high efficient calibration procedure for flow injection flame...

Chem. Anal. (Warsaw), 41,587 (1996)

High Efficient Calibration Procedure for Flow InjectionFlame Atomic Absorption Spectrometry

by Pawet Koscielniak!, Michael Sperling2 and B~mhardWelz2

IDepartment ofAnalytical Chemistry, Jagiellonian University,ul. R. Ingardena 3, 30-060 Krakow, Poland

2Departmel11 ofAppliedResearch, BodenseewerkPerkin-Elmer GmbH,Postfach 101761, D-88647 iiberlingen, Germany

Key words: calibration method, flow injection technique, flame atomic absorption

spectrometry

The calibration method, known in flow injection analysis as a "gradient ratio calibrationmethod", has been verified in respect of capability of applying to determinations byflame atomic absorption spectrometry. It has been revealed that the performance of thismethod is possible to be different and rather unpredictable if the interference effectoccurs in the analytical system examined. The calibration procedure has been attemptedto be improved by means of such approaches as the integration of signals processed andthe use of spectrochemical buffers. The mcthod in the version proposed is of low timeand reagent consuming as well as giving more reliable analytical results, acccptable interms of accuracy and precision.

Metodt( kalibracji, znan'l we wstrzykowej analizie przeptywowej pod nazw'l "gradientratio calibration method", sprawdzono pod k'ltem mozliwosci zastosowania w ptomieniowej atomowej spektrometrii absorpcyjnej. Okazato sit(, ze skutecznose metody mozebye rozna i w duzym stopniu nieprzewidywalna w przypadkach, gdy w badanymukladzie analitycznym ujawnia sit( efekt illterferencyjllY. Podjt(to probt( usprawnieniaprocedury kalibracyjnej, stosuj'lc integracjt( sygnalow pomiarowych i wykorzystuj,\cdzialanie buforow spektralnych. Metoda w zaproponowanej wersji jest szybka, ekonomiczna i bardziej wiarygodlla - zapewnia wylliki analityczlle 0 dobrej doktadnosci iprecyzji.

The flow injection analysis (FIA) combined with flame atomic absorption spectrometry (FAAS} is proved as very useful and attractive system. Flow injectiontechnique offers a lot of improvements in various stages of FAAS analysis, particu-

588 P. Koscielniak, M. Sperling and B. Welz

larly in sampling (e.g., increment in the nebulization efficiency, highest tolerance ofsalt content in samples), pretreatment (e.g, on-line digestion and separation processes), data handling, and conversion of spectrometer response to estimation of theanalyte concentration (calibration procedure). Consequently, the FI-FAAS has beendeveloped as fully automated instrumental method having the possibility to investigate tens samples per hour with a minimal sample consumption and, in principle,with good precision and accuracy.

However some analytical problems typical for FAAS are still expected to besolved by using the FI technique. One of them it is to develop the calibrationprocedure being more effective and reliable than those conventionally performed.Calibration strategy has a special importance in FAAS as the response of a flameatomic spectrometer is heavily dependent on the presence of various substances(interferents) accompanying analyte in the sample, as well as on the operatingconditions established. Both effects are able to be sources of serious systernatic errorsin the analytical results. Besides, conventional calibration procedures involve thepreparation of a set of separate standard solutions, hence consume too much timeespecially when covering the linear range of instrumental response and/or matchingthe standards to the sample are desired.

The FI technique has been widely attempted to be used for improving thecalibration procedures in FAAS [1,2]. In most cases it is suggested to be exploitedeither as a solution processing system offering automated dilution and addition ofreagents or as a source of information on variations in shape and size of the analyticalresponse. However, the calibration approaches recommended reveal usually somespecific drawbacks, namely being neither conducive enough to the fast, automated,on-line analysis, nor resistant enough to the interference effects appearing in ananalytical system assayed. The other question is that they generally use only a singlemeasuring estimator, mostly peak height or peak area, for evaluation of the analyteconcentration in the saomple examined; by doing so a lot of information potentiallyoffered by the flow injection analytical signal is lost.

