Automatic calibration and dilution in unsegmented flow systems

265

Analynca ChrmccaActa, 264 (1992) 265-273 Elsevler Science Pubhshers B V , Amsterdam

Automatic calibration and dilution in unsegmented flow systems M Agudo, A Rios and M Valdrcel Department of Anulytrcal Chemtstry, Untverslty of Cckdobo, E-14004 Cbrdoba (Spa& (Received 22nd November 1991, revised manuscript received 3rd March 1992)

Abstract An open-closed flow system allowmg variable volumes of standard calibration solution to be introduced and automatlcally diluted was used to cany out automatic cahbratlons m unsegmented flow systems A ddutlon loop was thus established m which its final homogenized volume was used as diluted sample or cahbrant solution m the mam flow system The performance of the ddutlon loop was tested both m mlectlon and m completely continuous flow systems and was found to be appropriate for analytical process control

Keywords Flow mJection, Flow system, UV-Visible

spectrophotometry,

Flow systems have proved to be powerful tools for automatmg various steps of the analytical process [l-4] Calibration 1s a major step m this process, which mvolves the often time-consuming preparation of a set of appropriate standards, recording then signals and constructing a cahbratlon graph Some procedures allow cahbratlon to be accomplished m an mtegrated manner with the flow manifold used for the determmatlon of the analyte Tyson and Appleton [5] used concentration gradients for cahbratlon purposes m atomic absorption spectrometry The principle behind that apphcatlon lies m the use of a muuapparatus for integrated contmuous dllutloncahbratlon by means of four valves and a minichamber of 7 2 ml filled with dlluent solution The signal-time response thus obtained 1s an exponential function that is used as a calrbratlon equation (the approach resembles the exponential calibration method used m process conCorrespondence to Prof M Valc&rcel, Department of Analytical Chemistry, Faculty of Sciences, University of Gkdoba, E-14004 C%rdoba (Spam)

Cahbratlon,

Dllutlon, Process analysis

trol [6]) A different approach 1s based on coupling a completely continuous system for analyte determmatlon with periodic inJections of standards of dtierent concentrations in order to unplement a flow-inJection standard-addltlon method [7] Also, a review of flow-inJectron cahbratlon techniques has been pubhshed [8] Conventional routine work m flow-mJectlon analysis (FIA) systems mvolves manual preparation of the cahbratlon solutions from a stock standard solution and usually also manual mJectlon of both cahbratlon solutions and samples mto the manifold (Fig 1A) A higher degree of automation, including dllutlon when required, can be obtained if these operations are incorporated in the flow system (Fig lB), as Tyson [8] and Frenzel[9] have shown The aim of this work was to develop a processing flow unit for automatic dilution of standards and/or samples The key to this unit 1s a system for msertlon of variable volumes of a standard solution (or samples) into a fixed volume of dlluent Therefore, ordmary mJectlon valves are inappropriate m this instance Use of a closed system [lo] to provide a constant

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

M Agudo et al /Anal

266

Sulphamlamlde solution was prepared by dlssolvmg 2 5 g of sulphamlamlde 1n 13 ml of concentrated HCl and diluting to 250 ml with dlstilled water N-(l-Naphthyl)ethylened1anune solution was prepared by dlssolvmg 0 25 g of the drhydrochloride and 10 g of sodium chlonde 1n 250 ml of distilled water Aqueous carrier solution was prepared by mix1ng 0 4 M ammonium chloride and 0 3 M sodium chloride solutions Chlorine standard solutions contalnlng 0 155 g 1-l of active chlorine was prepared from sodium hypochlorlte (Carlo Erba) and standardized 1od1metrically Nitrite standard solution (0 100 g 1-l) was prepared from sodium nitrite

Al

n FIA

Chm Acta 264 (1992) 265-273

Apparatus MANIFOLD

Fig 1 (A) ConventIonal and (B) completely automatic procedures for cahbratlon and ddutlon of samples m an FIA mamfold

final volume and hydrodynamic 1nJect1on [ll] or a swltchmg valve [12] are two possible approaches to accompllsh1ng this alfn Spectrophotometrlc determination of both residual chlorrne with otolldine and nitrite with sulphanllarmde and N(l-naphthyl)ethylenedlamlne 1n water were used as chermcal systems to illustrate the approach

EXPERIMENTAL

Reagents

Reagents were obtained from Merck, unless indicated otherwise A Bromocresol Green stock solution was prepared by dlssolvmg 0 10 g of dye 1n 6 25 ml of ethanol and diluting to 25 ml with 1 X lo-’ M sodium tetraborate solution A 3 mM o-tohdme solution was prepared from o-tohdme dihydrochlorlde (Aldrich) 1n 2 M HCl and stored 1n dark-coloured glass flasks

