Analytica Chimica Acto, 200 (1987) 35-49 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands

FUNDAMENTAL AND PRACTICAL CONSIDERATIONS IN THE DESIGN OF ON-LINE COLUMN PRECONCENTRATION FOR FLOW- INJECTION ATOMIC SPECTROMETRIC SYSTEMS

ZHAOLUN FANG*, SHUKUN XU and SUCHUN ZHANG

Institute of Forestry and Soil Science, Academia Sinica, Box 417, Shenyang (China)

(Received 2nd April 1987)

SUMMARY

Fundamental considerations and practical points in the development of on-line pre- concentration flow-injection with atomic spectrometric detection are discussed, with emphasis on overall efficiency and accuracy. The terms concentration efficiency (CE) expressed as the enrichment factor achieved per minute by the system, and retention efficiency (%E), together with recovery studies on real samples, are recommended as criteria for the evaluation of the efficiency and reliability of such systems. Time-based sampling and double-column systems without intermediate column washing are recom- mended for improving efficiency. Large columns (3 mm i.d., 45 mm long) with high sample-loading rates of 9.5 ml min-’ are proposed for achieving high efficiency and accu- racy with atomic absorption detection; more careful optimization of column dimensions is needed for detection with inductively-coupled plasma/atomic emission spectrometry. A procedure for the on-line preconcentration of cobalt in water samples was developed under the guidelines presented. An enrichment factor of 48 was achieved at a sampling frequency of 60 h-’ with good recoveries for all the water types studied. The precision was 1.7% r.s.d. at the 40 fig 1-l level, and the detection limit (30) was 0.2 pg 1-l.

In recent years, flow injection analysis (f.i.a.) has been repeatedly proved to be a powerful means of extending the capabilities of conventional atomic spectrometry, particularly flame atomic absorption spectrometry (a.a.s.) and inductively coupled plasma/atomic emission spectrometry (i.c.p./a.e.s.) in trace analysis [ 11. Improvements in terms of sample throughput, economy in sample and reagent consumption, tolerance of salt content and interfer- ences, calibration procedures and pretreatment procedures have been repor- ted [l] . In the last category, the on-line preconcentration of trace metals by using micro-columns has been shown to enhance the sensitivity of trace

*Zhaolun Fang, Research Professor of Analytical Chemistry in Soil Science and Environ- mental Sciences at the Institute of Forestry and Soil Science of the Chinese Academy (Academia Sinica) is a graduate of Beijing University. He is author of over 60 research papers. His main research interests include development of atomic spectrometrm and flow-injection methods and their applications to soil science and environmental sciences. He is chairman of the Chinese Society for Development of Flow Injection Analysis.

0003-2670/87/$03.50 0 1987 Elsevier Science Publishers B.V.

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element determinations by flame a.a.s. and i.c.p./a.e.s. by factors of 10-100 at sampling frequencies of lo-60 h- l . Extensive interest was stimulated by the first publication on the subject by Olsen et al. [2] in 1983, and there has been a roughly exponential increase of related publications per annum in the ensuing years [l-17] . Various modifications have been proposed to improve the performance of the system [5] and the detection system has been gradu- ally extended from flame a.a.s. to i.c.p./a.e.s. The technique has been success- fully applied to the determination of trace elements in a large variety of real samples including tap water [3, 81, natural waters [ 4, 6, 111, soil extracts [l] , plants [ 14-161, bovine liver [14], and steels [lo], and also in synthetic sea water [ 2, 51 and EPA quality-control water samples [ 81 .

Before the advent of on-line column preconcentration, numerous papers reported on the preconcentration of trace elements by off-line batch pro- cedures either with columns or by static equilibration prior to atomic spectrometric determination [ 181. Such methods offer prominent advan- tages in achieving a large gain in sensitivity as well as separation of the inter- fering sample matrix, but they are tedious to operate, especially when com- pared to the final atomic spectrometric step, and several litres or hundred millilitres of sample are often required for each determination. These short- comings have largely been overcome by using on-line column preconcen- tration whilst the advantages of the off-line procedures are preserved.