Reviewing critically the FI calibration procedures developed hitherto the mostpromising seems to be that named the gradient ratio calibration method (GRCM)[3,4]. According to this method two transient peaks, one for the reference standardsolution and the other for unknown sample, are recorded undder the same experimental conditions and compared point by point in entire time range defined. By that meansmuch information about the calibration dependence (absorbance vs. concentration)and the interference effects can be achieved and efficiently used for the estimationof analyte concentration in the sample.

The GRCM has been applied to FAAS analysis only once on the example of thedetermination of calcium in the presence of phosphate [3] giving quite accurateresults. In the work reported here it is however proved that the method is not fullyreliable when used for analysis in some another analytical systems typified by verystrong interference effects. Therefore the calibration procedure has been attemptedhere to be improved. For this aim the integration of analytical signals appeared as auseful approach. The efficient way for reducing of interferences with the aid of a

High efficient calibration procedure... 589

spectrochemical buffer added in low concentration has been suggested. Some newaspects of GRCM have been also revealed and discussed.

Theoretical background

In the gradient ratio calibration method two flow injection peaks are considered(see Fig. la): one, Ar(t), corresponding to the reference standard solution of theanalyte concentration CO,r, and the other,As(t), produced by the sample with unknownconcentration of the analyte, Co,s, and interferents, CO,M' Every i-th point of the

a) b)

Ar.hAbsorbanceo

c.2~Co,.c Cap va. ArCDocoo ..

Time, S

CD A~ho •CaI.0(;

"~ A••h·....····..........·

Figure 1. Gradient ratio calibration method: a) two peaks produced for the reference solution, An andfor the sample, As, then adjusted to each other in a time window defined; b) the calibrationcurve in GRCM, i.e. the relationship between the apparent concentration Cap al}d the absorbanceA r

reference peak represents the analyte concentration Cr(fi) being the result of dispersion process taking place in the FI system. The concentration gradient can beexpressed by the current value of dispersion coefficient D(ti):

(1)

Thus, the absorbance Ar(ti) is related to the initial concentration CO,r as follows:

(2)

where fICr(ti)] is an analytical function (generally non-linear) describing the detectorresponse and hence implying the shape of the calibration curve. In the relationbetween analogous sample signal As(ti) and concentration Co,s (being expected to bedispersed in the moment ti to the same extent as CO,r) the interference effect isnecessary to be additionally considered as the function g[Cs(t),CM(t)] of both theanalyte and interferent concentrations. It can be included in the multiplicative formusually valid in FAAS:


A.(tj) = { CO.'x D~ti) + CO.' X D~ti) X g[C.(ti),CM(ti)]} X nC.(ti)] (3)

The ratio A s(ti)/Ar(ti) is then given by:

and then

As(ti) [[Cr(ti)] 1Co,s = Co r x -(-) x ( ) x (- (), Ar ti [[Cs ti] 1 + g[Cs ti),CM ti ]

(4)

(5)

There are two basic assumptions of GRCM creating it as a calibration method,namely: a) with decreasing analyte concentrations Cr(t) and Cs(t) the functionsfICr(t)] and fICs(t)] are directed towards the same value (i.e., the calibration curvebecomes linear in the region of small analyte concentrations):

(6)

as well as, b) with decreasing interferent concentration CM(t) the interference effectis eliminated:

(7)

If both assumptions are valid it is evident from eq. (5) that the concentration of theanalyte in the sample, Co,s, can be calculated on the condition that it will be possibleto estimate the ratjo As(t)/Ar(t) in the sample of infinite dilution.

