The authors would like to acknowledge the financial support of this project by The J.M. Smucker company.

<ol>
	<li>Ara&#250;jo, W. L., Nunes-Nesi, A., Nikoloski, Z., Sweetlove, L. J., Fernie, A. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Metabolic+Control+and+Regulation+of+the+Tricarboxylic+Acid+Cycle+in+Photosynthetic+and+Heterotrophic+Plant+Tissues:+TCA+Control+and+Regulation+in+Plant+Tissues.">Metabolic Control and Regulation of the Tricarboxylic Acid Cycle in Photosynthetic and Heterotrophic Plant Tissues: TCA Control and Regulation in Plant Tissues.</a> Plant Cell Environ. 35 (1), 1-21 (2012).</li><li>Finkemeier, I., Konig, A. C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Transcriptomic+Analysis+of+the+Role+of+Carboxylic+Acids+in+Metabolite+Signaling+in+Arabidopsis+Leaves.">Transcriptomic Analysis of the Role of Carboxylic Acids in Metabolite Signaling in Arabidopsis Leaves.</a> Plant Physiol. 162 (1), 239-253 (2013).</li><li>Doyle, M. P., Buchanan, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Food Microbiology: Fundamentals and Frontiers. , (2013).</li><li>T&#367;ma, P., Samcov&#225;, E., &#352;tul&#236;k, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Determination+of+the+Spectrum+of+Low+Molecular+Mass+Organic+Acids+in+Urine+by+Capillary+Electrophoresis+with+Contactless+Conductivity+and+Ultraviolet+Photometric+Detection-An+Efficient+Tool+for+Monitoring+of+Inborn+Metabolic+Disorders.">Determination of the Spectrum of Low Molecular Mass Organic Acids in Urine by Capillary Electrophoresis with Contactless Conductivity and Ultraviolet Photometric Detection-An Efficient Tool for Monitoring of Inborn Metabolic Disorders.</a> Anal Chim Acta. 685 (1), 84-90 (2011).</li><li>L&#243;pez-Bucio, J., Nieto-Jacobo, M. F., Ram&#305;&#769;rez-Rodr&#305;&#769;guez, V., Herrera-Estrella, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Organic+Acid+Metabolism+in+Plants:+From+Adaptive+Physiology+to+Transgenic+Varieties+for+Cultivation+in+Extreme+Soils.">Organic Acid Metabolism in Plants: From Adaptive Physiology to Transgenic Varieties for Cultivation in Extreme Soils.</a> Plant Sci. 160 (1), 1-13 (2000).</li><li>Cebolla-Cornejo, J., Valc&#225;rcel, M., Herrero-Mart&#236;nez, J. M., Rosell&#242;, S., Nuez, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High+Efficiency+Joint+CZE+Determination+of+Sugars+and+Acids+in+Vegetables+and+Fruits:+CE+and+CEC.">High Efficiency Joint CZE Determination of Sugars and Acids in Vegetables and Fruits: CE and CEC.</a> Electrophoresis. 33 (15), 2416-2423 (2012).</li><li>Rosello, S., Galiana-Balaguer, L., Herrero-Martinez, J. M., Maquieira, A., Nuez, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Simultaneous+Quantification+of+the+Main+Organic+Acids+and+Carbohydrates+Involved+in+Tomato+Flavour+Using+Capillary+Zone+Electrophoresis.">Simultaneous Quantification of the Main Organic Acids and Carbohydrates Involved in Tomato Flavour Using Capillary Zone Electrophoresis.</a> J Sci Food Agr. 82 (10), 1101-1106 (2002).</li><li>Wasielewska, M., Banel, A., Zygmunt, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Capillary+Electrophoresis+in+Determination+of+Low+Molecular+Mass+Organic+Acids.">Capillary Electrophoresis in Determination of Low Molecular Mass Organic Acids.</a> Int J Environ Sci Dev. 5 (4), 417-425 (2014).</li><li>Galli, V., Garc&#236;a, A., Saavedra, L., Barbas, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Capillary+Electrophoresis+for+Short-Chain+Organic+Acids+and+Inorganic+Anions+in+Different+Samples.">Capillary Electrophoresis for Short-Chain Organic Acids and Inorganic Anions in Different Samples.</a> Electrophoresis. 24 (1213), 1951-1981 (2003).</li><li>Klampfl, C. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Determination+of+Organic+Acids+by+CE+and+CEC+Methods.">Determination of Organic Acids by CE and CEC Methods.</a> Electrophoresis. 28 (19), 3362-3378 (2007).</li><li>Kenney, B. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Determination+of+Organic+Acids+in+Food+Samples+by+Capillary+Electrophoresis.">Determination of Organic Acids in Food Samples by Capillary Electrophoresis.</a> J Chromatogr A. 546, 423-430 (1991).</li><li>Galli, V., Barbas, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Capillary+Electrophoresis+for+the+Analysis+of+Short-Chain+Organic+Acids+in+Coffee.">Capillary Electrophoresis for the Analysis of Short-Chain Organic Acids in Coffee.</a> J Chromatogr A. 1032 (1-2), 299-304 (2004).</li><li>Schmitt-Kopplin, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Capillary+Electrophoresis:+Methods+and+Protocols.">Capillary Electrophoresis: Methods and Protocols.</a> Methods in Molecular Biology. , 384 (2008).</li><li>Nollet, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Chromatographic analysis of the environment 3rd ed. , (2006).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> ElixerOA Organic Acids/Anions Operating and Instruction Manual, MicroSolv Technology Corperation. , (2001).</li><li>Dahlen, J., Hagberg, J., Karlsson, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Analysis+of+low+molecular+weight+organic+acids+in+water+with+capillary+zone+electrophoresis+employing+indirect+photometric+detection.">Analysis of low molecular weight organic acids in water with capillary zone electrophoresis employing indirect photometric detection.</a> Fresenius J. Anal. Chem. 366 (5), 488-493 (2000).</li><li>Ibanez, A. B., Bauer, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Analytical+method+for+the+determination+of+organic+acids+in+dilute+acid+pretreated+biomass+hydrolysate+by+liquid+chromatography-time-of-flight+mass+spectroscopy.">Analytical method for the determination of organic acids in dilute acid pretreated biomass hydrolysate by liquid chromatography-time-of-flight mass spectroscopy.</a> Biotech. For Biofuels. 7 (145), (2014).</li></ol>

The authors declare that they have no competing financial interests.

