Below detection, limit width

One approach to this problem is to devise a set of operating parameters, i.e., sample size, current and voltage values, exposure time, slit width, etc., so the maximum allowed concentration is the detection limit for the spectral plates used. If the spectral line is observed on the plate, the concentration exceeds specifications and if it does not appear, the concentration is below that specified. 

It is interesting how the width of the isotopic pattern increases as X+2, X+3, X+4. .. become detectable. In principle, the isotopic pattern of C expands up to X+w, because even the composition is possible. As a result, the isotopic pattern of w atoms of a di-isotopic element consists at least theoretically of w+l peaks. However, the probability of extreme combinations is negligible and even somewhat more probable combinations are of no importance as long they are below about 0.1%. In practice, the interpretation of the carbon isotopic pattern is limited by experimental errors in relative intensities rather than by detection limits for peaks of low intensity. Such experimental errors can be due to poor signal-to-noise ratios (Chap. 1.4.3), autoprotonation (Chap.7), or interference with other peaks. 

The typical peak width with analytical columns of 4.6 mm i.d. and a 1 ml/min flow rate is of the order of 10-30 s. The acquisition of NMR spectra with a short relaxation delay and an acquisition time of below 1 s allows the acquisition of 8-24 transients for one spectrum during the presence of a peak in the NMR cell. This low number of transients limits the detectable amount of sample to 5-10 xg per compound. 

Raman spectroscopy can be used to detect normal modes of target molecules and also to monitor spectra of Raman labels that are used for one of the spectroscopic bar-codes. Raman bands in the vibrational Raman spectmm have intrinsically narrow bandwidths of ca. 10 cm, which, for example, correspond to less than 0.5 nm width in the visible region below 800 nm. The fluorescence of dye molecules has a broad bandwidth of 100 nm more or less. Hence, spectral overlap between fluorescence bands is inevitable and limits their use for multiplexed analysis. Quantum dots (QDs) have narrower bandwidth than dye-based fluorescence but stUl have broad bands that are several tens of nanometers. Light scattering of noble metal nanoparticles caused by surface plasmon resonance is also... 

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