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 

## EXAMPLE 18 

## EXAMPLE 19 

## EXAMPLE 20 

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. 

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. 

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 

## EXAMPLE 21 

## EXAMPLE 22 