The advantages of on-line column preconcentration over conventional batch procedures can be summarized as follows. First, the columns offer much greater efficiency; enrichment factors are one or two orders of magni- tude higher for a defined time interval, or a larger number of samples can be processed to achieve a certain enrichment factor in a fixed time period. Secondly, sample consumption is usually 5-10 ml per determination for enrichment factors of 10-100, while reagent consumption is usually only a few hundred microlitres of eluant per sample with <lOO mg of column packing; the latter will normally last for hundreds of samples. Thirdly, the closed preconcentration system decreases the risks of contamination from the laboratory environment. Finally, the continuous monitoring of the baseline provides better checks on the column performance.

However, the application of on-line column preconcentration is not with- out problems. In some cases, the improvement in efficiency is only marginal and, occasionally, interference effects seem even to be enhanced. In this paper, an attempt is made to assess fundamental and secondary aspects of the design of on-line column preconcentration flow-injection systems with atomic spectrometric detection. The scattered information on the tech- nique is at least partially integrated in order to present some guidelines for future improvements. Recent developments of the technique in this laboratory serve to support some of the viewpoints.

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EXPERIMENTAL

Apparatus The atomic absorption spectrometer was a Nippon Jarrell-Ash AA-1 MK II

model connected to a Yanaco YR-101 chart recorder. The sample uptake rate of the nebulizer was adjusted to 4 ml min-’ . The wavelengths used for cobalt and cadmium were 240.8 and 228.8 nm, respectively.

A Gilson Minipuls &channel peristaltic pump was used with tygon pump tubes. A two-layer multi-functional valve following a previous design [ 51 was used, but the push-fit tube connections were replaced by connections with threaded fittings and flanged tube-ends furnished with silicone rubber washers to form a seal at the ends. This improved the reliability of the con- nections under the slightly elevated hydrodynamic pressures caused by the incorporation of a packed column in the line. A pneumatic system similar to that described by Jdrgensen et al. [9] controlled by an electronic timer was used to operate the valve automatically. A two-way pneumatic cylinder (Festo DSN, 8-mm diameter, 40-mm stroke) and two solenoid valves (F-to MFH-3-M5) were used to construct the automated system.

The columns were made from Plexiglas with threaded fittings at both ends (Fig. l), teflon washers with conical holes were inserted at both ends to ensure a streamlined flow through the column. The teflon tubes for connection were flanged at the ends and furnished with silicone rubber washers as in the connections for the valve. The tube and washers were slipped through the threaded fitting and connected to a column with a piece of nylon gauze inserted between the flanged tube-end and teflon washer to keep the column packing in position. The column design is more complex than a previously reported version with push-fit connections but the relia- bility is much improved, completely avoiding leakages and severance at the connections.

The manifold for the on-line preconcentration system was basically the same as that used previously [ 51 except that a single pump was used without timing. The loading (preconcentration) and elution periods were governed by settings on the timer which controlled the operation of the multifunc- tional valve via the pneumatic system. A simple two-way valve was included in the eluant line and operated manually to switch the eluant from one column to the other to permit sequential elution of the columns during the elution stage (Fig. 2).

Reagents and procedures All chemicals were of analytical-reagent grade. Deionized water was used

throughout. Standard solutions of cobalt and cadmium were made by two- or three-

stage dilutions of aqueous 1000 mg 1-l stock solutions. The ammonium acetate buffer solutions were prepared by diluting a 2 M ammonium acetate solution adjusted to the appropriate pH with 25% ammonia solution.

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1 A

7 65

Fig. 1. Construction of on-line preconcentration column: (1) Plexiglas column body; (2) Plexiglas fitting; (3) teflon tube with flanged end; (4) silicone rubber washer; (5) nylon gauze; (6) teflon washer with conical hole; (7) column packing.

-1 w--___..__

ml mm - AASI- I_--,.--

SA f&s ___..__ B

___.._-

P VI w

LOAD

LOAD ELUTE

1 90s ;15sr15s, A A A A Tl TI T2 T2

Fig. 2. Schematic diagram of flow-injection atomic absorption spectrometric system with on-line ion-exchange : P, pump; VI, multifunctional valve, Vu, elution-stage column selection valve; SA, SB, samples A and B; CA, CB, columns A and B; B, buffer solution ; E, eluant; W, waste; AAS, atomic absorption spectrometer; T, , T, , turning points for valves VI and Vu.

