TABLE 2.
Survey of representative methods of analysis for acrylamide from different food matrices
| Detection method | Samples | Sample preparation | Internal standard | Column used | Method parameters (LC or GC) | Detector parameters (MS or MS/MS) | References |
|---|---|---|---|---|---|---|---|
| LC-MS | Roasted coffee, barley, potato crisps | QuEChers procedure followed by treatment with Al2O3 and purification on HLB column | AA-d3 | X-Select HSS T3 (2.5 μm particle size, 150 × 2.1 mm i.d., Waters Corp., Milford, MA, USA) | Water and MeCN (both acidified with 0.2% formic acid) as solvent A and B, respectively. Flow rate 0.2 ml min−1, 100% solvent A 3 min, linear gradient to 15% solvent B within 4 min, isocratic 2 min, conditioning column 7 min | ESI interface in positive mode, spray capillary voltage 4.2 kV, sheath and aux gas 35 and 12 psi, respectively, skimmer 12 V, temperature of heated capillary 270 °C, argon pressure 1.2 mTorr at CE of 12 V | Bertuzzi et al. (2017) |
| Dried fruits and edible deeds | Samples ground, QuEChers extraction, filtration through 0.22 μm PES membrane | None? | Gemini RP C18 column (Phenomenex, Torrance, CA, USA) (25 cm × 2 mm i.d. × 5 μm particle size × 110 A pore size) | Water and MeOH acidified with 0.1% formic acid as solvent A and B, respectively. Isocratic elution 7 min with flow rate of 0.25 ml min−1 | Positive ESI with curtain gas 20.0 pse, collision-activated dissociation 7.0, ionspray 5,500/0 V, 700.0 °C, nebulizer gas 70.0 pse, heater gas 30.0 psi, declustering potential 22.0 V, CE 14.1 V, CXP 4.1 V, CEP 6.0 V | De Paola et al. (2017) | |
| LC-MS/MS | Blood samples, serum (DNA-GA adducts) | GA-DNA isolation with Blood and Call Culture Maxi kits (Qiagen Co., Valencia, CA) or thawed blood serum sample purified with solid-phase extraction | (13C)3-Substituted AA, racemic (13C)3-GA | Aquasil column (2.1 mm × 150 mm, 5 μm particle size) (Thermo Hypersil-Keystone, Bellefonte, PA) | 300 μL min−1, step gradient 0.5% MeOH in 0.01% formic acid in water for 2.5 min, 30% MeOH in aqueous 0.1% formic acid for 2 min, re-equilibration for 5.5 min. | Ion source temperature 120 °C, desolvation gas temperature 400 °C, constant cone voltage 35 V, gas pressure (ar) 2–4 × 10−3 mbar | Doerge et al. (2005) |
| Hemolyzed erythrocytes | Cleave N-terminal valine with AA or GA attached with Edman reagent, followed by liquid–liquid extraction | n/a | n/a | n/a | n/a | (Ferrari et al., 2013) | |
| Food samples | Stable isotope dilution method (SIDA) | AA-d3 | Adapted from Ruenz. Et al with 1 μL injection volume | Ruenz et al. | DP 36 V, CEP 10 V, CE 15 V, CXP 7 V, ionspray voltage 4500 V, curtain gas 40 psi, temperature 550 °C | (Goempel et al., 2017) | |
| Blood samples | Hb isolation described by Schettgen et al. 2003; derivatization by modified Edman degradation generating pentafluorophenyl thiohydantoin derivatives | D7-AA-Val-PFPTH, D7-GA-Val-PFPTH | Kintex PFP column (100 × 3.1 mm, 100 Å, 2.6 μm, Phenomenex, Aschaffebburg, Germany) | 0.1% aqueous acetic acid and MeCN as solvents A and B, respectively, at 40 °C, MeCN concentration 10% for 0.5 min, increased to 40% for 0.6 min, and to 45% for 4.8 min, and reconditioning | DP −125/75 V, exit potential −10 V, CE −22 V/−22 V, CXP −17/−15 V, curtain gas 50 psi, ionspray voltage −4,500 V, temperature 500 °C | (Goempel et al., 2017) | |
