n t i o n o f i o n i c s o l u t e s .

A further

p o t e n t i a l a p p l i c a t i o n o f c a r t r i d g e columns i s t h e i r use f o r a d j u s t i n g t h e pH o f a sample i n t h e same manner as t h a t described e a r l i e r f o r i o n exchange r e s i n s used i n t h e b a t c h mode.

Most o f t h e abovementioned

p o s s i b i l i t i e s have been r e a l i s e d i n p r a c t i c e and Table I V l i s t s some examples o f successful a p p l i c a t i o n s . Several

practical

c a r t r i d g e columns,

aspects

should

namely column

receive

pretreatment,

attention flow-rate,

when

using

method

of

sample a p p l i c a t i o n , and sample pH. F i r s t , t h e columns almost i n v a r i a b l y r e q u i r e pretreatment packing m a t e r i a l ,

to

i n o r d e r t o remove very f i n e p a r t i c l e s of elute

any

contaminants,

or

to

condition

the the

s t a t i o n a r y phase t o improve t h e e f f i c i e n c y o f sample b i n d i n g . S i g n i f i c a n t l e v e l s o f i n o r g a n i c contaminants a r e commonly encountered i n c a r t r i d g e columns (see s e c t i o n 4 . 3 ) , g e n e r a l l y as a r e s u l t o f r e s i d u a l reagents from t h e manufacturing process. Hydrophobic s t a t i o n a r y phases u s u a l l y r e q u i r e pretreatment w i t h an o r g a n i c s o l v e n t such as methanol i n o r d e r t o wet t h e s t a t i o n a r y phase surface so t h a t e f f e c t i v e b i n d i n g o f hydrophobic s o l u t e s i s achieved from aqueous sample s o l u t i o n s . The f l o w r a t e o f sample o r f l u s h i n g s o l u t i o n through t h e precolumn should be kept as low as p r a c t i c a b l e so t h a t mass t r a n s f e r e f f e c t s minimised, and as r e p r o d u c i b l e as p o s s i b l e . Most column c a r t r i d g e s a r e

are

52 designed f o r use w i t h disposable syringes and t h e low packing density of the s t a t i o n a r y phase permits very h i g h flow-rates (e.9.

50 ml/min) t o be

e a s i l y achieved. Experience w i t h a n a l y t i c a l chromatographic columns suggests t h a t such a high f l o w - r a t e i s u n l i k e l y t o produce t h e degree of s e l e c t i v e separation required, so i t i s advisable t o use flow-rates l e s s than 10 ml/min. TABLE I V

Applications o f cleanup w i t h c a r t r i d g e columns

Matrix

Solute ions

P1ant e x t r a c t

NO2-, NO3 , SOq 2-

Urine Urine Soil extract P1asma

Stationary phase

Reference

C18 C18

4,34

t h i o s u l phate oxalate

C18

33 34

-

sot-

32

High c h l o r i d e

anions

NaOH Leachate

anions A s ( I I I ) , As(V)

C18 C18 silica AG+ catex H+ catex catex

Brine

sodium anions

H+ catex charcoal

43 40

metal 0x0-anions anions

anex

41

amino

42

anions anions

polymer

43 43

Plant e x t r a c t

A i r samples

Digests Natural waters Surf act ants Aromatics

polymer

36 35 37 38 39

The t h i r d important p r a c t i c a l consideration i s t h e manner i n which t h e sample i s applied t o and e l u t e d from t h e c a r t r i d g e column. It i s p o s s i b l e t o apply a known volume o f t h e sample t o t h e head o f t h e column and t o e l u t e the sample band through t h e column w i t h a s u i t a b l e eluent. However, t h i s method i s d i f f i c u l t

i n p r a c t i c e because o f the d i f f i c u l t y

in

applying an accurate volume o f sample using t h e syringes compatible w i t h the c a r t r i d g e column, and i s recommended o n l y when the sample volume i s small o r t h e concentration o f t h e sample i s h i g h enough t o q u i c k l y saturate t h e c a r t r i d g e . I t i s g e n e r a l l y more appropriate t o pass sample continuously through t h e column, d i s c a r d i n g t h e f i r s t two o r t h r e e column volumes and then c o l l e c t i n g s u f f i c i e n t e f f l u e n t f o r analysis. F i n a l l y , the sample pH has an important bearing on t h e s e l e c t i o n of a

s u i t a b l e s t a t i o n a r y phase. Apart from t h e obvious c o n s i d e r a t i o n t h a t some s t a t i o n a r y phases a r e i n t o l e r a n t o f a c i d i c o r a l k a l i n e s o l u t i o n s , sample pH i s o f t e n a very useful

the

i n d i c a t o r o f the i o n i c strength.

cases where t h e i o n i c s t r e n g t h i s unacceptably high, i t may

In

be necessary

t o use a second c a r t r i d g e column, o r an a l t e r n a t i v e cleanup procedure, t o remove some o f t h e i o n i c components from t h e sample. I n conclusion, i t should a l s o be noted t h a t c a r t r i d g e columns packed w i t h hydrophobic s t a t i o n a r y phases can a l s o be used t o r e t a i n i o n i c

s o l u t e s ( r a t h e r than n e u t r a l , o r g a n i c s o l u t e s ) i f they a r e f i r s t cond i t i o n e d w i t h an i o n - i n t e r a c t i o n reagent. The success o f t h i s approach i s dependent on r e t e n t i o n o f t h e i o n - i n t e r a c t i o n reagent on t h e s t a t i o n a r y phase d u r i n g sample e l u t i o n , t h u s i t i s d e s i r a b l e t h a t r e l a t i v e l y hydrophobic i o n - i n t e r a c t i o n reagents be used and t h e sample volume be limited. Tetramethylammonium hydroxide employed as i o n - i n t e r a c t i o n

and pentanesulphonic

acid

have been

reagents f o r t h e removal o f a n i o n i c and

c a t i o n i c s u r f a c t a n t s , r e s p e c t i v e l y , u s i n g a c a r t r i d g e column packed w i t h a polymeric divinylbenzene s t a t i o n a r y phase ( r e f . 43). CHEMICAL REACTION OF SOLUTES

3.3.4

f o r some samples, cleanup can be b e s t achieved u s i n g an a p p r o p r i a t e chemical r e a c t i o n t o e l i m i n a t e a m a t r i x component. A l t e r n a t i v e l y , i t may be necessary t o d e r i v a t i s e a s o l u t e i n o r d e r t o enhance i t s d e t e c t a b i l i t y o r t o convert i t i n t o a form s u i t a b l e f o r separation.

Much has been

w r i t t e n on t h e p r i n c i p l e s o f chemical d e r i v a t i s a t i o n o f o r g a n i c s o l u t e s ( r e f . 44), and t h e same p r i n c i p l e s apply here t o i n o r g a n i c s o l u t e s . Table V

l i s t s some r e a c t i o n s which have been employed as sample t r e a t m e n t

methods f o r i o n chromatography, o r as m o b i l e phase r e a c t i o n s designed t o modify t h e n a t u r e o f t h e s o l u t e i n an i o n chromatographic d e t e r m i n a t i o n . 4. 4.1

CONTAMINATION EFFECTS

INTRODUCTION One o f t h e most important c o n s i d e r a t i o n s i n sample h a n d l i n g i s t h e

p o s s i b i l i t y o f contamination a r i s i n g from v a r i o u s sources such as t h e h a n d l i n g procedures used, t h e v o l u m e t r i c ware employed,

filtration or

cleanup

itself.

devices

and

the

chromatographic

hardware

Such

contamination may a l t e r t h e t r u e c o n c e n t r a t i o n o f s o l u t e s o f i n t e r e s t , e i t h e r d i r e c t l y by c o n t r i b u t i n g d e t e c t a b l e l e v e l s o f t h e a n a l y t e s t o t h e f i n a l s o l u t i o n , o r by promoting chemical r e a c t i o n s which cause l e v e l s o f analytes t o a l t e r . F u r t h e r t h e sample i t s e l f may be a source of

contamination of the chromatographic system, causing column poisoning or memory effects resulting from adsorption of sample constituents on chromatographic components. In this section, the chief sources of contamination are discussed. TABLE V