The Chelex-100 and Squinolinol chelating exchangers were as previously reported [5] ; the weakly acidic 122 resin in the previous report [ 51 was replaced by a 501 resin, which has the same properties as the 122 resin.

Procedures were as reported previously [5] , except that the timer was used to control the valve instead of the pump and 2 M hydrochloric acid was used as eluant. Samples were buffered to pH 8 for the CPGSQ column and to pH 9 for the other columns.

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PRIMARY CONSIDERATIONS IN THE DEVELOPMENT OF ON-LINE COLUMN PRECONCENTRATION SYSTEMS

The ultimate purpose of developing an on-line column preconcentration system for flame a.a.s. or i.c.p./a.e.s. must be considered in relation to the particular application involved. In principle, each of the advantages for on- line preconcentration cited in the introductory section might be regarded as part of the purpose; however, the advantages in material consumption, lack of contamination and baseline monitoring, taken individually, may not stimulate enough interest to construct an on-line preconcentration system, as they might either be achieved in other ways or be considered of minor importance. Low sample consumption can be an important feature which could prompt the use of an on-line system, either when only limited amounts of samples are available or to save effort in collection and/or transportation of large sample loads; but in such situations, some workers might prefer to use electrothermal atomization methods. Thus, it is safe to say that for most applications the efficiency offered by the on-line system will be the principal purpose of adopting the procedure. This does not imply that the other merits are unimportant and can be ignored in design- ing on-line preconcentration systems but that they show their real value only when they are combined in an efficient system.

It is important therefore to set some criterion for evaluating the efficiency of different on-line ion-exchange systems, so that the favourable aspects from different designs may be located and integrated or enhanced, providing stimulation for further development. Commonly, both the enrichment factors and the sampling frequencies are mentioned in evaluating the efficiency of on-line preconcentration systems. This is reasonable, as both criteria are equally important in the assessment of efficiency and neither is independent of the other. Fang et al. [5] proposed the integration of the two criteria by using the term “concentration efficiency”, which was defined as the product of enrichment factor (EF) and the sampling frequency in number of samples analyzed per minute; they used this term to compare different on-line column preconcentration systems for illustration of the development of the technique [5] . In the present paper, the term is abbre- viated as CE, expressed in EF min- I. Thus, a system capable of attaining 25-fold enrichment at a sampling frequency of 25 h-i (0.417 min-‘) will have a CE value of 10.4, whilst one achieving loo-fold enrichment at 60 h-’ will have a CE value of 100 EF min-‘. The CE value of a typical manual batch procedure is usually less than 4 even when operated in batches of ten, and so is obviously much less efficient than an on-line system. It should be taken into account, however, that with conventional batch preconcen- trations the spectrometer will be required for measurements only after com- pletion of the preconcentration step, whereas with on-line column precon- centration, the spectrometer will be working full-time. As the preconcen- tration step normally takes longer than the elution stage, the spectrometer

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is not in use for a large percentage of the total working period, even when two columns are used alternately for preconcentration (see, e.g. [4, 91). Thus, an assessment of the overall efficiency of this type of on-line pre- concentration system will not be complete without consideration of the most efficient usage of the spectrometer involved. This is especially impor- tant when a multichannel i.c.p. spectrometer capable of delivering large amounts of analytical data and consuming large amounts of argon is used for detection. Therefore it is suggested here that the CE value for an on-line preconcentration system based on atomic spectrometry should be at least twice as high as the conventional batch procedure in order to outweigh the loss in efficient usage of the spectrometer. In practice, this means that an on-line system with CE < 8 probably will not show much advantage over conventional batch procedures in terms of overall efficiency of the system. Of course, even with the most inefficient on-line system, the analytical readout will be available within a few minutes, which is much faster than any efficient conventional batch method, yet this favourable feature may have little significance except in process control. In most circumstances, the sample throughput per day and overall costs are of greater concern. Conven- tional batch procedures are then favoured because numerous columns can be processed in parallel; simultaneous use of more than 3-4 columns in an on-line preconcentration system would require highly sophisticated control.