| HPLC-UV | Vending machine snacks | Solid–liquid water extraction with water and derivatization with 2-mercaptobenzoic acid, followed by extraction with ethyl acetate, resolve residue in MeOH | AA (AA 99%, SIGMA-Fluka, St. Louis, MO, USA) | C18 RP column (250 × 4.6 mm, Phenomenex Inc, Torrance, CA, USA) | Detection wavelength of 238 nm, 20:80 MeCN and acetic acid as mobile phase | n/a | (Haouet, Pistolese, Branciari, Ranucci, & Altissimi, 2016) |
| UPLC-MS | Ground and homogenize samples. Spike with IS and perform solid–liquid extraction with Carrez I and II solutions, cation exchange with SPE column | AA-d3 (Sigma-Aldrich, Steinheim, Germany) | Waters Acquity BEH C18 column (50 mm × 2.1 mm i.d., particle size 1.7 μm) | Isocratic elution with 10% MeOH in 0.1% aqueous formic acid, flow rate 300 μL/min | Cone voltage 48 V, capillary voltage 3.5 kV, desolvation temperature 350 °C, source temperature 120 °C, desolvation gas flow rate 600 L h−1 | (Khan et al., 2017) | |
| LC-MS/MS | Infant and toddler foods | Homogenize samples, spike with IS, solid–liquid extraction with water, anion exchange with SPE column | AA-d5 (Cluzeau Info Labo) | Hypercarb chromatographic column (50 mm × 2.1 mm i.d., 5 μm particle size) (Thermo Fisher Scientific) | 1% MeOH in water with 0.2% formic acid and pure MeOH as solvents A and B, respectively, 200 μL min−1 flow rate with re-equilibration | ESI+, spray voltage 5 kV, curtain gas 25 psi, spray gas 15 psi, auxiliary gas 20 psi, source temperature 500 °C, CEP 10 V, CXP 15 V, CE 15 eV | (Lambert et al., 2018) |
| LC-MS/MS | Food samples *potato chip, sweet potato chip, crackers and snacks, peanut better, chocolate, chocolate syrup) | Spike sample with IS, solid–liquid extraction with MeCN and hexane (for defatting) | AA-d3 (Cambridge Isotope Laboratories Inc., Andover, MA, USA) | Aqua C18 Column (150 × 3 mm, 5 μm particle size, 125 A pore size), C18 4 × 3 mm guard column (Phenomenex; Torrance, CA) | 0.5% aqueous MeOH and 0.1% formic acid in 50% aqueous MeOH as solvents A and B, respectively, 200 μL min−1 flow rate for 8 min | ESI+, no other information available | (Mastovska & Lehotay, 2006) |
| HPLC | Food samples (potato, baked bread, potato chips, french fries, sweet bread, bun, rusk and biscuit) | Finely ground samples, solid–liquid extraction with MeOH, defatting with hexanes | L-asparaginase (Nocozymes, Acrylaway L), AA (Sigma-Aldrich, St. Louis, USA) | C18 column (250 mm × 4.6 mm × 5 μm i.d.) | Isocratic conditions with 97% MeCN, 3% acetic acid as mobile phase at 1 ml min−1 flow rate and detection at 210 nm at 28 °C | n/a | (Meghavarnam & Janakiraman, 2018) |
| LC-MS/MS | Food samples (wheat flour and biscuits) | Spike with IS, solid–liquid extraction with water, defatting with a spatula | AA-d3 (Cambridge Isotope Laboratories, Inc.) and asparagine-15N2 (Sigma-Aldrich, The Netherlands) | Hypercarb column (100 × 3 mm, 5 μm particle size) | 100% water and 100% MeOH as solvents A and B, respectively; 16 min per sample with a flow rate of 0.4 mL min−1 | ESI− for Asp, ESI+ for AA | (Nguyen et al., 2017) |
| LC-MS/MS | Food samples (biscuit powder) | Spike with IS, solid–liquid extraction with water | AA-d3 (Cambridge Isotope Laboratories, Inc.) and asparagine-15N2 (Sigma-Aldrich, The Netherlands) | Hypercarb column (100 × 3 mm, 5 μm particle size) | 100% water as mobile phase with 0.4 mL min−1 flow rate and 10 min run time per sample | ESI− for Asp, ESI+ for AA | (Nguyen, Van Der Fels-Klerx, Peters, & Van Boekel, 2016) |
