We have developed and demonstrated a high-sensitivity trace gas instrument employing two mid- infrared quantum cascade lasers and an astigmatic Herriott sample cell with up to a $240\mathrm{m}$ path length. Several aspects of astigmatic Herriott cell optics have been addressed to enable operation at a high pass number (up to 554), including aberrations and pattern selection to minimize interference fringes. The new instrument design, based on the $200\mathrm{m}$ cell, can measure various atmospheric trace gases, depending on the installed lasers, with multiple trace gases measured simultaneously. Demonstrated concentration noise levels ( $1\mathrm{s}$ average) are $40\mathrm{parts}$ per trillion [(ppt) ${10}^{-12}$ ] for formaldehyde, $10\mathrm{ppt}$ for carbonyl sulfide, $110\mathrm{ppt}$ for hydrogen peroxide ( ${\mathrm{H}}_2{\mathrm{O}}_2$ ), and $180\mathrm{ppt}$ for nitrous acid (HONO). High-precision measurements of nitrous oxide and methane have been recorded at the same time as high-sensitivity measurements of HONO and ${\mathrm{H}}_2{\mathrm{O}}_2$ .