Editing Digital microfluidics (section)

=== Mass spectrometry ===
The coupling of digital microfluidics (DMF) and [[Mass spectrometry|Mass Spectrometry]] can largely be categorized into indirect off-line analysis, direct off-line analysis, and in-line analysis<ref name="Kirby_2013">{{cite journal | vauthors = Kirby AE, Wheeler AR | title = Digital microfluidics: an emerging sample preparation platform for mass spectrometry | journal = Analytical Chemistry | volume = 85 | issue = 13 | pages = 6178–84 | date = July 2013 | pmid = 23777536 | doi = 10.1021/ac401150q }}</ref> and the main advantages of this coupling are decreased solvent and reagent use, as well as decreased analysis times.<ref name="Wang_2015">{{cite journal | vauthors = Wang X, Yi L, Mukhitov N, Schrell AM, Dhumpa R, Roper MG | title = Microfluidics-to-mass spectrometry: a review of coupling methods and applications | journal = Journal of Chromatography A | volume = 1382 | pages = 98–116 | date = February 2015 | pmid = 25458901 | pmc = 4318794 | doi = 10.1016/j.chroma.2014.10.039 | series = Editors' Choice IX }}</ref>

Indirect off-line analysis is the usage of DMF devices to combine reactants and isolate products, which are then removed and manually transferred to a mass spectrometer. This approach takes advantage of DMF for the sample preparation step but also introduces opportunities for contamination as manual intervention is required to transfer the sample. In one example of this technique, a [[Grieco three-component condensation]] was carried out on chip and was taken off the chip by micropipette for quenching and further analysis.<ref name="Dubois_2006"/>

Direct off-line analysis is the usage of DMF devices that have been fabricated and incorporated partially or totally into a mass spectrometer. This process is still considered off-line, however as some post-reaction procedures may be carried out manually (but on chip),  without the use of the digital capabilities of the device. Such devices are most often used in conjugation with [[Matrix-assisted laser desorption/ionization|MALDI-MS]]. In MALDI-based direct off-line devices, the droplet must be dried and recrystallized along with matrix – operations that oftentimes require vacuum chambers.<ref name="Kirby_2013" /><ref>{{cite journal | vauthors = Chatterjee D, Ytterberg AJ, Son SU, Loo JA, Garrell RL | title = Integration of protein processing steps on a droplet microfluidics platform for MALDI-MS analysis | journal = Analytical Chemistry | volume = 82 | issue = 5 | pages = 2095–101 | date = March 2010 | pmid = 20146460 | doi = 10.1021/ac9029373 | url = https://figshare.com/articles/Integration_of_Protein_Processing_Steps_on_a_Droplet_Microfluidics_Platform_for_MALDI_MS_Analysis/2788444 | url-access = subscription }}</ref> The chip with crystallized analyte is then placed in to the MALDI-MS for analysis. One issue raised with MALDI-MS coupling to DMF is that the matrix necessary for MALDI-MS can be highly acidic, which may interfere with the on-chip reactions<ref>{{cite journal | vauthors = Küster SK, Fagerer SR, Verboket PE, Eyer K, Jefimovs K, Zenobi R, Dittrich PS | title = Interfacing droplet microfluidics with matrix-assisted laser desorption/ionization mass spectrometry: label-free content analysis of single droplets | journal = Analytical Chemistry | volume = 85 | issue = 3 | pages = 1285–9 | date = February 2013 | pmid = 23289755 | doi = 10.1021/ac3033189 }}</ref>

Inline analysis is the usage of devices that feed directly into mass spectrometers, thereby eliminating any manual manipulation. Inline analysis may require specially fabricated devices and connecting hardware between the device and the mass spectrometer.<ref name="Kirby_2013" /> Inline analysis is often coupled with [[electrospray ionization]]. In one example, a DMF chip was fabricated with a hole that led to a microchannel<ref>{{cite journal | vauthors = Jebrail MJ, Yang H, Mudrik JM, Lafrenière NM, McRoberts C, Al-Dirbashi OY, Fisher L, Chakraborty P, Wheeler AR | display-authors = 6 | title = A digital microfluidic method for dried blood spot analysis | journal = Lab on a Chip | volume = 11 | issue = 19 | pages = 3218–24 | date = October 2011 | pmid = 21869989 | doi = 10.1039/c1lc20524b }}</ref> This microchannel was, in turn, connected to an electrospray ionizer that emitted directly into a mass spectrometer. Integration ambient ionization techniques where ions are formed outside of the mass spectrometer with little or no treatment pairs well with the open or semi-open microfluidic nature of DMF and allows easy inline couping between DMF and MS systems. Ambient Ionization techniques such as Surface Acoustic Wave (SAW) ionization generate surface waves on a flat piezoelectric surface that imparts enough acoustic energy on the liquid interface to overcome surface tension and desorb ions off the chip into the mass analyzer.<ref>{{cite journal | vauthors = Yeo LY, Friend JR | title = Ultrafast microfluidics using surface acoustic waves | journal = Biomicrofluidics | volume = 3 | issue = 1 | pages = 12002 | date = January 2009 | pmid = 19693383 | pmc = 2717600 | doi = 10.1063/1.3056040 }}</ref><ref name="Kirby_2013" /> Some couplings utilize an external high-voltage pulse source at the physical inlet to the mass spectrometer <ref>{{cite journal | vauthors = Heron SR, Wilson R, Shaffer SA, Goodlett DR, Cooper JM | title = Surface acoustic wave nebulization of peptides as a microfluidic interface for mass spectrometry | journal = Analytical Chemistry | volume = 82 | issue = 10 | pages = 3985–9 | date = May 2010 | pmid = 20364823 | pmc = 3073871 | doi = 10.1021/ac100372c }}</ref> but the true role of such additions is uncertain.<ref>{{cite journal | vauthors = Ho J, Tan MK, Go DB, Yeo LY, Friend JR, Chang HC | title = Paper-based microfluidic surface acoustic wave sample delivery and ionization source for rapid and sensitive ambient mass spectrometry | journal = Analytical Chemistry | volume = 83 | issue = 9 | pages = 3260–6 | date = May 2011 | pmid = 21456580 | doi = 10.1021/ac200380q }}</ref>