A Hewlett-Packard Model 8451A diode-array spectrophotometer equipped with an HP 9121 floppy disk drive, an HP 98155A keyboard and an HP 7470A plotter was used A Rheodyne Model 5041 injection valve (used as a svvltchlng valve here), controlled by a laboratory-made timer device, and several normal Rheodyne Model 5041 injection valves were also employed to select the different channels indicated 1n the manifolds A G1lson Mmlpuls-3 perlstaltlc pump controlled by a Commodore-64 microcomputer through a laboratory-made interface was used to provide the pump flow-rate pulses A G1lson Mmlpuls-2 penstaltlc pump, a Tecator TM III chemlfold and a Hellma 178 12 QS flow cell (inner volume 18 ~11) were also employed Man folds

Figure 2 shows the two possible configurations, which are placed before the FIA manifold where the analytical determination 1s carried out Hydrodynamic

mJectlon approach

(Fig

2A)

Only pump 1 1s workmg when the process 1s started The switching valve (SW 1s the key to creating the closed flow system In 1ts initial posltlon, the carrier stream (dlluent here) 1s selected, driven along the flow system and returned to SV, which directs 1t to the FIA manifold (the unit operates as an open flow system 1n this instance)

M Agudo et al /Anal

267

Chun Acta 264 (1992) 265-273

At this pomt, pump 1 is stopped and pump 2 IS sunultaneously started to mtroduce a fixed volume of the standard solution during the Interval when It 1s functlonmg As SV is switched to the other posltlon, the closed flow system 1s established as a result of pump 1 bemg mcluded m the system Then, dllutlon of the standard solution continues until complete homogemzatlon SV is then swltched again to Its first position, where the solution held in the system is unloaded mto the FIA manifold As the volume of the closed flow system is previously calculated and that of the standard solution introduced by pump 2 1s controlled through the tune during which it is kept workmg, the dllutlon factor 1s known and can be altered as required Selectuzg valve approach (Fig 2B) Pump 2 m Fig 2A is now replaced with a second switching valve W-2) that introduces variable volumes of

A) INITIAL

POSITION

SAMPLE 01 STANDARD

8)

SAMPLING SAMPLE

ANDARD ASTE

POSITION or

FLOW

Cl

RETURN

TO

INITIAL

SYSTEM

POSITION SAMPLE 1 STANDARD SOLUTION WASTE

SOLUTION

Fig 3 Detalled functlonmg of the swtchmg-dwertmg valve (SV) for mtroducmg small volumes of sample or stock standard solution (A) mltlal posltlon of the valve, (B) sampling position, (0 return to initial posltion

B) STANDARD

PUMP

the standard solution SV-1, the functioning of which is identical with that of SV in Fig 2A, IS used to establish the closed flow system Once established, the first positron of SV-2 (which functioning is explained m more detail in Fig 3) allows the carrier stream to be selected (closed flow system with the carrier/ dlluent solution) By controlling the time dunng which SV-2 remains m its second posltlon, a known volume of standard solution is introduced into the closed system as an equivalent volume of carner/dlluent solution 1s simultaneously sent to the closed system

TO FIA MANIFOLD

RESULTS AND DISCUSSION Fig 2 SchematIc diagrams for the lmplementatlon of an automatlc dllutlon loop (A) by usmg a pump or (B) a swtchmg-drvertmg valve to Introduce the stock standard solution or the samples mto the open-closed flow system SV = surltchmg-dlvertmg valve For details, see text

First the performances of the two approaches described above were assessed in order to choose that which provides higher preclslon

M Agudo et al /Anal Chrm Acta 264 (1992) 265-273

268

Apphcatlon of the first approach reqmred programmed functlonmg of pump 2 (Fig 2A) This was accomplished by using a Commodore-64 mlcrocomputer and a custom-made interface [13] fitted to the pmpp Thus, by using a BASIC program a set of pulses were generated, the mtenslty (flow-rate) and amplitude (the pump workmg interval) of which are related to the volume of standard solution introduced The mterface was activated from wlthm the program by POKE mstructlons and the intensity and amphtude of the pump pulses were controlled by using FOR NEXT loops Thus, the program actlvates the interface and switches on the pump which works at a fixed flow-rate during a selected period of time, after which it switches off The performance of this system was tested on a stock dye solution (Bromocresol Green) that was propelled by the programmable pump, and a sodium tetraborate buffer that was used as carrier The volume of the closed system was also measured, and was found to be 10 ml The results obtained by mtroducmg various volumes of the dye are listed m Table 1 (flow-rate 0 2 ml mm-‘) As can be seen, the procedure allowed the mtroductlon of very small volumes mto the closed system (less than 1 ~1 for a confidence limit of +O 1 ~11, but the precision was not very good as a result of the small volume used One of the mam factors affecting the preclslon m this approach was the inertia of the pump drum to

