Detectors range

Detectors range from the universal, but less sensitive, to the very sensitive but limited to a particular class of compounds. The thermal conductivity detector (TCD) is the least sensitive but responds to all classes of compounds. Another common detector is the flame ionization detector (FID), which is very sensitive but can only detect organic compounds. Another common and very sensitive detector is called electron capture. This detector is particularly sensitive to halogenated compounds, which can be particularly important when analyzing pollutants such as dichlorodiphenyltrichloroethane (DDT) and polychlorobiphenyl (PCB) compounds. Chapter 13 provides more specific information about chromatographic methods applied to soil analysis. 

This is still the most serious problem at present, as the time constant of ost detectors ranges from 0.3 to 3 sec including the time constant of the detector electronics to which the analyst has little access. Only a few commercial detectors have a time constant less than 0.3 sec whereas a few detectors, which are used mostly to build inexpensive homemade liquid chromatographs, have a time constant well in excess of 3 sec. 

Figure 7. Detectability comparisons among 4.6-mm, 2.0-mm, and 1.0-mm i.d. columns using 20- and 5-pL flow cells at a full-scale detector range of 0.05 absorbance units. The same seven-component sample was used for all injections but was diluted by one-half for chromatograms A, B, and C. (Reproduced with permission from reference 25. Copyright 1984...

One must remember that there are important spectral bands that do not fit in the detector range... 

Gas Chromatograph Conditions. The GC was a VARIAN 6000 with a flame ionization detector. The injection port was heated at 200°, the detector at 250°. The detector range was 10-12 AFS, attenuation 64 except as noted. The column was 25m X 0.32mm fused silica DB5 with a 1.0 micron film thickness (bonded SE54, J W Scientific, Rancho Cordova, CA). The carrier gas was hydrogen at 47 cm/s. The column was temperature programmed from an initial temperature of 35 C, held for 4 min., to a final temperature of 200 C at 4°C/min. 

Current IPC detectors are on-stream monitors. HPLC detectors range from (1) non selective or universal (bulk property detectors such as the refractive index (RI) detector), characterized by limited sensitivity, (2) selective (discriminating solute property detectors such as UV-Vis detectors) to (3) specific (specific solute property detectors such as fluorescence detectors). Traditional detection techniques are based on analyte architecture that gives rise to high absorbance, fluorescence, or electrochemical activity. Mass spectrometry (MS) and evaporative light scattering detectors (ELSDs), can be considered universal types in their own right... 

The output from most detectors ranges from 0 to 10 mV whereas the input to most A/D converters is considerably greater e.g. 0 to 1.0 V. Thus, the instantaneous measurement of 2 mV assumed from the detector must be scaled up to 0.2 volt which is carried out by a simple linear scaling amplifier. This is achieved by a simple linear amplifier with an appropriate gain. 

The detector range - this must be set to ensure that the largest peaks do not go off the top of the chart. Adjustment may be based on the expected quantity of analyte, or by a trial-and-error process. Use the maximum sensitivity that gives intact peaks. If peaks are still too large on the minimum sensitivity, you may need to reduce the amount of sample used, or prepare and analyse a diluted sample. 

If the solute is injected in the plant without the column, the dead time of the plant can be estimated at the inflection point of the breakthrough curve. The resulting plateau also allows one to verify that the signal resulting from the feed concentration does not exceed the detector range. 

Wrong detector range, signal output (in mV) is not taken over correctly by the control unit ... 

The monochromated neutrons are scattered by the sample and recorded by He detectors ( 3.3.1.1). The azimuthal low angle bank consists of eight radial arms of detectors ranging from 3° to 12°. The rest are arranged in a vertical scattering plane and cover from 12° to 135°. 

The mass detection limits for the above detectors range from 10 to 10 moles based on a lOnl injection. 

The entire synthesis from collection of irradiated target water to delivery of sterile N02" solution for biological experimentation took less than 10 min and resulted in useful yields of about 120 mCi in a small volume. The specific activity of the concentrated N02" measured by HPLC with serial radioactivity and UV-absorbance detectors ranged from 2500 to 5000 Ci/mmol without any special precautions to maximize it. 

Figure 4. Characteristic separation pattern of (1) phenol, (2) p-nitrophenol, (3) m-nitrophenol, (4) o-nitrophenol column. Cyclobond II (gamma-CD) (25 cm x 4.6 mm i.d.) mobile phase, 1/4 methanol-water flow rate, 1.0 mL/min detector range, 0.04 a.u.f.s.

Figure 9. Separation of (A) dansyl-DL-leucine and (B) dansyl-DL-norleucine enantiomers column, Cyclobond I ( -CD) (25 cm X 4.6 nm i.d.) mobile phase, 45/55 methanol-water with 0.05 M acetate buffer (pH 4.6 flow rate 1.0 mL/min detector range, 0.08 a.u.f.s.

Fig. 1. Chromatograms of the LC assay for GABA by isocratic elution and electrochemical detection. (A) Blank injection. (B) 0.050 xM GABA standard, (C) Dialysate obtained from rat striatum. Peak 1 (B,C) is GABA. The detector range was 2 nA, full scale. The large, late-eluting peak is because of the denvatizing reagent. The injection volume in each case was 10 pL.

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