Spectrophotometer performance parameters

The performance of a spectrophotometer is dependent on a number of parameters that affect the quality of the data, namely, wavelength accuracy, photometric (absorbance) accuracy, spectral bandwidth variations, extent of stray light, and linear response of the instrument. The performance parameters can be evaluated from the specifications provided by the manufacturer. A standard list of specifications of a typical double-beam spectrophotometer is provided in Table 5. 

To verify that a spectrophotometer is performing satisfactorily, the device must be shown to be able to operate within the specifications provided for it. Parameters to be tested include (1) wavelength accuracy, (2) spectral bandwidth, (3) stray light, (4) linearity, and (5) photometric accuracy. 

As described previously, the deconvolution method of UV spectra allows to measure quantitative parameters (nitrates, surfactants for example) as well as to estimate some physico-chemical parameters (TOC, COD, BOD5, TSS). The obtained information (qualitative and quantitative) are enough relevant and robust to be integrated in the decision making process and water resources management. Indeed, the performances of Portable UV spectrophotometer have been evaluated and compared to a reference method and have shown good correlation (Gonzalez et al., 2007). In addition, performance criteria have been verified in field conditions in order to assess the impact of these conditions and to demonstrate the portability of the instrument. 

Colour determination can be carried out with several reference methods, generally based on the sample optical properties in the visible region (Table 2). The examination of the different procedures leads to the conclusion that, except for the USEPA 1 method, which uses several sets of three wavelengths, the others are limited to the choice of the wavelengths to be considered and give less useful results, which are apparently not very close to the significance of the parameter. Almost all methods can be automatically performed by a PC-controlled spectrophotometer, provided the bandwidth of the instrument is adapted for the measurement. 

The influence of the Schlieren effect on the analytical output depends on the optical performance of the spectrophotometer [83], the number of mirror interfaces and the intensity and direction of the concentration gradients. The latter parameters are closely related to the concentrations of the solutions involved, the mixing conditions and the kinetics of solution inter-mixing and hence the transport number of the related chemical species, as discussed below. 

The fluorescence lifetime measurements were performed with a streak camera (Agat SF 3M, VNIIOFI, Russia). A Nd YAG laser with excitation wavelengths of 532 and 266 nm (the second and fourth harmonic of fundamental radiation) was used as a light source. The laser radiation parameters of the fluorimeter were as follows (for 532 nm) pulse energy 160 (ij, duration 20 ps (fwhm), beam diameter 5 mm. The error in determining the fluorescence lifetimes in time intervals of several nanoseconds did not exceed 5 %. In addition to the laser equipment, the Cary 100 spectrophotometer (Varian, Inc., USA) and the Cary Eclipse spectrofluorimeter (Varian, Inc., USA slits width was 5 nm) were used for optical density measurements and fluorescence registration, respectively. 

Sensitivity in flame atomic absorption is defined as the characteristic concentration of an element required to produce a signal of 1% absorbance (0.0044 absorbance units). Sensitivity values are listed for each element by the atomic absorption spectrophotometer manufacturer and have proved to be a very valuable diagnostic tool to determine if instrumental parameters are optimized and if the instrument is performing up to specification. The sensitivity of the spectrophotometer used in the validation of the flame AAS anal ical technique agreed with the manufacturer specifications (5.6.) the 2 xg/mL cadmium standard gave an absorbance reading of 0.350 abs. units. 

---