DETERMINATION OF FREE SELENOMETHIONINE IN NUTRITIONAL SUPPLEMENTS BY HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY COUPLED WITH THERMOCHEMICAL HYDRIDE GENERATION ATOMIC-ABSORPTION SPECTROMETRY
G. Matni et al., DETERMINATION OF FREE SELENOMETHIONINE IN NUTRITIONAL SUPPLEMENTS BY HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY COUPLED WITH THERMOCHEMICAL HYDRIDE GENERATION ATOMIC-ABSORPTION SPECTROMETRY, Analyst, 120(2), 1995, pp. 395-401
Selective isolation and HPLC-AAS protocols were developed to determine
free selenomethionine (SeMet) in complex matrices such as nutritional
supplements and mixtures of free amino acids. The selenoamino acid in
alkaline solution was first derivatized with 1-fluoro-2,4-dinitrobenz
ene (FDNB, Sanger's reagent). After removal of excess of reagent by-pa
rtitioning with diethyl ether, the N-dinitrophenyl (DNP)-derivatized S
eMet was protonated by acidification and extracted with diethyl ether.
DNP-SeMet was determined selectively by HPLC-thermochemical hydride g
eneration (THG) AAS. A selective chromatographic mechanism based on pi
-electron interactions was optimized using a silica stationary phase d
erivatized with p-nitrophenyl moieties. Primary analytical validation
parameters including stability, linearity and limit of detection were
obtained using a purified DNP-SeMet standard which was characterized b
y H-1/C-13/Se-77 NMR and fast atom bombardment MS techniques. The cali
bration graphs for sequential dilutions of this standard were linear (
averge r = 0.997) and the limit of detection from the resultant calibr
ation graphs was estimated to be 17 ng as Se or 81 ng as DNP-SeMet. Th
e standard was found to decompose slowly (0.08% h(-1)) via photolytic
cleavage of a Se-C bond. MS and HPLC-THG-AAS studies demonstrated that
the cleavage intermediates (CH3Se.) combined to form dimethyl diselen
ide. Ascorbic acid present in large amounts in nutritional supplements
was revealed to be the major interferent during the DNP derivatizatio
n step.:This interference was removed using a cation-exchange treatmen
t of the crude extract prior to derivatization. The recovery of the :
SeMet from stock solutions and nutritional supplements was virtually q
uantitative (99 +/- 4.5%). In the presence of a 500-foId excess of oth
er amino acids, this recovery decreased (95 +/- 2.6%), but remained in
an acceptable range to allow the accurate determination of SeMet in a
protein hydrolysate.