Purnendu K. Dasgupta

Analytical Chemistry in a Drop. Solvent Extraction in a Microdrop

An organic microdrop (∼1.3 μL) is suspended inside a flowing aqueous drop from which the analyte is extracted. The drop-in-drop system is achieved by a multitube assembly. The aqueous phase is continuously delivered to the outer drop and is aspirated away from the bottom meniscus of the drop. After the sampling/extraction period, a wash solution replaces the sample/reagent in the aqueous layer, resulting in a clear outer aqueous drop housing a colored organic drop containing the extracted material. This also results in an automatic backwash. The color intensity of the organic drop, related to the analyte concentration, is monitored by a light-emitting diode based absorbance detector. After the analytical cycle, the organic drop is removed and replaced by a new one. The performance of the system is illustrated with the determination of sodium dodecyl sulfate (a methylene blue active substance) extracted as an ion pair into chloroform. This unique microextraction system is simple and flexible, permits automated backwashing, consumes only microquantities of organic solvents, and is capable of being coupled with other analytical systems. This concept should prove valuable for preconcentration and matrix isolation in a microscale.

Light emitting diode-based detectors: Absorbance, fluorescence and spectroelectrochemical measurements in a planar flow-through cell

Abstract Light emitting diodes (LEDs) were first used for chemical analysis three decades ago. They are finally making their appearance in commercial analytical systems and dedicated detectors. LEDs are the most energy-efficient means of producing monochromatic light, and provide a concentrated small cool emitter ideal for miniature analytical devices. Although they rank behind fluorescent and halogen discharge lamps in total conversion efficiency (lm/W), new efficiency records are being set every year such that by next decade broadband (white) LED sources are not only likely in analytical instrumentation, but for general illumination. This paper begins with a review of analytical use of LEDs that has been advanced in the last decade. LED-based absorbance measurement and its use in pedagogy, titrations, in providing immunity to refractive index and turbidity effects, in field and process analysis, in capillary electrophoresis (CE), in liquid–liquid extraction systems, in film and drop-based analytical systems and with liquid core waveguides (LCWs) are discussed. LED-based fluorescence and spectroelectrochemical detection follows next. Multipurpose LED-based analytical instrumentation and special analytical applications and general applications are discussed. A listing of (mostly web-based) resources for fabricating LED-based detectors is then provided. Detector circuits and available components are considered and different modes of driving LEDs are compared. The temperature dependence of LED characteristics and strategies to ameliorate this problem are discussed. The review and general resource material is followed with the construction details, operation and performance observed for a simple-to-fabricate multipurpose cell that allows simultaneous multiwavelength absorbance, fluorescence and spectroelectrochemical detection.

Recent developments in cyanide detection: a review.

The extreme toxicity of cyanide and environmental concerns from its continued industrial use continue to generate interest in facile and sensitive methods for cyanide detection. In recent years, there is also additional recognition of HCN toxicity from smoke inhalation and potential use of cyanide as a weapon of terrorism. This review summarizes the literature since 2005 on cyanide measurement in different matrices ranging from drinking water and wastewater, to cigarette smoke and exhaled breath to biological fluids like blood, urine and saliva. The dramatic increase in the number of publications on cyanide measurement is indicative of the great interest in this field not only from analytical chemists, but also researchers from diverse environmental, medical, forensic and clinical arena. The recent methods cover both established and emerging analytical disciplines and include naked eye visual detection, spectrophotometry/colorimetry, capillary electrophoresis with optical absorbance detection, fluorometry, chemiluminescence, near-infrared cavity ring down spectroscopy, atomic absorption spectrometry, electrochemical methods (potentiometry/amperometry/ion chromatography-pulsed amperometry), mass spectrometry (selected ion flow tube mass spectrometry, electrospray ionization mass spectrometry, gas chromatography-mass spectrometry), gas chromatography (nitrogen phosphorus detector, electron capture detector) and quartz crystal mass monitors.

