Detector stability

From a practical point of view it is better to use the maximum volume of sample possible, assuming there is no mass overload. This will allow the detector to be operated at the lowest possible sensitivity and in doing so provide the greatest detector stability and, as a consequence, the highest accuracy. 

The detector shall be a thermal conductivity type with high sensitivity and stability. The selection of detector temperature depends on sample composition, but temperatures of 10-50 °C above analysis colunm temperature have been found to provide adequate sensitivity. Regular checks on detector hnearity are essential in order to detect signs of filament ageing. The detector should be brought up to temperature and the current switched on several hours before analysis runs to ensure stability. Periodic cleaning, once every six months with srritable non corrosive solvent is recormnended to further ensure detector stability. 

Abb rev. Selectivity Flow dependence Destructive or not Detector stability Linear dynamic range Sensitivity Importance to TEQA Used by author... 

If detector stability is the problem it may be possible to study the effect of changing the conditions. 

The flame ionization detector (FID) is the most widely used GC detector. Its operation is simple, sensitivity is of the order of lOpg carbon per second with a linear range of 10, the response is fast, and detector stability is excellent. The carrier gas is mixed with hydrogen, and this mixture is combusted in air at the exit of a flame jet. Ions are formed that are collected at an electrode producing a current that is proportional to the amount of sample compound in the flame. Analytes such as permanent gases, nitrogen oxides, sulfur oxides, carbon oxides, carbon disulfide, water, formic acid, formaldehyde, etc., do not provide a significant FID response. The flows of carrier and combustion gas should be set properly for optimal FID operation. Typical flow rates are 1 1 12... 

The filament life is extended by tnming off the current when the detector is not in use. However, leaving the cnrrent on for extended periods of time (along with the oven temperature) gives the highest detector stability in the morning. 

Measurement precision incorporates a combination of sources of instability. In typical instrumentation, the magnitude of the precision is dependent on several factors including the stability of the detector, stability of the control electronics, the thermal stability of the operating environment, the stability of the ICP (including the regulation of gas flow rates), and the stability of the sample introduction system. By far the most important source... 

HPLC with dielectric constant detection [3] was considered by ASTM, but problems with detector stability prevented standardization. 

Detector—Either a flame ionization or a thermal conductivity detector may be used. The detector must have sufficient sensitivity to detect 1.0 % dodecane with a peak height of at least 10 % of full scale on the recorder under conditions prescribed in this method and without loss of resolution as defined in 8.3.1. When operating at this sensitivity level, detector stability must be such that a baseline drift of not more than 1 % of full scale per hour is obtained. The detector must be capable of operating continuously at a temperature equivalent to the maximum column temperature employed. Connection of the column to the detector must be such that no temperature below the column temperature exists. 

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