Tyson, Julian

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Flow-injection Techniques in Atomic-absorption Spectrometry

University of Massachusetts AmherstScholarWorks@UMass AmherstChemistry Department Faculty Publication Series Chemistry1981Flow Injection Techniques in Atomic AbsorptionSpectrometryJulian TysonUniversity of Massachusetts AmherstFollow this and additional works at: https://scholarworks.umass.edu/chem_faculty_pubsPart of the Chemistry CommonsThis Article is brought to you for free and open access by the Chemistry at ScholarWorks@UMass Amherst. It has been accepted for inclusion inChemistry Department Faculty Publication Series by an authorized administrator of ScholarWorks@UMass Amherst. For more information, pleasecontact scholarworks@library.umass.edu.Recommended CitationTyson, Julian, "Flow Injection Techniques in Atomic Absorption Spectrometry" (1981). Analytical Proceedings. 1361.Retrieved from https://scholarworks.umass.edu/chem_faculty_pubs/1361542 LOW COST CONTINUOUS FLOW ANALYSIS Anal. PYOC. Low Cost Continuous Flow Analysis The following is a summary of one of the papers presented at  a Meeting of the Midlands Region held on January 21st, 1981, at the University of Birmingham. Flow-injection Techniques in Atomic-absorption Spectrometry J. F. Tyson Department of Chemistry, University of Technology, Loughborough, Leicestershire, L E 1 1  3TU In this paper the combination of a flow injection sample introduction system with an atomic- absorption spectrometer is described, in which low, medium and high dispersion of the sample in the carrier stream have been utilised. A simple model of the dispersion is proposed to account for the absorbance - time behaviour observed and for the results of preliminary experiments, in which sample solution is continuously pumped. Apparatus The carrier was pumped by a Gilson minipuls peristaltic pump via an Anachem 2 x 4 port slider injection valve to the nebuliser of either a Pye Unicam SP90 or a Shandon Southern A3300 atomic-absorption spectrometer. The absorbance was recorded as a function of time. Preliminary Experiments The effect of pumping a solution continuously into the spectrometer nebuliser, rather than allowing the nebuliser to operate naturally (i.e., by aspiration under the action of the oxidant flow) is shown in Fig. 1, curve A. This shows that although the absorbance increases as the flow-rate is increased, the efficiency of the nebuliser (in terms of introducing sample solution into the flame) passes through a maximum. This is also demonstrated by the fact that the peak absorbance obtained when a 200-pl sample plug was injected via a distilled water carrier stream, under low dispersion conditions (valve very close to nebuliser), also December, I981 LOW COST CONTINUOUS FLOW ANALYSIS 543 passes through a maximum [Fig. 1, curve ( B ) ] ,  which it will be noted, does not coincide with the nebuliser’s natural flow-rate. Low Dispersion Basic model When a step change in concentration is made at the nebuliser then, because of the nature of the response of the nebuliser and the response times of the instrument and recorder, an exponential increase in absorbance with time is observed. This continues until a plateau is reached and as the concentration is stepped back to zero, the absorbance decreases exponentially to zero. As the volume of the concentration step decreases so the length of the plateau decreases, until eventually the maximum steady-state value is no longer reached and a peak with a sharp maximum is obtained instead (the flow-injection condition). If a step concentration change from C, to C, is made immediately prior to a mixing chamber of volume V in a stream flowing with velocity zc (what chemical engineers call a continuously stirred tank reactor, see for example reference 1) then the resulting concentration (C) - time ( t )  profile, is given by the following equation: When C, is zero this reduces to .. .. * (2) C = C, [l - exp(- ut/V)] . . and when C, is zero the equation becomes . .  .. * * (3) C = C,exp(- u t / V )  . . . .  The combined effect of the action of the nebuliser and the instrument and recorder response time is the same as though the response were instantaneous, and a mixing chamber of volume V were introduced into the flow system. If it is assumed that absorbance is a linear function of concentration, then the absorbance - time graphs can be conveniently analysed by re- arranging the absorbance forms of equations (2) and (3) to give t = V/u[ln A/(Am - A)] and t = V/u(ln A, /A) ,  where A is the absorbance at time t and A m  is the maximum absorbance, so that a volume, V ,  is obtained if the flow-rate u is known. If a volume, Vi,  is injected into the flowing stream then the time for the absorbance maximum to be reached will be Vi/u;  substituting in the absorbance form of equation (2) gives APIA, = 1 - exp(--Vi/V) (where A ,  is the peak absorbance and A m  is the steady-state value). Therefore the value of V can be calculated very easily. The term Ap/Am is the reciprocal of the dispersion of a flow-injection system as defined by RiiiiEka (see for example reference 2). If the value of V is established for a set of conditions, the effect of the sample volume can be readily predicted, as can the ratio APIArn. For the low dispersion flow injection experiment the analysis of the curve is simpler. Experimental and results The injection valve was located as close to the nebuliser as possible and 100-pl volumes of calcium solutions were injected into a stream of distilled water, flowing at  0.0822 ml s-l. For a step change in concentration, the Shandon Southern instrument gave an absorbance - time profile that corresponded to a hypothetical mixing chamber volume of 43 p1. Thus for a 100-p1 injection volume the value of APIA, is calculated to be 0.90 (i.e.,  a dispersion of 1.1). The experimentally determined value obtained was 0.92 0.02, for a series of calcium