The Influence of Complexing Agents on the Retention/Elution of

complexing agents, with respect to retention of the analytes by activated alumina. The complexing agent selected must ensure that the analytes Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, V and Zn are maintained in solution, in a form accessible to activated alumina without inhibiting their reaction with the column packing material. Hence, analyte complexes formed on the addition of the complexing agent to the sample and standard solutions, must prevent precipitation of the analyte and the formation of anionic species. The passage of these solutions through the column, must then result in the retention of the analytes by activated alumina, as detailed in section 3.1. The complexing agent should not inhibit retention by forming very strong complexes with the analytes. Hence, activated alumina must remain able to retain the trace analytes, when present in solution as analyte complexes. o I! C — OH n o — c — h I + Mn+ HO — C — H C — OH II o

H O I / / H — C — C I \ H OH H O O H n+ \ / / ^ I + M —^ II— C — C C — C — H + 2H ^ _H 1 \ 0 _{U \ n + ^ °} 0 / _H1 M +

Possible Analyte-Acetate Complex Formation

OH O

/ n+ . / \ n+

A1 + M - C E z _ A1 M + C E

^ OH N s‘ O ^

* CE is the complexing agent

Proposed Retention of Analytes by Activated Alumina

This reaction is proposed for the majority of the analyte complexes formed which are of neutral charge and hence not in a form that alumina can retain under alkaline conditions. The exceptions are Fe^+ and Cr^+ It is possible therefore, that these analytes may also be retained as analyte complexes. This suggests that Fe and Cr may be retained by alumina in a different form to the remaining analytes. Hence differences may be observed for these analytes. Furthermore, investigations by Kummert and Stumm (164) have suggested that organic acids are retained by activated alumina. Therefore, as the complexing agent is present in excess, it may also be retained.

. OH / 0 H _{HO— C — H} c — OH _{v \} A1 + I H — C ---OH + 1^0 \ _{>■ ATT} HO— C — H_\ OH ^ C — OH O

Tartaric Acid Retention by Alumina

H — C OH / HO— C II O 146

Tartaric acid is a dicarboxylic acid, it is therefore possible, under the alkaline experimental conditions utilised for optimum analyte retention, that the second COOH group may de-protonate, producing an alternative reactive site for analyte retention. This will not occur for acetic acid. In fact, if acetic acid is used, it may hinder retention of the analytes, as it will compete for active sites. It is therefore possible that differences will be observed, with respect to analyte retention, with the use of different complexing agents. OH OH o Al ° A1 \ _{x o — C \} 11 X o II_\ H — C OH n+ H — C ---- OH j + M - buffer ^ | + buffer H—y C--- OH H —^ OH ' o — c o — c ii OH Al \ O OH

Possible Alternative Site for Analyte Retention o OH H s O O ---C H / I / / v / \ / Al + H — C C — ± AY C \ _{N OH H} 1 \ _OH \ _{x OH} h_H H_H

Acetic Acid Retention by Alumina

The capability of activated alumina to retain the trace analytes Ba, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, V and Zn, in the presence of Buxton Mineral Water, was

t

investigated. Using the procedure detailed in section 2.3.2, a volume of 0.5 ml of the sample, (Buxton Mineral Water, multi-element spike, Ba, Ca, Cd, Co, Cr, Cu, Fe, Mg, Mn, Ni, Pb, V and Zn, 50 pg 1"*) and a standard solution, adjusted to pH 8, were processed using the FI system. The procedure was repeated for Buxton Mineral Water

containing either 0.025 M tartaric acid or acetic acid. Typical time/elution profiles are displayed in Fig.4.2, 4.3, 4.4 and 4.5.

Fig.4.2 shows the time/elution profiles for Fe. The analyte Cr indicated similar retention/elution behaviour to the analyte Fe. The plots show that only background levels of the analyte Fe is detected, following the injection of sample. This indicates effective retention of the analyte from the standard solution, Buxton Mineral Water and Buxton Mineral Water which contains either 0.025 M tartaric acid or acetic acid. A transient signal was observed for the analyte, following the injection of eluent, indicating that the retained analyte could be subsequently eluted. However, the transient signal detected, differed for each matrix. A peak height of 1557 was obtained for 50 jig 1"* Fe, present in the standard solution. The peak height for Buxton Mineral Water (50 pg 1"* Fe spiked) was 1788. This increase suggests that the sample contained Fe at a detectable level. Buxton Mineral Water with added acetic acid, produced a peak height of 1761. This slight decrease may be due to competition for active sites by acetic acid, as discussed above, which resulted in incomplete retention of the analytes and/or slight suppression of the analyte signal in the presence of acetic acid. Buxton Mineral Water containing added tartaric acid produced a peak height of 750. However, although the height has more than halved, the peak width has doubled. It is therefore possible that Cr and Fe are retained as analyte-tartrate complexes, as discussed above. Thus these analytes form stronger complexes with alumina in the presence of tartrate, as shorter, broader peaks are observed on elution. It should be noted that the peak areas calculated for the profiles Fe (Buxton), Fe (Buxton/acetate) and Fe (Buxton/tartrate) in Fig.4.2 were virtually equivalent, that peak area is normally utilised for results generated in this study (see section 2.3.2) and hence peak heights are discussed here for comparison purposes only.

