A photoionization detector

Summary A photoionization detector (P.I.D.) with separated discharge and detection compartments, enabling detection of eluted substances under normal pressure, has been developed. A detection mechanism was formulated for these conditions and relations for the P.I.D. signal were derived. The detector can measure all substances with ionization potentials less than 11.4 eV. A detection limit of 10^–14 mol/s for benzene, with a linear dynamic range of 10⁴, was attained.     

Combination of amperometric detector & UV detector for capillary electrophoresis

A new electrochemical cell assembly with the combination of UV and amperometric detector (AD) based on their complementarity was described. A Nafion tubing junction was used to decouple the high voltage from the separation capillary in the rear of on-column UV detector. In this mode, the electroactive and inert compounds could be detected by UV and AD at the same time. Aromatic amines were determined with the UV and the end-column AD detection to evaluate the performances of the cell assembly. Such an improved electrochemical detector could match the capillary with different diameters. By simple adjustment of the screws, the positioning of the working electrode and the detection capillary was easily gained without microscope. It is also very easy to assemble and disassemble the working electrode when needed.     

Pulsed discharge emission detector: an element-selective detector for gas chromatography

An element-specific pulsed discharge emission detector (PDED) has been coupled directly with a vacuum UV monochromator so that vacuum UV atomic emissions from Cl, Br, I and S can be observed. The observed sensitivities for the elements are in the range of mid to high pg/s, but can be lowered by direct absorption of the radiation using a vacuum UV radiation photomultiplier tube. A helium pulsed discharge photoionization detector (He-PDPID) was run simultaneously in parallel with the PDED. The chromatograms recorded with the two detectors had similar peak shapes, suggesting that there is no peak tailing in the PDED. The ratio of the detector responses PDED/He-PDPID can be used for qualitative identification of the Cl-, Br-, I- or S-containing compounds.     

A radio-frequency moving-boundary detector

A new type of moving-boundary detector is described in which boundaries are detected at platinum ring electrodes situated on the outside of a moving-boundary tube. These rings are part of a bridge circuit operating at radio frequencies. Impedance changes in the region of the rings are converted by a phase-sensitive detector to a voltage output which, when operating in the differential mode, takes the form of a sharp peak as a boundary passes through the rings. Boundary displacements of less then 20 m can be detected, and a precision of better than 0.02% is obtained in the measurement of boundary velocities in typical transference measurements. The fact that only three electrical leads are required in the detecting circuit gives this detector a decided advantage for high-pressure work. Equally important is the elimination of electrode polarization problems associated with other types of electrical detectors.Taken, in part, from a Ph.D. thesis, Carnegie-Mellon University, 1971.     

Directional characteristics of a detector

Some rarely used detector characteristics which characterize the directional properties of a detector are introduced in the paper. Attention is particularly paid to relative and absolute directiveities of a detector and their connection to other important detector characteristics.     

A sensitive photo-acoustic mercury detector

Apparatus for the photo-acoustic determination of mercury vapour is described. A detection limit of better than 0.2 ng of mercury and a linear working range up to 150 ng can be realised in the presence of substantial levels of ambient noise. Nitrogen carrier gas effectively quenches the mercury fluorescence in a tubular cell attached alongside a modulated germicidal mercury discharge lamp. The released heat results in pressure fluctuations, in a partially closed cell, which are detected by using a microphone and simple electronics. A narrow-band amplifier and a lock-in detector allow signals to be recorded. Very low cost and simplicity are important features. A gold foil collection system is used to increase selectivity. The apparatus is suitable for laboratory and field applications.     

Pulsed discharge helium ionization detector

Summary A pulsed discharge helium ionization detector (PDHID) (patent pending) for gas chromatography has been developed. This detector uses a non-radioactive pulsed high voltage discharge source for generation of electrons and pulsed collection of these electrons. We have evaluated this detector for the analysis of a wide range of chemical compounds. In this paper the analytes are passed through the discharge since the permanent gases are difficult to ionize. The initial results for the permanent gases indicate that the PDHID can be used as a universal detector of contaminant traces at detection levels on the order of 1–20 pg. The response in this mode of operation is linear over four orders of magnitude.Dedicated to Professor Leslie S. Ettre on the occasion of his 70th birthday.     

