Linear and dynamic range

In routine spectrophotometers, photomultiplier tubes are replaced by photodiodes (Fig. 11.11), which have excellent sensitivity, linearity and dynamic range. The photoelectric threshold, in the order of 1 eV, allows detection up to wavelengths of 1.1 pm. In diode array systems, each rectangular rectangular diode (15 pm x 2.5 mm) is associated with a capacitor. The electronic circuit sequentially samples the charge of each capacitor. While a photomultiplier tube measures the instant intensity in watts, a diode measures the emitted energy in joules over a time interval. 

Figure 4-12 Calibration curve illustrating linear and dynamic ranges.

The MALDI triple-quadrupole mass spectrometer generates calibration curves with linearity and dynamic ranges similar to those typically expected from a triple-quadrupole mass spectrometer. In most cases, linearity is established over three orders of magnitude with suitable accuracy and precision. Figure 11.5 depicts calibration curves obtained for some common drugs using neat standards. 

The performance of all HPLC detectors can be characterised by certain parameters such as sensitivity, noise, drift, limit of detection, linear and dynamic range, and detection volume. Other factors are more specific to individual types of detectors, and are discussed in their respective sections. 

The figures of merit of primary interest to analytical chemist that are discussed are noise characteristics and powers of detection, linearity and dynamic range, spectral stripping and interpolative correction for variable underlying background, compromise between resolution and spectral coverage and application to nonideal "real" samples. 

Linearity And Dynamic Range. There is extensive evidence that emission intensities of many atomic lines excited in an ICP source are linearly related to concentration of the corresponding analytes over a range of at least one million. Trace and major constituents are therefore determinable without changes in the operating condition of the plasma. Ideally, the detection system should have a comparable linear dynamic range performance. As previously discussed (45), there are several definitions of dynamic range that are applicable to the SPD detection system. 

For conventional analysis by ICP or DCP, liquid samples are used, which are either easily prepared or commercially available. Interference problems are reduced to a minimum if the cahbration solutions are matched to the samples with respect to their content of acids and easily ionisable elements (see above). Calibration curves obtained with sparks, arcs, and laser ablation systems are usually curved so that 8—15 calibration samples or more are needed to define a suitable calibration. In the case of liquid analysis by DCP and ICP, fewer cahbration samples can be used due to the better linearity and dynamic range and absence of selfabsorption effects. With the introduction of hquids, the spray chamber is the major source of flicker noise due to aerosol formation and transport. While shot noise can easily be compensated by longer integration times, the flicker noise is of multiplicative nature so that any element can be used as an internal standard provided that a true simultaneous measurement of the analyte and internal standard line intensity is possible. 

ITMS can also be used for pantothenic acid analysis in multivitamin dietary supplements. Although the quality of the quantitative analysis using ITMS is highly dependent on the analytes, pantothenic acid displays good linearity and dynamic range when the LCQ series (Thermo Fisher Scientific Inc. Waltham, MA, USA) ITMS is used (unpublished data). Both the SIM mode and SRM mode (SRM transition 220- 90) can be used. 

Seismic accelerometers sense the ground or stmcture seismic vibrations and, together with a suitable recorder, are called accelerographs. Most modem seismic accelerometers are of force-balance type (EBA), a servo system in which a feedback force is applied to the suspended inertial mass to keep its motion as small as possible. This improves the instmment linearity and dynamic range. Usually the mass motion is measured by a sensitive capacitive transducer. 

By monitoring the current at +0.90V we have made a calibration curve, shown in Figure 7 winch establishes the high sensitivity in addition to the linearity and dynamic range of this approach. 

The key performance indicator for any instrument is the overall SNR, which evaluates the ratio of diffracted tight to unwanted tight. Because this is an instrument function there is no clear rule-of-thumb that indicates what grating type, ruled or holographic, might provide the higher SNR. It is the SNR of an instrument that will determine the system linearity and dynamic range. 

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