1. Introduction
1.3 Capillary electromigration techniques
1.3.3 Detection methods
The narrow-bore capillary that is used in CE makes the detection of analytes challenging. Low analyte concentrations in a sample with high detection sensitivity and narrow analyte zone widths require innovative methods to detect low concentrations from nanoliter (or even smaller) sample volumes. Careful choice of the detector enables separations of analytes in low concentrations from many different and complex matrices. [165] The most common detection methods used in CE are summarized in Table 14 along with their detection limits. The detection methods used in this study are presented later on.
Table 14.Most common detection methods for CE. [166]
(UV/Vis) 10-12–10-15 10-5–10-7 Universal and most common detection method, diode array detection (DAD) provides spectral information.
Laser induced
fluorescence (LIF) 10-18–10-20 10-9–10-12 The most sensitive detection method, usually requires derivatization.
Amperometry (AD) 10-18–1019 10-10–10-11 Sensitive and selective but only for electroactive analytes. Not robust.
Conductivity (CD) 10-15–10-16 10-6–10-7 Universal.
Mass spectrometry
(MS) 10-16–10-17 10-8–10-9 Sensitive and gives structural information. Interfacing CE and MS than that of the direct method.
*Assume 10 nL injection
UV/Vis detection. UV/Vis detection is the most common detection method in CE and is compatible with all the modes of CE. However, some restrictions in the BGE composition exist due to the optical properties of the BGE itself. With UV/Vis detection, any molecule that possesses a UV chromophore in its structure can be detected. A chromophore is a functional group that absorbs light in the UV/Vis region of the electromagnetic spectrum. If the analyte does not contain a chromo-phore or it absorbs weakly, the analyte can either be derivatized or detected with indirect UV detection. In indirect UV detection, a highly UV absorbing chemical is added to the BGE to produce a high and constant background signal. When an analyte with no UV absorbance reaches the detector, the analyte is seen in the electropherogram as a negative peak. The detection with UV/Vis is based on the Beer-Lambert law which indicates that absorbance is proportional to the concen-tration of the analyte. The magnitude of the signal is also dependent on the ana-lyte; analytes with high molar absorptivities of 104 to 105 are strong absorbers and analytes with molar absorptivities of 103 are weak absorbers. Furthermore, the pH and composition of the BGE, and the degree of ionization have an effect on the molar absorptivity value and on the maximum wavelength of absorption. [162] The Beer-Lambert law is presented as:
A= lc
, (5)where A is absorbance, is molar absorption coefficient (in cm-1M-1), l is optical path length and c is concentration.
Fluorescence detection. Fluorescence is the most sensitive and second most common detection method in CE. The detection limits are below 10-21 mol, making it almost 1000 times more sensitive than UV detection. [162] Fluorescence can be divided into three stages: 1) excitation, 2) excited-state lifetime, and 3) fluorescence emission. These stages can be illustrated by the Jablonski diagram (Figure 9A) which represents the electronic states of each stage. In stage 1, the fluorophore absorbs a photon of energy h EX that is delivered by a light source such as a laser, and moves to the excited electronic single state (S1´). The excited state exists for a limited time, typically 1–10 ns (stage 2). First, the energy of S1´ is partially dissipated, yielding a relaxed singlet excited state (S1) which is the origin of fluorescence emission. Second, the molecules that were not excited by the absorption in stage 1 return to the ground state S0. After this, stage 3 occurs when electronically excited fluorophore emits a photon of energy h EM and returns to the ground state. Because of the energy dissipation during stage 2, the energy of this photon is lower than in stage 1. The difference between these photon energies (h EX – h EM) is called Stokes shift (Figure 9B). [162, 167]
Figure 9.A) Jablonski diagram, and B) Stokes shift. [164]
The intensity of fluorescence detection is also defined by the Beer-Lambert law.
The most important parameters of a fluorophore are molar absorption coefficient ), fluorescence quantum yield and photostability of the molecule. Fluorescence quantum yield illustrates the number of fluorescence photons emitted per excita-tion photo absorbed, thus accounting for the efficiency of fluorescence detecexcita-tion with certain fluorophores. Theoretical maximum of the quantum yield is 1.0.
However, most of the analytes do not have native fluorescence and therefore the use of this detection method involves attaching fluorescent probe molecules to the analyte. Most fluorescent probes can be designed to contain a reactive moiety capable of coupling to a specific functional group of a biomolecule. With this deri-vatization procedure it is possible to enhance the selectivity of the detection. [162]
Common fluorescent labels in capillary electrophoresis and their properties and applications are listed in Table 15.
Table 15. Common fluorescent labels used in CE.
Label MW
(g/mol)
Excitation/
emission maximum wavelength
(nm)
Quantum
yield [164] Reactivity Ref.
8-Aminonaphtalene-1,3,6-trisulfonic acid
(ANTS) 427.34 356/512 na 7 200 carbohydrates [168]
8-Aminopyrene-1,3,6-trisulfonic acid (APTS) 523.40 420/500 na 19 000 carbohydrates [165]
Fluorescamine 278.26 391/464 0.11 7 800 primary aliphatic
amines [165]
Fluorescein 332.31 490/514 0.93 93 000 amines
Fluorescein
isothiocyanate (FITC) 389.38 492/518 na 77 000 amines [165,
169]
Oregon green®
succinimidyl ester
5-isomer 509.38 495/521 na 76 000 primary and
secondary aliphatic amines Oregon green®
succinimidyl ester
6-isomer 509.38 496/516 na 82 000 primary and
secondary aliphatic amines 1-Pyrenyldiazomethane
(PDAM) 242.27 340/375 na 41 000 carboxylic acid [165]
MW molecular weight na not available
molar absorption coefficient (in cm-1M-1)
Oregon green™ 488 succinimidyl ester (OG-SE) used in the Publication IV is the fluorinated analogue of fluorescein. The molecular structure of OG-SE is presented in Figure 10A. Conjugates of Oregon green fluorophores are more photostable than those of fluorescein and are not as pH sensitive. The excitation and emission maximums wavelengths of the label are at 496 nm and 524 nm, respectively, (Figure 10B) and are well suited for detection with argon-ion laser.
Reaction of OG-SE with primary and secondary amines creates stable amide and imide linkages, respectively, and the reaction occurs rapidly in mild conditions.
[170]
Figure 10. A) Structure of Oregon green® 488 succinimidyl ester [167], and B) Absorption and fluorescence emission spectra of Oregon Green® 488 [164]. The blue line is absorption and the red line is emission.
2. Aims of the study
The overall object of this study was to develop novel analytical methods to measure and control the productivity of microorganisms in bioreactors during cultivations. For this purpose, capillary electrophoresis with UV- and LIF-detection was used to assist research of industrial biotechnology and validation of biotechnological manufacturing processes. In addition, an online CE system was assembled from modified commercial capillary electrophoresis device and an in-house constructed flow-through sample vial.
Specifically the aims were:
- to develop CE methodologies and technologies to study carboxylic acids that are present in bioprocesses in both offline (I) and online (III) mode - to develop CE methodologies to study phenolic compounds with
inhibito-ry effects on bioprocesses (II)
- to develop CE methodologies to study the consumption of amino acids by yeast during beer fermentation (IV).