In practice the set of values As(t;)IAr(ti) is calculated in entire time rangeconsidered (to,tr) taking into account the absorbances obtained for the sample and thereference solution in the same moment i (0 sis 1). Then the apparent analyteconcentrations, [Cs(ti)]ap, i.e. the analyte concentrations affected in every time ti byboth the curvature of the calibration curve and the interference effect, are calculated:

(8)

and they are expressed as a function of absorbance Ar(ti) what is depicted in Fig. lb.Since the decre~sed absorbance Ar(ti) is corresponding to the sample more and more


diluted and the value ArCt) =0 corresponds to the sample of infinite dilution theconcentration Co,s can be estimated by the extrapolation of function [CsCt)]ap vs.ArCt)toAlt) =O. It is worth to notice that the zero absorbance can be achieved by followingboth the rising and falling parts of the sample peak, thus two sets of experimentalpoints may be obtained being expected to be overlapped to each other and usedsimultaneously in calculation of the final analytical results.

That is a principle of GRCM that the analyte concentration is searched andestimated in the sample more and more dispersed, i.e. more and more diluted in fact.Exactly the same is a basis of so called "dilution method" which has been brought inthe analytical practice much earlier [5] and developed for calibration in FAAS quiterecently [6,7]. Therefore the important suggestion is not to consider the GRCM asquite new calibration method but rather as the dilution method performed by the flowinjection technique only.

EXPERIMENTAL

Reagents

Standard stock solutions of Mg, Ca, AI, and Si containing each metal at a concentration of 1 mg ml-1

were prepared from Titrisol standards (E. Merck). Stock solution of lanthanum chloride was made bydissolving of 1.16 g La203 in 10 ml of hydrochloric acid and finally diluting to 500 ml with water. Stocksolution of strontium chloride was prepared by water-dissolving 1.51 g of SrCI2'6H20 and diluting to500 ml with water. The working solutions were made from the stock solutions by mixing, diluting withwater and acidifying to pH =1 with hydrochloric acid. All reagents were of an analytical-reagent gradeas well as doubly distilled water was used throughout.

Instrumentation

The atomic absorption spectrometer used was a Perkin-Elmer 3100. It was equipped with magnesium and calcium hollow-cathode lamps operated at 10 mA and 15 mA, respectively. The wavelengthswere set to 285.2 nm and 422.7 nm, respectively, with a spectral slit width of 0.7 nm in both cases. Anair-acetylene flame was used in all experiment'). The nebulizer uptake rate was adjusted to be 9.6 mlmin-I. An IBM compatible Epson PC-XTcomputer was connected to the spectrometer for data handlingwith a time resolution of 20 ms. The calibration algorithm was performed according to the programwritten in Basic.

The PI technique was executed with the aid of a Perkin-Elmer FIAS 400 analyser equipped withtwo peristaltic pumps and four-port injector valve. It was operated under computer control. The FImanifold used in the study is presented in Fig. 2.

In order to restrict the sample dispersion, to produce relatively symmetrical peak, and consequentlyto improve precision the sample loop was a three-dimensional microline tubing (0.35 mm i.d.) and thedistance between the valve and the nebulizer was possibly shortest (15 em). The carrier flow rate(2.3 ml min-I) and the volume of the sample loop (60 I.d) were matched to each other giving a dispersioncoefficient at the peak maximum nearly 1 (1.1) and reasonable time window (8 s) for measuring FI peak.

592

cPl

P. Koscielniak, M. Sperling and B. Welz

Load Inject

L

v

ss v

L

Figure 2. FI-FAAS manifold for performing GRCM: S - sample, C - carrier, Ph P2 - peristaltic pumps,V - injector valve, L - sample loop, FAAS - flame atomic absorption spectrometer

RESULTS AND DISCUSSION

Basic calibration procedure

The calibration procedure in the basic mode was performed as recommendedpreviously [3]. At first fiye peaks from repeated injections of a reference solutionwere produced. The peaks were smoothed by a Savitzky-Golay quartic filter [8] andpositioned to each other. They were summed point by point to each other and averagedcreating finally a single peak representative for the standard. Then the peak for asample injected was produced in the same way as for the standard. After that thereference and sample peaks were adjusted to each other and the apparent concentrations were calculated according to eq. (8). The experimental relationships Cap VS. Arwere approximated by four-parametric hyperbolic function of the following form:

bA; + cArCap = a + --=---

cAr + 1(9)

A weighted least-squares algorithm was used for fitting. The analytical result wasfound by the extrapolation of function (9) to zero sjgnallevel (signals creating lessthan 1 % of the peak area on both sides of the reference peak were neglected).

