Fast capillary electrophoresis (CE) hyphenated to time-of-flight mass spectrometry (TOF-MS) of four organoarsenic species (glycerol oxoarsenosugar, sulfate oxoarsenosugar, arsenobetaine, arsenocholine) are presented using short length CE capillaries under high electric field strengths of up to 1.3 kV cm−1 with small inner diameter (ID). The separation of arsenosugars by CE is demonstrated for the first time. An aqueous formic acid solution was employed as the background electrolyte (BGE) for the separation. Various acid concentrations were evaluated for their influence on migration times, separation efficiency as well as with regard to controlling the charge of the arsenic species. A 0.1 M formic acid/ammonium formate buffer (pH 2.8) proved to be suitable for the separation of the four species. A non-aqueous BGE was tested as an alternative buffer system for fast speciation analysis. Separation of arsenobetaine and arsenocholine could even be achieved within 10 s by pressure-assisted CE. Application of the optimized method for the analysis of extracts of a seagrass and a Wakame algae sample as well as the brown algae homogenate reference material IAEA-140/TM revealed a clear signal for the glycerol arsenosugar.
Abstract Besonders für Biomoleküle ist die Kapillarelektrophorese eine effektive und unaufwendige Alternative zur HPLC. Auch die Kapillarelektrophorese lässt sich mit Massenspektrometrie koppeln und ergänzt für geladene Analyte die HPLC.
Fast Capillary Electrophoresis–Mass Spectrometry. Measurements with capillary electrophoresis (CE) coupled to mass spectrometry (MS) are desirable when both the high separation efficiency of capillary electrophoresis and the identification capabilities of mass spectrometry are required. With current instrumentation, analyses require between 10 min and more than one hour to complete. Fast CE–MS measurements are generally preferable; however, they become mandatory when a high sample throughput is required.
Using an experimental setup of simple and straightforward design, a methodology to separate analytes with CE–MS both fast and efficiently has been developed, which successfully employs both aqueous and non-aqueous background electrolytes (BGEs). High electric field strengths of up to 1.25 kV•cm−1 and the use of short-length capillaries were found to be key in achieving fast separations.
Hyphenation of CE and MS was accomplished using a coaxial sheath liquid electrospray ionisation interface. Its commercial availability and simple experimental design aid in the rapid implementation of fast CE–MS methodology by other researchers and laboratories. The influence of parameters inherent to this interface was studied in more detail as to its effects on both separation and detection, including suction pressure and dilution. It was shown that parameter settings different from those routinely recommended have to be used in order to achieve highly efficient separations. General conclusions could be drawn from these findings, which allow rapid method development for fast CE–MS.
The influence of the capillary inner diameter (ID) on separations was investigated. In addition to standard capillaries of 75 and 50 µm ID, separations in capillaries with IDs of 25, 15, and 5 µm have been successfully applied to this setup. The analytical performance is compared over this range of capillary dimensions and both advantages and disadvantages are discussed. Usage of reduced ID capillaries allows for the analysis of sub-nL samples, the use of high conductivity BGEs, and can eliminate problems arising from certain parameter combinations in CE–MS experiments based on non-aqueous BGEs. Most significantly, it could be shown that reducing the separation capillary ID can have great potential in improving the separation efficiency.
The often-cited assumption of increased dilution at the ESI interface (and, hence, decreased signal intensity) with decreased separation capillary ID was found to be far less dramatic. Together with the increase in separation efficiency and the correspondingly sharper analyte signals, this dilution effect either was not present at all, or was considerably lower than what would be expected from the volumetric ratio of separation capillary outflow to sheath liquid flow rate.
The fast CE–MS methodology has been successfully applied to the separation of cationic and anionic analytes (co- and counter-electroosmotic separation conditions, respectively), namely catecholamines, hyaluronan oligomeres, organoarsenic compounds, and organotin compounds. For relevant sets of analytes, it could be shown that matrix-containing samples can be injected with only minimal sample pretreatment, with no negative effect on the CE–MS method. Most of these analytes could be separated in less than one minute; using a pressure-assisted approach, a separation within 10 s was possible.
While the injection process employed here was manual, it could be easily automated. At the time of writing, work towards an automated setup for fast CE–MS is being conducted in the same working group.
Small Samples. Efficient sample usage is of key importance in many bioanalytical questions for which a CE–MS separation is sought. The concept of injection efficiency is introduced to give a figure of merit to the injection process in analytical systems. It represents the ratio of injected sample and the amount of sample needed for carrying out the injection process (v/v). Typical values for the injection efficiency in CE range from 10−3 to 10−7.
Based on the concept of capillary batch injection (CBI), the development of an automated system is presented. This device is capable of running true multi-sample measurement series, using minimal sample volumes in the lower nL range and delivering an injection efficiency of up to 100%. It is compatible with both aqueous and non-aqueous background electrolytes. Design and specifications of the injection device are shown, and all parameters relevant for achieving both high injection efficiency and high separation efficiency are discussed. These parameters include liquid handling of sample volumes with the injection capillary, relative positioning of injection and separation capillary, and convection effects in the injection cell.
Furthermore, a procedure is presented to coat the tip of a fused silica capillary with a silicone elastomer acting as a seal between two capillaries in mid-solution. This allows for the injection of sample solution from the injection capillary directly into the separation capillary.
As an additional benefit, very short separation capillaries down to 15 cm in length can be used with this device. CE–MS separations of a catecholamine model system in capillaries of only 15 cm length under conditions of high electric field strength could be completed in 20 s with high separation efficiency.