i

Fig.4.3 shows the time/elution plots for Cd. The analytes Co, Cu, Mn, Ni, Pb and Zn exhibited similar retention/elution behaviour to Cd. The plots indicate effective retention and elution of the analyte Cd, in the presence of the standard solution, Buxton Mineral Water and Buxton Mineral Water containing either 0.025 M acetic acid or

tartaric acid. The differences in peak height observed for the different matrices in Fig.4.2, were detected for Cd also. The peak height for the standard was 758, Buxton Mineral Water, 788, Buxton Mineral Water containing acetic acid, 762 and tartaric acid, 689. The peak width for sample containing tartaric acid was slightly broader than the remaining matrices. It is possible therefore, that the alternative active site proposed above, is produced and utilised. Hence, these analytes form slightly stronger complexes with alumina in the presence of tartrate, as slightly shorter, broader peaks are produced. It should be noted that the peak areas calculated for the four profiles in Fig.4.3 were virtually equivalent.

Fig.4.4 shows the time/elution profiles for Ca. The analytes Ca, Mg and Ba indicate retention/elution behaviour similar to Ca. Effective retention of Ca is observed for the standard solution only. Incomplete retention of Ca is observed for the remaining matrices. This is mainly due to the high concentration of this analyte (mg 1“* level) in Buxton Mineral Water. The peak height for the standard solution was 714, Buxton Mineral Water, 40841, Buxton Mineral Water containing acetic acid, 45041 and tartaric acid, 47268. The observed increase in peak height from 40841 to 47268, is due to increased retention of the analyte, which confirms that tartaric acid is an efficient complexing agent. It is able to prevent the precipitation of Ca, Ba and Mg at high pH and/or the formation of a species which alumina can not retain.

The analyte V behaves differently to every other analyte. The time/elution plots are shown in Fig.4.5. The analyte shows complete retention from every matrix. However, in the presence of the standard solution, Buxton Mineral Water and Buxton Mineral Water containing 0.025 M tartaric acid, the carrier solution (0.02 M NH4OH), which follows the sample plug, elutes a proportion of the analyte retained. The proportion eluted decreases in the series Buxton + tartrate < Buxton < standard. It is

i

possible that at pH 8, a proportion of the V is converted to species such as vanadate, which are weakly retained by alumina, in the presence of these matrices. Hence, in this situation, acetic acid is a more efficient complexing agent. It should be noted that the V retained, in the presence of Buxton Mineral Water containing tartrate, demonstrates

1S57 934 311 17 0_{1 21 40 60 80} Fe ( aqueous standard) 17 Fe ( Buxton ) 1409 i1 V 1057 704 352 17 0 CO 300 ISO 17 0 Fe ( Buxton/acetate ) Fe ( Buxton/tartrate )

Fig.4.2 The emission/time response for Fe, for a 0.5 ml volume of sample. 1:- standard solution. 2:- (Buxton Mineral Water, multi-element spike, Ba, Ca, Cd, Co, Cr, Cu, Fe, Mg, Mn, Ni, Pb, V and Zn, 50 pg/1). 3:- sample 2 containing acetic acid (0.025 M). 4:- sample 2 containing tartaric acid (0.025 M).

•40 68 88 sec 100 Cd ( aqueous standard) 780 3 IS 1SB 21 0 sec 100 1 21 48 80 Cd ( Buxton ) 762 305 152 21 0_{1 21 60 80} Cd ( Buxton/acetate) 551 d_I 9 413 276 13B 21 0 Cd ( Buxton/tartrate )

Fig.4.3 The emission/time response for Cd, for a 0.5 ml volume of sample. 1:- standard solution. 2:- (Buxton Mineral Water, multi-element spike, Ba, Ca, Cd, Co, Cr, Cu, Fe, Mg, Mn, Ni, Pb, V and Zn, 50 fig/1). 3:- sample 2 containing acetic acid (0.025 M). 4:- sample 2 containing tartaric acid (0.025 M).