As with any analytical technique, there are several critical factors that can significantly impact the quality and reliability of the data generated. First, it is important to process samples efficiently, with a minimum of freeze/thaw cycles. Repeated freezing and thawing can compromise the chemical composition of the sample before processing or analysis. Second, it is critical to apply the steps of this protocol to all samples consistently and evenly. Technical errors arising from inconsistent sample preparation and handling can significantly impact the quality of the data generated, and result in increased &#34;noise&#34; in the SCCA measurements. For example, the particle size of the sample post-grinding will impact the speed and efficiency of SCCA extraction. It is therefore critical to ensure that the samples are ground to a uniform particle size, as this will maintain extraction efficiency across samples and help to reduce sample-to-sample variability. Similarly, accurate measurement and pipetting during the preparation of standard solutions, accurate measurement of sample masses, and careful monitoring (and normalization) of extraction times will result in the generation of more uniform data. Taking time to carefully normalize, measure, and record each of these parameters will ensure the reliability of the data and allow for accurate post-run correction of data for any processing errors which may occur. Third, it is critical to select and use an appropriate internal standard. Calculations of the final analyte concentration will rely heavily on accurate correction based on the peak area of the internal standard observed in each sample. Because of this, it is critical that the internal standard be both accurately and precisely measured; and should ideally not occur naturally in the sample or co-elute with other compounds. Not only does the proper selection and use of an internal standard in all samples enable the researcher to control for changes in metabolite levels observed due to differences in extraction efficiency; it also greatly simplifies downstream processing and peak identification. Finally, it is necessary to monitor the peak identification and integration process. While automated processing algorithms are useful, they are not perfect. The data generated by this technique rely on the accuracy of integration, and the traces must be checked manually to ensure the quality of integration events, particularly the definition of peak boundaries and baselines.
As with any analytical procedure, it is important to establish that the CZE method presented here is: a. accurate: able to determine the amount of a given SCCA present; b. precise: capable of reproducibly quantifying SCCAs within the same day and in measurements made across several days of analysis; and c. robust: capable of recovering most of the SCCAs present in the samples being analyzed. As shown above in the representative results, the method described here is capable of accurately determining the amounts of SCCAs present in samples. All the SCCAs measured (citric, malic, acetic, and lactic acid) and the internal standard (adipic acid) exhibited a linear response in the 0-80 ng/&#181;l concentration range. Additionally, the LOQs and LODs for these acids ranged from 2-4 ng/&#181;l and 1-2 ng/&#181;l, respectively. The method was also precise, as all of the SCCAs measured and the adipic acid internal standard exhibited low intra-day variability, falling between 2.0-5.5% of the peak area (or between 1.0-5.2% of the calculated concentration) for all SCCAs measured. The inter-day variability of SCCA measurements was also relatively low, falling between 3.05-10.10% of the peak area (or between 3.30-10.95% of the calculated concentration) for all SCCAs measured. Interestingly, with the exception of acetic acid, which was consistently at or close to the limit of detection/quantification in all coffee samples analyzed, correcting samples using adipic acid as an internal standard resulted in more reproducible measurements and a decrease in the coefficient of variability (see Representative Results, above). These data are consistent with previously published results indicating that correction with an internal standard is essential in maintaining precision in CE analyses over time16. While some methods have employed inorganic ions as an internal standard, we felt that adipic acid was a more appropriate standard for the CZE method presented here, as it is an organic acid and therefore more likely to experience loss in the sample preparation steps similar to that experienced by the SCCAs of interest in the sample. Adipic acid had the additional advantages of not being present in the coffee samples being examined, exhibiting a linear response across the same concentrations used for SCCAs in this study, exhibiting relatively low LOD/LOQ values (1 ng/&#181;l and 4 ng/&#181;l, respectively), and a high percent recoverability from the coffee matrix (100.19% &#177; 2.57), making it a useful standard for quantifying SCCAs in coffee samples. Interestingly, the high percent recovery observed for adipic acid was achievable for all organic acids measured in this study, which exhibited percent recovery values ranging from 91.94-106.08%. These data indicate that the CZE method presented here is capable of recovering most of the SCCAs present in coffee samples.
Adapting this protocol to new sample types requires the collection of preliminary data, based on literature or empirical evidence, concerning the expected amounts of targeted analytes. This information will help guide the experimental design, sample preparation, and standard curve construction. Determining the correct sample dilution to use can be easily accomplished empirically by running a dilution series of a test extract to find a proper working dilution. This protocol was designed to detect the SCCAs outlined in the example results, but it could easily be adapted to detect other SCCAs by altering the separation parameters, such as separation voltage, pressure, or time. It is important to remember, however, that altering the separation parameters may alter the number of runs that can be accomplished on a single set of separation buffers as these buffers degrade with use. Baseline degradation or changes in current are often manifestations of over-used/depleted buffers. Replacing the running buffer and reconditioning the capillary can often resolve these problems. After extended use, the ability of the capillary to resolve analytes will decrease as well. Decreased baseline stability, drastic shifts in retention time, and reduced peak resolution are all signs that the capillary may need to be replaced. It is also possible to receive a pressure error resulting from improper sealing between the sample vial and the capillary due to an ill-fitting vial cap or a defective vial. Ensure the caps fit snugly into the sample vials to prevent this error.
Traditionally, chromatographic methods such as GC-MS, GC-FID, or HPLC have been used to detect SCCAs in a wide range of matrices, including air, water, soil, and plant samples14. While effective, a major drawback in these methods is the need to derivatize SCCAs before detection. Derivatization uses hazardous reagents the limit of detection is often affected by the efficiency of the derivatization reaction13,14. The detection of non-derivatized SCCAs through CZE avoids analyte loss during processing, exposure to hazardous derivatizing agents, and reduces sample preparation time. These benefits can be seen in the high percent recovery rates (ranging between 91.94-106.08%) calculated for all SCCAs measured in this study. In addition, CZE quantification of SCCAs achieves limits of detection comparable to those reported in literature for GC-MS, GC-FID, and HPLC, in the range of parts per million to parts per billion13,14. When combined with high-speed run times, high-resolution separation power, the straight-forward sample processing protocols employed by this method of analysis, the sensitivity and convenience of CZE make it an attractive alternative to traditional GC or HPLC methods.
However, it is important to note that, while CZE provides several advantages over GC-MS HPLC, or LC-MS/MS based methods of SCCA analysis, it is not appropriate for all SCCA analyses. For example, since the CZE method described here relies solely on spectrophotometric detection, it does not allow for the identification of unknown SCCAs present in the sample, or the confirmation of SCCA identity via mass spectral profiling. Because of this, GC-MS or LC-MS/MS methods would be better suited for SCCA analyses in samples containing large numbers of unknown SCCAs, or samples in which it is critical to confirm the identity of all SCCAs present. Additionally, as previously reported LC-MS/MS based methods allow for lower limits of detection (LODs)-on average, 0.05 &#181;g/ml for LC-MS/MS17 vs. the 1 &#181;g/ml for CZE obtained using the method presented here-LC-MS/MS based quantification of SCCAs may be more appropriate for samples in which SCCAs are present in very low or trace amounts.
The protocol presented here focuses on the extraction and quantification of short chain carboxylic acids (SCCAs) from plant tissues. Obtaining proficiency in this technique will open the door to a wide range of applications of this technology for the individual researcher. This technique, with only slight variations in methodology, has been successfully employed in the analysis of SCCAs in a diverse population of samples and substrates, ranging from food products and tissue samples to environmental water and soil samples8. Additionally, gaining familiarity with the chemical principals underlying this technology though this protocol will provide researchers with the knowledge base needed to attempt the analysis of other compounds, such as carbohydrates or metals8,13. The analytical flexibility of capillary electrophoresis makes it an extraordinarily powerful tool suitable for a myriad of experimental applications.

