Optimising sensitivity

Under conditions where there is complete decay of transverse magnetisation between scans (i.e. the FID decays to zero) the optimum tip angle for a pulse repetition time h, known as the Ernst angle, Oe, is given by [1,2]  

Fignre 4.1. Ihe essential elements of the single-pulse NMR experiment the relaxation (recovery) delay, the pulse excitation angle and the data acquisition time. 

Editing of heteionuclide spectra (notably carbon-13) according to resonance multipUcity. 

Enhancement of a low-y nuclide through polarisation transfer fiom a high-y nuclide, e.g. H, P, Editing of spectra according to resonance multiplicity. 

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Even with all these chromatographic parameters optimised, sensitivity may not be enough. The simplest way to increase sensitivity further is to inject more sample. Valve injectors are usually supplied with 20-pl loops but there are few assays for which 100-200 pi of sample cannot be injected simply by changing the loop size. A further way is to use minibore (1 mm or 2mm i.d.) or even microbore (< 1 mm) columns. Such columns give increases in sensitivity due to the narrower peaks eluted but this may reduce column loading volumes and with some equipment the extra sensitivity gained may be lost due to extra column band broadening. 

Therefore, for some well-known matrices and some perfectly stable substances or elements, common sense is often better than any measurement. Also linked to the precision of the analytical method, it is always better to look for the appearance of degradation products with a method with optimised sensitivity rather than checking for the disappearance of few percent of a highly concentrated substance. Unfortunately, degradation products are not always known or they may already exist naturally in the sample at high concentration levels. Nevertheless, it is sometimes possible to find cases where degradation products are of help e.g. degradation of p,p -DDT into p,p -DDE in BCR-CRM 115 [12] and in BCR-CRM 598 [14] or arsenobetaine into dimethyl arsinic acid in BCR-CRM 627 [1,2]. 

Detection limits down to 1 ng g can be reached for certain isotopes. The choice of ion type and energy provide versatility for optimising sensitivity and selectivity to suit particular problems. Proton energies from 5 to 20 MeV are needed to obtain a sufficiently high cross-section for good sensitivity in CPAA. CPAA has excellent sensitivity especially for light elements, such as B, C, N and O. CPAA is unique amongst activation techniques its ability to determine of H and He contents at levels below 1 jjg g ... 

The model LS-2B has all these features arranged to optimise sensitivity for micro-samples. It can also be connected to a highly sensitive 7 pi liquid chromatographic detector for detecting the constituents in the column eluent. It has the capability of measuring fluorescence, time-resolved fluorescence, and bio- and chemiluminescence signals. A 40-portion autosampler is provided. An excitation filter kit containing six filters - 310, 340, 375, 400, 450, and 480 pm - is available. 

Both FAB and LSIMS can be used in conjunction with continuous-flow inlet systems to optimise sensitivity, to improve reproducibility and to allow the use of in-line liquid chromatography for LC-MS. In the flow technique an LC pump delivers a continuous flow of solvent (containing a low concentration of the FAB/LSIMS matrix) to the point where the atom or ion beam is applied. The sample is introduced into this solvent flow by an LC injection valve and is delivered to the point of ionisation in a sharp concentrated slug. A good account of the method of continuous flow FAB is given in reference [5]. 

The carbohydrate (again often molasses, 15 - 25%) and added nutrients are pH-adjusted to below 4.0 and, for Otis process, have to be sterilised. It is necessary to add potassium hexacyanoferrate but greater care is required in this process compared to surface culture. The A. niger seems to be more sensitive to and more easily inhibited by hexacyanoferrate in submerged culture. It is essential however to lower the ferrous and manganese concentrations, probably below 200 and 5 pg l1 respectively, to optimise the performance of A. niger. 

As an example, Baitz et al7 focused on different technologies and peripheral system conditions to reduce dust and heavy metal emissions from a refinery. They stressed that the knowledge of the sensitive life cycle parameters and a suitable database, and thus the possibility to quantify impacts, enables a sustainable decision-making in process design and process optimisation. 

Cool sample transfer to column No discrimination Little optimisation needed Simple to use, robust Handles difficult samples Good repeatability High sensitivity Rugged... 

C, is one of the most critical parameters in TSP operation, and should be optimised for different samples, wherever possible. This is considered to be a considerable drawback in routine operation of unknown polymer/additive extracts. Too low a vaporiser temperature results in the solute and solvent spraying into the ionisation source in their liquid form, without formation of gas-phase ions. Too high a vaporiser temperature causes premature evaporation of the solute and solvent before the outlet of the capillary is reached. This causes an unstable, pulsing ion beam. As ion formation in TSP operation depends very critically on the extent of desolvation and the energy of the nebulised droplets, it is clear that an inappropriate vaporiser temperature will cause loss of sensitivity. 

Flow limitations restrict application of the DFI interface for pSFC-MS coupling. pSFC-DFI-MS with electron-capture negative ionisation (ECNI) has been reported [421], The flow-rate of eluent associated with pSFC (either analytical scale - 4.6 mm i.d. - or microbore scale 1-2 mm, i.d.) renders this technique more compatible with other LC-MS interfaces, notably TSP and PB. There are few reports on workable pSFC-TSP-MS couplings that have solved real analytical problems. Two interfaces have been used for pSFC-EI-MS the moving-belt (MB) [422] and particle-beam (PB) interfaces [408]. pSFC-MB-MS suffers from mechanical complexity of the interface decomposition of thermally labile analytes problems with quantitative transfer of nonvolatile analytes and poor sensitivity (low ng range). The PB interface is mechanically simpler but requires complex optimisation and poor mass transfer to the ion source results in a limited sensitivity. Table 7.39 lists the main characteristics of pSFC-PB-MS. Jedrzejewski... 

Satisfactory performance of the SFE-SFC-HRMS instrumentation (resolution 1200) was only possible after optimisation (temperatures, restrictor and quartz tube positions, flow characteristics and sample transfer conditions). Mass spectra obtained for Irganox 1010/1076/1330 and Irgafos 168/P-EPQ by SFC-HRMS were identical with those obtained by use of DIP [431]. However, the sensitivity of the SFE-SFC-MS interface is low (at best 4 % of that obtained with sample introduction via DIP). An enormous amount of sample is lost in all parts of the coupling system (SFE, SFC and... 

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