Abstract
The performance characteristics of the particle beam interface (PB) coupled to
supercritical fluid chromatography (SFC) and mass spectrometry (MS) were assessed with
pure and methanol-modified C02. Factors which affect the nebulization and the subsequent
desolvation of the droplets were assessed. The quantitative performance was evaluated
yielding a limit of detection (LOD) of 40 ng (caffeine) for scan data which is some 3 to 25
times lower than previously reported studies. Operation of the SFC-PB-MS system was
found to be highly dependent on the mobile phase characteristics (flow rate and
composition). Between 0.1 to 0.64 mL/min liquid C02 flow, relatively stable operation of
the system was determined; whereas beyond this range significant losses in sensitivity
were observed. Mobile phase composition was shown to have a dramatic effect with 4%
methanol-modified C02 yielding the most sensitive results; whereas, no detection was
possible with pure C02. This lack of sensitivity with pure C02 and dependence of
sensitivity on mobile phase composition, presented a problem in method development.
The SFC-PB-MS system was therefore modified by employing a particle forming solvent
(PFS). The purpose of the PFS was to aid in the formation of an aerosol. With the PFS
solvent, mobile phase composition had no effect on sensitivity and detection of analytes
eluted with pure C02 was achieved. The nature, composition, and flow rate of the PFS
were found to be crucial to the optimum operation. Quantitative performance of the system
was improved by a factor of 4 to 5 over the prior system. The analysis of pesticides,
steroids, and polyaromatic hydrocarbons was achieved with the SFC-PB-MS system. The
resulting EI spectra were artifact-free and gave good matches on comparing with on-line
library spectra.
Packed column SFC, however, is only able to handle directly non-polar to medium
polar analytes. Thus for polar to highly polar analytes (peptides, proteins) liquid
introduction (infusion, flow injection, chromatography) is the preferred method of sample
delivery to the MS. Furthermore because of the polar nature and thermal lability of these
compounds conventional ionization methods (EI, CI) are not suitable. Factors which have
ramification on sample handling (flow rate, solution composition) were studied. The high
sample flow rate capability was dependent on effective nebulization and desolvation. Thus,
needle distance/angle and bath gas flow setting played a critical role in the performance of
the ES-MS. The utility of the system was demonstrated by analysis of gramicidin s,
myoglobin, and tryptic peptides of cytochrome c.
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