| LC-MS | Baby food | Homogenized samples, extracted with MeCN, anion exchange with PSA sorbent | MethylAA (Sigma-Aldrich, St. Louis, MO, USA) | C8 column (150 mm × 2.1 mm i.d., 5 μm particle size; Zorbax Eclipse XDB, Agilent) at 30 °C | DI water and MeCN as solvent A and B, respectively, both acidified with 0.01% formic acid: 0 min, 5% B, 5 min, 50% B, 5.1 min, 100% B, flow rate 0.2 ml min−1 | ESI+ mode, source temperature 120 °C, desolvation temperature 400 °C, capillary voltage, 3 kV, cone voltage 25 V, gas flow 500 L h−1 (desolvation), 54 L h−1 (cone) and 0.2 mL min−1() | (Petrarca et al., 2017) |
| LC-MS/MS | Cocoa and cocoa products | Homogenize samples, spike with IS extract with MeCN followed by clean up with Carrez I and II solutions | AA-d3 (LGC Standards GmbH (Wesel, Germany) | Lichrospher 100 CH 5 μm (250 × 4 mm) (Merck, Darmstadt, Germany) | Isocratic elution with 0.5% MeCN in water with 0.1% formic acid; flow rate 0.25 ml min−1 and 20 μL injection volume | ESI+, capillary voltage 3 kV, cone voltage 40 V, source temperature 450 °C | (Raters & Matissek, 2018) |
| LC-MS/MS | Food samples (potato chips, mashed potatoes, rye flour) | Homogenize samples, spike with IS, solid–liquid extraction with MeCN, and cleanup with SPE sorbent | AA-d3 (Polymer Source Inc., Dorval, Quebec, Canada) | Hypercarb 5 μm (50 × 2.1 mm, guard column 5 μm, 10 × 2 mm) (Thermo Hypersil-Keystone, UK) | Water mobile phase with flow rate 400 μL min−1, 10 μL injection volume | ESI+, source temperature 125 °C, capillary voltage 2 kV, cone voltage 20 V, energies 9, 16, 20, 0, and 14 eV | (Rosén & Hellenäs, 2002) |
| LC-MS | Food samples | Stable isotope dilution method (SIDA) after homogenization | AA-d3 (Toronto Research Chemicals; Toronto, Canada) | Luna C8(2) (150 mm × 4.6 mm; 3 μm) and Luna C8(2) guard column (4 mm × 3 mm, Phenomenex, Aschaffenburg, Germany) | Injection volume 50 μL, flow rate 0.5 mL min−1; 0.05% aqueous formic acid and MeOH as solvents A and B, respectively; concentration was increased from 1% to 20% over 12 minutes | ESI+, DP 36 V, CEP 10 V, CE 15 V, CXP 7 V, curtain gas 40 psi, ionspray voltage 4,500 V, temperature 550 °C | (Ruenz, Bakuradze, Eisenbrand, & Richling, 2016) |
| HPLC | Fried potatoes | According to Wenzal et al. (2003), defatted with hexane, adding of MeCN and Carrex I and II solutions | AA | n/a | 50% MeCN in aqueous 1% formic acid | n/a | (Sanghvi et al., 2016) |
| LC-MS/MS | Foodstuffs; fish fillet (cod), lean beef, lean pork, chicken fillet, soy flour, grated potato, grated beetroot, spinach | Homogenized samples, add IS, solid–liquid extraction with water, anion exchange with SPE sorbent, add MeCN and add water | AA-13C3 | Hypercarb column (50 × 2 mm; ThermoHypersil) | Water as mobile phase, flow rate 0.2 mL min−1 for 6.1 min, wash with 80% aqueous MeCN 0.2 ml min−1, 10 min, reconditioning with water | ESI+, capillary voltage 3.2 kV, cone voltage 50 V, source temperature 125 °C, desolvation temperature 350 °C, cone gas flow 211 L h−1, desolvation gas flow 653 L h−1, 2.5 mbar gas psi | (Tareke et al., 2002) |
| HPLC-MS/MS | Blood samples (hemoglobin) | Cleave N-terminal Val of Hb with AA attached via Edman degradation, spike with IS, liquid-liquid extraction | AA-Val(13C515N)-HLTPEEK | n/a | n/a | n/a | (Vesper et al., 2008) |
| GC-MS | Potato flour, french fries | Homogenize samples, spike with IS, solid–liquid extraction with hexane, water and MeCN | AA (Sigma-Aldrich, Milquakee, WI, USA) | PEG column, no specifics given | n/a | Single ion monitoring mode, no other specifics given | (Adedipe et al., 2016) |
