Internal Standard (IS)
Definition
A compound chemically near-identical to the analyte, often a deuterated isotopologue (d3-morphine, d5-EDDP, d5-alprazolam, pyrene-d10), spiked into every sample at a known concentration before extraction. Quantitation is reported as analyte peak area divided by IS peak area, which corrects for losses during clean-up, injection-volume variation and matrix-effect ion suppression at the source.
- Added to
- Every sample, calibration standard, and quality control before analysis
- Chemical form
- Isotopically labelled analogue (deuterium or 13C) of the target compound
- Quantitation method
- Analyte peak area divided by IS peak area, not absolute signal
- Common examples (drugs)
- d3-morphine, d5-EDDP, d5-alprazolam, pyrene-d10
- Common examples (ethanol)
- n-propanol, t-butanol
Common questions
Why add an internal standard if you can just measure the analyte peak?+
Because losses happen everywhere. As your sample moves through extraction, cleanup, and injection, some of it gets left behind or absorbed into container walls. An internal standard traces that loss. When you measure the ratio of analyte peak to IS peak, the losses cancel out. Both compounds lose the same percentage, so the ratio stays true even if the actual signal dropped.
How is an internal standard different from the drug I'm trying to measure?+
It is chemically almost identical, usually a deuterated isotopologue. Deuterium makes it slightly heavier. So in a mass spectrometer, a sample of deuterated morphine (d3-morphine) and real morphine show up as separate peaks even though they behave the same way during extraction and chromatography. That chemical near-identity is the whole point. Your internal standard must suffer the same matrix effects and injection losses as the target compound.
What does the internal standard correct for?+
Three main things: variation in how much sample actually gets injected (injection volume difference between runs), losses during sample preparation (extraction and cleanup), and matrix effects (molecules in the sample that suppress or enhance the ionization of your target in the mass spectrometer). The internal standard absorbs all those problems so your final number is reliable.
Related terms
- Certified Reference Material (CRM)
- A sample with one or more analyte concentrations established by an accredited body and traceable to a national metrology institute. NIST SRM...
- Measurement Uncertainty (MU)
- A quantified parameter associated with a result that characterises the dispersion of values reasonably attributable to the measurand. Estimated by the GUM...
- 2100:1 Partition Ratio
- The conventional blood/breath partition assumption used by breath alcohol analysers worldwide: 2100 mL of venous blood contains the same mass of ethanol...
- ATR (Attenuated Total Reflectance)
- An FTIR sampling mode in which the sample is pressed against a diamond or ZnSe crystal and the evanescent wave probes the...
- Beer-Lambert Law
- A = εbc. Absorbance equals molar absorptivity times path length times concentration. The linear relation between absorbance and concentration that makes UV-Vis...
- Beta Elimination Rate (Beta)
- The rate at which BAC falls during the post-absorptive phase of alcohol metabolism, expressed as mg/100 mL/hour. Population mean approximately 15 mg/100...
- Blood Alcohol Concentration (BAC)
- Mass of ethanol per unit volume of whole blood, expressed as mg per 100 mL (equivalent to mg/dL or mg%), or as...
- Bracketing Standards
- Calibration standards or quality control samples injected before, during and after a batch of case samples. They track instrument drift through the...
- Calibration Curve
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- Category a Method
- An analytical method that provides the highest level of discriminating power in drug identification, generating structural or molecular fingerprint information. Category A...
- Category B Method
- An analytical method with intermediate discriminating power, measuring a characteristic but less structurally specific property. Includes UV-Vis spectroscopy, TLC, GC without MS,...
- DB-ALC1 / DB-ALC2
- A matched pair of Agilent fused-silica GC columns used for dual-column orthogonal confirmation of blood ethanol. DB-ALC1 uses a polyethylene glycol stationary...
Explained in these topics
- Ethanol Analysis by GC-Headspace and the Blood-Alcohol BaselineA compound added at fixed known concentration to all vials before analysis. The analyte/IS peak area ratio is the quantity used for quantification, compensatin...
- Reference Standards, Certified Reference Materials and the Quantification BaselineA compound added at a known concentration to every analytical sample and calibration standard before preparation, used to correct for variations in extraction...
- Sample Preparation, Purification and Instrument CalibrationA compound chemically near-identical to the analyte, often a deuterated isotopologue (d3-morphine, d5-EDDP, d5-alprazolam, pyrene-d10), spiked into every sampl...
- Spectrometric Detection: UV-Vis, FTIR and Mass SpectrometryA chemically related compound, usually a deuterated analogue (d3-morphine, d5-EDDP, d5-alprazolam), spiked into every sample at a known concentration. Quantita...
- SWGDRUG Identification Tiers and the UV / TLC / GC-MS / LC-MS/MS WorkflowA compound added at known concentration to every sample, calibration standard, and quality control before analysis. Corrects for variation in extraction effici...