Chromatographic deflection

The next FCC gas used was steam, with corresponding results shown in Table VIII. In a comparison of absolute concentrations by measuring chromatographic deflection amplitudes, the total product yield of hydrocarbon increased with increasing flow rate of steam. As with the water-gas tests, short time intervals with steam produced appreciable methane under conditions which would have yielded only a small amount of acetylene or no hydrocarbons if the same amount of contained hydrogen were fed as pure hydrogen in equivalent rates to the FCC arc. 

In general, the signal from a gas chromatograph is recorded continuously as a function of time by means of a potentiometric device. Most frequently, a recorder of 1-10 mV full-scale deflection ( 10 inches) and having a response time 1 second or less is quite adequate. 

RI detectors measure this deflection, and are sensitive to all analytes that have a different R1 than the mobile phase. There are two major limitations First, Rl detectors are very sensitive to changes in the temperature, pressure, and flow rate of the mobile phase, and so these measurement conditions must be kept stable in order to obtain low background levels. Second, Rl detectors are incompatible with chromatographic separations using gradient elution. Furthermore, because Rl detectors are nonselective, they must be used in conjunction with other detection methods if specificity is required. Nevertheless, they have found wide application in isocratic chromatographic analysis for analytes that do not have absorptive, fluorescent, or ionic properties, such as polymers and carbohydrates. 

Hippe et al. discussed numerical operations for computer processing of (gas) chromatographic data. Apart from a baseline correction method, a method of reco -tion of peaks is described. The relationship between the convexity of an isolated peak and the monotonic nature of its first derivative is used to find the most probable deflection points. The munber of maxima and shoulders are used for a decision if the segment of the chromatogram contains an isolated peak or an unresolved peak complex. The number of shouders and maxima determine the total number of component peaks. 

Potentiometric recorder. A continuously recording device whose deflection is proportional to the voltage output of the chromatographic detector. 

ADCs digitize at different rales. What conversion rale is required if a chromatographic peak is 10 be sampled and digitized 20 limes between the first positive deflection form the baseline until the peak returns lo the baseline The total... 

Because of refractive-index effects, an unretained solvent used to dissolve the sample— if different from the chromatographic mobile phase—often deflects the base-line when passing through an ultraviolet detector cell. This indicates the void volume or the void time. Consider the chromatogram in Figure 21.19. (a) Determine the capacity factors for each nitroaniline isomer, (b) Determine the selectivity factor for the m- and p-substituted isomers relative to the o-nitro-aniline. 

Fig. 9. Norepinephrine and dopamine release from brain slices (slices of rat nucleus accumbens). (A) Dopamine standard (10 pmol injected onto column—volume 100 pi) (B) norepinephrine standard (10 pmol injected onto column—volume 100 pi) (C) supernatant obtained after incubation of one sliced nucleus accumbens in Krebs buffer containing pargyline (10 M) at 37°C for 10 min, followed by centrifugation (Bennett et ai, 1981fl). The supernatant is deproteinized by addition of 20 pi 0.1 M perchloric acid per 2 ml supernatant and then centrifuged, and 100 pi is injected onto the column. Note the small norepinephrine and large dopamine peaks (D) same as C, except the nucleus accumbens (the other accumbens from the same animal as in G) was incubated in the presence of d-amphetamine (10 M). Note the increased norepinephrine and dopamine peaks. Chromatographic conditions column, cation-exchange Nucleosil (10 p) 30 cm X 2.1 mm mobile phase, 0.05 M acetate/citrate, pH 4.8 flow rate, 1.2 ml/min electrode potential, +0.65 V sensitivity, 2 nA/V full scale deflection volume injected, 100 pi.

Fig. 10. Measurement of norepinephrine and epinephrine in human plasma. Explanation of traces from left to right. A (1) Norepinephrine standard (10 pmol injected) (2) epinephrine standard (10 pmol injected) (3) dopamine standard (10 pmol injected). B (1) Plasma (1.1 ml) collected during a normoglycemic baseline period. Dopamine (10 pmol) added as internal standard prior to aluminum oxide adsorption. Sensitivity changes from 100 nA to 1 nA full scale deflection immediately after elution of the solvent front. Note the small norepinephrine (0.57 nmol/liter) and epinephrine (0.74 nmol/liter) peaks (2) plasma (1.1 ml) from the same subject, insulin-induced hypoglycemia. Note the marked increase in the epinephrine (5.16 nmol/liter) peak and the small rise in norepinephrine (0.75 nmol/liter). Dopamine (10 pmol) added as internal standard. Chromatographic conditions column, Nucleosil (10 [x), 30 cm X 2.1 mm mobile phase, acetate/citrate, 0.1 M, pH 5.2 flow rate, 1.2 ml/min electrode potential, +0.65 V (carbon paste) volume injected, 100 xl plasma extraction, alumina adsorption (Fig. 8).

Fig. 11. 5-HT spinal cord assay. Sample recordings of (A) 5-HT standards (10 pmol injected onto the column) (B) rat lumbar spinal cord saline injected tissue weights 61.0 and 61.3 mg (C) rat lumbar spinal cord tetrabenazine (75 mg/kg) administered 6 hr before sacrifice tissue weights 60.0 and 56.4 mg (D) rat lumbar spinal cord tetrabenazine (75 mg/kg) administered 24 hr before sacrifice tissue weights 70.0 and 68.3 mg. Chromatographic conditions column, cation exchange (Vydac 40 x), 50 cm x 2.1 mm mobile phase, acetate/ citrate, 0.2 M, pH 4.8 flow rate, 0.9 ml/min electrode potential, +0.60 V sensitivity, 2 nA/ V full scale deflection volume injected onto the column, 20 xl tissue extraction, acidified butanol (Ponzio and Jonsson, 1979). Note the rapid depletion of cord 5-HT caused by tetrabenazine followed by partial recovery within 24 hr. Full recovery is seen after 48 hr (Marsden, Bennett, Emson, and Gilbert, in preparation).

Chromatograph the sample and standard solutions using the following operating conditions column length, 6 ft. column temperature, 80° stationary phase, 30 per cent of glycerol on 100/120-mesh Celite hydrogen flow rate, 45 ml per minute flash heater temperature, 125° detector, thermal conductivity cell detector current, 180 mA sample size, 30 x recorder, Honeywell-Brown with 1 mV full-scale deflection and chart speed of 12 inches per hour. 

Chromatograph-—Any chromatograph having either a thermal conductivity or flame ionization detector may be used. The detector system shall have sufflcient sensitivity to obtain a deflection of at least 2 mm at a signal-to-noise ratio of at least 5 1 for 0.01 weight % of butadiene dimer and styrene under the operating conditions prescribed in this test method. 

Methane and Ethane—Typical operating conditions for the analyses for methane and ethane are summarized in Table 3. Slowly flush the sample to be analyzed through the gas sample valve on the chromatograph until all extraneous vapor has been purged from the sample loop. Turn the gas valve to introduce the sample into e carrier gas stream. Record the deflection of each component peak at the mini-... 

Recorder—A recording potentiometer with a full-scale deflection of 10 mV or less is suitable for obtaining the chromatographic data. Full-scale response time should be 2 s or less, and with sufficient sensitivity to meet the requirements of 5.1. 

Chromatograph— Any chromatographic instrument that has a backflush system and thermal conductivity detector, and that can be operated at the conditions given in Table 1, can be employed. Two backflush systems are shown. Hgure 1 is a pressure system and Fig. 2 is a switching valve system. Either one can be used. The detector-recorder combination must produce a 4-mm deflection for a 2-pL sample containing 0.1 volume % MEK when operated at maximum sensitivity. 

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