Pentafluoropropionyl derivatives

The relative response of the electron-capture detector to some haloalkylacyl derivatives is sumi rized in Table 8.17 [451]. In general terms, the monochloroacetyl and chlorodifluoroacetyl derivatives provide a greater response than the trifluoroacetyl derivatives. Increasing the fluorocarbon chain length of the fluorocarbonacyl derivatives increases t ir electron-capture detector response without inconveniently increasing their retention times. The heptafluorobutyryl and pentafluoropropionyl derivatives are considered to be the best compromise between detector sensitivity and volatility for most applications. 

Fluorination of caesium heptafluoropropoxide at —40°C with nitrogen-diluted fluorine exploded violently after 10 h. This may have been caused by ingress of moisture, formation of some pentafluoropropionyl derivative and conversion of this to pentafluoropropionyl hypofluorite, known to be explosive if suitably initiated. Other possible explosive intermediates are peroxides or peresters. 

Figure 1.10. Chromatograms obtained by monitoring total ion current (tic) and m/e 617 (M), 603 and 602 (M-Me) in the detection of the pentafluoropropionyl derivative of the alkaloid salsolinol in an extract of urine from a Parkinsonian patient. Response ratios for 617/602 and 603/602 at the indicated point on the chromatograms correspond to these ratios in the authentic substance (adapted from reference [90])...

False neurotransmitters. A number of substances, in some cases structurally related to the biogenic amines, are considered to act as false neurotransmitters and GC-MS methods have been applied to their detection. Thus, p-hydroxynorephedrine, a metabolite of (-l-)-amphetamine, can apparently replace noradrenaline and be taken up and released by a similar mechanism to that involved for the catecholamine. A quantitative SIM assay for this metabolite (as the pentafluoropropionyl derivative) has been used in studies of its formation and localisation in brain regions of the rat [473]. Similar methods have been applied to the detection in neural tissue of the N-dealkylated metabolites of methamphetamine, which depletes brain noradrenaline and fenfluramine which depletes serotonin [474]. 

Figure 7-2 Fragmentation patterns for the pentafluoropropionyl (A) and carbethoxy-hexafluorobutyryl (B) derivatives of methamphetamine (R = CH3) and amphetamine (R = H masses in parentheses). Compare the predicted masses with the spectrum shown in Figure 7-1. Note that for the pentafluoropropionyl derivative, only one ion (204, 190 m/z) is characteristic of the aliphatic portion of the molecule.

Pentafluoropropionyl derivatives have been successfully employed for the determination of ll-nor-A -tetrahy-... 

Heptafluorobutyrate derivatives are popular in isotope dilution reference methods for steroids such as testosterone [141], 17 -oestradiol [142-144] and progesterone [145-147]. Even difficult steroids such as aldosterone can be quantified in serum as the 3-enol-HFB ester by stable isotope dilution GC-MS [I4I] or after a new derivatization procedure with HFB anhydride [148). Pentafluoropropionyl derivatives have a)so found application to the quantification of oestriol [141] and urinary progesterone [149]. 

Pentafluoropropionyl derivative by pentafluoropropionic acid anhydride (PFPA). 

Amino acid enantiomers can be separated on a chiral stationary phase after derivatization with chloroformates (Abe et al., 1996). The derivatization procedure is quite simple and rapid, but the derivatizing reagent must be synthesized, which complicates the assay. Another method for the analysis of amino acid enantiomers uses N,0-pentafluoropropionyl isopropyl derivatives and a chiral column with NPD detection (Hashimoto et al., 1992). 

Pentafluoropropionyl and heptafluorobutyryl derivatives have also been used for the GC analysis of amino acids in combination with esterification with various alcohols. In comparison with TFA derivatives, they are much more stable and resitant to hydrolysis, their retention times are shorter and all twenty protein amino acids can be separated on common phases (OV, SE, etc.). The price and restricted availability of the acylation reagents obviously hinder their wider application. 

To be able to determine nicotine in sub-picomole quantities Neelakantan and Kostenbauer11 used an electron-capture detector in connection with derivatization of nicotine Nicotine was hydrogenated catalytically to yield N-methyl-4-(3 -piperidyl)-n-butylamine (= octahydro-nicotine), the two secondary amino-functions of which were treated with perfluoropropionic anhydride to give the di-pentafluoropropionyl octahydronicotine. The perfluoropropionyl derivative of octahydronicotine provides an electron capture derivative which represents a means of detecting nicotine in amounts of approximately l/100th of the minimum quantity of nicotine that is detectable by FID. 

Perfluoroacyl derivatives such as trifluoroacetyl (TF.A), pentafluoropropionyl (PFP) and heptafluorobutyryl (HFB) are very commonly employed in GC-MS. Much of their popularity arose from their ease of preparation and their useful employment in GC with ECD. Workers then employed them directly in the GC-MS system and in many cases excellent results were obtained—high mass increments which can be conveniently adjusted by choice of derivative (see Table 3) and high abundance fragment ions. All this is in addition to their advantages for negative-ion chemical ionization mass spectrometry, where their electron capturing properties can enable highly sensitive analyses to be carried out. 

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