Continuous liquid-phase fluorometry coupled to a diffusion scrubber for the real-time determination of atmospheric formaldehyde, hydrogen peroxide and sulfur dioxide

Abstract An aqueous scrubber liquid is pumped through a filament-filled narrow bore microporous hydrophobic membrane tube while sample air flows around it. A constant fraction of the analyte gas, dependent on its diffusion coefficient and the effective mass accommodation coefficient in the scrubber liquid and on the diffusion scrubber dimensions and sampling rate, is collected in the liquid. One or more reagents are added to the effluent liquid for specific determination of the gas of interest. The Hantzsch reaction, peroxidase-mediated oxidation of p-hydroxyphenylacetic acid by peroxide, and bisulfite addition to 9-N-acridinylmaleimide, all direct adaptations of previously described continuous flow aqueous analytical procedures, are used for the determination of HCHO, H2O2 and SO2, respectively. A permeation-based SO2 source and previously described Henrys Law-based porous membrane sources of HCHO and H2O2 are integral to the respective instruments for calibration. Programmed inert valves allow the instruments to perform any sequence of zero, calibrate and sample functions. Each instrument occupies a standard two-tier (50×100 cm) laboratory cart and has been successfully field tested. With a filter fluorometer as detector, LODs are 100 pptv HCHO, 30 pptv H2O2 and 175 pptv SO2. There is no significant dependence on relative humidity. Specific interference testing ( SO 2 O 3 for HCHO and H2O2, O 3 H 2 O 2 H 2 SCH 3 SH for SO2) reveals no major interferences.

Luminescence detector with liquid-core waveguide

A new fluoropolymer tube is proposed as the basis of a novel class of liquid core waveguide-based luminescence detectors. Both chemiluminescence and photoluminescence detectors are possible. In the latter case, illumination is transverse to the main axis of the tube. With such a geometry, it is even possible to operate without monochromators, although limits of detection do improve with the incorporation of monochromators. The nature of the design is such that it is particularly simple to fabricate detectors in a flow-through configuration and where the light from the cell is coupled to a photodetector by an optical fiber. No focusing optics are necessary. A number of applications are illustrated. Attainable limits (LODs, S/N = 3) of detection include 150 pM fluorescein with a 254-nm excitation source, 200 amol of fluorescein in a capillary electrophoresis setup with excitation by two blue light-emitting diodes, 35 nM NH(3) as the isoindole derivative in a flow injection analysis system using a photodiode detector, 50 nM methylene blue and 1 nM Rhodamine 560 using respectively red and green LED arrays and an avalanche photodiode and a PMT in a FIA configuration, 100 parts per trillion by volume gaseous formaldehyde as the Hantzsch reaction product with cyclohexanedione using a diffusion scrubber, 2.7 μM and 17 nM hypochlorite based on its chemiluminescence reaction with luminol with photodiode and PMT detectors, respectively, and 1 ppm SO(4)(2)(-) based on nephelometric detection at 470 nm. The approach described herein leads to particularly simple and inexpensive luminescence detectors with excellent sensitivity.

Solubility of gaseous formaldehyde in liquid water and generation of trace standard gaseous formaldehyde

Stable concentrations of gaseous formaldehyde (atm) may be generated at the 10/sup -9/-atm level and up by equilibration of air with an aqueous formaldehyde (M) solution through a microporous membrane tube. The solubility behavior of formaldehyde at 293.2 K obeys the equation (HCHO(aq)) = 16650(HCHO(g))/sup 1.0798/ for HCHO(g) ranging from 3.3 x 10/sup -9/ to 1.3 x 10/sup -6/ atm or HCHO(aq) ranging from 1.0 x 10/sup -5/ to 6.9 x 10/sup -3/ M. When the aqueous concentration is expressed in mole fraction units, the predictions of the corresponding equation are in good agreement with the vapor pressure data obtained by Blair and Ledbury at aqueous concentrations as high as X/sub HCHO/ = 0.254 (13.4 M). At low HCHO concentrations, the temperature dependence between 278.2 and 298.2 K is given by (HCHO(aq)) = 10/sup ((4538/T)-11.34)/ (HCHO(g))/sup ((252.2/T) + 0.2088) with an uncertainty of less than 10%.

