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

Abstract

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...

Charge Exchange Reaction in Dopant-Assisted Atmospheric Pressure Chemical Ionization and Atmospheric Pressure Photoionization

“…Since DA-APPI has been reported to give the best performance at lower solvent flow rates [8,[28][29][30] than those typically used in APCI, we tested if this is the case also in DA-APCI. The flow rate of the dopant was kept at 10% of the solvent flow rate.…”

Quantitative aspects of and ionization mechanisms in positive-ion atmospheric pressure chemical ionization mass spectrometry

“…In the absence of experimental thermodynamic data for analyte and solvent clusters and to obtain the ⌬H°and ⌬G°values for the charge exchange (16) and the proton transfer (17,18) reactions when considering solvation, the enthalpy (H) and Gibbs free-energy (G) of S n (n ϭ 1-3), A, (S) n S ϩ· (n ϭ 0 -2), (S) n SH ϩ (n ϭ 0 -2), (S) n A ϩ· (n ϭ 0,1), (S) n AH ϩ (n ϭ 0,1), and S n [SϪH] · (n ϭ 0,1) (where A represents DBT, PY, CAR, or FLU, and S represents CH 3 CN or CH 3 OH) were estimated using density functional theory. Thermodynamic data were calculated for the solvent clustering Reactions (19) to (22) in the gas phase, Table 8:…”

“…Although the mechanisms of ionization associated with these techniques [13][14][15], and with APPI [16,17], are not necessarily the same as that of APCI, certain similarities do exist, and any information obtained regarding APCI may be helpful in understanding these other methods.…”

Atmospheric pressure photoionization mass spectrometry of polyisobutylene derivatives

“…Considering eqs 4 and 5, values of ϳ1 ϫ 10 Ϫ3 , ϳ5 ϫ 10 Ϫ3 , and ϳ4 ϫ 10 Ϫ3 s Ϫ1 can be calculated for k 1 in CH 2 Cl 2 , CHCl 3 , and CCl 4 , respectively. The photoabsorption cross-sections for CH 2 Cl 2 (1.3 ϫ 10 Ϫ16 cm Ϫ2 ) and CHCl 3 (4.2 ϫ 10 Ϫ17 cm Ϫ2 ) were taken from references [24,32]. Based on the model calculations, it can be expected that the resulting APPI(Ϫ)-MS signal intensities increase in the order of CH 2 Cl 2 Ͻ CCl 4 Ͻ CHCl 3 .…”

“…Very recently, a new ionization method called atmospheric pressure photoionization (APPI) has been developed to broaden the range of analytes that are useful for MS analysis, especially for compounds of low polarity [21,22]. This method has been used to successfully analyze several classes of nonpolar compounds, including polyaromatic hydrocarbons [23][24][25], lipids [26 -28], and steroids [29 -31]. This technique is based on the direct or indirect photoionization of the analyte by vacuum UV (VUV) photons generally emitted from a Kr discharge lamp.…”