Obviously, any trace method requires not only sensitivity and efficiency but also reasonable accuracy. This is a vital factor in considering the selec- tion of a system because a careful balance between efficiency and accuracy is often necessary. An effective measure for improving efficiency might also degrade the accuracy through deterioration of retention efficiency, defined by Hartenstein et al. [7] as the percentage of total amount of analyte retained on the column and abbreviated as SE. Under the controlled con- ditions of a flow-injection system, low retention efficiencies do not neces- sarily imply a serious loss in precision, but effective control over matrix effects and interferences from competing trace elements cannot be guaran- teed if the separation cannot be considered reasonably “clean”. Even for trace elements which show 100% retention efficiency, there may be large drops in %E when matrices contain significant amounts of interfering species. Unlike the retention efficiency, the elution efficiency, defined as the percentage elution of the retained analyte from the column, is normally high, particularly with chelating ion-exchangers, provided that the appro- priate eluant is chosen. Hence, for critical evaluation of an on-line column preconcentration system, the features to be considered are not only the con- centration efficiency CE but also the retention efficiency %E, recovery data obtained in the presence of the sample matrix, and sometimes also the elution efficiency,

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SECONDARY CONSIDERATIONS IN THE DEVELOPMENT OF AN ON-LINE COLUMN PRECONCENTRATION SYSTEM

In this section, some tactical considerations on the development of on-line column preconcentration spectrometric systems which are both efficient and interference-free will be discussed. The various factors involved are often supplementary and sometimes even contradictory, so that care is needed not to overemphasize some favourable aspect as it might often lead to harmful effects in other factors.

Time-based vs. volume-based sample loading The process of sample loading on packed columns is a key link in the

entire preconcentration procedure. Proper choice of manifold design and operational parameters in relation to sample loading are essential for achiev- ing good efficiency and accuracy.

Different modes involving either the injection of a defined volume of sample from a sample loop on to the column (volume-based loading) or the pumping of sample at a definite flow rate through the column for a defined time interval (time-based loading) have been proposed by different workers. When other factors are similar (e.g., number of columns, column-packing material, etc.), time-based loading systems [3, 5, 7, 9, 131 usually give higher CE values than volume-based systems. Obviously, it is simpler and more straightforward to load the sample directly on to a column without prior introduction into a loop. This also obviates the need for a washing stage to remove the somewhat dispersed sample completely from the loop, which is necessary for volume-based loading. Admittedly, with time-based loading, the sample remaining in the pump tube at the end of the loading period will have to be washed out of the pump tube by the next sample, but this can be done during the elution state and will not require extra time as in the volume-based mode [4, 81. Samples can normally be changed in lo-20 s, depending on the sample flow rate, during the elution period so that the next sample-loading cycle can be initiated immediately after the elution period without appreciable carry-over.

A disadvantage of time-based sample loading is its larger dependence on the stability of the flow rate. Because of the relatively large back-pressures produced in flow systems incorporating packed columns compared to open tube systems, the flow rates obtained with the commonly used peristaltic pumps tend to be sensitive to pressure changes in the flow system, created by partial line blockages (e.g., from accidentally released resin particles, etc.) and also by swelling and shrinking of certain ion-exchangers such as Chelex- 100. Thus, with time-based loading, regular checks of the sample flow rate are needed, particularly when a new column of different column dimensions or different packing material is used for the first time or when large fluctu- ations in sensitivity are observed.

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Single-column vs. double-column on-line preconcentration systems Both single and double column systems have been proposed. The obvious

