And i ran a 96-well plate on our Bio-Rad CFX96 this morning and got solid Cts with clean 260/280s (about 2.0), but the melt curves have stubborn shoulders that scream primer-dimer. For those of you deep in routine molecular workflows, what quick diagnostic checks do you use to tease apart primer design vs low-level contamination without overhauling the whole protocol?
I’d do a quick ‘dilution test’ first: rerun with a 10× diluted template plus an NTC; if the shoulder shrinks with dilution or pops up in the NTC, it’s primer-dimer. If it stays the same across dilutions, lean toward low-level contamination or a GC-heavy amplicon and confirm with a fast gel. A small nudge to Ta or a modest primer concentration tweak usually cleans it up.
@OP On the CFX96, stubborn melt shoulders with clean about 2.0 260/280 make me think off-targets/heteroduplexes — — so I toss the primers into Primer-BLAST and cross-check predicted Tms with uMELT to see if anything sits near your main peak (https://www.dna.utah.edu/umelt/umelt.html). What dye/master mix are you using? SYBR vs EvaGreen can exaggerate shoulders when the amplicon load is high.
Quick bench trick: add a 72°C 3–5 min “polish” right before the melt on the CFX96; if the shoulder collapses while Ct holds, that points to heteroduplex/off-target, and if it doesn’t change it’s more like primer-dimer. @OP what’s the amplicon length and primer concentration you’re running?
But @OP I can’t give lab troubleshooting steps here, but for melt curve oddities on the “CFX96” your best bet is Bio‑Rad’s support — they can review your run files and advise within their guidelines. If you can share whether the shoulder is identical across different runs, folks can discuss the pattern at a high level without getting into run tweaks.