Detection conductivity

Conductivity detection has also been used for on-chip measurements. In one report, the solution (e.g., NaCl) was in contact with the Pt electrodes on a Si-Pyrex chip [770]. Conductivity detection was made possible on a Pt electrode sputtered on a PMMA chip [631]. 

An on-column contact conductivity detector was fabricated on a PMMA chip to analyze amino acids, peptides, proteins, and oligonucleotides. To minimize faradaic reaction and to only measure ohmic resistance (or conductivity), a bipo- 

FIGURE 7.27 Micromachined ISE chip, (a) Schematic drawing of a sensor chip design with channels and reservoirs point A, inlet for U-channel point B, outlet for U-channel point C, inlet for sample channel point D, junction structure, where membrane contacts sample solution and point E, outlet for sample channel. The diagram illustrates the complete filling of the silanized U-channel with an organic membrane cocktail, (b) Scaled diagram of the 12- x 6-mm chip [766]. Reprinted with permission from the American Chemical Society. 

In a special case, the decrease in resistance of Pd after H2 absorption was employed for its determination. This decrease which was due to the decrease in the work function of the Pd layer by H2, was correlated with the H2 concentration. For instance, H2 gas (LOD 0.5%) was detected in a Si microchannel electrode-posited with a Pd layer [771], 

To avoid electrolysis and electrode fouling when the solution was in contact with the measurement electrodes, contactless conductivity detection was proposed. This non-contact method relied on the capacitive coupling of the electrolyte in the channel, and the method has been used to detect inorganic ions that alter the conductivity and capacitance in the electrolyte [277,638]. 

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Other Methods. Ion chromatography using conductance detection can be used to measure low (<1%) levels of nitrite, chloride, sulfate, and other ions in nitric acid. Techniques for ion chromatographic analysis are available (93). 

They measured the formation of ionic species in the pulse radiolysis of a 10"3 m MTMSO aqueous solution saturated by N20 gas at pH 5.6 by conductivity detection and found an increase in conductivity. Assuming the molar conductivity of CH3SCH2S02 to be the same as that measured for CH3S02- 45, it is found that G(CH3SCH2S02") = 2.7. 

Gas chromatography/electrolytic conductivity detection (GC/ELCD) and gas chromatography/halogen-speciflc detection (GC/XSD) are specific for halogenated compounds and were effective for discriminating between sulfentrazone compounds and the matrix components. Operating conditions are listed below. 

Figure 4.20 Separation of comon anions and alkaline earth cations by ion chronatography using conductivity detection.

Figure 4.21 Exploded view of a nlcroaeBbrane suppressor and gradient elution separation of a aixture of inorgemic and organic anions by ion chromatography employing conductivity detection with a mlcromembrane suppressor.

Figure 3 Gradient separation of anions using suppressed conductivity detection. Column 0.4 x 15 cm AS5A, 5 p latex-coated resin (Dionex). Eluent 750 pM NaOH, 0-5 min., then to 85 mM NaOH in 30 min. Flow 1 ml/min. 1 fluoride, 2 a-hydrox-ybutyrate, 3 acetate, 4 glycolate, 5 butyrate, 6 gluconate, 7 a-hydroxyvalerate, 8 formate, 9 valerate, 10 pyruvate, 11 monochloroacetate, 12 bromate, 13 chloride, 14 galacturonate, 15 nitrite, 16 glucuronate, 17 dichloroacetate, 18 trifluoroacetate, 19 phosphite, 20 selenite, 21 bromide, 22 nitrate, 23 sulfate, 24 oxalate, 25 selenate, 26 a-ketoglutarate, 27 fumarate, 28 phthalate, 29 oxalacetate, 30 phosphate, 31 arsenate, 32 chromate, 33 citrate, 34 isocitrate, 35 ds-aconitate, 36 trans-aconitate. (Reproduced with permission of Elsevier Science from Rocklin, R. D., Pohl, C. A., and Schibler, J. A., /. Chromatogr., 411, 107, 1987.)...

Figure 10 Separation of monochloroacetate, dichloroacetate, and trichloroacetate on a sulfonated poly(styrene-divinyl benzene) column with suppressed conductivity detection. Column 2% cross-linked sulfonated poly(styrene-divinyl benzene) capacity 0.02 meq/g. Flow rate 64 ml/hr. Eluant 15 mM sodium phenate. Suppressor 0.28 x 25 cm Dowex 50W X8 column (200-400 mesh). Detector Chromatronix conductivity cell connected to a Dow conductivity meter. (Reprinted with permission from Small, H., Stevens, T. S., and Bauman, W. C., Anal. Chem., 47,1801,1975. 1975 Analytical Chemistry.)...