advantage for a single-column system is its simple construction; the simplest version can be built with a few pieces of basic flow-injection hardware [2, 8, lo]. Yet when other factors are considered, such as reversal of flow direction during elution, intermediate washing of the column between elution and sample-loading sequences, avoiding discharge of sample waste into the nebulizer, etc., the system becomes more complicated [2,3,13]. A double-column system was proposed by Fang et al. [4] to enhance the efficiency of an on-line preconcentration system. Sample loading and elution were performed on the two columns alternately, i.e., while one column was being loaded, the other was being eluted, using a multiftinctional valve (commutator). In this way, a CE value of 13-18 was attained for Ni, Cu, Pb, Cd even when the relatively inefficient volume-based sample loading was used. The double-column system was later improved by using time-based sample loading and more efficient column packing; furthermore, with the consideration that the elution period (15-20 s) is usually much shorter than the sample-loading period, a double-column system which allowed simultaneous sample loading and sequential elution of the analyte from the two columns was proposed and CE was increased to 50-100 [5]. Such a system with 90-s loading period and 15-s elution period takes 120 s to com- plete a cycle (two samples analyzed), whilst an alternately operated double- column system with the same loading period will take 180 s for a complete cycle, and a single-column system will take 210 s to analyze two samples. Thus the efficiency of the preconcentration system can be almost doubled by using a double-column system. Operation is not complicated if suitable valves with adequate timing facilities are available.

A possible drawback of double-column systems is that, although elution peaks from a single column are usually reproducible, often with precisions of better than 2% r.s.d., columns which give identical peak heights are difficult to prepare because the peak heights are extremely sensitive to variations in tightness of packing materials and differences in the geometry of the flow systems. Yet this difficulty can be overcome either by using the average of the peak heights or by constructing separate calibration curves for each of the columns. It was even suggested that two columns with different packing materials can be used to provide a cross-check on the accuracy [5] .

The effect of intermediate washing of columns In conventional batchwise column preconcentration, it is common prac-

tice that after each elution the column is washed with water and equili- brated with a buffer solution to restore the pH to that optimized for the retention of the analyte or separation of matrix elements. This policy has been adopted in most on-line column systems either by using the buffer as the carrier for the eluant [ 2, 3, 81 or by using a separate line for the

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equilibration [13, 151. This was considered to be necessary because elution by a strong acid or alkali immediately followed by introduction of the next sample to the column will result in loss of analyte in the initial stage of load- ing owing to unsuitable pH conditions. Yet, in some on-line column precon- centration systems, the washing stage was completely omitted to simplify the flow system and improve the efficiency [ 5, 71. No degradation in the preconcentration capability or precision was observed in these systems.

It is, therefore, essential to consider if the washing (equilibration) of columns is really necessary in an on-line column preconcentration system, and if omitting the washing degrades sensitivity or precision. A detailed study of these points was made in this laboratory with the system outlined in Fig. 2. The columns were packed with CPGSQ exchanger and an ammo- nium acetate buffered (0.025 M, pH 8) sample contaimng 0.6 mg 1-l cobalt was pumped through the columns immediately after the elution of the pre- vious sample with 2 M HCl. The outlet from the column was connected to the nebulizer of the atomic absorption spectrometer, and the cobalt signal recorded. A peak appeared in the initial stage of sample loading which lasted for a total of 12 s. The area of this peak was compared to that of the sample aspirated in the conventional mode at an identical pumping rate for a period of 90 s. The short duration of the peak show that the columns can be rapidly equilibrated by the buffer in the sample under the flow conditions used. The area of the peak, which was considered to be the amount of cobalt lost because of the lack of an equilibration sequence, constituted only 5% of the total amount. Ten replicate recordings of the breakthrough peak gave an r.s.d. of 2%, which shows that the leakage is reproducible. The percen- tage leakage may vary with different elements and column-packing materials and the duration of the sample loading period. But with sample-loading periods of over 1 min, which is the case with most on-line column systems, the omission of a washing/equilibration sequence seems not seriously to affect the sensitivity or precision of the determination, whereas the advan- tages gained by simplifying the manifold and improving the overall efficiency are obvious.

Washing the CPG-8Q columns with deionized water after sample loading before elution with 2 M HCl also did not show any advantage in terms of sensitivity and precision for cobalt. On the contrary, the lowering of pH during the washing caused some loss of cobalt so that the elution peak was lower than when no washing was done.

Optimization of sample-loading rate and column dimensions As stated above, the sample-loading sequence is the key link in the entire

procedure for achieving high efficiency and accuracy. The principal factors to be optimized for this key link are the loading (flow) rate and column dimensions including the ion-exchange capacity, although pH and eluant composition can also be important in some situations. A joint evaluation of the concentration efficiency CE and retention efficiency %E of an ana-

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lyte, together with recovery studies of real samples and potential interferents is recommended for the optimization of the factors. This is important because pursuance of efficiency without simultaneous consideration of its effects on accuracy is rarely justifiable in the context of real samples.