Fast fluorometric flow injection analysis of formaldehyde in atmospheric water

Formaldehyde can be determined in aqueous solution at a rate of 45 samples/h with a small sample requirement (100 ..mu..L). The fluorescence of 3,5-diacetyl-1,4-dihydrolutidine formed upon reaction of formaldehyde with ammonium acetate and 2,4-pentanedione (25 s, 95 /sup 0/C) is monitored with a filter fluorometer. The detection limit is 0.1 ..mu..M (3 ..mu..g/L) or 10 pmol of HCHO. The response is linear up to 3.3 ..mu..M (100 ..mu..g/L), the departure from linearity at 0.33 mM is 21%, but high levels are satisfactorily determined with a second-order calibration equation. Interference from S(IV) has been investigated in detail and completely eliminated by addition of H/sub 2/O/sub 2/ before rendering the sample alkaline. There are no effects from commonly occurring metal ions and anions; the method is very selective to formaldehyde compared to other carbonyl compounds. A S(IV)-containing preservative has been formulated for the stabilization of low concentrations of HCHO. Results are presented for fogwater samples. 8 figures, 41 references.

Solubility of ammonia in liquid water and generation of trace levels of standard gaseous ammonia

Abstract Equilibrium gas phase concentration of ammonia in dilute solution has been measured as a function of total ammonia + ammonium concentration (0.002–0.10 M), pH (6–10) and temperature (278.8−290.6 K). Henrys Law is obeyed under these conditions and may be expressed as In KH(M atm−1) = 4092/T −9.70 with a relative standard error of less than 5 %, in good agreement with NBS thermodynamic data. Convenient generation of trace levels of ammonia (1.33 × 10−8–7.77 × 10−4 atm) using a porous membrane tube is described.

Microfabrication and characterization of teflon AF-coated liquid core waveguide channels in silicon

The fabrication and testing of Teflon AF-coated channels on silicon and bonding of the same to a similarly coated glass wafer are described. With water or aqueous solutions in such channels, the channels exhibit much better light conduction ability than similar uncoated channels. Although the loss is greater than extruded Teflon AF tubes, light throughput is far superior to channels described in the literature consisting of [110] planes in silicon with 45/spl deg/ sidewalls. Absorbance noise levels under actual flow conditions using an LED source, an inexpensive photodiode and a simple operational amplifier circuitry was 1/spl times/ 10/sup -4/ absorbance units over a 10-mm path length (channel 0.17-mm deep /spl times/0.49-mm wide), comparable to many commercially available macroscale flow-through absorbance detectors. Adherence to Beers law was tested over a 50-fold concentration range of an injected dye, with the linear r/sup 2/ relating the concentration to the observed absorbance being 0.9993. Fluorescence detection was tested with fluorescein as the test solute, a high brightness blue LED as the excitation source and an inexpensive miniature PMT. The concentration detection limit was 3 /spl times/ 10/sup -9/ M and the corresponding mass detection limit was estimated to be 5 /spl times/ 10 /sup -16/ mol.

Light emitting diode based flow-through optical absorption detectors.

Simple inexpensive high performance optical absorption detectors are possible using light emitting diodes (LEDs) as light sources. The designs presented in the literature are reviewed. Designs used by the investigators are described in detail with respect to construction, electronic design, performance and cost; these have not previously been described in the literature. Characteristics of commercially available LEDs are tabulated. At the simple end, a single beam, dc driven, transmittance output detector can be constructed within the body of a LED. At the high performance end, fully referenced, computer interfaced detectors are described that are pulsed at high speeds to attain measurement standard deviations in the range of 2-3 x 10(-6) absorbance.