Chemical modification of the sample

Additive

Effect

Reference

Boric acid

prevent oxidation of ascorbic acid H3B03 in suppressor) (borate

N, N dimethylphenylenedi amine

reacts with H2S to form methylene blue 46

Boric acid

fluoride=+BF4to eliminate interference of F- on silica analysis

47

Iodine

I2 + HCN*H+ + I- + ICN iodide used as a measure of CN-

48

EDTA

complex interfering metal ions

49

Formaldehyde

S032-*

50

Methanol

oxidised to formate by Cr042-

51

Barium ions

precipitate sulphate

52

4.2

+

hydroxymet hane-su 1 phonate

45

CONTAMINATION FROM PHYSICAL HANDLING OF THE SAMPLE The prime sources of sample contamination in physical operations such as weighing and volumetric manipulations are contact of the sample or apparatus with the skin, and leaching of contaminants from volumetric ware. Contact with the skin introduces detectable levels o f sodium and chloride to the sample and in cases where trace determination of these solutes is desired, high background levels will invariably occur unless protective gloves are worn. Volumetric ware should be made from polyethylene or some other inert material and should be washed in non-ionic detergent (sulphate-free) and

rinsed thoroughly before use. Standard solutions used for calibration of the ion chromatograph should be stored in inert containers. There is ample evidence to show that even brief exposure of aqueous solutions to conventional 1 aboratory glassware results in significant contamination, particularly by sodium and silicate. It is also noteworthy that samples for anion analysis can be readily contaminated by bicarbonate ion produced by absorption of carbon dioxide from the atmosphere, particularly under alkaline conditions. Care should therefore be taken to exclude carbon dioxide wherever possible. 4.3

CONTAMINATION FROM FILTRATION D E V I C E S AND CARTRIDGE COLUMNS As mentioned in the earlier discussion on the use of disposable filtration devices and cartridge columns for the clarification and chemical cleanup of samples for ion chromatography, contamination from these devices must be considered. In most cases, these devices have been manufactured for the general HPLC market where sample contamination by inorganic ions would be a minor problem unless the particular contaminants involved were capable of participating in chemical reactions with the sample components. For this reason, it is not uncommon for inorganic reagents to be employed during the manufacturing process. Disposable filters and both C18 and alumina cartridge columns produced by the major HPLC supplier have been evaluated for contamination effects (refs. 53, 54) and some of the results obtained are summarised in Tables VI and V I I . Whilst the results shown are specific to one brand of product, it is expected that similar contamination levels would exist in alternative products, unless specific means were employed by the manufacturer t o remove inorganic contaminants. Table V I shows that disposable filtration devices release appreciable quantities of nitrate, and lesser amounts o f chloride and sulphate, into the initial fraction of solution passed through them. However, the leachable ions are very labile and are essentially removed completely if the filter is pre-washed with 20 ml of water. Care should therefore be taken that such filters are adequately washed before they are used on samples to be subsequently analysed by ion chromatography. Detectable levels of chloride, nitrate, sulphate and lead are leached from C 1 8 cartridge columns by water (Table V I I ) , and a reduced, but still detectable, level of these ions persists after the column has been washed with 20 ml of water. The levels of ions leached from the cartridge are sufficiently low that that they would present a problem only for ultratrace analyses using sample preconcentration methods. I n such cases, it

56

would be necessary to run a blank solution. Alumina columns produce much more severe contamination, undoubtedly due to residues of the reagents used to modify its surface properties during manufacture. TABLE VI

Contamination from filtration devices Data taken from ref. 53 ~~

ION

~

~~

Conc. (ppb) in successive 20 ml fractions HA filters Fraction 1

F-

<0.2

c1-

84.6 698.8 17.8

N03;so4

TABLE V I I Ion

HA filters Fraction 2

Fraction 1

c0.2 13.6 c0.4 2.2

23.4 73.2 409.5 111.9

2

c0.2 13.2 c0.4 8.7

Contamination from cartridge columns Conc. (ppb) in successive 20 ml fractions Fraction 1

Fraction 2

c0.2

c0.2

c1-

101.2

29.4

NOg-

69.9

36.4

so42-

99.4

39.0

Pb2+

76.3

21.4

F-

Fraction

Notes 1. No contamination observed for Cd2+, Cu2+, Mn2+, Ni2+ and Zn2+ 2. Other stationary phases (particularly alumina) contain significant quantities of 1 eachabl e materi a1 3. Data taken from ref. 53. 4.4

CONTAMINATION FROM CHROMATOGRAPHIC HARDWARE COMPONENTS Over recent years there has been considerable discussion relating to the suitability of conventional HPLC hardware components for use in ion chromatographic applications. It i s clear that the major instrumental components of an ion chromatograph, namely the pump, injector, data

57 management system, and o f t e n a l s o t h e detector, are i d e n t i c a l t o those used i n a t y p i c a l HPLC system.

The c h i e f d i f f e r e n c e between t h e two

techniques l i e s i n the column used f o r each method. I n view o f t h i s , i t has been common f o r HPLC hardware t o be used i n i o n chromatographic systems and t h i s raises t h e question o f t h e s u i t a b i l i t y o f s t a i n l e s s s t e e l components f o r use w i t h t h e aqueous eluents and sample types employed i n i o n chromatography. Types 304 and 316 s t a i n l e s s s t e e l s are t y p i c a l l y used i n t h e construction o f solvent-wetted HPLC components. Studies w i t h reversedphase eluents ( r e f . 55) have shown t h a t components w i t h small diameter openings,

such

as

capillary

tubing,

are

susceptible

to

corrosion

r e s u l t i n g from mechanical erosion o f t h e p r o t e c t i v e surface oxide l a y e r due t o high f l u i d v e l o c i t y . There i s a l s o evidence t h a t sample components such as p r o t e i n s and metal chelates can undergo complexation o r l i g a n d exchange reactions a t s t a i n l e s s s t e e l surfaces ( r e f s . 5 6 , 57). Aqueous buffers used f o r the analysis o f anions and cations i n i o n chromatography provide a s u i t a b l e environment f o r corrosion and i n some cases a l s o e x h i b i t strong complexation p r o p e r t i e s .

It i s therefore possible t h a t

contamination e f f e c t s could a r i s e from t h e use o f these eluents on s t a i n l e s s steel hardware components. Several p o s s i b i l i t i e s e x i s t where contamination o f t h e eluent by metal ions leached from m e t a l l i c components i n t h e chromatographic system could present serious problems. The f i r s t o f these i s the d i r e c t e l e v a t i o n o f detector baseline l e v e l s i n c a t i o n analyses using post-column r e a c t i o n detection.

Secondly,

i n t e r f e r e n c e e f f e c t s r e s u l t i ng from complexat i o n

reactions w i t h solutes can be expected t o occur i n some anion-exchange separations.

For example, i r o n (111) forms a strong complex w i t h many

common inorganic anions and t h i s complex would have d i f f e r e n t chromatographic and detection c h a r a c t e r i s t i c s t o those e x h i b i t e d by t h e f r e e anions. A f u r t h e r e f f e c t r e s u l t i n g from eluent contamination occurs

in the ion-exchange separation o f monovalent cations where t h e presence o f m u l t i p l y charged cations i n t h e eluent would lead t o r a p i d column d e t e r i o r a t i o n caused by i r r e v e r s i b l e b i n d i n g o f these ions onto t h e exchange s i t e s o f the low capacity cation-exchange columns used. I n such methods, i t i s common p r a c t i c e t o include an ion-exchange guard column between the pump and i n j e c t o r i n order t o remove any m u l t i v a l e n t c a t i o n s from the eluent. However, t h i s approach would be i n e f f e c t i v e against ions produced as corrosion products w i t h i n t h e i n j e c t o r o r t h e column i t s e l f . Table V I I I shows t h e compositions o f types 304 and 316 s t a i n l e s s s t e e l . These m a t e r i a l s are corrosion r e s i s t a n t by v i r t u e o f a p r o t e c t i v e

58

coating of chromium-rich oxides which forms on the surface (ref. 55). This coating can develop gradually during usage or can be formed rapidly by exposing the surface to relatively strong nitric acid solutions. If the latter method is used, the surface is said to be "passivated". Consideration of the composition of the steel suggests that the species most likely to be produced by corrosion reactions are iron, chromium, manganese, molybdenum and nickel. These metal ions could be leached from the metallic surface through either direct oxidation or by complexation reactions with eluent components. The latter mechanism could be expected to be most prevalent with eluents containing strong complexing agents such as citrate, tartrate, phthalate and ethyl enedi ami ne. Side-react i on coefficients for compl exat i on of the above metal ions with the eluents typically used for ion chromatography suggest that significant complexation of iron, chromium and, to a lesser extent, nickel can be expected (ref. 58). TABLE