Optimization studies have shown that an increase in sample-loading rate within an upper limit enhances the enrichment factor by increasing the sample volume passing through the column within a certain time interval 17, 13, 161. However, the effects of sample-loading rate on the tolerance for interferences in relation to column dimensions and capacity seem not to have been studied hitherto.

Such an investigation was undertaken in this laboratory, and the results are tabulated in Tables 1 and 2. Cobalt, which forms medium-strong com- plexes with the functional groups of the chelating ion-exchangers studied, was used as a model element, and cadmium, representing the metals which form the weakest complexes, was used for comparison. Although high loading rates are expected to yield oetter concentration efficiencies, their optimum values are limited by the column dimensions and the capacity of the ion-exchanger. Ignoring factors associated with tolerance capacity of interferences, the loading rate should not be so high as to produce excessive back-pressure in the system, because this will create problems in leakages and fluctuations in the flow rate; nor should it produce excessively low retention efficiencies. Considering these factors and the range of column dimensions often used for on-line preconcentrations, a loading rate of 9.5 ml min-’ was chosen for these studies. For the larger columns (3 X 45 mm), this is close to the upper limit of ensuring a uniform flow; for the smaller columns (2.3 X 12 mm), this is also the upper limit before the preconcentration pro- cess becomes counterproductive, with retention efficiencies below 50% [7]. Columns with different dimensions, representative of those reported earlier, were compared, with CPGSQ as the packing. The largest columns (3 X 45 mm) were also packed with Chelex-100 and Resin 501 for comparison.

The results of these tests (Tables 1 and 2) allow some conclusions to be drawn. With the sample-loading rate at its highest practicable limit, the con- centration efficiency of the on-line system remains relatively constant within a factor of 2, for different column dimensions, sample residence times, ex- change capacities and even with different ion-exchangers and analyte metals, provided of course that reasonably strong complexes are formed between the metals and the functional groups at the pH used. It is inferred, therefore, that other factors in the design of an on-line preconcentration system (time- based or volume-based sampling, single or double columns, etc.) will have more influence on the concentration efficiency of the system.

An explanation for this behaviour could be that the higher retention efficiencies obtained with larger columns, and hence with larger exchange capacities and longer residence times, were counteracted by losses in peak height during elution because of increased dispersion in the larger dead volume of the columns. With the smaller columns, lower dispersion in

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TABLE 1

The effects of column dimensions and capacity of CPG-8Q ion-exchange columns and effect of different ion-exchangers on concentration efficiency retention efficiency and recoveries of cobalt and cadmium in water samples

Column Columna packing

i.d. length

Cobalt CPG-8;$ 3.0 45

2.3 20 2.3 12

Resin 501 3.0 45 Chelex-100 3.0 45

Residence CE En time (EF min-‘) (%) (s)

1.0 48 95 96 99 102 98 0.3 58 95 90 66 98 83 0.15 50 80 73 56 98 72 1.0 35 96 37 89 100 99 1.0 34 87 98 101 - 105

RecoveryC (%)

Sea Tap Tap Waste water waterd water water

Cadmium CPG-8Q 3.0 45 1.0 55 98 95 42 104 60

2.3 20 0.3 55 87 58 30 100 50 2.3 12 0.15 56 65 46 18 85 32

Resin 501 3.0 45 1.0 52 97 28 87 102 81 Chelex-100 3.0 45 1.0 28 81 68 88 100 94

aDimensions in mm. b%E evaluated by passing 1 mg 1-l (each) Co and Cd solution through column. ‘Loading rate 9.5 ml min.‘; 90-s preconcentration period; samples spiked with 100 pg 1-l Co and 20 pg 1“ Cd. dTap water collected from a new pipeline with high zinc content.