| GC-MS | Food samples (potato chip, sweet potato chip, crackers and snacks, peanut better, chocolate, chocolate syrup) | Spike sample with IS, solid–liquid extraction with MeCN and hexane (for defatting) | AA-d3 (Cambridge Isotope Laboratories Inc., Andover, MA, USA) | Stabilwax-DB capillary column (20 m, 0.32 mm i.d., 1 μm film thickness; Restek) | Analysis time 11 min (AA r.t. 9.3 min), inlet temperature 100 °C held for 3.1 min, 200 °C min−1 ramp to 150 °C, held for 2 min, return to 100 °C, split ratio 50:1 for 2.5 min, then 15:1 for rest of analysis, oven temperature 80 °C for 6.35 min, then 70 °C min−1 ramp to 200 °C for 2.94 min | MS transfer line temperature 170 °C, ion trap temperature 200 °C, manifold temperature 50 °C, MeOH chemical ionization reagent | (Mastovska & Lehotay, 2006) |
| GC-MS | Bread, biscuits and bakery products | Homogenize samples, solid–liquid extraction in water pH 4–5 with glacial acetic acid, clean up with Carrex I and II solutions, derivatization with KBr, HBr, sodium thiosulfate, extraction with ethyl acetate and hexane | AA-13C3 | PEG column (30 × 0.25 μm, 0.25 μm i.d., TraceGOLD TG-WaxMS, Thermo Fisher Scientific, USA) | He mobile phase with constant flow rate 1.6 ml min−1, 1 μL sample volume, split ratio 1:10, injector temperature 220 °C, AA r.t. 10.65 min | n/a | (NegoiţĂ & Culeţu, 2016) |
| GC-MS | Instant coffee | QuEChERS method with addition of hexane for defatting | AA-d3 (Sigma-Aldrich Chemi GmbH, Germany) | DB-5MS column (30 m × 0.25 μm; Agilent Technologies, USA) | Initial temperature 50 °C, 3 °C min−1 to 100 °C, and then 25 °C min−1 until 250 °C (takes 5 min), solvent delay 8 min, He carrier gas at flow rate 1.0 ml min−1 | Internal ionization mode, scan m/z from 45 to 500, emission current of ionization filament 15 μM, trap line temperature 180 °C, transfer line temperature 220 °C, SIM mode | (Surma et al., 2017) |
| GC-MS | Foodstuffs; fish fillet (cod), lean beef, lean pork, chicken fillet, soy flour, grated potato, grated beetroot, spinach | Homogenize samples, solid–liquid extraction with water, purification on carbon black column, spike with IS | N,N-dimethylAA (Sigma-Aldrich; Stockholm, Sweden) | BPX-10 fused silica capillary column (30 mx 0.25 mm i.d., 0.25 μm film thickness; SGE, Ringwood, Australia) | Isothermal for 1 min at 65 °C, 15 °C min−1 to 250 °C, isothermal 10 min | Splitless injection, injector temperature 250 °C | (Tareke et al., 2002) |
| UV spectrophotometry | Bread samples (white and brown) | Homogenize samples, spike with IS, add MgSO4 and NaCl, hexanes for defatting, and MeCN and water for extraction, anion exchange with PSA sorbent, followed by microextraction with MeCN and ultrasound, add HNO3 in EtOH | AA (Sigma) | n/a | Perform spectrophotometric determination at 530 nm against a blank | (Altunay et al., 2018) | |
| Fluorescence quenching biosensor | Potato fries | Homogenize and extract with water, defat with hexanes, purify with Carrex I and II solutions, spike with IS, incubate with ssDNA, add FAM-csDNA followed by AuNPs with incubation | AA (Sigma-Aldrich, USA) | n/a | Detect at 490 nm | n/a | (Asnaashari et al., 2018) |
| Glutathione S-transferase | Food samples: bread, potato chips, boiled potatoes | Homogenize samples, extract with EtOH and defat with hexane, spike with IS | AA (Sigma-Aldrich, Germany) | n/a | Detect spectrometrically at 340 nm using CDNB to produce a colored compound with GSH, get inhibition constant | n/a | (Bucur, Bucur, & Radu, 2018) |
