Internal standards fluorine

Proton magnetic resonance (carbon tetrachloride) S 3.75 (singlet with fine structure) infrared (neat) cm. 2985, 2273, 1667, 1527, 1515 fluorine magnetic resonance (carbon tetrachloride) p.p.m. (CFCI3 internal standard) 142.4 (symmetrical multiplet, 2 ortho F), 153.8 (triplet with flne structure, 1 para P, J = 20 Hz), 161.7 (multiplet, 2 meta F). 

FIGURE 3. The NMR spectra of the two racemic diastereomers of lV-(4-methyl-2-pentyl)-a-methoxy-a-trifluoromethylphenylacetamide prepared from racemic a-methoxy-a-(trifluoromethyl)phenylacetic acid [MTPA, ( )-83] and racemic 4-methyl-2-pentylamine [( )-84] (A) 60-MHz proton spectrum in chloroform-4 with tetramethylsilane (TMS) as the internal standard (B) 94.1-MHz fluorine-19 spectrum in chloroform-4 with trifluoroacetic acid as the internal standard. Reprinted with permission from Reference 76. Copyright (1969) American Chemical Society... 

The reactants and products show only singlets in their fluorine magnetic resonance spectra with the following chemical shifts (downfield from fluorotrichloromethane Internal standard) 8 TFA, -76.3 TfOH, -77.3 TFAT, -73.3 and -74.8 TFAA, -75.9 triflic anhydride, -72.6 ppm. 

Internal standards are brominated, fluorinated or stable isotopically labelled analogs of specific target compounds or other closely related compounds not found in environmental samples that are added to all standards, field, and laboratory QC samples as part of internal standard calibration procedure. The addition of internal standards takes place after the samples have been prepared and before they are analyzed. 

Chemical shifts of 19F attached to a 1,3,4-dioxazolidine ring were reported in CHEC-II(1996), where other 19F chemical shifts for fluorinated substituents are also included <1996CHEC-II(4)498>. Some new data for the chemical shifts of trifluoromethyl substitutents of several 1,4,2-dithiazolidine derivatives are as expected with from -62.5 to -67.5 (CCI3F as internal standard). Thus for the adamantyl dithiazolidines 35 SF appears at -65.2 to -67.5 depending on the aryl substituents, while the 19F chemical shifts of dithiazolidines 36-38 are as follows compound 36 S at -63.44 and -67.07 compound 37 6t- at -63.75 <1998EJ0459> compound 38 6 at -62.5 for X=0 and -62.58 for X = S <2000T4231>. 

Some of the above alkyl halides contain fluorine. In the case of CF3C1 the activation energy indicated that two negative-ion curves contribute to the dissociative thermal electron attachment. In 1997 S. R. Sousa and S. E. Bialkowski carried out a classic study of the ECD temperature dependence of alternative fluorocarbon freon replacements [16]. The study used a commercial ECD and the fundamental concepts discussed in this book to obtain rate constants and energies for compounds. One of these, CF2C12, was used as an internal standard in all the measurements. The... 

The shifts were observed for the fluorine resonances in several solutions of the radical TTBP in hexafluorobenzene, and were measured relative to tetramethylsilane as an internal standard. The experiments were performed in a constant magnetic field of 12,500 gauss by changing from proton to fluorine resonances and vice versa, several times for each solution to eliminate errors in the shifts arising... 

Since all sample manipulations are associated with the loss of analytes, internal standards are usually added to the sample before workup. In polycyclic aromatic carbon (PAC) work with mass spectrometric methods, deuterated aromatics have long been used for this purpose. Fluorinated PACs also offer several advantages. Andersson et al. used 5-fluorobenzothiophene for benzothiophenes and 2-fluorodibenzothiophene for three-ring PAHs. These fluoroderivatives were chosen because they are well separated from other sample constituents typically occurring in complex oil samples and because they show suitable elution characteristics in the various separation steps of sample workup. 

The last method to be presented for the analysis of the carbon-bonded fluorine in fluorouracil is F Nuclear Magnetic Resonance spectrometry (3). A fluorouracil reference standard and an internal standard. 