VIII

Typical composition o f types 304 and 316 stainless steel Data taken from ref. 58 ~

Element

Carbon Manganese Phosphorus Sulphur S i 1 icon Ch romi um Nickel Molybdenum

~

~~~

~~

Percentage Type 304

0.08 (max) 2.00 0.045 0.030 1 .oo 18.00 - 20.00 8.00 12.00

-

-

Type 316 0.08 (max) 2.oo 0.045 0.030

1 .oo 16.00 - 20.00 10.00 - 14.00 2.00 - 3.00

In a recent study (ref. 58), corrosion products were allowed to accumulate to detectable levels by recirculating aqueous eluents through a HPLC system, and the eluents were then analysed by inductively coupled plasma atomic emission spectrometry. The experiment was performed on the chromatographic hardware alone and with a stainless steel column included in the flow-path. The results o f this study are summarised in Table IX, and show clearly that the levels of contaminant metal ions (especially iron) found in the eluent were extremely low when the eluent passed only through the chromatographic hardware (pump and injector), but increased

59 markedly when t h e column was incorporated i n t o t h e flow-path. I t i s i n t e r e s t i n g t o speculate on t h e source o f t h e c o r r o s i o n products

observed w i t h t h e s t a i n l e s s s t e e l column. The most probable source i s t h e column f r i t s which have a very high surface area i n comparison t o t h e r e s t o f the column and the external chromatographic system and i n e v i t a b l y a l s o contain s t r e s s p o i n t s a t which t h e r a t e o f c o r r o s i o n would be accelerated. Calculations show t h a t i f t h e e l u e n t wets t h e e n t i r e surface o f the column f r i t s , then these f r i t s account f o r approximately 96% o f t h e t o t a l metal surface i n contact w i t h t h e eluent and should t h e r e f o r e be the prime source o f eluent contamination. The l e v e l s o f metal ions given i n Table I X should be viewed from a standpoint which considers t h e t o t a l l e v e l s o f these species from a l l sources. The reagents used f o r t h e preparation o f eluents can be expected t o contain residual l e v e l s o f metal ions: examination o f t h e manufact u r e r s ' s p e c i f i c a t i o n s shows t h a t t h e l e v e l s o f i r o n and n i c k e l would be i n the range o f 1-5 ppb i n t h e eluents tested.

I t can t h e r e f o r e be

concluded t h a t o x i d i s i n g o r complexing eluents used f o r cation-exchange separations can be contaminated w i t h detectable l e v e l s o f i r o n , chromium and n i c k e l

from s t a i n l e s s

steel

column

frits.

Accordingly,

cation-

exchange columns designed f o r use w i t h these eluents should be f i t t e d w i t h non-metallic

frits,

or alternatively,

m e t a l l i c f r i t s should be

deactivated by passivation o r s i l a n i s a t i o n reactions. Eluents t y p i c a l l y used f o r i o n chromatography o f anions do n o t show any contamination from t h e metal 1 i c components o f t h e chromatographic system and t h i s r e s u l t indicates t h a t m e t a l l i c f r i t s are s u i t a b l e f o r use i n anion-exchange columns

.

Chromatographic hardware components such as pumps and i n j e c t o r s do n o t c o n t r i b u t e any s i g n i f i c a n t l e v e l s o f t h e metal ions. It should be noted t h a t the contamination l e v e l s shown i n Table I X were obtained on an unpassivated HPLC system and lower l e v e l s occurred when m e t a l l i c surfaces were passivated by treatment w i t h n i t r i c a c i d ( r e f .

58). Corrosion of

these components therefore does n o t represent a problem i n i o n chromatographic methods. 4.5

CONTAMINATION OF THE COLUMN I n t h e preceding section i t was pointed out t h a t metal ions introduced

i n t o the eluent from the chromatographic hardware could cause poisoning of

cation-exchange

columns

through

irreversible binding o f

exchange

s i t e s . This example serves t o i l l u s t r a t e t h e p o s s i b i l i t y of column contamination, and i n the f o l l o w i n g discussion such contamination from the sample w i l l be considered.

TABLE IX

Contamination from stainless steel Data taken from ref. 58.

Eluenta

Metal concentration (ppb) per eluent cycle FE Cr Mn Mo Ni

HN03 HN03

311.8 <1 .o

4.0

EDA-TA EDA-TA EOA-CA EDA-CA KHP

9.5 1.9 20 .Q

4.5 c1.3 4.4 4.6

3.8 4.2

4.4

3.2

Column

33.6 c0.3 c0.3

c0.8 4.5

11.7 4.5

c1.6

4.6

yes no yes

0.4

c1.5 4.7 4.2 4.7

4 . 5

no

4.7 <3.2

yes

4.7

yes

c0.3

c0.6 c0.3

no

a. Kev to eluent identities. HN03: 10 mM nitric acid. EDA-TA: 0.5 mM ethylenediamine and 1.3 mM tartaric acid. EDA-CA: 3.5 mM ethylenediamine and 10 mM citric acid. KHP: 1 mM potassium hydrogen phthalate. Perhaps the most commonly encountered example of column poisoning by the sample is the binding on the column of organic sample components. This problem can be easily circumvented by pretreatment of the sample with a cartridge column, use of an ion-exchange guard column, and periodical flushing of the analytical column with a water-methanol mobile phase when the column packing is sufficiently resistant to the use of organic solvents. A more insidious problem is the effect of metal ions on both anion and cation separations. I n the former case, metal ions such as calcium and magnesium may form complexes with mobile phase components (e.9.. gluconate), leading to system peaks or an unstable baseline in the final chromatogram (ref. 59). Silica-based anion-exchangers can also show retention of metal ions, with iron (111), aluminium (111) and mercury (11) being strongly retained, whilst copper(II), lead(I1) and zinc(I1) may elute at retention times similar to those observed for anions on the same column (ref.60). It has recently been shown (ref. 61) that the latter group of metal ions form complexes with phthalate eluents which are retained to varying degrees on silica and polymeric anion-exchangers. It i s therefore evident that polyvalent metal ions should be removed from samples on which anion determinations are to be performed, and this may be achieved conveniently by passage o f the sample through a cation-exchange cartridge column. Fig. 1 1 shows the effect of treating a drinking water sample in this manner and it is clear that the removal of calcium and magnesium resulted in an improved baseline.

61

I

0

"

1

5

I

I

Time (minl

15

10

Fig. 11 E f f e c t o f calcium and magnesium on t h e baseline produced i n t h e determination o f anions i n d r i n k i n g water. Chromatogram (a) i s f o r a Sam l e contaminated w i t h calcium and magnesium and chromatogram (by i s the same sample a f t e r treatment by passage through a cation-exchange c a r t r i d g e column. Column: Waters I C PAK A. Eluent: 1.3 mM Na2Bq07 5.8 mM3B03 1.4 mM K-gluconate in w a t e r - a c e t o n i t r i l e (88:12).

-

-

Polyvalent cations can e x e r t a detrimental e f f e c t on t h e determination of monovalent cations by cation-exchange. The r e l a t i v e l y weak eluents used f o r the monovalent cations are unable t o e l u t e p o l y v a l e n t cations, w i t h the r e s u l t t h a t these species remain s t r o n g l y bound t o t h e column. The n e t r e s u l t o f t h i s i s t h a t chromatographic performance o f t h e column f o r monovalent cations i s degraded i n terms of decreased r e t e n t i o n , l o s s o f e f f i c i e n c y , poor r e s o l u t i o n and reduced peak heights ( r e f . 62).

The

l a t t e r three c h a r a c t e r i s t i c s are d i r e c t l y a t t r i b u t a b l e t o t h e f a c t t h a t the s o l u t e ions do not compress i n t o a compact band a t t h e head o f t h e column when ion-exchange s i t e s are occupied by polyvalent metal ions. For these reasons i t i s therefore advisable t h a t a cation-exchange

guard

