Lower detection limit, chromatographic detectors

Lee [42] determined pentachlorophenol and 19 other chlorinated phenols in sediments. Acidified sediment samples were Soxhlet extracted (acetone-hexane), back extracted into potassium bicarbonate, acetylated with acetic anhydride and re-extracted into petroleum ether for gas chromatographic analysis using an electron capture or a mass spectrometric detector. Procedures were validated with spiked sediment samples at 100,10 and lng chlorophenols per g. Recoveries of monochlorophenols and polychlorophenols (including dichlorophenols) were 65-85% and 80-95%, respectively. However, chloromethyl phenols were less than 50% recovered and results for phenol itself were very variable. The estimated lower detection limit was about 0.2ng per g. 

Interfacing the TEA to both a gas and a HPLC has been shown to be selective to nitro-based explosives (NG, PETN, EGDN, 2,4-DNT, TNT, RDX and HMX) determined in real world samples, such as pieces of explosives, post-blast debris, post-blast air samples, hand swabs and human blood, at picogram level sensitivity [14], The minimum detectable amount for most explosives reported was 4-5 pg injected into column. A pyrolyser temperature of 550°C for HPLC-TEA and 900°C for GC/TEA was selected. As the authors pointed out, GC uses differences in vapour pressure and solubility in the liquid phase of the column to separate compounds, whereas in HPLC polarity, physical size and shape characteristics determine the chromatographic selectivity. So, the authors reported that the use of parallel HPLC-TEA and GC-TEA techniques provides a novel self-confirmatory capability, and because of the selectivity of the technique, there was no need for sample clean-up before analysis. The detector proved to be linear over six orders of magnitude. In the determination of explosives dissolved in acetone and diluted in methanol to obtain a 10-ppm (weight/volume) solution, the authors reported that no extraneous peaks were observed even when the samples were not previously cleaned up. Neither were they observed in the analysis of post-blast debris. Controlled experiments with handswabs spiked with known amounts of explosives indicated a lower detection limit of about 10 pg injected into column. 

Gas chromatographic methods measure the carbon monoxide content of blood. Y/hen blood is treated with potassium ferricyanide, carboxyhemoglobin is converted to methemoglobin, and the carbon monoxide is released into the gas phase. Measurement of the released carbon monoxide may be performed by GC using a molecular sieve column and a thermal conductivity detector. A lower detection limit is achieved by incorporating a reducing catalyst (e.g., nickel) between the GC column and the detector to convert... 

The lower detection limit of a detector is defined as the minimum amount of compound detectable at a given signal-to-noise ratio. Ideally the detection limit should be determined independently of the chromatographic separation system using standard dilution devices. Detection limits are often expressed in g/s (mass flow-sensitive detectors) or g/mL (concentration-sensitive detectors) to obtain a value which is independent of the measuring conditions (flow rate, etc.). 

The lower detection limit is influenced by the noise frequency and amplitude distribution compared with the signal band width and height. The noise level is usually measured over a given time period which is a multiple of the signal width. The noise can be expressed as peak-to-peak (p-t-p) or root-mean-square (rms) values. The latter gives about 70-80% lower noise levels. Figure 15-7 summarizes the detection limits of some selected chromatographic detectors. 

Electrochemical detection can be employed with most chromatographic modes in HPLC although polar mobile phases containing dissolved electrolytes must be used for electrochemical detection to operate. This work has demonstrated that it is possible to use electrochemical detection with non-aqueous solvents, such as acetonitrile, thereby expanding the areas of possible use of the technique. The application of the detector for monitoring sterols, organic acids and some non-ionic surfactants, in many cases using non-aqueous mobile phases, has been demonstrated. For many of these compounds electrochemical detection offers lower detection limits than any other direct detection system. 

CEAD permits efficiencies in the oxidation of the analyte of nearly 100% oxidation, and lower detection limits are possible. Two analytes could thus be separated if they have different redox properties even though they present similar chromatographic properties. Therefore, CEAD shows high sensitivity in comparison to other detectors, although it is especially dependent on appropriate sample pretreatment. 

For an analyte of molecular weight 5000 and good chromatographic conditions, most photometric detectors can be expected to provide detection limits of 2—5 ng. Improvement into the mid-picogram or lower range normally requires the use of more sensitive detection means such as fluorescence or electrochemical detectors. 

HPLC-UV-NMR can now be considered to be a routine analytical technique for pharmaceutical mixture analysis and for many studies in the biomedical field. HPLC-UV-NMR-MS is becoming more routine with a considerable number of systems now installed worldwide, but the chromatographic solvent systems are limited to those compatible with both NMR spectroscopy and mass spectrometry. The increased use of HPLC-UV-IR-NMR-MS is possible, but it is unlikely to become widespread, and the solvent problems are more complex. The future holds the promise of new technical advances to improve efficiency, and to enhance routine operation. These approaches include the use of small-scale separations, such as capillary electrochromatography, greater automation, and higher sensitivity and lower NMR detection limits through the use of NMR detectors cooled to cryogenic temperatures. 

Difficulties have been observed in the preservation of samples for speciation of chromium. Chromium speciation in seawater was determined on board ship shortly after samples had been collected (Abollino et at., 1991). Some samples were frozen, and analysed later in a laboratory. However, significantly lower concentrations of Crvl were observed in these latter samples. Thus, sea-going analytical methods for the determination of Crm and total chromium are of particular importance (Mugo and Orians, 1993). The volatile trifluoroacetyl-acetone derivative of Crm was formed and then concentrated by extraction into toluene. Chromium was determined by means of a gas chromatograph equipped with an electron capture detector. Total chromium was determined as Cr111 after reduction. The detection limits were 0.062 and 0.255 nmol dm 3 total chromium. A useful method was described for sampling natural water in the field, and for the preservation of Crm and Crvl species for subsequent analyses in a laboratory (Cox and McLeod, 1992). Water samples were drawn through small columns packed with activated alumina, which had been prepared previously. Chromium species were retained on the columns. 

Alberti and Jonke [60] describe a gas chromatographic method for the determination of vinyl chloride in surface waters using a flame ionisation detector and a Poropak Q or Chromosorb 101 column. The detection limit is 0.3mg L 1 and samples of waste waters from vinyl chloride or PVC factories can be injected direct into the gas chromatograph, while water samples with lower concentrations require preliminary enrichment for which a gradient-tube method is described. 

This method was used for the first time by Ray [6] to determine non-olefinic impurities in ethylene. The sample (10-25 ml) was first fed into a reactor (19 x 1.1 cm) filled with activated charcoal saturated with bromine (40%). The resulting liquid bromina-tion products of ethylene were securely retained on charcoal at room temperature. The zone of non-olefinic impurities (permanent and saturated hydrocarbon gases) moved in a flow of carbon dioxide (carrier gas) from the reactor into a chromatographic column (40 X 0.2 cm I.D.) packed with activated charcoal. A nitrometer was used as the detector [39, 40]. The method permitted the determination of trace concentrations of 10" -10" % in ethylene. The use of a more sensitive detector should substantially lower the detection limit. 

When large samples are used in isothermal analysis using PEG, water forms a chromatographic zone in the form of a step, which simplifies calibration and improves the accuracy [66]. This method permits the determination of water at concentrations of 0.2—1.2% (w/w) using a thermal conductivity cell. The application of reaction methods for the conversion of water in organic compounds using a flame-ionization detector or selective electrochemical detectors that are highly sensitive to water will undoubtedly enable the detection limit to be lowered. 

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