Mass Spectrometry & Spectroscopy


Determination of the instrument detection limit of the ISQ 7610 single quadrupole GC-MS with the ExtractaBrite electron ionisation source Authors: Amit Gujar1


, Tim Anderson1 , Kenneth M. Free1 and Adam Ladak2 1. Thermo Fisher Scientifi c, Austin, TX, USA. 2. Thermo Fisher Scientifi c, Hemel Hempstead


To determine whether a GC-MS system is suitable for the detection of analytes occurring in trace and ultra-trace levels, the calculation of the instrument detection limit should be used. Historically, signal-to-noise ratio (S/N) has been employed to convey the detection limit. However, advancements of MS technologies have resulted in reduction of noise and increased sensitivity. When combined with lower bleed GC columns producing sharper peaks, noise levels go to zero in certain time segments. This results in very high signal-to-noise numbers and does not give a true representation of the limit of detection (LOD). Therefore, a more accurate or statistically signifi cant method of calculating the detection limit of an instrument is required.


According to the most recent U.S. Code of Federal Regulations,1


MDL is defined as: “the minimum Instrument Detection Limit


According to the most recent U.S. Code of Federal Regulations [1], MDL is defi ned as: “the minimum concentration of a substance that can be measured and reported with 99% confi dence that the analyte concentration is greater than zero and is determined from analysis of a sample in a given matrix containing the analyte.” This methodology can be seamlessly transferred when calculating IDL. Unlike MDL, the IDL uses solvent-based standards containing the test chemical at concentrations that give a consistent response over several repeat injections (e.g., %RSD <10). Therefore, the IDL is a statistically rigorous method that uses the precision of a measurement at low analyte levels and accurately refl ects the true detection limit of an instrument, ultimately defi ning how sensitive an analytical system is. Simply, the IDL can be determined by using the following equation [2].


5000 4500 4000 3500 3000 2500 2000 1500 1000 500


IDL = t × Amount × %RSD IDL = t × Amount × %RSD


Where, t = student t-value for one-tailed distribution: for example, for n = 8 injections; t = 2.998 Amount = amount of analyte (on-column) %RSD = relative standard deviation of the response


0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Injection number


Where, t = student t-value for one-tailed distribution: for example, for n = 8 injections; t = 2.998 Amount = amount of analyte (on-column) %RSD = relative standard deviation of the response


In this study, the IDL for octafl uoronaphthalene (OFN), a commonly used reference compound in GC-MS systems, was determined using the Thermo Scientifi c™ ISQ™ 7610 single quadrupole GC-MS system with the Thermo Scientifi c™ ExtractaBrite™ source operating in EI mode. The ExtractaBrite source is an electron ionisation source with the fi lament positioned orthogonally with respect to the ion beam. Thermo Scientific™


Experimental


An ISQ 7610 system equipped with the ExtractaBrite ion source was coupled with a Thermo Scientifi c™ TRACE™ 1610 GC and a Thermo Scientifi c™ AS 1610 liquid autosampler was used for the assessment of the instrument detection limits. The IDL was derived using repeated injections (n=8) of OFN at 10 fg/µL. Instrumental conditions are detailed in Table 1.


7.0e3 6.5e3


8.0e3 7.5e3 counts


6.0e3 5.5e3


5.0e3 4.5e3


4.0e3 3.5e3


3.0e3 2.5e3


2.0e3 1.5e3


1.0e3 5.0e2


RT: 3.863 – OFN


In this study, the IDL for octafluoronaphthalene (OFN), a commonly used reference compound in GC-MS systems, was determined using the Thermo Scientific™ ISQ™


7610 single quadrupole GC-MS system with the ExtractaBrite™


source operating in


EI mode. The ExtractaBrite source is an electron ionization source with the filament positioned orthogonally with respect to the ion beam.


Experimental An ISQ 7610 system equipped with the ExtractaBrite ion source was coupled with a Thermo Scientific™ TRACE™


1610 GC and a Thermo Scientific™ AS 1610


liquid autosampler was used for the assessment of the instrument detection limits. The IDL was derived using


To determine the instrument detection limit (IDL), the standard deviation of the response of an analyte of choice at a concentration close to the estimated detection limit is used. This approach gives much more reliable LOD values as it considers not only the signal intensity, but also the consistency of the response. Method detection limits (MDL) are fi rmly established with many regulatory bodies and can be defi ned in numerous ways.


%RSD=7 .


concentration of a substance that can be measured and reported with 99% confidence that the analyte concentration is greater than zero and is determined from analysis of a sample in a given matrix containing the analyte.” This methodology can be seamlessly transferred when calculating IDL. Unlike MDL, the IDL uses solvent-based standards containing the test chemical at concentrations that give a consistent response over several repeat injections (e.g., %RSD <10). Therefore, the IDL is a statistically rigorous method that uses the precision of a measurement at low analyte levels and accurately reflects the true detection limit of an instrument, ultimately defining how sensitive an analytical system is. Simply, the IDL can be determined by using the following equation2


2.6e3 2.5e3 2.4e3 2.3e3 2.1e3 2.0e3 1.9e3 1.8e3 1.6e3 1.5e3 1.4e3 1.3e3 1.1e3 1.0e3 8.8e2 7.5e2 6.3e2 5.0e2 4.0e2


2.8e3 2.8e3


Table 1. Experimental conditions for ISQ 7610 with Trace 1610 GCRT: 3.865 – OFN


Table 1. Experimental conditions for ISQ 7610 with Trace 1610 GC AS 1610 autosampler


counts Syringe Injection volume


10 µL, 25 gauge, 50 mm length, cone tip (P/N 36500525)


1 µL


Pre-injection solvent and cycles None Sample rinses


5


Post-injection solvent and cycles None TRACE 1610 GC system


Thermo Scientific™ Column Liner SSL mode


AS 1610 autosampler Syringe


Inlet temperature Split flow


Splitless time Injection volume


Pre-injection solvent and cycles None Sample rinses


Septum purge flow Carrier flow


Oven program Column Liner SSL mode Split flow Splitless time


Septum purge flow Carrier flow


Oven program


Inlet temperature Detector gain


ISQ 7610 GC-MS system MS transfer line temperature Ion source temperature Ionisation mode Emission current Scan start Scan mode Dwell time


5


Post-injection solvent and cycles None TRACE 1610 GC system


TraceGOLD™


TG-5MS 30 m × 0.25 mm × 0.25 µm (P/N 26098-1420)


Splitless Liner single taper with wool, 4 mm ID, 78.5 mm length (P/N 453A0924)


Splitless 220 ˚C


10 µL, 25 gauge, 50 mm length, cone tip (P/N 36500525)


50 mL/min 0.5 min


1 µLConstant flow of 5.0 mL/min Constant He flow of 1.2 mL/min


45 ˚C (0.5 min), 40 ˚C/min to 190 ˚C (0 min)


Thermo Scientific™ TraceGOLD™


TG-5MS 30 m × 0.25 mm × 0.25 µm (P/N 26098-1420)


250 ˚C 200 ˚C EI


Splitless Liner single taper with wool, 4 mm ID, 78.5 mm length (P/N 453A0924)


50 µA 3.10 min


Splitless 0.1 s


220 ˚C 1.00E+06


50 mL/min 0.5 min


Constant flow of 5.0 mL/min Constant He flow of 1.2 mL/min 45 ˚C (0.5 min), 40 ˚C/min to 190 ˚C


LAB ASIA - APRIL 2022 SIM m/z 272 min


3.80 3.81 3.82 3.83 3.84 3.85 3.86 3.87


3.88 3.89 3.90 3.91


3.92 3.93


Peak area, counts*s


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