The performance of GRCM has been tested in the three analyte-interferentsystems: Mg-AI, Ca-AI, Ca-Si. The composition of all samples examined is shownin Table 1. Each sample was analyzed five times under the same experimentalconditions. The analyte concentrations in the reference standard solutions were 1 flg ml-1

for Mg and 10 flg ml-1 for Ca, both chosen in such a way that the signal could bemeasured with the best precision attainable [3]. Some examples of the relationships


Cap vs.ArobselVed are depicted in Fig. 3. In Table 1 the analytical results are collectedobtained by conventional means (i.e. comparing the standard and sample peaks interms of maximum points only) and by GRCM (column NIM).

Table 1. Results of the determination of a.lcium and magnesium in the presence of stro:lg interferents .obtained by means of conventional a.libration and GRCM executed in non-integrated (NIM) andintegrated (1M) modes

Analyte concentration found, I-tg mr!Sample comP,Osition

GRCMI-tgmr! conventional calibrationNIM 1M

0.25 Mg + 0.8 AI 0.244 ± 0.012 0.260 ± 0.022 0.258 ± 0.0280.25 Mg+8 AI 0.241 ± 0.007 0.249 ± 0.034 0.251 ± 0.0180.25 Mg + 80 AI 0.181 ± 0.010 0.232 ± 0.028 0.238 ± 0.0291 Mg + 0.8 AI 1.03 ± 0.02 1.03 ± 0.10 1.10 ± 0.031 Mg+8A1 0.88 ± 0.04 1.04 ± 0.08 0.98 ± 0.061 Mg+80AI 0.79 ± 0.02 1.07 ± 0.09 1.03 ± 0.084Mg+0.8AI 3.35 ± 0.12 3.92 ± 0.20 3.90 ± 0.224Mg+8A1 2.22 ± 0.08 4.01 ± 0.38 4.00 ± 0.214Mg+80AI 2.07 ±0.1O 3.88 ± 0.29 3.91 ± 0.19

4Ca+0.8AI 3.72 ± 0.07 3.99 ± 0.21 3.92 ± 0.274Ca+8AI 2.19 ± 0.'10 1.45 ± 0.15 1.55 ± 0.224 Ca +80Al 1.46 ± 0.08 0.82 ± 0.45 0.98 ±0.25lOCa + 0.8 AI 9.40 ± 0.22 9.96 ± 0.47 9.92 ± 0.3210 Ca + 8 AI 7.22 ± 0.13 7.38 ± 0.62 7.09 ± 0.48lOCa +80AI 4.49 ± 0.18 2.14 ± 0.87 1.98 ± 0.6725 Ca + 0.8 AI 21.48 ± 0.34 26.30 ± 1.51 25.89± 0.8925 Ca + 8 AI 19.54 ± 0.24 23.48 ± 0.94 23.76 ± 0.7425Ca+80AI 14.69 ± 0.47 6.56 ± 1.78 6.08 ± 1.27