Carboxylic acids are organic compounds containing one or more terminal carboxyl functional groups, each attached to an R-group containing one or more carbons (R-C[O]OH). Short chain, low molecular weight carboxylic acids (short chain carboxylic acids, SCCAs) containing between one and six carbons, are essential components of cellular respiration, and function in several biochemical pathways necessary for cell growth and development. SCCAs play critical roles in cellular metabolism1, cell signaling2, and organismal responses to the environment (such as antibiosis3). Because of this, SCCAs can serve as useful indicators of disruptions to cellular metabolism, plant stress responses4,5, and fruit quality6,7. To date, SCCAs have been quantified primarily through chromatographic techniques such as high performance liquid chromatography (HPLC) or gas chromatography-mass spectroscopy (GC-MS). While these methods, are capable of achieving very low limits of detection, they can be expensive, require the derivatization of target SCCAs using caustic and/or toxic reagents, and include lengthy separation runs on the GC or HPLC. Because of this, interest in the use of free zonal capillary electrophoresis (CZE), which does not require sample derivatization, to quantify organic acids has steadily increased8.
Free zonal capillary electrophoresis (CZE) is a chromatographic separation methodology that, due to its high number of theoretical plates, speed, and relative ease-of-use, is increasingly replacing both GC-MS and high-pressure liquid chromatography as an analytical method for the quantification (particularly for quality control purposes) of anions, cations, amino acids, carbohydrates, and short chain carboxylic acids (SCCAs)8,9,10. CZE-based separation of small molecules, including SCCAs, is based two primary principles: the electrophoretic movement of charged ions in an electrical field established across the buffer filling the capillary; and the electro-osmotic movement of the entire buffer system from one end of the capillary to the other, generally towards the negative electrode. In this system, small molecules move towards the negative electrode at varying speeds, with the speed of each molecule determined by the ratio of the net charge of the molecule to the molecular mass. As the movement of each individual molecule in this system is dependent on the charge state of the molecule and the overall rate of electro-osmotic flow (which is itself based on the ion content of the buffer used to fill the capillary), the buffer pH and ionic composition heavily impact the degree to which molecules can be efficiently separated using CZE. Because of this, SCCAs, with their relatively high charge-to-mass ratios, are ideal targets for CZE-based separation. Metabolites separated using CZE can be detected using a variety of methods, including UV absorbance, spectral absorbance (which is generally performed using a photo-diode array [PDA]), and/or mass spectroscopy (CE-MS or CE-MS/MS)8. The diversity of separation and detection methods provided by CZE makes it an extremely flexible and adaptable technique. Because of this, CZE has been increasingly applied as a standard method of analysis in the areas of food safety and quality11,12, pharmaceutical research13, and environmental monitoring13,14. 
Capillary electrophoresis has been used to detect and quantify short chain carboxylic acids for nearly two decades13. The resolving power (particularly for small, charged molecules), short run time, and low per sample cost of CZE analyses make CZE an ideal technique for the separation and quantification of SCCAs13. This method presents a protocol to utilize CZE to measure the concentration of organic acids from plant tissues. Example data was generated through the successful implementation of this protocol to measure the change in organic acid levels in coffee seeds following a secondary fermentation treatment. The protocol details the critical steps and common errors of CZE-based separation of SCCAs, and discusses the means by which this protocol can be successfully applied to quantify SCCAs in additional plant tissues.

