Blank concentration
Subtract blank concentration toward NativeConc; Blank use; vs Blank offset; Solvent notes.
Blank concentration correction (Blank corr on the Quants grid) subtracts the method-blank concentration after the curve has been read. The corrected amount goes into NativeConc.
It is not Blank offset. Blank offset is a Curve origin: it uses blank response as intercept b. Blank corr never changes the fit and never changes on-curve Concentration.
What it is vs Blank offset
| Setting | Space | What it changes |
|---|---|---|
| Origin = Blank offset | Response / intercept | Uses blank response on the curve (y = m·x + b). On-curve Concentration already follows that curve. |
| Blank corr | Concentration | After quantitation: sample concentration minus blank concentration → NativeConc (then minus Dose when a dose is present). Concentration and FConc stay on-curve. |
If both are on, Blank offset wins. Blank offset already removes blank background on the curve. Subtracting blank concentration as well would correct twice. Analyze still runs: it keeps Blank offset and skips concentration subtraction. The Quants grid shows Blank warn, and every result of that Quant gets a Warnings entry. Turn off Blank corr if you only want the offset, or turn off Origin = Blank offset if you want concentration subtraction.
A Type Blank bottle is still not a calibration level. Blank corr reads its on-curve Concentration like any other vial, then uses that amount as the blank concentration to subtract.
When Blank corr is on or off
Blank corr defaults to off.
| Blank corr | What Analyze writes |
|---|---|
| Off | No blank concentration is stored. NativeConc is the usual path: Concentration, or Concentration − Dose when a dose is present. Standard-addition groups still set NativeConc from the addition line. |
| On | Same Blank use rules as Blank offset pick the bottles. Blank concentration = average of those blanks’ on-curve Concentration (0 if none, or Blank use = None). NativeConc uses the formulas below. A Warnings entry is written when Blank use is None or no usable blanks were found. |
| On, no eligible blanks | Blank concentration = 0. NativeConc matches the uncorrected path. Warning on the result. |
| On + Origin = Blank offset | Analyze still runs. Blank offset is applied. Concentration subtraction is skipped. Blank concentration stays empty. NativeConc stays on the Blank-offset curve. Warning on the Quant and on every result. |
Turning Blank corr off and re-Analyze clears the stored blank-concentration list. Origin = Blank offset does not turn Blank corr on by itself.
Formula (what happens after Analyze)
Analyze first reads the curve to get on-curve Concentration. Then:
- Collect non-rejected blanks with the Quant’s Blank use (see Blank use).
- Blank concentration = average of those blanks’ on-curve Concentration. None (or Blank use = None) →
0. - Blank-corrected = Concentration − blank concentration.
- Normal path: NativeConc = blank-corrected when there is no dose; blank-corrected − Dose when a dose is present.
- Standard-addition CalGroups: keep the shared native from the addition line, then subtract blank concentration. Recovery % still uses the spike increment.
Negative NativeConc is allowed (same as dose subtraction). FinalConcentration stays on-curve (Concentration × dilution) — it is not blank-corrected.
How NativeConc is filled
NativeConc is the unspiked amount in the vial. Blank corr changes how that number is derived; it does not replace on-curve Concentration.
| Path | NativeConc when Blank corr is on |
|---|---|
| No dose | Concentration − blank concentration |
| Dose present (not standard addition) | (Concentration − blank concentration) − Dose |
| Standard-addition CalGroup | Shared standard-addition native − blank concentration |
On the result grid you see:
- BConc — blank concentration that was subtracted
- NConc — NativeConc after the subtract
- Blanks used — which bottles supplied BConc
- Conc — still the on-curve reading
Why subtract in concentration space
Detector response is often not a straight line with amount (especially with electrospray / electron multiplier). Subtracting responses and then reading the curve is only exact when sensitivity is constant.
A method blank is an amount of analyte (process or reagent contamination). Amounts add and subtract in concentration space.
So:
- use Blank offset when you want the curve to sit on a measured blank response;
- use Blank corr when you want to remove a measured blank concentration from NativeConc after the curve is read.
Do not do both at once. When both settings are on, Analyze prefers Blank offset, skips concentration subtraction, and writes a warning.
Blank use (which blanks)
The same Blank use column on the Quants grid picks bottles for Blank offset and Blank corr. Only non-rejected Type Blank bottles contribute.
Type Solvent is different:
- Solvent is read from the curve like Sample.
- Solvent is not a blank source for Blank use.
- Even when Blank corr is on, blank-concentration subtraction is not applied to Solvent results.
- Blank offset may still apply through the shared curve.
A bottle is skipped when the sample is rejected, that Quant’s result is rejected, or the blank has no usable response. After the curve is read, those bottles’ on-curve Concentration values are averaged.
| Blank use | Selection |
|---|---|
| None | No blanks. Blank concentration = 0. NativeConc is unchanged. |
| Average (CalGroup) | Average of eligible blanks in the same CalGroup as the curve. Unlabeled samples share the default group. |
| Average (overall) | Average of eligible blanks in the whole batch, all CalGroups. |
| Previous, else closest | The blank before the bracket span in run order; if none, the closest blank. Stays inside the CalGroup. |
| Bracket (previous + next) | Average of the blank before and after the span. One side is enough if the other is missing; if both are missing, same fallback as Previous, else closest. Stays inside the CalGroup. |
The span is the same run-time range Blank offset uses.
Use Average (CalGroup) or a sequence mode when groups should keep their own blanks. Use Average (overall) when one set of blanks should serve every group.
Rejected blanks never enter blank concentration. Type Solvent never enters it either.
After Analyze: what you see
Each result (and its calibration bracket) stores:
- BConc / BlankConcentration — the on-curve blank concentration subtracted toward NativeConc
- Blank use — how the blanks were picked
- Blanks used — sample names (or file names) that contributed
One blank-concentration value is computed per bracket, then copied onto every result that used that curve.
Each result also stores:
- Warnings — Analyze warnings for that result (both options on, missing blanks / Blank use = None, Blank offset fallback
b = 0, standard-addition failure, and similar) - Outliers — which flags fired (a consolidated list, not the individual flag columns). Both lists are cleared and rebuilt on Analyze.
Method editor
On the Quants grid:
- Blank corr → checked
- Blank use → how to pick blanks (shared with Blank offset)
- Blank warn → filled when Origin is also Blank offset (Analyze still runs; offset is applied and subtract is skipped)
ISTD Quants can use Blank corr the same way; ISTD still does not use Blank offset.
What this is not
- Not a curve-origin option. Origin = Blank offset does not turn Blank corr on. If both are on, Analyze still runs, prefers Blank offset, and skips concentration subtraction with a warning.
- Not a change to on-curve Concentration / FConc.
- Not subtraction of blank response from the sample before reading the curve.
- Not a zero-concentration calibration level.
- Not applied to Type Solvent results (those stay on the normal NativeConc path). Solvent is also not a Blank use source.
Next: Blank offset, ESTD, ISTD, bracketing, method design.