4 Ca + 1 Si 3.71 ± 0.08 3.95 ± 0.21 4.02 ± 0.184Ca + 4Si 2.76 ± 0.13 4.08 ± 0.35 4.10 ± 0.254 Ca + lOSi 1.24 ± 0.05 3.87 ± 0.45 3.95 ± 0.32lOCa + 1 Si 9.32 ± 0.17 10.18 ± 0.77 10.06 ± 0.47lOCa + 4Si 7.13 ± 0.18 9.88 ± 0.78 9.39 ± 0.5210Ca + lOSi 6.04 ± 0.09 10.33 ± 0.57 9.98 ±0.6725 Ca + 1 Si 20.46 ± 0.28 24.01 ± 1.32 25.87 ± 1.1425 Ca +4Si 18.72 ± 0.37 25.91 ± 1.99 26.89 ± 1.4325 Ca + lOSi 18.01 ± 0.42 24.21 ± 1.68 24.67 ± 0.98

It is seen in Fig. 3 that the analyte concentration in the diluted sample may bechanged very differently and it does not always approach true concentration. Thetendency obselVed depends not only on the analytical system investigated but alsoon the ratio of analyte-to-interferent concentrations in,the sample. However, it is herepostulated again [7] that for certain low analyte concentrations the interferences arereduced completely ill fact irrespective of the kind and amount of the inteferent. Thepoint is that the region of such low analyte concentrations being sensitive enough to


A

b--:..::.;:..:..:..:.:.: ,., ,.: : : , w-.-

a~_-:-,_ .-..:..: :.:.:.:.:.:.:.: .'.:.: :,',:::,',: ·.·.·.t:::~ ,." .•-:,.,~-

8

8

4

4

10 ...-...._-__

10

8

8

0.4

a

: ~~~;i:::i;~~,:-~·_--_.0.8

co

..cCD

ocoo

-.IE

Ab80rbance

Figure 3. Relationships Cap VS. Ar obtained for the selected samples examined: (A) 1 I-tg ml-1 Mg with0.8 (a), 8 (b), and 80 (c) I-tg 1111-1 AI; (B) 10 I-tg 1111-1 Ca with 0.8 (a), 8 (b), and 80 (c) ~g ml-1

AI; (C) 10 I-lg ml-1 Ca with 1 (a), 4 (b), and 10 (c) ~g 1111-1 Si (in each case two sets ofexperimental points are seen resulting from the comparison of the rising parts and the fallingparts of the peaks)

reflect the elimination of interferences may be different in particular analyticalsystems assayed and it cannot be always experimentally attainable. This suppositionis supported especially by the results presented in Fig. 3B dealing with the detennination of calcium in the presence of aluminium. It is seen that in the region accessiblefor the experimental investigation the total elimination of the aluminium influenceis observed only for the sample of lowest interferent concentration (0.8 llg ml-1 AI)while it is only partly detected for 8 llg ml-1 Al and completely unnoticed for 80llg ml-1 AI.

It is also seen in Fig. 3 that both rising and falling parts of FI peaks can be usedfor the calibration purpose as expected but the falling part is more useful in practice,being always longer and giving especially much more reliable data in very lowmeasuring region than the rising one. Therefore the falling peak-side is suggested tobe only considered in GRCM.

The most critical feature of GRCM as a dilution method is a necessity to considervery low absorbances. In this measurement region the dependence of Cap on A r canreveal quite considerable irregularities (see Fig. 3) what becomes potentially a source

.High efficient calibration procedure... 595

ofserious random errors ofthe analytical results. Therefore, the fundamental questionis to operate under the conditions ensuring the signal-to-noise ratio as high aspossible. Both the filtration an ensemble summation procedures have been appliedfor this aim. After all it was proved that five injection peaks summed and thenaveraged create as the result the peak which is not reduced but of the noise beingdiminished almost by a theoretical factor of V5 [9,10]. The choice of a propernonlinearfunction for approximation of the experimental points has a special roletoo. Four-parametric function (9) used here is sensitive enough to reflect the generaldistribution of most points but not so sensitive to fit exactly some accidentalfluctuations which were usually averaged as a consequence. Due to the above effortsquite acceptable repeatability of the analytical results has been achieved when usingGRCM for calibration; it is indicated by the results presented in Table l.

