Obtaining and processing data from laboratory instruments

trends in analytical chemistry, vol. 72, no. 2, 1993

Obtaining and processing data from laboratory instruments Il. Processing the data using EXCEL Benne D.J.R. Fennema”, Robert J. Forster, Johannes G. Vos, Greg Hughes and Dermot Diamond** Dublin, Ireland In the first part of this article, we discussed the use of data acquisition cards for upgrading analytical information, and strategies for getting digitised information into the WINDOWS environment. In Part II, we discuss the software options for processing the digitised information and show how it is possible to develop customised solutions to achieve quite complex data transformations using standard applications packages such as the spreadsheet EXCEL.

Introduction In this article, we demonstrate how the approach described in Part I [ 11 can be used for more specialised research applications through the development of customised software.

Dedicated software for polymer modified electrodes In this application the analog output facility of the RTI-815 card was used to generate the particular waveform needed for the experiment. Parameters such as pulse height, pulse width and delay between pulses can

*Present address: School of Chernistry, University of Illinois, 1209 W. California, St., Urbana, IL 61801, USA. **To whom correspondence should be addressed.

0165.9936/93/$06.00

be set through the software. This represents a real improvement on the model 175 programmer, which can only generate manually triggered single short pulses. Hence the RTI-815 card can be used as a waveform programmer, enabling analog instruments to perform pulse train experiments that before only could be carried out on expensive instruments based on digital technology such as the EG&G PAR 273 model potentiostat.

Sampled current voltammetry In this experiment a pulse train is applied to the system under study and the current response is monitored. A typical pulse train is depicted schematically in Fig. 1. When the system is excited by a pulse which reaches a value located around the half-wave potential (E112) value for the particular redox couple present, a current transient will be induced. In sampled current voltammetry this transient is captured and sampled at various time intervals. In this example, the transient is captured over a 40-ms timescale using the parallel data transfer facility of the RTI-815 card, and the sampling is carried out just before (background) and at 1,2,4, 8 and 10 ms after application of the pulse to the redox system, as measured from the onset of the pulse. A plot of the resulting time-current data is presented in Fig. 2. Sixty pulses were applied to an Os(II/III) couple present as an [Os(bpy)2(PVP)l&l]CCl polymer film on a glassy carbon substrate with aqueous 0.1 M toluene sulphonic acid as supporting electrolyte. The sixty data points for each of the sampling times were captured and subsequently transferred to Microsoft EXCEL and graphed using a customised macro. From Fig. 2, it is evident that El/2 is around 0.58 V as measured against the normal hydrogen electrode (NHE), and one can clearly see that at the fastest sampling time of 1 ms,

0 1993 Elsevier

Science

Publishers

B.V. All rights

reserved

38

trends in analytical chemistry, vol. 72, no. 2, 1993

at low sweep rates, or evaluated from the selected pulse

window, using a 5point Savitsky-Golay second-order derivation algorithm to find the point of inflection. Background correction is applied based on linear regression of the data before the slope, assuming a linear response of background with applied potential. The first step in the analysis of the data is based on the well-known Cottrell [2] equation:

!I;!

TT 8

10

nFAC,m

timelms

i(T)

=

(1) *

ow (b)

AH

H=llJ-20mV P=2Os W=50ms

Fig. 1. Current sampling points (a) and pulse train waveform details (b) used in sampled current experiments. The sample is subjected to a voltage pulse every 20 s (pulse width 50 ms) with the pulse height being stepped up by a fixed step in the range lo-20 mV. The current is sampled just before the pulse is applied (B) for background correction, and then at 1,2, 4, 8 and 10 ms after the pulse is triggered. While the model 175 programmer can give analog ramp and single pulse outputs (fixed pulse height and width) it cannot produce the type of waveform shown above. This waveform was generated using one of the RTI-815 analog-out channels. The rise for a one-volt output step on these channels is ~30 ps, which is similar to that of the PAR-363 potentiostat.