<table border="0" cellpadding="0" cellspacing="0" width="782">
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		<col />
	</colgroup>
	<tbody>
		<tr height="19">
			<td height="19" style="height:19px;width:296px;">Ceramic Moarter and Pestle</td>
			<td style="width:123px;">Coorstek</td>
			<td style="width:119px;">60310</td>
			<td style="width:247px;"></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:296px;">Beckman Coulter P/ACE MDQ CE system</td>
			<td style="width:123px;">Beckman Coulter</td>
			<td style="width:119px;">Various</td>
			<td style="width:247px;"></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:296px;">Glass sample vials</td>
			<td style="width:123px;">Fisher Inc.</td>
			<td style="width:119px;">033917D</td>
			<td style="width:247px;"></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:296px;">1.5 ml microcentrifuge tubes&#160;</td>
			<td style="width:123px;">Fisher Inc.</td>
			<td style="width:119px;">02-681-5</td>
			<td style="width:247px;"></td>
		</tr>
		<tr height="38">
			<td height="38" style="height:38px;width:296px;">LC/MS grade water</td>
			<td style="width:123px;">Fisher Inc.</td>
			<td style="width:119px;">W6-1</td>
			<td style="width:247px;">Milli-Q water (18.2 M&#937;.cm) is also acceptable</td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:296px;">15 ml glass tube/ Teflon lined cap&#160;</td>
			<td style="width:123px;">Fisher Inc.</td>
			<td style="width:119px;">14-93331A</td>
			<td style="width:247px;"></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:296px;">Parafilm M</td>
			<td style="width:123px;">Fisher Inc.</td>
			<td style="width:119px;">13-374-12</td>
			<td style="width:247px;"></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:296px;">CElixirOA detection Kit pH 5.4&#160;</td>
			<td style="width:123px;">MicroSolv</td>
			<td style="width:119px;">06100-5.4</td>
			<td style="width:247px;"></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:296px;">BD Safety-Lok syringes</td>
			<td style="width:123px;">Fisher Inc.</td>
			<td style="width:119px;">14-829-32</td>
			<td style="width:247px;"></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:296px;">17 mm Target Syringe filter, PTFE</td>
			<td style="width:123px;">Fisher Inc.</td>
			<td style="width:119px;">3377154</td>
			<td style="width:247px;"></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:296px;">32 Karat, V. 8.0 control software</td>
			<td style="width:123px;">Beckman Coulter</td>
			<td style="width:119px;">285512</td>
			<td style="width:247px;"></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:296px;">capillary electrophoresis (CE) sample vials&#160;</td>
			<td style="width:123px;">Beckman Coulter</td>
			<td style="width:119px;">144980</td>
			<td style="width:247px;"></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:19px;width:296px;">caps for CE vials&#160;</td>
			<td style="width:123px;">Beckman Coulter</td>
			<td style="width:119px;">144648</td>
			<td style="width:247px;"></td>
		</tr>
		<tr height="38">
			<td height="38" style="height:38px;width:296px;">Liquid Nitrogen</td>
			<td style="width:123px;">N/A</td>
			<td style="width:119px;">N/A</td>
			<td style="width:247px;">Liquid nitrogen is available from most facilities services</td>
		</tr>
	</tbody>
</table>

using capillary electrophoresis to quantify organic acids from plant tissue: a test case examining coffea arabica seeds

Carboxylic acids are organic acids containing one or more terminal carboxyl (COOH) functional groups. Short chain carboxylic acids (SCCAs; carboxylic acids containing three to six carbons), such as malate and citrate, are critical to the proper functioning of many biological systems, where they function in cellular respiration and can serve as indicators of cell health. In foods, organic acid content can have significant impact on taste, with increased SCCA levels resulting in a sour or &#34;acid&#34; taste. Because of this, methods for the rapid analysis of organic acid levels are of particular interest to the food and beverage industries. Unfortunately, however, most methods used for SCCA quantification are dependent on time-consuming protocols requiring the derivatization of samples with hazardous reagents, followed by costly chromatographic and/or mass spectrometric analyses. This method details an alternate method for the detection and quantification of organic acids from plant material and food samples using free zonal capillary electrophoresis (CZE), sometimes simply referred to as capillary electrophoresis (CE). CZE provides a cost-effective method for measuring SCCAs with a low limit of detection (0.005 mg/ml). This article details the extraction and quantification of SCCAs from plant samples. While the method provided focuses on measurement of SCCAs from coffee beans, the method provided can be applied to multiple plant-based food materials.