| Fluormetric assay | Food samples: potato chips, fried bread sticks, bread and rice crust | Solid–liquid extraction with MeOH, defatting with hexanes, spike with AA | AA (supplier in supplementary) | ZnS quantum dots (QD) doped with Mn (II) ions are fluorescent source, are conjugated to AA to serve as fluorescent probe | Add AA-MIP, add AA extraction, let incubate. Record fluorescence at 470–750 nm upon excitation at 235 nm, slit width 10 nm, scan speed 200 nm min−1, excitation voltage 750 V | n/a | (Y. Liu et al., 2018) |
| HS-SPME coupled with IMS (Headspace solid-phase microextraction with ion mobility spectrometry) | Food samples: potato chips, fried potatoes | Homogenize samples, solid–liquid extraction with MeOH | AA (Aldrich, Steinheim, Germany) | Nanostructured polypyrrole fiber | n/a | [IMS] Corona voltage 3.0 kV, drift field 636 V cm−1, drift gas flow, N2 450 ml min−1, injection temperature 200 °C, shutter grid pulse 150 us, drift tube length 11 cm, calibrant ion NH4+(H2O)n, drift time (7.50 ms), reduced mobility 1.89 cm2V−1s−1 | (Pourmand, Ghaemi, & Alizadeh, 2017) |
| Hemoglobin nanoparticles for detection | Food samples: biscuits, cakes, chips, fried cereals, nuts, corn puffs | Homogenize, solid–liquid extraction in water, clean up with Carrez I and II solutions, measure in sodium acetate buffer | AA (SISCO Research Lab, Mumbai, India) | n/a | [three electrode system] Immobilize covalently HbNPS onto polycrystalline Au electron, which acts as a biosensor to detect AA concentration in processed foods | Current measured at 0.26 V with scan rate 20 mVs−1 vs Ag/AgCl | (Yadav et al., 2018) |
| AA-specific aptamers | n/a (methods paper with no validation with other methods yet) | n/a | AA (Sigma-Aldrich, St. Louis, MO, USA) | n/a | Use SELEX to identify AA-specific aptamers, incubate gold nanoparticles | Record on microplate reader from rage 400 to 750 nm | (Hu et al., 2018) |
| Hollow fiber liquid phase microextraction and chromatography-electron capture detection (HF-LPME) | Waste water samples | AA (Aldrich, Buchs, Switzerland) | (Sobhi et al., 2017) | ||||
| GC-MS | Waste water samples | Derivatization with bromine | AA (Aldrich, Buchs, Switzerland) | HP-5 MS capillary column (30 m × 0.25 mm, 0.25 μm, film thickness) (Agilent J&W Scientific, Folsom, CA, USA) | TIC mode, He carrier gas flow rate 1.0 ml min−1, 60 °C for 1 min, increased to 280 °C at 10 °C min−1 and held 5 min, increase to 300 °C at 50 °C min−1 held for 3 min. MS quadrupole source temperature 150 °C, MS source temperature 230 °C, m/z range 40–300 | (Sobhi et al., 2017) | |
| GC-MS | Food samples: biscuits, chips and cake | Homogenize, spike with IS, solid–liquid extraction with water, defat with petroleum ether, cleanup with Carrex I and II solutions, salt out with NaCl, extract with ethyl acetate | AA-d3 (Cambridge Isotope Laboratories, Andover, MA, USA) | DB-23 capillary column (30 m × 0.25 mm i.d.d., 0.25 μm film thickness; J&W Scientific Products GmbH, Koln, Germany) | He flow rate 1.0 ml min−1, column temperature 80 °C for 2 min, increase from 80 °C-220 °C at a rate of 10 °C min−1 | Splitless injection 240 °C, ion source temperature 200 °C | (Razia et al., 2016) |
| HS-SPME/GC-FID | Potato chips, french fries | Homogenize, extract with HS-SPME at 60 °C for 30 min, desorp at 230 °C for 2 min | AA (Merck; Darmstadt, Germany) | BP20 polar capillary column (PEG, 30 m × 0.25 mm × 0.25 μm; SGE Company) | N flow rate 2.3 ml min−1, start at 45 °C for 2 min, raise to 150 °C at a rate of 10 °C min−1 | Detector temperature 200 °C, injector temperature 230 °C | (Ghiasvand & Hajipour, 2016) |
