The world leader in serving science Bruce Bailey, Ph.D. Thermo Fisher Scientific, Chelmsford, MA Pittcon ™ Conference & Expo 2014 March 2-6, 2014 Expanding Your HPLC and UHPLC Capabilities with Universal Detection: Shedding Light on Compounds That Lack a Chromophore
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1 The world leader in serving science
Bruce Bailey, Ph.D. Thermo Fisher Scientific, Chelmsford, MA Pittcon™ Conference & Expo 2014 March 2-6, 2014
Expanding Your HPLC and UHPLC Capabilities with Universal Detection: Shedding Light on Compounds That Lack a Chromophore
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Outline
• Introduction to Charged Aerosol Detection • How Charged Aerosol Technology Works • Comparison with Evaporative Light Scattering Detectors
(ELSD) • Examples of Applications • Inverse Gradient Solution for Uniform Response
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Introduction to Charged Aerosol Detection
Comparison of Charged Aerosol Detection to UV and MS
• Used to quantitate any non-volatile and many semi-volatile analytes with LC
• Provides consistent analyte response independent of chemical structure and molecule size
• Neither a chromophore, nor the ability to ionize, is required for detection
• Dynamic range of over four orders of magnitude from a single injection (sub-ng to µg quantities on column)
• Mass sensitive detection – provides relative quantification without the need for reference standards
• Compatible with gradient conditions for HPLC, UHPLC, and micro LC
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The liquid eluent from the LC column enters the detector (1) where it undergoes nebulization by combining with a concentric stream of nitrogen gas or air (2).
The fine droplets are carried by bulk gas flow to the heated evaporation sector (3) where desolvation occurs to form particles, while any larger droplets are drained to waste (4).
The dry particles exit from evaporation (5) and are combined with another gas stream that first passes over a high voltage Corona charger (6). The charged gas then mixes with the dry particles, where excess charge transfers to the particle’s surface (7).
Charged Aerosol Detection – How It Works
Any high mobility species are removed by an ion trap (8) while the remaining charged particles pass to a collector where the passing particles charges are measured with a very sensitive electrometer (9). The resulting signal is then conveyed to a chromatographic data software for quantitation.
Signal is directly proportional to the analyte quantity
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Particle Charging for Charged Aerosol Detection
Mixing Chamber
• Particle size proportional to mass of analyte + background residue
• Charge per particle proportional to particle size
• Charged particles are measured, not gas phase ions as in MS
• A major consequence of ELSD sigmoidal response is that the dynamic range is relatively small and analyte signal rapidly decreases and completely disappears as the amount of analyte decreases.
• Unlike ELSD, Charged Aerosol Detector response does not
simply disappear for the same lower levels of analytes. Subsequently charged aerosol detection performs better for measurement of lower analyte levels and is generally more sensitive and provides a wider dynamic range than ELSD.
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Calibration of the Charged Aerosol Detector
• Over short ranges, the Charged Aerosol Detector is linear.
• Over wider ranges it is parabolic in behavior. To deal with this, several approaches are available. Which is the most appropriate will depend upon the data. Selection includes: • Log-Log
• Quadratic
• Power function
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Working with Non-Linear Data
• Limits of Detection (LoD) data by extrapolation from Signal / Noise data is only practical when working with a linear response.
• Both charged aerosol and ELS detector are non-linear. LoDs cannot be extrapolated from the response of high levels of analyte and can only be determined through the generation of calibration curves.
• Extrapolation of non-linear data produces major errors and should be avoided.
• Charged aerosol detectors performs better for the measurement of low levels of analytes, and have a wide dynamic range of four orders of magnitude. The analyte’s physicochemical properties affect the detector much less than ELSD.
• Charged aerosol detectors uses a single nebulizer to address a wide flow rate range. ELSD requires multiple nebulizers adding to expense and downtime.
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Working with Non-Linear Data
• The only way to estimate the LoD when response is non-linear is to construct a calibration curve.
• Comparisons are completely meaningless when the response of a non-linear detector to a high concentration of standard is used to imply that the performance of one detector is superior to the other.
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Comparisons Charged Aerosol vs. ELS Detectors
Charged Aerosol Detector ELSD
Response Curvilinear Sigmoidal Dynamic Range >4 orders 2–3 orders
LoQ and LoD LoQ and LoD often lower (better) than that estimated by SNR
LoQ and LoD often higher (worse) than that estimated by SNR
Sensitivity (LoD) <1 ng >10 ng Semivolatility Range Similar Similar Analyte Response Independent of structure Variable - dependent on compound Ease of Operation Simple Can be complex
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Charged Aerosol Applications: Shedding Light on Compounds
That Lack a Chromophore
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Determination of Adjuvants
Column: Thermo Scientific™ Hypersil GOLD™ PFP 1.9 um, 2.1 × 100 mm Mobile Phase A: 0.1% Formic acid in water Mobile Phase B: 0.1% Formic acid in 10:90 acetonitrile:reagent alcohol Gradient: 35% B to 83% B in 6 min to 90% B in 10 min Flow Rate: 0.5 mL/min Inj. Volume: 2 μL Col. Temp: 45 ºC Evap. Temp: 50 ºC
Analysis of Plant Saponins
UV @ 210 nm
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Glycan Analysis for Bovine Fetuin
Column: Thermo Scientific™ GlycanPac AXH-1™, 1.9 μm, 2.1 × 150 mm Mobile Phase A: 80% Acetonitrile Mobile Phase B: 80 mM Ammonium formate, pH 4.4 Gradient: 2.5% B to 25% B from 1 to 40 min Flow Rate: 0.4 mL/min Inj. Volume: 5 μL Col.Temp: 30 ºC Evap. Temp: 50 ºC
Separation of Oligosaccharide Alditols
Native Glycans
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Determination of Carbohydrates in Juice
Column: Amino, 3 μm, 3 × 250 mm Mobile Phase: Acetonitrile:water (92:8) Flow Rate: 0.8 mL/min Inj. Volume: 2 μL Col. Temp: 60 ºC Post-column Temp: 25 ºC Evap. Temp: 75 ºC Sample Preparation: Add 20 mL of 85% acetonitrile to 1 gram juice