This protocol has been successfully utilized to measure the effects of seed treatments on the SCCA content of green coffee seeds. In this experiment, the six treatments were: a saturated microbial suspension of Leuconostoc pseudomesenteroides strain GCP674 in its growth medium (1), an aqueous suspension of GCP674 microbes in water (2), an aqueous solution of acetic and lactic acids (0.15 and 0.4 mg/ml respectively) (3), a spent M1 growth medium treatment (4), dH2O water (5), and an un-treated control (no substance added to seeds) (6). Treatments were applied and allowed to ferment for 24 hours. Four independent replicates were assessed for each treatment. Analysis was conducted for citric (C), malic (M), acetic (A) and lactic (L) acid levels using adipic acid as an internal standard.
For each SCCA and the internal standard, standard curves spanning the range of concentrations predicted to be in coffee samples (5, 10, 20, 40, 60, and 80 ng/&#181;l) were generated. As expected, each acid exhibited a linear response for this concentration range with R-squared values of 0.9876 for citric acid, 0.9987 for malic acid, 0.9998 for acetic acid, and 0.9999 for lactic acid. The internal standard (adipic acid) also exhibited a linear response over this concentration range, yielding an R-squared value of 0.9984. Prior to sample analyses, limits of detection (LOD) and limits of quantitation (LOQ) were determined for each SCCA and the internal standard (adipic acid). The limit of detection of citric, malic, acetic, and adipic acid was 1 ng/&#181;l; and the LOD for lactic acid was 2 ng/&#181;l. The limit of quantitation of citric, adipic, acetic, and lactic acid was 4 ng/&#181;l; and the LOQ for malic acid was 2 ng/&#181;l. To calculate the percent recovery SCCAs and adipic acid, coffee was spiked with individual SCCAs or adipic acid and processed using the protocol described above. The percent recovery was then calculated by using the following formula ([{peak area spiked sample - peak area control sample}/theoretical peak area of calculated concentration of spiked sample {generated by analyzing buffer + SCCA/adipic acid spike}] x 100). Using this method, we calculated percent recoveries of 106.08% &#177; 12.66 for citric acid, 98.35% &#177; 1.15 for malic acid, 91.94% &#177; 3.07 for acetic acid, 97.42% &#177; 1.48 for lactic acid, and 100.19% &#177; 2.57 for adipic acid.
To determine the precision of the method, the intra- and inter-day variability of measurements made using CZE was also calculated. To determine intra-day variability, samples of each SCCA and adipic acid were measured from a single coffee sample five times across a 24-hour period, and coefficients of variability (COV; calculated by dividing the standard deviation in peak area [or concentration] for each SCCA by the average peak area [or concentration] for that SCCA, and then multiplying the result by 100) for each compound were calculated using the peak area. To assess the importance of correcting samples using the internal standard, the coefficient of variance for each SCCA was also calculated using the concentrations of each SCCA calculated using adipic acid as an internal standard. As expected, the CZE method was able to precisely measure SCCAs and adipic acid, with COVs of 3.02% for citric acid, 2.64% for malic acid, 3.74% for acetic acid, and 2.01% for adipic acid (lactic acid was below LOD/LOQ for all coffee samples measured). These measurements were repeated across a five-day period, and the CVs ranged from 2.11-5.25% for citric acid, 2.01-5.32% for malic acid, 1.72-3.74% for acetic acid, and 1.26-3.82% for adipic acid. When peak areas were corrected using the internal standard and calculated concentrations of SCCA were used to calculate COVs, the intra-day COVs ranged from 1.08-4.17% for citric acid, 1.47-3.40% for malic acid, and 2.68-5.10% for acetic acid. To assess inter-day variability, SCCAs in a single coffee sample were measured once per day across a five-day period. This experiment was then repeated five times for each SCCA and adipic acid. The COV for each SCCA was then calculated by dividing the standard deviation in peak area (or concentration) for each SCCA by the average peak area (or concentration) for that SCCA and multiplying by 100. To assess the importance of correcting samples using the internal standard, COVs were also calculated using the concentrations of each SCCA calculated using adipic acid as an internal standard. The CZE method was able to reliably reproduce SCCA measurements, with COVs ranging from 5.24-10.02% for citric acid, 6.55-9.47% for malic acid, 7.67-8.63% for acetic acid, and 3.08-6.57% for adipic acid. When peak areas were corrected using the internal standard and calculated concentrations of SCCA were used to calculated COVs, the inter-day COVs ranged from 4.56-6.23% for citric acid, 3.39-4.99% for malic acid, and 9.5-10.94% for acetic acid.
<img alt="Figure 2" src="/files/ftp_upload/54611/54611fig2.jpg" /> 
Figure 2: Example PDA traces with peaks indicated. Green coffee samples were diluted 1:10 before loading into CE vials. Acids of interest are indicated: acetic acid (A), citric acid (C), lactic acid (L), malic acid (M) and the internal standard, adipic acid (IS). <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54611/54611fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Statistical analysis was conducted on corrected acid concentrations using a general linear model (GLM) to determine whether or not treatments altered organic acid levels. A model incorporating &#34;treatment&#34; x &#34;run&#34; (coded as a random factor) was implemented for the GLM. Tukey pairwise comparisons at 95% confidence were used to determine the directionality of changes.
Treatment altered SCCA concentrations in the green coffee. Acetic acid was significantly enriched in all treatments over the no treatment control (GLM, p &#60;0.0001).
<img alt="Figure 3" src="/files/ftp_upload/54611/54611fig3.jpg" /> 
Figure 3: Impact of treatment on organic acid levels in green coffee. Treatment had no impact on (a) citric or malic acid levels in green coffee. However, all treatments significantly increased (b) acetic acid levels compared to no treatment controls (GLM, p &#60;0.001). Increases in acetic acid levels were greatest with the microbe + medium treatment. Letters indicate significant difference by Tukey pairwise comparisons at 95%. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54611/54611fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
The greatest increase was observed in the microbe + media treatment, which increased 0.25-fold (0.5 vs 0.4 mg/g coffee in GCP674 vs. NT). A similar trend was observed in lactic acid levels, though they were not significantly different. There were no significant changes or trends in the other acids tracked (citric and malic acid).

Watch this Scientific Journal Video about Using Capillary Electrophoresis to Quantify Organic Acids from Plant Tissue: A Test Case Examining Coffea arabica Seeds at JoVE.com

Using Capillary Electrophoresis to Quantify Organic Acids from Plant Tissue: A Test Case Examining Coffea arabica Seeds

This article presents a method for the detection and quantification of organic acids from plant material using free zonal capillary electrophoresis. An example of the potential application of this method, determining the effects of a secondary fermentation on organic acid levels in coffee seeds, is provided.

Using Capillary Electrophoresis to Quantify Organic Acids from Plant Tissue: A Test Case Examining Coffea arabica Seeds

Carboxylic acids are organic acids containing one or more terminal carboxyl (COOH) functional groups. Short chain carboxylic acids (SCCAs; carboxylic ...