TABLE

1

Volume of dye Introduced mto the open-closed dllutlon system by usmg the programmable pump approach as a functlon of the duration of the pump pulses Duration of pump pulse (s)

Volume of dye (Pl) a

Repeatability (RSD,%ja

210 16 6 96 52 28 12

281*08 221+07 132*05 65f03 45*03 08fOl

31 31 41 43 63 125

an=11

TABLE

2

Volume of dye introduced mto the open-closed system by using the swltchmg valve approach Flow-rate of the dye stream (ml mm-‘)

Time during which the swltchmg valve IS kept m the sampling

ddutlon

Volume of dye introduced (~1) a

Repeatability (RSD %)a

103 *100 459*070 292+050 174*040 61&020 38*015 09*007

09 15 16 25 28 42 77

pos1t10n (9

06 06 04 04 01 01 01

70 30 50 30 50 30 10

an=ll

spm when small amounts of sample were mtroduced The approach mvolvmg use of the swltchmg valve (Fig 2B) to introduce variable volumes of the standard solution was applied by usmg a timer to control swltchmg of the valve This device allowed one to select the interval over which the valve was kept m one or the other position Its functioning 1s depicted m Fig 3 The volume introduced into the closed flow system was thus controlled through the tnne during which the swltchmg valve remained m the “samphng” posltlon, where it delivered the standard solution stream at a fixed flow-rate Table 2 gives results obtained by applying this approach to a stock dye solution According to the results given m Tables 1 and 2, the latter approach provides good preaslon (even at low dye volumes) and better repeatability than the former Homogemzatwn

wzthm the closed system

Introduction of the standard solution (or the samples, if required) must be followed by a period m which the dilution and complete homogemzatlon of the volume introduced mto the closed flow system are effected In order to determine the mmlmum dilution time required for this purpose, a spectrophotometrlc detector was mcorporated mto the closed system to monitor the dllu-

M Agzub et al /Anal

269

Chum Acta 264 (1992) 265-273

AUTOMATIC LOOP SAMPLE STREAM I CARRIER STREAM-

PUMP

Fig 4 Generic determmatlon

mamfold

combmmg

automatic

preparation

of the cahbratlon

(or sample ddutlon,

If requued)

and FIA

tometrlc detector and finally to waste Obviously, the loop of the mJectlon valve must be smaller than that of the dilution unit (278 ~1 m this instance for the loop of I V against 106 ml for the dilution loop), m order to ensure that the loop of the inJectIon valve is completely filled with the dilute solution Table 3 gives the time-concentration and absorbance-concentration cahbratlon results obtamed by applymg the procedure based on the use of the swltchmg valve to introduce the stock solution mto the dllutlon loop, which provides better precision It 1s interesting to compare the cahbratlon graph obtained by applymg this procedure and that provided by the conventlonal procedure mvolvmg manual preparation of the cahbration solutions The result was as follows

tlon process by mtroducmg different volumes of the dye solution A time of 500 s was found to be optimum (this time can be reduced by including a packed bed or a single-bead string reactor m the closed system) After this time had elapsed, the volume held m the closed system was transferred to the manifold for the determmatlon of a standard solution of known concentration In order to check the overall system, a cahbratlon graph was automatically constructed from the dye solution The manifold used for this purpose 1s depicted m Fig 4 It 1s able to dilute both the standard solution and the sample (if necessary) by usmg a swltchmg valve (the three-way valve S m Fig 4) located before the pump The stream selected by S 1s driven to the automatic dilution loop, namely the closed flow system described above When dilution 1s finished, the loop is opened and the dilute solution 1s used to load the loop of the injection valve (I V ) The volume is then inJected mto the carrier stream, driven to the spectrophoTABLE

solutions

y = 0 955x + 0 0005

(n = 6, r = 0 9998)

by usmg a regression line to compare both methods The equation shows perfect agreement be-

3

Relationship between the time during which the swltchmg valve is kept in the samplmg posltlon solutlon thus prepared m the open-closed dllutlon loop a Time (s) Concentration of dye (pg ml-‘) Absorbance of signal obtamed m FIA manifold

10 18

20 37

30 61

50 91

0048

0 095

0 160

0 235

a Cabbratlon equations t = 4(concentration) t -0524 C+OO21 Absorbance = +(concentratlon) A=0025 C+OOO6 where C IS the concentration of dye m fig ml-’