solutions containing 3-15 p.p.m. of calcium. The flow-rate can be calculated from the time taken to reach the absorbance maximum and the volume injected. This enables the time taken for the absorbance to fall to a specified fraction of the maximum value to be calculated. In this case, the absorbance will have dropped to 1% of the maximum in a further 2.4 s. Low-dispersion injection is the flow-injection analogue of discrete nebulisation and the precision obtained in this study (on peak heights) of 1% relative standard deviation, com- pares favourably with other ~ t u d i e s . ~ - ~  Medium Dispersion This mode of operation was used to provide the flow injection analogue of the standard additions method. The sample was pumped continuously into the nebuliser and discrete volumes of standards were injected into the sample stream. Basic model The expected concentration - time behaviour is given by equation (1), where C1 is the concentration of the sample, and C2 is the concentration of the standard. At the peak maximum t = V1/u so that equation (1) becomes (4) where Cp is the concentration at the peak maximum. The difference between the concentra­tion at the peak maximum and the concentration of the sample stream is given by Cp - C1 ( = �C) and an expression for this is obtained by subtracting C1 from both sides of equation (4) as follows(5) If it is assumed that absorbance is proportional to concentration then, from the absorbancc form of equation (5), when �A = 0, C2 = C1 • Equation (5) therefore, predicts that LiA will be a linear function of C2, which intercepts the concentration axis at C1 • Experimental and resuUs The dispersion was increased by incorporating a 100-cm length of 0. 76-mm bore tubing between the injection valve and the nebuliser. The sample volume injected was 200 pJ. When distilled water was injected into a stream containing 10 p.p.m. of calcium, the absorbance - time profile corresponded to a mixing chamber volume of 182 pJ, and the dispersion had increased to 1.5. For the injection of 5, 15 and 20 p.p.m. into the 10 p.p.m. stream (absorbance 0.56) the model predicts peak absorbances of 0.38, 0.74 and 0.92, respec­tively. The experimental values were 0.40, 0.71 and 0.85. Part of the discrepancy is due to the non-linearity of the absorbance - concentration relationship. The standard addition calibrations obtained for (A) 5 p.p.m. of calcium, and (B) 5 p.p.m. of calcium in the presence of 200 p.p.m. of phosphate, are shown in Fig. 2. 0.6 A Q) 0.4 u C: "' -e 0 !II .s:, <( 0.2 0 2 4 6 Flow-rate/ml min-1Fig. 1. Relationship between absorbance and flow-rate. (A), Solution continuously pumped into nebuliser and (B), 200 µ.l injected into carrier stream. 0 2 4 6 8 10 Concentration of standard, p.p.m. Fig. 2. Standard addition analysis of 5 p.p.m. of calcium. Plot of t.A against concentra­tion of standard. (A), 5 p.p.m. in distilled water and (B\, 5 p.p.m. in 200 p p.m. of phos­phate. High Dispersion Basic model If a real mixing chamber is introduced into the flowing stream then a known concentration -time profile may be generated when a step change is made in the concentration. If this is from zero, then equation (2) applies, and if suitable values of V and C2 are chosen the profiles generated may be almost linear. For example, with V = IO ml, C2 = 50 p.p.m. and u = 4 ml min-1, the calibration over the first 33 s (0-10 p.p.m.) would give a correlation coefficient of 0.999 for a least-squares fit of the best straight line. Experimental and results A small glass chamber containing a magnetic stirrer was incorporated into the line between the injection valve and the nebuliser. The agreement between the actual and the predicted behaviours for a profile generated from a mixing chamber of volume 8.4 ml with C2 = IO p.p.m. of calcium and a flow-rate of 4.38 ml Inin-1, is shown in Fig. 3. Results for the high dispersion case have been somewhat variable and it is felt that the single Inixing chamber model may be an oversimplification in this case. However, it is suggested that the genera­tion of curves of this type would be a useful way of calibrating the instrument, particularly if it were interfaced with a computer-based data handling facility. Q) u C: "' ..0 0 .. ..0 <I: 0.6 0.5 0.4 0.3 0.2 0.1 0 30 60 90 120 150 180 Time/s Fig. 3. Ab�orbance • time profiles genera­ted by a mixing chamber. Continuous line, real behaviour; broken line, theoretical behaviour. Conclusion It has been demonstrated that the manipulation of the concentration of a sample by dispersion in a flowing carrier stream has potential as a sample introduction technique for atoinic-absorption spectrometry. The experimental set up is simple and inexpensive. Low dispersion systems provide the analogue of discrete nebulisation with precision better than 1 % relative standard deviation and a total peak base width of less than 5 s. Medium dispersion systems provide the analogue of the standard additions method and can give base widths of about 8 s. High dispersion systems provide known concentration - time profiles that may be useful for calibration purposes. In all instances the concentration - time behaviour observed can be modelled as the passage of concentration step changes through a single mixing chamber. References 1. Levenspiel. 0., "Chemical Reaction Engineering," Second Edition, John Wiley, New York, 1972. 2. Ruzicka, J., and Hansen, E. H., Anal. Chim. Acta, 1980, 114, 19.3. Uchida, T., Kojima, I., and Iida, C., Anal. Chim. Acta, 1980, 116, 205.4. Fry, R. C., Northway, S. J., and Denton, M. B., Anal. Chem., 1978, 50, 1719.5. Wolf, W. R., and Stewart, K. K., Anal. Chem., 1979, 51, 1201.6. Fukamachi, K., and Ishibashi, N., Anal. Chim. Acta, 1980, 119, 383. 

Flow Injection Techniques in Atomic Absorption Spectrometry

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Flow Injection Techniques in Atomic Absorption Spectrometry

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