## 2 Critical Analytical Parameters
### Sample Cleanup or No Cleanup  
Sample cleanup has many advantages over direct analysis, but comes at the cost of increased time, expense, potential analyte losses, and waste generation. Cleanup is usually required for complex and 'dirty' matrices such as food, soil, petroleum, and biological matrices to separate matrix co-extractives from target compounds. The sample cleanup also serves the purpose of analyte concentration and can include a solvent exchange step suitable to the instrumental technique of choice. For water samples, cleanup may not be necessary if the instrument has sufficient sensitivity to meet detection limit requirements and if the samples are relatively clean (e.g., drinking water, groundwater, and surface water). Complex samples such as OSPW can also be analyzed without prior cleanup by using LC separation before the MS, or by using HRAM MS instruments that can differentiate the masses of target ions from the background. However, ion suppression may occur in these samples and can bias results. The presence of high salt concentrations in OSPW may lead to sodium-bridged dimers and can complicate data analysis. The choice ultimately comes down to having an injectable sample or extract that contains all the compounds of interest at sufficient concentrations to measure with minimum interference and low ion suppression. Comparison of samples with and without cleanup is an important step in method development and is particularly true for the analysis of the thousands of organic compounds present in OSPW [23; 26; 27]. Selectivity in OSPW cleanup is generally carried out using one of two extraction techniques. Liquid-liquid extraction is a simple procedure where a strong base is added to the samples to ionize the weakly acidic NAFCs and keep analytes dissolved in the aqueous phase while neutrals are partitioned into DCM and discarded, removing much of the dissolved organic content (DOC) [23]. The water is then acidified to pH-1.5 using H\({}_{2}\)SO\({}_{4}\) and re-partitioned to extract the NAFCs into fresh DCM, then dried and concentrated by \begin{table}
\begin{tabular}{c c c c} \hline
**Compound** & **Additive** & **Intensity** & **Apparent pH** \\ \hline Biphenyl-4-carboxylic acid & 0.1\% NH\({}_{4}\)OH & 5.34 \(\times\) 10\({}^{10}\) & 8.91 \\ C\({}_{13}\)H\({}_{10}\)O\({}_{2}\) & 0.1\% HCOOH & 4.79 \(\times\) 10\({}^{8}\) & 3.52 \\ \hline Anthraquinone-2-carboxylic acid & 0.1\% NH\({}_{4}\)OH & 5.46 \(\times\) 10\({}^{10}\) & 9.19 \\ C\({}_{15}\)H\({}_{8}\)O\({}_{4}\) & 0.1\% HCOOH & 3.31 \(\times\) 10\({}^{9}\) & 3.68 \\ \hline Trimesic acid & 0.1\% NH\({}_{4}\)OH & 2.81 \(\times\) 10\({}^{10}\) & 8.75 \\ C\({}_{9}\)H\({}_{6}\)O\({}_{6}\) & 0.1\% HCOOH & 1.86 \(\times\) 10\({}^{10}\) & 3.59 \\ \hline \end{tabular}
\end{table} 
Table 2: Spectrum abundances by FT-ICR-MS (ESI-) on model NAs with acidic and basic pH conditions [17]. evaporation. The neutralized acids are reconstituted into a suitable solvent such as MeOH or ACN: water for MS analysis. Neutral compounds that are not ionized in either extraction procedure will be extracted into the final DCM phase based on their K\({}_{\mathrm{ow}}\) coefficients, whereas salts remain in the aqueous waste fraction. The second cleanup procedure uses SPE (described above) to selectively remove acids from samples using neutral sorbents such as divinyl benzene (DVB) [24]. A study that used a highly selective, mixed-mode polymer with additional anion exchange properties for acidic compounds (Oasis(r) MAX) [28] required no acidification of the OSPW. Also, for the MAX column cleanup procedure, the organic compounds were separated into two fractions, first using MeOH to elute the non-acidic compounds followed by acidified MeOH to elute neutralized NAs. Methanol alone eluted 10% of the DOC from the total MAX recoveries. The highest DOC recovery (95.4%) was achieved when using universal sorbent divinylbenzene hydrophilic-lipophilic balanced (DVB-HLB) cartridges and MeOH elution.