Indirect photometric detection

A practical method for low level perchlorate analysis employs ion chromatography. The unsuppressed method using a conductivity detector has a lower detectable limit of about 10 ppm. A suppression technique, which suppresses the conductivity of the electrolyte but not the separated ions, can further improve sensitivity (110,111). Additionally, ion chromatography can be coupled with indirect photometric detection and appHed to the analysis of perchlorates (112). 

Weston, A., Brown, P. R., Jandik, P., Heckenberg, A. L., and Jones, W. R., Optimization of detection sensitivity in the analysis of inorganic cations by capillary ion electrophoresis using indirect photometric detection,. Chromatogr., 608, 395, 1992. 

Shi, H., Zhang, R., Chandrasekher, G., and Ma, Y., Simultaneous detection and quantitation of sodium, potassium, calcium and magnesium in ocular lenses by high-performance capillary electrophoresis with indirect photometric detection, ]. Chromatogr. A, 680, 653 1994. 

An alternative to derivatizing carbohydrates is the use of indirect photometric detection. In this method, a detectable co-ion in the electrolyte is added to the buffer system generating a steady state absorbance signal in the detector. As the analyte ions migrate in front of the detector window, they displace the detectable co-ion and cause a decrease or negative response in the detector signal. This method provides universal detection of all anions or cations. Since most carbohydrates are not ionized... 

Although AS and AES can be detected at a low UV wavelength, sensitivity is lacking and a more suitable detection was achieved using indirect photometric detection, post-column colour formation reactions, or a pre-column derivatisation, suppressed conductivity detection and refractive index detection [1,42,43]. A comparison of detection limits for the determination of these anionic surfactants shows that photometric and conductivity detectors are better (picomole or nanogram range) than refractive index or fluorometry detectors by about a factor of 1000 [40],... 

Jamali, B., and Nielsen, H. M. (2003). Development and validation of a capillary electrophoresis-indirect photometric detection method for the determination of the non-UV-absorbing 1,4-dideoxy-l,4-imino-d-arabinitol in active pharmaceutical ingredients, solutions and tablets using an internal standard. J. Chromatogr. A 996(1—2), 213-223. 

Foret, R, Fanali, S., Ossicini, L., and Bocek, P. (1989). Indirect photometric detection in capillary zone electrophoresis. /. Chromatogr. 470, 299—308. 

Johns, C., Macka, M., and Haddad, P. R. (2003). Enhancement of detection sensitivity for indirect photometric detection of anions and cations in capillary electrophoresis. Electrophoresis 24, 2150-2167. 

P. Doble, and P. R. Haddad, Indirect Photometric Detection in CE Using Buffered Electrolytes, LCGC 2001,19, 38, 178. 

In one study, a modified Monier-Williams method has been utilized as a preparative procedure to obtain both the free and bound sulfite fractions. The two fractions were analyzed by HPLC with indirect photometric detection using a 250 X 4.6-mm LC-SAX column eluted with potassium hydrogen phtalate (0.15 g/L, pH 5.7) and detected at 280 nm. Levels of 5-10 ppm of S02 in foods, corresponding to 30 - 60 ng injected, were reliably detected by this method. The results confirmed that the chromatographic method, unlike the Monier-Williams method, is able to avoid the potential interference of volatile substances derived from matrices or utilized chemicals (36). The HPLC conditions are summarized in Table 3. 

J. H. Zou, S. Motomizu, and H. Fukutomi, Reversed-phase ion-interaction chromatorgaphy of inorganic anions with tetraalkylam-monium ions and divalent organic anions using indirect photometric detection, Analyst, 776 1399 (1991). 

Wang and Yang [261] used liquid chromatography with indirect photometric detection to determine down to 0.8pg triorganotin compounds in non saline waters. 

Takeuchi et al. [343] described an indirect photometric detection method for anions in micro high performance liquid chromatography. Micro anion exchange octadecylsilica columns permanently coated with the hydrophobic cetyltrimethylammonium ion (cetrimide) were used in the high performance liquid chromatography of chloride, nitrate... 

Ion chromatography can be used to measure ionic concentrations in solution. It requires a fairly large amount of solvent and usually a suppressor column to neutralize the acidity prior to detection by conductivity. For analytical purposes, most analyses previously performed by ion chromatography are now performed using capillary electrophoresis. However, Katagi et al. used ion chromatography with indirect photometric detection to analyze for nerve agents and hydrolysis products in human serum (8). 