column be i n s e r t e d i n t o the flow-path p r i o r t o t h e a n a l y t i c a l column. 5.

SAMPLE HANDLING FOR ULTRA-TRACE ANALYSIS

5.1

INTRODUCTION

The s e n s i t i v i t y o f an i o n chromatographic method i s s t r o n g l y dependent

on the type o f detector used, w i t h amperometric and d i r e c t UV absorption detection being among t h e most s e n s i t i v e ,

and r e f r a c t i v e index being

r e l a t i v e l y i n s e n s i t i v e ( r e f . 3). Most o f the universal d e t e c t i o n modes such as c o n d u c t i v i t y and i n d i r e c t UV absorption have comparable p r a c t i c a l detection l i m i t s o f about 100 ppb f o r a 100 pl i n j e c t i o n . This means t h a t f o r r e l i a b l e q u a n t i t a t i o n t o be achieved, analysis by d i r e c t sample i n j e c t i o n can be most conveniently used f o r s o l u t e concentrations

of

about 1 ppm o r higher. Below t h i s concentration l e v e l , e i t h e r very l a r g e i n j e c t i o n volumes must be employed o r a sample preconcentration method i s

62 necessary. Fig. 12 shows a schematic representation o f t h e sample concent r a t i o n ranges applicable t o t h e abovementioned approaches, each o f which i s discussed below.

5.2

USE OF LARGE INJECTION VOLUMES Because o f t h e zone compression e f f e c t occurring on

ion-exchange

columns, i t i s possible t o i n j e c t very l a r g e volumes o f sample onto an a n a l y t i c a l column without s i g n i f i c a n t l o s s o f chromatographic e f f i c i e n c y . Several authors have u t i l i s e d t h i s approach ( r e f s .

63

-

65) and t h e

r e s u l t s suggest t h a t an upper p r a c t i c a l l i m i t o f 2 m l e x i s t s f o r t h e sample i n j e c t i o n volume, otherwise t h e l a r g e solvent peak i n t h e f i n a l chromatogram may mask e a r l y e l u t i n g solutes.

Moreover,

i n d i r e c t UV

absorption d e t e c t i o n has been shown t o be superior t o c o n d u c t i v i t y detection because t h e former method shows a more r a p i d r e t u r n t o baseline a f t e r passage o f the l a r g e i n j e c t i o n peak ( r e f s . 64, 65). Fig. 13 shows a chromatogram obtained using i n d i r e c t UV absorption d e t e c t i o n and a 1 m l sample i n j e c t i o n volume. To increase the volume of sample above t h e p r a c t i c a l l i m i t mentioned above, i t i s essential t h a t t h e s i z e o f t h e solvent peak be decreased so t h a t interference w i t h e a r l y e l u t i n g solutes does n o t occur. One method t o achieve t h i s i s t o f l u s h t h e i n t e r s t i t i a l sample from t h e column by pumping a measured volume o f eluent through t h e column i n t h e reverse d i r e c t i o n t o t h a t used f o r sample e l u t i o n ( r e f . 65).

The s o l u t e ions

remain bound t o t h e column during t h i s operation, b u t t h e i n j e c t i o n peak i s reduced because t h e sample solvent has been displaced from t h e column. This method permits t h e use o f sample volumes up t o 5 m l , b u t requires the use o f two switching valves and t h e r e f o r e seems t o o f f e r

little

advantage over t h e use o f s i m i l a r hardware w i t h preconcentration columns, as discussed below. 5.3

USE OF PRECONCENTRATION COLUMNS The most widely used approach t o t r a c e enrichment i n i o n chromato-

graphy involves the use o f a separate precolumn designed t o t r a p t r a c e l e v e l s o f solutes from a l a r g e volume o f sample ( r e f . 5). The precolumn method i s popular because i t i s simple and convenient t o apply, amenable t o automation and o f f e r s h i g h enrichment f a c t o r s .

is

I n precolumn sample enrichment, an accurately known volume of sample i s pumped a t a p r e c i s e f l o w - r a t e through a small ion-exchange precolumn ( o r a reversed-phase precolumn coated w i t h an i o n i c i o n - i n t e r a c t i o n reagent), c a l l e d the concentrator column. Solute ions contained i n t h e

63 sample are s e l e c t i v e l y trapped on t h e concentrator column and are then e l u t e d onto an ion-exchange a n a l y t i c a l column f o r separation and q u a n t i t a t i o n . This procedure can be e f f e c t i v e

as an a n a l y t i c a l method

only i f t h e processes o f b i n d i n g o f s o l u t e ions on t h e concentrator column and t h e i r

subsequent

transfer

to

the analytical

column

are

quantitative. > 100

ppm DIRECT INJECTION ZONE (sample z 100 i t l )

1 PPm

100 ppb

Fig. 12 Working concentration ranges i n i o n chromatography.

5.3.1

HARDWARE CONSIDERATIONS

The simplest form o f sample preconcentration device u t i l i s e s a s i n g l e , six-port,

high pressure, switching valve which can be actuated e i t h e r

manually

or

automatically

(ref.

66).

14 shows

Fig.

the

plumbing

In t h e f i r s t step,

the

concentrator and a n a l y t i c a l columns are e q u i l i b r a t e d w i t h eluent.

The

arrangement

and operation

of

this

system.

valve i s then r o t a t e d and t h e concentrator column i s removed from t h e flow-path w h i l s t eluent continues t o be pumped through t h e a n a l y t i c a l column. A measured volume o f sample i s passed through t h e concentrator column using e i t h e r a pump o r a l a r g e volume syringe, w i t h t h e e f f l u e n t being d i r e c t e d t o waste. A t t h i s stage, t h e s o l u t e ions from t h e sample are assumed t o be retained on t h e concentrator column and i n t h e subsequent step, r o t a t i o n o f the valve permits eluent t o be pumped i n t h e reverse d i r e c t i o n ( i e . the f l o w d i r e c t i o n opposite t o t h a t used f o r sample loading) through t h e concentrator column. This operation i s known as backflushing and i s designed t o t r a n s f e r t h e s o l u t e ions t o t h e a n a l y t i c a l column i n a small volume of eluent.

I n t h e f i n a l step, t h e

valve i s again r o t a t e d and eluent then c a r r i e s t h e s o l u t e ions through the a n a l y t i c a l column f o r separation and detection.

64 This system has the advantages of simplicity and ease of operation. The backflush volume is generally selected to be high enough to guarantee that all the ions are transfered. It should be noted here that the volume of eluent used to backflush the solute ions to the analytical column will necessarily have lower concentration of eluent ions than that present in the bulk eluent. This is because some eluent ions are required to reequilibrate the concentrator column which has been depleted of eluent ions during the passage of sample. The result of this is that severe baseline disturbances often occur in the final chromatogram when the detection method employed is sensitive to the background level of eluent ions in the mobile phase. Conductivity detection falls into this category and when this is used, the initial baseline disturbance in the chromatogram often masks early eluting solutes. A more flexible preconcentration system is produced by the combination of a single, programmable pump with two high pressure switching valves and a low pressure solvent selection valve (refs. 67, 68). Here, the same pump can be used to deliver eluent and to load the sample onto the concentrator column. Fig. 15 shows the interconnections used for these valves and Fig. 16 illustrates some of the flow-paths achievable with this system. Using a suitable configuration of the valves, the pump tubing and interconnecting lines can be flushed with sample solution or eluent with both the concentrator and analytical columns removed from the flow-path. In Fig. 16a a measured volume of sample is loaded onto the concentrator column at a precise flow-rate, after which a small, accurately known volume of eluent is pumped through the concentrator column in the same flow direction as that used for sample loading (Fig. 16b). This is termed a "wash" step and serves to partially re-equilibrate the concentrator column with eluent ions without loss of bound solute ions. Fig. 16c shows the sample stripping step in which the solute ions are backflushed from the concentrator column onto the analytical column using an accurately known volume of eluent. In the final step of the analysis, the concentrator column is removed from the flow-path and the eluent is pumped directly to the analytical column. Clearly, this multi-valve system is more complex than the single-valve approach and requires the use of a sophisticated pump. It does however offer the advantages of unlimited and precise control over the volumes of eluent used for the washing and stripping steps, and these may be readily manipulated to adapt to the requirements of a particular sample. In addition, the baseline produced in the final chromatogram is superior to that obtained with a single-valve preconcentration system.

