An apparatus (100; 300) to provide an indication of constituents of interest in a sample having a first sample portion and a second sample portion, said apparatus comprising: a first analyzer including a first filter (121; 321) having a first area for accumulating the constituents of interest in the first sample portion, and a first sensor (131; 330, 331) to measure intensity of light transmitted through the first filter ; a first pump operable to draw the first sample portion through the first filter at a first flow rate (F1); a second analyzer including a second filter having a second area for accumulating the constituents of interest in the second sample portion, and a second sensor (133; 330, 333) to measure intensity of light transmitted through the second filter (123; 323); a second pump operable to draw the second sample portion through the second filter at a second flow rate (F2); and a computer to accept two sequential measurements from the first sensor and two sequential measurements from the second sensor, calculate, for each of said measurements, a corresponding attenuation, and provide an indication of the constituents of interest in the sample based on the attenuation corresponding to the first measurement by the first sensor, a change between the attenuations corresponding to the second and first measurements by the first sensor, the attenuation corresponding to the first measurement by the second sensor, and a change between the attenuations corresponding to the second and first measurements by the second sensor; and wherein the apparatus is configured such that said first filter and said second filter, in use, have different rates of constituent accumulation per unit filter area. a first analyzer including a first filter (121; 321) having a first area for accumulating the constituents of interest in the first sample portion, and a first sensor (131; 330, 331) to measure intensity of light transmitted through the first filter ; a first pump operable to draw the first sample portion through the first filter at a first flow rate (F1); a second analyzer including a second filter having a second area for accumulating the constituents of interest in the second sample portion, and a second sensor (133; 330, 333) to measure intensity of light transmitted through the second filter (123; 323); a second pump operable to draw the second sample portion through the second filter at a second flow rate (F2); and a computer to accept two sequential measurements from the first sensor and two sequential measurements from the second sensor, calculate, for each of said measurements, a corresponding attenuation, and provide an indication of the constituents of interest in the sample based on the attenuation corresponding to the first measurement by the first sensor, a change between the attenuations corresponding to the second and first measurements by the first sensor, the attenuation corresponding to the first measurement by the second sensor, and a change between the attenuations corresponding to the second and first measurements by the second sensor; and wherein the apparatus is configured such that said first filter and said second filter, in use, have different rates of constituent accumulation per unit filter area. <EOS>