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Nanyang Technological University

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9.8K Views.  The Ohio State University, OARDC. This article presents a method for the detection and quantification of organic acids from plant material using free zonal capillary electrophoresis. An example of the potential application of this method, determining the effects of a secondary fermentation on organic acid levels in coffee seeds, is provided.

Video: Using Capillary Electrophoresis to Quantify Organic Acids from Plant Tissue: A Test Case Examining Coffea arabica Seeds

Qualitative Identification of Carboxylic Acids, Boronic Acids, and Amines Using Cruciform Fluorophores

Molecular cruciforms are X-shaped systems in which two conjugation axes intersect at a central core. If one axis of these molecules is substituted with electron-donors, and the other with electron-acceptors, cruciforms&#39; HOMO will localize along the electron-rich and LUMO along the electron-poor axis. This spatial isolation of cruciforms&#39; frontier molecular orbitals (FMOs) is essential to their use as sensors, since analyte binding to the cruciform invariably changes its HOMO-LUMO gap and the associated optical properties. Using this principle, Bunz and Miljani&#263; groups developed 1,4-distyryl-2,5-bis(arylethynyl)benzene and benzobisoxazole cruciforms, respectively, which act as fluorescent sensors for metal ions, carboxylic acids, boronic acids, phenols, amines, and anions. The emission colors observed when these cruciform are mixed with analytes are highly sensitive to the details of analyte&#39;s structure and - because of cruciforms&#39; charge-separated excited states - to the solvent in which emission is observed. Structurally closely related species can be qualitatively distinguished within several analyte classes: (a) carboxylic acids; (b) boronic acids, and (c) metals. Using a hybrid sensing system composed from benzobisoxazole cruciforms and boronic acid additives, we were also able to discern among structurally similar: (d) small organic and inorganic anions, (e) amines, and (f) phenols. The method used for this qualitative distinction is exceedingly simple. Dilute solutions (typically 10-6 M) of cruciforms in several off-the-shelf solvents are placed in UV/Vis vials. Then, analytes of interest are added, either directly as solids or in concentrated solution. Fluorescence changes occur virtually instantaneously and can be recorded through standard digital photography using a semi-professional digital camera in a dark room. With minimal graphic manipulation, representative cut-outs of emission color photographs can be arranged into panels which permit quick naked-eye distinction among analytes. For quantification purposes, Red/Green/Blue values can be extracted from these photographs and the obtained numeric data can be statistically processed.

Cross-conjugated cruciform fluorophores based on 1,4-distyryl-2,5-bis(arylethynyl)benzene and benzobisoxazole nuclei can be used to qualitatively identify diverse Lewis acidic and Lewis basic analytes. This method relies on the differences in emission colors of the cruciforms that are observed upon analyte addition. Structurally closely related species can be distinguished from each other.

Molecular cruciforms are X-shaped systems in which two conjugation axes intersect at a central core. If one axis of these molecules is substituted with ...

Thin-layer Chromatographic (TLC) Separations and Bioassays of Plant Extracts to Identify Antimicrobial Compounds

A common screen for plant antimicrobial compounds consists of separating plant extracts by paper or thin-layer chromatography (PC or TLC), exposing the chromatograms to microbial suspensions (e.g. fungi or bacteria in broth or agar), allowing time for the microbes to grow in a humid environment, and visualizing zones with no microbial growth.&#160;The effectiveness of this screening method, known as bioautography, depends on both the quality of the chromatographic separation and the care taken with microbial culture conditions.&#160;This paper describes standard protocols for TLC and contact bioautography&#160;with a novel application to amino acid-fermenting bacteria. The extract is separated on flexible (aluminum-backed) silica TLC plates, and bands are visualized under ultraviolet (UV) light.&#160;Zones are cut out and incubated face down onto agar inoculated with the test microorganism.&#160;Inhibitory bands are visualized by staining the agar plates with tetrazolium red.&#160;The method is applied to the separation of red clover (Trifolium pratense cv. Kenland) phenolic compounds and their screening for activity against Clostridium sticklandii, a hyper ammonia-producing bacterium (HAB) that is native to the bovine rumen.&#160;The TLC methods apply to many types of plant extracts&#160;and other bacterial species (aerobic or anaerobic), as well as fungi, can be used as test organisms if culture conditions are modified to fit the growth requirements of the species.

Thin-layer Chromatographic TLC Separations and Bioassays of Plant Extracts to Identify Antimicrobial Compounds

Methods are described for thin-layer chromatographic (TLC) separation of plant extracts and contact bioautography to identify antibacterial metabolites. The methods are applied to the screening of red clover phenolic compounds inhibiting hyper&#160;ammonia-producing bacteria (HAB) native to the bovine rumen.

A common screen for plant antimicrobial compounds consists of separating plant extracts by paper or thin-layer chromatography (PC or TLC), exposing the ...

The Use of Gas Chromatography to Analyze Compositional Changes of Fatty Acids in Rat Liver Tissue during Pregnancy

Gas chromatography (GC) is a highly sensitive method used to identify and quantify the fatty acid content of lipids from tissues, cells, and plasma/serum, yielding results with high accuracy and high reproducibility. In metabolic and nutrition studies GC allows assessment of changes in fatty acid concentrations following interventions or during changes in physiological state such as pregnancy. Solid phase extraction (SPE) using aminopropyl silica cartridges allows separation of the major lipid classes including triacylglycerols, different phospholipids, and cholesteryl esters (CE). GC combined with SPE was used to analyze the changes in fatty acid composition of the CE fraction in the livers of virgin and pregnant rats that had been fed various high and low fat diets. There are significant diet/pregnancy interaction effects upon the omega-3 and omega-6 fatty acid content of liver CE, indicating that pregnant females have a different response to dietary manipulation than is seen among virgin females.

Pregnancy leads to significant changes to the fatty acid composition of maternal tissues. Lipid profiles can be obtained via gas chromatography to allow identification and quantification of fatty acids in individual lipid classes among rats fed various high and low fat diets during pregnancy.