65 1A

H

0 001 AU

I

0

LI I

12

8

L

1

16

Time ( m i n )

F i g . 13

Use o f l a r g e i n j e c t i o n volumes f o r anion a n a l y s i s . Column: Vydac 302 IC 4.6. Eluent: 2.5 mM potassium hydrogen p h t h a l a t e a t pH 4.0. I n j e c t i o n volume: 1 m l . Detection: UV a b s o r p t i o n a t 285 nm. S o l u t e concentrations: 50 - 200 ppb. Peak i d e n t i e s : A - s o l v e n t peak, B - dihydrogenphosphate, C - c h l o r i d e , D - n i t r i t e , E -bromide, F - n i t r a t e , G - i o d i d e , H - system peak.

w j i E ! i ) cJk3L-o nConcenlrator

Concenlrala

Column Detecio~

P"V"

A

Pump

Equilibrale

wa

Sample

Concentratof

Pump

Column

Backllush

F i g . 14

Column Delecior

Load Sample

Concenlralor

Detector

Column Deleclor

Pump

Analysis

Sample p r e c o n c e n t r a t i o n u s i n g two pumps, a s i n g l e s w i t c h i n g v a l v e and a c o n c e n t r a t o r column.

high-pressure

66

--

‘SAMPLE TO DETECTOR

Fig. 15 Apparatus for sample preconcentration using a single pump, a low pressure solvent selection valve ( A ) , two high-pressure switching valves (B and C) and a concentrator column ( D ) . E is the analytical column.

I

F i g . 16

WASTE

I

67

WASTE

I

B

C

I I PUMP

(c)

i TO DETECTOR

Fig. 16 Important flow-paths in the preconcentration o f a sample using the apparatus shown in Fig. 15. (a) sample loading, (b) concentrator column washing, (c) sample stripping. Reproduced with permission from ref. 68.

68 5.3.2 CHOICE OF ELUENT

The most important realisation in selection of an appropriate eluent for a preconcentration method is that eluents which are perfectly suitable for direct injection ion chromatography may be quite inappropriate for use with preconcentration techniques. In the latter case, the eluent must perform three distinct functions: it must permit solute ions to bind onto the concentrator column during the sample loading step, it must transfer quantitatively these solute ions from the concentrator column to the analytical column during the stripping step, and it must provide adequate resolution of the sample components on the analytical column. Clearly these multiple requirements will limit the number of eluents which are suitable for preconcentration methods. The following desirable eluent characteristics may be enumerated for preconcentration using conductivity detection: (1) Selectivity. The solute ions should elute within a range of capacity factors of 4-30. The lower limit is chosen to minimise interference of early eluting solutes from the relatively large solvent peak which invariably results in preconcentration chromatograms, whilst the upper limit ensures that excessive retention does not preclude re1 i able quanti tation. (2) Sensitivitv. Since the purpose of sample preconcentration is to improve the sensitivity of an ion chromatographic method, the eluent should be chosen to maximise the detectability of the solute ions. For this reason, an eluent anion with low limiting equivalent ionic conductance is prefered. In addition it is desirable that the eluent anion be singly charged in order to provide the greatest detector response to the elution o f univalent solute anions. (3) Eluent DH. Apart from the above considerations, the pH of the eluent exerts two additional important effects on the final chromatogram. In the first place, the presence of neutral, protonated forms of the eluent can result in the appearance of system peaks due to elution of these neutral components under a reversed-phase mechanism. Secondly, bicarbonate ion is present in the majority of samples due to absorption o f carbon dioxide from the atmosphere and if quantitation of this species is not required, the resultant large peak can represent a major interference to early eluting solutes. Both of these problems can be circumvented through the use of a fully ionised eluent operated at pH <6. Under these conditions, bicarbonate becomes fully protonated and elutes from the column with the solvent front.

69 A l a r g e number o f aromatic c a r b o x y l i c and sulphonic a c i d s has been evaluated f o r use as e l u e n t s i n p r e c o n c e n t r a t i o n methods ( r e f . 69).

Aromatic

monosulphonic

acids

such

as

p-toluenesulphonic

acid

and

2-naphthylamine-1-sulphonic a c i d have proved t o be t h e most s u i t a b l e f o r

use w i t h c o n d u c t i v i t y d e t e c t i o n , methyl-,

heptyl- o r octyl-side

w h i l s t a l i p h a t i c sulphonic acids w i t h chains a r e a p p l i c a b l e when d i r e c t UV

absorption d e t e c t i o n i s employed. I n a d d i t i o n t h e l o n g e r c h a i n a l i p h a t i c sulphonic acids can a l s o be used w i t h c o n d u c t i v i t y d e t e c t i o n , p r o v i d e d that t h e i r surfactant properties are not intrusive.

5.3.3

CONCENTRATOR COLUMN CHARACTERISTICS

I t i s c l e a r t h a t sample p r e c o n c e n t r a t i o n i s n o t an open-ended technique and t h a t p r a c t i c a l l i m i t a t i o n s must e x i s t on t h e amount o f

sample which can be loaded and recovered q u a n t i t a t i v e l y . I t i s d e s i r a b l e t h a t l a r g e sample volumes can be accommodated and t h a t t h e sample be loaded a t a h i g h f l o w - r a t e i n o r d e r t o minimise t h e t i m e r e q u i r e d f o r t h e analysis.

Studies w i t h f i x e d - s i t e

anion exchange c o n c e n t r a t o r columns

( r e f . 70) have shown t h a t t h e maximum p e r m i s s i b l e f l o w - r a t e s and sample volumes

are

dependent

on

the

nature

of

the

ion-exchange a f f i n i t y o f t h e e l u e n t increases, retained solutes

eluent

used.

As

the

then b i n d i n g o f weakly

onto t h e c o n c e n t r a t o r column reduces markedly w i t h

l a r g e r sample volumes. However i f t h e e l u e n t conforms t o t h e requirements l i s t e d above,

sample volumes

as h i g h as

100 m l may be loaded a t a

f l o w - r a t e o f 8 ml/min w i t h q u a n t i t a t i v e b i n d i n g o f s o l u t e i o n s b e i n g maintained. I t might appear a t f i r s t s i g h t t h a t t h e ion-exchange c a p a c i t y o f t h e

c o n c e n t r a t o r column should be as h i g h as p o s s i b l e i n o r d e r t o p r o v i d e ample ion-exchange s i t e s f o r t h e b i n d i n g of s o l u t e ions. C e r t a i n l y t h i s s i t u a t i o n does encourage q u a n t i t a t i v e b i n d i n g , b u t as t h e ion-exchange c a p a c i t y o f t h e c o n c e n t r a t o r column increases, i t becomes more d i f f i c u l t t o t r a n s f e r t h e bound i o n s o n t o t h e a n a l y t i c a l volume o f e l u e n t .

column u s i n g a small

Attempts t o use a h i g h e l u e n t s t r e n g t h f o r sample

s t r i p p i n g and a lower s t r e n g t h f o r sample e l u t i o n