12

C a ( aqueous standard ) Ca ( Buxton )

1BB16

37B14

Ca ( Buxton/acetate) Ca ( Buxton/tartrate )

Fig.4.4 The emission/time response for Ca, for a 0.5 ml volume of sample. 1:- standard solution. 2:- (Buxton Mineral Water, multi-element spike, Ba, Ca, Cd, Co, Cr, Cu, Fe, Mg, Mn, Ni, Pb, V and Zn, 50 jig/I). 3:- sample 2 containing acetic acid (0.025 M). 4:- sample 2 containing tartaric acid (0.025 M).

13 V ( aqueous standard ) ©t_A 1 V 217 189 13 e V ( Buxton ) 13 V ( Buxton/acetate ) V ( Buxton/tartrate )

Fig.4.5 The emission/time response for V, for a 0.5 ml volume of sample. 1:- standard solution. 2:- (Buxton Mineral Water, multi-element spike, Ba, Ca, Cd, Co, Cr, Cu, Fe, Mg, Mn, Ni, Pb, V and Zn, 50 jig/I). 3:- sample 2 containing acetic acid (0.025 M). 4:- sample 2 containing tartaric acid (0.025 M).

elution behaviour similar to Cr and Fe (see Fig.4.2). In summary:

1. Every analyte (Ba, Ca, Cd, Co, Cr, Cu, Fe, Mg, Mn, Ni, Pb, V and Zn) present in the standard solution is effectively retained by activated alumina. However, a proportion of the analyte V is eluted by the carrier solution (0.02 M NH^OH), which follows the sample zone. This is assumed to be due to the formation of V species at pH 8 which are weakly retained by alumina, and can therefore be easily eluted.

2. Every analyte retained in the presence of the standard solution is subsequently eluted from alumina with the injection of 250 pi of eluent (2 M HN03).

3. The analytes Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, V and Zn are effectively retained by activated alumina, in the presence of Buxton Mineral Water. The remaining analytes (Ba, Ca and Mg), indicate incomplete retention. The analytes Ba, Ca and Mg are present in Buxton Mineral Water at the mg 1“* level (0.4, 55 and 19 respectively). The remaining analytes are present at the 50 pg f* level in this study. Therefore the analytes Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, V and Zn are selectively retained by activated alumina in the presence of high concentrations of Ba, Ca, Mg. The analyte V shows similar behaviour as detailed in 1. above. The proportion of V eluted by the carrier however was notably lower.

4. Every analyte retained in the presence of Buxton Mineral Water is subsequently eluted from alumina with the injection of 250 pi of eluent (2 M HN03). The peak height obtained for each analyte however, is notably greater than those found for the standard solution. This is assumed to be due to either a detectable presence of the analytes in the original sample and/or a clearer more controlled desorption process for the analytes retained in the presence of the sample.

5. The analytes Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, V and Zn are effectively retained by activated alumina, in the presence of Buxton Mineral Water containing added acetic acid (0.025 M). The remaining analytes (Ba, Ca and Mg), indicate incomplete retention as detailed in 3. above. However, the proportion of Ba, Ca and Mg retained by activated alumina, notably improves with the addition of acetic acid. Also the analyte V is not

eluted by the carrier. Hence the presence of the complexing agent acetic acid improves retention.

6. Each analyte retained in the presence of Buxton Mineral Water containing added acetic acid (0.025 M), is subsequently eluted from alumina with the injection of 250 pi of eluent (2 M HN03). The peak height obtained for each analyte however, is slightly lower than those found for Buxton Mineral Water. This may be due slight suppression of the analyte signal in the presence of acetic acid.

7. The analytes Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, V and Zn are effectively retained by activated alumina, in the presence of Buxton Mineral Water containing added tartaric acid (0.025 M). The remaining analytes (Ba, Ca and Mg), indicate incomplete retention as detailed in 3. above. However, the proportion of Ba, Ca and Mg retained by activated alumina, significantly improves with the addition of tartaric acid. A proportion of the analyte V is eluted by the carrier as detailed in 1. However the proportion eluted is significantly lower than that found for the standard and Buxton Mineral Water. Hence the presence of the complexing agent tartaric acid improves retention.

8. Each analyte retained in the presence of Buxton Mineral Water containing added tartaric acid (0.025 M), is subsequently eluted from alumina with the injection of 250 pi of eluent (2 M HN03). The peak height obtained for each analyte however, is significantly lower than those found for Buxton Mineral Water, but the peak width has significantly increased accordingly. It is proposed that this is due to the formation of an alternative reactive site on activated alumina, in the presence of tartaric acid, which binds the analytes more strongly.