Pulse Polarographic Techniques

Studies using a differential-pulse polarographic technique, which is recommended in place of classical direct current polarography, yielded the following results. For... 

Pulse Polarographic Determination of Trinitro-glycerin in Explosives , BullBelg 86 (12), 17—21 (1977) CA 86,192063 (1977) [Reported is a quant pulse polarographic technique for the measurement of NG with a reproducibility of 0.5%] 2) W.L. RoUwitz D.J. King, Funda-... 

Polarographic AC and pulse polarographic techniques as well as stripping analysis are effective tools for the determination of trace levels of metal ions. Table 3.1 provides a comparison of the sensitivity and usefulness of the various methods.12,21 23 For metal ions, stripping voltammetry usually is the method... 

The analysis obtained with classical polarographic methods corresponds roughly with those reached by the pulse-polarographic technique, but the sensitivity is much lower. The difference between the polarographic behaviour of single-stranded and double-helical form of polynucleotides makes possible the study of the conformation of nucleic acids [81,82,108-113]. Polarography can be utilized also in the study of structural changes of polynucleotides under the influence of the temperature [112,114,115] or irradiation [116]. The photodynamic destabilisation of DNA has been described [117]. 

As a general rule, quantification limits of lO moll can be readily obtained with conventional (d.c.) polarography, whereas these limits may be reduced to lO moll or lower by using modern pulse polarographic techniques and particularly square wave (Osteryoung) polarography. 

The pulse polarograph designed and built by Barker at Harwell served as the prototype for an instrument marketed by Southern Analytical, Ltd. in the I960 s. This vacuum-tube based instrument reportedly sold for about 25,000 in 1969 (23). In 1961 Milner and coworkers at Harwell demonstrated beautifully the power of the differential pulse polarographic technique as implemented by the Southern Analytical instrument for the determination of uranium in seawater. After separation the determination was done at the level of 10 nM with precision of several per cent. A few years later in the United States, Gilbert reported on using this instrument for determination of nickel and vanadium in petroleum stocks ( ). 

The Metrohm 646 VA Processor is another microprocessor based instrument. Used in tandem with the 647 VA Electrode Stand and 675 VA Sample Changer, this system is capable of performing automated data acquisition, including the use of the standard additions method. Data analysis features include smoothing and differentiation, and a peak shape analysis routine that performs independently of the base current. Pulse polarographic techniques that can be performed include dp, which can be optimized for reversible and irreversible systems staircase, with current measurement during the final 20 ms of each current step and Barker square wave, which employs a waveform composed of five square wave oscillations superimposed upon a staircase, with currents measured for 2 ms at the end of each half cycle of the second, third and fourth oscillations. The 1988 price of this instrument is 14,000. 

Betso and McLean [11] have described a differential pulse polarographic method for the determination of acrylamide and acrylic acid in polyacrylamide. A measurement of the acrylamide electrochemical reduction peak current is used to quantify the acrylamide concentration. The differential pulse polarographic technique also yields a well-defined acrylamide reduction peak at 2.0 V versus SCA (reduction potential), suitable for qualitatively detecting the presence of acrylamide. The procedure involves extraction of the acrylamide monomer from the polyacrylamide, treatment of the extracted solution on mixed resin to remove interfering cationic and anionic species, and polarographic reduction in an 80/20 v/v) methanol/water solvent with tetra-n-butylammonium hydroxide as the supporting electrolyte. The detection limit of acrylamide monomer by this technique is less than 1 ppm. 

Fig. 2. Response of the polarographlc detector. Differential pulse polarographic technique, pulse amplitude 25 mV, constant potential values denoted. Other conditions the same as in Figure 1.

The realization that current sampling on a step pulse can increase the detection sensitivity by increasing the faradaic/charging ratio is the basis for the development of various pulse voltammetric (or polarographic) techniques. Also, the pulses can be applied when it is necessary and can reduce the effect of diffusion on the analyte. Figure 18b. 11 shows the waveform and response for three commonly used pulse voltammetric techniques normal pulse voltammetry (NPY), differential pulse voltammetry (DPV), and square-wave voltammetry (SWV). 

The detection of products derived from the N-oxygenation of C=N functionalities presents many problems, which illustrate difficulties that are associated with the isolation, identification and quantification of small amounts of water-soluble metabolites. Spectrophotometric methods19 as well as differential pulse polarographic techniques20 previously used to determine oximes, nitrones and N-oxides frequently lack sensitivity and/or specificity. Improved analytical methods for the quantification of these N-oxy compounds include chromatographic techniques taking into account the chemical peculiarities of the individual N-oxygenated C=N functionalities. These procedures usually require the chemical synthesis of authentic material for comparison with data obtained with the isolated metabolites, and also for the construction of calibration curves. 

Direct assays for SOD have relied on pulse radiolysis (K12.M15), stopped-flow spectroscopy (M4,M17), rapid-flow electron paramagnetic resonance (EPR) (Bl,05, SI), polarographic techniques (R4), and nuclear magnetic resonance (NMR) spectroscopy (R3). 

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