Detectors sensitivity enhancements

The sensitivity enhancement achieved by VPD is determined by the ratio of the substrate area to the area of the detector aperture (analyzed area), provided there is fiill collection of the impurities. This has been demonstrated for Fe and Zn. For Cu and Au, however, only a small percentage can be collected using this technique, due to electrochemical plating. An example comparing direct TXRF with VPD-TXRF on the same substrate is shown in F%ure 4. 

Sensitivity enhancement due to the fact that we are using two uncorrelated detectors. 

Continuous Sampling and Determination. There are no truly continuous techniques for the direct determination of sulfuric acid or other strong acid species in atmospheric aerosols. The closest candidate method is a further modification of the sensitivity-enhanced, flame photometric detector, in which two detectors are used, one with a room-temperature de-nuder and one with a denuder tube heated to about 120 °C. Sulfuric acid is potentially determined as the difference between the two channels. In fact, a device based on this approach did not perform well in ambient air sampling (Tanner and Springston, unpublished data, 1990). Even with the SF6-doped H.2 fuel gas for enhanced sensitivity, the limit of detection is unsuitably high (5 xg/m3 or greater) because of the difficulty in calibrating the two separate FPD channels with aerosol sulfates. 

Narrow-bore columns of between 1.0 and 2.5 mm ID are available for use in specially designed liquid chromatographs having an extremely low extracolumn dispersion. For a concentration-sensitive detector such as the absorbance detector, the signal is proportional to the instantaneous concentration of the analytes in the flow cell. Peaks elute from narrow-bore columns in much smaller volumes compared to those from standard-bore columns. Consequently, because of the higher analyte concentrations in the flow cell, the use of narrow-bore columns enhances detector sensitivity. The minimum detectable mass is directly proportional to the square of the column radius (107) therefore, in theory, a 2.1-mm-ID column will provide a mass sensitivity about five times greater than that of a 4.6-mm-ID column of the same length. 

Improvement in sensitivity can be obtained by increasing the temperature of the sample or by the salting-out effect, which is particularly useful for compounds such as phenols and fatty acids which form strong hydrogen bounds in aqueous solutions. With some compounds, the use of a more sensitive detector such as an electron-capture detector or an element-specific detector will enhance sensitivity. 

Dolan and Hall [148] have described a Coulson electrolytic conductivity detector of enhanced sensitivity for the gas chromatographic determination of chlorinated insecticides in the presence of PCBs. The detector was modified by the replacement of the silicone-rubber septum and stainless steel fitting at the exit of the pyrolysis furnace with PTFE fitting, by the reduction in diameter of the PTFE transfer tube, and by the... 

The use of the HPLC method provides some unique solutions to the problem of determination of product formation at low substrate concentrations. For example, the sensitivity of the detector can be enhanced even during the course of an analysis. Next, the volume of the incubation mixture assayed may be increased to provide more product, and finally, it is possible to use analogs such as radiolabeled or fluorescent compounds, for which there is greater detector sensitivity. 

Use of HMQC and HSQC for sensitivity enhancement is most appropriate where the detector has a high y relative to the insensitive nuclide. HMQC/HSQC caimot compensate for low natural abundance of the insensitive spin since only the satellites from coupling to S are detected. 

The first identification of solar neutrons at Earth took place on June 3,1982 (Fig. 2a), by neutron monitor measurements at Jungfraujoch, Lomnicky Stit, and Rome [Debrunner et al., 1983 Chupp et al., 1987], This was two years after the discovery of solar neutrons in near-Earth space by the Gamma Ray Spectrometer (GRS) aboard the Solar Maximum Mission (SMM) satellite [ Chupp et al., 1982], Thereafter, standardized neutron monitors were set up at favorable observational locations at Earth, such as Haleakala, Hawaii [Pyle and Simpson, 1991], Additionally, new ground-based detectors with enhanced sensitivity to solar neutrons were developed [e.g. Shibata et al., 1991 Muraki et al., 1993]. 

The reasons for derivatizing carbamates, apart from their thermolability during the gas chromatographic analysis, include increased detector sensitivity (particularly with electron capture detection), increased volatility, better chromatography separation, apphcability to multiresidue and confirmatory analysis, and enhanced compound stability. There are two general approaches to the analysis of Af-methylcarbamates by derivatization, namely derivatization of the intact pesticides and derivatization of a hydrolysis product (one of which will always be the volatile methylamine). The reactions typically used to obtain derivatives of both intact and hydrolysis products of A-methylcarbamates are methylation, silylation, halogenation, acylation, and esterification. 

Spatial resolution currently achievable with hard X-ray microbeam techniques, down to the 100 nm range, is quite satisfactory. In many cases, spatial resolution at the beam size cannot be realized because of the properties of the sample, notable thickness (because of the penetrating nature of the X-rays, 100 nm resolution can only be achieved in samples <100 nm thick). Improvements in sensitivity can be achieved in two principal ways, increase in incident flux or improvement in detection efficiency. Major increases in incident flux will be impractical in many cases because of the radiation sensitivity of the earth and environmental samples of interest. Improvements in fluorescence detection efficiency will be a more fruitful avenue because current detection schemes use very small solid angles. Energy dispersive detectors that intercept large solid angles would be a major advance in sensitivity enhancement. 

Use of HMQC and HSQC for sensitivity enhancement is most appropriate where the detector has a high y relative to the insensitive nuclide. HMQC/HSQC cannot compensate for low natural abundance of the insensitive spin since only the satellites from coupling to S are detected. Isotopic enrichment can be used where a suitable detector has low natural abundance but is readily available in enriched form, for example, CO in metal carbonyl clusters, or to boost the number of insensitivie spins, for example,... 

Other reagents commonly used include pentafluorobenzyl bromide developed for the analysis of acids, amides and phenols using an electron capture detector for enhanced sensitivity. Organics in surface waters have been successfully analysed by this procedure [47,48]. 

ELSDs respond to any non-volatile solute and thus approach the ideal of a universal detector. Sensitivity is enhanced using lasers as light sources with detection limits of 5 g per 25 pi reported, a considerable improvement compared to RI detection. In addition, ELSDs are not as sensitive to ambient conditions such as temperature and pressure and eluant flow rate. 

FI preconcentration system compared to one without preconcentration. For example, the response of a flame AAS detector may be influenced by a change in the solution introduction rate (cf. Sec, 2.4,2, equation 2.1). These effects should be differentiated from enrichment effects in order to obtain a valid evaluation of the preconcentration performance. This may be realized by separately determining the enhancement factor under similar operational conditions but without preconcentration. In this book the two factors will be differentiated whenever possible, otherwise the total enhancement factor, expressed as will be used instead of EF. Fang ei al.[10] have shown that when sensitivity enhancement factors exist, other than an increase in concentration of the analyte in solution, the enhancement effects will be multiplicative on EF. Provided that different factors have independent enhancement mechanisms, the total enhancement factor Nt, will be the product of the individual enhancement factors, N, and... 

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