Katagi, M., Nishikawa, M., Tatsuno, M and Tsu-chihashi, H. Determination of the main hydrolysis products of organophosphorous nerve agents, methylphosphonic Acids, in human serum by indirect photometric detection ion chromatography, J. Chromatogr., B, 698, 81 (1997). 

T. L. Wang and S. F. Y. Li (1998). Separation of synthetic inorganic polymers of condensed phosphates by capillary gel electrophoresis with indirect photometric detection. J. Chromatog. A, 802, 159-165. 

Interestingly a crystal violet dye was used as a IPR for indirect photometric detection. Common inorganic anions were separated and detected at the absorption maximum of the dye with classical reversed phase columns. The affinity of the analytes decreased in the order of 8203 > 804 > I" > N03" > Br > N02 > CL, which is the same as that observed with classical anion exchange chromatography [136]. 

The nature of the counter ion is important because it establishes the exact value of the potential difference between the stationary phase and the bulk eluent, and also, if it exhibits suitable absorption characteristics, indirect photometric detection of UV-inactive analytes is feasible. Moreover, the choice of the counter ion of the potential determining ion allows a tailor-made separation of the analytes since adsorbophilic counter ions may compete with the analyte for interaction with the potential determining ion, thereby decreasing analyte retention. Different counter ions may alter the elution sequences of a series of analytes with potential advantage for resolution and identification purposes [143]. 

Markuszewski, M.J., Szczykowska, M., Siluk, D., Kaliszan, R. Human red blood cells targeted metabolome analysis of glycolysis cycle metabolites by capillary electrophoresis using an indirect photometric detection method. J. Pharm. Biomed. Anal. 39, 636-642 (2005)... 

R. Thompson, N. Grinberg, H. PerpaU, G. Bicker, and P. Tway, Separation of organophosphonates by ion chromatography with indirect photometric detection, /. Liq. Chrom. 17 (1994), 2511-2531. 

In its broadest sense, selectivity has also been achieved through the development of ion chromatography and indirect photometric detection. Apart from opening up a completely new application of HPLC these provide highly selective techniques for the analysis of inorganic and organic ions. 

Marsh DB, Nuttall KL. Methylmalonic acid in clinical urine specimens by capillary zone electrophoresis using indirect photometric detection. J Cap Elec 1995 2 63-7. 

Fig. 5-14. Ion-pair chromatographic separation of inorganic anions with indirect photometric detection. — Separator column Waters C,% Radial-PAK (5 pm) eluent 0.0004 mol/L tetrabutylam-monium salicylate (pH 4.62) flow rate 2 mL/min detection UV (288 nm, indirect) injection volume 50 pL solute concentrations 4 ppm orthophosphate, 2 ppm chloride, 4 ppm nitrite, bromide, nitrate and iodide, 6 ppm sulfate, and 6 ppm thiosulfate (taken from [28]).

The precise measurement of eluent ion absorption is also extremely important in indirect photometric detection. It is known from classical spectrophotometry [31] that the photometric error is only small in an absorbance range between 0.2 and 0.8. There-... 

Fig. 6-16. Indirect photometric detection of various inorganic anions. - Separator column 250 mm x 4 mm I.D. SAR-40-0.6 eluent 0.001 mol/L sodium phthalate (pH 7 to 8) flow rate 2 mL/min detection UV (285 nm, indirect) injection volume 20 pL solute concentrations 106 ppm chloride, 138 ppm nitrite, 400 ppm bromide, 310 ppm nitrate, and 480 ppm sulfate (taken from [29]).

Fig. 6-18. Ion-pair chromatographic separation of chloride and sulfate with indirect photometric detection. - Separator column 200 mm x 4.6 mm l.D. Zorbax C8 eluent 0.001 mol/L /V-methyloctylam-monium-p-toluenesulfonate flow rate 1.6 mL/min injection volume 20 pL solute concentrations 35 ppm chloride and 96 ppm sulfate (taken from [32]).

The principle of indirect photometric detection can also be applied to ion-pair chromatography provided the counter ion of the ion-pair reagent exhibits the respective absorption properties. For example, A-methyloctylammonium-p-toluenesulfonate was utilized as the ion-pair reagent to analyze chloride and sulfate [32] upon application of indirect photometric detection. The corresponding chromatogram is shown in Fig. 6-18. 

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