Regarding the analytical results from Table 1 in terms of accuracy, the systemcalcium-silicon revealed to be most convenient for the calibration by GRCM amongthose examined. If even the influence of silicon was very great (e.g. ca. 67% for thesample containing 4 ~g ml-1 Ca and 10 ~g ml-1 Si) the relative error of calciumdetermination did not exceed 7%. The determination of magnesium in the presenceof aluminium can also be performed with acceptable accuracy. Only when theinterference effect was almost 50 % it could be reduced to ca. 15 % giving still quiteinaccurate analytical result. Anyway it seems that the GRCM is capable of reducingthe interference effect in both systems Ca-Si and Mg-Al irrespective of the ratioanalyte-to-interferent concentrations hence giving always more accurate results thanthose obtained by traditional calibration approach.

However, the GRCM used in the system calcium-aluminium is able to compen- .sate only for very small interferences as, for instance, those revealed in the sampleswith 0.8 ~g ml-1 AI. If this effect is more serious than ca. 20% calcium is determinedwith an error approaching even 80%, i.e. with even smaller accuracy than when usingthe conventional calibration.

Integration approach

The FI technique offers the peak area measurements as additional form ofinstrumental response on the element determined. It has been shown that the integration process is a simple and fast procedure which may successfully improve thesignal-to-noise ratio [11]. Recent investigations confirmed this approach as fullyvaluable in FI-FAAS technique and especially effective when applying the signalsummation procedure [10].

The integration process can be easily and originally adapted to the calibration byGRCM. The method is schematically shown in Fig. 4. The injection peaks obtainedfor the standard, Ap and for the sample, As, both averaged from five peaks andpositioned to each other (Fig. 4a), were integrated point by point from two sides ofthe time window up to the highest dat~ points. The integrated peaks, I r andIs, obtainedby this means (Fig. 4b) were then interpreted similarly as peaks Rr and Rs assumingthat the conditions expressed by eqs. (6) and (7) (dealing with the elimination of thecalibration curvature and the interference effect, respectively) could be still fulfilled


(10)

after integration. Thus, the apparent concentration of the analyte are now calculatedaccording to the equation:

Is{ti)[Cs{ti)]ap = Ir{ti) x CO,r

and true analyte concentration in the sample, Co,s, is estimated by the extrapolationof apparent concentrations calculated from eq. (10) to infinite dilution (zero absorbance).

0.70

CD 0.560cat.a 0.420.a.a 0.26-<

0.14

CD0c 36at.a0

27.a.aat'tI 18CDa;OJ 9CD

:E0

0

Time, 8

6.4

a)

b)

8.0

Figure 4. Typical flow injection peaks exploited in GRCM: a) two raw signals,Ar and As, produced fromthe reference solution (10 f.lg ml-1 Ca) and the sample (10 f.lg ml-1 Ca + 4 f.lg ml-1 Si)respectively, smoothed and adjusted to each other; b) the same two peaks integrated from bothrising and falling sides

The examples of experimental relationships Cap VS. AT obtained before and afterintegration process are presented in Fig. 5. The smoothing effect which can be clearlyseen is caused by the fact that every successive integrated signal comprises all signalsmeasured just now and added to each other. Consequently, if the integrated signalsare rationed [according to eq. (9)] the phenomenon is manifested which consists ingradual compensation of the values of apparent concentrations calculated. As everyapparent concentration contains the information about the interferent influence andcalibration curvature, both existing in a given moment, these effects are then compellSated during the integration procedure.

co·

III

c

•ocoo

High efficient calibration procedure...

A10.8

9.2 ,~-....~ b

7.8 ~ . ...................... .. --.

8.4 S······ ..·· _

B

1j .

