APPI-MS: effects of mobile phases and VUV lamps on the detection of PAH compounds.

The technique of atmospheric pressure photoionization (APPI) has several advantages over electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI), including efficient ionization of nonpolar or low charge affinity compounds, reduced susceptibility to ion suppression, high sensitivity, and large linear dynamic range. These benefits are greatest at low flow rates (i.e.,

Liquid chromatography-atmospheric pressure photoionization-mass spectrometry analysis of triacylglycerol lipids--effects of mobile phases on sensitivity.

In this work, we evaluate the performance of liquid chromatography-atmospheric pressure photoionization-mass spectrometry (LC-APPI-MS) for non-aqueous reversed phase analysis of six triacylglycerol model compounds using six binary mobile phases including MeOH/iPrOH, MeOH/CHCl(3), MeOH/CH(2)Cl(2), CH(3)CN/iPrOH, CH(3)CN/CHCl(3), and CH(3)CN/CH(2)Cl(2). All mobile phases give comparably good separation performance on a Gemini C(18) column with carefully adjusted gradient elution programs. APPI sensitivity varies from one mobile phase to the other without dopants; however use of dopants brings sensitivity to comparable levels for all mobile phases. MeOH/iPrOH offers high sensitivity without dopants due to self-doping effect and dopants are not necessary for this mobile phase. Dopants enhance analyte sensitivity to a varying degree for each of the mobile phases tested. Photo-induced chemical ionization (PCI) of solvent may play a significant role in achieving high sensitivity. Two critical parameters affecting sensitivity are photoabsorption cross-sections and ionization potentials of mobile phase solvents. How these mobile phase solvents affect APPI sensitivity and their dependency on dopant use are discussed. All six mobile phases offer comparable overall limits of detection for the analytes tested. These results indicate that LC-APPI-MS is a successful tool for neutral lipid analysis, giving high sensitivity with a variety of non-aqueous mobile phases.

Simultaneous measurement of nitric oxide (NO) and nitrogen dioxide (NO2) in simulated automobile exhaust using medium pressure ionization-mass spectrometry.

In previous papers we have demonstrated two different, two-color resonance-enhanced multiphoton ionization (REMPI) schemes for the simultaneous measurement of trace amounts (ppbV to pptV) of nitrogen monoxide (NO) and nitrogen dioxide (NO(2)). The goal of this study is to provide a laser ionization-mass spectrometric scheme capable of measuring ppmV to ppthV concentrations of NO and NO(2) within vehicle exhaust containing up to ppthV of aromatic hydrocarbons and a time frame of seconds. Two ionization schemes are used here to measure NO and NO(2) in simulated automobile exhaust with three different sources. REMPI Scheme 1 uses broad-bandwidth light and an effusive source to measure NO (limit of detection (LOD) 300 ppmV), NO(2) (LOD 100 ppmV), and aromatic hydrocarbons (via photoionization) along with fragments (via electron impact). REMPI Scheme 2 uses narrow-bandwidth light and a medium pressure laser ionization (MPLI) source to measure NO (LOD 60 ppmV), NO(2) (LOD 3 ppmV), and fragments (via electron impact). The LOD is determined using 10-second sampling times. A newly developed delayed-ion extraction technique for MPLI is then applied to REMPI Scheme 2, dramatically reducing the electron impact signal, so that only NO and NO(2) are observed. We conclude that Scheme 2 with delayed-electron extraction is best suited for measuring in situ NO and NO(2) within engine exhaust.

Real-time analysis of exhaled breath via resonance-enhanced multiphoton ionization-mass spectrometry with a medium pressure laser ionization source: observed nitric oxide profile.

An elevated concentration of nitric oxide (NO) in alveolar ventilation is indicative of inflammatory stress within the lung. We present here the first description of time-resolved measurement of NO in breath using photoionization mass spectrometry, providing new capabilities for the medical investigator, such as isotopic tracing. Here we use resonance-enhanced multiphoton ionization (REMPI) with time-of-flight mass spectrometry (TOF-MS) coupled with a medium pressure laser ionization (MPLI) source for the selective detection of NO in breath. To demonstrate this technology, a single male subject breathes NO-free air for several minutes, and then the exhaled breath is monitored. The ability of REMPI to differentiate among three different isotopomers of NO is demonstrated, and then the concentration profile of NO in exhaled breath is measured. A similar time-dependence concentration is found as observed by previous techniques. The advantages of this approach compared to other techniques are: (1) parts-per-billion by volume (ppbV) mixing ratios of NO can be measured on a sub-second time scale, (2) since the technique operates optically as well as mass-resolved, isotopomers of NO are discernable, permitting the use of isotopic tracing, and (3) other biologically significant gas molecules can be measured via REMPI.

Selective measurement of HCHO in urine using direct liquid-phase fluorimetric analysis.

Quantification of formaldehyde (HCHO) in urine was recently shown to be a promising tool in the investigation of cancer, particularly bladder cancer. Development of a low-maintenance, inexpensive and rapid analyzer for HCHO in urine would greatly facilitate future research and the potential diagnosis of bladder cancer. We examine here the application of an off-the-shelf system, originally designed for gas-phase atmospheric monitoring of HCHO, for the quantification of HCHO in urine. Under strict dietary protocols, e.g., avoidance of foods rich in free or chemically bound HCHO, an increase in HCHO in urine is an indirect indicator of cancer in the urogenital system. The concentration of HCHO in urine samples from an individual over a several-month period was determined, with a range from 39 to 1400 microM and a mean of 600 microM. The limit of detection for the present method was 0.1 microM. The proposed technique provides a direct, low-cost and greatly simplified analytical method for the quantification of HCHO in urine compared to other available techniques.