(n=7, (n=7,

r=O9995) r=O9998)

70 13 5 0 341

and the concentration

100 18 9 0 470

of the dye

120 229 0 572

270

M Agudo et al /Anal Chm Acta 264 (1992) 265-273

tween the manual (x-ax& and automatic (y-ax& procedures (slope close to 1, mtercept close to zero and good regresslon) Analytrcal

STANDARD

-

apphcatwns

The analytIca posslblhtles of the generic manifold depicted m Fig 4, as applicable to specific determmatlons, are automatic preparation of standard solutions from a single stock solution for subsequent constructlon of the cahbratlon graph and automatic ddutlon of samples, If required, prior to mJectlon mto the manifold designed for the analytical determmatlon This auxlhary flow unit can be very useful m process control apphcatlons, where calibration would be done between successive samples Thus, the determmatlon of residual chlorine m water was carried out by performing cahbratlon slmultaneously with sample analysis The manifold used 1s shown m Fig 5, it was applied to the classical reactron with o-tohdme, with spectrophotometrlc detection at 438 nm The whole system was controlled by three swltchmg-dlvertmg valves (A, B and C m Fig 5), plus the ordinary mjectlon valve (I V ), which was used to introduce the samples or standards mto the mamfold for the determmatlon of residual chlorine Valves A and B established the open-closed loop for the automatic

SAMPLE

“20

PIIOTOMETER

0-TOLIOINE PUMP

Fig 5 Mamfold used for the automatic determmatlon and cahbratton of residual chlorme m water A, B and C are swltchmg valves, I U IS the mjectlon umt For details, see text

dilution (preparation) of standards, and worked independently of the determination manifold Valve C selected the sample or standard stream for loading the loop of the mjectlon valve For slmphclty, the waste hnes for valves A, B and C (deplcted m Fig 3) have been omitted from Fig 5 The arrows m Fig 5 denote the selected channel m relation to the two mcommg streams (without arrows> The chief assets of this mamfold are automatic preparation of standards for calibration and simultaneous preparation with the sample analysis

TABLE 4 Steps mvolved m the automatic preparation and mtroductlon of a standard solution between two samples m the mamfold m Fig 5 Step

Process

PosItIon of the valves

Valves sv&ched between steps

Analysis of sample 1 Preparation of the ddutlon loop (filhng wrth dduent) Analysis of sample 1 Introduction of the standard mto the ddutlon loop Rephcates of sample 1 or analysis of other samples Homogemzatlon of standard m the ddutlon loop Analysis of standard 1 Ddutlon loop open Rephcates of standard 1

C3, I V -3 (filling)

A, B, I V

B2, A2 C3, I V -3 (mjectlon)

A, I V

Analysis of sample 2 (see step 1) (See step 2), etc

B2 (from A), Al C3, I V -3 (filhng-mjectlon) B2 (from A), A2 C2 (wah l), I V -2 (fdhng) B2, A2 C2 (with 0, I V -2 (fdhng-mJectlon) B2, A2 C3, I V (filling) B2, A2

B, C, I V

IV

c, IV

h4 Agudo et al /Anal

Chm

271

Acta 244 (1992) 265-273

Table 4 hsts the steps involved and the role of the valves m the procedure used to mtroduce a standard between two successive samples Each swltchmg-diverting valve (A, B and C) 1s assigned a number (1, 2 or 3) m the table to denote the chlorme standard solution (11, the dduent solution (2) and the sample stream (31, wluch were selected as required m each step The number followmg IV indicates the corresponding solution used to load the loop of this valve As can be

TABLE

5

Results obtained m the determmatton of residual chlorme Hrlth automatic preparatton of the cahbratlon solution Concentration added (pg ml-‘)

Concentratlon found (pg ml-‘)

Concentratlon added (c1gnll--‘)

Concentratlon found (CLgml-‘)

