High power play It's not far from the truth to say that laser diodes have revolutionized Raman spectroscopy. These solid-state lasers are attractive as Raman excitation sources because of their size, frequency, efficiency, and low cost. Unfortunately, typical lowpower laser diodes are unsuitable for Raman measurements of gases. High-power laser diodes are available, but they produce an astigmatic beam with multiple modes, each of which produces Raman scattering, complicating the Raman spectrum. M. Fink and co-workers at the University of TexasAustin describe a system that circumvents these problems. In their system, an external feedback cavity is used to mode-lock the laser. The diode beam is directed to a grating, and the first-order diffracted beam is reflected back into the laser cavity. The frequency of the laser is tuned by tilting this feedback mirror. All Raman systems need a way to reject Rayleigh scattering. The trick is ffnding a method to filter the Rayleigh band without removing the immediately adjacent Raman bands. In this setup, that task is accomplished by judiciously selecting the laser frequency—the D1 spectral line of rubidium—and inserting two rubidium gas cells between the sample and the spectrometer. The first cell absorbs most of the Rayleigh scattering from the sample. A second rubidium cell—pressure-broadened with argon—removes the edges of the Rayleigh scattering. These two cells significantly decrease the spectral background. This particular system has a spectral range of 210 cm -1 at 1.0 cm -1 resolution and 340 cm -1 at 2.0 cm -1 resolution. Rotational Raman spectra of C0 2 and N2 were recorded. For C0 2 , the Rayleigh line was completely removed, and the first rotational transition of the molecule could still be seen. A calculation of the temperature from the intensity of the Stokes bands indicated that the rotational lines were assigned properly. Similar experiments were performed for N 2 and air. The Raman spectrum of air had a slight Rayleigh feature, which the authors ascribe to a slight detuning of the laser from the rubidium D1 frequency. The authors suggest that their setup can measure the Raman spectrum 0.5 cm - 1 from the Rayleigh line, which will allow the analysis of Van der Waal's complexes, soft phonons, and spin waves. {Appll Spectrosc. 1 9 9 9 , 53, 491-96)
The Raman system consists of a diode laser, the sample chamber, two rubidium cells to suppress the Rayleigh scattering, and a 0.5-m single grating spectrometer. (Adapted with permission. Copyright 1999 Society for Applied Spectroscopy.)
SIMS of cellular membranes The function of cellular membranes is determined by the physical and chemical properties of the lipid bilayers that form them. Andrew Ewing, Nicholas Winograd, and co-workers at Pennsylvania State University use secondary ion MS (SIMS) to determine not only the chemical composition of the membranes but also their orientation. The spectral signature can identify whether the headgroup or the tailgroup of the phospholipid molecule is exposed to the vacuum. Images are constructed by rastering a Ga+ ion beam over the sample. Ions are desorbed from the sample and analyzed by time-of-flight MS. If the film is in a headgroup-up configuration, a peak at m/z 184, which is associated with the phosphocholine headgroup, predominates. On the other hand, if the tailgroup points up, m/z 311 is the major mass spectral component. In unoriented films, both groups are represented in the mass spectrum.
To investigate intact biological membranes, freeze-fractured, frozen-hydrated red blood cells were analyzed. When the samples were fractured, three possibilities existed for the exposed surface: a fracture at the membrane-water interface exposed the headgroups, splitting the bilayer revealed the tailgroups, and presenting a cross section of the cell caused the lipid bilayer to appear as a ring around the cell. In their experiments, the Penn State researchers found two characteristic mass spectra featuring peaks for phosphatidylcholine dipalmitoyl, cholesterol and water clusters. A heads-up fracture appeared most frequently Some spectra exhibited a strong feature at m/z 311 which is representative of palmitic acid tailgroups (the most common lioid fatty acids in human red blood cells) suggesting that the membrane bilayer was SDlit during fracture The fragmentation Dattern of the cholesterol associated with the membranes indicates that the cholesterol mav not be oriented as com monlv rhoiicht (I Am Chem SnS IQOl)
797 471 fill 7)
Positive ion SIMS spectra of freeze-fractured, frozen-hydrated red blood cell outer membranes, with (a) heads-up fracture and (b) tails-up fracture.
Analytical Chemistry News & Features, August 1, 1999 5 1 3 A