| FAAS (flame atomic absorption spectrometry) | Food samples: chips, crackers, cereal-based baby foods | Homogenize, defat with hexane, extract with MeCN and water with NaCl and MgSO4, add IS as preconcentration with ammonia buffer at pH 9.0, separate aqueous phase, dilute in MeOH | AA (Sigma, St. Louis, MO, USA) | n/a | n/a | Wavelength 232.0 nm, 0.7 nm slit, lamp current 6 mA, burner height 7 mm, nebulizing flow rate 6 mL min−1 | (Altunay et al., 2016) |
| Microchip electrophoresis | Food samples; potato chip and french fry | Homogenize, spike with IS and extract with water, defat with hexane, dilute in borate buffer, and derivatize with Cy5 | AA (Sinopharm Chemical Reagent Co. Ltd; Shanghai, China) | Glass microchip with separation channel 60 mm × 45 mm, etched to depth of 25 μm and width 70 μm | n/a | n/a | (Wu, Chen, Wang, He, & Wang, 2016) |
| HS-GC-MS | Brewed coffee | AA (Sigma-Aldrich, St. Louis, MO, USA) | Mega-FFAP-EXT column (50 m × 200 μm × 20 μm; Legnane, MI, Italy) | HS at 205 °C, sample loop and transfer line temperatures 215 °C and 225 °C, respectively, injector temperature 250 °C with 5:1 split ratio, He gas flow 1.0 ml min−1, initial temperature 50 °C held 1 min, ramp of 2 °C min−1 to 165 °C, then increased to 250 °C at 7.5 °C min−1 held 2 min | EI mode at 70 eV, SIM (single ion monitoring) acquisition mode | (Zhang, Cagliero, Pierson, & Anderson, 2017) | |
| HPLC-UV | Food samples; potato chips, coffee beans, french fries | Homogenize, extract with water, defat with hexane, clean up with Carrez I and II solutions | AA (Sigma-Aldrich, Hamburg, Germany) | Zorbax C18 steel analytical column (250 × 4.6 mm, 4.5 μm, Hewlett Packard, Houston, USA) | Six mobile phases utilized: 1. Water/MeOH (90:10 v/v), 2. MeOH/MeCN/Water (10:10:80 v/v/v), 3. Water/MeCN (90:10 v/v), 4. Water/formic acid (99.9: 0.1 v/v), 5. Water/formic acid (90:10 v/v) 6. Water (100%) | DAD wavelength set to 190 and 240 nm | (Oroian et al., 2016) |
| LC-MS/MS | Food samples; rice porridge, apple juice and peanut butter | Homogenize sample, spike with IS, extract with water, clean up with sorbent | aa- 13 c 3 | Atlantis T3 column (2.1 × 100 mm i.d., 3 μm; Waters Corporation, Dublin, Ireland) | Mobile phase 30 mm ammonium formate in water and 100% MeCN as solvents A and B, respectively; 90% A for 2 min, 2–2.5 min held at 87% A, 2.5–4 min 87%–86% A, 7.5–8.5 min 66–50% A, 8.5–11 min held at 50% A, 11–13 min 50–0% A, flow rate 0.25 ml min−1, 5 μl injection volume | Ionspray voltage 55 kV at 550 °C, curtain gas 25 psi, nebulizing gas 50 psi, heating gas 50 psi, DP 31 V, CEP 4.5 V, CE 15 V, CEP 18 V, CXP 4 V | (Lee et al., 2015) |
| GC-NCD | Food samples; dry and unheated potato, french fries, potato chips | Homogenize samples, extract with MeCN and water, add NaCl and Mg SO4, defat with hexane, clean up with PSA sorbent | Acrylamide (Dr. Ehrenstorfer, Augsburg, Germany) | DBWaxEtr pre-column (5 m × 0.53 mm × 1 μm) and DBWaxEtr analytical column (15 m × 0.53 mm × 1 μm) (Agilent) | Injection volume 20 μl, splitless mode, 3 min purge time, MMI inlet temperature 70 °C (1 min), increased by 600 °C min−1 to 300 °C, oven temperature 60 °C (1 min), increased by 25 °C min−1 to 150 °C and held 2.5 min, flow rate 20 ml min−1 He, initial source pressure 9.1 psi, switching flow from pressure control started 45 ml min−1 (1 min), increased by 2 mL min−1 to 40 ml min−1 | Nitrogen chemiluminescence detector conditions 930 °C burner temperature, 5 ml min−1 hydrogen and 10 ml min−1 oxygen | (Weijun, 2015) |