Instrument - Particle-into-liquid sampler (PILS) + ultra-performance liquid chromatography/electrospray ionization quadrupole time-of-flight mass spectrometry (UPLC/ESI-Q-TOF-MS)
Particle-into-liquid sampler (PILS) + ultra-performance liquid chromatography/electrospray ionization quadrupole time-of-flight mass spectrometry (UPLC/ESI-Q-TOF-MS)
What is being measured:
Negative (-) and positive (+) ion mass spectra from m/z 40 to 1000 of SOA molecular constituents in PILS samples. Analytes are detected as [M-H]- ions in (-) mode, and as both [M+H]+ ions as well as adducts with Na+ and NH4+ in (+) mode.
Custom-modified PILS; Waters (UPLC/ESI-Q-TOF-MS)
Custom-modified (PILS); ACQUITY I-Class (UPLC) with Xevo G2-S (ESI-Q-TOF-MS)
2006 (PILS); 2014 (UPLC/ESI-Q-TOF-MS)
Data recording software:
N/A (PILS); MassLynx v4.1 (UPLC/ESI-Q-TOF-MS)
Data analysis software:
N/A (PILS); MassLynx v4.1 (UPLC/ESI-Q-TOF-MS)
Raw data time resolution:
PILS samples are collected with 5-min duty cycle.
Analysis data averaging:
Low (1-10) ppbm in PILS samples.
Sensitivity to temperature (and correction method, if applicable):
Sensitivity to relative humidity (and correction method, if applicable):
injection of liquid extract
Sample preparation method:
The Caltech PILS is based on a modification of the original design of Weber et al. Briefly, chamber aerosol is sampled into the instrument through a 1 um cut size impactor at a flow rate of 12.5 L/min and passed successively through individual acid and ba
Sample residence time (chamber to instrument) (seconds):
Length of tubing (cm):
Instrument flow rate:
Tubing inner diameter:
Chemical identification method:
PILS samples are analyzed by a Waters ACQUITY UPLC I-Class system coupled to a Xevo G2-S Q-TOF-MS equipped with an ESI, source and operating at a mass resolution of 20,000-34,000 and a mass accuracy of <5 mDa. . . An ACQUITY BEH C18 column (1.7 um, 2.1 mm x 50 mm) kept at 30 C is used to separate SOA molecular constituents. The polar (A) and nonpolar (B) eluents are 0.1% v/v formic acid (98%, Fluka) in ultra-pure water (<3 ppb TOC, Millipore Milli-Q) and 100% acetonitrile (Optima LC/MS, Fisher Scientific), respectively. The 12-min eluent program is: (0-2.0 min) 99% A and 1% B; (2.0-10.0 min) linear gradient to 10% A and 90% B; (10.0-10.2 min) 10% A and 90% B; (10.2-10.5 min) linear gradient to 99% A and 1% B; (10.5-12 min) 99% A and 1% B. The total flow rate is 0.3 mL/min and the injection volume is 10 uL. The sample temperature is 4 C. Optimized ESI conditions are: 2.0 kV capillary voltage, 40 V sampling cone, 80 V source offset, 120 C source temperature, 400 C desolvation temperature, 30 L/h cone gas flow, and 650 L/h desolvation gas flow.
Data analysis method:
(-) and (+) ion mass spectra are acquired from m/z 40 to 1000, following calibration using sodium formate clusters prepared from formic acid (98%, Fluka) and sodium hydroxide (50% w/w in H2O, Fisher Scientific). The calibrated mass axis islocked to the [M-H]- or [M+H]+ ion of a lock spray of leucine enkephalin (>95%, AnaSpec). MS/MS spectra are collected in tandem with MS measurements (50% duty cycle) via collision-induced dissociation (CID; Ar collision gas) of parent ions, quadrupole-selected with an isolation width of 3 Da during specified retention time (RT) ranges, using a collision energy ramping program (15-50 V). Instrumental stability (i.e., chromatographic and mass spectral reproducibility) is verified to within 3% using standard solutions of representative analytes [e.g., cis-pinonic acid (98%, Sigma-Aldrich) for pinene SOA] run periodically (one standard every 10 samples) during routine analysis. Data are acquired and processed using MassLynx v4.1, software. Molecular formulas (CxHyOz) of parent and fragment ions in MS and MS/MS spectra are assigned with mass tolerances of <7 ppm and supported by the associated 13C isotope distributions.
