A method for the detection or quantitation of an analyte in a biological
sample is disclosed. The method comprises the steps of selecting a
biological sample suspected of containing a target analyte and binding
the target analyte in the sample to a binding partner conjugated to an
acridinium compound, wherein the acridinium compound comprises an
acridinium nucleus having an electron-donating substituent directly
attached to the acridinium nucleus at the C.sub.2 position.
Chemiluminescent acridinium compounds useful in the method have emission
maxima close to or in the near infrared (NIR) region (>590 nm). These
chemiluminescent acridinium compounds when used in conjunction with short
wavelength-emitting acridinium esters (with emission maxima below 450 nm)
can be highly useful labels for the simultaneous detection of multiple
target analytes in a single assay.