Physical measurements, chromatographic detector

Bulk property detectors function by measuring some bulk physical property of the mobile phase, e.g., thermal conductivity or refractive index. As a bulk property is being measured, the detector responses are very susceptible to changes in the mobile phase composition or temperature these devices cannot be used for gradient elution in LC. They are also very sensitive to the operating conditions of the chromatograph (pressure, flow-rate) [31]. Detectors such as TCD, while approaching universality in detection, suffer from limited sensitivity and inability to characterise eluate species. 

Before the development of efficient chromatographic separation techniques and selective and sensitive detectors, analytical methods for the determination of specific analytes in environmental samples were very limited. Those methods depended on highly selective chemical reactions that are relatively rare and difficult to discover, or on very selective physical measurements such as atomic absorption or emission techniques for elemental analytes. Therefore only a relatively few analytical methods for the most common and amenable organic and inorganic compounds or... 

The objective of the ECD and NIMS experiments is to measure the molar response of different compounds as a function of temperature. From these data the fundamental kinetic and thermodynamic properties of the reaction of thermal electrons with molecules and negative ions can be determined. The measurement is carried out in the same manner as the calibration of any detector. Known amounts of a compound are injected into the chromatograph and purified on a column, they then enter the detector. The response of the detector is normalized to the number of moles injected. When obtaining physical parameters, the detector temperature is changed and the procedure repeated. Since the molar response can vary by three to four orders of magnitude, the concentrations of the test molecule and the conditions in the detector at different temperatures must be taken into account. 

Another important physical property of liquids is the refractive index. Since the refractive index is a constant for a particular liquid at a given temperature, it can be used to help identify substances, check for purity, and measure concentrations. One type of detector found in some liquid chromatograph instruments (Chapter 13) uses refractive index. 

Detectors which measure a physical property of the eluent such as the refractive index, rate of radioactive decay or the speed of sound waves can be used with chromatographic systems. However, in most instances the detection limits are relatively high, which reduces the application range to preparative and semi-preparative separations. 

Once the chromatographic separation on the column has been conducted, the composition of the eluent at the column end must be determined using a detector. In all HPLC detectors, the eluent flows through a measuring cell where the change of a physical or chemical property with elution time is detected. The most important parameter of the detector is sensitivity, which is influenced by the noise and baseline drift, the absolute detection limit of the detector, the linearity, the detector volume (band broadening), and the effects of pressure, temperature and flow (pulsation, gas bubbles). 

For the internal standard method, a substance is added at the earliest possible point in the analytical scheme to compensate for sample losses during extraction, cleanup, and final chromatographic analysis. The internal standard must be completely resolved from all other peaks in the chromatogram, having chemical and physical properties as similar as possible to those of the analyte of interest, so that the detector response is similar to the solute to be quantified. However, it is sometimes difficult to find the proper internal standard, as this must be stable over the time of the measurement period, absent from real samples, not reactive with the analytes, and eluted within a reasonable retention time. Analogs, homologs, and isomers are usually preferred. In the presence of analytes with different chemical or physical properties, two or more internal standards representing these analytes should be used. 

Quantitation of N-acetylnorheroin in heroin hydrochloride samples was by gas chromatography on a Perkin Elmer 900 Gas Chromatograph (column 6 ft. x 1/4 in.) temperature initial 180°C, final 270°C, program rate 4°C/min. Flame Ionization Detector, interfaced with Spectra-Physics Autolab System I Computing Integrator, measured peak areas in y volts Carrier gas nitrogen 60 ml/min. Authentic heroin hydrochloride samples were prepared by dissolving ca. 60 mg/ml in methanol. Standard N-acetylnorheroin (prepared by procedure described below) was 5.0 mg/ml. Sample... 

Physical detectors, such as those used for gas chromatographic detection are sometimes fitted to detect evolved gases, but they do not identify them. Moisture can be measured quantitatively using a capacitance moisture meter or by absorption and electrolytic determination. 

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