027 030 0 45 081 0 10

026 032 049 0 82 0 17

060 0 63 0 26 0 20 050

052 061 026 023 052

seen from Table 4, only five different steps were needed For routine analytical control, this sequence can be implemented durmg the normal analysrs of samples as many times as required to perform cahbratlon without stopprng sample analysis for an unduly long tune Figure 6 shows graphical examples of application and Table 5 gNes the results obtained m the determmatlon of residual chlorme m synthetic samples Occasionally, analytical process control requires contmuous or nearly continuous monitormg of the analyte The proposed methodology can be very useful for this purpose as It allows a completely continuous flow system to be developed by incorporating the automatic ddutlon loop The modified manifold should be smular to that depicted m Fig 5, except that the inJection valve 1s replaced with a merging point When required, valve C can be switched to select the cahbratlon solution held m the dllutlon loop Figure 7 shows the recordmgs obtamed for a sunulated nitrite control m a synthetic sample where its concentration was changed over me by using both a flowmjectlon and a completely contmuous system NItrite was deternuned with spectrophotometrlc monitoring at 540 nm by using sulphamlamlde and N-(l-naphthyl)ethylenedlamme as reagents Several stock standard solutions of sodnun mtnte at different concentrations were employed and water was used as dlluent Figure 7 shows the varlatlon of the nitrite concentration m relation

111 TIME

0

(mln)

10

5 TIME (mu?)

Fig 6 (a) Recordmg obtamed by usmg the mamfold m l?g 5 to control residual chlorme m waters, where peaks A, B, C, D and E correspond to the cahbratlon solutions automatxally prepared (0 82, 0 27, 0 64, 0 45 and 0 26 pg ml, respectively), and (b) the correspondmg vanatlon of chlonne reslduat concentratlon durmg this period of trme

272

M Aguab et aL /Anal

Chtm Acta 264 (1992) 265-273

OOOO-’ 0

10

20

0

30

10

20

TIME (mln)

30 TIME (mm)

OOJ 1

2

3

4

5

6

7 TIME (hours)

Fig 7 Recordmgs obtalned for mtnte momtormg by usmg (a> flow-mjectlon and (b> completely contmuous modes m a synthetic sample of variable concentration The slgnal ylelded by automatically prepared cahbratlon solutions IS also shown Concentrations A=016, B=O02, C=O12, D=O05, A’=O82, B’=Oll, C’=O35, D’=O58, E/=035, A”=O53, B”=O29, C”=O41, D” = 0 17 fig ml-’

the response of the cahbratlon solution A large volume of the cahbratlon solutions was reqmred for the completely contmuous system to to

obtam a constant slgnal Thus, the volume of the dllutlon loop was 105 ml m this instance Table 6 lists some results obtained by applymg these methods

TABLE 6 Results obtamed m the determmatlon of mtrlte with automatic preparation of the cahbratlon solution by usmg flow-mJectlon and completely contmuous modes Completely contmuous system

Flow-inJectIon system Concentratlon added (pg ml-l)

Concentration found (pg ml-‘)

Concentratlon added @g ml-‘)

Concentration found (pg ml-‘)

0.50 0 82 011 034 0 58 008 0 10 003

0 45 081 0 12 032 062 006 0 12 0 02

053 0 10 0 30 028 040 0 50 0 17 070

0 5.5 0 11 029 028 041 052 0 18 0 67

Conclusion

The straightforward flow concentration developed m thrs work for the automatic preparation of calibration solutions can be incorporated mto a manifold by means of a swltchmg-diverting valve and allows the cahbratlon and determmatlon steps to be coupled and automated The system can be of great use m process control, whether perlodlc or continuous momtormg of an analyte 1s required

REFERENCES 1 J Ruzlcka and E H Hansen, Flow InJectIon Analysis, Wiley, New York, 2nd edn , 1988

M Agudo et al /Anal 2 M Valclrccl

3 4

5 6

Chm

Acta 264 (1992) 265-273

and MD Luque de Castro, Flow InJectIon Analysis Prmclples and Apphcatlons, Horwod, Chlchester, 1987 M Valcircel and MD Luque de Castro, Automatic Methods of Analysis, Elsevler, Amsterdam, 1988 M Valcircel and M D Luque de Castro, Non-Chromatographic Continuous Separation Techniques, Royal Society of Chemistry, Cambndge, 1991 J F Tyson and J M H Appleton, Anal Proc ,22 (1985) 17 G L Baker, m D P Manka, (Ed 1, Automated Stream Analysis for Process Control, Vol 2, Academic, London, 1984, p 36

273

7 8 9 10

J F Tyson and A B Idns, Analyst, 109 (1984) 23 J F Tyson, Fresenms’ Z Anal Chem , 329 (1988) 663 W Frenzel, Fresemus Z Anal Chem , 329 (1988) 668 A. Rios, MD Luque de Castro and M Valdrcel, Anal Chem , 57 (1985) 1803 11 J Ruzlcka and E H Hansen, Anal Chum Acta, 145 (1983) 1 12 A Rios, MD Luque de Castro and M ValcLrcel, J Autom Chem , 9 (1987) 30 13 M Agudo, J Marcos, A Rios and M Valcircel, Anal Chum Acta, 239 (1990) 211