Gas chromatography (GC) is a highly sensitive method used to identify and quantify the fatty acid content of lipids from tissues, cells, and plasma/serum, ...

Analysis of Volatile and Oxidation Sensitive Compounds Using a Cold Inlet System and Electron Impact Mass Spectrometry

This video presents a protocol for the mass spectrometrical analysis of volatile and oxidation sensitive compounds using electron impact ionization. The analysis of volatile and oxidation sensitive compounds by mass spectrometry is not easily achieved, as all state-of-the-art mass spectrometric methods require at least one sample preparation step, e.g., dissolution and dilution of the analyte (electrospray ionization), co-crystallization of the analyte with a matrix compound (matrix-assisted laser desorption/ionization), or transfer of the prepared samples into the ionization source of the mass spectrometer, to be conducted under atmospheric conditions. Here, the use of a sample inlet system is described which enables the analysis of volatile metal organyls, silanes, and phosphanes using a sector field mass spectrometer equipped with an electron impact ionization source. All sample preparation steps and the sample introduction into the ion source of the mass spectrometer take place either under air-free conditions or under vacuum, enabling the analysis of compounds highly susceptible to oxidation. The presented technique is especially of interest for inorganic chemists, working with metal organyls, silanes, or phosphanes, which have to be handled using inert conditions, such as the Schlenk technique. The principle of operation is presented in this video.

This video presents a protocol for the mass spectrometrical analysis of volatile and oxidation sensitive compounds using electron impact ionization. The presented technique is especially of interest for inorganic chemists, working with metal organyls, silanes, or phosphanes which have to be handled using inert conditions, such as the Schlenk technique.

This video presents a protocol for the mass spectrometrical analysis of volatile and oxidation sensitive compounds using electron impact ionization. The ...

Synthesis and Characterization of Functionalized Metal-organic Frameworks

Metal-organic frameworks have attracted extraordinary amounts of research attention, as they are attractive candidates for numerous industrial and technological applications. Their signature property is their ultrahigh porosity, which however imparts a series of challenges when it comes to both constructing them and working with them. Securing desired MOF chemical and physical functionality by linker/node assembly into a highly porous framework of choice can pose difficulties, as less porous and more thermodynamically stable congeners (e.g., other crystalline polymorphs, catenated analogues) are often preferentially obtained by conventional synthesis methods. Once the desired product is obtained, its characterization often requires specialized techniques that address complications potentially arising from, for example, guest-molecule loss or preferential orientation of microcrystallites. Finally, accessing the large voids inside the MOFs for use in applications that involve gases can be problematic, as frameworks may be subject to collapse during removal of solvent molecules (remnants of solvothermal synthesis). In this paper, we describe synthesis and characterization methods routinely utilized in our lab either to solve or circumvent these issues. The methods include solvent-assisted linker exchange, powder X-ray diffraction in capillaries, and materials activation (cavity evacuation) by supercritical CO2 drying. Finally, we provide a protocol for determining a suitable pressure region for applying the Brunauer-Emmett-Teller analysis to nitrogen isotherms, so as to estimate surface area of MOFs with good accuracy.

Synthesis, activation, and characterization of intentionally designed metal-organic framework materials is challenging, especially when building blocks are incompatible or unwanted polymorphs are thermodynamically favored over desired forms. We describe how applications of solvent-assisted linker exchange, powder X-ray diffraction in capillaries and activation via supercritical CO2 drying, can address some of these challenges.

Metal-organic frameworks have attracted extraordinary amounts of research attention, as they are attractive candidates for numerous industrial and ...

Capillary Electrophoresis to Monitor Peptide Grafting onto Chitosan Films in Real Time

Free-solution capillary electrophoresis (CE) separates analytes, generally charged compounds in solution through the application of an electric field. Compared to other analytical separation techniques, such as chromatography, CE is cheap, robust and effectively requires no sample preparation (for a number of complex natural matrices or polymeric samples). CE is fast and can be used to follow the evolution of mixtures in real time (e.g., chemical reaction kinetics), as the signals observed for the separated compounds are directly proportional to their quantity in solution.
Here, the efficiency of CE is demonstrated for monitoring the covalent grafting of peptides onto chitosan films for subsequent biomedical applications. Chitosan&#39;s antimicrobial and biocompatible properties make it an attractive material for biomedical applications such as cell growth substrates. Covalently grafting the peptide RGDS (arginine - glycine - aspartic acid - serine) onto the surface of chitosan films aims at improving cell attachment. Historically, chromatography and amino acid analysis have been used to provide a direct measurement of the amount of grafted peptide. However, the fast separation and absence of sample preparation provided by CE enables equally accurate yet real-time monitoring of the peptide grafting process. CE is able to separate and quantify the different components of the reaction mixture: the (non-grafted) peptide and the chemical coupling agents. In this way the use of CE results in improved films for downstream applications.
The chitosan films were characterized through solid-state NMR (nuclear magnetic resonance) spectroscopy. This technique is more time-consuming and cannot be applied in real time, but yields a direct measurement of the peptide and thus validates the CE technique.

Free solution capillary electrophoresis is a fast, cheap and robust analytical method that enables the quantitative monitoring of chemical reactions in real time. Its utility for rapid, convenient and precise analysis is demonstrated here through analysis of covalent peptide grafting onto chitosan films for improved cell adhesion.

Free-solution capillary electrophoresis (CE) separates analytes, generally charged compounds in solution through the application of an electric field. ...