where i(z) is the current at time 7, F the Faraday constant, n the number of electrons, A the electrode

0014

-

0012

-

001

3

1

0006

I

0004

I 002 0

+

0

the current does not reach its limiting value as compared with the other traces. This is due to the fact that the potentiostat cannot compensate quickly enough for the difference between the applied and set potential. This potential drop across the (resistive) electrolyte, known as ZR drop, is a common problem in electrochemistry. At slower sampling times, the hardware can counteract this drop by applying a larger potential difference to compensate for the effect. From the data presented in Fig. 2, important kinetic parameters such as the transfer coefficient (a) and the heterogeneous rate constant (k,) can be determined as described below.

Determination

of kinetic parameters

The standard electrode potential (E,) value can either be fixed at the value obtained from voltammetry

-c

p

0002

..-+ ...

-I

wp”~...~~...~_,

01

02

03

04

05

06

07

08

09

1

1 E(appl)iV

vs.NHE

Fig. 2. Results obtained with the sampled current technique as described in Fig. 1. Sixty pulses (15 mV step increase, 50 ms pulse width, 20 s delay) were applied to the Os(ll/lll) couple present as an [Os(bpy)2(PVP)loCI]CI polymer film on a glassy carbon substrate with aqueous 0.1 M toluene sulphonic acid as supporting electrolyte. Sixty data points for each sampling time (background, 1, 2, 4, 8 and 10 ms after triggering the pulse) were captured and subsequently transferred to Microsoft EXCEL and graphed using a customised macro. El12 is around 0.56 V vs. NHE, and the effect of IR drop is clearly visible on the 1-ms trace which fails to reach a steady state above 0.7 V vs. NHE.

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trends in analytical chemistv, vol. 12, no. 2, 1993

surface area, Co the bulk concentration of the electroactive species, Do the diffusion coefficient, and T the sampling time. The analysis consists of a plot of current i(T) vs. l/~:“~ with z the sampling time in seconds which gives a line of slope proportional to a, with all factors contributing to the proportionally constant known. Hence, from this plot the diffusion coefficient D, may be evaluated. Using Matsuda’s approach [3] and the assumption that the diffusion coefficients for oxidation and reduction are identical (i.e., E, = El/z), the transfer coefficient (a) and the heterogeneous rate constant (k,) can be determined. The transfer coefficient is the fraction of reduced species, and is similar to a symmetry factor describing which process is favoured, oxidation or reduction (for an ideally reversible reaction a should be 0.5). The heterogeneous rate constant gives the rate at which electrons can cross the electrode-electrolyte junction, or, for a modified electrode, the electrodepolymer interface. The analysis continues with plotting; 1.75 + x2[ 1 + exp (- r)2] 1 - x[ I + exp (- <)I

(2)

where;

After linear regression, the best fit line has the characteristics; Y = MEappl + B

(3)

where M is the slope and B the intercept. Matsuda has shown (see ref. 3, eqns. 44,45 and 49) that the transfer coefficient (a) is related to the slope of the regression line (M> and several other known quantities by the expression;

(4)

The heterogeneous rate constant mined from (see ref. 3, eqn. 49);

(ko) may be deter-

4G &iz L

lnkO+ln

= i ‘/I

=ln

x

1.75 + x2[ 1 + exp (- L)2]

(5) 11 When Eappl = E. and with substitution and rearrangement we get; (i

k

0

a

=

t

4G

I-x[l

1

+ exp (-<)I

e-(MEo+B)

