Huamin Cai

Pulsed discharge helium ionization detector universal detector for inorganic and organic compounds at the low picogram level

Abstract The pulsed discharge helium ionization detector is a universal ionizalion detector with sensitivity in the low picogram range. The response is approximately constant for saturated hydrocarbons on a per gram basis. Unsaturated and aromatic hydrocarbons have lower sensitivities, by about 10–20%. Heterocyclic substituents such as oxygen, chlorine and bromine tend to lower the response on a per gram basis. The dependence of the response on various parameters such as pulse interval and power, voltage, flow-rate and detector volume has been investigated. The response is linearly related to concentration over five orders of magnitude. The detector volume can be made small enough for high-speed microbore chromatography.

Characterization of the Pulsed Discharge Electron Capture Detector

A new version of the pulsed discharge electron capture detector (PDECD) has been developed and characterized. Changes to the old version include a slightly altered detector geometry, replacement of the polymer insulation with sapphire and quartz, and the use of methane dopant gas instead of nitrogen or hydrogen. Various operating parameters have been investigated and optimized, including discharge current, dopant gas, bias voltage, and sample introduction position. The resulting detector is more inert and more sensitive (a limit of 36 fg for lindane) and capable of operation at temperatures as high as 400 °C. By running 23 halocarbon compounds on the improved PDECD and on a (63)Ni-ECD using the same GC system, we find that the PDECD is superior to (63)Ni-ECD in terms of sensitivity, linearity, and response time. We attribute the enhanced sensitivity to a lower positive ion concentration, which in turn lowers the electron-positive ion rate of recombination. Pesticides (including some real-world samples) have also been analyzed on the PDECD. The results demonstrate that the PDECD can replace the radioactive ECD typically used in these analyses.

Non-radioactive electron-capture detector

Abstract This paper is a review of the research that has been performed on the development and applications of a non-radioactive electron-capture detector (ECD). The ionization in the ECD, normally supplied by a radioactive foil, is supplied by the electromagnetic radiation emanating from a high voltage pulsed discharge in pure helium. This emission consists of a broad band in the vicinity of 13.5–17.5 eV. This is a well know emission arising from a transition in an excited He2 molecule to a dissociative ground state. The importance of having a high energy process to initiate the ionization in an ECD is emphasized and is the principal reason why the non-radioactive ECD performs in a similar manner to the radioactive ECD. The principal advantage of the non-radioactive ECD is the cleanliness of the detector since the gas chromatography (GC) column effluent does not come in contact with the ionization source. Most radioactive ECDs generally pass the GC column effluent directly over the radioactive foil where the electrons are produced from the emission. Furthermore, the internal volume of the non-radioactive ECD can be made smaller than that of radioactive ECD using 63Ni foils. For this reason, lower make-up gas flow-rates can be used and the sensitivity of the non-radioactive ECD is slightly greater than that for radioactive ECD. The temperature dependence of the non-radioactive ECD closely parallels that of the radioactive detector. This similar behavior is the most definitive criterion that the detectors operate in a similar manner, displaying the different types of electron attachment mechanisms. Applications to the analysis of pesticides, polychlorinated biphenyls and metal complexes have been demonstrated along with their temperature dependence.

Pulsed discharge helium ionization detector with multiple combined bias/collecting electrodes for gas chromatography.

A pulsed discharge ionization detector (PDHID) with multiple combined bias/collecting electrodes (MC-PDHID) has been developed. Unlike most ionization detector designs with only one collecting electrode, the MC-PDHID builds multiple electrodes inside the detector cell. Each electrode serves as both a bias and a collecting electrode, thus gathering more information from the detector cell and improving PDHIP performance. The advantages of the MC-PDHID are: (1) sensitivity is increased by a factor of 2-3 times as compared with a single collecting electrode PDHID; (2) peak symmetry is improved, especially for narrow peaks; (3) it is possible to use a lower helium flow rate without compromising peak tailing; (4) linear dynamic range is increased by an order of magnitude through the calibration of electron and ion response factors; (5) certain groups of compounds can be identified. For example, if a trace amount of water is used as a dopant, the detector can identify alcohols and compounds with a hydrogen bond, since these compounds interact with the water coated on the wall in the detector cell which makes them stay in the detector cell longer than other compounds. In this research, the detector is characterized with different detector temperatures, flow rates, bias electrical potential arrangements, and bias potential polarities.

A comprehensive two-dimensional gas chromatography valve modulation method using hold-release primary column flow for long secondary separation time with 100% transfer

A valve modulator with a hold-release primary column flow method has been proposed. This method uses a valve plumbed so that the head and tail of the primary column are connected during the secondary separation, to reverse, slow down, and finally stop the primary column flow. This keeps the sample in the primary column longer, gaining more time for secondary separation. Applying this method, a 60-second secondary retention time with 100% transfer, 133 ms secondary peak width at base, and no primary peak profile loss has been achieved. A standard sample with 19 compounds in 3 groups has been tested. The relative standard deviations for the retention time of this standard mixture are <0.26% for primary dimension and <0.94% for secondary dimension. A gasoline sample analysis with 5, 15, and 30-meter-long secondary columns has been demonstrated. With the secondary column length increase, the secondary separation has greatly improved even though the total run time increases to 45, 110, and 120 min and the toluene primary peak width at base increases to 0.51, 1.81, and 1.85 min respectively. A diesel samplerun by this method has also been demonstrated.