:: ~::::::::::::""',:.:::~~:::::-==::.::::0.8

.Absorbance

Integrated absorbance

597

Figure 5. Relationships Cap "S. Ar obtained on the basis of non-integrated (a) and integrated (b) si§nalsobtained for some selected samples: (A) 10 I-lg ml-1 Ca with 10 I-lg ml-1 Si; (B) 1 I-lg ml- Mg

. with 8 I-lg ml-1 AI (only the falling parts of peaks were considered in calculations)

The compensation effect is relatively week when the integration procedure starts,e.g. when the signals measured are very small, and gradually increases with increas=ing absorbances. Hence the changes of the interference and curvature effects revealing in most critical region of very small concentrations are detected with sensitivityalmost so great as without integration (see Fig. SA) but all fluctuations possible tobe quite strong in further concentration regions can be effectively suppressed (seeFig. SB). Owing to those features a nonlinear function used for approximation iscapable of fitting the integrated data Cp VS. A r generally better than those non-integrated but finally approaching the same concentration value in both cases. As a resultthe performance of GRCM can be improved in terms of precision and accuracy. Thisfact has been experimentally proved what is clearly seen when comparing someresults in columns NIM and 1M in Table 1.

Chemical elimination of interferences

In order to improve the performance of GRCM with respect to elimination of theinterference effects typical spectrochemical buffers have been tested with the use ofthe flow injection manifold presented in Fig. 2. The sample assayed was injected intothe stream of a buffer solution flowing continuously through the carrier tubingtowards the flame. By doing so the sample becomes dispersed within the buffersolution and the local concentrations of both the analyte and interferent in the samplezone become supplemented by the local concentration of the buffer. Thus, during


dilution process followed along two parts of the sample peak towards zero absorbancea gradual decreasing in concentrations of the analyte and interferent is associatedwith a successive increasing in the buffer concentration. For the compensation ofpossible influences between the buffer a'nd analyte the standard solution is insertedinto the flame together with the buffer solution under the same experimental conditions as it is made for the sample.

Figure 6 illustrates the GRCM curves obtained when calcium is determined inthe presence of alumini.um using lanthanum as a spectrochemical buffer. The capability of reducing the interference effect during dilution of the sample is widelydependent on the amount of lanthanum used. In the initial stage of dilution by thecarrier with 20 Ilg ml-1 La the interference effect is not reduced at all (compare linesa and b) but beginning from a certain quantitative composition of the sample thereduction becomes very sharp and effective. Still more and more favorable situationcan be observed using the carrier with 50 and 100 Ilg 1111-1 La. Irrespective of thebuffer concentration used quite reliable determination of calcium can be achieved asthe experimental data always approach true analyte concentration.

12

10!.."E

80:::a.a. 6•u

4

2

Absorbance

Figure 6. Relationships Cap VS. A r obtained for the sample containing 10 J..lg ml-1 Ca with 80 J..lg ml-lAIwhen lanthanum chloride in concentration of 0 (a), 20 (b), 50 (c), and 100 (d) J..lg ml-1 La waspresent as a spectrochemical buffer in a carrier stream (the falling parts of peaks wereconsidered only in calculations)

The high efficiency of the procedure applied results from the fact that the bufferis not kept in the sample in a constant ammount (as usually is done) but it is insertedin such a way that the ratio buffer-to-interferent concentrations increases rapidlyduring the sample dilution. The supposed tendency that the interference effect isreduced "naturally" in the region of very great dilution is then supported by activityof higher and higher amount of a buffer. Even if the initial concentration of a bufferis not very great its activity becomes strong enough to eliminate the interferenceeffect in this experimental region easily observed in practice. The possibility ofeffective using a buffer of a concentration much lower than this recommendedtraditionally 'is worth to be noticed in itself as attractive from the economical pointof view.


In Table 2 some analytical results are presented obtained when lanthanum as wellas strontium were used as spectrochemical buffers in the system calcium-aluminium.It is seen that calcium was determined with much better accuracy but with no reducingof precision if compared with this procedure excluding buffers (compare Table 1).

