The overall goal of this procedure is to visualize the gas stream of an ambient ionization source used with mass spectrometry, for the purpose of understanding and optimizing the ionization source. This method can help answer key questions in the ambient ionization mass spectrometry field, such as ionization efficiency versus source position. The main advantage of this technique is using visualized gas streams with an ambient ionization source to better understand the physical phenomenon associated with the detection technique.
Though this method can provide insight into ambient mass spectrometry, it can also be applied to other systems, such as physical phenomenon, which involve gas streams. An example would be the aerodynamics field. Generally, individuals new to this method will struggle, because initially it's difficult to figure out how to manually adjust the system to achieve proper alignment of the camera.
First, clamp a spherical concave mirror in a ring stand clamp large enough to support the mirror. Attach the ring stand clamp with the mirror to a three foot ring stand perpendicular to the floor. Have the ring stand and mirror placed to the side of the mass spectrometer source.
Adjust the face of the mirror so that it is parallel to, and at the same height, as the source. Then, position the mirror so that it's center is aligned with the center source region of the mass spectrometer. Next, attach a metal plate to top of the tripod.
Attach a razor blade, which is known as the cut off, to the metal plate using a magnet, so that the sharp edge is vertical. Place the tripod in line with the mirror, at twice the focal length of the mirror. Then, align the razor blade orthogonal to the path of light reflected from the mirror.
Manually adjust the height of the tripod so that the sharp edge of the razor blade is approximately aligned with the center of the mirror. Following this, mount a digital camera, with a 300 millimeter telephoto lens on a separate tripod. Position the camera so the lens is four centimeters directly behind, and at the same height, as the razor blade.
Connect the video output of the camera to a computer monitor to easily view the Schlieren phenomenon in real time. Drill a small hole approximately zero point six millimeters in diameter into the center of a vial cap in order to attach it to a flashlight. Affix the vial cap over a 200 lumen LED flashlight, using high temperature foil tape.
The experiment will only work if alignment of all the components is correct. Please take care to ensure proper positioning. To ensure proper positioning of the flashlight, place a laser pointer on the metal plate next to the razor blade.
Manually move the laser pointer so the bean is hitting the center of the mirror. Adjust as necessary to ensure the reflected beam intersects orthogonally to the razor blade, so that approximately half of the beam is blocked. If the beam is not aligned, manually adjust the mirror's position to aim the beam of the laser pointer directly at the razor blade.
After ensuring that the laser beam is centered on the lens, replace the laser pointer with a covered flashlight while everything is aligned. Next, turn on the flashlight. Using a piece of white paper, observe the reflected light at the cut off, ensuring that the beam is a small, focused spot.
Make any vertical adjustments necessary to block approximately half of the reflected light beam with the cutoff. Then, remove the lens cap on the camera and focus on the mirror. Manually align the mass spectrometry ion source within the test region, with a distance of 10 millimeters between the end of the nozzle, and the inlet of the mass spectrometer.
Following this, manually open the needle valve to the ambient source, allowing nitrogen to flow through it. Open the software used to control the mass spectrometer. Click on File, and select Open Tune.
Then, select the appropriate Tune file. After opening the manual Tune, apply all voltages and temperatures to the ambient source. Observe the appearance of flow exiting the nozzle with the Schlieren apparatus, on the view screen of the digital camera at the temperature increases.
Once the desired image is visualized on the camera, collect it by taking a picture of the gas stream. At this point, open the acquired image using image viewing software, and print it. Manually draw a line using a pencil and a ruler on the printed image, defining the center axis of the gas stream parallel to the direction of flow.
Using the ruler, manually draw a line along the edge of the visualized gas stream on the printed image. Mark the outer edges of the gas stream to obtain a range for the spray half angle. Finally, measure the angle produced between the center axis and the line using a protractor.
When all components in the Schlieren set up are properly aligned, gasses within the test region can be seen as contrasting dark and light regions. This contrast can be used to observe how the shape of the nitrogen jet flow from the mass spectrometry source changes as nozzle size decreases. A full, uncropped, Schlieren image of the source, and gas flow, illustrates the orientation of the test objects relative to the mirror, and shows what should be expected when the proper amount of light is cut off.
If the cut off is either too high, or too low, poor images will result. With constant nozzle size the half angle increases accordingly with an increase in pressure, signifying an overall size increase of the gas stream. The half angle increases with larger nozzle diameter at constant pressure, which signifies an overall scaling increase in size of the nitrogen jet, coming out of the source as the nozzle diameter is increased.
Once mastered, this technique can be done in one to two hours, if it is performed properly. After watching this video, you should have a good understanding of how to visualize the gas stream of an ambient ionization source. Don't forget that working with laser pointers can be extremely hazardous, and protective eyewear should always be worn while performing this procedure.