Mini-electrochemical detector for microchip electrophoresis

This paper presents the development of a mini-electrochemical detector for microchip electrophoresis. The small size (3.6 x 5.0 cm2, W x L) of the detector is compatible with the dimension of the microchip. The use of universal serial bus (USB) ports facilitates installation and use of the detector, miniaturizes the detector, and makes it ideal for lab-on-a-chip applications. A fixed 10 M ohm feedback resistance was chosen to convert current of the working electrode to voltage with second gain of 1, 2, 4, 8, 16, 32, 64 and 128 for small signal detection instead of adopting selectable feedback resistance. Special attention has been paid to the power support circuitry and printed circuit board (PCB) design in order to obtain good performance in such a miniature size. The working electrode potential could be varied over a range of +/-2.5 V with a resolution of 0.01 mV. The detection current ranges from -0.3 x 10(-7) A to 2.5 x 10(-7) A and the noise is lower than 1 pA. The analytical performance of the new system was demonstrated by the detection of epinephrine using an integrated PDMS/glass microchip with detection limit of 2.1 microM (S/N = 3).     

A differential photoacoustic mercury detector

A sensitive instrument for the detection of mercury is described. The difference in mercury concentration between two identical flow-through silica cells is measured by means of the photoacoustic effect. The cells are illuminated laterally by a 20-W germicidal mercury discharge lamp. A microphone in each cell detects pressure changes produced when mercury vapour absorbs resonance radiation from the lamp and releases the energy to nitrogen flowing in the partially closed cells, as heat derived from quenched fluorescence. A differential microphone connection discriminates against ambient noise and contributes to a detection limit of less than 0.02 ng of mercury with a linear response up to 70 ng. Variables such as gold collector design and heating rate, inlet filters and traps, changes in carrier gas flow, carrier gas composition, lamp operating current and modulation frequency which all affect instrument performance are discussed. Photochemical loss of mercury can be avoided if dry nitrogen, carbon dioxide, or air is used as the carrier gas provided contamination by avid vapours and water is prevented by a calcium oxide trap at the inlet.     

Broad band infrared thermal detector

A broad band thermal detector based on pyroelectric materials with temperature-dependent electrical conductivity is described. This detector combines the properties, notably the frequency response, of a thermistor bolometer and a pyroelectric detector. Suitable choice of the detector parameters provides a uniform frequency response above and below the thermal relaxation frequency.     

The plasma emission detector (PED) as an element-selective detector for supercritical fluid chromatography

The plasma emission detector (PED) has been shown to be an element specific detector for supercritical fluid chromatography (SFC). The commonly used eluents (CO₂ and N₂O) tend to disturb the He plasma; two different discharge tubes were tested in an attempt to overcome this problem. Promising results were obtained with a concentric dual flow torch design for the element specific detection of Cl, H, and C containing analytes using N₂O as SFC mobile phase.     

On-line coupling of packed column supercritical fluid chromatography with multiple wavelength detector and radioactivity flow-through detector

Summary The coupling of packed column supercritical fluid chromatography with a commercial Radioactivity Flow-Through Detector for the sensitive and highly selective detection of radioactive nuclides (e.g.¹⁴C is described. The radioactivity flow-through detector showed no baseline shift when pressure or modifier (methanol) gradients were applied. The detector cell was pressure resistant even at high flow rates, over a period of approx. 100 hours and showed no leakage problems even at 40°C, 340 bar, 4.0 ml/min and a modifier content up to 20%.     

Dual electrode electrochemical detector for HPLC

Summary A method for determination of phenolic compounds in distilled alcoholic beverages has been developed. After separation by reversed phase chromatography these compounds are detected coulometrically in a dual electrode detector. The hydrodynamic electrochemical behaviour of the substances in oxidative and reductive mode was investigated. For quantitative determination phenolic compounds are oxidized at the first working electrode (+0.65 V); then the oxidation products are reduced at the second working electrode (0.0 V). The current due to these processes is recorded. By the high selectivity of the detection mode matrix interferences can be eliminated in several alcoholic beverages. In this way qualitative information is improved. The detection limits of phenolic acids and aldehydes are between 0.01 and 1 ng (S/N=3).

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Dualelektroden-Detektor für die HPLCBestimmung von phenolischen Verbindungen in Spirituosen

     

CE detector based on light-emitting diodes

CE detectors based on light-emitting diodes (LEDs) as light sources are receiving considerable attention due to their exceptionally high stability, high intensity, low cost, and a variety of wavelengths in the UV and visible spectrum. This article is a comprehensive review on CE methods using LED-based detectors with absorbance and fluorescence detection, and several applications on the determination of riboflavin, bacteria, drug, and amino acids in biological samples are described.