Chemical derivatization for the enhancement of sensitivity in electron capture negative ion chemical ionization mass spectrometry
The sensitivity enhancement which can be achieved by the use of electron capture negative ion chemical ionization (ECNICI) in mass spectrometry (MS) is well documented. While some halogenated or highly conjugated molecules have inherent electron affinity, most compounds must first be derivatized to impart the ability to capture electrons. The two primary goals of this work were (1) develop suitable reagents for alcohols which produce structure specific anions in ECNICI/MS and (2) if possible, carry out the derivatization of biologically relevant compounds directly in biological fluids. With these two main objectives in mind, several novel reagents were synthesized and their reactivity and mass spectral properties studied. The reagents included pentafluorophenyl boronic acid (FPBA), anthraquinone-2-carbonyl chloride (ACCl), and pentafluorobenzyl chloroformate (PFBCF). Studies of electrophillic isocyanates were also conducted. Pentafluorophenyl boronic acid exhibited excellent reactivity, specificity and the derivatives had good chromatographic properties. Unfortunately, the ECNICI signal enhancement of the derivatives was not sufficient and the equilibrium of the derivatization did not allow for quantitation at the pg to fg level. Anthraquinone-2-carbonyl chloride (ACCl) was synthesized and used for the derivatization of alcohols with subsequent analysis by particle beam LC/MS. ACCl proved to be a useful reagent, where good transport properties of the derivatives, combined with the electron affinity of the anthraquinone, allowed for detection limits in the low pg. The use of pentafluorobenzyl chloroformate for the derivatization of alcohols, amines and carboxylic acids was investigated. Aqueous PFBCF derivatization was applied to amino acid standards, whole blood, plasma and protein hydrolysates. The ECNICI mass spectra of the derivatives are dominated by the (M-181) $\sp-$ ion and the derivatization is accomplished in seconds, in one step at room temperature. Detection limits are in the low femtomole range. PFBCF derivatization of alcohols, under anhydrous conditions, resulted in carbonate esters which yielded molar responses 2-6 times larger than those of the analogous pentafluorobenzoate esters. Finally, the addition of a Hewlett-Packard particle beam LC/MS interface to a Finnigan TSQ-70 mass spectrometer is described. The optimization of particle path and analyte ionization is reported.