F NMR has also been used to quantitate resin-bound species in solid-phase synthesis, by use of of a polymeric support bearing F as an internal standard [206], and has been reported as a rapid screening tool to quantify the biotransformations of fluorine-containing model drugs [207]. 

Another report demonstrates the capability of providing quantitative information on the extent of conversion of a solid-phase material. This method may be applicable to a wide range of reaction chemistries and resin-supported materials. These authors use NMR to quantify the extent of the derivatization of functionalized polymeric material with a fluorinated substrate. Thus the rate of the reaetion of Merrifield resin with 2-lluo-rophenol was examined. At various time intervals portions of the reacting resins were removed and were exhaustively reacted with excess 4-lluo-rophenol. Analysis of the resin samples shows two signals (Scheme 2.9), one for the 2-fluoro derivative at -134 ppm and the other for the 4-fluoro derivative at -124ppm. Quantitative information on the extent of the reaction was obtained by measuring the relative intensities of the resin bound fluorine and a known concentration of an internal standard in solution, such as fluorobenzene. It was noted that there needed to be a sufficient time delay between the RF pulses so that the solid-phase and solution-phase nuclei would be similarly relaxed in the F NMR spectra. This method provided excellent correlation for resin-bound F atom content in samples containing 30 mg resin samples blended to various extents with 100% 2-fluorophenyl Merrifield ether and unfunctionalized Merrifield resin. This fluorophenol based assay was then used to optimize the attaehment of an epoxide to the Merrifield resin (Scheme 2.10). The alkylation reaetion was monitored at... 

The analysis has been performed by means of NMR with the sodium salt of trifluoroacetic acid as internal standard for both chemical shift and fluorine content [44]. The notable advantage of the NMR method is the avoidance of interference from fluorine-containing impurities in the polymer. Any such impurities would give rise to signals quite distinct from those originating from the fluorines at the chain-ends. The relaxation times for the end-of-chain fluorines and those in the standard are 0.3 and 1.1s, respectively, and so to ensure valid comparison between peak areas and fluorine contents for standard and polymer, the delay between scans was extended to 5.6 s. 

The catalysts used in the present study were prepared from anunonium tetrathiotungstate (ATT), containing 10 wt% tungsten. The content of fluorine in the fluorinated catalyst was 1 wt%. Details of the catalyst preparation can be found elsewhere [2]. The HDN reactions were carried out in a continuous-flow microreactor. 0.4 to 1.2 g of the catalyst sample diluted with 8 g SiC was sulfided in situ with a mixture of H2S (10 mol%) and H2 (90 mol%) at 400°C and 1.5 MPa for 4 h. After sulfidation the temperature was cooled to 370°C, the pressure was increased to 3.0 MPa, and the liquid feed was introduced to the reactor by means of a high-pressure pump, with n-octane as the solvent and n-hqjtane as internal standard. Dimethyldisulfide was added to the feed to generate H2S (6 kPa) in the reaction stream. The partial pressure of o-toluidine varied fipom 1 to 9 kPa. 

The quantitative aspect of hydroxyl determination is illustrated in Table 3.7 for the analysis of hydroxyl in some polymeric materials. In most cases, the fluorine resonance from n-h xty trifluoroacetate (at 7463 Hz in Figure 3.8) was used as internal standard to calibrate the spectral integral. From the data shown in Table 3.7, it seems that the adduct... 

Isotope Dilution. This technique is based on the fact that most elements have several stable isotopes if only one stable isotope exists, as for phosphorus or fluorine, the technique cannot be used. Let the ratio of two isotopes of an element in the sample be R. and the ratio for the same isotopes in the internal standard R. From the ratios, the percentage peak heights can be calculated for the sample (S] and 52) and the standard (ij and /2>. Then, a known amount / of the standard is added to the sample S and the two signals are measured (Fig. 5). The ratio of the two peaks is then... 

Since the fluorine concentration in the internal standard is known, eq. (3.1) simplifies to... 

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