A significant barrier to the widespread integration of DMF with mass spectrometry is biological contamination, often termed bio-fouling.<ref name="Kirby_2013" /> High throughput analysis is a significant advantage in the use of DMF systems,<ref name="Wang_2015" /> but means that they are particularly susceptible to cross contamination between experiments. As a result, the coupling of DMF with mass spectrometry often requires the integration of a variety of methods to prevent cross contamination such as multiple washing steps,<ref>{{Cite journal| vauthors = Zhao Y, Chakrabarty K |date=June 2010|title=Synchronization of washing operations with droplet routing for cross-contamination avoidance in digital microfluidic biochips|url=https://ieeexplore.ieee.org/document/5523385|journal=Design Automation Conference|pages=635–640}}</ref><ref name="Shih_2012">{{cite journal | vauthors = Shih SC, Yang H, Jebrail MJ, Fobel R, McIntosh N, Al-Dirbashi OY, Chakraborty P, Wheeler AR | display-authors = 6 | title = Dried blood spot analysis by digital microfluidics coupled to nanoelectrospray ionization mass spectrometry | journal = Analytical Chemistry | volume = 84 | issue = 8 | pages = 3731–3738 | date = April 2012 | pmid = 22413743 | doi = 10.1021/ac300305s }}</ref> biologically compatible surfactants,<ref>{{cite journal | vauthors = Aijian AP, Chatterjee D, Garrell RL | title = Fluorinated liquid-enabled protein handling and surfactant-aided crystallization for fully in situ digital microfluidic MALDI-MS analysis | journal = Lab on a Chip | volume = 12 | issue = 14 | pages = 2552–2559 | date = July 2012 | pmid = 22569918 | doi = 10.1039/C2LC21135A }}</ref> and or super hydrophobic surfaces to prevent droplet adsorption.<ref>{{cite journal | vauthors = Samiei E, Tabrizian M, Hoorfar M | title = A review of digital microfluidics as portable platforms for lab-on a-chip applications | journal = Lab on a Chip | volume = 16 | issue = 13 | pages = 2376–2396 | date = July 2016 | pmid = 27272540 | doi = 10.1039/C6LC00387G | url = https://escholarship.mcgill.ca/concern/articles/vt150p551 }}</ref><ref>{{cite journal | vauthors = Lapierre F, Piret G, Drobecq H, Melnyk O, Coffinier Y, Thomy V, Boukherroub R | title = High sensitive matrix-free mass spectrometry analysis of peptides using silicon nanowires-based digital microfluidic device | journal = Lab on a Chip | volume = 11 | issue = 9 | pages = 1620–1628 | date = May 2011 | pmid = 21423926 | doi = 10.1039/C0LC00716A }}</ref> In one example, a reduction in cross contaminant signal during the characterization of an amino acid required 4-5 wash steps between each sample droplet for the contamination intensity to fall below the limit of detection.<ref name="Shih_2012" />

==== Miniature Mass Spectrometers ====
Conventional mass spectrometers are often large as well as prohibitively expensive and complex in their operation which has led to the increased attractiveness of miniature mass spectrometers (MMS) for a variety of applications. MMS are optimized towards affordability and simple operation, often forgoing the need for experienced technicians, having a low cost of manufacture, and being small enough in size to allow for the transfer of data collection from the laboratory into the field.<ref>{{cite journal | vauthors = Ouyang Z, Cooks RG | title = Miniature mass spectrometers | journal = Annual Review of Analytical Chemistry | volume = 2 | issue = 1 | pages = 187–214 | date = 2009-07-19 | pmid = 20636059 | doi = 10.1146/annurev-anchem-060908-155229 | bibcode = 2009ARAC....2..187O }}</ref> These advantages often come at the cost of reduced performance where MMS resolution, as well as the limits of detection and quantitation, are often barely adequate to perform specialized tasks. The integration of DMF with MMS has the potential for significant improvement of MMS systems by increasing throughput, resolution, and automation, while decreasing solvent cost, enabling lab grade analysis at a much reduced cost. In one example the use of a custom DMF system for urine drug testing enabled the creation of an instrument weighing only 25&nbsp;kg with performance comparable to standard laboratory analysis.<ref>{{cite journal | vauthors = Kirby AE, Lafrenière NM, Seale B, Hendricks PI, Cooks RG, Wheeler AR | title = Analysis on the go: quantitation of drugs of abuse in dried urine with digital microfluidics and miniature mass spectrometry | journal = Analytical Chemistry | volume = 86 | issue = 12 | pages = 6121–6129 | date = June 2014 | pmid = 24906177 | doi = 10.1021/ac5012969 }}</ref>