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., Յ100 L/min), while at a higher flow, photon absorption and ion-molecule reactions become significant. Under certain circumstances, APPI signal and S/N have been observed to excel at higher flow, which may be due to a nonphotoionzation mechanism. To better understand APPI at higher flow rates, we have selected three lamps (Xe, Kr, and Ar) and four mobile phases typical for reverse-phase, high-pressure liquid chromatography: acetonitrile, methanol, (1:1) acetonitrile:water and (1:1) methanol:water. As test compounds, three polyaromatic hydrocarbons are studied: benzo[a]pyrene, indeno [1,2,3-c, d]pyrene and benz [a]anthracene. We find that solvent photoabsorption cross-section is not the only parameter in explaining relative signal intensity, but that solvent photo-ion chemistry can also play a significant role. Three conclusions from this investigation are: (1) [4,5]. Recent work has focused on the direct comparison of different sources (APPI, APCI, and ESI) with specific target compounds, e.g., polyaromatic hydrocarbons [6,7], hydrophobic peptides [8], pesticides [9,10], as well as fatty acids and lipids [4,5]. In many cases atmospheric pressure photoionization (APPI) [1,7] has demonstrated extended linear dynamic range [11], enhanced sensitivity and thus lower detection limits [6,9,[12][13][14][15], and reduced or no off-line sample cleanups [6,9] in comparison with direct APCI or ESI. Adding a dopant (dopant-assisted, DA) to the mobile phase in many cases can further increase sensitivity [2].A related analytical technique is atmospheric pressure laser ionization (APLI), which has shown excellent sensitivity for certain non-or low-polar compounds [3]. The enhanced sensitivity is a direct result of the high photon flux associated with a laser system, often several orders of magnitude higher than the noncoherent light sources, e.g., lamps used with APPI. The APLI source relies on one-color, two-photon (1 ϩ 1) resonantlyenhanced multiphoton ionization (REMPI), typically using a KrF excimer laser emitting light at 5.0 eV, efficiently ionizing nonpolar molecules. However, APLIis not yet widely used due to the large size, expense, and maintenance associated with the laser system. Furthermore, changing the wavelength of an excimer laser requires the replacement of the gas mixture and often re-tuning of the laser cavity, whereas with an APPI system, the wavelength can be changed by simply switching the lamp.To improve the capabilities of APPI for LC-MS, we have focused on the ion chemistry involved in and following the photoionization process. Although direct, single-photon ionization (SPI) of a compound [M] however, there is litt...
In this paper we demonstrate the feasibility of ultra-trace resonance enhanced multiphoton ionization (REMPI) detection employing a small broad-bandwidth solid state laser system. The results reported here are compared with measurements carried out with a conventional excimer pumped dye laser combination. Mass selected broad-bandwidth REMPI spectra for the environmentally relevant nitrogen oxides NO and NO2 are presented. Tunable broad-bandwidth laser radiation with a spectral resolution of > 10 cm(-1) in the wavelength range 560-400 nm was employed for the detection of NO2. For NO detection, the range 230-224 nm was covered. Laser radiation was generated using an optical parametric oscillator pumped by an unseeded Nd:YAG laser. A mobile time-of-flight mass spectrometer equipped with an atmospheric pressure laser ionization source allowed for mass selective parent ion detection at m/z 30 for NO and m/z 46 for NO2. The limit of detection was 10 pptV for NO and 20 pptV for NO2. A selectivity of > 2000 for both compounds with respect to N2O5, organic nitrates and NO2 in the case of NO is reported. An improved laser system currently under construction is expected to provide detection limits below pptv mixing ratios for both nitrogen oxides in a 20 s integration interval.
Analysis of several polar and non-polar compounds is performed with a newly developed dual electrospray ionization/atmospheric pressure photoionization (ESI/APPI) or ESPI source. Several variables are considered in the source, such as ESI probe heater temperature, solvent flow, dopant effects, repeller plate voltage, source geometry and photon energy (Kr vs. Ar lamp). Direct photoionization resulting in a molecular radical cation [M](*+) dominates at high temperatures (>400 degrees C) and low flow rates (<200 microL/min). Indirect photo-induced chemical ionization (PCI) involving solvent molecules becomes important at lower temperatures and higher solvent flow rates. Indirect PCI is enhanced using an Ar lamp, which yields comparable [M+H](+) signal but poorer [M](*+) signal than the Kr lamp at lower temperatures and higher flow rates. This is in support of our recent finding that the Ar lamp results in a solvent-dependent enhancement of analyte molecules via PCI. Analysis of 12 compounds in methanol under low-flow conditions (10 microL/min) demonstrates that the dual ESPI source performs favorably for most compounds versus the standard ESCI source, and significantly better than ESCI for the analysis of unstable drugs, like flurbiprofen. Several factors contributing to the benefits of the ESPI source are the shared optimal geometry for ESI and APPI sources and soft ionization of APPI versus APCI.
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.
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