Detection limit and background noise

If the eluent has a background conductance, then temperature control is important to reduce detector noise and improve detection limits. Good quality conductivity detectors have temperature control, temperature compensation, or both. An oven can keep the temperature of the fluid constant by the time it reaches the conductivity ceU, and this also helps to improve detector noise and detection limits. Some column ovens are designed to preheat the eluent to a pre-set temperature as it enters the column. Use of a column oven permits 1C separations to be run at temperatures ranging from sub-ambient to around 85 °C. 

The concentration of analytes that can be measured in various materials has been decreasing over the years as sensitivity and detection limits of analytical techniques have improved. The method detection limit (MDL) is the order of magnitude of the smallest quantity or concentration of substance which can be detected in principle the limit of detection (LOD), on the contrary, is a precisely calculable statistical value for a particular, defined analytical procedure. The instrument detection limit (IDL) is the smallest signal above background noise that an instrument can detect reliably. It is expressed either as an absolute limit (in units of mass, eg, ng), or as a relative limit (in terms of concentration, eg, g mL 1). 

The macrolide antibiotics clarithromycin (and its metabolite 14-hydroxyclari-thiomycin) and azithromycin (roxithromycin internal standard) were isolated from plasma and quantitated on a cyanopropyl column (electrochemical detection at +0.85 V). Azithromycin was eluted in 12 min with a 500/600/50 water (50 mM phosphate at pH 6.8)/acetonitrile/methanol mobile phase [1345]. The clarithiomy-cins were separated and eluted m 20 min with a 450/300/50 water (50 mM phosphate at pH 7.5)/acetonitrile/methanol mobile phase. The authors noted that phosphate buffers were chosen over ammonium buffers because the background noise was considerably higher with the latter. Linear ranges in the 0.025-5 pg/mL range and detection limits of 0.5-1.5 ng injected (S/N = 3) were reported (analyte dependent). 

A detector s limit of detection (LOD) is the lowest concentration level that it can identify with a certain degree of confidence. There are many definitions of LOD, such as the concentration at which the response signal generated is three times the instrument noise level. Here LOD is referred to as the minimum detection level (MDL) of concentration that will consistently cause the detector to alarm. It is affected by background noise and blank signals. The LOD of a detector may vary widely for different chemicals. Environmental and operational conditions could drastically affect the LOD. Manufacturers normally provide LOD information obtained under optimum conditions. 

The detection limits in the table correspond generally to the concentration of an element required to give a net signal equal to three times the standard deviation of the noise (background) in accordance with lUPAC recommendations. Detection limits can be confusing when steady-state techniques such as flame atomic emission or absorption, and plasma atomic emission or fluorescence, which... 

Detection limit. The detection limit of an instrument should be differentiated from its sensitivity. The detection limit reflects the smallest flow of sample or the lowest partial pressure that gives a signal that can be distinguished from the background noise. One must specify the experimental conditions used and give the value of signal-to-noise ratio corresponding to the detection limit. 

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