Detection limition chromatography

Volatile impurities, eg, F2, HF, CIF, and CI2, in halogen fluoride compounds are most easily deterrnined by gas chromatography (109—111). The use of Ftoroplast adsorbents to determine certain volatile impurities to a detection limit of 0.01% has been described (112—114). Free halogen and haHde concentrations can be deterrnined by wet chemical analysis of hydrolyzed halogen fluoride compounds. 

A practical method for low level perchlorate analysis employs ion chromatography. The unsuppressed method using a conductivity detector has a lower detectable limit of about 10 ppm. A suppression technique, which suppresses the conductivity of the electrolyte but not the separated ions, can further improve sensitivity (110,111). Additionally, ion chromatography can be coupled with indirect photometric detection and appHed to the analysis of perchlorates (112). 

Gas Chromatography Analysis. From a sensitivity standpoint, a comparable technique is a gas chromatographic (gc) technique using flame ioni2ation detection. This method has been used to quantify the trimethylsilyl ester derivative of biotin in agricultural premixes and pharmaceutical injectable preparations at detection limits of approximately 0.3 pg (94,95). 

Since 1970, new analytical techniques, eg, ion chromatography, have been developed, and others, eg, atomic absorption and emission, have been improved (1—5). Detection limits for many chemicals have been dramatically lowered. Many wet chemical methods have been automated and are controlled by microprocessors which allow greater data output in a shorter time. Perhaps the best known continuous-flow analy2er for water analysis is the Autoanaly2er system manufactured by Technicon Instmments Corp. (Tarrytown, N.Y.) (6). Isolation of samples is maintained by pumping air bubbles into the flow line. Recently, flow-injection analysis has also become popular, and a theoretical comparison of it with the segmented flow analy2er has been made (7—9). 

The method of detecting dimethylterephthalate (DMTP), dibuthyl-phthalate (DBP) and diocthylphthalate (DOP) in aqueous extract is based on their extraction with an organic solvent (hexane) and subsequent concentration using gas-liquid chromatography and an electron-absorbing detector. The detection limit is 0.05 mg/dirf for DMTP and DBP, and 0,01 mg/dm for DOP. 

Figure 12.1 Analysis of Tinuvin 1577 in 30% virgin olive oil (in hexane), showing (a) the gas cliromatogram comparing the pure oil with a sample at the Tinuvin 1577 detection limit concentration, and (b) the coixesponding liquid chromatogram. Reprinted from Journal of High Resolution Chromatography, 20, A. L. Baner and A. Guggenberger, Analysis of Tinuvin 1577 polymer additive in edible oils using on-line coupled HPLC-GC , pp. 669-673, 1997, with pennission from Wiley-VCH.

Simple mixtures—like in alkyl sulfosuccinates—can be run using only one solvent. For more complex systems (e.g., ethoxylated fatty alcohol sulfosuccinates) a gradient technique is strongly recommended Technical mixtures of disodium laureth sulfosuccinate could be separated [68]. The separation was so effective that resolution of single homologs of ethoxylates was possible. The detection limit of this method lies at around 0.5 pg. Therefore reverse phase ion pair chromatography seems to be an excellent tool to analyze sulfosuccinates directly without the use of any kind of manipulation. 

In addition the role played by the sorbent on which the chromatography is carried out must not be neglected. For instance, it is only on aluminium oxide layers and not on silica gel that it is possible to detect caffeine and codeine by exposure to chlorine gas and treatment with potassium iodide — ben2idine [37]. The detection limits can also depend on the sorbent used. The detection limit is also a function of the h/ f value. The concentration of substance per chromatogram zone is greater when the migration distance is short than it is for components with high h/ f values. Hence, compounds with low h/ f values are more sensitively detected. 

The method of choice for the determination of a- and P-endosulfan in blood, urine, liver, kidney, brain, and adipose tissue is gas chromatography equipped with an electron capture detector (GC/ECD) (Coutselinis et al. 1976 Demeter and Heyndrickx 1979 Demeter et al. 1977 Le Bel and Williams 1986). This is because GC/ECD is relatively inexpensive, simple to operate, and offers a high sensitivity for halogens (Griffith and Blanke 1974). After fractionation of adipose tissue extracts using gel permeation chromatography, detection limits of low-ppb (1.2 ng/g) were achieved for endosulfan and other chlorinated pesticides using GC/ECD (Le Bel and Williams 1986). 

Several methods are available for the analysis of trichloroethylene in biological media. The method of choice depends on the nature of the sample matrix cost of analysis required precision, accuracy, and detection limit and turnaround time of the method. The main analytical method used to analyze for the presence of trichloroethylene and its metabolites, trichloroethanol and TCA, in biological samples is separation by gas chromatography (GC) combined with detection by mass spectrometry (MS) or electron capture detection (ECD). Trichloroethylene and/or its metabolites have been detected in exhaled air, blood, urine, breast milk, and tissues. Details on sample preparation, analytical method, and sensitivity and accuracy of selected methods are provided in Table 6-1. 

There are many proteins in the human body. A few hundreds of these compounds can be identified in urine. The qualitative determination of one or a series of proteins is performed by one of the electrophoresis techniques. Capillary electrophoresis can be automated and thus more quantified (Oda et al. 1997). Newer techniques also enable quantitative determination of proteins by gel electrophoresis (Wiedeman and Umbreit 1999). For quantitative determinations, the former method of decomposition into the constituent amino acids was followed by an automated spectropho-tometric measurement of the ninhydrin-amino add complex. Currently, a number of methods are available, induding spectrophotometry (Doumas and Peters 1997) and, most frequently, ELISAs. Small proteins can be detected by techniques such as electrophoresis, isoelectric focusing, and chromatography (Waller et al. 1989). These methods have the advantage of low detection limits. Sometimes, these methods have a lack of specifidty (cross-over reactions) and HPLC techniques are increasingly used to assess different proteins. The state-of-the-art of protein determination was mentioned by Walker (1996). 

When pushed to the limit by overriding human health concerns, residue chemists have achieved detection limits of Ippt (Ingkg ) or even into the low ppqr (1 pg kg ) range. An example at the 1 ppt level is provided by methods for 2,3,7,8-tetrachlorodibenzodioxin (TCDD) in milk and TCDD in adipose tissue. Eor relatively clean matrices such as water and air, preconcentration on solid-phase adsorbents followed by GC or gas chromatography/mass spectrometry (GC/MS) can provide detection limits of 1 ng m and less for air (examples in Majewski and Capel ) and 1 ngL and less for water (examples in Larson et A summary of units of weight and concentration used to express residue data is given in Table 1. 

Liquid chromatography/mass spectrometry Lower limit of detection Limit of detection Limit of quantitation Florseshoe crab hemocyanin Liquid scintillation counting Matrix-assisted laser desorption/ ionization mass spectrometry m -Maleimidobenzoy 1-A -Hydroxysuccinimide 1 -Cyclohexyl-3-(2-Morptiolino-ethyl)carbodiimide rnetlio-/ -Toluenesulfonate (same as CDI)... 

Comparing equations (10) and (5), the lUPAC definition for detection limit, the difference is that RMSE is used instead of For dynamic systems, such as chromatography with autointegration systems, RMSE is easier to measure and more reliable than for reasons discussed earlier. Both are measures of variance and, although dissimilar, provide similar information. This is apparent in the equations used to calculate the values of. Tb and RMSE ... 

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