TABLE 2

The effects of sample-loading rate, column dimensions and capacity of CPG-8Q ion- exchange column on recoveries of cobalt and cadmium in a tap-water sample with high zinc contenta

Loading rate (ml mine’) Column i.d. (mm) Column length (mm) Residence time (s) Recovery (%)

Cobalt Cadmium

9.5 9.5 9.5 1.5 1.5 1.5 3.0 2.3 2.3 3.0 2.3 2.3

45 20 12 45 20 12 1 0.3 0.15 6 2 1

99 66h 56 59b 58 31 42 30 18 29 12 8

aThe water samples were spiked with 100 pg l-’ cobalt and 20 rg 1-l cadmium. A 14-ml aliquot of sample was taken in each case for the reconcentration. The zinc content of the sample was 8 mg 1-l unless stated otherwise. g Sample, collected on a separate day, contained 12 mg 1-l zinc.

elution would be counteracted by losses of analyte caused by insufficient contact time (as low as 0.15 s) with the column packing. The relatively low retention efficiency for cadmium with the smaller columns was probably compensated by the faster elution producing a high narrow peak. The

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relatively low retention efficiency and concentration efficiency for the Chelex-100 columns is attributed to the swelling and shrinking properties as well as the coarser particle size of the Chelex-100 (50-100 mesh compared to 90-120 mesh for the CPG-8Q exchanger). The %E measurements were made with pure aqueous standards at a relatively high concentration of 1 mg 1-l; the situation will be different with lower concentrations, and could be extremely different in the presence of certain sample matrices which would influence the concentration efficiency.

The column dimensions and exchange capacity have a strong influence on the tolerance for interferences of the on-line preconcentration system. This can be seen from the recovery studies on the water samples in Table 1. Whereas the large column (3 X 45 mm) of CPG-8Q produced good recoveries of 90-99% for cobalt in all the samples studied, few of the recoveries with the smaller column (2.3 X 12 mm) could be considered satisfactory. The same tendency can be seen in the case of cadmium, but the recoveries are worse. A study on the retention efficiencies for cobalt and cadmium in the water samples (not included in the table) showed that the low recoveries were almost without exception due to breakthrough of the analyte during the loading stage. This behavior was at first attributed mainly to the short residence time of the sample in the column which would be unfavourable for retention of the analyte in the presence of a large population of com- peting ions from the sample matrix. The flow rate was therefore varied to produce identical or longer residence times with the smaller columns without altering the total volume of sample loaded on the columns. The results are shown in Table 2. The tap-water sample showing the most serious interfering effects in Table 1 was used for the study. Surprisingly, the recoveries at the lower flow rate were even worse than those at the higher flow rate, although the elution peaks for both standards and samples were 20-50% taller than at the higher flow rate with 2.3 X 12 mm columns. The increase in peak height showed that the retention efficiency was definitely improved but at the expense of poorer concentration efficiency. Yet the increased residence time seemed to favour retention of the competing ions, which apparently is the reason for the low recovery. Analysis of the tap-water sample, which was from a new pipeline, revealed an extraordinarily high zinc content of 8 mg 1-r ; this was suspected to be the main interferent creating the low recovery. A separate study conducted by adding similar amounts of zinc to standard solutions of cobalt and cadmium confirmed this suspicion. The stability constants of the zinc complexes of the functional groups studied are similar to those of cobalt and higher than those of cadmium. Thus, if insufficient exchange capacity is reserved for the competition, recovery of the more weakly complexed metals will be low. This phenomenon was also observed by Malamas et al. [3] in a study on the interfering effects of copper on cadmium also with a CPGSQcolumn. These observations further emphasize the importance of column capacity in the design of on-line column pre- concentration systems.

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Although columns with higher ionexchange capacities are more tolerant to chemical interferences in the sample solution, this may not be true in relation to spectral interferences in the eluate, particularly when i.c.p.1a.e.s. is used for detection. A high column capacity is also beneficial for the col- lection of matrix elements such as calcium and magnesium which usually form weak complexes with chelating ion-exchangers. Although a sample buffered at a pH favouring the analyte is generally used for preconcentra- tion, considerable amounts of the alkaline earth metals are still retained on the columns which are later eluted with the analyte metals. Hirata et al. [ 131 recently reported spectral interferences from magnesium and manganese in the determination of chromium(III), using an on-line column preconcen- tration/i.c.p. system.