have n o t proved 68), thus t h e same e l u e n t should be used f o r b o t h purposes. I n t h i s case, t h e optimal ion-exchange c a p a c i t y o f t h e c o n c e n t r a t o r column i s approximately 40% o f t h a t o f t h e a n a l y t i c a l column ( r e f . 71). I n c r e a s i n g t h e ion-exchange c a p a c i t y o f t h e c o n c e n t r a t o r column beyond t h i s value leads t o t h e requirement f o r l a r g e r s t r i p successful

(ref.

volumes, causing i n t e r f e r e n c e broadening e f f e c t s

.

with

early

eluting

solutes

and

band

70

The n a t u r e of

t h e r e s i n used t o support t h e bonded ion-exchange

f u n c t i o n a l i t i e s can a l s o e x e r t a c o n s i d e r a b l e e f f e c t on t h e preconcent r a t i o n . Studies have shown ( r e f . 71) t h a t c o n c e n t r a t o r columns packed w i t h aminated methacrylate and aminated s t y r e n e - d i v i n y l benzene r e s i n s of s i m i l a r ion-exchange breakthrough

in

c a p a c i t y gave markedly d i f f e r e n t performance

experiments

using

a

mixture

of

chloride,

nitrate

and

sulphate. Both c h l o r i d e and n i t r a t e showed very poor r e t e n t i o n on t h e styrene-divinylbenzene methacryl a t e r e s i n .

r e s i n i n comparison t o t h a t obtained u s i n g t h e

F i n a l l y , i t i s p e r t i n e n t t o comment on t h e p o s s i b i l i t y o f r e p l a c i n g t h e f i x e d - s i t e ion-exchanger i n t h e c o n c e n t r a t o r column w i t h a n e u t r a l I reversed-phase m a t e r i a l which has been coated w i t h a v e r y hydrophobic i o n - i n t e r a c t i o n reagent.

I n t h e case o f

anion p r e c o n c e n t r a t i o n ,

this

i o n - i n t e r a c t i o n reagent c o u l d be c e t y l p y r i d i n i u m i o n o r c e t y l t r i m e t h y l ammonium

ion.

Provided

that

the

ion-interaction

reagent

remains

permanently bound t o t h e s t a t i o n a r y phase s u r f a c e d u r i n g sample l o a d i n g , then an ion-exchange column can be produced. The main a t t r a c t i o n s o f t h i s approach a r e t h a t t h e n a t u r e o f t h e f u n c t i o n a l group may be v a r i e d by r e c o a t i n g t h e s t a t i o n a r y phase w i t h an a l t e r n a t i v e i o n - i n t e r a c t i o n reagent, and t h e ion-exchange c a p a c i t y o f t h e c o n c e n t r a t o r column can be e a s i l y manipulated by a l t e r i n g t h e c o n d i t i o n s under which t h e concentrator column i s coated. I t has been shown ( r e f . 72) t h a t concent r a t o r columns prepared i n t h i s way do n o t show t h e degree o f b i n d i n g o f s o l u t e ions expected from c o n s i d e r a t i o n o f t h e i r ion-exchange c a p a c i t i e s alone. A comparison o f a f i x e d - s i t e c o n c e n t r a t o r column w i t h one prepared by t h e permanent c o a t i n g method showed t h a t e q u i v a l e n t s o l u t e i o n s was obtained o n l y when t h e ion-exchange

retention o f

capacity o f the

l a t t e r column was a f a c t o r o f f i f t e e n times h i g h e r t h a n t h a t o f t h e former column. 5.3.4

APPLICATION TO SAMPLES OF LOW I O N I C STRENGTH

Most o f t h e samples used f o r p r e c o n c e n t r a t i o n methods c o n s i s t o f very d i l u t e aqueous s o l u t i o n s . Examples i n c l u d e r a i n water, p u r i f i e d water and b o i l e r feed water f o r power s t a t i o n generators. I n each case t h e sample contains p a r t s - p e r - b i l l i o n

l e v e l s o f i o n i c species and i s r e l a t i v e l y f r e e

from organic i m p u r i t i e s . Despite t h e low l e v e l s o f i o n s present, accurate a n a l y s i s may be e s s e n t i a l i n o r d e r t o prevent damage t o expensive p l a n t such as steam t u r b i n e s . For such samples I a f i x e d - s i t e ion-exchange c o n c e n t r a t o r column used w i t h a s i n g l y charged aromatic sulphonic a c i d e l u e n t a t pH 6 p r o v i d e s optimal r e s u l t s , e x p e c i a l l y when coupled w i t h c o n d u c t i v i t y d e t e c t i o n . A

71 typical chromatogram obtained for the preconcentration of a standard mixture of anions is shown in Fig. 17, which illustrates the excellent separation obtained with this eluent. When applied to the analysis of deionised water containing low ppb levels of ions (Fig. 18), peaks were evident for chloride, nitrate and sulphate. it is noteworthy that the peak widths are similar for Figs. 17 and 18, despite the wide disparity in sample volumes used. An alternative eluent, 2-naphthylamine-lsulphonic acid, has been applied in Fig. 19 t o the determination o f anions in water purified by reverse osmosis. The analysis of samples of the above type is relatively straightforward and provided that the correct eluent is chosen, the condition of the concentrator column is periodically monitored, and the performance of the system is routinely assessed using recovery experiments, then a high level of confidence can be assigned to the results.

r

O

1

1

G

I

8

12

Time (min.)

16

20

24

Fig. 17 Preconcentratim o f a trace solution of anions. Waters IC PAK A analytical column and anion concerttrator. Eluent: 3 . 5 mM toluenesulphonic acid at pH 6.0. Wash volume: 100 p1 . Strip volume: 500 pl . Sample: 10 ml of a solution contaicing 100 ppb of each of the indicated anions, except acetate (Ac ) which was present at 503 ppb. Detection: conductivity. Reproduced with permission from ref. 73.

72

CI-

I

0

1

1

1

1

1

8

1

1

1

1

12 16 Time (min.)

1

1

20

1

1

21

Fig. 18 Preconcentration o f anions i n d e i o n i s e d water. Sample volume: 100 ml. S o l u t e c o n c e n t r a t i o n s : 4 ppb c h l o r i d e , 0.5 ppb n i t r a t e and 3 ppb sulphate. Other c o n d i t i o n s as f o r F i g . 17. Reproduced w i t h permission from r e f . 73. CI’

F i g . 19 Preconcentration o f anions i n water p u r i f i e d by reverse-osmosis. Eluent: 0.4 mM 2-naphthylamine-1-sulphonic a c i d a t pH 6.0. Sample volume: 6 ml. S o l u t e c o n c e n t r a t i o n s : 5 ppb f l u o r i d e , 20 ppb c h l o r i d e and 3 ppb n i t r a t e . Other c o n d i t i o n s as f o r F i g . 17. Reproduced w i t h permission from r e f . 73.

73 5 .3 .5

APPLICATION TO SAMPLES OF H I G H IONIC STRENGTH

It i s a challenging prospect t o attempt preconcentration analysis o f

t r a c e components

i n samples which c o n t a i n h i g h l e v e l s o f o t h e r i o n i c

species. I n such cases i t i s l i k e l y t h a t b i n d i n g o f t h e t r a c e components t o t h e c o n c e n t r a t o r column w i 11 be a d v e r s e l y a f f e c t e d by m a s s - a c t i o n influences o f t h e bulk constituents.

Indeed, s u c c e s s f u l p r e c o n c e n t r a t i o n

can be a n t i c i p a t e d o n l y a f t e r sample cleanup o r when t h e i o n s o f i n t e r e s t have a much h i g h e r a f f i n i t y f o r t h e ion-exchange r e s i n t h a n do t h e m a t r i x components. An example o f t h e l a t t e r case i s t h e d e t e r m i n a t i o n o f phosphate, s u l p h a t e and o x a l a t e vegetation ( r e f .

in a

leaf

litter

extract

taken

from

coastal

7 3 ) . P r e l i m i n a r y a n a l y s i s o f t h e e x t r a c t showed h i g h

l e v e l s o f c h l o r i d e and n i t r a t e . Two s t r a t e g i e s were t h e r e f o r e employed t o s u c c e s s f u l l y a n a l y s e t h i s sample. F i r s t , a h i g h e l u e n t pH was s e l e c t e d i n o r d e r t o c o n v e r t t h e phosphate t o HP04’-and

so i n c r e a s e i t s a f f i n i t y f o r

t h e ion-exchange r e s i n i n t h e c o n c e n t r a t o r column. Second, a s t y r e n e - d i vinylbenzene based ion-exchange m a t e r i a l was used i n t h e c o n c e n t r a t o r column because t h i s r e s i n has been shown t o have p o o r a f f i n i t y

