US 9,583,325 B2  28  27  size determination can be a important predictor of other  transport phenomena and can be used to inform cellular  uptake rates and toxicity studies.  The method allows for monitoring changes in the com› positional distribution of compounds. The distribution of  individual components in a complex mixture can provide  useful information when monitoring the extent of chemical  weathering, the effectiveness of certain processes (both  natural and industrial) and can be applied to source identi› fication.  A study was conducted in which the effect of pH upon the  observed CP-MIMS signals for naphthenic acids (NA). Two  spike levels of NA (0.4 and 1.4 ppm total Merichem NA)  were added to a 40 mL stirred (magnetic stir bar) deionized  water sample, and the CP-MIMS signal responses were  recorded, as well as the pH of the solution. For this experi› ment, the J-Probe type CP-MIMS interface was used with a  2.0 cm length of PDMS HFM that was 170  µm  thick. FIG.  18  is an example of typical results observed for NAm/z 171.  After the signals for the 1.4 ppm total NA reached a steady  state at pH 5.2, HCl was added to the sample to yield pH 2.  Without being bound to theory, acidification of the sample  results in protonation of the naphthenic carboxylate ions to  their neutral (acidic) forms, which are considerably more  hydrophobic and thus readily partition into the PDMS  membrane. This dramatically increases the observed signal  intensity. Because the PDMS is selective for neutral ana› lytes, a simple pH adjustment yields superior analytical  results (FIG.  18).  This can be taken further, by measuring the  observed signals before and after acidification. By measur› ing the signals in a weakly buffered sample at pH 8, it is  anticipated that only neutral species present in the sample  (not NA) would be able to pass through the PDMS. A  subsequent measurement at lower pH (e.g. 2) would be the  sum of the neutral species and NA. Subsequent subtraction  of the data obtained at pH 8 from those obtained at pH 2  would yield a representation of the species capable of  accepting acid protons (e.g. just NA). As well, the measure› ment of signals obtained at several known, intermediate pH  values will allow the determination of the fractional abun- dance of protonated/not protonated species, providing a  route for the calculation of pKa values for various NA  isomer classes (m/z values). Using the data presented in  FIG.  18,  the fractional abundance of the C$_{10}$H$_{20}$ 0$_{2 }$ at  pH=5.2 is determined to be 0.33. This can be shown to  correspond to an effective pKa=4.9 for this isomer class.  Furthermore, it is anticipated that further improvements in  the transport and ionization of NAs can be obtained by  adjusting the pH of the methanol acceptor phase. Adding a  small amount base to the acceptor phase will establish a pH  gradient across the membrane, increasing the driving force  for NA transport and may also improve the ionization by  negative ion ESL  To demonstrate the capabilities of CP-MIMS for the  detection and measurement of NAs in a wide range of  typical environmental samples, 40 mL EPA type water  sample vials were filled with a variety of different water  samples obtained from northern Alberta, including oil sands  Dilute aqueous solutions of Merichem naphthenic acids  (50-1000 ppb) were analyzed using an immersion ’J-probe’  membrane interface coupled to CP-MIMS with a methanol  acceptor phase. Because naphthenic acids are a complex  mixture of structurally related isomer classes, typically  ranging in molecular weight from -150-400 atomic mass  units (amu), membrane performance (response time and  To demonstrate the capabilities of CP-MIMS for the  determination of intrinsic molecular properties such as dif› fusion coefficients and hydrodynamic volumes, the response  times of various isomer classes to a step-function increase in  upstream NA concentration was determined. The chromato- grams displayed in FIG.  20  illustrate that smaller NA  components exhibit faster response times. By establishing a  trend line with a set of training compounds of known  hydrodynamic volumes, it is anticipated that CP-MIMS can  be used to determine volumes of many components in a  complex mixture. These data are useful in predicting isomer  structures, diffusion coefficients, environmental distribution,  cellular uptake and toxicity.  process water (OSPW-1,2), ground water (GW-1,2) and  natural surface waters (SW-1,2,3). The samples were acidi› fied to pH 4.0 with 2M HCl, then stirred (magnetic stir bar)  and sequentially measured with the CP-MIMS system using  the ’J-Probe’ type interface equipped with a 2.0 cm length of  PDMS HFM that was 170 µm  thick. The sample probe was  removed from each sample when steady signals were  obtained, and the probe washed with methanol between each  sample until baseline signals were achieved. FIG.  19a  shows  SIM signal intensities (chromatograms) at m/z 221, 223 and  235 for the measurement sequence of the seven samples.  Naphthenic acid concentrations in these samples have been  verified by a solid phase extraction, chemical derivitization,  and liquid chromatography tandem mass spectrometry (LC- MS/MS) and range from very high (>10 ppm) for OSPWs to  very low (<10 ppb) for the SW samples. The signal inten› sities on the chromatograms obtained by CP-MIMS (FIG.  19a)  shows good agreement with the total NA concentra› tions obtained by a verified off-line technique. FIG.  19b.  compares the distribution of naphthenic acid isomer classes  of OSPW-2 for CP-MIMS and the liquid chromatography  tandem mass spectrometry method. Signal intensities of  each isomer class have been converted to an equivalent  concentration of pyrene butyric acid (PBA) giving rise to the  same signal. The bottom panel illustrates the reconstructed  NA concentration data from M.B. Woudneh et al. (Journal of  Chromatography A, 2013, 1293, 36) also in terms of PBA.  The CP-MIMS total NA concentration obtained directly  from sample in roughly 10 minutes agrees remarkably well  with that obtained by LC-MS/MS, which is both time and  labour intensive and requires consumable reagents and sol› vents. Results from CP-MIMS are observed to over-predict  total NA concentrations, which is acceptable for a rapid  screening method. We can further refine CP-MIMS screen- ing by running samples at pH 7 followed by an acidification  step and analysis at pH<4. At pH 7, naphthenic acids will be  present in aqueous solution in their ionized forms and will  not permeate the membrane. After the acidification step, the  NAs can be detected. By subtracting the spectral intensities  obtained at pH 7 from those at pH<4, one can remove the  non-naphthenic acid components.  EXAMPLE 21  EXAMPLE 20  EXAMPLE 22  EXAMPLE 18  EXAMPLE 19 