Negative chemical ionization detection

Liquid chromatography was developed to analyze carbonyl (2,4-dinitro-phenyl) hydrazones with detection by diode array ultraviolet spectroscopy (DA-UV) and by atmospheric pressure negative chemical ionization (APNCI) mass spectrometry [716]. In addition, LC can be combined with electrospray ionization coupled on-line with a photolysis reactor for better detection and confirmation of photo degradation products [717]. 

Simpson JT, Torok DS, Girard JE, Markey SP. 1996. Analysis of amino acids in biological fluids by pentafluorobenzyl chloroformate derivatization and detection by electron capture negative chemical ionization mass spectrometry. Anal Biochem 233 58. 

Goto J, Watanabe K, Miura H, Nambara T, Iida T (1987) Trace analysis of bile acids by gas chromatography-mass spectrometry with negative ion chemical ionization detection. J Chro-matogr 388 379-387... 

EC = electron capture detection FID = flame ionization detector GC = gas chromatography hexa = hexabrominated biphenyl HRGC = high resolution gas chromatography HRMS= high resolution mass spectrometry LC = liquid chromatography MS = mass spectrometry NCI = negative chemical ionization RED = plasma emission detection PBBs = polybrominated biphenyls... 

Derivatization was conducted by the addition of a 10% H-ethyl-diiso-propylethylamine solution and a-bromo-2,3,4,5,6-pentafluorotoluene. Sample obtained from the derivatization procedure were dissolved in ethyl acetate prior to injection in splitless mode using a DB-1 capillary column. Helium was used as the mobile phase, and the injector temperature was set at 290 °C with a transfer line temperature of 270 °C. Sample detection used ion trap MS for detection, with the detector being set at negative chemical ionization with m/z = 262 (for CCA) and m/z = 286 (for the internal standard). The limit of quantitation was 5 ng/ml, and the average recovery ranged from 92.0% to 114%. In addition, the extraction efficiency ranged from 48.2% to 55.6% for concentrations of 5, 50, and 250 ng/ml. Samples were reported to be stable for up to 6 months when stored at 18 °C. 

Negative chemical ionization (NCI) mass spectrometry detects ABA with a high sensitivity since the negative molecular ion M of methyl ester of ABA is more stable than the positive molecular ion [M] + due to the high electrophilicity of ABA. The NCI mass spectrum shows [M]- at m/z 278 as a base peak, and other fragment ions at m/z 310, 260, 245, 141, and 152.601 The combination of SIM with NCI gives highly selective and sensitive detection of ABA the lowest detection limit is 0.3 pg, which is 200 times lower than that... 

Buser HR (1976), Anal. Chem. 58 2913-2919.. .Selective detection of brominated aromatic compounds using gas chromatography/negative chemical ionization mass spectrometry"... 

In Table 4 are listed each compound detected with their specific conditions of derivatization and instrument requirements. Electronic impact (El) detection is widely documented. To enhance the limit of detection (LOD), negative chemical ionization (NCI) and tandem MS were recently proposed. ... 

Both electron impact (El) and negative chemical ionization (NCI) techniques can be used for MS detection of PCTs. The use of El results in a more fragmented spectrum, which may provide more structural information [34]. NCI provides a higher sensitivity [8]. Only lower chlorinated congeners (<4 chlorine atoms) show a lower sensitivity with NCI. However, lower chlorinated PCTs are hardly present in Aroclor 5442, and even less so in Aroclor 5460. Hale et al. [17] reported the presence of lower chlorinated PCTs in sediments and shellfish from the Chesapeake Bay due to a specific discharge of Aroclor 5432. Monsanto has, however, produced substantially less Aroclor 5432 than Aroclor 5460 [8]. El and NCI spectra of a deca PCT are shown in Fig. 3. 

Retinoic acid, an endogenous retinoid, is a potent inducer of cellular differentiation. Because cancer is fundamentally a loss of cellular differentiation, circulating levels of retinoic acid could play an important role in chemoprevention. However, physiological concentrations are typically below the limits of HPLC detection. Sensitive techniques, such as negative chemical ionization (NCI) GC/MS have been employed for quantification, but cause isomerization and also fail to resolve the cis and trans isomers of retinoic acid. Normal phase HPLC can resolve the cis and trans isomers of retinoic acid without isomerization, and mobile phase volatility makes it readily compatible with the mass spectrometer. Based on these considerations, a method combining microbore normal phase HPLC separation with NCI-MS detection was developed to quantify endogenous 13-cis and all-trans retinoic acid in human plasma. The limit of detection was 0.5 ng/ml, injecting only 8 pg of retinoic acid onto the column. The concentration of 13-cis retinoic acid in normal, fasted, human plasma (n=13) was 1.6 +/- 0.40 ng/ml. 

Napoli et al. (23) developed a sensitive assay based on negative chemical ionization mass spectrometry to quantify retinoic acid in human plasma. Endogenous levels of all trans retinoic acid in plasma were 4.9 ng/ml, using a 0.1 ml sample. The limit of detection was less than 1 ng/ml. Direct quantification of 13-cis retinoic acid was impossible due to the inability of the GC to resolve the isomers. Barua and Olson (33) described a method to quantify all trans retinoic acid in serum using reverse phase HPLC. They detected 1.8 ng/ml of the all trans isomer, using a 2 ml serum sample and a non-acidic extraction procedure. 

Biological specimen extraction can be accomplished by liquid-liquid, solid-phase or solid-phase microextraction with subsequent detection of GHB or GBL by gas chromatography-mass spectrometry (GC-MS) using electron ionization (El), positive or negative chemical ionization (CI) or gas chromatography with flame ionization detection (GC-FID). LeBeau et al. (1999) describes a method that employs two ahquots of specimen. The first is converted to GBL with concentrated sulfuric acid while the second is extracted without conversion. A simple liquid-liquid methylene chloride extraction was utilized, and the ahquots were then screened by GC-FID without derivatization. Specimens that screened positive by this method were then re-aliquoted and subjected to the same extraction with the addition of the deuterated analog of GBL. The extract was then analyzed by headspace GC-MS in the full-scan mode. Quantitation was performed by comparison of the area of the... 

Derivatization is also useful to detect volatile metabolites. Liu et al. [282] described a specific, rapid, and sensitive in situ derivatization solid-phase microextraction (SPME) method for determination of volatile trichloroethylene (TCE) metabolites, trichloroacetic acid (TCA), dichloroacetic acid (DCA), and trichloroethanol (TCOH), in rat blood. The metabolites were derivatized to their ethyl esters with acidic ethanol, extracted by SPME and then analyzed by gas chromatography/negative chemical ionization mass spectrometry (GC-NCI-MS). After validation, the method was successfully applied to investigate the toxicokinetic behavior of TCE metabolites following an oral dose of TCE. Some of the common derivatization reagents include acetyl chloride and TV-methyl-iV- ft-b u (y Idi methyl si I y I) tro (1 uoroacctam i nc (MTBSTFA) for phenols and aliphatic alcohols and amines, dansyl chloride and diazomethane for phenols, dansyl chloride for amines, acidic ethanol and diazomethane for carboxylic acids, and hydrazine for aldehydes. 

Hemoglobin adducts of MBOCA and its metabolites have been detected in animals dosed with the chemical (Chen et al. 1991 Sabbioni and Neumann 1990). The methods used were HPLC/ED, gas chromatography/mass spectrometry (GC/MS), and GC/ECD. Sample preparation for all three methods required hemoglobin to be isolated from the blood of the test animals and hydrolyzed to release the bound MBOCA. Insufficient data were provided to compare the different methods, but MBOCA was detected and quantified in the blood of dosed rats by all three methods. GC/MS in the negative chemical ionization mode, with a detection limit of 2 pg, appeared to be the most sensitive of the methods tested (Sabbioni and Neumann 1990). 

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