Which trace-metal acid grade do you actually need?
ACS, TraceGrade, 1 ppb, 100 ppt, 10 ppt - the ladder is about the blank you can tolerate, not about the assay on the label.
Most laboratories over-specify acid grade, and a few badly under-specify it. Both cost money. The question is not "how pure is the acid" but "how low is the blank I need to measure against".
Start from your detection limit
Every acid contributes trace metals to your digest. If you are quantifying lead at 1 µg/L, a nitric acid carrying 100 ppt of lead contributes a blank you can comfortably subtract. If you are chasing sub-ppt lead in a semiconductor rinse, that same acid is your result.
Work backwards:
- Write down the lowest analyte concentration you must report.
- Divide by ten. That is roughly the blank you can live with.
- Account for your dilution factor - a 1:50 digest dilutes the acid's contribution too.
The grades in practice
- ACS Reagent - the workhorse. Correct for titrimetry, gravimetry, pH work and general wet chemistry. Trace metals are controlled but not certified at ultratrace level.
- TraceGrade (1 ppb) - suitable for most environmental and food ICP-OES work, and for ICP-MS above about 1 µg/L.
- Ultrapure (100 ppt) - the usual choice for ICP-MS trace element work, including drinking water and clinical matrices.
- Ultrapure (10 ppt) - semiconductor, photovoltaic and high-purity materials work, where the specification is set by the wafer rather than the method.
Packaging matters as much as the acid
An ultrapure acid in the wrong bottle is no longer ultrapure. We pack trace-analysis acids in fluoropolymer, not glass, because borosilicate leaches boron, sodium and aluminium into the acid over its shelf life. Decant with a clean PFA pipette, never a metal spatula, and never return unused acid to the bottle.
A 10 ppt acid handled carelessly at the bench performs worse than a 1 ppb acid handled well.
A note on hydrofluoric acid
HF deserves separate thought. It attacks glass and silicon, which is exactly why it is used in wafer processing and silicate digestion - and exactly why it must never touch borosilicate labware. Keep calcium gluconate gel within arm's reach of the bench, not in a cupboard down the corridor.
Ask us for the lot-specific certificate before you commit a method to a grade. We report the actual measured values, not just the specification limits.