Table 2. Analytical result.. obtained by means of GRCM (executed in integrated mode) in the systemcalcium-aluminium with the use of spectrochemical buffers

Sample comp'osition Buffer concentration Analyte concentration foundJ-lgml-1 J-lg ml-1 J-lgml-1

4Ca +80Al SOb 4.08 ± 0.15

25 Ca + 80Al SOb 26.03 ± 0.76

lO Ca + 80Al 20L, lO.20 ± 0.45

10 Ca -t 80 Al SOL, 10.13 ± 0.34

lO Ca + 80Al 100 L, 10.35 ± 0.24

lOCa + 80Al 20 Sr 9.88 ± 0.66

lOCa + 80Al 50 Sr lO.21 ± 0.38lOCa + 80Al 100Sr 9.96 ±0.21

Conclusions

Most advantages of the gradient ratio calibration method reported previously[3,4] have been confirmed by the examination carried out here. The GRCM offerspossibility to perform the analysis using a single standard solution instead of a set ofstandards usually needed to prepare according to the conventional FAAS calibrationprocedures. The compensation of curvature of the calibration graph can be alsoachieved whereby the preliminary matching the standards and the samples to thelinear range of instrumental responses is avoided. Moreover the method uses the totalinformation contained in the transient peaks avoiding evaluation of the dispersioncoefficients. Finally, the GRCM is executed using very simple flow injection manifold which is easily operated and controlled.

.The basic GRCM procedure is not sufficiently reliable if an analyte has to bedetermined in the presence of interferent(s). Depending on the qualitative andquantitative composition of a sample the analytical results of various accuracy canbe expected (the same has been observed when the conventional dilution method forcalibration in FAAS was used [7]). The performance of GRCM can be radicallyimproved in a manner including the integration of the signals measured and the useof a spectrochemical buffer. Both treatments are proposed to be executed in such away preserving all GRCM advantages and those general facilities offered by the flowinjection technique such as high frequency of sample introduction and low reagentconsumption. The GRCM in the expanded version allows to obtain the satisfactorilyaccurate results even in the presence of very strong interference effects.

The application of GRCM to routine analysis is still limited in a sense that theexperimental parameters and procedural conditions should be strictly defined inrespect of maximum signal-to-noise ratio. Moreover, even under optimum conditions


the method cannot be expected to be better in term of precision than the conventionalcalibration method due to the needful operation in the region of very small signals.However, in those determinations when the systematic error caused by the interference effect is supposed to be higher then the random error typical of GRCM thismethod is recommended to be used instead of another alternative calibration approaches such as, for instance, the standard addition method.

REFERENCES

1. FlowInjectionA tomic Spectroscopy (Burguera J.L., Ed.), Marcel Dekker Inc., New York and Basel 1989.2. Tyson J., Fresenius Z. Anal. Chem., 329, 663 (1988).3. Sperling M., Fang Z. and We1z B., Anal. Chem., 63, 151 (1991).4. Fan S. and Fang Z.,Anal. Chim. Acta, 241, 15 (1990).5. Gilbert Jr. P.l:,Anal. Chem., 31,110 (1959).6. Thompson M. and Ramsey M.H.,J. Anal. Atom. Spectrom., 5, 701 (1990).7. Koscielniak P., Zesz. Nauk. UJ, Acta Chimica X\XVI, 27 (1993).8.Savitzky A and Golay M.l.E.,Anal. Chem., 36,1627 (1964).9. Barzev A, Dobreva D., Futekov L., Rusev V:, Bekjarov G. and Toneva G., Fresenius Z. Anal. Chem,

325, 255 (1986).10. Sperling M., Koscielniak P. and Welz B., Anal. Chim. Acta, 261, 115 (1992).11. Synovec R.E and Yeung E.S.,Anal. Chem., 57,2162 (1985).

Received October 1995Accepted March 1996

high efficient calibration procedure for flow injection flame...

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