The effect of column capacity on the concentration of calcium and mag- nesium in the elution peak maxima was studied here by passing a solution containing 1000 mg 1-l magnesium and 500 mg 1-l calcium through CPG-SQ columns with different capacities. The results are shown in Table 3. Much less of the alkaline earth metals was collected on the smaller columns, so that spectral interferences would be less. Clearly, the tolerance capacity of interferences in both the sample-loading stage and in the elution/detection stage must be carefully balanced in the optimization of column capacity.

The primary and secondary considerations presented above were used as guidelines for the optimization of experimental parameters for the deter- mination of trace amounts of cobalt in water samples by a.a.s. after on-line column preconcentration, with special emphasis on the overall performance, including preconcentration factor, speed, precision and accuracy. The recom- mended procedure involves on-line preconcentration of cobalt with time- based sampling and a double-column system. Columns (3 mm i.d., 45 mm long) of CPG-8Q ion-exchanger are used with a loading rate of 9.5 ml min-’ and a loading period of 90 s. An enrichment factor of 48 is thus achieved at a sampling frequency of 60 h-l. Good recoveries were obtained for all the water sample types studied (see Table 1). The precision was 1.7% r.s.d. (n = 11) at the 40 pg 1-l level, and the detection limit (30) was 0.2 ug 1-l)

TABLE 3

The effects of dimensions of a CPGSQ column on the retention of calcium and mag- nesiuma

Column 1.d. (mm) 2.3 2.3 3.0 Column length (mm) 12 20 45 Ca (mg 1-l) 2.0 13.2 77 Mg (mg 1-l ) 4.7 21 138

‘Concentrations given refer to those of the eluate peak maxima. Original concentrations in sample were 500 mg 1“ Ca and 1000 mg 1-l Mg. Sample loading rate 9.5 ml min-’ ; preconcentration period 90 s.

48

which is close to that of electrothermal atomization a.a.s. with a lo-$ sample.

Preliminary attempts to develop an efficient and reliable preconcentration procedure for cadmium were not as successful, as can be seen from Table 1. Although fairly satisfactory results can be obtained with a Chelex-100 column for most of the sample types studied, the concentration efficiency is not impressive, and a better chelating ion-exchanger is needed. The Japa- nese chelating ion-exchanger Muromac A-l, which exhibits the same chel- ating properties as Chelex-100 but without its swelling properties, was used successfully by Japanese workers [13, 141 in on-line preconcentration sys- tems, and seems to be a good choice.

CONCLUSIONS

One of the principal drawbacks of on-line column-preconcentration flow- injection spectrometric systems in comparison to electrothermal atomization spectrometric systems is their relatively large sample consumption of lo-20 ml per determination for a 20-100 fold signal enhancement, although this volume is much smaller than that needed for conventional batch preconcen- tration. A study of a typical elution curve for on-line column preconcentra- tion shows that the eluate zone containing the highest concentration of analyte and corresponding to over 90% of the peak maximum, is often only 30-50 ~1. As an injection of a 30+1 sample via the shortest possible con- nection into the nebulizer of an atomic absorption or i.c.p. spectrometer will produce only about 30% of the stable-state signal, decreased dispersion by improvement of the nebulization system would obviously also improve the performance of the overall system. The direct injection nebulizer, recently introduced by Fassel and coworkers [19, 201 and designed for f.i.a./i.c.p./ a.e.s. systems, is capable of attaining a peak response almost equivalent to the stable state signal by injecting 30-~1 samples. Thus, a dramatic increase in concentration efficiency could be expected if such a system were coupled to an on-line column preconcentration system. This direct injection nebu- lizer should also permit further miniaturization of the on-line preconcentra- tion system so that concentration efficiencies of 20-50 EF min-’ may be achieved with l-2 ml of sample.

Gosnell et al. [21] have recently shown that controlled-pore glass can be plasticembedded on the walls of teflon or tygon tubing to construct open tubular reactors with immobilized enzymes. The back-pressure in such reactors is greatly reduced, permitting higher flow rates and longer column lengths. Chelating agents such as 8quinolinol might be immobilized similarly for use in on-line preconcentration.

Financial support for this work was provided by the Research Founda- tions of Academia Sinica. The authors are grateful to Pierce Chemicals for the donation of a sample of the CPGSQ ion-exchanger.

49

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