(6)

which gives k. in terms of known quantities. Hence values for k. and a can be calculated for each of the sampling times z during application of the pulse. The software allows the user to change settings such as total number of pulses applied (via the pulse height) and the pause between pulses during data acquisition. During a scan, the operator is guided by the real time plotting of the raw data and is able to stop the acquisition at any time. In addition to the processing described above, the software calculates the difference between the potentials E3/4 and E1/4. These are the potentials at which the current reaches 25% and 75% of the maximum limiting current, respectively. From this difference it can be deduced whether the redox couple is reversible by applying Tomes criterion for reversibility, i.e., Ey4-E114 < 56.4 mV, when measured at 298 K [4]. Obviously, wading through this type of data processing manually is difficult, time consuming and prone to operator error. It is also unlikely that electrochemical software packages would be flexible enough to allow this particular series of operations to be carried out. However, since we have direct access to the experimental data in digital format, it is a relatively simple matter to produce a routine to perform this particular analysis and to display the results. Alternatively, one can use a spreadsheet to perform the necessary operations on the data and generate a macro to automate the process. Fig. 3. shows the analysis result for the 2-ms trace shown in Fig. 2. It should be stressed that this approach gives the user access to the data and the formulae used at every stage, from the raw experimental data to the final result. Hence the researcher is in complete control of the data processing and can follow the sequence of operations performed on the data and observe the effect. In addition, to implement the processing requires a precise grasp of the mathematics involved.

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trends in analytical

Fig. 3. Best-fit line and equation using Matsuda’s approach for the 2-ms trace shown in Fig. 2. Data points on the rising portion of the curve were extracted from the array and the various mathematical operations carried out in Microsoft EXCEL. Linear regression analysis was perfomred on the data using the LINEST function in EXCEL. This returned the slope (M = 16.412) and intercept (B = -9.412) of the best-fit line and thus enabled a and kOto be calculated. The various key presses and mouse movements required were recorded using the EXCEL macro facility. Processing of other data sets can be readily achieved by importing the data into an EXCEL template which contains the appropriate mathematical expressions in the correct spreadsheet cells and activating the macro with a single keypress.

Conclusions

chemistry, vol. 1.2,no. 2, 1993

processing, and to provide gateways for importing the data into commercial packages for desk-top publishing and report generation. We have found that as expertise and the number 01 problems solved grows, modification ofthe routines for specialised research techniques becomes easier and quicker. For example, using the same hardware we have developed software for performing chronoamperometry and data processing/analysis for experiments on polymer-modified electrodes. Software for these applications is not readily available from commercial sources at an affordable price. In addition, software has been written to enable the RTI-815 card to completely control flow injection analysis systems and for monitoring sensor arrays using in-house developed software [.5].

References I

B.D.J.R. Fennema, R.J. Forster, J.G. Vos, G. Hughes and D. Diamond, Trends in A&. Chem., 12 (1993) 1-3. 2 A.J. Bard and L.R. Faulkner, Electrochemical Methods, Wiley, New York, 1980, p. 143. 3 H. Matsuda, Bull. Chem. Sot. Jp., 53 (1980) 3439-3446. 4 A.J. Bard and L.R. Faulkner, Electrochemical Methods, Wiley, New York, 1980, p. 160. 5 R. Forster and D. Diamond, Ajzal. Chenz. 64 (1992) 1721-1728.

I/O cards have proven themselves useful intermediates at the interface between laboratory instruments and PCs. The result presented show that a PC and I/O card can be used to significantly improve the performance of existing analog instrumentation. Our approach is to develop routines for data acquisition and specialised

Benne D. J. R. Fennema and Greg Hughes are at the School of Physical Sciences, Dublin City University, Glasnevin, Ireland. Robert J. Forster, Johannes G. Vos and Dermot Diamond are at the School of Chemical Sciences, Dublin City University, Dub/in 9, Ireland.

Computer Corner Contributions Contributions are welcome in the following categories: matical tools and interfacing. Please send your papers either to:

hardware, software, chemical applications,

TrAC Computer Corner, Prof. G. Gauglitz, Institut fur Physikalische und Theoretische Tubingen, Auf der Morgenstelle 8, W-7400 Tiibingen 1, Germany; or

mathe-

Chemie, Universitat

TrAC Computer Corner, Dr. A. P. Wade, Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, B.C. Canada V6T 1Y6.