Grafting Multiwalled Carbon Nanotubes with Polystyrene to Enable Self-Assembly and Anisotropic Patchiness

We demonstrate a straightforward protocol to graft pristine multiwalled carbon nanotubes (MWCNTs) with polystyrene (PS) chains at the sidewalls through a free-radical polymerization strategy to enable the modulation of the nanotube surface properties and produce supramolecular self-assembly of the nanostructures. First, a selective hydroxylation of the pristine nanotubes through a biphasic catalytically mediated oxidation reaction creates superficially distributed reactive sites at the sidewalls. The latter reactive sites are subsequently modified with methacrylic moieties using a silylated methacrylic precursor to create polymerizable sites. Those polymerizable groups can address further polymerization of styrene to produce a hybrid nanomaterial containing PS chains grafted to the nanotube sidewalls. The polymer-graft content, amount of silylated methacrylic moieties introduced and hydroxylation modification of the nanotubes are identified and quantified by Thermogravimetric Analysis (TGA). The presence of reactive functional groups hydroxyl and silylated methacrylate are confirmed by Fourier Transform Infrared Spectroscopy (FT-IR). Polystyrene-grafted carbon nanotube solutions in tetrahydrofuran (THF) provide wall-to-wall collinearly self-assembled nanotubes when cast samples are analyzed by transmission electron microscopy (TEM). Those self-assemblies are not obtained when suitable blanks are similarly cast from analogous solutions containing non-grafted counterparts. Therefore, this method enables the modification of the nanotube anisotropic patchiness at the sidewalls which results into spontaneous auto-organization at the nanoscale.

A procedure for the synthesis of polystyrene-grafted multiwalled carbon nanotubes using successive chemical modification steps to selectively introduce the polymer chains to the sidewalls and their self-assembly via anisotropic patchiness is presented.

We demonstrate a straightforward protocol to graft pristine multiwalled carbon nanotubes (MWCNTs) with polystyrene (PS) chains at the sidewalls through a ...

Using Cyclic Voltammetry, UV-Vis-NIR, and EPR Spectroelectrochemistry to Analyze Organic Compounds

Cyclic voltammetry (CV) is a technique used in the analysis of organic compounds. When this technique is combined with electron paramagnetic resonance (EPR) or ultraviolet-visible and near-infrared (UV-Vis-NIR) spectroscopies, we obtain useful information such as electron affinity, ionization potential, band-gap energies, the type of charge carriers, and degradation information that can be used to synthesize stable organic electronic devices. In this study, we present electrochemical and spectroelectrochemical methods to analyze the processes occurring in active layers of an organic device as well as the generated charge carriers.

In this article, we describe electrochemical, electron paramagnetic resonance, and ultraviolet-visible and near-infrared spectroelectrochemical methods to analyze organic compounds for application in organic electronics.

Cyclic voltammetry (CV) is a technique used in the analysis of organic compounds. When this technique is combined with electron paramagnetic resonance ...

Microprobe Capillary Electrophoresis Mass Spectrometry for Single-cell Metabolomics in Live Frog (Xenopus laevis) Embryos

The quantification of small molecules in single cells raises new potentials for better understanding the basic processes that underlie embryonic development. To enable single-cell investigations directly in live embryos, new analytical approaches are needed, particularly those that are sensitive, selective, quantitative, robust, and scalable to different cell sizes. Here, we present a protocol that enables the in situ analysis of metabolism in single cells in freely developing embryos of the South African clawed frog (Xenopus laevis), a powerful model in cell and developmental biology. This approach uses a capillary microprobe to aspirate a defined portion from single identified cells in the embryo, leaving neighboring cells intact for subsequent analysis. The collected cell content is analyzed by a microscale capillary electrophoresis electrospray ionization (CE-ESI) interface coupled to a high-resolution tandem mass spectrometer. This approach is scalable to various cell sizes and compatible with the complex three-dimensional structure of the developing embryo. As an example, we demonstrate that microprobe single-cell CE-ESI-MS enables the elucidation of metabolic cell heterogeneity that unfolds as a progenitor cell gives rise to descendants during development of the embryo. Besides cell and developmental biology, the single-cell analysis protocols described here are amenable to other cell sizes, cell types, or animal models.

Microprobe Capillary Electrophoresis Mass Spectrometry for Single-cell Metabolomics in Live Frog Xenopus laevis Embryos

We describe steps that enable fast in situ sampling of a small portion of an individual cell with high precision and minimal invasion using capillary-based micro-sampling, to facilitate chemical characterization of a snapshot of metabolic activity in live embryos using a custom-built single cell capillary electrophoresis and mass spectrometry platform.

The quantification of small molecules in single cells raises new potentials for better understanding the basic processes that underlie embryonic ...

Microfluidic Preparation of Liquid Crystalline Elastomer Actuators

This paper focuses on the microfluidic process (and its parameters) to prepare actuating particles from liquid crystalline elastomers. The preparation usually consists in the formation of droplets containing low molar mass liquid crystals at elevated temperatures. Subsequently, these particle precursors are oriented in the flow field of the capillary and solidified by a crosslinking polymerization, which produces the final actuating particles. The optimization of the process is necessary to obtain the actuating particles and the proper variation of the process parameters (temperature and flow rate) and allows variations of size and shape (from oblate to strongly prolate morphologies) as well as the magnitude of actuation. In addition, it is possible to vary the type of actuation from elongation to contraction depending on the director profile induced to the droplets during the flow in the capillary, which again depends on the microfluidic process and its parameters. Furthermore, particles of more complex shapes, like core-shell structures or Janus particles, can be prepared by adjusting the setup. By the variation of the chemical structure and the mode of crosslinking (solidification) of the liquid crystalline elastomer, it is also possible to prepare actuating particles triggered by heat or UV-vis irradiation.

This article describes the microfluidic process and parameters to prepare actuating particles from liquid crystalline elastomers. This process allows the preparation of actuating particles and the variation of their size and shape (from oblate to strongly prolate, core-shell, and Janus morphologies) as well as the magnitude of actuation.

This paper focuses on the microfluidic process (and its parameters) to prepare actuating particles from liquid crystalline elastomers. The preparation ...