Mass concentrations of individual organic compounds in chamber-generated SOA collected by PILS and analyzed off-line by UPLC/ESI-Q-TOF-MS are calculated from the following expression: C = (Ql*rho*DF*R)/(Qs*CE*IE), where C is the particle-phase mass concentration of the compound (ug/m3), Qs is the aerosol sampling flow rate (12.5 L/min), Ql is the rate of the washing flow (0.15 mL/min), DF is the dilution factor that accounts for water vapor condensation on the PILS impaction plate (assumed to be 1.1), rho is the density of the collected PILS sample (assumed to be the density of the washing flow, 1.0 g/mL), CE is the overall PILS collection efficiency for the SOA. (estimated from empirical correlation with O:C ratio), R is the UPLC/(-)ESI-Q-TOF-MS response (i.e., chromatographic. peak area) for the compound, and IE is the compound-specific ESI efficiency (1/ppb). From comparison of the resulting particle-phase mass concentrations to the SMPS-derived suspended SOA mass loading, mass fractions of identified molecular products in SOA are determined.
In the case of small molecules (<1000 Da), such as those analyzed in the current work, the ESI process is described by the ion evaporation model (IEM). Prior separation of analytes from the complex SOA matrix via UPLC precludes potential ion-source artifacts (e.g., signal suppression and noncovalent clustering), ensuring the quantitative nature of the method. Due to a lack of authentic standards, ESI efficiencies of SOA molecular constitutents are typically quantified using commercially available surrogates (e.g., cis-pinic acid and cis-pinonic acid for pinene SOA). 5-point calibration curves (n = 3) are generated from aqueous solutions of the surrogates spanning a concentration range from 50 to 1000 ng/mL (ppb); a linear response (R2 > 0.99) is generally observed. The detection limits for surrogates used to date, calculated as three times the standard deviation of the blank, are in the low (1-10) ppb range. The concentrations of the SOA constituents measured in the PILS samples typically fall well above this threshold, and well within the calibrated linear range of the instrument., . ESI efficiencies have also been estimated using a hybridized empirical and theoretical approach based on the linear model of Kruve et al. (see Kenseth et al. 2018 for details).
Calibration drift estimate:
Uncertainty estimation method:
Uncertainty in the PILS method (dPILS) arises mainly from variation in the collected liquid volume due to the existence of air bubbles, and has been estimated to be less than +/- 11%. Uncertainty associated with the chromatographic and mass spectral reproducibility of the UPLC/(-)ESI-QTOF-MS (dUPLC) is determined to be less than +/- 3% by performing replicate injections of standard solutions of representative analytes [e.g., cis-pinonic acid for pinene SOA]., Uncertainty in the measured ESI efficiency of commercial surrogates (dESI) is typically small, less than +/- 1%. An uncertainty of +/- 20% is assumed for SMPS-derived suspended SOA mass loadings (dSMPS). For molecular products in SOA quantified using commeercial surrogates, propagation of these individual uncertainties yields a total estimated relative uncertainty in the reported SOA. mass fractions (dtotal) of +/- 23%:. dtotal = SQRT[(dPILS)^2+(dUPLC)^2+(dESI)^2+(dSMPS)^2] = SQRT[(0.11^2)+(0.03^2)+(0.01^2)+(0.20^2)] = 0.23.. . For the mass fractions of SOA molecular constitutents quantified via hybridized empirical and theoretical approach based on the linear model of Kruve et al., see Kenseth et al. 2018 for details.
Link to supplemental information: