A continuous film-recirculable drop gas-liquid equilibration device. Measurement of trace gaseous ammonia

A miniature gas-liquid equilibrator or a gas collector, intended as a low-volume interface between a soluble gaseous sample and a liquid phase analyzer or between a liquid phase sample and a detector designed for use with gas samples, is described. This paper addresses the application of the device for the measurement of trace atmospheric ammonia. Gas collection occurs solely by diffusive sampling such that aerosol particles are not collected. The device essentially consists of a tube surrounded externally by a jacket. Gas flows through the jacket and contacts a liquid film flowing on the surface of the tube. The flowing film forms a drop at the tube terminus and is aspirated off through the inner bore of the tube. The collected analyte can be (a) directly sent to an analysis system or (b) preconcentrated on a suitable stationary phase; the preconcentrator effluent can be recycled, if desired. With a fluorometric flow injection analysis system harnessed to measure ammonia with such a collector, the limit of detection (LOD, S/N = 3) for a sample drawn for 18 min at 200 mL/min was 4.5 parts per trillion by volume, with the linear range extending up to 30 parts per billion.

Transversely illuminated liquid core waveguide based fluorescence detection Fluorometric flow injection determination of aqueous ammonium/ammonia.

The analytical performance of a new type of fluorescence detector, based on a transversely illuminated liquid core waveguide (LCW), has been investigated using the determination of NH(3)/NH(4)(+) as the 1-sulfonatoisoindole. With a very inexpensive combination of a miniature Hg blacklight as an excitation source, a colored plastic sheet as the emission filter, and an integrated blue sensitized photodiode-operational amplifier as the detector (totaling <$100 in hardware cost), we were able to achieve a limit of detection (LOD) of 35 nM (1.6 pmol) NH(3) with a linear dynamic range up to 60 muM NH(3). Details of detector construction and performance are given.

Luminescence detection with a 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.

Measurement of diffusive flux of ammonia from water.

An instrument was developed for the measurement of gaseous ammonia concentration, NH(3(sw,eq)), in equilibrium with surface waters, notably ocean water. The instrument measures the ammonia flux from a flowing water surface under defined conditions and allows the calculation of NH(3(sw,eq)) from the principles of Fickian diffusion. The flux collector resembles a wetted parallel plate denuder previously developed for air sampling. The sample under study runs on one plate of the device; the ammonia released from the sample is collected by a slow flow of a receptor liquid on the other plate. The NH(3) + NH(4)(+) (hereinafter called N(T)) in the effluent receptor liquid is preconcentrated on a silica gel column and subsequently measured by a fluorometric flow injection analysis (FIA) system. With a 6-min cycle (4-min load, 2-min inject), the analytical system can measure down to 0.3 nM N(T) in the receptor liquid. Coupled with the flux collector, it is sufficiently sensitive to measure the ammonia flux from seawater. The instrument design is such that it is little affected by ambient ammonia. In both laboratory (N(T) 0.2-50 µM), and field investigations (N(T) 0.18-1.7 µM) good linearity between the ammonia flux and the N(T) concentration in seawater (spiked, synthetic, natural) was observed, although aged seawater, with depleted N(T) content, behaves in an unusual fashion upon N(T) addition, showing the existence of an "ammonia demand". NH(3(sw,eq)) levels from ocean water measured in the Coconut Island Laboratory, HI, ranged from 6.6 to 33 nmol/m(3) with an average of 17.4 ± 6.9 nmol/m(3), in comparison to 2.8-21 nmol/m(3) (average 10 ± 7 nmol/m(3)) NH(3(sw,eq)) values previously reported for the Central Pacific Ocean (Quinn, P. K.; et al. J. Geophys. Res. 1990, 95, 16405-16416).

A pulse amperometric sensor for the measurement of atmospheric hydrogen peroxide.

Gaseous H(2)O(2) is sampled through a Nafion membrane diffusion scrubber while 1 mM HCl is maintained stationary in the scrubber. After a preselected preconcentration time (typically, 5-10 min), a valve is opened to allow the scrubber liquid to flow by gravity over an electrochemical H(2)O(2) sensor for a brief period. The miniature flow-over sensor consists of a Pt/Rh wire working electrode and a Pt wire counter electrode wound respectively on separate segments of a Nafion solid polymer electrolyte tubing supported on a Ag/AgCl wire reference electrode. A simple electronic interface and a personal computer are used to control and record the electrochemical measurement. The liquid phase detection limit for this sensor is ∼30 nM H(2)O(2) in the anodic oxidation mode. For a 9 min gas sample preconcentration period, the LOD (S/N = 3 criterion) is 0.11 ppbv H(2)O(2)(g). Ambient H(2)O(2) data obtained with this instrument were in excellent agreement with those obtained by an established fluorometric technique in a blind intercomparison.

Hematin as a peroxidase substitute in hydrogen peroxide determinations.

Hematin can substitute for horseradish peroxidase (HRP) as the catalyst in the determination of hydrogen peroxide using phenolic substrates such as p-hydroxyphenylacetate or p-cresol. Although the peroxidatic activity of hematin from bovine blood is not as great as HRP in terms of unit iron content, the activity per unit weight is substantially greater. Hematin is 500 times less expensive than HRP per unit peroxidatic activity. In hematin-catalyzed systems, reaction development and fluorescence measurement can both be conducted optimally in the same ammoniacal buffer. Hydroxyalkyl hydroperoxides are rapidly hydrolyzed to H2O2 at this pH and are also determined. On the other hand, for methyl hydroperoxide, hematin exhibits only approximately 10% of the sensitivity exhibited by HRP. Hematin is significantly more stable in solution than HRP. The use of hematin as catalyst and p-cresol as the substrate leads to a particularly inexpensive and sensitive system, permitting a limit of detection (LOD) of 7 nM H2O2 in a flow-injection configuration.