A method of ionisation comprising: heating a sample such that analyte is desorbed and/or evaporated from the sample; accumulating analyte desorbed and/or evaporated from the sample; and then passing the accumulated analyte to an ionisation region and ionising the analyte; wherein the method comprises: heating the sample and accumulating analyte desorbed and/or evaporated from the sample in one or more heating and/or accumulation regions, wherein said one or more heating and/or accumulation regions comprise a first inlet configured to allow a gas flow to enter said one or more heating and/or accumulation regions and a first outlet configured to allow the gas flow to leave said one or more heating and/or accumulation regions and pass to said ionisation region; operating a first inlet valve and a first outlet valve in a closed state while accumulating the analyte; passing the accumulated analyte to the ionisation region by opening the first inlet valve and the first outlet valve; operating a bypass valve in an open state while accumulating the analyte, thereby providing gas to the ionisation region via a bypass while accumulating the analyte; and operating the bypass valve in a closed state while passing the accumulated analyte to the ionisation region. heating a sample such that analyte is desorbed and/or evaporated from the sample; accumulating analyte desorbed and/or evaporated from the sample; and then passing the accumulated analyte to an ionisation region and ionising the analyte; wherein the method comprises: heating the sample and accumulating analyte desorbed and/or evaporated from the sample in one or more heating and/or accumulation regions, wherein said one or more heating and/or accumulation regions comprise a first inlet configured to allow a gas flow to enter said one or more heating and/or accumulation regions and a first outlet configured to allow the gas flow to leave said one or more heating and/or accumulation regions and pass to said ionisation region; operating a first inlet valve and a first outlet valve in a closed state while accumulating the analyte; passing the accumulated analyte to the ionisation region by opening the first inlet valve and the first outlet valve; operating a bypass valve in an open state while accumulating the analyte, thereby providing gas to the ionisation region via a bypass while accumulating the analyte; and operating the bypass valve in a closed state while passing the accumulated analyte to the ionisation region. heating the sample and accumulating analyte desorbed and/or evaporated from the sample in one or more heating and/or accumulation regions, wherein said one or more heating and/or accumulation regions comprise a first inlet configured to allow a gas flow to enter said one or more heating and/or accumulation regions and a first outlet configured to allow the gas flow to leave said one or more heating and/or accumulation regions and pass to said ionisation region; operating a first inlet valve and a first outlet valve in a closed state while accumulating the analyte; passing the accumulated analyte to the ionisation region by opening the first inlet valve and the first outlet valve; operating a bypass valve in an open state while accumulating the analyte, thereby providing gas to the ionisation region via a bypass while accumulating the analyte; and operating the bypass valve in a closed state while passing the accumulated analyte to the ionisation region.