Linear range of detectors

Peaks are outside detector s hnear range. —> Dilute or enrich sample to reach linear range of detector. 

Linearity Range Linearity Linear range of detector ... 

We take intensity I to mean x-ray quanta per second measured by a detector. An instantaneous detector measures / directly that is, the detector gives a reading always proportional to intensity over the linear range of the detector. The reading could appear on an ammeter after suitable amplification. 

Transverse moving head of four-quadrant position detectors (Fig. 2), precise measuring lateral displacement which corresponds to lateral voltage F/ when it varies at the linear range of 10 V, thus we can compute the rotated angle a of the reflection ray ... 

A second source of error may be in the detector. Detector linearity is an idealization useful over a certain concentration range. While UV detectors are usually linear from a few milliabsorbance units (MAU) to 1 or 2 absorbance units (AU), permitting quantitation in the parts per thousand level, many detectors are linear over only one or two decades of operation. One approach in extending the effective linear range of a detector is high-low injection.58 In this approach, an accurate dilution of a stock sample solution is prepared. The area of the major peak is estimated with the dilution, and the area of the minor peak is estimated with the concentrated stock. This method, of course, relies on linear recovery from the column. Another detector-related source of error that is a particular source of frustration in communicating... 

Fluorescence detectors can be made much more sensitive than uv absorbance detectors for favourable solutes (such as anthracene) the noise equivalent concentration can be as low as 10 12 g cm-3. Because both the excitation wavelength and the detected wavelength can be varied, the detector can be made highly selective, which can be very useful in trace analysis. The response of the detector is linear provided that no more than about 10% of the incident radiation is absorbed by the sample. This results in a linear range of 103-104. 

Refractive index detectors are not as sensitive as uv absorbance detectors. The best noise levels obtainable are about 1CT7 riu (refractive index units), which corresponds to a noise equivalent concentration of about 10-6 g cmT3 for most solutes. The linear range of most ri detectors is about 104. If you want to operate them at their highest sensitivity you have to have very good control of the temperature of the instrument and of the composition of the mobile phase. Because of their sensitivity to mobile phase composition it is very difficult to do gradient elution work, and they are generally held to be unsuitable for this purpose. 

Some commercially available detectors have a number of detection modes built into a single unit. Fig. 2.4o is a diagram of the detector used in the Perkin Elmer 3D system, which combines uv absorption, fluorescence and conductivity detection. The uv function is a fixed wavelength (254 nm) detector, and the fluorescence function can monitor emission above 280 nm, based on excitation at 254 nm. The metal inlet and outlet tubes act as the electrodes in the conductance cell. The detection modes can be operated independently or simultaneously, using a multichannel recorder. In the conductivity mode, using NaCl, a linear range of 103 and a noise equivalent concentration of 5 x 10 8 g cm-3 have been obtained. 

Due to the rather narrow linear range of the MS detector and owing to the strongly varying concentrations of the target analytes in environmental samples, dilutions have to be prepared in most instances. 

Linearity is the measurement of the linear range of detectability that obeys Beer s Law and is dependent on the compound analyzed and the detector used. In short there is a linear relationship between absorbance and concentration. To be within the linear range of the method you should be working within the absorbances and concentrations that form the linear part of the curve. 

In contrast with UV methods where the linear range is approximately 1-2 orders of magnitude at best, the HPLC method with UV detection typically has a linear range of 3-4 orders of magnitude due to the narrow path length of the detector flow cell. For example, a simple... 

The absorptivity and concentration of the probe should ideally be as high as possible, though they must be in the linear range of the detector. These parameters also have an impact on the noise. 

The linear range of such detectors is between 0.0001 and 2 absorbance units and samples have to be diluted sufficiently to fall within the range. Although the exact concentration of a sample passing through the flow cell is not known, a suitable concentration can be approximated as shown in Calculation example 12.3. 

Tables II-VI further confirmed the accuracy of the in-line GC as a monitor. However, it should be pointed out that for atmospheres of very high concentrations i.e., at levels where the amount of contaminants in the sampling loop exceeds the linear range of the detector, the in-line GC would be of little use.

Table 24-5 Detection limits and linear ranges of gas chromatography detectors... 

The linear range of a detector is defined as the range of concentration over which the sensitivity is constant to within a defined tolerance. S is obtained by performing a linear regression analysis of the data. A simple way to determine the linear portion is to draw a second line having a slope equal to 95% of that of the best-fit line. It intersects the response curve and determines the Unear range (Fig. 17). 

Fig. 17 Plot of recorder deflection versus concentration to determine the linear range of a detector...

Cassidy and Frei [23] designed a microflow cell for the Turner Assoc. Model III fluorimeter for use with HPLC. Nanogram quantities of fluorescent materials could be detected. The volume of the flow cell was only 7.5 jul. The detector was unaffected by the flow-rate or composition of the solvent. This gives this detector a decided advantage over refractive-index or UV detectors. The peak shapes were symmetrical and the linear range of response was 2-3 orders of magnitude. 

Another common reason for having separate assay and impurity methods is the need to use more concentrated samples with the impurity assay to increase sensitivity for minor impurities. Modern HPLC systems have been shown to adequately detect low-level impurities (i.e., 0.05%) in chromatograms where the parent peak is still on scale (that is, within the linear range of the detector). This level of detection is usually adequate for screening methods therefore, the assay for loss of parent compound and the measurement of the increase in impurities can typically be done using a single HPLC method. 

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