for

c h l o r i d e and n i t r a t e . The f i n a l chromatogram o b t a i n e d u s i n g g l u c o n a t e b o r a t e b u f f e r a t pH 8.5 as e l u e n t w i t h a c o n c e n t r a t o r column packed w i t h aminated polystyrene-divinylbenzene r e s i n i s shown i n F i g . 20. I n many cases i t i s b e n e f i c i a l t o c o u p l e s e l e c t i v e d e t e c t i o n w i t h sample p r e c o n c e n t r a t i o n t o a c h i e v e t h e d e s i r e d s e n s i t i v i t y . An example o f t h i s approach i s t h e d e t e r m i n a t i o n o f a n i o n s u s i n g d i r e c t UV a b s o r p t i o n d e t e c t i o n i n t h e wavelength range 200-220 nm. Only a r e l a t i v e l y sma number o f anions show absorbance under t h e s e c o n d i t i o n s and i f a s u i t a b UV-transparent e l u e n t i s chosen, t n e n s e l e c t i v e a n a l y s i s i s p o s s i b l e .

5 .3 .6

CONCLUSIONS

Sample p r e c o n c e n t r a t i o n

i s a complex procedilre and s h o u l d n o t

c o n s i d e r e d t o be a s i m p l e e x t e n s i o n o f d i r e c t i n j e c t i o n i o n chromatography. Care must be a p p l i e d t o t h e s e l e c t i o n o f t h e e l u e n t and t h e c o n c e n t r a t o r column t o ensure q u a n t i t a t i v e r e t e n t i o n on t h e c o n c e n t r a t o r column o f t h e s o l u t e i o n s of i n t e r e s t , and t h e i r subsequent q u a n t i t a t i v e t r a n s f e r o n t o t h e a n a l y t i c a l column. The a b i l i t y t o o p t i m i s e t h e wash and s t r i p volumes o f e l u e n t f o r i n d i v i d u a l samples p r o v i d e s a h i g h l e v e l o f f l e x i b i l i t y i n a t t a i n i n g these g o a l s .

The t e c h n i q u e can be a p p l i e d t o

v e r y d i l u t e aqueous samples o f low i o n i c s t r e n g t h and t o more complex samples c o n t a i n i n g h i g h l e v e l s o f i n t e r f e r i n g i o n i c s p e c i e s .

74 5.4

USE OF DIALYTIC PRECONCENTRATION METHODS

Trace enrichment using c o n c e n t r a t o r columns s u f f e r s from t h e d i s advantage o f being m a t r i x dependent. I t i s t h e r e f o r e o f i n t e r e s t t o n o t e t h a t preconcentration can a l s o be performed by Oonnan d i a l y s i s ; moreover, t h i s approach r e s u l t s i n t h e sample i o n s b e i n g t r a n s f e r r e d t o a s o l u t i o n o f known composition,

regardless of

t h e n a t u r e o f t h e sample m a t r i x

i t s e l f . That i s , m a t r i x n o r m a l i s a t i o n occurs. The p r i n c i p l e s o f Donnan d i a l y s i s wherein s o l u t e ions a r e t r a n s f e r r e d from t h e sample t o a r e c e i v i n g e l e c t r o l y t e s o l u t i o n v i a an ion-exchange membrane have been discussed i n s e c t i o n 3.3.2.

When a p p l i e d t o preconcen-

t r a t i o n , a l l t h a t needs t o be done i s t o ensure t h a t t h e volume of t h e r e c e i v e r s o l u t i o n i s considerably l e s s than t h a t o f t h e sample. T r a n s f e r o f t h e s o l u t e i o n s i s never q u a n t i t a t i v e ,

so t h e enrichment f a c t o r s

achieved a r e somewhat l e s s than t h e volume r a t i o e x i s t i n g between t h e sample and r e c e i v e r s o l u t i o n s . The volume o f t h e r e c e i v e r s o l u t i o n should t h e r e f o r e be kept as low as p o s s i b l e and t y p i c a l apparatus ( r e f s . 74, 75) c o n s i s t s of a membrane-covered t u b e i n s e r t e d i n t o t h e s t i r r e d sample (as shown schematically i n Fig. 21).

I

0

Fig. 20

I

4

1

8

1

12

I

16 l i m e Imin.)

Z'O

$4

z'8

Preconcentration o f an aqueous e x t r a c t o f c o a s t a l v e g e t a t i o n l e a f l i t t e r . Concentrator column packed w i t h aminated styrenedivinylbenzene r e s i n . E l u e n t : 1.0 mM t e t r a b o r a t e , 4.2 mM b o r i c a c i d and 1.0 mM g l u c o n i c a c i d . Wash volume: 200 PI Strip volume: 650 pl , Solute concentrations: 50 p p i phosphate, 150 ppb sulphate and 200 ppb o x a l a t e . A n a l y t i c a l column and d e t e c t i o n as f o r F i g . 17. Reproduced w i t h permission from ref. 73.

.

75

0

Receiver

solution

Sample solution Ion-exchange membrane

Fig. 21 Schematic r e p r e s e n t a t i o n o f apparatus f o r Donnan d i a l y s i s concentration.

pre-

W h i l s t sample p r e c o n c e n t r a t i o n i s r e l a t i v e l y s t r a i g h t f o r w a r d t o accomplish, some important steps must be taken b e f o r e t h e r e c e i v e r s o l u t i o n can be i n j e c t e d onto an i o n chromatograph. The reason f o r t h i s i s the high i o n i c strength o f the receiver, e f f e c t i v e Donnan d i a l y s i s t o occur.

which i s e s s e n t i a l

for

I n t h e case of c a t i o n s , t h e o p t i m a l

r e c e i v e r s o l u t i o n c o n s i s t s o f 0.1 M A12(S04)3 adjusted t o pH 0.5 w i t h s u l p h u r i c a c i d ( r e f . 74). When i n j e c t e d i n t o an e l u e n t comprising 0.01 M t a r t r a t e b u f f e r a t pH 5 , t h e b u f f e r c a p a c i t y o f t h e e l u e n t g r e a t l y reduces t h e hydrogen i o n c o n c e n t r a t i o n and t h e aluminium(II1)

forms a

strong, a n i o n i c complex w i t h t a r t r a t e and so i s e f f e c t i v e l y e l i m i n a t e d from i n t e r a c t i n g w i t h t h e cation-exchange column. Anions may b e d i a l y s e d i n t o a r e c e i v e r s o l u t i o n comprised o f 0.04 M Na2C03 and 0.16 M NaHC03 ( r e f . 75) and a f t e r d i a l y s i s , t h e r e c e i v e r s o l d t i o n i s s u b j e c t e d t o dual ion-exchange treatment (see s e c t i o n 3.3.2)

w i t h a cation-exchange r e s i n

i n t h e hydrogen form. This process c o n v e r t s t h e r e c e i v e r components t o water and carbon d i o x i d e , which i s removed by a p p l i c a t i o n o f a vacuum. The f i n a l s o l u t i o n t h e r e f o r e c o n s i s t e d o f t h e s o l u t e anions i n water. The enrichment

factors

achieved

i n the

above

examples

were

ap-

proximately 80 f o r c a t i o n s a f t e r 1 hour o f d i a l y s i s , and approximately 15 for anions a f t e r 30 min o f d i a l y s i s . Donnan d i a l y s i s t h e r e f o r e o f f e r s u s e f u l preconcentration b u t i s c l e a r l y very l i m i t e d i a i t s a p p l i c a t i o n because of c o n s t r a i n t s on t h e n a t u r e o f t h e r e c e i v e r s o l u t i o n and t h e e l u e n t t o be used f o r t h e f i n a l i o n chromatographic a n a l y s i s .

76 6.

M A T R I X ELIMINATION METHODS

6.1

INTRODUCTION To conclude t h i s c h a p t e r , t h e t e c h n i q u e o f m a t r i x e l i m i n a t i o n w i l l b e

b r i e f l y discussed. M a t r i x e l i m i n a t i o n can be d e f i n e d as any i n s t r u m e n t a l method whereby m a t r i x components a r e removed f r o m t h e sample.

It i s

t h e r e f o r e q u i t e d i s i i n c t f r o m t h e removal o f m a t r i x components u s i n g such sample t r e a t m e n t procedures as t h e use o f c a r t r i d g e columns. M a t r i x u s u a l l y w i t h t h e a i d of s w i t c h i n g

e l i m i n a t i o n can be a p p l i e d on-column,

v a l v e s , o r post-column, and has t h e c h i e f advantage t h a t i t can be e a s i l y automated. These approaches a r e d e s c r i b e d below. ON-COLUMN M A T R I X ELIMINATION

6.2

One s i m p l e way of e ' i m i n a t i n g t h e d e t e c t o r s i g n a l produced b y i n t e r f e r i n g m a t r i x compor.ents i s t o d i v e r t t h e column e f f l u e n t t o waste d u r i n g e l u t i o n of t h e s e m a t r i x com?onents. T h i s approach has been d e s c r i b e d f o r t h e d e t e r m i n a t i o n o f ppm l e v e l s of chloride (ref.