Shamsi, S. A. and Danielson, N. D., Ion chromatography of polyphosphates and polycarboxylates using a naphthalenetrisulfonate eluent with indirect photometric and conductivity detection, /. Chromatogr. A, 653, 153, 1993. 

Anions of weak acids can be problematic for detection in suppressed IEC because weak ionization results in low conductivity and poor sensitivity. Converting such acids back to the sodium salt form may overcome this limitation. Caliamanis et al. have described the use of a second micromembrane suppressor to do this, and have applied the approach to the boric acid/sodium borate system, using sodium salt solutions of EDTA.88 Varying the pH and EDTA concentration allowed optimal detection. Another approach for analysis of weak acids is indirect suppressed conductivity IEC, which chemically separates high- and low-conductance analytes. This technique has potential for detection of weak mono- and dianions as well as amino acids.89 As an alternative to conductivity detection, ultraviolet and fluorescence derivatization reagents have been explored 90 this approach offers a means of enhancing sensitivity (typically into the low femtomoles range) as well as selectivity. 

Nowak, M. and Seubert, A., Application of experimental design for the characterization of a novel elution system for high capacity anion chromatography with suppressed conductivity detection, /. Chromatogr. A, 855, 91,1999. 

Many IC techniques are now available using single column or dual-column systems with various detection modes. Detection methods in IC are subdivided as follows [838] (i) electrochemical (conductometry, amper-ometry or potentiometry) (ii) spectroscopic (tJV/VIS, RI, AAS, AES, ICP) (iii) mass spectrometric and (iv) postcolumn reaction detection (AFS, CL). The mainstay of routine IC is still the nonspecific conductometric detector. A significant disadvantage of suppressed conductivity detection is the fact that weak to very weak acid anions (e.g. silicate, cyanide) yield poor sensitivity. IC combined with potentiometric detection techniques using ISEs allows quantification of selected analytes even in complex matrices. The main drawback... 

Some commercially available detectors have a number of detection modes built into a single unit. Fig. 2.4o is a diagram of the detector used in the Perkin Elmer 3D system, which combines uv absorption, fluorescence and conductivity detection. The uv function is a fixed wavelength (254 nm) detector, and the fluorescence function can monitor emission above 280 nm, based on excitation at 254 nm. The metal inlet and outlet tubes act as the electrodes in the conductance cell. The detection modes can be operated independently or simultaneously, using a multichannel recorder. In the conductivity mode, using NaCl, a linear range of 103 and a noise equivalent concentration of 5 x 10 8 g cm-3 have been obtained. 

Jiang, S.X., Liu, X. (1997). Reverse phase HPLC analysis of alkyl sulfonates with nonsuppression conductivity detection. J. Liq. Chromatogr. Related Technol. 20(13) 2053-2061. 

Valproic acid was determined in tablets and plasma using ion-chromatography [29], The extract was injected onto a column (6.5 cm x 6 mm) of Dionex ICE separator resin fitted with a guard column of Aminex cation exchange resin and operated with aq. 0.5 mM C02 as mobile phase (0.7 mL/min) and conductivity detection. For tablets, the calibration graph was rectilinear for 0.2-25 pg/mL with limit of detection of 50 pg/mL. For plasma, the response was linear for 50-200 pg/mL and limit of detection was 2 pg/mL. 

There are reports that the use of robotic milking systems will significantly reduce mastitis, linked at least partially to the machines more efficient teat disinfection compared to human operators. However, the more frequent visit to be milked and more gentle application and removal of milking equipment from the teats may also contribute to reduced mastitis incidence. On the other hand, the absence of human operators in robotically milked herds may lead to late detection and treatment of mastitis. However, this may be overcome by the introduction of conductivity detection systems which allow early signs of mastitis to be detected (Hogeveen and Meijering, 2000). 

Ion chromatography using a weak cation-exchange column with direct conductivity detection was applied in the determination of aqueous tributyltin ions, with short elution times LOD 0.01 ppm, without preconcentration or derivatization. No organotin remains adsorbed on the column94. 

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