761, Here,

s u l p h a t e i n t h e presence o f 0.05%

t h e sample was i n j e c t e d o n t o an a n a l y t i c a l

column anti a s i x - p o r t v a l v e was used t o d i r e c t t h e column e f f l u e n t e i t h e r t o waste o r t o a second a n a l y t i c a l column. The e l u e n t f r a c t i o n c o r r e s ponding

LO

c h l o r i o e was vented t o waste, w h i l s t t h a t c o r r e s p o n d i n g t o

s u l p h a t e was passed t o t h e second column and t h e n c e t o t h e d e t e c t o r . M a t r i x e l i m i n a t i o n can be combined w i t h s a n p l e p r e c o n c e n t r a t i o n u s i n g a c o n c e n t r a t o r column, p r o v i d e d t h e m a t r i x components do n o t i n t e r f e r e w i t h t h e c o n c e n t r a t i o n process. The method i s t h e r e f o r e e s p e c i a l l y s u i t e d t o samgles in which t h e a n a i y t e s show v e r y s t r o n g r e t e n t i o n on t h e concent r a t o r coiumn. P.n example o f such an a p p l i c a t i o n i s t h e d e t e r m i n a t i o n of aurocyanide i n t a i l i n g s s o l u t i o n s produced f r o m a c y a n i d a t i o n process f o r t h e e x t r a c t i o n o f g o l d from i t s o r e s . These t a i l i n g s s o l u t i o n s c o n t a i n v e r y low l e v e l s o f z u r o r y a n i d e (10-20 ppb) i n t h e presenc? o f much h i g h e r l e v e l s o f o t h e r metal cyano complexes,

and r e q u i r e sample preconcenliable a n a l y s i s . P r e c o n c e n t r a t i o n of t h e t a i l i n g s s o l u t i o n on a C18 c o n c e n t r a t o r column c o n d i t i o n e d w i t h tetrabutylammonium i o n as t h e i o n - i n t e r a c t i o n reagent y i e l d s q u a n t i t a t i v e b i n d i r i g o f g o l d f o r sample vo1dmc.s l e s s than 2 m l ( r e f . 7 7 ) . T h i s occurs becduse o f t h e v e r y s t r o n g a f f i n i t y of aiirocyanide f o r t h e ion-exchange s i t e s on t h e column t r a t i o n fs:.

(ref.

78j, h o i w e r stile,- metal c y a n i d e s a r e a l s o r e t a i n e c t o some e x t e n t

and producs severe i n t e : * f e r e n c e i n t h e f i n a l chromatcgran: ( F i g . 22a). M a t r i x e l i m i n a t i o n on t h e c o n c e n t r a t o r

column

CE,I

be achieved u s i n g

s i m i l a ? apparatus t r J t h a t shcwn i n F i g . 15, b u t adapced t o p e r m i t t h e use o f two e i u e n t s . By c a r e f u l c o n t r o l o f t h e s t r e n g t h o f t h e e l u e n t and t h e volume de:ivet-ed

t h e c o n c e n t r a t o r column, i t i s possib;e t o l a r g e l y

77 e l i m i n a t e t h e i n t e r f e r e n c e s w i t h o u t loss o f aurocyanide ( F i g . 22b). process can be r e a d i l y a p p l i e d t o a v a r i e t y o f o t h e r sample t ypes.

This

0

.;Fi g . 22

M a t r i x e l i m i n a t i o n i n t h e a n a l y s i s of g o l d ( 1 ) cyanide i n mine process l i q u o r s . Chromatogram(a) shows t h e i n t e r f e r e n c e o f 5 ppm o f bexacyanocobalt ( I I I ) on t h e p r e c o n c e n t r a t i o n d e t e r m i n a t i o n o f 2 m l o f 50 ppb g o l d ( 1 ) cyanide. Chromatogram (b) shows a process l i q u o r c o n t a i n i n g 25 ppb g o l d and l a r g e excesses o f o t h e r metal cyano complexes p r e c o n cent rat ed u s i n g t h e two v a l v e system shown i n F i g . 15. S e p a r a t o r column: Waters Nova Pak C18. Co nc ent r a t o r column: Waters C18 Guard Pak. E l u e n t : 30:70 ( v / v ) a c e t o n i t r i l e - w a t e r c o n t a i n i n g 5 mM Waters Low UV PIC A . Wash volume: 800 pl S t r i p volume: 1600 pl D e t e c t i o n : UV abs o r p t i o n a t 214 nm. Reproduced w i t h p e r m i s s i o n f rom r e f . 77.

.

.

6.3.

POST-COLUMN M A T R I X ELIMINATION

I n some cases, i t i s p o s s i b l e t o e l i m i n a t e a rr.atrix peak a f t e r i t has e l u t e d from t h e a n a l y t i c a l column. For example, Brown e t a l . ( r e f . 79) have observed t h a t t h e i n t e r f e r e n c e o f doparnine on t h e d e t e r m i n a t i o n o f anions i n a pharmaceutical p r o d u c t u s i n g a p h t h a l a t e e l u e n t and i n d i r e c t UV

absorption

detection

could

be e l i m i n a t e d

by passing

the

column

e f f l u e n t through a h o l l o w - f i b r e suppressor d evice. Under t h e c o n d i t i o n s used, t h e dopamine became p r o t o n a t e d and d i f f u s e d t hrough t h e c a t i o n exchange membrane c o m p r i s i n g t h e h o l l o w f i b r e suppressor, and no i n t e r f e r i n g peak was m o n i t o r e d by t h e UV d e t e c t o r . When t h e i n t e r f e r i n g m a t r i x component i s UV absorbing and t h e i n d i r e c t UV a b s o r p t i o n d e t e c t i o n mode i s employed, i t may be p o s s i b l e t o e l i m i n a t e t h e m a t r i x peak by a j u d i c i o u s c h o i c e o f d e t e c t i o n wavelength. The

78 d e t e c t o r response i s given by r e f . 3. A

=

( € s -€E).

..........(8)

CS.1

where A s t h e absorbance change monitored by t h e d e t e c t o r d u r i n g passage o f t h e sample, and a r e t h e molar a b s o r p t i v i t i e s o f t h e s o l u t e and e l u e n t ions, r e s p e c t i v e l y , Cs i s t h e molar c o n c e n t r a t i o n of t h e s o l u t e and 1 i s t h e path l e n g t h o f t h e d e t e c t o r f l o w c e l l . I f a

4

9

d e t e c t i o n wavelength i s s e l e c t e d such t h a t : E s =

......... . ( 9 )

€ E

then no peak should be observed f o r t h a t p a r t i c u l a r s o l u t e . For example, t h e peak f o r n i t r a t e i o n i n samples prepared by n i t r i c a c i d d i g e s t i o n can be e l i m i n a t e d u s i n g a benzenesulphonate e l u e n t and a d e t e c t i o n wavelength of 239 mn ( r e f . 80). F i g . 23 shows t h e chromatograms o b t a i n e d f o r a m i x t u r e o f anions a t 239 nm and 225 nm. Note t h a t t h e d e t e c t i o n s e n s i t i v i t i e s f o r c h l o r i d e and n i t r i t e change w i t h wavelength and t h a t t h e peak d i r e c t i o n f o r n i t r i t e reverses a t 239 nm because o f a change i n s i g n f o r eqn. 8 a t t h i s wavelength.

I A

B

L

Time

Fig. 23

_ic

E l i m i n a t i o n o f t h e peak o f an absorbing anion ( n i t r a t e ) . Column: TSK-GEL I C Anion PW. E l u e n t : 1 mM benzenesulphonate. Detection: UV absorption a t 225 nm (a0 and 239 nm (b). Peak i d e n t i t i e s : ( l ) c h l o r i d e , (2) n i t r i t e , (3) n i t r a t e . The arrow i n (b) shows t h e e l u t i o n p o s i t i o n o f n i t r a t e . Reproduced w i t h permission from r e f . 80.

79

7.

CONCLUSION Sample hand1 i n g i n i o n chromatography i n c o r p o r a t e s many o f t h e same

p r i n c i p l e s g ov ern i n g c o n v e n t i o n a l HPLC methods. Nevert heless, some u n i q u e procedures apply t o i o n chromatography i n terms o f t h e sample d i s s o l u t i o n methods employed, use o f c a r t r i d g e columns f o r sample cleanup, cont am i n a t i o n e f f e c t s from cleanup d e v i c e s and f r o m chromatographic hardware, and sample p r e c o n c e n t r a t i o n u s i n g ion-exchange c o n c e n t r a t o r columns o r d i a l y t i c methods. Coupling o f t h e s e sample h a n d l i n g methods w i t h t h e powerf u l s e p a r a t i o n a b i l i t y o f i o n chromatography

should ensure t h a t t h e

tech nique can be a p p l i e d t o a v e r y wide range o f sample t ypes. 8.

ACKNOWLEDGEMENTS The a u t h o r wishes t o thank O r .

A l l a n Heckenberg o f t h e M i l l i p o r e

C o r p o r a t i o n and O r . A r t F i t c h e t t o f t h e Dionex C o r p o r a t i o n f o r t h e p r o v i s i o n o f unpublis h e d documents r e l a t i n g t o sample h a n d l i n g . REFERENCES

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